Formulate differential diagnosis for patients with hip pain Know how to take history for patients with hip problems Recognize radiological features of common hip disorders Formulate suitable management plan

Hip arthritis is a bread-and-butter orthopaedic topic. For exams, you need to be able to:

• Distinguish mechanical vs inflammatory hip pain
• Interpret hip X-rays systematically (Shenton's line, joint space, OA features, AVN staging)
• Know indications for conservative vs surgical management (especially THR)
• Recognize septic arthritis as a surgical emergency
• Understand AVN etiology and Ficat staging — this is tested repeatedly

The femoral head has a precarious blood supply. Understanding the vascular anatomy explains why certain traumas (femoral neck fractures, hip dislocations) and certain systemic conditions (steroid use, alcohol) lead to AVN.

Blood Supply to the Femoral Head ^[1]:

• 1. Profunda Femoris Artery
• 2. Medial Femoral Circumflex Artery (MFCA)
• 3. Ligamentum teres artery (from obturator artery)
• 4. Ascending branch of medial circumflex artery

60% of patients have medial and lateral femoral circumflex arteries originating from the Profunda Femoral Artery Majority of blood supply comes from the Medial Femoral Circumflex Artery and its retinacular branches These run posteriorly and superiorly along the femoral neck MFCA and its branches are the MAIN blood supply

Why this is important: The retinacular branches of the MFCA are end-arteries that run along the posterior-superior femoral neck. They are vulnerable to disruption in:

• Displaced femoral neck fractures (capsule disrupted → vessels torn)
• Hip dislocations (vessels stretched/torn)
• Increased intramedullary pressure (steroid/alcohol → fat emboli → vessel occlusion)

The ligamentum teres artery is small and unreliable — it cannot salvage the femoral head if the MFCA is disrupted.

Broken Shenton's Line ^[1]

Shenton's line is a smooth arc drawn on an AP pelvis X-ray from the inferior border of the superior pubic ramus to the medial border of the femoral neck. A broken (disrupted) Shenton's line indicates:

• Femoral neck fracture
• Hip subluxation/dislocation
• DDH
• Any displacement of the femoral head relative to the acetabulum

M/55, construction site worker, good past health, 3-month right groin pain, unable to work. Drinker (2 cans beer/day).

Etiology: Trauma, Nature of work, Drug/alcohol Function: Walking (level ground/stairs), Sitting tolerance, Limp, Shoes and socks, Cutting toenails, Getting on/off public transport Harris Hip Score

This patient's functional status ^[1]:

• Mechanical right hip pain
• Walks with stick, walking tolerance 15 minutes
• Limping+
• Stairs step by step
• Sitting tolerance 30 minutes in high chair
• Difficulty putting on shoes and socks
• Drinker, 2 cans of beer everyday

Key points:

• All movements restricted on the affected (right) side
• Internal rotation and abduction are typically the first and most restricted movements in hip pathology (because the capsule is tightest in these positions)
• Fixed flexion deformity of 10° (detected by Thomas' test)
• True leg length discrepancy — femoral head collapse causes shortening

Avascular necrosis is a result of impairment of circulation to the femoral head → subsequent bone death and repair → bone disease

The key concept: bone is living tissue that requires blood supply. When blood supply is interrupted, osteocytes die. The body attempts to repair the dead bone by:

1. Resorption of necrotic bone (→ cysts, lucency on X-ray)
2. Creeping substitution — new bone laid down on dead trabecular scaffolding (→ sclerosis on X-ray)
3. If the reparative process cannot keep up with mechanical loading, the subchondral bone collapses (→ crescent sign, femoral head flattening)
4. Eventually, joint incongruity leads to secondary OA

Trauma (femoral neck fracture 15-50%, hip dislocation 10-25%) Alcohol abuse Steroid ( > 20mg/day increases the risk) Caisson disease Sickle cell anaemia

Additional causes (from senior notes ^[2]): SLE, osteomyelitis, septic arthritis, radiation.

