GC230 Knee Sport Injuries: Part 6
Sport-related knee injuries encompassing complex or multi-ligament injuries, including knee dislocations and combined ligamentous damage, requiring comprehensive evaluation and often surgical reconstruction.
Patella-Femoral Joint (PFJ) Instability — Knee Sport Injuries Part 6
Lecture Map
This is Part 6 of 6 in the GC 230 "Knee Sports Injuries" series, delivered by Professor WP Yau [1]. The six-part series covers: (1) Introduction, (2) Ligament injury including knee dislocation, (3) Meniscus injury, (4) Cartilage injury and osteochondral fracture, (5) Extensor mechanism injury, and (6) Patella-femoral joint instability [2].
The Big Idea: The patella sits in a groove on the front of the femur and is inherently at risk of lateral dislocation because the combined quadriceps pull vector drags it laterally. Understanding the anatomy, the stabilizers (especially the MPFL), the risk factors that worsen the lateral pull, and the classification/management of PFJ instability is the core of this lecture.
- Anatomy and kinematics of the patella-femoral joint
- Stabilizers against patellar dislocation and risk factors for PFJ instability
- Classification of patella-femoral joint dislocation
- Symptoms, signs, and management of PFJ instability
- PFJ instability is a favourite SAQ/minicase/MCQ topic because it involves anatomy, biomechanics, clinical examination signs (apprehension test, J-sign, Q-angle), and management decisions.
- The lecture emphasizes first-principles reasoning (why the patella dislocates laterally, why MPFL is critical at 20–30° flexion, why deep flexion is protective).
- Connects to Part 1's management principles: non-operative treatment for first-time dislocation (tissue can heal), surgical reconstruction for recurrent dislocation (function cannot be restored by healing alone) [2].
1. Anatomy and Kinematics of the Patella-Femoral Joint
There are three joints in the region of the knee: the knee joint, the patella-femoral joint, and the proximal tibiofibular joint. [1]
| Joint | Type | Surfaces | Primary Motion |
|---|---|---|---|
| Knee joint | Synovial | Distal femur ↔ Proximal tibia | Flexion/extension (sagittal plane); secondary: AP translation, internal/external rotation |
| Patella-femoral joint | Synovial | Patella ↔ Trochlea (anterior distal femur) | Translational movement of patella over trochlear groove ("patellar tracking") |
| Proximal tibiofibular joint | Fibrous | Fibular head ↔ Posterolateral proximal tibia | Minimal; extra-articular |
Why this matters: The PFJ is a distinct articulation. Its pathology (instability, chondromalacia, OA) is separate from tibiofemoral joint disease. The knee joint is divided into medial and lateral compartments. The PFJ has its own compartment on the anterior surface of the distal femur.
The patella is a sesamoid bone within the extensor mechanism of the knee joint. [1]
Why sesamoid matters: A sesamoid bone is embedded within a tendon. The patella sits within the quadriceps tendon above and the patellar tendon below. It increases the moment arm of the quadriceps, improving mechanical efficiency of knee extension. But this means the patella's position is entirely governed by the pull of the quadriceps and the constraints of the trochlear groove.
The combined vector of the pull of the quadriceps muscles results in a lateral displacing force on the patella, putting the patella at risk of lateral dislocation. [1]
First-principles explanation: The quadriceps has four heads, but the net pull vector is not perfectly in line with the patellar tendon. The quadriceps inserts more laterally relative to the tibial tuberosity, creating a valgus vector (the "Q-angle"). This means every time the quadriceps contracts, there is a component of force pushing the patella laterally. This is the fundamental reason why patellar dislocations are almost always lateral.
