PATELLOFEMORAL INSTABILITY
MPFL | Trochlear Dysplasia | TT-TG Distance | Surgical Options
DEJOUR TROCHLEAR DYSPLASIA
Critical Must-Knows
- MPFL is the PRIMARY soft tissue restraint to lateral patellar translation (50-60%)
- MPFL ruptures in almost ALL acute patellar dislocations
- TT-TG greater than 20mm indicates lateral malalignment needing tibial tubercle osteotomy
- Trochlear dysplasia is the most common anatomical risk factor
- First-time dislocators: conservative unless loose body or osteochondral fragment
Examiner's Pearls
- "MPFL attaches just distal to adductor tubercle on femur
- "Patella alta (Insall-Salvati greater than 1.2) increases instability
- "Recurrence rate 50% after first dislocation, 80% after second
- "Combined procedures often needed for significant anatomical abnormalities
Clinical Imaging
Imaging Gallery
Critical Exam Concepts
MPFL is KEY
MPFL provides 50-60% of restraint to lateral patellar translation in the first 30° of flexion. It ruptures in almost all acute dislocations and is the target of surgical reconstruction.
Anatomical Risk Factors
Must assess ALL factors: Trochlear dysplasia, patella alta, increased TT-TG distance, femoral anteversion, genu valgum, rotational malalignment. Failure to address leads to recurrence.
TT-TG Distance
Normal is less than 20mm. Greater than 20mm = pathological lateral offset. Indicates need for tibial tubercle osteotomy (medialization +/- distalization) in addition to MPFL reconstruction.
First Dislocation Management
Most first-time dislocators: conservative treatment. Surgery indicated for: osteochondral loose body, large chondral/MPFL avulsion, recurrent instability, or high-risk anatomy.
Quick Decision Guide
| Clinical Scenario | Key Factor | Treatment | Additional Procedures |
|---|---|---|---|
| First dislocation | No loose body | Conservative 6 weeks | None usually |
| First dislocation | Osteochondral fragment | Surgery - fix fragment + MPFL | Address fragment |
| Recurrent instability | Normal anatomy | MPFL reconstruction | May be sufficient alone |
| Recurrent instability | TT-TG greater than 20mm | MPFL + tibial tubercle osteotomy | Medialization required |
| Recurrent instability | Severe dysplasia (B-D) | MPFL + consider trochleoplasty | Bony correction needed |
PATELLAAnatomical Risk Factors
Memory Hook:The PATELLA itself tells you all the risk factors!
SAFEMPFL Femoral Attachment
Memory Hook:SAFE femoral tunnel placement is essential for MPFL reconstruction!
FLAPIndications for Surgery After First Dislocation
Memory Hook:Fix the FLAP - surgical indications after first dislocation!
Overview and Epidemiology
Recurrence Risk
First dislocation: 50% recurrence risk. Second dislocation: 80%+ recurrence risk. This dramatic increase after second dislocation justifies earlier surgical intervention in recurrent cases. Anatomical risk factors predict higher recurrence.
Epidemiology
- Incidence: 7-77 per 100,000/year
- Peak age: 15-19 years (adolescents)
- Females greater than males in some studies
- Often during sports/pivoting activities
- Strong association with anatomical abnormalities
Mechanism
- Twisting on planted foot with knee flexed
- Valgus force with external rotation
- Direct blow to medial patella (rare)
- May occur with minimal trauma if dysplastic
- MPFL ruptures in essentially all cases
Pathophysiology and Anatomy
Medial Patellofemoral Ligament
Primary restraint: Provides 50-60% of restraint to lateral patellar translation in first 30° of flexion.
Femoral attachment: Between adductor tubercle and medial epicondyle (Schoettle point).
Patellar attachment: Proximal 2/3 of medial patella border, blends with VMO.
Length: Approximately 55mm.
Function: Most important in early flexion (0-30°) when patella not yet in trochlea.