X-ray findings:

• Cyst (resorption of dead bone + replacement with fibrous & granulation tissue)
• Sclerosis (thickened trabeculae due to direct deposition of new bone onto dead bone)
• Crescent sign (subchondral collapse of the necrotic segment)

MRI: 99% sensitivity and specificity

Why MRI is so important: X-rays are often NORMAL in early AVN (Ficat Stage 1). MRI detects marrow edema and the characteristic "double-line sign" on T2-weighted images (outer dark line = sclerotic bone, inner bright line = granulation tissue) before any structural collapse occurs ^[2].

MRI of contralateral hip — because AVN is frequently bilateral (especially in steroid/alcohol users), always scan both sides ^[1].

M/50, Caucasian, bilateral mechanical hip pain for 6 months, constant non-remitting, requires 2 elbow crutches, daily analgesics. Previously professional kickboxer.

Radiological features (remember: LOSS)

• Osteophyte
• Subchondral sclerosis
• Subchondral bone cyst
• Narrowing of joint space

WEIGHT BEARING X-rays Kellgren & Lawrence scoring system

Why each feature occurs:

• Joint space narrowing: Loss of articular cartilage → bones get closer together
• Osteophytes: Attempted repair/stabilization at joint margins → new bone growth
• Subchondral sclerosis: With cartilage loss, bone bears more load → thickens in response
• Subchondral cysts: Synovial fluid forced through cracks in denuded bone → cyst formation; OR ischemic necrosis of subchondral bone

Primary/Idiopathic:

• Uncommon in Chinese population

Secondary:

• Trauma
• Infection (e.g., old TB hip)
• Inflammatory joint diseases (e.g., RA)
• Crystal deposition diseases (e.g., gout)
• Neuropathic (e.g., Charcot joint)
• Metabolic/endocrine

M/45, unemployed, known IVDU, sudden onset left groin pain. Previous groin pseudoaneurysm, Fournier's gangrene with skin grafting.

Tenderness in left groin Localized swelling with discharge Hip held in flexion, abduction and external rotation Hip joint irritable, resisting all passive movements

Why this position? The hip capsule has maximum volume in flexion, abduction, and external rotation. When there is an effusion or pus in the joint, the hip assumes this position to minimize intracapsular pressure and pain. This is a classic examination finding for septic arthritis of the hip.

Blood tests: WBC, CRP, ESR Image-guided Hip Aspiration:

• Cell count
• Gram smear
• Bacterial / fungal / AFB cultures
• ± Crystals Blood cultures Imaging

X-rays: soft-tissue swelling, hip joint capsular distension (widening of joint space or subluxation), proximal femoral metaphysis changes → osteomyelitis MRI: most useful test to distinguish proximal femoral osteomyelitis from septic arthritis Radioisotope scan

Antibiotics — AFTER joint fluid/synovium obtained for culture, unless hemodynamic instability Early initiation of antibiotics before obtaining specimens risks negative culture → affects antibiotic regime

Surgical Drainage:

• Anterior approach in young to preserve blood supply
• Posterior approach when hip destruction not salvageable (need for replacement)
• Surgical drainage with antibiotic cement spacer

Secondary OA Pain due to joint incongruency and chondral damage Stiffness due to ankylosis and soft tissue contracture (flexion and adduction contracture) Deformity: angulation, coxa vara, shortening Instability, dislocation Leg length discrepancy

F/50, right hip pain for many years. Childhood right hip infection → cast for 1 year. Right femoral osteotomy in Shanghai as teenager. Now: significant right hip pain, walks with stick, 30min walking tolerance, daily analgesics, difficulty with shoes/socks, significant LLD and back pain.