This is a critical concept tested in exams — understanding why the patella dislocates in early flexion, not in deep flexion.
| Knee Position | Patella-Trochlea Relationship | Stability | Clinical Relevance |
|---|---|---|---|
| 0° (full extension) | Patella sits proximal to the trochlea; they do not articulate. Patella slightly lateral to centre of groove. | Patellar glide (passive medial/lateral translation) is possible and normal — no discomfort or apprehension | Patellar glide at terminal extension is NOT a sign of instability |
| 20–30° flexion | Patella begins to articulate with trochlea. Moves slightly medial, aligns with groove. Lateral pulling force increases beyond 30° | MPFL serves as a checkrein to prevent excessive lateral translation. If MPFL deficient → lateral subluxation/dislocation | This is the critical danger zone. Apprehension test is performed at 20–30° flexion because this is where dislocation occurs |
| Deep flexion | Contact surface area between patella and trochlea increases. PFJ pressure increases | Patella is compressed into the groove by the posterior vector of the extensor mechanism → inherently stable | Patellar dislocation uncommon in deep flexion unless severe trochlear dysplasia |
High Yield
Patellar dislocation nearly always occurs during early knee flexion (20–30°) because that is when the patella first engages the trochlea and the MPFL is the primary restraint. In deep flexion, the bony conformity and compression forces make dislocation rare. [1]
2. Stabilizers of the Patella-Femoral Joint
The MPFL is a ligament located on the medial aspect of the knee, which connects the medial distal femur to the medial aspect of the patella. It acts as a static stabilizer of PFJ and serves as a checkrein to prevent excessive lateral translation of the patella, especially during early knee flexion. [1]
Lateral patellar dislocation nearly always occurs during early knee flexion and MPFL is almost always ruptured at the time of patellar dislocation. [1]
Why MPFL is so important:
- At 0° the patella isn't articulating with the trochlea, so there is no bony constraint. As the knee begins to flex to 20–30°, the patella starts engaging the groove but hasn't yet seated deeply. The only thing stopping the patella from sliding laterally under the pull of the quadriceps is the MPFL.
- The MPFL provides approximately 50–60% of the total medial restraining force against lateral patellar displacement (a well-established biomechanical principle).
- When the MPFL ruptures, the primary restraint is lost → recurrent instability.
The tension of the intact extensor mechanism results in a posterior vector, which stabilizes the PFJ by compressing the patella onto the trochlea. [1]
The patella and trochlea's conforming anatomy prevents lateral translation of the patella. [1]
In deeper flexion, two things protect against dislocation:
- Posterior compression vector: As the knee flexes more, the angle between the quadriceps tendon and patellar tendon becomes more acute, generating a net force that pushes the patella posteriorly into the groove.
- Bony conformity: The trochlea has a lateral wall that is higher than the medial wall, acting as a buttress. The patella's articular surface is shaped to match. If the trochlea is dysplastic (flat or shallow), this bony constraint is lost → instability even in deeper flexion.
3. Risk Factors for PFJ Instability
The combined vector of the pull of the quadriceps muscles results in a lateral displacing force on the patella... The lateral pulling force of the quadriceps increases as the knee flexes beyond 30 degrees. [1]
The lateral displacing force of quadriceps on the patella is aggravated by developmental bony abnormalities [1]:
| Bony Abnormality | Plane | Mechanism of Increasing Lateral Force |
|---|---|---|
| Genu valgum deformity | Coronal | Increases the Q-angle by lateralizing the direction of the patellar tendon relative to the quadriceps pull |
| Excessive femoral anteversion | Rotational (axial) | Internal rotation of the distal femur means the trochlear groove faces more medially, so the relative pull of the quadriceps is more lateral |
| Lateralization of tibial tuberosity (compensatory tibial external torsion) | Rotational (axial) | Moves the distal attachment of the patellar tendon more lateral, increasing the Q-angle |
First-principles explanation of Q-angle: The Q-angle is formed between the line from ASIS to centre of patella and the line from centre of patella to tibial tuberosity. Any condition that moves the tibial tuberosity laterally or the quadriceps pull line medially will increase this angle, increasing the lateral displacing vector on the patella.
