Septic arthritis is an acute infection of a joint space, most commonly caused by bacteria such as *Staphylococcus aureus*, leading to rapid cartilage destruction and joint damage if not promptly treated.

Exam domain	One-glance essentials
Definition / diagnosis	*Septic arthritis is infection of the joint space and a rheumatological emergency* ^[1] Rapid bacterial and neutrophil-mediated injury can destroy cartilage within days ^[2]
Epidemiology / risks	Extremes of age, RA, prosthetic joint, diabetes, immunosuppression, IVDU and STI exposure increase risk ^[1] *S. aureus is the commonest organism*; consider gonococcus in sexually active young adults ^[2]
Core mechanism	Haematogenous seeding (most common) or direct/contiguous spread → purulent synovitis → irreversible cartilage loss ^[1]^[2] Hip effusion under pressure may compromise femoral-head perfusion → AVN ^[2]
Clinical picture	*A hot, swollen, tender joint is septic arthritis until proven otherwise* ^[1] Usually acute monoarthritis; knee is commonest overall, while children may refuse to walk with hip disease ^[2]
Investigations	*Aspirate before antibiotics: cell count, Gram stain, culture and crystals; take blood cultures ^[1]^[3] Synovial WBC > 50,000/mm³ with > 90% neutrophils* strongly supports infection but is not independently diagnostic ^[3]
Management	*Urgent joint drainage + empirical IV antibiotics, then tailor to culture* ^[1]^[4] Involve orthopaedics early; hip infection usually needs operative drainage, followed by early physiotherapy ^[4]
Complications	Irreversible cartilage destruction, secondary OA, ankylosis and contracture ^[2] Osteomyelitis, sepsis and metastatic infection; specifically look for co-existing osteomyelitis ^[1]^[2]
Prevention / follow-up	Prompt aspiration and treatment prevent structural damage; maintain asepsis for injections and surgery ^[1]^[2] Address bacteraemic sources and begin early range-of-motion physiotherapy once safe ^[4]
Exam traps	*Crystals do not exclude concurrent septic arthritis—always send microbiology ^[3] Normal fever, WCC, ESR or CRP does not* safely exclude infection; never inject intra-articular steroid before sepsis is excluded ^[1]^[4]

References

^[1] Lecture slides: GC 075. Pain red joint.pdf (septic arthritis, pp.18–24) ^[2] Senior notes: Adrian Lui Pediatrics Notes.pdf (Section 13.2.4 Septic Arthritis, pp.453–454) ^[3] Senior notes: Maksim Medicine Notes.pdf (Section 13.8 Septic arthritis and synovial-fluid table, pp.309, 331) ^[4] Senior notes: Handbook of Internal Medicine 2024.pdf (Septic Arthritis management, p.429)

Septic Arthritis

Septic arthritis (from Latin septicus = putrefying, Greek arthron = joint, -itis = inflammation) is an infection within a joint space — a rheumatological and orthopaedic emergency characterised by an inflammatory, destructive monoarthritis ^[1]^[2]^[3]. The key concept is that bacteria (most commonly) or other organisms invade the normally sterile synovial space, triggering an intense neutrophilic inflammatory response that, if untreated, destroys articular cartilage within days through a combination of direct bacterial enzymatic damage and host-mediated inflammatory proteolysis ^[2]^[3].

The Golden Rule — GC Lecture Slide

"A hot, swollen, tender joint is septic arthritis until proven otherwise, even without fever, leukocytosis, or elevated ESR/CRP" ^[1]^[2]^[3]. This means you MUST perform synovial fluid analysis on any acute inflammatory monoarthritis — you cannot "rule it out" clinically.

Why is this an emergency?

Hyaline cartilage is avascular and aneural — it relies on diffusion of nutrients from synovial fluid. When pus fills the joint, the cartilage is bathed in proteolytic enzymes (from both bacteria and neutrophils) and is physically cut off from its nutrient supply.
Cartilage has no capacity for regeneration in adults. Once destroyed, the damage is permanent — leading to secondary osteoarthritis, ankylosis, or need for joint replacement.
Prompt and proper treatment leaves the joint without permanent structural damage ^[2]^[3].

Parameter	Detail
Incidence	~2–10 per 100,000 person-years in the general population; much higher in those with RA (~30–70 per 100,000) or prosthetic joints
Age distribution	Bimodal: children < 5 years and elderly > 65 years ^[1]^[2]
Sex	Slight male predominance overall; gonococcal arthritis more common in young women
Most affected joint	Knee ( > 50% of cases), followed by hip, wrist, ankle, shoulder ^[3]^[5]
Mortality	~10–15% overall; higher in polyarticular septic arthritis (~30%) and in elderly/immunocompromised
Morbidity	Up to 40–50% have some residual joint dysfunction if treatment is delayed

Hong Kong Context

Staphylococcus aureus remains the dominant organism in HK, as elsewhere ^[1]^[4]^[7].
Methicillin-resistant S. aureus (MRSA) is an important consideration in HK hospitals — empirical cover must account for local antibiograms.
Disseminated gonococcal infection (DGI) is less common in HK than in Western populations but should be considered in sexually active young adults.
TB arthritis is an important differential in HK given the intermediate TB burden — typically presents as a chronic, indolent monoarthritis (see later differentiation from pyogenic arthritis) ^[7].
Burkholderia pseudomallei (melioidosis) can cause septic arthritis in Southeast Asia/HK in diabetic or immunocompromised patients — rare but important.

Risk factors are critical to identify because they determine both the likelihood of septic arthritis and the probable causative organism ^[1]^[2]^[3].

Risk Factor Category	Specific Factors	Pathophysiological Basis
Extremes of age	< 5 years or > 65 years ^[1]	Immature or declining immune function; children have vascular metaphyseal anatomy that predisposes to haematogenous seeding
Chronic arthritic syndromes	RA, crystal-induced arthritis (gout/pseudogout), severe OA, Charcot (neuropathic) joints, haemarthroses ^[1]	Damaged synovium has abnormal vascularity → bacteria adhere more easily; RA patients on immunosuppressants; Charcot joints have impaired sensation → injuries go unnoticed
Prosthetic joints	Joint replacement surgery ^[1]^[3]	Foreign body provides surface for biofilm formation; bacteria adhere to prosthetic materials and become resistant to both antibiotics and immune clearance
Intra-articular injection or arthrocentesis	Steroid injections, joint aspiration ^[1]^[3]	Direct inoculation breaches sterile barrier; corticosteroids locally suppress immune response
Parenteral drug use (IVDU)	^[1]	Bacteraemia from non-sterile injection → haematogenous seeding; unusual organisms (Pseudomonas, Serratia, Candida)
Sexually transmitted disease	Predisposes to localised gonococcal infection, which may disseminate to cause joint disease ^[1]	N. gonorrhoeae has tropism for synovium; complement deficiency (C5–C9) predisposes to disseminated Neisseria infections
Chronic skin infections	Cellulitis, ulcers, wounds ^[1]^[2]	Portal of entry for bacteria → bacteraemia → haematogenous seeding
Chronic systemic illness	DM, SLE, malignancy, immunocompromised state ^[1]^[2]^[3]	Impaired neutrophil function (DM), generalised immunosuppression; malignancy and its treatment cause immune deficiency
Diabetes mellitus	^[3]	Impaired chemotaxis, phagocytosis, and intracellular killing by neutrophils; peripheral neuropathy → unnoticed skin breaks
Infective endocarditis	^[3]	Continuous bacteraemia seeds multiple joints
Recent joint surgery	^[3]	Breach of sterile joint space; post-operative haematoma provides culture medium
Immunosuppressants	Corticosteroids, DMARDs, biologics ^[3]	Suppress both innate and adaptive immune responses
Alcoholism	^[3]	Immune suppression, malnutrition, increased fall risk, liver dysfunction

High Yield — Why RA Patients Get Septic Arthritis

RA patients are at particularly high risk for several compounding reasons:

Chronically inflamed synovium has abnormal vasculature → easy bacterial adhesion
Immunosuppressive medications (methotrexate, biologics like anti-TNF agents) impair host defence
Joint deformity → functional impairment → skin breakdown → portal of entry
RA itself impairs immune function This is why septic arthritis must always be excluded in an RA patient presenting with a flare of a single joint — it can mimic an RA flare exactly.

Anatomy and Functional Relevance

Understanding the anatomy of a synovial joint is essential to comprehend why septic arthritis behaves as it does.

A synovial (diarthrodial) joint consists of:

Articular (hyaline) cartilage: Covers the bone ends; avascular, aneural, alymphatic — entirely dependent on diffusion from synovial fluid for nutrition. This is why it is so vulnerable to destruction in septic arthritis.
Synovial membrane (synovium): Lines the inner surface of the joint capsule but does NOT cover the articular cartilage. It is:
- Highly vascular (fenestrated capillaries)
- Produces synovial fluid (ultrafiltrate of plasma + hyaluronic acid + lubricin)
- Contains type A synoviocytes (macrophage-like, phagocytic) and type B synoviocytes (fibroblast-like, produce hyaluronic acid)
Synovial fluid: Normally a clear, viscous fluid; < 200 WBC/mL; acts as lubricant and nutrient medium for cartilage.
Joint capsule: Fibrous outer layer; provides mechanical stability.
Subchondral bone: Beneath the cartilage; well-vascularised.

The synovial membrane has a rich blood supply with fenestrated capillaries and no limiting basement membrane on its synovial surface — this means bacteria in the bloodstream can easily cross into the joint space.
Once bacteria enter, the joint space is relatively sequestered — there is limited lymphatic drainage and the avascular cartilage creates a "privileged site" where immune cells have difficulty completely clearing infection.

In infants < 1 year: The growth plate (physis) has transphyseal blood vessels connecting the metaphysis to the epiphysis. This means a metaphyseal osteomyelitis can spread across the growth plate into the epiphysis and then into the joint → septic arthritis can complicate osteomyelitis ^[7].
In children > 1 year: The growth plate acts as a vascular barrier — infection typically remains in the metaphysis and does NOT cross into the joint (unless the metaphysis is intracapsular, as in the proximal femur/hip, proximal humerus, proximal radius, and distal lateral tibia).
In adults: Growth plates have fused; vertebral bodies are the most common site of haematogenous osteomyelitis (but septic arthritis still occurs via haematogenous seeding of the synovium itself).

The proximal femoral metaphysis is intracapsular (i.e., within the hip joint capsule). This means:

Osteomyelitis of the proximal femur can directly decompress into the hip joint → septic arthritis
In children, the hip is the most commonly affected joint in septic arthritis ^[2]
The hip joint cannot be easily examined visually or palpated → clinical detection is harder
The characteristic posture of a child with hip septic arthritis: leg held flexed, abducted, and externally rotated (this position maximises the intracapsular volume, thereby minimising intracapsular pressure and thus pain) with no spontaneous movement (pseudoparesis/pseudoparalysis) ^[2]

Etiology (Focus on Hong Kong)

There are four main routes by which organisms reach the joint ^[2]:

Haematogenous spread (most common): Bacteria in the bloodstream from a distant focus (e.g., skin, urinary tract, respiratory tract, endocarditis) seed the highly vascular synovial membrane.
Contiguous spread from osteomyelitis: Particularly important in the hip (intracapsular metaphysis) and in infants (transphyseal vessels).
From adjacent soft tissue infection: Cellulitis, wound infection, bursitis → direct extension through the capsule.
Direct inoculation: Penetrating trauma, arthroscopy, arthrocentesis, intra-articular injection, joint surgery.

Microbiology

The causative organism varies by age and clinical context ^[1]^[2]^[3]^[4]^[7]:

Age Group	Most Common Organisms	Notes
Neonates (< 1 month)	Group B Streptococcus, Gram-negative bacilli (e.g., E. coli), S. aureus ^[2]	Acquired during birth or from neonatal bacteraemia
Infants/children < 2 years	H. influenzae serotype B ^[2]	Now rare in vaccinated populations (Hib vaccine); S. aureus and Kingella kingae more common in vaccinated children
Children > 2 years and adults	S. aureus ^[2]^[4]^[7]	The single most common organism across all ages (except neonates)
Elderly or chronically ill	Gram-negative bacilli ^[2]	E. coli, Klebsiella, Pseudomonas — reflect urinary/biliary sources of bacteraemia

Population	Organism	Explanation
Sexually active young adults	Neisseria gonorrhoeae ^[1]^[2]^[3]^[4]	Part of disseminated gonococcal infection (DGI) — usually polyarticular ^[2]; presents with migratory polyarthralgia, tenosynovitis, vesiculopustular skin lesions → then may localise to purulent monoarthritis
IVDU	Gram-negative bacilli (e.g., P. aeruginosa), S. aureus ^[2]	Non-sterile injection; unusual portals of entry
Late prosthetic joint infection	S. aureus ^[2]; also coagulase-negative staphylococci (e.g., S. epidermidis) ^[4]	Biofilm formation on prosthetic material; S. epidermidis for early/delayed prosthetic joint infection
Skin/soft tissue infection	Streptococcus spp ^[2]	Contiguous spread from cellulitis
Foul-smelling or gas-forming	Anaerobes (e.g., Clostridium) ^[2]	Suggest tissue necrosis or penetrating/bowel-related source
Immunocompromised (HK context)	M. tuberculosis ^[4]^[7]	Indolent, chronic monoarthritis; important in HK with intermediate TB burden
Sickle cell disease	Salmonella species (for osteomyelitis); S. aureus still most common for septic arthritis	Functional asplenia predisposes to encapsulated organisms; Salmonella has tropism for infarcted bone

GC Lecture Slide — Causes of Septic Arthritis

Causes: Bacterial, Mycobacterial, Fungal, Viral ^[1]. In practice, pyogenic bacterial infection (especially S. aureus) accounts for the vast majority. Mycobacterial (TB), fungal (Candida in immunocompromised/IVDU), and viral causes are less common but must be considered in the right clinical context.

S. aureus is the most common organism because:

It has multiple surface adhesins (e.g., clumping factor, fibronectin-binding proteins) that allow it to bind to joint tissues and prosthetic materials
It produces biofilm that protects it from antibiotics and immune cells
It secretes proteases, lipases, and toxins (including Panton-Valentine Leukocidin, PVL) that damage tissue and evade immune responses
It is a common skin commensal → easy access to the bloodstream through skin breaks

This distinction is clinically important because their presentations and management differ ^[2]^[3]:

Feature	Gonococcal	Non-gonococcal
Demographics	Young, sexually active	Any age; elderly, immunocompromised
Presentation	Triad: migratory polyarthralgia/arthritis + tenosynovitis + dermatitis (vesiculopustular lesions) → may localise to purulent monoarthritis	Acute monoarthritis (80–90%)
Joint distribution	Distal joints: knees, wrists, ankles, small joints of hands	Large joints: knee, hip
Synovial fluid culture	Often negative (organism is fastidious)	Usually positive
Blood culture	Usually negative	Positive in ~50%
Diagnosis	Clinical + NAAT (PCR) of urogenital/pharyngeal/rectal swabs + synovial fluid PCR	Synovial fluid Gram stain + culture
Prognosis	Excellent — rarely causes permanent joint damage if treated	Worse — significant risk of cartilage destruction
Treatment	IV/IM ceftriaxone (shorter course)	Prolonged IV antibiotics (≥ 2 weeks IV, then PO 2–4 weeks)

Pathophysiology

Understanding the pathophysiology explains every clinical feature and guides rational management.