MPFL Femoral Tunnel Placement
Correct femoral tunnel placement is CRITICAL for MPFL reconstruction. The Schoettle point is just distal and anterior to the adductor tubercle, between the posterior cortex of the medial femoral condyle and the posterior edge of Blumensaat line. Malposition leads to graft failure or patellofemoral overload.
Classification Systems
Dejour Trochlear Dysplasia Classification
| Type | Radiograph/CT Finding | Trochlear Morphology | Surgical Implication |
|---|---|---|---|
| A | Crossing sign only | Shallow trochlea | MPFL may suffice |
| B | Crossing sign + supratrochlear spur | Flat trochlea | Consider bony procedure |
| C | Crossing sign + double contour | Asymmetric facets | Bony procedure likely needed |
| D | All above + cliff pattern | Convex trochlea | Trochleoplasty may be needed |
Crossing sign: Trochlear groove crosses anterior femoral cortex on lateral radiograph (most sensitive finding).
Clinical Assessment
History
- Mechanism: Twisting, pivoting injury
- Sensation: Often describe patella moving laterally
- Spontaneous reduction: Usually reduces on extension
- Swelling: Immediate hemarthrosis common
- Previous dislocations: Recurrence history
Examination
- Effusion: Hemarthrosis if acute
- Apprehension test: Positive with lateral pressure
- J-sign: Lateral tracking in terminal extension
- Q-angle: Increased suggests malalignment
- Generalized laxity: Beighton score
Apprehension Test
With the knee in 30° flexion, apply lateral force to the patella. Positive test: patient resists or shows apprehension (fear of dislocation). This is the most reliable clinical sign of patellofemoral instability.
Examination Checklist
Alignment: Valgus, femoral anteversion, tibial torsion.
Patella tracking: J-sign, lateral tilt, subluxation.
Ligament laxity: Beighton score for generalized hypermobility.
Contralateral knee: Often bilateral predisposition.
Investigations
Standard Views
AP weight-bearing: Valgus alignment assessment.
Lateral: Trochlear dysplasia (crossing sign, supratrochlear spur), patella alta (Insall-Salvati).
Skyline/Merchant: Patellar tilt, subluxation, trochlear morphology.
Long leg alignment: If considering osteotomy for valgus.
Key radiograph findings: Crossing sign on lateral is most sensitive for dysplasia.
Classic MRI Findings
Acute patellar dislocation MRI: MPFL tear (usually femoral side), bone marrow edema medial patella AND lateral femoral condyle (kissing contusion pattern), possible osteochondral fragment, hemarthrosis. This pattern is pathognomonic.
Management Algorithm
First-Time Patellar Dislocation
Management Pathway
Usually self-reduces with knee extension. Aspiration if tense effusion. Radiographs to rule out fracture/loose body. MRI to assess MPFL and osteochondral injury.
If no loose body: brace in extension 2-4 weeks, then progressive ROM and quadriceps strengthening. Full recovery 6-12 weeks. 50% recurrence risk with conservative treatment.
Osteochondral loose body (fix or remove), large bony MPFL avulsion, high-risk anatomy, or professional athlete may warrant early MPFL repair or reconstruction.
Conservative treatment is appropriate for most first-time dislocators without loose bodies.
Surgical Technique
MPFL Reconstruction Technique
Surgical Steps
Gracilis or semitendinosus autograft (most common). Allograft option. Quadriceps tendon strip gaining popularity.
Two tunnels in proximal 2/3 of medial patella. Suture anchors or interference screws. Avoid full-thickness tunnels.
Schoettle point: between adductor tubercle and medial epicondyle. Fluoroscopy to confirm (intersection of posterior femoral cortex and Blumensaat line).
Tension with knee at 30-60° flexion. Avoid overtensioning (causes PF overload). Confirm full flexion and patella tracking before final fixation.
Femoral Tunnel Position
Correct femoral tunnel placement is THE most important technical factor. The Schoettle point on fluoroscopy: where the posterior femoral cortex line meets the posterior extent of Blumensaat line. Anterior or distal malposition causes graft failure or patellofemoral overload.