Local Factors: [1]
- Patella alta (high-riding patella — patella sits more proximal, takes longer to engage the trochlea during flexion, so spends more time in the "unprotected" zone)
- Mal-alignment (coronal plane: genu valgum; rotational plane: femoral anteversion, tibial torsion)
- Issue of retinaculum — medial MPFL injury + lateral para-patellar retinaculum tightness (a tight lateral retinaculum pulls the patella laterally)
- Muscle wasting — specifically vastus medialis obliquus (VMO) (the VMO is the only dynamic medial stabilizer; its fibres pull the patella medially. Weakness → loss of this counterforce)
- Joint geometry problem — trochlear dysplasia (shallow or flat trochlea → no bony buttress)
Systemic Factors: [1]
- Generalized ligamentous laxity (e.g., Down syndrome) — all ligaments including MPFL are lax
- Neuromuscular disease (e.g., cerebral palsy) — abnormal muscle tone, spasticity of quadriceps, poor VMO control
Exam Trap
Students often forget patella alta and trochlear dysplasia. These are the two most important anatomical risk factors on imaging. Patella alta means the patella is high-riding and doesn't engage the trochlea until later in flexion, leaving a longer vulnerable window. Trochlear dysplasia means even when the patella engages, the groove is too shallow to constrain it.
4. Classification of Patella-Femoral Joint Dislocation
The lecture provides three classification axes [1]:
| Direction | Notes |
|---|---|
| Lateral PFJ instability | By far the most common; nearly always the direction of dislocation |
| Medial PFJ instability | Usually iatrogenic (e.g., over-tightened medial repair, over-released lateral retinaculum) |
| Cause | Features |
|---|---|
| Congenital patellar dislocation | Present at birth; can accompany arthrogryposis multiplex congenita, Down syndrome; often familial and bilateral; uncommon; persistent and irreducible (chronic); surgery should be performed as soon as possible |
| Traumatic patellar dislocation | 50–60% associated with sport or significant trauma; 40–50% during ADLs; nearly always lateral; accounts for 3% of all knee injuries; many patients have underlying developmental abnormalities (patella alta, increased Q-angle, increased TT-TG distance, trochlear dysplasia) |
High Yield
"Traumatic" patellar dislocation — 40–50% occur during ADLs without significant trauma. This means the majority of patients labelled "traumatic" actually have underlying predisposing anatomy. Only 50–60% have a direct association with sport or significant trauma. [1]
| Type | Definition | Key Distinguishing Feature |
|---|---|---|
| Acute | Patella dislocated for a short duration | Currently dislocated or recently reduced |
| Chronic | Patella remains dislocated for an extended period | Persistently dislocated (e.g., congenital, neglected) |
| Recurrent | Patella repeatedly dislocates from normal position | Patellar tracking is normal between episodes — this is the key discriminator from habitual |
| Habitual | Repeated dislocation occurring during each cycle of knee flexion and extension | Dislocates with every flexion-extension cycle — more severe; continuous mechanical problem |
Exam Trap — Recurrent vs Habitual
The examiner may test the distinction: Recurrent = normal tracking between episodes, dislocation is episodic. Habitual = every single flexion-extension cycle causes dislocation, indicating a severe structural problem. Don't confuse them.
5. Clinical Presentation — Symptoms and Signs
5.1 Acute Patellar Dislocation
Typical history of injury: [1]
- Can happen with or without significant injury
- Significant injury usually involves low-energy trauma with active quadriceps contraction and concomitant twisting motion of the knee
- Without significant injury: e.g., "attempt to pick up something from the floor with bending of the involved knee"
- Mechanical pain
- Up to 50% experience spontaneous reduction when the knee is extended
- Gave history of "giving way," "knee cap pop or clunk in or out of place," "abnormal shape of the knee"
Why spontaneous reduction occurs: When the knee extends, the lateral pull of the quadriceps decreases, and the patella naturally slides back medially toward the groove. Many patients present already reduced and may not realize they had a dislocation.
If the patella is still dislocated: [1]
- Deformity of knee
- Patella locked in a lateral position
- Knee in flexion deformity (because extension would push the patella back, the knee stays flexed with the patella locked out laterally)
If the patella has reduced spontaneously: [1]
- Significant swelling at the suprapatellar pouch (haemarthrosis from MPFL tear and chondral/osteochondral damage)
- Bruises at the medial parapatellar gutter (from MPFL rupture — the MPFL is on the medial side, so it tears/stretches when the patella goes laterally)
- Diffuse tenderness
- Reduced range of motion
- Frank blood found on knee aspiration (haemarthrosis)
X-ray (AP + Lateral +/- tangential view of PFJ) should be arranged: [1]
- To confirm the diagnosis of patellar dislocation
- To look for osteochondral fracture
Why tangential (skyline) view: This is a special view taken with the knee flexed ~30° that shows the patella sitting in the trochlear groove en face. It reveals subluxation, lateral tilt, and trochlear dysplasia. However, if the patella has already reduced, the standard views may look normal — the tangential view may show residual lateral tilt or osteochondral fragments.