Entry: Bacteria cross the fenestrated synovial capillaries (no basement membrane barrier) into the joint space. The synovial fluid is an excellent culture medium — warm (37°C), nutrient-rich, and relatively immune-privileged.
Inflammatory cascade: Bacterial surface molecules (e.g., lipoteichoic acid from Gram-positives, LPS from Gram-negatives) activate pattern recognition receptors (TLRs) on synoviocytes and resident macrophages → release of pro-inflammatory cytokines (IL-1β, IL-6, TNF-α) → massive recruitment of neutrophils from the bloodstream.
Purulent effusion: The hallmark. Neutrophils flood the joint (WBC > 50,000–100,000/mL, > 90% neutrophils). These neutrophils phagocytose bacteria but also undergo degranulation, releasing:
- Proteases (elastase, cathepsin G) — digest cartilage matrix
- Matrix metalloproteinases (MMPs) — degrade collagen and proteoglycans
- Reactive oxygen species (ROS) — oxidative damage to chondrocytes
- Neutrophil extracellular traps (NETs) — trap bacteria but also damage surrounding tissue
Cartilage destruction: Occurs through two synergistic mechanisms:
- Direct bacterial damage: Bacterial enzymes (e.g., hyaluronidase from streptococci, proteases from staphylococci) directly degrade cartilage matrix components
- Host-mediated ("bystander") damage: Neutrophil-derived proteases and cytokine-driven chondrocyte apoptosis — this is actually the dominant mechanism of cartilage destruction
Raised intra-articular pressure: The effusion increases pressure within the closed joint capsule → compresses the synovial vasculature → reduces blood supply to the already avascular cartilage (further compromising its nutrient supply) → can cause avascular necrosis of subchondral bone (especially in the femoral head in hip septic arthritis).
Systemic response: Cytokines (IL-1, IL-6, TNF-α) enter the systemic circulation → fever, malaise, elevated CRP/ESR, leukocytosis (the acute phase response). However, these may be absent — particularly in the elderly, immunocompromised, or those on immunosuppressants ^[1]^[2]^[3].

Why Cartilage is Destroyed So Quickly

Cartilage is avascular — once the joint is filled with pus, the cartilage loses its only source of nutrition (synovial fluid diffusion). Simultaneously, it is attacked from two fronts: bacterial enzymes from above and ischaemia from raised intra-articular pressure compressing subchondral vessels from below. This is why cartilage can be irreversibly destroyed within 48–72 hours — making septic arthritis a true emergency.

Classification

Category	Examples
Pyogenic (bacterial)	S. aureus, Streptococcus spp, N. gonorrhoeae, H. influenzae, Gram-negative bacilli
Mycobacterial	M. tuberculosis, non-tuberculous mycobacteria (NTM)
Fungal	Candida spp, Coccidioides, Histoplasma, Sporothrix
Viral	Parvovirus B19, Hepatitis B/C, Rubella, HIV, Chikungunya, Dengue

Type	Common Organisms	Key Points
Native joint	S. aureus; TB (immunocompromised); N. gonorrhoeae (STD) ^[4]	Most common presentation
Prosthetic joint	S. epidermidis (early/delayed); S. aureus (acute/late haematogenous) ^[4]	Biofilm formation is central; usually requires surgical management (DAIR, one-stage, or two-stage revision)

Pattern	Frequency	Associations
Monoarthritis	80–90% ^[2]^[3]	Typical pyogenic SA
Oligo/polyarthritis	~20% ^[2]	Especially in those with underlying systemic arthritis or in sepsis ^[2]; gonococcal arthritis

Type	Onset	Typical Organisms
Acute	Hours to days	Pyogenic bacteria (S. aureus, Streptococcus, N. gonorrhoeae)
Chronic/indolent	Weeks to months	M. tuberculosis, NTM, fungi, Brucella

Feature	Pyogenic	TB Arthritis (may involve spine as Pott's disease)
Clinical onset	Acute	Insidious
X-ray: disc space	Disc space narrowing (relevant for spine)	Disc space relatively spared
Common site	Anywhere (knee > hip)	Spine: thoracic (closest to lymph nodes); hip, knee for peripheral
Synovial fluid	Purulent, high WBC, neutrophil predominant	Lymphocyte predominant; AFB smear/culture; tissue biopsy shows granulomas
Treatment duration	4–6 weeks	9–12 months (standard TB regimen)

Clinical Features

The clinical presentation of septic arthritis is driven by the acute inflammatory response within the joint and the systemic response to infection.

Symptoms

Symptom	Description	Pathophysiological Basis
Joint pain	Severe, constant, exacerbated by any movement	Inflammatory mediators (PGE₂, bradykinin) stimulate nociceptors in the synovium and joint capsule; raised intra-articular pressure stretches the capsule (which IS innervated, unlike cartilage)
Joint swelling	Rapid onset	Purulent effusion + inflammatory exudate from increased vascular permeability (IL-1, TNF-α → endothelial activation → vascular leak)
Inability to bear weight / use the joint	Cannot walk (lower limb) or grip (upper limb)	Pain + effusion → mechanical limitation + reflex muscle guarding (splinting) to protect the joint
Fever / chills / rigors	High fever ^[2] (but may be absent in elderly or immunocompromised)	Systemic cytokine release (IL-1, IL-6, TNF-α) → hypothalamic set-point elevation via PGE₂ in the thermoregulatory centre
Malaise / anorexia	Non-specific systemic symptoms	Cytokine-mediated illness behaviour
Preceding skin infection / wound / UTI / URTI	May identify the portal of entry	Source of bacteraemia → haematogenous seeding

Symptom	Description	Pathophysiological Basis
Pseudoparesis / pseudoparalysis	No spontaneous movement of the affected limb ^[2]	Pain inhibits voluntary movement; child "guards" the limb — gives the appearance of paralysis but neurological exam is normal
Irritability / crying with handling	Non-specific but important in non-verbal children	Pain on passive movement of the infected joint
Refusal to walk / limp	Lower limb involvement	Antalgic gait to offload the painful joint
Effusion difficult to detect in toddlers	Covered by subcutaneous fat ^[2]	Physical exam is harder; maintain high index of suspicion

Phase	Symptoms
Bacteraemic phase	Migratory polyarthralgia (flitting joint pains), tenosynovitis (especially wrists/fingers/ankles), vesiculopustular skin rash (scattered, non-tender, on extremities and trunk), fever
Localised phase	Purulent monoarthritis (usually knee, wrist, or ankle); skin lesions and tenosynovitis may have resolved

Sign	Description	Pathophysiological Basis
Joint warmth (calor)	Palpable heat over the joint	Increased blood flow to the inflamed synovium (vasodilation driven by histamine, NO, PGE₂)
Joint erythema (rubor)	Redness of overlying skin	Vasodilation of dermal vessels; more obvious in superficial joints (knee, ankle, wrist); may be subtle in deep joints (hip, shoulder)
Joint swelling (tumor)	Visible and palpable enlargement	Intra-articular effusion + synovial thickening + periarticular soft tissue oedema
Joint tenderness	Exquisitely tender to palpation	Sensitisation of nociceptors by inflammatory mediators; distension of the capsule
Restricted range of motion (ROM)	Both active and passive ROM markedly reduced	Pain, effusion (mechanical block), reflex muscle spasm
Effusion	Fluctuant swelling; positive bulge sign (knee), ballottement/patellar tap (knee)	Accumulation of purulent synovial fluid
Held in position of comfort	Leg held flexed, abducted, and externally rotated (hip) ^[2]	This position maximises intracapsular volume (and therefore minimises intracapsular pressure), reducing pain ^[2]
Systemic signs	Fever (often > 38.5°C), tachycardia, occasionally hypotension (if septic)	Systemic inflammatory response; may progress to sepsis/septic shock
Overlying skin changes	Cellulitis, sinus tract (chronic)	Spread of infection to periarticular soft tissues; chronic infection may drain spontaneously

GC Lecture Slide — Position of Comfort in Hip Septic Arthritis

In children with hip septic arthritis, the characteristic posture is leg held flexed, abducted, and externally rotated to decrease intracapsular pressure ^[2]. This is because the hip joint capsule has maximum volume in this position (similar to how you'd hold a balloon at its most expanded shape). Any attempt to extend, adduct, or internally rotate the hip causes pain because it compresses the distended capsule.

Septic arthritis most commonly affects: ^[2]^[3]^[5]

Joint	Frequency	Notes
Knee	> 50% (in adults: > 90% in some series) ^[2]^[3]	Superficial → easier to examine but also more exposed to direct inoculation
Hip	Second most common in children; important in adults	Most commonly affected joint in paediatric septic arthritis ^[2]; deep → harder to examine clinically, requires imaging
Ankle	~10–15%
Wrist	~5–10%	More common in gonococcal arthritis
Shoulder	~5%	Deep joint; can be difficult to diagnose
Elbow	Less common
Small joints of hands/feet	Uncommon for non-gonococcal; more common in gonococcal

On clinical examination, systematically assess:

Look:

Skin: erythema, warmth, swelling, scars (previous surgery/injection), sinuses (chronic infection), skin rash (gonococcal vesiculopustules)
Posture: position of comfort (hip: flexed, abducted, externally rotated)
Muscle wasting (if chronic)

Feel:

Temperature: increased local warmth
Tenderness: exquisite over the joint line
Effusion: patellar tap, bulge sign (knee); less obvious in deep joints
Crepitus (chronic → cartilage destruction)

Move:

Active ROM: severely restricted
Passive ROM: severely restricted (distinguishes from periarticular pathology where passive ROM may be relatively preserved)
Pain at end-range and throughout ROM (unlike mechanical problems where pain is typically at end-range only)

Special:

Log roll test (hip): Gentle internal and external rotation of the hip with the patient supine — exquisitely painful in hip septic arthritis
Assess for adjacent osteomyelitis: Bony tenderness, draining sinuses
Systemic examination: Vital signs, skin (look for primary source of infection), heart murmur (endocarditis), IV access sites (IVDU)

Clinical Feature	Underlying Mechanism
Acute onset severe joint pain	Bacterial products + inflammatory mediators activate nociceptors; capsular distension
Swollen, warm, red joint	Vascular dilation + increased permeability → exudate + effusion
Restricted ROM (active and passive)	Effusion (mechanical), pain (reflex guarding), synovial thickening
Fever, malaise	Systemic cytokine response (IL-1, IL-6, TNF-α → PGE₂ → hypothalamic thermoregulation)
Position of comfort (hip)	Maximises intracapsular volume → minimises pressure → reduces pain
Pseudoparesis (children)	Pain inhibits voluntary movement; not a neurological deficit
Cartilage destruction (if untreated)	Neutrophil proteases + bacterial enzymes + ischaemia from raised pressure
May lack systemic signs	Elderly/immunocompromised patients may not mount an adequate febrile/inflammatory response

High Yield Summary

Septic arthritis is a rheumatological emergency — bacterial infection can destroy cartilage within 48–72 hours ^[1]^[2]^[3].
"A hot, swollen, tender joint is septic arthritis until proven otherwise" — even if fever, WCC, and inflammatory markers are normal ^[1]^[2]^[3].
S. aureus is the most common organism in children > 2 years and adults ^[2]^[4].
Routes of infection: haematogenous (most common), contiguous (osteomyelitis), adjacent soft tissue, direct inoculation ^[2].
Risk factors: extremes of age, chronic arthritis (esp RA), prosthetic joints, IVDU, STD, DM, immunosuppression ^[1].
Knee is the most commonly affected joint overall ( > 50%); hip is most common in children* ^[2]^[3].
Gonococcal arthritis presents as part of DGI with the triad of migratory polyarthralgia, tenosynovitis, and vesiculopustular rash ^[2].
In children: look for pseudoparesis, refusal to walk, held in flexed/abducted/externally rotated position (hip) ^[2].
Native joint: S. aureus, N. gonorrhoeae (STD), TB. Prosthetic joint: S. epidermidis (early), S. aureus (late haematogenous) ^[4].
Pyogenic vs TB arthritis: Pyogenic = acute onset, disc narrowing; TB = insidious, disc spared, thoracic spine ^[7].
Prompt treatment prevents permanent structural damage ^[2]^[3].
Must perform synovial fluid analysis (cell count, Gram stain, C/ST, crystal microscopy) on any acute inflammatory monoarthritis ^[1]^[3].

Active Recall - Septic Arthritis (Definition to Clinical Features)

^[1] Lecture slides: GC 075. Pain red joint.pdf (Causes and risk factors of septic arthritis slide) ^[2] Senior notes: Adrian Lui Pediatrics Notes.pdf (Section 13.2.4 Inflammatory Joint Conditions — Septic Arthritis, p.453); Ryan Ho Rheumatology.pdf (Section 2.8 Septic arthritis, p.67) ^[3] Senior notes: Maksim Medicine Notes.pdf (Section 13.8 Septic arthritis, p.331; Section 13.1 Clinical approach, p.309) ^[4] Senior notes: Maksim Surgery Notes.pdf (Musculoskeletal infection table — Septic arthritis, p.275) ^[5] Senior notes: MBBS Final MB (Medicine) (Felix PY Lai).pdf (Septic arthritis, p.1686–1688) ^[6] Senior notes: Ryan Ho Fundamentals.pdf (Section 3.7.1 Acute Monoarthritis, p.406) ^[7] Senior notes: Maksim Surgery Notes.pdf (Pyogenic osteomyelitis vs TB spine table, p.275); Lecture slides: GC 237. Musculoskeletal infection [Updated in 2025].pdf

Differential Diagnosis of Septic Arthritis

When a patient presents with an acute, hot, swollen, painful joint, the clinician faces a critical diagnostic fork: this presentation is shared by several conditions, but septic arthritis must be excluded first because delayed treatment leads to irreversible cartilage destruction ^[1]^[2]^[3]. The differential diagnosis is essentially the differential of acute monoarthritis — because septic arthritis presents as monoarthritis in 80–90% of cases ^[2]^[3].

The key mental framework: any acute inflammatory monoarthritis = septic arthritis until proven otherwise → perform synovial fluid analysis (SFA) ^[1]^[3]^[4].

The differential can be organised by pattern of joint involvement and by acuity. The most useful clinical approach is to think in terms of monoarthritis differentials (since that's how septic arthritis usually presents), then broaden if oligo/polyarticular.

Comprehensive Differential Diagnosis Table

These are the diagnoses you must actively consider and systematically exclude in every case of an acute hot swollen joint ^[6]^[5]^[3].

