Patellofemoral Instability

Patellofemoral instability (PFI) encompasses a spectrum from acute first-time patellar dislocation to chronic recurrent instability. The patella dislocates laterally in the vast majority of cases (97%), and the medial patellofemoral ligament (MPFL) is the primary soft tissue restraint to lateral patellar translation — it is injured in virtually all lateral patellar dislocations. Understanding the anatomical risk factors for instability (bony and soft tissue) and how to assess and address them is central to modern patellofemoral surgery.

• Incidence: approximately 5–77 per 100,000 per year; peak incidence in adolescents and young adults (15–25 years); recurrence rate after first dislocation approximately 15–45%; higher recurrence in younger patients and those with anatomical risk factors
• MPFL anatomy: runs from the medial femoral epicondyle (slightly proximal and anterior to the adductor tubercle) to the superomedial border of the patella; provides approximately 50–60% of the resistance to lateral patellar translation between 0° and 30° of knee flexion; almost universally torn in lateral patellar dislocation (97% of cases); the MPFL is the primary restraint in the critical early arc of motion when the patella has not yet engaged the trochlear groove
• Mechanism of first dislocation: typically a non-contact twisting injury with the knee near extension; valgus with external tibial rotation; the patella translates laterally over the lateral trochlear facet; spontaneous reduction usually occurs as the knee extends

• Dejour classification of trochlear dysplasia: Grade A — sulcus shallower than normal but still present; Grade B — flat or convex trochlea (supratrochlear spur on lateral X-ray); Grade C — asymmetric trochlear facets (medial facet hypoplastic); Grade D — cliff pattern (asymmetric facets + supratrochlear spur); Grades B and D are the most severe and carry the highest risk of recurrence

• History: acute dislocation event (heard/felt pop, medial knee pain, visible deformity if not spontaneously reduced); recurrent subluxation (giving way without full dislocation, catching); fear of dislocation during activity (avoidance behaviour)
• Patellar apprehension test: patient supine; knee at 20–30° flexion; examiner applies lateral force to the medial edge of the patella; positive = patient becomes anxious, grabs the examiner`s hand, or contracts quadriceps to prevent dislocation; most sensitive clinical test for PFI
• J-sign: the patella tracks from a laterally displaced position at near-extension, then suddenly moves medially (into the trochlear groove) as the knee flexes — looks like the letter "J" in reverse; indicates significant lateral maltracking or patella alta with delayed trochlear engagement; assessed during active knee flexion from extension
• MPFL tenderness: medial retinaculum and MPFL are tender at the medial femoral epicondyle and along the superomedial patellar border; site of MPFL tear
• Lateral retinacular tightness: tight lateral retinaculum limits patellar medial tilt — normal medial tilt should be ≥15°; tight lateral retinaculum contributes to lateral tracking but is rarely the primary cause of instability
• Associated osteochondral injury: acute dislocation causes an osteochondral shear injury at the medial patellar facet or lateral femoral condyle in approximately 50–90% of cases; haemarthrosis in the acute setting is common; osteochondral fragments may require arthroscopic removal or fixation

• Weight-bearing AP and lateral knee radiographs: assess bony alignment; lateral X-ray for patellar height (Insall-Salvati), trochlear dysplasia signs (crossing sign, supratrochlear spur, double contour); Merchant/skyline view for patellar tilt and subluxation
• CT scan of the knee: gold standard for TT-TG measurement; also measures patellar tilt; provides accurate bony anatomy for surgical planning; TT-TG measured on axial cuts by superimposing the image at the trochlear groove level on the image at the tibial tubercle level; measures the offset between the two points
• MRI knee: essential after acute dislocation; demonstrates MPFL tear (superomedial patellar border or femoral insertion are most common sites); osteochondral injury (medial patellar facet, lateral femoral condyle); trochlear dysplasia; bone bruising (lateral femoral condyle and medial patellar facet = classic dislocation pattern); assesses articular cartilage
• Patellar height measurement: Insall-Salvati (patellar tendon length / patellar length — normal ≤1.2; >1.3 = alta); Caton-Deschamps (articular patellar surface to anterior tibial plateau — normal ≤1.2; >1.2 = alta) on lateral X-ray

• First dislocation (without osteochondral fragment, without significant anatomical risk factors): non-operative management is appropriate — rehabilitation, VMO (vastus medialis oblique) strengthening, patellar bracing, proprioception training; recurrence rate approximately 15–45%
• First dislocation in a patient with significant anatomical risk factors (severe trochlear dysplasia Dejour B/D, patella alta, TT-TG >20 mm) or a loose osteochondral fragment: consider early surgical management as recurrence risk is very high; a second dislocation causes further chondral damage; discuss risks vs benefits with patient
• Rehabilitation: quadriceps and VMO strengthening, hip abductor and external rotator strengthening (reduce dynamic valgus), neuromuscular training, functional activity progression; 3–6 months of structured physiotherapy
• Patellar bracing: medially directed patellar brace for activity; reduces perceived instability; does not correct the underlying anatomy

The goal of surgery is to address the anatomical risk factors driving instability. Procedures are divided into soft tissue (MPFL reconstruction) and bony (tibial tubercle osteotomy, trochleoplasty). The surgical plan is determined by the specific anatomy of each patient.