Annual risk for first-time patellar dislocation: 5.8 / 100,000 people [1]
Females between the ages of 10 and 17 years old are at the highest risk [1]
15–50% of patients who suffer from first-time dislocation may develop recurrent patellar dislocation [1]
The MPFL almost always ruptures in acute patellar dislocation [1]
Osteochondral fracture is present in 39% of cases [1]
High Yield — Osteochondral Fracture
39% of acute patellar dislocations have an osteochondral fracture. This occurs because as the patella dislocates laterally, the medial patellar facet or the lateral femoral condyle can shear off a fragment of cartilage and bone. This is one of the indications for early surgery. Always look for loose bodies on X-ray.
5.2 Recurrent Patellar Dislocation
Typical history: [1]
- Confirmed history of previous dislocation by X-ray; OR subjective feeling that patella "moves in and out"
- +/- pain
- +/- giving way
- Symptom of APPREHENSION — feeling or worry that "knee cap is coming out" during at-risk position
- Loss of confidence in walking downslope, downstairs or participation in sports
Why apprehension dominates: In recurrent dislocation, the actual dislocation events may become less painful over time (because the MPFL is already chronically deficient and the medial structures are stretched). But the patient lives in constant fear that the kneecap will slip out again. This apprehension is often the primary complaint and the main indication for surgery.
Signs to assess: [1]
- J-sign — abnormal patellar tracking
- Apprehension sign — patellar instability
- Q-angle — concomitant bony abnormality
- Lower limb coronal plane alignment
- Quadriceps wasting
- Knee effusion
- Range of motion
- Patellar glide test
- Patellar tilt test
- Signs of generalized ligamentous laxity
6. Special Tests — Detailed Examination Techniques
Position: Sitting, knee flexed to 90° [1] Manoeuvre: Instruct the patient to actively extend the involved knee from 90° to full extension [1] Finding: Observe abnormal patellar tracking — lateral displacement of the patella at terminal extension [1] This signifies significant patellar mal-tracking and the presence of underlying trochlear dysplasia [1]
Why "J": As you watch the patella from the front while the patient extends the knee, it normally tracks in a straight line up the groove. In pathological cases, the patella suddenly jerks laterally at the last 20–30° of extension (as it exits the trochlea), tracing an inverted "J" shape. This lateral jump at terminal extension indicates that the trochlea is too shallow to guide the patella smoothly.
Position: Supine, knee flexed to around 20–30° [1] Manoeuvre: Apply a lateral pushing force on the medial border of the patella to sublux it [1] Finding: The patient will feel pain and impending dislocation and may try to stop the examiner [1]
Why 20–30° flexion: This is the position where the patella is just engaging the trochlea and is most vulnerable to lateral displacement. The test reproduces the exact mechanism of dislocation.
Clinical pearl: A truly positive apprehension test is when the patient grabs your hand or pulls away — it's the fear (apprehension) response, not just pain. Pain alone without apprehension may indicate other pathology (e.g., patellofemoral pain syndrome).
Position: Supine, knee in full extension [1] Manoeuvre: Measure the angle between:
- Line from ASIS to centre of patella
- Line from centre of patella to tibial tuberosity [1]
Normal Q-angle: Males 14 ± 3°, Females 17 ± 3° [1]
Increased Q-angle associated with: [1]
- Genu valgum
- Increased femoral anteversion
- Increased external tibial torsion
- Laterally positioned tibial tuberosity
Why females have higher Q-angle: The wider female pelvis means the line from ASIS approaches the knee at a more oblique angle, naturally increasing the Q-angle. This partly explains why young females are at highest risk for patellar dislocation.
7. Management
7.1 First-Time Acute Patellar Dislocation
Closed reduction should be performed as soon as possible to relieve symptoms and minimize secondary damage to articular cartilage.