Differential	Key Distinguishing Features	Why It Mimics Septic Arthritis	How to Differentiate
Crystal-induced arthritis — Gout ^[5]^[6]	Single joint in lower extremities (usually 1st MTP, midfoot, ankle, knee); severe pain, redness, swelling; often intensely inflammatory ± fever ^[6]	Can cause fever, leukocytosis, elevated CRP — almost identical acute presentation; podagra (1st MTP) is classic but gout can affect the knee too	SFA with polarising microscopy: needle-shaped, negatively birefringent crystals (MSU) ^[3]^[6]; NB: gout and septic arthritis can co-exist — always send for Gram stain and culture even if crystals are found ^[3]
Crystal-induced arthritis — Pseudogout (CPPD) ^[5]^[6]	Older age ( > 60), more proximal joints (knee, shoulders), positively birefringent, rhomboid-shaped crystals ^[3]; XR may show chondrocalcinosis	Acute presentation mimics gout and septic arthritis; elderly patients often have overlapping risk factors for infection	SFA: CPPD crystals (positive birefringence, rhomboid); XR: chondrocalcinosis (calcification of fibrocartilage e.g. knee menisci) ^[3]
Haemarthrosis ^[6]	Trauma history +ve; onset typically seconds to minutes after injury; may be associated with intra-articular fractures, dislocations, ligamentous injury, meniscal tear	Rapid onset of swollen painful joint with restricted ROM	History of trauma; SFA shows bloody fluid (haemorrhagic); XR may show fracture; check coagulation screen (haemophilia, anticoagulant use); may be a/w coagulopathies and intra-articular tumours ^[6]
Osteoarthritis (OA) flare ^[5]^[6]	Weight-bearing joints; history of overuse/mechanical symptoms; evening stiffness; hard bony swelling (Heberden's/Bouchard's nodes); can present with acutely painful synovitis mimicking septic arthritis ^[5]	An OA joint can develop acute inflammatory synovitis → looks red, hot, swollen	SFA: non-inflammatory or mildly inflammatory fluid ( < 2000 WBC); XR: joint space narrowing, osteophytes, subchondral sclerosis/cysts; no crystals, culture negative
Monoarticular onset of RA / SpA / SLE ^[3]^[6]	RA or SpA can begin as monoarthritis before evolving into polyarthritis; look for other features (morning stiffness > 1h, symmetry, extra-articular features)	Early presentation may be indistinguishable from septic arthritis clinically	SFA: inflammatory but sterile; serology (RF, anti-CCP for RA; HLA-B27 for SpA; ANA, dsDNA for SLE); follow-up: evolves to polyarticular pattern
Bursitis / Tendonitis	Periarticular tenderness (not intra-articular); pain with specific resisted movements; swelling may be localised to bursa	Can appear similar to joint inflammation superficially	Anatomical localisation: tenderness is outside the joint line; passive ROM relatively preserved (unlike true arthritis where passive ROM is also restricted); USG can differentiate

GC Lecture Slide — Crystal Arthropathy Can Co-exist with Septic Arthritis

Crystal arthropathy may present with monoarthritis and leukocytosis ^[5]. Finding crystals in synovial fluid does NOT exclude septic arthritis — always send for Gram stain and C/ST even when crystals are identified, because the two conditions can co-exist ^[3]. This is a classic exam pitfall.

These are considered when septic arthritis presents as oligoarthritis (~20% of cases) or when polyarticular infective arthritis (e.g. gonococcal, endocarditis-associated) is suspected ^[2]^[3].

Differential	Key Distinguishing Features	How to Differentiate from Septic Arthritis
Reactive arthritis (Reiter's syndrome) ^[3]^[5]^[7]	"Can't see, can't pee, can't climb a tree" — conjunctivitis + urethritis/cervicitis + asymmetrical lower limb large joint arthritis ^[7]; extra-articular: circinate balanitis, keratoderma blennorrhagica ^[7]; history of preceding GI (Shigella, Salmonella, Campylobacter, Yersinia) or GU (Chlamydia, gonococcal) infection 1–4 weeks prior	Reactive arthritis is sterile — the organism triggered the immune response but is NOT in the joint; SFA: inflammatory but culture negative; patients usually present with conjunctivitis, skin or mucous membrane lesions, GI and GU symptoms ^[5]
Disseminated gonococcal infection (DGI) ^[2]^[6]	Young, sexually active patient; triad of migratory polyarthralgia + tenosynovitis + vesiculopustular dermatitis; may localise to purulent monoarthritis; usually polyarticular ^[2]	Urogenital/pharyngeal/rectal NAAT for N. gonorrhoeae; SFA culture often negative (fastidious organism); blood culture usually negative; responds rapidly to ceftriaxone
Rheumatoid arthritis (RA) ^[3]^[5]	Chronic symmetrical polyarthritis of small joints (MCP, PIP); morning stiffness > 1h; extra-articular features (nodules, interstitial lung disease, episcleritis); RF/anti-CCP positive	SFA: inflammatory but sterile; serology (RF, anti-CCP); RA patients can develop superimposed septic arthritis — always exclude infection in an RA patient with a single swollen joint disproportionate to other joints ^[3]^[5]
Spondyloarthropathies (SpA) ^[3]	Asymmetrical oligoarthritis, usually men < 45y ^[6]; includes AS, PsA, reactive arthritis, IBD-associated arthritis; HLA-B27 positive; look for axial symptoms (back pain), enthesitis, dactylitis, psoriasis, IBD	SFA: inflammatory but sterile; HLA-B27; sacroiliac joint imaging (MRI); specific extra-articular features
Viral polyarthritis ^[5]	Self-limiting; often symmetric small joint involvement; associated with viral prodrome (fever, rash, myalgia); common viruses: HBV, HCV, Parvovirus B19, rubella, HIV, chikungunya, dengue	History: viral prodrome, recent travel (chikungunya/dengue in HK context); serology: viral panel; SFA: inflammatory but culture negative; usually self-resolving within weeks
Infective endocarditis (IE) ^[8]	Persistent fever + new/changing murmur + embolic phenomena (Janeway lesions, Osler nodes, splinter haemorrhages, Roth spots); arthralgia/arthritis occurs via immune complex deposition or septic embolism	Blood cultures (≥ 3 sets before antibiotics); echocardiography (vegetations); evaluate for IE in patients with polyarticular septic arthritis without a clear source ^[5]
Rheumatic fever ^[3]	Children/adolescents; follows Group A streptococcal pharyngitis by 2–4 weeks; migratory polyarthritis (classically flits from joint to joint); Jones criteria; associated with carditis, Sydenham chorea, erythema marginatum, subcutaneous nodules	ASO titre, throat culture; migratory pattern is key (septic arthritis doesn't migrate); Jones criteria; responds to aspirin
Lyme arthritis ^[6]	Late-stage Lyme disease; most commonly affects knee; look for tick bite history and erythema migrans rash; endemic areas (not typical in HK but important for travellers)	Lyme serology (ELISA then Western blot); SFA: inflammatory but routine culture negative; responds to doxycycline/ceftriaxone

Differential	Key Features	Reasoning
Pigmented villonodular synovitis (PVNS) ^[6]	Young adults; chronic recurrent haemarthrotic effusions; knee most common; MRI: "blooming" on GRE sequences (haemosiderin)	Rare neoplastic condition of synovium; brown bloody fluid on aspiration; biopsy diagnostic
Avascular necrosis (AVN) ^[6]	Risk factors: corticosteroids, alcohol, SLE, sickle cell; groin/hip pain; XR: crescent sign; MRI: early detection	Pain is typically mechanical (worse with weight-bearing); no signs of acute inflammation (no warmth/erythema unless complicated)
Osteomyelitis with adjacent septic arthritis ^[4]	Especially in children (hip) where metaphysis is intracapsular; draining sinus tract; bony tenderness	MRI best for diagnosis; contiguous spread mechanism; in infants < 1y: epiphysis lacks growth plate → septic arthritis possible ^[4]
Coagulopathy with haemarthrosis ^[6]	Haemophilia (especially A and B); anticoagulant use; spontaneous or post-minor trauma	Coagulation screen: prolonged APTT (haemophilia A/B); factor levels; bloody joint fluid
Leukaemia/Malignancy ^[6]	Bone pain, nocturnal pain, constitutional symptoms (weight loss, night sweats); may present with joint pain from periosteal involvement or leukaemic infiltration of synovium	CBC: blasts on blood film; bone marrow biopsy; XR: periosteal reaction, lytic lesions
Erythema nodosum ^[6]	Tender erythematous nodules on shins; may be associated with arthralgia (especially ankles); causes: sarcoidosis, IBD, infections, drugs	Periarticular rather than true arthritis; skin biopsy: septal panniculitis
Foreign body reaction ^[6]	History of penetrating injury or prior surgery; chronic monoarthritis	Imaging (XR/CT/MRI) may show foreign body; surgical exploration

Differentiating Features — Systematic Approach

The clinical approach to differentiating septic arthritis from its mimics relies on four pillars: History, Examination, Synovial Fluid Analysis, and Imaging.

Synovial fluid analysis is the single most important investigation for distinguishing septic arthritis from other causes of acute monoarthritis ^[1]^[3]^[5].

Parameter	Normal	Non-inflammatory	Inflammatory	Septic	Haemorrhagic
Clarity	Transparent	Transparent	Translucent	Opaque	Bloody
Colour	Clear	Yellow	Yellow	Yellow/green	Red
Viscosity	High	High	Low	Variable	Variable
WBC/mm³	< 200	< 2,000	2,000–100,000	50,000–300,000	Variable
% Neutrophils	< 25%	< 25%	25–75%	> 90%	50–75%
Culture	-ve	-ve	-ve	+ve	-ve

^[3]^[5]

High Yield — Synovial Fluid Interpretation

Key thresholds to remember:

WBC > 50,000/mm³ with > 90% neutrophils is highly suggestive of septic arthritis ^[3]^[5]
But there is overlap: severe gout can produce WBC > 50,000; and partially treated septic arthritis can have WBC < 50,000
Always send for: (1) Gram stain, (2) C/ST, (3) crystal microscopy, (4) WBC count with differential ^[3]
Polarising microscopy must be performed even in suspected septic arthritis — crystal arthropathy may co-exist ^[3]
SFA must be performed BEFORE administration of antibiotics ^[5] to maximise culture yield

Feature	Septic Arthritis	Gout	Pseudogout	Reactive Arthritis	RA Flare	OA Flare
Onset	Hours	Hours	Hours–days	Days–weeks	Days–weeks	Days
Joints	Large (knee, hip)	1st MTP, ankle, knee	Knee, shoulder	Asymmetric large LL	Symmetric small	Weight-bearing
Fever	Common (but may be absent)	May occur	Uncommon	Low-grade	Uncommon	No
SFA WBC	> 50,000	2,000–100,000+	2,000–100,000	2,000–50,000	2,000–50,000	< 2,000
Crystals	Absent*	MSU (neg birefringent)	CPPD (pos birefringent)	Absent	Absent	Absent
Gram stain	+ve (~50–75%)	-ve	-ve	-ve	-ve	-ve
Culture	+ve (~70–90%)	-ve	-ve	-ve	-ve	-ve
Systemic signs	Bacteraemia, sepsis	Tophi	Chondrocalcinosis	Urethritis, conjunctivitis, rash	Extra-articular RA features	None

*Crystals absent unless co-existing crystal arthropathy

Special Differential Diagnosis Scenarios

This is a classic exam scenario. An RA patient presents with one joint that is disproportionately swollen compared to the others.

Differential:

Septic arthritis (RA patients are at high risk — immunosuppression + abnormal synovium)
RA flare of that particular joint
Crystal arthropathy (gout or pseudogout — can co-exist with RA)

Approach: You MUST aspirate the joint ^[1]^[3]. You cannot distinguish these clinically. Send SFA for Gram stain, C/ST, and crystal microscopy. Do not assume it is "just a flare."

Differential of acute hip pain in a child (all presenting with limp or refusal to bear weight):

Septic arthritis — emergency; child usually systemically unwell, fever
Transient synovitis ("irritable hip") — most common cause; self-limiting viral-related; child usually afebrile or low-grade fever, looks well
Osteomyelitis — often metaphyseal tenderness rather than joint tenderness; may coexist with septic arthritis
Perthes disease — avascular necrosis of femoral head; insidious onset, typically 4–8 years
Slipped capital femoral epiphysis (SCFE) — adolescent, obese, groin/thigh/knee pain
JIA — chronic; morning stiffness; may have systemic features
Malignancy (leukaemia, bone tumour) — night pain, constitutional symptoms

The Kocher criteria (for children) help distinguish septic arthritis from transient synovitis: fever > 38.5°C, non-weight-bearing, ESR > 40, WBC > 12,000. Presence of ≥ 3/4 criteria → high probability of septic arthritis (discussed further in diagnosis section).

Differential:

Disseminated gonococcal infection (DGI) — triad of migratory polyarthralgia, tenosynovitis, vesiculopustular rash ^[2]^[6]
Reactive arthritis — sterile joint; preceding urethritis/cervicitis; conjunctivitis; keratoderma blennorrhagica ^[7]
Non-gonococcal septic arthritis — less likely in young healthy adults but possible
Viral arthritis — HIV seroconversion, HBV

Key differentiator: In gonococcal arthritis, the organism is IN the joint (though difficult to culture); in reactive arthritis, the joint is sterile — the immune response is triggered by a distant infection ^[7].

Differential:

Prosthetic joint infection (PJI) — early ( < 3 months, S. epidermidis), delayed (3–12 months, low-virulence organisms), late haematogenous ( > 12 months, S. aureus) ^[2]^[4]
Aseptic loosening — most common cause of painful prosthetic joint; mechanical symptoms; no fever
Periprosthetic fracture — trauma history
Crystal arthropathy — can occur around prostheses
Metal hypersensitivity / adverse local tissue reaction — more common with metal-on-metal implants

Approach: Any acutely painful prosthetic joint with signs of inflammation → aspirate under fluoroscopic guidance; send SFA for cell count, Gram stain, C/ST; check serum CRP and ESR ^[4].

Condition	Why It's Different from Septic Arthritis
Cellulitis overlying a joint	Skin and soft tissue infection; the joint itself is not involved; ROM relatively preserved (especially passive); no effusion on aspiration
Septic bursitis (e.g. olecranon, prepatellar)	Bursa, not joint space, is infected; swelling is over the bursa (anterior to joint line for prepatellar); ROM at the joint itself relatively preserved
Periarticular abscess	Soft tissue collection near but outside the joint; imaging (USG/MRI) delineates; may require drainage but joint space is sterile

Exam Pitfall — Periarticular vs Intra-articular

A common mistake is to confuse septic bursitis (e.g. olecranon or prepatellar bursitis) with septic arthritis. The key clinical difference: in bursitis, passive ROM of the underlying joint is relatively preserved because the joint space itself is not involved. In septic arthritis, both active and passive ROM are severely restricted. Anatomical localisation of the swelling is critical.