• MPFL reconstruction: the cornerstone of surgical management of patellofemoral instability; gracilis or semitendinosus autograft (or allograft) reconstructs the MPFL between its patellar and femoral attachments; femoral insertion is critical — placed at the Schottle point (radiographic landmark: 1 mm anterior to the posterior cortex extension line, at the level of the posterior femoral cortex, proximal to the medial femoral condyle); misplacement of the femoral tunnel causes non-isometric graft behaviour and either excessive tightness in flexion or failure to provide stability in extension
• Tibial tubercle osteotomy (TTO — anteromedialization, Fulkerson osteotomy): oblique osteotomy of the tibial tubercle; medialization reduces TT-TG (corrects lateral tracking); anteriorization unloads the patellofemoral joint (reduces patellar compressive force); used when TT-TG >20 mm; can be combined with MPFL reconstruction for combined instability and chondral disease
• Distal tubercle transfer (medialization only — Elmslie-Trillat): for isolated lateral maltracking with increased TT-TG; less commonly used than Fulkerson as anteromedialisation also addresses articular chondral overload
• Trochleoplasty: surgical deepening of the trochlear groove; indicated for severe trochlear dysplasia (Dejour B/D) where MPFL reconstruction alone is insufficient to provide stability; technically demanding; deepening sulcus trochleoplasty (Bereiter technique) or elevation trochleoplasty (Goutallier); reserved for experienced surgeons; high satisfaction in appropriately selected patients
• Lateral retinacular release: historically overused and now controversial — isolated lateral release does not correct instability and may cause medial instability if the lateral retinaculum is the only remaining restraint; should be combined with MPFL reconstruction if lateral tightness is contributing; never used as a sole procedure for instability
• Patellar height correction (tibial tubercle distalization): for patella alta (Caton-Deschamps >1.2–1.3); the tibial tubercle is moved distally to allow the patella to engage the trochlea earlier in flexion; combined with MPFL reconstruction in most cases

• Schottle point for femoral MPFL tunnel: the critical landmark for MPFL femoral tunnel placement; on a true lateral knee X-ray, the Schottle point is located 1 mm anterior to the posterior cortex extension line, at the level of the posterior border of the medial femoral condyle, and proximal to the posterior femoral condyle; proximal and posterior misplacement causes graft tightening in flexion (flexion loss); distal and anterior misplacement causes laxity in extension (instability); isometric placement is the goal
• Skeletally immature patients: MPFL reconstruction with soft tissue techniques (patellar suture anchor fixation; gracilis graft; avoiding physeal tunnels where possible); TTO in the skeletally immature risks physeal damage and growth disturbance — deferred until physeal closure; MPFL reconstruction is the primary surgical option in most skeletally immature patients with recurrent instability
• Osteochondral fragment fixation in acute dislocation: osteochondral fragments >1 cm detected on MRI or CT should be assessed for fixation viability at arthroscopy; small avascular fragments (medial patellar rim) are excised; large fragments from the lateral femoral condyle or central trochlea with viable cartilage are fixed with absorbable pins or headless screws; failure to recognise a large fragment requiring fixation leads to loose body and early arthritis
• Global laxity and MPFL reconstruction: patients with Ehlers-Danlos syndrome or generalised hyperlaxity are at high risk of MPFL graft stretching and failure; avoid aggressive MPFL tensioning (which may cause medial over-constraint and MPFL graft failure); address all anatomical factors including tibial tubercle and trochlear dysplasia; MPFL alone often insufficient in severe generalised laxity

• MPFL: primary restraint to lateral patellar translation 0–30°; torn in 97% of lateral dislocations; most important structure to reconstruct
• TT-TG >20 mm = abnormal; >20 mm = tibial tubercle osteotomy (TTO) indicated to correct lateralisation
• Dejour B and D trochlear dysplasia: highest recurrence risk; may require trochleoplasty in addition to MPFL reconstruction
• Patella alta: Insall-Salvati >1.3; Caton-Deschamps >1.2; delayed trochlear engagement → instability; address with tibial tubercle distalization
• J-sign: lateral to medial patellar movement during early flexion; indicates lateral maltracking and delayed trochlear engagement
• Schottle point: femoral MPFL attachment — 1 mm anterior to posterior cortex line, at level of posterior femoral condyle; misplacement = graft non-isometry = failure
• Lateral retinacular release alone: not an isolated treatment for PFI; risk of medial instability; combine with MPFL reconstruction if tight lateral structures contributing
• Osteochondral injury in acute dislocation: present in 50–90%; medial patellar facet and lateral femoral condyle; >1 cm viable fragment = fix; small/avascular = excise
• Skeletally immature: MPFL reconstruction preferred over TTO (growth plate risk); avoid physeal tunnels; defer TTO until physeal closure
• First dislocation management: non-operative if no osteochondral fragment and no severe anatomical risk factors; early surgery if Dejour B/D + severe anatomy + high-demand young patient