Technique (step-by-step from lecture):
- Analgesics for pain control
- Patient lies supine
- Surgeon supports the affected leg
- Slightly flex the hip on the involved side → this relaxes the rectus femoris (which crosses both hip and knee), reducing quadriceps tension
- Gently extend the involved knee → the patella may reduce spontaneously
- If not reduced: apply gentle force to the lateral edge of the displaced patella, pushing it medially back between the femoral condyles
- An obvious "clunk" felt on successful reduction
Why extend the knee: At full extension, the lateral pull of the quadriceps is minimized, and the patella naturally tends to sit proximal to the trochlea where it is mobile — making it easier to slide back into position.
Initial treatment should be non-operative: [1]
- Analgesics
- Bracing
- Quadriceps strengthening exercise (especially VMO)
- Range of motion training
Early surgery is ONLY indicated in: [1]
- Osteochondral fracture (loose body needs removal or fixation)
- Persistent patellar subluxation/dislocation despite closed reduction
Management Principle from Part 1
This aligns with the general principle from GC 230 Part 1: "If the injured tissue likely can heal by itself and function can be restored → non-operative. If chance of healing is low or normal function cannot be restored → surgery." First-time dislocation: MPFL can heal with non-operative treatment. BUT if osteochondral fracture or persistent subluxation → surgery needed. [2]
Patients nearly ALWAYS have repeated patellar dislocation or symptoms of patellar instability (i.e., apprehension) [1]
Repeated patellar dislocation will lead to progressive chondral damage, leading to premature symptomatic osteoarthritis, especially in the patella-femoral joint [1]
Surgical stabilization is advised to treat the symptoms of apprehension and to reduce the chance of further repeated knee dislocation [1]
Why surgery is needed for recurrent but not first-time: After the first dislocation, the MPFL has ruptured. In some patients (50–85%), it heals adequately and they don't re-dislocate. But in those who develop recurrent instability (15–50%), the MPFL has either healed in a lengthened position or has underlying anatomical risk factors that overwhelm any healing. Non-operative management won't fix a structurally deficient MPFL or a dysplastic trochlea.
Surgical Options [1]
| Scenario | Procedure |
|---|---|
| No significant bony developmental problem | MPFL Reconstruction (graft used to recreate the medial checkrein) |
| Significant bony developmental problem | Bony operations required: |
| — Lateralized tibial tuberosity | Tibial tubercle transfer (Fulkerson osteotomy) — moves the tubercle medially (and sometimes anteriorly) to reduce the Q-angle |
| — Trochlear dysplasia | Trochleoplasty — surgically deepens the trochlear groove to create a better bony constraint |
Why MPFL reconstruction and not repair: In the chronic setting, the ligament remnant is often attenuated, scarred, or in the wrong position. Reconstruction with a graft (e.g., gracilis tendon autograft) provides a stronger, more reliable restraint than attempting to repair damaged tissue.
Epidemiology of recurrent instability:
Annual risk for recurrent patellar instability: 3.8 / 100,000 people [1]
From Maksim Surgery Notes [3], risk factors for patellar dislocation are summarized as:
- Young obese female
- Patella alta
- Wide Q-angle, genu valgus
- Shallow patellofemoral groove
- Weak quadriceps
- Connective tissue disease
This aligns well with the GC lecture. The lecture adds the important nuances of femoral anteversion, tibial external torsion, lateral retinaculum tightness, neuromuscular disease, and the temporal classification system.
From the "Injuries to bone and joint" lecture [4]: A joint dislocation's effect on ligaments can lead to laxity (re-dislocation later) or stiffness (loss of motion) — which explains the clinical spectrum in PFJ instability. Post-dislocation arthrofibrosis is uncommon in PFJ but chondral damage and subsequent OA are the main long-term concerns.