Category	Condition	Key Clue
Infection	Septic arthritis	Hot swollen joint, fever, SFA: purulent, culture +ve
	DGI (gonococcal)	Young, sexually active, triad: polyarthralgia + tenosynovitis + dermatitis
	TB arthritis	Indolent, chronic, immunocompromised, HK endemic
	Lyme	Tick bite, erythema migrans, knee
	IE-associated	Persistent fever, murmur, embolic phenomena
Crystal	Gout	1st MTP, negatively birefringent needle crystals
	Pseudogout (CPPD)	Elderly, knee/shoulder, positively birefringent rhomboid crystals, chondrocalcinosis
Trauma	Haemarthrosis	Trauma history, bloody fluid, fracture on XR
Degenerative	OA flare	Weight-bearing joint, non-inflammatory fluid, osteophytes on XR
Inflammatory	RA	Symmetric polyarthritis, RF/anti-CCP +ve
	Reactive arthritis	"Can't see, can't pee, can't climb a tree"; sterile joint; recent GI/GU infection
	SpA	Asymmetric oligo, young male, HLA-B27, enthesitis, dactylitis
	Viral polyarthritis	Viral prodrome, self-limiting, serology
Other	PVNS, AVN, malignancy, foreign body	See uncommon differentials above

High Yield Summary — Differential Diagnosis

The differential of septic arthritis = differential of acute monoarthritis — septic arthritis, crystal arthropathy (gout/pseudogout), haemarthrosis, OA flare, monoarticular onset of polyarthritis ^[3]^[6].
All acute inflammatory monoarthritis must have SFA to exclude septic arthritis ^[1]^[3].
Crystal arthropathy and septic arthritis can co-exist — finding crystals does NOT exclude infection; always send for Gram stain and C/ST ^[3].
SFA must be performed BEFORE antibiotics to maximise culture yield ^[5].
Gonococcal arthritis (DGI): young, sexually active; triad of polyarthralgia, tenosynovitis, dermatitis; usually polyarticular ^[2]^[6].
Reactive arthritis: sterile joint inflammation triggered by distant GI/GU infection; "can't see, can't pee, can't climb a tree" ^[7].
RA patients with one disproportionately swollen joint → must aspirate to exclude superimposed septic arthritis ^[3].
OA of a single joint can present as acutely painful synovitis mimicking septic arthritis — aspirate to confirm ^[5]^[6].
Key SFA cutoffs: Septic: WBC > 50,000, > 90% neutrophils, opaque, culture +ve; Inflammatory: WBC 2,000–100,000, 25–75% neutrophils; Non-inflammatory: WBC < 2,000 ^[3]^[5].
In children with acute hip pain: differentiate septic arthritis from transient synovitis using Kocher criteria.

Active Recall - Differential Diagnosis of Septic Arthritis

References

^[1] Lecture slides: GC 075. Pain red joint.pdf (Causes and risk factors of septic arthritis slide; Septic arthritis slide p.18, p.20) ^[2] Senior notes: Adrian Lui Pediatrics Notes.pdf (Section 13.2.4 Inflammatory Joint Conditions — Septic Arthritis, p.453); Ryan Ho Rheumatology.pdf (Section 2.8 Septic arthritis, p.67) ^[3] Senior notes: Maksim Medicine Notes.pdf (Section 13.8 Septic arthritis, p.331; Section 13.1 Clinical approach — Joint pain differential, synovial fluid table, p.309) ^[4] Senior notes: Maksim Surgery Notes.pdf (Musculoskeletal infection table — Septic arthritis and Osteomyelitis, p.275) ^[5] Senior notes: MBBS Final MB (Medicine) (Felix PY Lai).pdf (Septic arthritis — Differential diagnosis, p.1688–1690; Diagnosis — Synovial fluid table, p.1689; OA differential, p.1667–1669; RA differential, p.1677–1679) ^[6] Senior notes: Ryan Ho Rheumatology.pdf (Section 2.1 Approach to Acute Monoarthritis — Differential Diagnoses table, p.28–30); Ryan Ho Fundamentals.pdf (Section 3.7.1 Acute Monoarthritis, p.406) ^[7] Senior notes: Maksim Medicine Notes.pdf (Reactive arthritis, p.326–328) ^[8] Senior notes: Ryan Ho Cardiology.pdf (Infective endocarditis — Presentation table, p.148)

Diagnostic Criteria, Diagnostic Algorithm, and Investigations for Septic Arthritis

Unlike many rheumatological conditions (e.g., RA with its 2010 ACR/EULAR criteria, or gout with the 2015 ACR/EULAR classification), septic arthritis has no universally accepted formal classification criteria or scoring system for adults. The diagnosis is fundamentally microbiological — it rests on identifying the causative organism in the synovial fluid. This makes sense from first principles: "septic" means infected, so proving infection = proving the diagnosis.

GC Lecture Slide — Definitive Diagnosis

"The definitive diagnostic test is identification of bacteria in synovial fluid" ^[5]^[9]. This means a positive Gram stain and/or culture from synovial fluid is the gold standard. However, clinical diagnosis must often be made before culture results return (takes 24–72 hours), based on the clinical picture plus Gram stain plus synovial fluid cell count.

In clinical practice, septic arthritis is diagnosed when any of the following are met:

Definitive: Positive synovial fluid culture or positive Gram stain
Highly probable: Clinical features of septic arthritis (acute inflammatory monoarthritis ± fever ± risk factors) PLUS synovial fluid WBC > 50,000/mm³ with > 90% neutrophils, even if initial Gram stain is negative (treat empirically while awaiting culture)
Probable: Compatible clinical picture + positive blood culture + inflammatory synovial fluid, even if synovial fluid culture is negative (e.g., partially treated infection, fastidious organisms like N. gonorrhoeae)

Why no formal criteria? Because the clinical presentation overlaps too heavily with crystal arthropathy and RA flares to allow a purely clinical scoring system. The microbiology is what settles it. This is precisely why synovial fluid analysis is non-negotiable in any acute inflammatory monoarthritis ^[1]^[6]^[9].

In children presenting with an acutely painful hip (the classic paediatric scenario is distinguishing septic arthritis from transient synovitis), the Kocher criteria provide a validated clinical prediction rule:

Criterion	Threshold
Fever	> 38.5°C
Non-weight-bearing	Unable to bear weight on affected side
ESR	> 40 mm/hr
WBC	> 12,000/mm³

Number of Criteria Present	Predicted Probability of Septic Arthritis
0	< 0.2%
1	3%
2	40%
3	93%
4	99.6%

Cavenagh modification adds CRP > 20 mg/L as a fifth criterion (further increases specificity).

Clinical application: ≥ 3 Kocher criteria = very high probability → proceed to urgent aspiration (hip aspiration under USG guidance) and empirical antibiotics. 0–1 criteria in an otherwise well child = transient synovitis more likely → observe, serial reassessment.

Why Kocher Criteria Exist for the Hip Specifically

The hip is a deep joint — you cannot easily see or palpate an effusion. In children, transient synovitis (a benign, self-limiting condition) is far more common than septic arthritis (~10:1 ratio), but misdiagnosing septic arthritis as transient synovitis is catastrophic (cartilage destruction, avascular necrosis of the femoral head). The Kocher criteria help stratify risk and guide the decision to aspirate.

The following algorithm represents the standard clinical approach to any patient presenting with an acute hot swollen joint where septic arthritis is in the differential ^[1]^[3]^[5]^[6]^[9].

GC Lecture Slide — Key Principles of the Algorithm

From GC 075 Pain red joint ^[1]:

"Synovial fluid aspiration is key"
"Fluid to be sent fresh for relevant stains and culture"
"Prior to commencement of antibiotic therapy"
"Warfarin does not contradict needle aspiration"
"Suspected hip sepsis requires US guidance"
"Blood cultures should always be taken"
"MRI is the most appropriate imaging where required — also sensitive in detecting osteomyelitis"

These are high-yield GC exam points. Note especially that anticoagulation with warfarin is NOT a contraindication to joint aspiration — the risk of missing septic arthritis far outweighs the risk of haemarthrosis from the procedure.

Investigation Modalities — Comprehensive Breakdown

A. Synovial Fluid Analysis (SFA) — THE Most Important Test

Joint fluid analysis is the MOST IMPORTANT TEST ^[6]^[9]. It is the single investigation that can definitively confirm or exclude septic arthritis.

Aseptic technique: Skin preparation with antiseptic, sterile drape, local anaesthetic
Site-specific approach: Knee (superolateral or medial parapatellar approach); ankle (anteromedial); wrist (dorsal); hip (requires USG guidance — too deep for blind aspiration) ^[1]
Send fluid immediately (fresh, not in formalin) to the lab for processing
Must be performed BEFORE administration of antibiotics to maximise culture yield ^[1]^[5]

From the GC lecture slides and senior notes, five key analyses ^[1]^[3]^[4]^[5]^[6]^[9]:

Analysis	Why	What You're Looking For
1. Gram stain (URGENT) ^[1]^[3]^[4]	Rapid result (within 1 hour); guides immediate empirical therapy	Gram-positive cocci in clusters (S. aureus), Gram-positive cocci in chains (Streptococcus), Gram-negative diplococci (N. gonorrhoeae), Gram-negative bacilli
2. Culture and sensitivity testing (C/ST) ^[3]^[4]^[5]	Gold standard for definitive diagnosis; provides antibiotic sensitivities	Growth of organism; takes 24–72 hours; positive in ~70–90% of non-gonococcal SA but often negative in gonococcal SA
3. WBC count with differential ^[3]^[5]^[6]	Quantifies degree of inflammation; helps classify fluid	Septic: WBC 50,000–300,000/mm³ with > 90% PMNs ^[3]^[5]
4. Crystal microscopy (polarising microscopy) ^[3]^[4]^[6]	Exclude (or identify co-existing) crystal arthropathy	MSU crystals (negative birefringence, needle-shaped = gout); CPPD crystals (positive birefringence, rhomboid = pseudogout); may co-exist with septic arthritis ^[3]
5. Glucose ^[9]	Bacteria consume glucose → low synovial fluid glucose suggests infection	Synovial fluid glucose < 50% of simultaneous serum glucose is suggestive of septic arthritis

Parameter	Normal	Non-inflammatory	Inflammatory	Septic	Haemorrhagic
Volume (knee)	< 3.5 mL	> 3.5	> 3.5	> 3.5	> 3.5
Clarity	Transparent	Transparent	Translucent	Opaque	Bloody
Colour	Clear	Yellow	Yellow	Yellow/green (purulent)	Red
Viscosity	High	High	Low	Variable	Variable
WBC/mm³	< 200	0–2,000	> 2,000	> 20,000 (typically 50,000–150,000)	Variable
PMNs (%)	< 25%	< 25%	≥ 50%	≥ 75% (typically > 90%)	50–75%
Culture	-ve	-ve	-ve	+ve	-ve

^[3]^[5]

Understanding the Synovial Fluid Table from First Principles

Why is viscosity low in inflammatory fluid? Normal synovial fluid is viscous because of hyaluronic acid produced by type B synoviocytes. In inflammation, neutrophil-derived enzymes (hyaluronidase, proteases) degrade hyaluronic acid → viscosity drops. This is why inflammatory fluid doesn't form a "string" when you drip it (the "string sign" or "mucin clot test" is poor/negative).

Why is septic fluid opaque? The massive number of neutrophils (> 50,000/mm³) makes the fluid turbid/purulent — think of it like pus (which is essentially dead neutrophils and bacteria).

Why is glucose low in septic fluid? Bacteria metabolise glucose for energy; neutrophils also consume glucose via glycolysis during phagocytosis. The net effect is depletion of synovial fluid glucose.

Exam Pitfall — Overlap Zones

The WBC thresholds are guidelines, not absolute cutoffs:

Severe gout can produce WBC > 50,000 with predominantly neutrophils — overlapping with the septic range
Partially treated septic arthritis or early infection may have WBC < 50,000
Crystal arthropathy and septic arthritis can co-exist ^[3]
Therefore: always send for BOTH crystal microscopy AND Gram stain/culture, regardless of which you suspect

Test	When to Send	Rationale
AFB smear and culture ^[3]^[4]^[6]	Immunocompromised, chronic indolent monoarthritis, HK/endemic TB setting	TB arthritis; AFB smear has low sensitivity (~20%); culture takes 4–8 weeks; consider synovial biopsy
Fungal stain and culture	Immunocompromised, IVDU	Candida, Coccidioides, etc.
Synovial biopsy	Undetermined diagnosis despite SFA ^[3]	Granulomas (TB, sarcoidosis), PVNS, synovial malignancy
PCR / NAAT	Suspected gonococcal (fastidious), TB, Lyme	Higher sensitivity than culture for fastidious organisms

Investigation	Key Findings in Septic Arthritis	Why It Matters
CBP (CBC with differential) ^[1]^[3]^[5]^[9]	Leukocytosis (raised WBC with neutrophilia)	Reflects systemic inflammatory/infective response; however, may be normal in elderly or immunocompromised
ESR ^[1]^[5]^[9]	Elevated (often > 40 mm/hr)	Non-specific marker of inflammation; useful for baseline and monitoring treatment response
CRP ^[1]^[5]^[9]	Elevated (often markedly)	More specific and responsive than ESR; rises within 6 hours of infection; useful for monitoring treatment response (should halve every 48–72 hours with effective treatment)
Blood culture (x2 sets) ^[1]^[3]^[5]	Positive in ~50% of cases ^[5]	Essential — identifies the organism even when synovial fluid culture is negative; always take blood cultures ^[1]
RFT and LFT ^[1]^[3]	May show end-organ damage (e.g., AKI from sepsis)	"Electrolytes and renal and liver function to detect end-organ damage and guide antibiotic choice" ^[1] — many antibiotics are renally cleared (e.g., vancomycin) and require dose adjustment
Procalcitonin	Elevated in bacterial infection	Helps distinguish bacterial from non-bacterial causes; not universally available; more useful in sepsis workup
Serum urate	May be elevated in gout; does NOT exclude gout if normal during acute attack	Taken to assess for co-existing gout; serum urate drops during acute attacks (cytokine-driven uricosuria) — check 2 weeks after resolution for accurate baseline
Coagulation screen (PT, APTT)	To assess bleeding risk before aspiration; to identify coagulopathy if haemarthrosis suspected	Warfarin does not contradict needle aspiration ^[1] — the risk of missing septic arthritis outweighs bleeding risk

GC Lecture Slide — Blood Tests

From GC 237. Musculoskeletal infection [Updated in 2025] ^[9]: "Investigations: CBP, ESR, CRP" — these are the basic mandatory blood tests.

From GC 075. Pain red joint ^[1]: "Blood cultures should always be taken" "Electrolytes and renal and liver function to detect end-organ damage and guide antibiotic choice"

If gonococcal infection is suspected: swabs of pharynx, urethra, cervix, and anorectum ^[3]^[5].