Avoid Overtensioning MPFL
Overtensioning the MPFL graft leads to increased patellofemoral contact pressures and accelerated cartilage wear. Tension with knee in 30-60° flexion, ensure patella can still translate one quadrant laterally, and confirm full flexion before final fixation.
Complications
| Complication | Cause | Prevention | Management |
|---|---|---|---|
| Recurrent instability | Missed anatomical factor, tunnel malposition | Address all risk factors, correct tunnel placement | Revision surgery addressing missed factors |
| Patellofemoral pain/OA | Overtensioned graft, tunnel malposition | Proper tensioning, correct tunnel position | Graft release, revision if severe |
| Stiffness | Overtensioning, prolonged immobilization | Early ROM, proper tensioning | Physical therapy, MUA if needed |
| Patellar fracture | Full-thickness patellar tunnels | Partial thickness tunnels or anchors | ORIF |
| Graft failure | Tunnel malposition, early return to sport | Correct technique, appropriate rehab | Revision reconstruction |
Most Common Cause of Failure
Femoral tunnel malposition is the most common cause of MPFL reconstruction failure. Anterior or distal placement changes graft isometry, leading to graft laxity in flexion or patellofemoral overload. Always confirm Schoettle point with intraoperative fluoroscopy.
Postoperative Care
Rehabilitation Protocol
Brace locked in extension for ambulation. ROM exercises 0-90° out of brace. WBAT with crutches. CPM if available.
Progressive ROM to full. Unlock brace. Wean crutches. Quadriceps strengthening, VMO focus. No resisted knee extension 0-45°.
Full weight-bearing, no brace. Progressive closed chain strengthening. Stationary bike, pool exercises.
Running at 3-4 months if strength adequate. Sport-specific training. Full return 6-9 months.
Tibial Tubercle Osteotomy Modifications
If TTO performed: protected weight-bearing for 6-8 weeks until osteotomy healed. Confirm radiographic healing before advancing to full weight-bearing. May delay return to sport.
Outcomes and Prognosis
Prognostic Factors
Good prognosis:
- Isolated MPFL rupture without dysplasia
- Normal TT-TG distance
- First or second dislocation
- Compliance with rehabilitation
- Correct surgical technique
Poor prognosis:
- Severe trochlear dysplasia (Dejour C-D)
- Multiple dislocations with cartilage damage
- Uncorrected anatomical abnormalities
- Generalized ligamentous laxity
- Tunnel malposition
Evidence Base and Key Studies
Natural History of First Dislocation
- Prospective study of 189 first-time dislocations
- 17% overall redislocation rate with conservative treatment
- Younger patients (less than 15) had higher recurrence
- No difference in outcomes between operative and non-operative initially
MPFL Reconstruction Outcomes
- Meta-analysis of 25 studies, 1130 knees
- Redislocation rate 1.1% after MPFL reconstruction
- Complications 10.5%, most were minor
- High satisfaction and return to sport rates
Combined MPFL and TTO
- Systematic review of combined procedures
- 0-4.5% redislocation rate with combined approach
- Better outcomes when addressing bony abnormalities
- TTO reduces recurrence in patients with elevated TT-TG
Trochleoplasty Outcomes
- Sulcus-deepening trochleoplasty in 45 knees
- 2% redislocation rate at 7-year follow-up
- Significant improvement in functional scores
- Complications included stiffness and cartilage damage
Schoettle Point Validation
- Cadaveric study defining optimal femoral tunnel position
- Intersection of posterior cortex and Blumensaat line on lateral fluoroscopy
- 1mm proximal to medial epicondyle, 2.5mm posterior
- Isometric point minimizes length change with flexion
Exam Viva Scenarios
Practice these scenarios to excel in your viva examination
Scenario 1: First Patellar Dislocation
"A 16-year-old netball player presents after her knee gave way while pivoting. She felt the patella move laterally and it reduced when she extended her knee. There is a moderate effusion. Radiographs show no fracture. How would you manage this?"