• True LLD: -4cm (right shorter)
• Right hip: Flexion 110°, FFD 0°, Abduction 30°, Adduction 20°, ER 30°, IR 20°
• Old ischium scar

Unstable, poor functioning hip Significant leg length discrepancy Proximal femur deformity Management: Total Hip Replacement

Joint: Restore pain-free stable joint Bone:

• Acetabular side: Restore anatomical hip centre
• Femoral side: Correct femoral side deformity Soft tissue:
• Contracted muscles, ligaments and joint capsule
• Sciatic nerve (risk of stretch injury with leg lengthening)
• Femoral shortening (to protect nerve)

The lecture demonstrates robotic-arm assisted THA and 3D-printed metal cutting jig for proximal femur wedge osteotomy — these are advanced techniques used for complex cases with significant deformity ^[1].

Dysplasia → Low Dislocation → High Dislocation

This classifies the severity of DDH in adults for surgical planning.

Based on percentage of proximal migration of medial neck-neck junction from inferior margin of acetabulum (teardrop)

• Crowe I: < 50%
• Crowe II: 50-75%
• Crowe III: 75-100%
• Crowe IV: > 100%

Assumes femoral head height is 20% of pelvic height

Periacetabular Osteotomy (PAO):

• Indication: Symptomatic dysplasia in young adult with concentrically reduced hip and congruent joint space
• Before OA changes develop

Total Hip Replacement:

• Secondary OA changes
• Hip subluxation

M/22, referred for "left hip AVN". Left hip stiffness for 3 years. Walking with stick 30 min, limping, difficulty sitting in normal chair, cannot manage shoes and socks. Minimal hip pain or back pain.

Left hip ankylosed at 40° flexion, 15° abduction, 15° external rotation Left hip abductor contraction palpable Schober test: excursion only 5mm (normal > 5cm) Chest expansion: 2cm (normal > 5cm)

Why Schober's test and chest expansion matter: These are screening tests for ankylosing spondylitis (AS). Reduced Schober's excursion indicates lumbar spine fusion; reduced chest expansion indicates costovertebral joint involvement. This patient was misdiagnosed as "AVN" — but the findings point to AS with hip ankylosis.

Multidisciplinary approach: Rheumatologist:

• Diagnosis of Ankylosing Spondylitis
• Medical treatment
• Screening of associated disorders Orthopaedic Surgeon:
• Hip (conversion to THR)
• Spine

Total hip replacement — conversion of ankylosed hip

THR involves replacing both the acetabular and femoral sides of the joint:

• Acetabular component: Metal shell + polyethylene or ceramic liner (reamed into the acetabulum)
• Femoral component: Metal stem inserted into the femoral canal + head (metal or ceramic)

GT (Greater Trochanter), Head, Acetabulum — these are the landmarks to identify on post-op X-rays.

• Cemented: Polymethylmethacrylate (PMMA) cement bonds implant to bone. Better for elderly/osteoporotic bone.
• Uncemented (press-fit): Implant has porous coating for bone ingrowth. Better for young, active patients with good bone quality.
• Hybrid: Cemented stem + uncemented cup (or vice versa).

Stem: "A 12-month-old boy presented with fever and refused to crawl for the recent 4 days. He had a recent history of otitis media with antibiotics prescribed yet compliance was poor for fear of side effects. Physical examination showed fever of 40 degrees Celsius with hip examination demonstrating irritable right hip upon light touch and minimal passive hip movement."

• (a) What is the MOST LIKELY diagnosis? → Septic arthritis of the right hip
• (b) Name two investigations with expected findings to confirm your diagnosis. → Joint aspiration: turbid/purulent fluid, WBC > 50,000–100,000, > 75% PMNs, positive Gram stain/culture. Blood tests: elevated WBC, CRP, ESR. Blood culture: positive. (Any 2 with expected findings)
• (c) Name one mechanism relating otitis media to the present symptoms of hip pain. → Haematogenous spread of bacteria from the infected middle ear to the hip joint (bacteraemia → seeding of the synovial membrane/metaphysis)
• (d) Give two management options for this patient. → (1) IV antibiotics (empirical then targeted to culture results) (2) Surgical drainage/washout of the hip joint

Rationale: This is classic septic arthritis — fever, irritable hip, refusing to bear weight, preceding infection with poor antibiotic compliance. The hip is the second most common joint for septic arthritis in children. Connects directly to Case 3 in GC 229.