9. Exam Intelligence
| Trap | Correct Understanding |
|---|---|
| "Patellar dislocation is always from high-energy trauma" | Wrong. 40–50% occur during ADLs; underlying anatomical risk factors are common |
| "Medial patellar dislocation is common" | Wrong. Nearly always lateral; medial is usually iatrogenic |
| "Apprehension test is done at full extension" | Wrong. Done at 20–30° flexion — the vulnerable zone |
| "First-time dislocation always needs surgery" | Wrong. Initial treatment is non-operative. Surgery only for osteochondral fracture or persistent subluxation/dislocation |
| "Recurrent = habitual" | Wrong. Recurrent = normal tracking between episodes. Habitual = every flexion-extension cycle |
| "Patellar glide at terminal extension is pathological" | Wrong. Patellar glide at terminal extension is normal and not associated with discomfort or apprehension |
| "Q-angle is higher in males" | Wrong. Normal Q-angle: Males 14 ± 3°, Females 17 ± 3° |
- Name the stabilizers of PFJ at different degrees of flexion
- What is the role of MPFL and when is it most important?
- List risk factors for patellar instability (local and systemic)
- Classify patellar dislocations (direction, cause, temporal)
- Describe the apprehension test technique and findings
- What is the J-sign and what does it signify?
- Management of first-time vs recurrent patellar dislocation
- When is early surgery indicated in first-time dislocation?
- Surgical options for recurrent dislocation with and without bony abnormality
After searching the indexed past papers (2016–2025), no questions directly testing PFJ instability were identified in the provided past paper context. This does not mean it hasn't been tested — it may appear as MCQ options or within minicases not fully indexed here.
However, the topic intersects with:
- 2025 Fourth Summative MCQ Q48 [5]: Tests knee OA management principles — the concept of progressive chondral damage from recurrent patellar dislocation leading to PFJ OA connects here.
- Orthopaedic examination technique questions (apprehension test, Q-angle) could appear in any year's SAQ or minicase.
High Yield Summary
PFJ Instability — The Essentials:
-
Anatomy: The patella is a sesamoid bone in the extensor mechanism. The PFJ is patella ↔ trochlea. The combined quadriceps vector creates a lateral displacing force.
-
Kinematics: At 0° — patella not articulating, glide is normal. At 20–30° — patella engages trochlea, MPFL is the critical stabilizer. In deep flexion — bony conformity + compression stabilize the PFJ.
-
MPFL is the most important structure: almost always ruptures in acute dislocation; its deficiency → lateral subluxation/dislocation.
-
Risk factors: Patella alta, genu valgum, femoral anteversion, tibial external torsion, trochlear dysplasia, MPFL injury, lateral retinaculum tightness, VMO wasting, generalized ligamentous laxity, neuromuscular disease.
-
Classification: Direction (lateral > > medial, which is iatrogenic), Cause (congenital vs traumatic), Temporal (acute/chronic/recurrent/habitual).
-
Highest risk group: Females aged 10–17.
-
Osteochondral fracture in 39% of acute dislocations — always image to look for it.
-
First-time dislocation → non-operative (analgesics, bracing, physio). Early surgery ONLY for osteochondral fracture or persistent dislocation.
-
Recurrent dislocation → surgical stabilization: MPFL reconstruction if no significant bony problem; tibial tubercle transfer and/or trochleoplasty if bony abnormality present.
-
Key exam tests: J-sign (trochlear dysplasia), Apprehension test (at 20–30°), Q-angle (males 14 ± 3°, females 17 ± 3°).
Active Recall - Knee Sport Injuries Part 6: PFJ Instability
[1] Lecture slides: GC 230. Knee Sport Injuries_Part 6.pdf (all pages) [2] Lecture slides: GC 230. Knee Sport Injuries_Part 1.pdf (p2, p70) [3] Senior notes: Maksim Surgery Notes.pdf (p259–260) [4] Lecture slides: Injuries to bone and joint WCS.pdf (p26) [5] Past papers: 2025 Fourth Summative MCQ.pdf (Q48)
GC230 Knee Sport Injuries: Part 5
Knee sport injuries Part 5 covers posterior cruciate ligament (PCL) injuries, characterized by disruption of the PCL typically due to a posterior-directed force on the proximal tibia, resulting in posterior knee instability.
GC231 High Energy Trauma Open Fracture: Part 1
An open fracture resulting from high-energy mechanisms (e.g., motor vehicle accidents, falls from height) in which bone is exposed through a wound, carrying significant risk of contamination, soft tissue damage, and complications requiring urgent surgical management.