Scenario 2: Recurrent Instability
"A 22-year-old woman has had 4 patellar dislocations in the past 2 years. Examination shows positive apprehension, no significant valgus. CT shows TT-TG of 24mm. Radiographs show Dejour Type A trochlear dysplasia. What would you recommend?"
Scenario 3: Failed MPFL Reconstruction
"A 19-year-old male had MPFL reconstruction 18 months ago but has had 2 further dislocations since. Examination shows persistent apprehension. CT shows TT-TG of 16mm. Review of operative notes shows the femoral tunnel was placed at the medial epicondyle. What is your assessment?"
MCQ Practice Points
MPFL Function
Q: What percent of restraint to lateral patellar translation does the MPFL provide? A: 50-60% of restraint in the first 30° of flexion. It is the primary soft tissue restraint and ruptures in almost all acute dislocations.
TT-TG Threshold
Q: What TT-TG distance is considered pathological? A: Greater than 20mm. This indicates need for tibial tubercle medialization in addition to MPFL reconstruction.
Recurrence Rate
Q: What is the recurrence rate after a first patellar dislocation treated conservatively? A: Approximately 50%. After a second dislocation, the rate increases to 80%+.
MPFL Attachment
Q: Where does the MPFL attach on the femur? A: Between the adductor tubercle and medial epicondyle at the Schoettle point. This is just distal and anterior to the adductor tubercle.
Patella Alta
Q: What Insall-Salvati ratio indicates patella alta? A: Greater than 1.2. Normal is 0.8-1.2. Patella alta delays patellar engagement in the trochlea.
Dejour Classification
Q: What radiographic finding is common to all Dejour types of trochlear dysplasia? A: The crossing sign - where the trochlear groove line crosses the anterior femoral cortex on lateral radiograph.
Australian Context
Clinical Practice
- MPFL reconstruction widely performed
- Gracilis autograft most common
- TTO for TT-TG greater than 20mm
- Trochleoplasty at specialist centres only
- Increasing use of fluoroscopy for tunnel placement
Funding and Access
- Allografts may incur additional cost
- Specialist referral for recurrent cases
- Public system wait times variable
- Private insurance covers most procedures
Orthopaedic Exam Relevance
Patellofemoral instability is a common viva topic. Know the MPFL anatomy, TT-TG threshold (20mm), Dejour classification, and be able to articulate a systematic approach to surgical planning addressing all anatomical abnormalities.
PATELLOFEMORAL INSTABILITY
High-Yield Exam Summary
MPFL Anatomy
- •Primary restraint 50-60% (first 30° flexion)
- •Femoral attachment: Schoettle point
- •Between adductor tubercle and medial epicondyle
- •Ruptures in almost all acute dislocations
Risk Factors (PATELLA)
- •Patella alta (Insall-Salvati greater than 1.2)
- •Anteversion (femoral)
- •Trochlear dysplasia (Dejour A-D)
- •Excessive TT-TG (greater than 20mm)
Measurements
- •TT-TG: Normal less than 20mm
- •Insall-Salvati: Normal 0.8-1.2
- •Crossing sign: Trochlear dysplasia marker
- •Sulcus angle: greater than 145° = dysplasia
Treatment Algorithm
- •First dislocation no loose body: Conservative
- •Recurrent, normal anatomy: MPFL reconstruction
- •TT-TG greater than 20mm: MPFL + TTO medialization
- •Patella alta: MPFL + TTO distalization
MPFL Reconstruction Keys
- •Femoral tunnel at Schoettle point (CRITICAL)
- •Tension at 30-60° flexion
- •Avoid overtensioning
- •Confirm full flexion before fixation
Prognosis
- •50% recurrence after 1st dislocation
- •80%+ recurrence after 2nd dislocation
- •80-95% success with MPFL reconstruction
- •Tunnel malposition = most common failure cause