Site	Rationale
Urethra/cervix	Primary site of gonococcal infection; NAAT is gold standard
Pharynx	Pharyngeal gonorrhoea is often asymptomatic; can be the only positive site
Anorectum	Rectal gonorrhoea; important in MSM

NAAT (nucleic acid amplification test / PCR) is the most sensitive test for N. gonorrhoeae and C. trachomatis — superior to culture at these sites
Also send synovial fluid for gonococcal NAAT (higher sensitivity than culture for this fastidious organism)

D. Imaging

Radiographs of the affected joints should be obtained ^[1]^[4]^[5]^[9].

Timing	Findings	Significance
Early (< 1 week)	Often normal or shows only soft tissue swelling and joint space widening (effusion)	XR is insensitive for early septic arthritis; a normal XR does NOT exclude the diagnosis
1–2 weeks	Joint space effusion, periarticular osteopenia ^[4]	Periarticular osteopenia results from hyperaemia of adjacent bone due to inflammation → increased osteoclast activity
Late (weeks–months)	Joint space narrowing (cartilage destruction), erosions, subchondral bone destruction, eventual ankylosis	Indicates established joint damage — a late finding indicating delayed/inadequate treatment

Primary roles of XR ^[1]^[5]:

Baseline radiograph is useful for comparison purposes to determine therapeutic response ^[5]
Exclude other diagnoses (e.g., fracture causing haemarthrosis, chondrocalcinosis suggesting pseudogout, osteophytes suggesting OA)
Identify gas in soft tissues (gas gangrene, anaerobic infection)
Identify underlying osteomyelitis (periosteal reaction, bone destruction — though this is a late finding)

MRI is the most appropriate imaging where required ^[1]^[5]^[9].

Role	What It Shows
Detection of co-existent osteomyelitis	"Useful to detect any co-existent osteomyelitis" ^[9]; "Also sensitive in detecting osteomyelitis" ^[1] — bone marrow oedema on T2/STIR sequences, periosteal reaction
Joint effusion in deep joints	"Detects inflammation and effusions in joints that are difficult to examine including the hip and sacroiliac joints" ^[5]
Soft tissue assessment	Periarticular abscess, myositis, fascial involvement
Synovial thickening/enhancement	Post-gadolinium enhancement of thickened synovium — indicates active synovitis
Cartilage damage	Cartilage loss, subchondral erosions
Adjacent bone	Bone marrow oedema suggests contiguous osteomyelitis

GC Lecture Slide — MRI Role

From GC 237. Musculoskeletal infection [Updated in 2025] ^[9]: "MRI: useful to detect any co-existent osteomyelitis — Not routine"

This is an important distinction: MRI is not a routine first-line investigation for straightforward septic arthritis (the diagnosis is made by SFA). MRI is reserved for:

Suspected co-existent osteomyelitis
Deep joints (hip, SI joint) where clinical assessment is difficult
Equivocal cases
Pre-operative planning

Role	Detail
Hip aspiration guidance	"Suspected hip sepsis requires US guidance" ^[1] — the hip is too deep for blind aspiration; USG confirms effusion and guides needle placement
Detection of effusion	Useful in any joint where effusion is clinically uncertain; high sensitivity for fluid
Guiding aspiration	Improves accuracy and safety of arthrocentesis, especially in obese patients or small joints
Not diagnostic	Cannot distinguish septic from non-septic effusion on imaging alone — the fluid must be aspirated and analysed

Role	Detail
CT-guided aspiration	Alternative to USG for deep joints (hip, SI joint, sternoclavicular)
Bone detail	Better than MRI for cortical bone destruction; useful in chronic osteomyelitis
Not first-line	Radiation exposure; MRI preferred for soft tissue and marrow oedema assessment

"Evaluate for suspected infective endocarditis in patients with known valvular heart disease or polyarticular involvement without a clear source of infection" ^[5].

When to Request	Rationale
Polyarticular septic arthritis without clear source	IE causes continuous bacteraemia → multiple joint seeding
Known valvular heart disease	Pre-existing valvular abnormality is a major risk factor for IE
Persistent bacteraemia despite treatment	Suggests an endovascular focus (IE, mycotic aneurysm)
Clinical features of IE	New murmur, embolic phenomena (Janeway lesions, Osler nodes, splinter haemorrhages)

The investigations can be grouped by urgency:

Priority	Investigation	Rationale
IMMEDIATE (before antibiotics)	Synovial fluid aspiration → Gram stain, C/ST, WBC count/diff, crystal microscopy, glucose ^[1]^[3]^[5]^[9]	Definitive test; must precede antibiotics
IMMEDIATE	Blood cultures x2 ^[1]^[3]^[5]	Positive in 50%; may be only positive culture in gonococcal or partially treated SA
URGENT	CBP, ESR, CRP ^[1]^[3]^[5]^[9]	Baseline inflammatory markers; guide monitoring
URGENT	RFT, LFT ^[1]^[3]	Detect end-organ damage; guide antibiotic dosing
SAME DAY	XR of joint ^[1]^[4]^[5]^[9]	Baseline; exclude other pathology
IF INDICATED	MRI (suspect osteomyelitis or deep joint) ^[1]^[9]	Not routine; for co-existent osteomyelitis
IF INDICATED	USG (hip aspiration guidance) ^[1]	Mandatory for hip; useful for other deep/difficult joints
IF INDICATED	Gonococcal swabs and NAAT (sexually active) ^[3]^[5]	Pharynx, urethra, cervix, anorectum
IF INDICATED	Echocardiography (polyarticular, no source, known VHD) ^[5]	Exclude IE
IF INDICATED	AFB smear/culture, synovial biopsy (chronic, immunocompromised) ^[3]^[4]^[6]	TB arthritis workup

Gram Stain Finding	Most Likely Organism	Clinical Context
Gram-positive cocci in clusters	S. aureus	Most common overall; any age/context
Gram-positive cocci in chains	Streptococcus spp (including Group B in neonates)	Skin/soft tissue source; neonates
Gram-negative diplococci	N. gonorrhoeae	Young, sexually active
Gram-negative bacilli	Enterobacteriaceae (E. coli, Klebsiella), Pseudomonas	Elderly, IVDU, immunocompromised
No organisms seen	Does NOT exclude septic arthritis	Gram stain sensitivity is only ~50–75%; culture is more sensitive; partially treated infection; gonococcal (low yield on Gram stain)

Why Gram Stain Sensitivity Is Only 50–75%

Gram stain requires a threshold bacterial concentration (~10⁵ organisms/mL) to be reliably detected. In early infection or fastidious organisms (especially N. gonorrhoeae), the bacterial load may be below this threshold. This is why a negative Gram stain does NOT exclude septic arthritis — you must treat empirically based on clinical suspicion and await culture results.

For prosthetic joint infection, the 2018 ICM (International Consensus Meeting) criteria / MSIS criteria are used. This is a separate but related entity:

Definitive PJI — one major criterion:

Two positive periprosthetic cultures with the same organism, OR
Sinus tract communicating with the joint

Probable PJI — minor criteria scoring ≥ 6 points (includes serum CRP, D-dimer, ESR, synovial WBC, synovial PMN%, synovial CRP, alpha-defensin, etc.)

This is more relevant for orthopaedic surgery exams but worth knowing the principle: PJI diagnosis relies heavily on multiple culture specimens because biofilm organisms are difficult to grow from a single aspiration.

High Yield Summary — Diagnosis of Septic Arthritis

No formal diagnostic criteria for native joint septic arthritis — diagnosis is microbiological: positive Gram stain and/or culture from synovial fluid ^[5].
Synovial fluid analysis is the MOST IMPORTANT TEST ^[1]^[6]^[9] — send for: Gram stain, C/ST, WBC count with differential, crystal microscopy, glucose.
SFA must be performed BEFORE antibiotics ^[1]^[5] — to maximise culture yield.
Warfarin does not contradict needle aspiration ^[1].
Suspected hip sepsis requires USG guidance ^[1].
Blood cultures should always be taken — positive in ~50% ^[1]^[5].
CBP, ESR, CRP — baseline markers; CRP most useful for monitoring response ^[1]^[9].
RFT/LFT — detect end-organ damage and guide antibiotic choice ^[1].
XR of joint: often normal early; baseline radiograph useful for comparison ^[4]^[5]^[9]; look for effusion, periarticular osteopenia.
MRI: not routine; useful to detect co-existent osteomyelitis ^[1]^[9].
Gonococcal suspected → swabs of pharynx, urethra, cervix, anorectum ^[3]^[5].
Echocardiography if polyarticular involvement without clear source, or known valvular heart disease → exclude IE ^[5].
Septic fluid: opaque, WBC 50,000–300,000/mm³, > 90% PMNs, culture positive ^[3]^[5].
Crystal arthropathy and septic arthritis can co-exist — always send for both crystal microscopy AND microbiology ^[3].
Kocher criteria (paediatric hip): fever > 38.5°C, non-weight-bearing, ESR > 40, WBC > 12,000 — ≥ 3/4 criteria strongly suggests septic arthritis over transient synovitis.

Active Recall - Diagnosis of Septic Arthritis

References

^[1] Lecture slides: GC 075. Pain red joint.pdf (Approach to septic arthritis — investigations slide, p.23; Septic arthritis routes slide, p.19) ^[2] Senior notes: Adrian Lui Pediatrics Notes.pdf (Section 13.2.4 Inflammatory Joint Conditions — Septic Arthritis, p.453) ^[3] Senior notes: Maksim Medicine Notes.pdf (Section 13.8 Septic arthritis — Investigations, p.331; Section 13.1 Synovial fluid table, p.309) ^[4] Senior notes: Maksim Surgery Notes.pdf (Septic arthritis investigations — Gram stain, C/ST, AFB smear, XR findings, p.275) ^[5] Senior notes: MBBS Final MB (Medicine) (Felix PY Lai).pdf (Diagnosis — Biochemical tests and Radiological tests, p.1689–1691) ^[6] Senior notes: Ryan Ho Fundamentals.pdf (Section 3.7.1 — Initial Ix, joint fluid analysis, p.407) ^[9] Lecture slides: GC 237. Musculoskeletal infection [Updated in 2025].pdf (Septic arthritis — Investigations slide, p.19) ^[10] Lecture slides: GC 075. Pain red joint [Notes].pdf (Key learning points)

Management of Septic Arthritis

The management of septic arthritis is driven by the understanding that every hour of delay = more cartilage destruction. The joint is a closed space filled with pus, bacteria, and destructive enzymes. Rational treatment therefore has four pillars:

Remove the pus (source control) — drain the joint to reduce bacterial load, remove destructive enzymes, and lower intra-articular pressure
Kill the bacteria (antibiotics) — systemic antibiotics to eradicate the organism
Relieve pain (analgesia) — enable early mobilisation
Restore function (physiotherapy) — prevent stiffness and muscle wasting

GC Lecture Slide — Management Overview

From GC 237. Musculoskeletal infection [Updated in 2025] ^[9]: "Management: Antibiotic + Surgical management"

From GC 075. Pain red joint ^[1]: "Bacterial septic arthritis should be treated promptly with antibiotics according to culture sensitivity" "Surgical intervention is indicated in some patients so always get the orthopaedics surgeon informed"

Treatment Modalities — Detailed Breakdown

A. Antibiotic Therapy

Antibiotics must achieve high concentrations within the joint space. Although the synovial membrane has fenestrated capillaries that allow relatively good antibiotic penetration, the purulent environment (low pH, high protein content, enzymatic degradation) reduces antibiotic efficacy. IV administration ensures high peak serum levels → high synovial fluid levels → bactericidal activity. Once the infection is controlled (clinical improvement, falling CRP), oral antibiotics with good bioavailability and bone/joint penetration can be used.

Start empirical IV antibiotics immediately according to suspected organisms and Gram stain. Modify according to culture and sensitivity results. Opinion from microbiologists is helpful. Refer to IMPACT guideline ^[11].

The choice of empirical antibiotic is guided by:

Gram stain result (available within ~1 hour)
Clinical context (age, risk factors, suspected organism)
Local antibiograms (MRSA prevalence in HK hospitals)

Clinical Scenario	Gram Stain	Empirical Antibiotic	Rationale
Standard adult, native joint	Gram +ve cocci in clusters (S. aureus likely)	IV flucloxacillin / cloxacillin ^[4]	Anti-staphylococcal penicillin; bactericidal against MSSA; excellent bone/joint penetration
MRSA risk (healthcare-associated, known MRSA carrier, IVDU, prosthetic joint)	Gram +ve cocci	IV vancomycin ^[12]	Glycopeptide; covers MRSA; requires therapeutic drug monitoring (trough 15–20 mg/L for serious infections)
Elderly / chronically ill / suspected Gram-negative	Gram -ve bacilli	IV 3rd-generation cephalosporin (e.g., ceftriaxone, cefotaxime)	Broad Gram-negative cover; good joint penetration
STD / sexually active / suspected gonococcal	Gram -ve diplococci	IV ceftriaxone ^[4]	"NG: IV ceftriaxone × 1 week" ^[4]; also covers most other Gram-negatives; treat presumptive Chlamydia co-infection with doxycycline
Penicillin allergy	Any	IV vancomycin ± IV 3rd-gen cephalosporin (if not cross-reactive) or IV meropenem	Avoid penicillins; vancomycin for Gram-positive cover
No organisms on Gram stain, high clinical suspicion	Negative Gram stain	IV flucloxacillin + gentamicin (or IV vancomycin + ceftriaxone if MRSA risk)	Must cover S. aureus (most common overall) + Gram-negatives; adjust when C/ST available
Prosthetic joint	Any	IV vancomycin + rifampicin ± Gram-negative cover	Biofilm-active combination; rifampicin penetrates biofilm; must always be used with another agent (rapid resistance if used alone)
Neonate ( < 1 month)	Variable	IV flucloxacillin + IV gentamicin (or IV cefotaxime)	Cover Group B Strep, Gram-negative bacilli, S. aureus ^[2]
Child 1 month – 5 years	Variable	IV flucloxacillin + IV 3rd-gen cephalosporin	Cover S. aureus and H. influenzae (if not fully Hib-vaccinated) / Kingella kingae

HK IMPACT Guidelines

In Hong Kong, the IMPACT guidelines (Interhospital Multi-disciplinary Programme on Antimicrobial ChemoTherapy) provide local antibiotic recommendations. The handbook states: "Refer to IMPACT guideline" ^[11]. In practice, for a straightforward native joint septic arthritis due to MSSA, the HK regimen is typically:

IV cloxacillin 2g Q6H (or IV flucloxacillin) × ≥ 2 weeks, then oral cloxacillin/flucloxacillin or an oral agent with good bone penetration (e.g., oral clindamycin, oral fluoroquinolone based on sensitivities) for an additional 2–4 weeks

IV antibiotics for at least 2 weeks or until signs improved for non-gonococcal arthritis, then orally for an additional 2–4 weeks ^[3]^[4]^[11].