Stem: "A 60-year-old lady presented with fever, pain and swelling in her right knee for 4 days. Septic arthritis is one of the causes of monoarthritis. Which of the following investigations can MOST LIKELY confirm the diagnosis?"

Options: A. Anti-nuclear antibody, B. Joint aspiration for synovial fluid analysis, C. Serum urate level, D. X-ray of the index joint

Answer: B. Joint aspiration for synovial fluid analysis

Rationale: Joint aspiration is the gold standard for diagnosing septic arthritis — it provides cell count, Gram stain, and culture. ANA diagnoses autoimmune conditions. Serum urate may suggest gout but doesn't confirm septic arthritis. X-ray may show soft tissue swelling but is neither sensitive nor specific for early septic arthritis. This directly tests the investigation approach in GC 229 Case 3.

Stem: "A 68-year-old lady suffered from left knee mechanical pain for 3 years. X-ray of the left knee showed reduced joint space in the medial compartment with marginal osteophyte formation. Which of the following is an effective and evidence-supported treatment of knee osteoarthritis?"

Options: A. Arthroscopic debridement, B. Glucosamine, C. Knee replacement, D. Paracetamol

Answer: D. Paracetamol (first-line analgesic for OA per AAOS guidelines and GC 229 lecture)

Rationale: Paracetamol is the first-line analgesic for OA (GC 229 confirms this). Glucosamine lacks strong RCT evidence (the lecture notes "? cartilage regeneration"). Arthroscopic debridement has been shown in RCTs to be no better than sham surgery for OA. Knee replacement is effective but is reserved for end-stage disease after failed conservative management — the question says "3 years" of mechanical pain with only moderate X-ray changes, suggesting conservative measures should be tried first. However, note that paracetamol's efficacy for OA is increasingly questioned in recent literature — but per GC exam framing, it remains first-line.

Stem: "A 50-year-old business man presented to the A&E with severe low back pain. He had noticed weight gain over the past few months. X-ray showed osteopenia and wedge fracture of the 3rd lumbar vertebra. Further questioning revealed that he had been receiving frequent injections for osteoarthritis of his knee for over 1 year."

Answer: D. Iatrogenic due to exogenous steroid

Rationale: "Frequent injections for OA" = intra-articular steroid injections. Repeated IA steroids have systemic absorption → Cushing's syndrome (weight gain) → osteoporosis → vertebral compression fracture. This connects to GC 229's discussion of IA steroid risks. Also connects to AVN risk with steroid use ( > 20 mg/day).

Stem: "A 30-year-old gentleman complained of lower back pain for 6 months. HLA-B27 was positive. You suspect a diagnosis of axial spondyloarthritis."

• (a) Name four spondyloarthritis features in addition to HLA-B27. → Inflammatory back pain (morning stiffness > 30 min, improves with exercise), peripheral arthritis, enthesitis (e.g., Achilles), dactylitis, anterior uveitis, psoriasis, IBD, family history of SpA, elevated CRP, good response to NSAIDs
• (b) Name two clinical examinations for assessment of spinal mobility. → Schober's test (lumbar flexion), Chest expansion (costovertebral mobility), ± occiput-to-wall distance, ± lateral flexion
• (c) Name two findings on conventional radiographs of sacroiliac joints. → Sclerosis, erosions, joint space narrowing, ankylosis (fusion), "bamboo spine" (in late disease on spinal X-ray)

Rationale: Directly connects to Case 5 in GC 229, where the patient with AS had Schober's excursion of only 5mm and chest expansion of 2cm.

Stem: "Gout and pseudogout are common causes of acute monoarthritis. Which of the following is correct?"

Answer: B. Monosodium urate is a negative birefringent crystal.

Rationale: Gout = MSU = negatively birefringent, needle-shaped. Pseudogout = CPPD = positively birefringent, rhomboid-shaped. This tests crystal arthropathy knowledge relevant to hip differential diagnosis (GC 229 Case 3 includes crystal analysis in joint aspirate workup).