Phase	Duration	Criteria for Transition
IV phase	≥ 2 weeks ^[3]^[4]^[11]	Continue until clinical improvement: resolution of fever, decreasing joint swelling/pain, falling CRP
PO phase	Additional 2–4 weeks ^[3]^[11]	Total duration typically 4–6 weeks for non-gonococcal native joint SA
Total	4–6 weeks ^[4]	Shorter for gonococcal (~1–2 weeks total); longer for prosthetic joint (6 weeks–3 months+)

Why 4–6 weeks total? Joint cartilage is avascular — antibiotics reach it only via diffusion from the synovial fluid. This slow diffusion means bacteria within cartilage matrix are exposed to lower antibiotic concentrations than in serum, requiring prolonged treatment to ensure eradication. Additionally, biofilm (especially relevant for S. aureus) is inherently resistant to short courses of antibiotics.

Phase	Regimen	Duration
IV/IM	IV ceftriaxone 1g daily (or 2g daily if DGI with purulent arthritis)	Until clinical improvement (usually 24–48 hours of IV → then switch to PO)
PO	Oral cefixime 400mg BD OR oral ciprofloxacin 500mg BD (if susceptible)	Total 7–14 days
Concurrent	Doxycycline 100mg BD × 7 days (or azithromycin 1g single dose)	Presumptive treatment of Chlamydia co-infection (> 40% co-infection rate)

Why is gonococcal SA treated for a shorter duration? N. gonorrhoeae rarely causes permanent cartilage destruction — the organism is less virulent to cartilage than S. aureus. It also responds rapidly to ceftriaxone. The excellent prognosis of gonococcal SA is well-established ^[2].

Parameter	Frequency	Interpretation
Clinical assessment	Daily	Fever, joint swelling/pain, ROM — should improve within 48–72 hours
CRP	Every 2–3 days	Should halve every 48–72 hours with effective treatment; failure to decline suggests inadequate drainage, wrong antibiotic, or complications (e.g., osteomyelitis)
ESR	Weekly	Slower to respond than CRP; useful for long-term trend
WBC	As needed	Should normalise
Drug levels	Per drug protocol	Vancomycin trough (target 15–20 mg/L); gentamicin pre-dose level (to avoid nephro/ototoxicity)
RFT	At least weekly	Monitor for antibiotic nephrotoxicity (especially vancomycin, gentamicin)

When Antibiotics Are Not Working — Red Flags

If after 48–72 hours of appropriate antibiotics the patient is not improving (persistent fever, rising CRP, persistent/re-accumulating effusion), consider:

Inadequate surgical drainage — the most common reason for failure; may need repeat aspiration or surgical washout
Wrong antibiotic — check C/ST; consider resistant organism (MRSA) or atypical pathogen (TB, fungal)
Co-existent osteomyelitis — requires MRI and potentially longer antibiotic course
Abscess formation — periarticular abscess requires drainage
Wrong diagnosis — reconsider crystal arthropathy, reactive arthritis, etc.

B. Joint Drainage — Surgical Management

Drainage is equally important as antibiotics — you cannot cure septic arthritis with antibiotics alone. The pus contains destructive enzymes, inflammatory debris, and fibrin that physically block antibiotic penetration and perpetuate cartilage damage.

From first principles:

Reduces bacterial load directly
Removes neutrophil proteases and MMPs that are destroying cartilage
Lowers intra-articular pressure → restores blood supply to synovium and subchondral bone → reduces ischaemic cartilage damage
Improves antibiotic penetration by removing purulent debris
Provides repeated synovial fluid for monitoring treatment response

GC Lecture Slide — Surgical Management

From GC 075. Pain red joint ^[1]:

"Confirm diagnosis by examination of joint aspirate"
"Urgent joint irrigation and debridement"
"Acute phase: arthroscopy"
"Delayed or chronic cases: open arthrotomy"
"Repeated surgical debridement until infection under control"
"Be prepared for co-existing osteomyelitis"

Method	Description	Indications	Advantages	Disadvantages
Repeated needle aspiration to dryness ^[3]^[11]	Serial percutaneous arthrocentesis (daily or as needed) until effusion resolves	First-line for most accessible joints (knee, ankle, wrist, elbow, shoulder)	Minimally invasive; can be repeated; performed at bedside; diagnostic AND therapeutic	May not achieve complete washout; loculations/fibrin may limit drainage; requires multiple procedures
Arthroscopic irrigation and debridement ^[1]	Surgical procedure; camera and instruments inserted through small portals; joint is irrigated with large volumes of saline, necrotic tissue and fibrin are debrided	"Acute phase: arthroscopy" ^[1]; when needle aspiration is inadequate; when loculations prevent complete drainage	Excellent visualisation; thorough washout; can debride synovium; less morbid than open surgery	Requires operating theatre, anaesthesia, surgical expertise
Open arthrotomy ^[1]	Formal surgical incision into the joint; allows direct inspection, complete drainage, thorough debridement of infected/necrotic synovium	"Delayed or chronic cases: open arthrotomy" ^[1]; hip joint (usual first-line); prosthetic joint infection; failed arthroscopic washout; thick purulent material/loculations	Most thorough drainage; complete access; can address osteomyelitis simultaneously	Most invasive; longer recovery; risk of surgical complications

Joint	Preferred Drainage Method	Why
Knee	Needle aspiration (first-line) → arthroscopic washout if inadequate	Superficial, easily accessible; knee arthroscopy is straightforward
Hip	Open drainage is usually necessary ^[3]^[11] "usually require OT for hip infection" ^[3]	Deep joint; needle aspiration often inadequate; raised intracapsular pressure can cause AVN of femoral head if not relieved promptly; high risk of concurrent osteomyelitis in children
Ankle / Wrist / Elbow	Needle aspiration → arthroscopy if needed	Accessible for needle aspiration; arthroscopy feasible
Shoulder	Arthroscopy preferred over needle aspiration	Deep soft tissue coverage; needle aspiration often incomplete
Prosthetic joint	Orthopaedic consult MANDATORY ^[3]^[11]	DAIR (Debridement, Antibiotics, and Implant Retention) if acute ( < 3 weeks from onset) and stable implant; otherwise one-stage or two-stage revision arthroplasty

Why the Hip Almost Always Needs Open Drainage

The hip joint is:

Deep — surrounded by thick muscles; needle aspiration is technically difficult and often incomplete
Intracapsular anatomy — the femoral head blood supply runs along the capsular retinaculum; raised intracapsular pressure compresses these vessels → AVN of the femoral head if not urgently decompressed
High risk of concurrent osteomyelitis — intracapsular metaphysis means infection easily spreads between bone and joint
Loculated effusions are common due to complex anatomy This is why "Consult orthopaedic surgeon for drainage especially for infected prosthetic joint. Open drainage is usually necessary for hip infection" ^[11].

"Repeated surgical debridement until infection under control" ^[1].

Indications for repeat washout/debridement:

Persistent or re-accumulating effusion despite initial drainage
Failure of CRP to decline
Persistent fever beyond 48–72 hours of appropriate antibiotics + drainage
Thick purulent material/loculations not adequately addressed initially
Evidence of co-existing osteomyelitis requiring further debridement

"Therapeutic joint aspiration to dryness" ^[3]^[11].

This deserves separate emphasis. The concept of aspirating "to dryness" means removing ALL fluid from the joint — not just a diagnostic sample. This is both diagnostic (the fluid is analysed) and therapeutic (removing the pus).

Why "to dryness"?

Every mL of pus left behind contains neutrophil proteases still actively destroying cartilage
Residual pus reduces antibiotic efficacy (high protein binding, low pH, enzyme degradation)
Persistent effusion maintains elevated intra-articular pressure

Frequency: Daily or every 1–2 days until no further re-accumulation. If effusion persistently re-accumulates despite repeated aspiration → escalate to arthroscopic or open drainage.

"NSAIDs for pain relief" ^[11].

Agent	Dose/Route	Mechanism	Considerations
NSAIDs	e.g., Naproxen 500mg BD PO, Diclofenac 50mg TDS PO, Ibuprofen 400mg TDS PO	Inhibit COX → reduce PGE₂ → reduce pain, inflammation, and fever	C/I: CKD (CrCl < 30), active PUD, severe CVD, allergy; use with PPI if GI risk; avoid in dehydration/sepsis-related AKI
Paracetamol	1g QDS PO/IV	Central COX inhibition + descending serotonergic pathways; anti-pyretic via hypothalamic PGE₂ inhibition	Safe first-line adjunct; hepatotoxic in overdose; avoid if severe hepatic impairment
Opioids	e.g., Tramadol 50mg QDS, Codeine, IV morphine PRN	μ-opioid receptor agonism in CNS → analgesia	For severe pain not controlled by NSAIDs/paracetamol; caution: sedation, respiratory depression, constipation

Why NOT intra-articular corticosteroid injection in septic arthritis? IA steroids are contraindicated in confirmed or suspected septic arthritis because they suppress the local immune response and can worsen infection. IA steroids are only used in crystal arthropathy after septic arthritis has been excluded ^[6]. This is a critical safety principle.

Exam Pitfall — IA Steroids and Infection

"Caution: must r/o septic arthritis before injection of steroid" ^[6]. Injecting corticosteroid into an infected joint is dangerous because it suppresses the already-struggling immune response within the joint. Always send synovial fluid for Gram stain and culture BEFORE considering IA steroid.

"Start physiotherapy early" ^[3]^[11].

Phase	Physiotherapy Goals	Rationale
Acute (during IV antibiotics)	Gentle passive ROM exercises; isometric muscle strengthening; position of comfort	Prevent joint stiffness (adhesions and fibrosis from inflammatory exudate); prevent muscle wasting (disuse atrophy occurs rapidly — quadriceps wasting visible within days of knee immobilisation)
Subacute (improving)	Active-assisted ROM → active ROM; progressive weight-bearing	Restore functional ROM; rebuild muscle strength
Recovery (post-discharge)	Full weight-bearing; strengthening exercises; functional rehabilitation	Return to normal activity; assess for residual damage

Why start early? Immobilisation beyond the first few days leads to joint fibrosis — the inflammatory exudate organises into fibrous adhesions that permanently restrict ROM. Early, gentle movement prevents this while still allowing the infection to be treated. The balance is: enough rest to reduce pain but enough movement to prevent stiffness.

PJI management is primarily surgical, with antibiotic therapy as an adjunct. The surgical approach depends on the timing of infection and implant stability:

Surgical Option	Indication	Description
DAIR (Debridement, Antibiotics, and Implant Retention)	Acute PJI ( < 3 weeks from symptom onset); stable, well-fixed implant; susceptible organism; adequate soft tissue	Open debridement + exchange of modular components (polyethylene liner, femoral head) + high-volume irrigation; IV antibiotics + prolonged oral suppressive therapy (including rifampicin for staphylococci)
One-stage revision	Selected cases; adequate bone stock; known organism with good susceptibility; healthy soft tissues	Remove prosthesis → thorough debridement → implant new prosthesis in same surgery; antibiotic-loaded cement
Two-stage revision	Chronic PJI; resistant organism; poor soft tissue; sinus tract	Stage 1: Remove prosthesis → debride → insert antibiotic-loaded cement spacer → 6–12 weeks IV/PO antibiotics; Stage 2: Remove spacer → reimplant new prosthesis after infection eradicated (confirmed by repeat aspiration, normalised inflammatory markers)
Permanent resection arthroplasty (Girdlestone)	Multiply failed revisions; uncontrolled infection; patient unfit for further surgery	Remove prosthesis; no reimplantation; functional result is poor (limb shortening, instability) but life-saving in refractory cases
Amputation	Last resort; life-threatening uncontrolled infection	Rare; only when all other options exhausted

Identify and treat the primary source of bacteraemia ^[12]:

Skin infection → wound care, drainage of abscess
Urinary tract infection → appropriate antibiotics
Dental infection → dental extraction (relevant for IE too)
IV line/catheter infection → remove the line
Endocarditis → prolonged IV antibiotics ± cardiac surgery

Special Scenarios

Aspect	Management
Antibiotics	Standard anti-TB regimen: RIPE (Rifampicin, Isoniazid, Pyrazinamide, Ethambutol) × 2 months, then RI × 7–10 months; total duration 9–12 months
Surgery	Synovial biopsy for diagnosis (granulomas, AFB, culture); synovectomy or debridement if needed; joint drainage if significant effusion
Key difference from pyogenic	Much longer antibiotic course; insidious presentation; do NOT use flucloxacillin (not effective against TB)

Same principles as adults with key modifications:

IV antibiotics: IV flucloxacillin + 3rd-gen cephalosporin empirically; adjust by age (cover Group B Strep and Gram-negatives in neonates ^[2])
Surgical drainage: Lower threshold for operative drainage, especially for hip (risk of AVN of femoral head, growth plate damage → limb length discrepancy)
Monitoring: Children respond more rapidly but also deteriorate faster; close monitoring essential
Urgent empirical IV antibiotics until signs improved, then PO ^[2]

Step	Action	Key Points
1. Resuscitate	IV fluids, analgesia, monitoring	Septic arthritis may present with sepsis
2. Investigate	Blood cultures, CBP/ESR/CRP/RFT/LFT, SFA BEFORE antibiotics	See Diagnosis section
3. Aspirate	Therapeutic aspiration to dryness ^[3]^[11]	Diagnostic AND therapeutic; daily until no re-accumulation
4. Antibiotics	Empirical IV antibiotics immediately based on Gram stain + clinical context; adjust according to C/ST ^[3]^[11]	IV ≥ 2 weeks, then PO 2–4 weeks ^[3]^[11]; refer to IMPACT guideline ^[11]
5. Orthopaedic consult	Always inform orthopaedics ^[1]^[10]; mandatory for hip and prosthetic joint ^[3]^[11]	Arthroscopic washout (acute) vs open arthrotomy (delayed/chronic) ^[1]; repeated debridement until infection under control ^[1]
6. Analgesia	NSAIDs ^[11]; paracetamol; opioids if needed	Avoid IA steroids until infection excluded
7. Physiotherapy	Start early ^[3]^[11]	Prevent joint stiffness and muscle wasting
8. Monitor	Daily clinical assessment; serial CRP; repeat aspiration if effusion re-accumulates	CRP should halve every 48–72 hours
9. Source control	Identify and treat primary source of bacteraemia	Skin, UTI, dental, IV line, endocarditis

High Yield Summary — Management of Septic Arthritis

Management = Antibiotic + Surgical management ^[9].
Therapeutic aspiration to dryness — both diagnostic and therapeutic ^[3]^[11].
Start empirical IV antibiotics immediately according to Gram stain and clinical suspicion; adjust according to C/ST; refer to IMPACT guideline ^[11].
IV cloxacillin/flucloxacillin for MSSA (most common); IV vancomycin if MRSA risk; IV ceftriaxone for gonococcal ^[4]^[11].
IV antibiotics ≥ 2 weeks, then PO for additional 2–4 weeks (total 4–6 weeks for non-gonococcal) ^[3]^[4]^[11].
Gonococcal SA: shorter course (~1–2 weeks total); IV ceftriaxone × ~1 week; excellent prognosis ^[4].
Always consult orthopaedics ^[1]^[10]; open drainage usually necessary for hip ^[3]^[11].
Surgical options: acute phase — arthroscopy; delayed/chronic — open arthrotomy; repeated debridement until infection under control ^[1].
Be prepared for co-existing osteomyelitis ^[1].
NSAIDs for pain relief ^[11]; do NOT inject IA steroids until infection excluded ^[6].
Start physiotherapy early — prevent stiffness and muscle wasting ^[3]^[11].
Prosthetic joint infection: DAIR (acute, stable implant) vs one/two-stage revision (chronic, unstable).
Contraindication to IA steroids = suspected or confirmed septic arthritis ^[6].

Active Recall - Management of Septic Arthritis

References

^[1] Lecture slides: GC 075. Pain red joint.pdf (Septic arthritis — surgical management slide, p.24; key learning points) ^[2] Senior notes: Adrian Lui Pediatrics Notes.pdf (Section 13.2.4 Inflammatory Joint Conditions — Septic Arthritis, p.453; Osteomyelitis management, p.447) ^[3] Senior notes: Maksim Medicine Notes.pdf (Section 13.8 Septic arthritis — Management, p.331) ^[4] Senior notes: Maksim Surgery Notes.pdf (Septic arthritis — Management: IV cloxacillin x 4-6 weeks, NG: IV ceftriaxone x 1 week, operative irrigation, p.275) ^[6] Senior notes: Ryan Ho Rheumatology.pdf (Management of acute gout — caution re IA steroids and septic arthritis, p.38, p.42; Role of surgery in RA — indications for emergency surgery including septic arthritis, p.56) ^[9] Lecture slides: GC 237. Musculoskeletal infection [Updated in 2025].pdf (Septic arthritis — Management slide, p.22) ^[10] Senior notes: Block A - Painful red joint_ monoarthropathy, gouty arthritis, septic arthritis, haemarthrosis.pdf (Key learning points, p.22) ^[11] Senior notes: Handbook of Internal Medicine 2024.pdf (Septic Arthritis management protocol, p.429) ^[12] Senior notes: MBBS Final MB (Surgery) (Felix PY Lai).pdf (Treatment of septic focus — antibiotic principles, p.41)

Complications of Septic Arthritis

Complications of septic arthritis arise from three interrelated mechanisms:

Local joint damage — the destructive inflammatory process within the joint itself (neutrophil-mediated and bacterial enzyme-mediated cartilage/bone destruction)
Spread of infection — local extension (osteomyelitis, soft tissue abscess) or systemic dissemination (bacteraemia, sepsis, metastatic infection)
Consequences of treatment — antibiotic side effects, surgical complications, immobilisation-related problems

The fundamental teaching point is that prompt and proper treatment leaves the joint without permanent structural damage ^[2], but delay in diagnosis and treatment can result in irreversible joint destruction or septicaemia ^[5]. In other words, every complication listed below is potentially preventable with timely intervention.

A. Local (Joint) Complications

Aspect	Detail
Mechanism	Neutrophil-derived proteases (elastase, cathepsin G, MMPs) + bacterial enzymes (hyaluronidase, collagenases) degrade the cartilage matrix. Cartilage is avascular and aneural — it cannot regenerate. Once the collagen-proteoglycan scaffold is destroyed, the smooth articular surface is lost permanently.
Consequence	Loss of articular cartilage → secondary osteoarthritis: chronic joint pain, stiffness, crepitus, reduced ROM. Weight-bearing joints (knee, hip) are most severely affected.
Timeline	Bacterial infection can destroy joint cartilage within a few days ^[2]. Clinically significant destruction can begin within 48–72 hours of symptom onset.
Clinical implication	Patients may need joint replacement surgery (total knee/hip arthroplasty) years later if cartilage destruction is severe.

The Central Teaching Point

The single most important complication of septic arthritis is irreversible cartilage destruction. This is why the condition is a rheumatological emergency ^[1]^[2] — the goal of all management is to prevent this outcome. Up to 40–50% of patients with delayed treatment develop some degree of permanent joint dysfunction.

Aspect	Detail
Mechanism	Severe cartilage loss exposes subchondral bone on both sides of the joint. The inflammatory exudate (rich in fibrin and growth factors) organises into fibrous tissue bridging the joint surfaces → fibrous ankylosis. If this progresses further, new bone forms across the joint space → bony ankylosis.
Consequence	Complete loss of joint motion; the joint is fused in whatever position it was held in during the infection (typically flexion for the hip/knee).
Relevance	Late complication of untreated or inadequately treated septic arthritis. Historically more common before the antibiotic era; still seen in delayed presentations or TB arthritis.

Aspect	Detail
Mechanism	Prolonged immobilisation in the position of comfort (e.g., hip flexed and externally rotated) leads to shortening of the joint capsule, ligaments, and periarticular muscles (myostatic contracture). Inflammatory fibrosis within the joint compounds this.
Consequence	Fixed flexion deformity; inability to fully extend the joint.
Prevention	Start physiotherapy early ^[3]^[11] — gentle passive ROM exercises during the acute phase prevent adhesion formation and capsular shortening. This is precisely why early physiotherapy is a pillar of management.

Aspect	Detail
Mechanism	Destruction of supporting ligaments and joint capsule by the inflammatory/infective process → loss of mechanical stability. Erosion of subchondral bone further destabilises the articulation.
Consequence	Joint subluxation (partial dislocation); chronic instability requiring bracing or surgical reconstruction.

B. Local (Peri-articular/Bone) Complications

Aspect	Detail
Mechanism	Infection spreads from the joint space to adjacent bone (contiguous osteomyelitis) or, conversely, pre-existing osteomyelitis spreads into the joint. In children, the intracapsular location of the proximal femoral metaphysis means that metaphyseal osteomyelitis and hip septic arthritis very commonly co-exist. In infants < 1 year: the epiphysis lacks a growth plate barrier → septic arthritis can complicate osteomyelitis ^[4].
Frequency	Co-existing osteomyelitis occurs in ~15% of septic arthritis cases ^[2].
Why it matters	Osteomyelitis requires a longer antibiotic course (typically 4–6 weeks IV) and may need surgical debridement of necrotic bone (sequestrum). If osteomyelitis is missed, the joint infection will appear to respond initially but then relapse.
Detection	MRI is the best investigation to detect co-existent osteomyelitis ^[1]^[9] — shows bone marrow oedema, periosteal reaction, cortical destruction. XR is often normal in acute osteomyelitis (changes take 10–14 days to appear).

GC Lecture Slide — Osteomyelitis

From GC 075. Pain red joint ^[1]: "Be prepared for co-existing osteomyelitis". This is a key GC exam point — always consider and investigate for osteomyelitis in any case of septic arthritis, especially in children.

Aspect	Detail
Mechanism	Pus under pressure within the joint can decompress through the capsule into surrounding soft tissues, forming a periarticular or intramuscular abscess. This is more common in delayed or inadequately drained cases.
Consequence	Persistent fever despite antibiotics; fluctuant soft tissue swelling; sinus tract formation if chronic. Requires surgical drainage in addition to antibiotics.
Detection	USG or MRI demonstrates a loculated fluid collection with rim enhancement.

Aspect	Detail
Mechanism	In chronic or inadequately treated cases, the abscess tracks through soft tissue to the skin surface → chronic discharging sinus. Analogous to chronic osteomyelitis with cloacae.
Consequence	Persistent infection, risk of secondary infection with different organisms, poor wound healing, social/functional disability.
Management	Surgical excision of the sinus tract, thorough debridement, prolonged antibiotics.

Aspect	Detail
Mechanism	In the hip joint, raised intra-articular pressure from the purulent effusion compresses the retinacular blood vessels (branches of the medial circumflex femoral artery) that supply the femoral head. If this pressure is not relieved urgently, the blood supply is interrupted → ischaemic necrosis of the femoral head. This is analogous to a compartment syndrome within the joint capsule.
Who is at risk?	Any patient with hip septic arthritis — but children are particularly vulnerable because the femoral head is still developing (cartilaginous epiphysis) and its blood supply is tenuous.
Consequence	Femoral head collapse → severe secondary OA → may require total hip replacement (adults) or have devastating effects on hip development in children.
Prevention	Urgent surgical drainage of hip septic arthritis — this is exactly why "open drainage is usually necessary for hip infection" ^[3]^[11]. Time is femoral-head-blood-supply.
Aetiology cross-reference	AVN of the hip can be caused by: trauma, long-term steroids, alcohol, SLE, infection: osteomyelitis, septic arthritis ^[13].

D. Paediatric-Specific Complications

Children are vulnerable to unique complications because their skeleton is still growing.

Aspect	Detail
Mechanism	In children, the infection can damage the physis (growth plate), either by direct bacterial invasion or by vascular compromise. The growth plate is the engine of longitudinal bone growth — its chondrocytes undergo ordered proliferation and endochondral ossification. Infection disrupts this process → premature growth plate closure (partial or complete).
Consequence	Limb length discrepancy (LLD) — the affected limb grows less than the contralateral side. If damage is asymmetric across the growth plate, angular deformity (valgus or varus) can result.
Relevance	Particularly important in neonatal/infant septic arthritis where transphyseal vessels allow infection to cross between metaphysis and epiphysis ^[4].

Aspect	Detail
Mechanism	Chronic infection and inflammation drive a catabolic state (cytokine-mediated); steroid use (if used for co-existing conditions like JIA) further suppresses growth via GH-IGF1 axis inhibition and direct effects on growth plate chondrocytes. Chronic joint inflammation → early fusion of growth plate ^[2].
Consequence	Short stature, delayed skeletal maturation.

Aspect	Detail
Mechanism	Asymmetric growth plate damage + cartilage destruction + muscle imbalance from prolonged guarding → progressive deformity. In the hip, damage to the proximal femoral epiphysis can lead to coxa vara (decreased neck-shaft angle), acetabular dysplasia, or subluxation.
Consequence	Functional impairment, cosmetic deformity, need for reconstructive surgery.

E. Systemic Complications

Aspect	Detail
Mechanism	Bacteria from the infected joint continuously seed the bloodstream (or the joint infection itself resulted from bacteraemia). Uncontrolled bacteraemia triggers a dysregulated host immune response (SIRS → sepsis → septic shock → multi-organ dysfunction).
Consequence	Delay in diagnosis and treatment can result in irreversible joint destruction or septicaemia ^[5]. Sepsis carries a mortality of 15–30% and requires ICU management (fluids, vasopressors, broad-spectrum antibiotics, source control).
Risk factors for progression	Extremes of age, immunocompromised state (DM, malignancy, steroids), polyarticular septic arthritis, S. aureus (especially PVL-producing strains or MRSA), delayed presentation.

Aspect	Detail
Mechanism	Sustained bacteraemia from the joint (or from the original source that also seeded the joint) can seed other organs and tissues. S. aureus has particular tropism for certain sites.
Sites of metastatic infection	Infective endocarditis (especially if pre-existing valvular disease), other joints (polyarticular septic arthritis), vertebral osteomyelitis/discitis, epidural abscess, brain abscess, splenic abscess, renal abscess, psoas abscess, lung (septic pulmonary emboli if right-sided IE)
Clinical clue	Persistent bacteraemia despite appropriate antibiotics + joint drainage → investigate for distant focus (echocardiography for IE, MRI spine for vertebral osteomyelitis, CT abdomen for visceral abscesses).

Aspect	Detail
Mechanism	Septic arthritis can be a manifestation of IE (the bacteraemia from IE seeds the joint), or the bacteraemia from septic arthritis can seed cardiac valves (especially if pre-existing valvular abnormality or prosthetic valve).
Clinical implication	Echocardiography should be performed in patients with polyarticular septic arthritis without a clear source, or in patients with known valvular heart disease ^[5].
Cross-reference	Musculoskeletal complications of IE include septic arthritis and vertebral osteomyelitis ^[14].

Aspect	Detail
Overall mortality	~10–15% for non-gonococcal native joint septic arthritis; rises to ~30% in polyarticular disease and in the elderly/immunocompromised.
Causes of death	Sepsis/septic shock with multi-organ failure; complications of IE (stroke, heart failure); overwhelming infection in the immunocompromised.
Gonococcal arthritis	Mortality is very low (~0%) with appropriate treatment — prognosis is excellent.

Antibiotic	Potential Complication	Monitoring
Flucloxacillin/Cloxacillin	Hepatotoxicity (cholestatic hepatitis, especially with prolonged courses > 2 weeks); hypersensitivity reactions; interstitial nephritis	LFT weekly during treatment
Vancomycin	Nephrotoxicity (acute tubular necrosis); ototoxicity (sensorineural hearing loss); "red man syndrome" (histamine-mediated infusion reaction — not true allergy; prevented by slow infusion)	Trough levels (target 15–20 mg/L); RFT at least twice weekly
Gentamicin	Nephrotoxicity; ototoxicity (vestibular > auditory)	Pre-dose trough levels; RFT
Prolonged IV antibiotics in general	IV line-related complications: phlebitis, line infection, line-associated thrombosis	Line care; rotate sites; consider PICC line for prolonged courses
C. difficile infection	Antibiotic-associated diarrhoea; pseudomembranous colitis — especially with broad-spectrum antibiotics	Stool C. difficile toxin if diarrhoea develops

Complication	Detail
Wound infection	Post-arthroscopy or post-arthrotomy wound infection
Haemarthrosis	Post-aspiration or post-surgical bleeding into the joint
Nerve/vessel injury	Rare with proper technique; more relevant in open arthrotomy near neurovascular structures
Stiffness	Post-operative joint stiffness from fibrosis; mitigated by early physiotherapy

Complication	Mechanism	Prevention
Muscle wasting/atrophy	Disuse atrophy; quadriceps wasting visible within days of knee immobilisation	Early physiotherapy, isometric exercises
Joint stiffness	Fibrosis of joint capsule and periarticular structures	Passive ROM exercises during acute phase
Deep vein thrombosis (DVT) / PE	Venous stasis from immobility + inflammatory prothrombotic state	VTE prophylaxis (LMWH); early mobilisation
Pressure injuries	Prolonged bed rest in elderly	Regular repositioning; pressure-relieving mattress
Osteoporosis	Prolonged immobilisation → increased bone resorption (via osteocyte-mediated mechanotransduction loss)	Weight-bearing as soon as safely possible

Complication	Detail
Implant loosening	Infection at the bone-implant interface → osteolysis → mechanical loosening; may require revision surgery
Periprosthetic fracture	Weakened bone around the infected prosthesis → fracture with minimal trauma
Persistent/recurrent infection	Biofilm on the prosthetic surface is extremely difficult to eradicate with antibiotics alone; may require implant removal (one-stage or two-stage revision)
Functional loss	Failed revision or permanent resection arthroplasty (Girdlestone) → significant limb shortening, instability, need for walking aids; amputation in worst case

Category	Complication	Pathophysiology	Prevention
Local — Joint	Cartilage destruction / secondary OA	Neutrophil + bacterial enzymes destroy avascular cartilage	Early antibiotics + drainage
	Ankylosis	Fibrous/bony fusion across denuded joint surfaces	Early treatment + physiotherapy
	Joint contracture	Capsular shortening from prolonged immobilisation	Early physiotherapy ^[3]^[11]
	Joint instability/subluxation	Ligament and capsule destruction	Adequate treatment; surgical reconstruction if needed
Local — Bone	Co-existing osteomyelitis (~15%) ^[2]	Contiguous spread; intracapsular metaphysis (hip)	MRI to detect; be prepared for it ^[1]
	Periarticular abscess	Pus decompresses through capsule	Adequate drainage; surgery
	Sinus tract	Chronic untreated abscess tracks to skin	Complete debridement
Vascular	AVN of femoral head	Raised intracapsular pressure → retinacular vessel compression	Urgent hip drainage ^[3]^[11]^[13]
Paediatric	Growth plate damage → LLD	Direct infection/vascular compromise of physis	Early aggressive treatment
	Growth faltering	Chronic inflammation + steroids	Disease control
	Joint deformity	Asymmetric growth plate damage	Early treatment + orthopaedic follow-up
Systemic	Septicaemia / sepsis	Bacteraemia → dysregulated immune response	Early source control + antibiotics
	Metastatic infection	Haematogenous seeding of distant sites	Eradicate source; serial blood cultures
	IE	Bacteraemia seeds cardiac valves	Echo if polyarticular/no clear source
	Death (~10–15%)	Sepsis, multi-organ failure	Timely diagnosis and treatment
Treatment	Antibiotic toxicity	Drug-specific (nephro/oto/hepatotoxicity)	TDM, serial RFT/LFT
	DVT/PE	Immobility + inflammatory state	VTE prophylaxis; early mobilisation

High Yield Summary — Complications of Septic Arthritis

Irreversible cartilage destruction is the defining complication — bacterial infection can destroy cartilage within a few days ^[2]; delay leads to irreversible joint destruction or septicaemia ^[5].
Co-existing osteomyelitis (~15%) — always consider and look for with MRI; "be prepared for co-existing osteomyelitis" ^[1]^[2].
AVN of the femoral head — unique to hip septic arthritis; raised intracapsular pressure compresses retinacular vessels; mandates urgent surgical drainage ^[3]^[11]^[13].
Paediatric-specific: growth plate damage → limb length discrepancy and angular deformity; growth faltering ^[2]^[4].
Sepsis and septicaemia — the joint is a continuous source of bacteraemia; can progress to multi-organ failure and death (~10–15% mortality) ^[5].
Metastatic infection: IE, vertebral osteomyelitis, distant abscesses — perform echo if polyarticular or no clear source ^[5].
Joint ankylosis, contracture, instability — late sequelae of delayed treatment; prevented by early physiotherapy ^[3]^[11].
Treatment complications: antibiotic nephro/oto/hepatotoxicity (especially vancomycin, gentamicin, prolonged flucloxacillin); DVT from immobility.
Prosthetic joint infection: implant loosening, persistent infection due to biofilm, may need revision surgery or even amputation.
Prompt and proper treatment leaves the joint without permanent structural damage ^[2] — this is the overarching message.

Active Recall - Complications of Septic Arthritis

References

^[1] Lecture slides: GC 075. Pain red joint.pdf (Septic arthritis — surgical management slide: "Be prepared for co-existing osteomyelitis", p.24) ^[2] Senior notes: Adrian Lui Pediatrics Notes.pdf (Section 13.2.4 Septic Arthritis, p.453–454: "bacterial infection can destroy joint cartilage in a few days"; "co-existing osteomyelitis 15%"; JIA complications including growth faltering, p.454) ^[3] Senior notes: Maksim Medicine Notes.pdf (Section 13.8 Septic arthritis — Management: "therapeutic aspiration to dryness", "start physiotherapy early", "usually require OT for hip infection", p.331) ^[4] Senior notes: Maksim Surgery Notes.pdf (Osteomyelitis — "Infants < 1y: epiphysis lacks growth plate → septic arthritis possible"; AVN of hip — aetiology includes infection, p.255, p.275) ^[5] Senior notes: MBBS Final MB (Medicine) (Felix PY Lai).pdf (Septic arthritis — "Delay in diagnosis and treatment will lead to irreversible joint destruction or septicemia", p.1688–1689) ^[9] Lecture slides: GC 237. Musculoskeletal infection [Updated in 2025].pdf (Septic arthritis slides, p.18–22) ^[11] Senior notes: Handbook of Internal Medicine 2024.pdf (Septic Arthritis management — "Open drainage is usually necessary for hip infection", "Start physiotherapy early", p.429) ^[13] Senior notes: Maksim Surgery Notes.pdf (AVN of hip — aetiology: osteomyelitis, septic arthritis, p.255) ^[14] Senior notes: MBBS Final MB (Medicine) (Felix PY Lai).pdf (Complications of IE — musculoskeletal: septic arthritis, vertebral osteomyelitis, p.442–444)

High Yield Summary

Septic arthritis is a rheumatological emergency — bacterial infection can destroy cartilage within 48–72 hours ^[1]^[2]^[3].
"A hot, swollen, tender joint is septic arthritis until proven otherwise" — even if fever, WCC, and inflammatory markers are normal ^[1]^[2]^[3].
S. aureus is the most common organism in children > 2 years and adults ^[2]^[4].
Routes of infection: haematogenous (most common), contiguous (osteomyelitis), adjacent soft tissue, direct inoculation ^[2].
Risk factors: extremes of age, chronic arthritis (esp RA), prosthetic joints, IVDU, STD, DM, immunosuppression ^[1].
Knee is the most commonly affected joint overall ( > 50%); hip is most common in children* ^[2]^[3].
Gonococcal arthritis presents as part of DGI with the triad of migratory polyarthralgia, tenosynovitis, and vesiculopustular rash ^[2].
In children: look for pseudoparesis, refusal to walk, held in flexed/abducted/externally rotated position (hip) ^[2].
Native joint: S. aureus, N. gonorrhoeae (STD), TB. Prosthetic joint: S. epidermidis (early), S. aureus (late haematogenous) ^[4].
Pyogenic vs TB arthritis: Pyogenic = acute onset, disc narrowing; TB = insidious, disc spared, thoracic spine ^[7].
Prompt treatment prevents permanent structural damage ^[2]^[3].
Must perform synovial fluid analysis (cell count, Gram stain, C/ST, crystal microscopy) on any acute inflammatory monoarthritis ^[1]^[3].

High Yield Summary — Differential Diagnosis

The differential of septic arthritis = differential of acute monoarthritis — septic arthritis, crystal arthropathy (gout/pseudogout), haemarthrosis, OA flare, monoarticular onset of polyarthritis ^[3]^[6].
All acute inflammatory monoarthritis must have SFA to exclude septic arthritis ^[1]^[3].
Crystal arthropathy and septic arthritis can co-exist — finding crystals does NOT exclude infection; always send for Gram stain and C/ST ^[3].
SFA must be performed BEFORE antibiotics to maximise culture yield ^[5].
Gonococcal arthritis (DGI): young, sexually active; triad of polyarthralgia, tenosynovitis, dermatitis; usually polyarticular ^[2]^[6].
Reactive arthritis: sterile joint inflammation triggered by distant GI/GU infection; "can't see, can't pee, can't climb a tree" ^[7].
RA patients with one disproportionately swollen joint → must aspirate to exclude superimposed septic arthritis ^[3].
OA of a single joint can present as acutely painful synovitis mimicking septic arthritis — aspirate to confirm ^[5]^[6].
Key SFA cutoffs: Septic: WBC > 50,000, > 90% neutrophils, opaque, culture +ve; Inflammatory: WBC 2,000–100,000, 25–75% neutrophils; Non-inflammatory: WBC < 2,000 ^[3]^[5].
In children with acute hip pain: differentiate septic arthritis from transient synovitis using Kocher criteria.

High Yield Summary — Diagnosis of Septic Arthritis

No formal diagnostic criteria for native joint septic arthritis — diagnosis is microbiological: positive Gram stain and/or culture from synovial fluid ^[5].
Synovial fluid analysis is the MOST IMPORTANT TEST ^[1]^[6]^[9] — send for: Gram stain, C/ST, WBC count with differential, crystal microscopy, glucose.
SFA must be performed BEFORE antibiotics ^[1]^[5] — to maximise culture yield.
Warfarin does not contradict needle aspiration ^[1].
Suspected hip sepsis requires USG guidance ^[1].
Blood cultures should always be taken — positive in ~50% ^[1]^[5].
CBP, ESR, CRP — baseline markers; CRP most useful for monitoring response ^[1]^[9].
RFT/LFT — detect end-organ damage and guide antibiotic choice ^[1].
XR of joint: often normal early; baseline radiograph useful for comparison ^[4]^[5]^[9]; look for effusion, periarticular osteopenia.
MRI: not routine; useful to detect co-existent osteomyelitis ^[1]^[9].
Gonococcal suspected → swabs of pharynx, urethra, cervix, anorectum ^[3]^[5].
Echocardiography if polyarticular involvement without clear source, or known valvular heart disease → exclude IE ^[5].
Septic fluid: opaque, WBC 50,000–300,000/mm³, > 90% PMNs, culture positive ^[3]^[5].
Crystal arthropathy and septic arthritis can co-exist — always send for both crystal microscopy AND microbiology ^[3].
Kocher criteria (paediatric hip): fever > 38.5°C, non-weight-bearing, ESR > 40, WBC > 12,000 — ≥ 3/4 criteria strongly suggests septic arthritis over transient synovitis.

High Yield Summary — Management of Septic Arthritis

Management = Antibiotic + Surgical management ^[9].
Therapeutic aspiration to dryness — both diagnostic and therapeutic ^[3]^[11].
Start empirical IV antibiotics immediately according to Gram stain and clinical suspicion; adjust according to C/ST; refer to IMPACT guideline ^[11].
IV cloxacillin/flucloxacillin for MSSA (most common); IV vancomycin if MRSA risk; IV ceftriaxone for gonococcal ^[4]^[11].
IV antibiotics ≥ 2 weeks, then PO for additional 2–4 weeks (total 4–6 weeks for non-gonococcal) ^[3]^[4]^[11].
Gonococcal SA: shorter course (~1–2 weeks total); IV ceftriaxone × ~1 week; excellent prognosis ^[4].
Always consult orthopaedics ^[1]^[10]; open drainage usually necessary for hip ^[3]^[11].
Surgical options: acute phase — arthroscopy; delayed/chronic — open arthrotomy; repeated debridement until infection under control ^[1].
Be prepared for co-existing osteomyelitis ^[1].
NSAIDs for pain relief ^[11]; do NOT inject IA steroids until infection excluded ^[6].
Start physiotherapy early — prevent stiffness and muscle wasting ^[3]^[11].
Prosthetic joint infection: DAIR (acute, stable implant) vs one/two-stage revision (chronic, unstable).
Contraindication to IA steroids = suspected or confirmed septic arthritis ^[6].

High Yield Summary — Complications of Septic Arthritis

Irreversible cartilage destruction is the defining complication — bacterial infection can destroy cartilage within a few days ^[2]; delay leads to irreversible joint destruction or septicaemia ^[5].
Co-existing osteomyelitis (~15%) — always consider and look for with MRI; "be prepared for co-existing osteomyelitis" ^[1]^[2].
AVN of the femoral head — unique to hip septic arthritis; raised intracapsular pressure compresses retinacular vessels; mandates urgent surgical drainage ^[3]^[11]^[13].
Paediatric-specific: growth plate damage → limb length discrepancy and angular deformity; growth faltering ^[2]^[4].
Sepsis and septicaemia — the joint is a continuous source of bacteraemia; can progress to multi-organ failure and death (~10–15% mortality) ^[5].
Metastatic infection: IE, vertebral osteomyelitis, distant abscesses — perform echo if polyarticular or no clear source ^[5].
Joint ankylosis, contracture, instability — late sequelae of delayed treatment; prevented by early physiotherapy ^[3]^[11].
Treatment complications: antibiotic nephro/oto/hepatotoxicity (especially vancomycin, gentamicin, prolonged flucloxacillin); DVT from immobility.
Prosthetic joint infection: implant loosening, persistent infection due to biofilm, may need revision surgery or even amputation.
Prompt and proper treatment leaves the joint without permanent structural damage ^[2] — this is the overarching message.

Septic Arthritis

Definition

Epidemiology

Risk Factors

Anatomy and Functional Relevance

Normal Synovial Joint Anatomy

Why Joints are Vulnerable to Haematogenous Infection

Paediatric Anatomy — Special Considerations

Intracapsular Metaphysis — Why Hip Septic Arthritis is Special

Etiology (Focus on Hong Kong)

Route of Infection

Microbiology

By Age Group

By Clinical Context / Special Populations

S. aureus — Why It Dominates

Gonococcal vs Non-gonococcal Septic Arthritis

Pathophysiology

Sequence of Events

Key Pathophysiological Concepts

Classification

By Causative Organism

By Joint Type

By Clinical Presentation

By Acuity

Pyogenic vs TB Arthritis [7]

Clinical Features

Overview of Presentation

Symptoms

In Children (Additional Symptoms)

In Gonococcal Arthritis (DGI)

Signs

Joint Distribution

Signs Grouped by Examination

Summary of Clinical Features vs Pathophysiology

Active Recall - Septic Arthritis (Definition to Clinical Features)

References

The Core Clinical Problem

Differential Diagnosis Framework

Comprehensive Differential Diagnosis Table

A. Common Differentials of Acute Monoarthritis

B. Common Differentials of Acute Oligo/Polyarthritis

C. Uncommon Differentials

Differentiating Features — Systematic Approach

Synovial Fluid Analysis — The Cornerstone

Key Clinical Differentiators — Quick Reference

Special Differential Diagnosis Scenarios

1. RA Patient with a Single Swollen Joint

2. Child with Acute Limp / Refusal to Walk

3. Sexually Active Young Adult with Joint Pain

4. Prosthetic Joint — Painful and Swollen

Conditions That Are NOT True Differential Diagnoses But Are Commonly Confused

Summary Differential Diagnosis Table

Active Recall - Differential Diagnosis of Septic Arthritis

Diagnostic Criteria — First Principles

Diagnostic Criteria — Working Definition

Kocher Criteria — Paediatric Decision Tool

Diagnostic Algorithm

Investigation Modalities — Comprehensive Breakdown

A. Synovial Fluid Analysis (SFA) — THE Most Important Test

How to Perform: Arthrocentesis

What to Send For

Synovial Fluid Interpretation Table

Additional Synovial Fluid Tests (When Indicated)

B. Blood Investigations

C. Microbiological Swabs (When Gonococcal Infection Suspected)

D. Imaging

1. Plain X-ray of the Joint

2. MRI

3. Ultrasound (USG)

4. CT

5. Echocardiography

E. Summary of Investigation Priorities

Gram Stain Interpretation — Practical Guide

Prosthetic Joint Infection (PJI) — Diagnostic Criteria

Active Recall - Diagnosis of Septic Arthritis

Principles of Management — First Principles

Management Algorithm

Treatment Modalities — Detailed Breakdown

A. Antibiotic Therapy

Why Antibiotics Are Given Intravenously First