Patellar Instability — MPFL Reconstruction

Medial patellofemoral ligament (MPFL) reconstruction is the gold standard surgical treatment for recurrent lateral patellar dislocation. The MPFL is the primary restraint to lateral patellar displacement, providing approximately 50–60% of the total medial restraining force. It is torn in virtually every acute patellar dislocation and, in patients with predisposing anatomical risk factors, does not heal sufficiently to prevent recurrence. MPFL reconstruction restores the medial passive restraint and, when combined with addressing coexisting anatomical abnormalities (trochlear dysplasia, elevated TT-TG distance, patella alta), achieves excellent results with very low recurrence rates.

• Indications for MPFL reconstruction: recurrent patellar dislocation (≥2 episodes); first-time dislocation with high-risk anatomy (severe trochlear dysplasia Dejour B/C/D, patella alta Caton-Deschamps >1.3, TT-TG >20 mm) and a young active patient; failed conservative management (VMO strengthening, bracing) for recurrent instability; the procedure should be tailored to address all contributing anatomical abnormalities — MPFL reconstruction alone may be insufficient if trochlear dysplasia is severe or TT-TG >20 mm without concurrent TTO
• Pre-operative workup: standing AP, lateral (assess trochlear crossing sign, patella alta), and skyline X-rays; CT scan to measure TT-TG distance and assess trochlear morphology (Dejour classification) and patellar tilt; MRI to confirm MPFL tear location, assess cartilage, and evaluate associated pathology

• MPFL anatomy: the MPFL runs from the medial border of the patella (upper two-thirds) to the medial femoral epicondyle at Scöttle`s point; Scöttle`s point is located on a true lateral radiograph approximately 1 mm anterior to the posterior femoral cortex and 2.5 mm distal to the posterior aspect of the medial femoral condyle; the MPFL is a flat, fan-shaped ligament within the second layer of the medial knee (between the superficial and deep layers of the medial retinaculum); it blends with the medial joint capsule and the VMO distally
• The MPFL is the primary restraint between 0° and 30° of knee flexion — the range during which the patella is not yet fully engaged in the trochlear groove and is most vulnerable to lateral displacement; beyond 30–40° of flexion, the trochlea provides bony containment and the MPFL becomes less critical
• The VMO is the primary dynamic medial stabiliser; VMO fibres insert into the medial patellar border at an angle of approximately 50–55° and provide active medial restraint during quadriceps contraction; VMO weakness is a modifiable risk factor for recurrence

• Graft choice: gracilis tendon autograft is the most commonly used graft (suitable length, appropriate stiffness, low donor site morbidity — mild knee flexion weakness); semitendinosus autograft (bulkier, may be too stiff — risk of over-constraining the patella); hamstring allograft (avoids donor site morbidity); quadriceps tendon autograft (increasingly used, no hamstring donor site, good tissue quality)
• Femoral tunnel placement — the most critical technical step: the femoral tunnel must be placed precisely at Scöttle`s point to achieve a graft that is isometric (same length) through the functional range of motion (0–90°); a tunnel placed too proximally (near the adductor tubercle) results in the graft tightening in flexion — the patient develops restricted flexion and medial patellar over-constraint, leading to medial compartment overload and accelerated OA; a tunnel placed too distally (near the joint line) tightens in extension; intraoperative fluoroscopic confirmation of the tunnel position on a true lateral view is mandatory
• Patellar fixation: the graft is fixed to the medial patellar border using suture anchors (placed in the upper two-thirds of the patella) or passed through bone tunnels; two anchors provide a better biomechanical reconstruction of the fan-shaped MPFL; avoid placing anchors in the articular surface
• Graft tensioning: the graft is tensioned with the knee at 30° of flexion in neutral rotation; at this position, the MPFL is at its physiological length; tensioning in full extension risks over-tightening; the correctly tensioned graft should allow approximately 1 cm of lateral patellar translation at 20–30° of flexion (the patella should move but with a firm end-point and no apprehension); over-tightened graft = medial compartment overload and arthrosis; under-tensioned graft = continued instability
• Concurrent procedures: if TT-TG >20 mm, add tibial tubercle osteotomy (AMZ Fulkerson or Elmslie-Trillat) at the same setting; if patella alta (Caton-Deschamps >1.3), add tibial tubercle distalisation; if severe trochlear dysplasia (Dejour B/C/D with supratrochlear bump), consider trochleoplasty (usually a separate staged procedure)

• Recurrence rate after MPFL reconstruction: approximately 2–5% for isolated MPFL reconstruction in appropriately selected patients without severe trochlear dysplasia; significantly higher if the TT-TG is not addressed (>20 mm without TTO) or if severe trochlear dysplasia (Dejour B/C/D) is present without concurrent bony correction; the majority of published series report >90% good to excellent outcomes with recurrence rates <5% at short- to medium-term follow-up
• Patient-reported outcomes: significant improvement in Kujala score (the primary PROM for patellofemoral instability — 30 questions, 0–100, 100 = best), Banff Patella Instability Instrument (BPII), and IKDC scores after MPFL reconstruction; return to sport in approximately 70–85% of cases
• Complications: graft over-tightening (most significant technical complication — leads to medial OA); recurrent instability (graft failure or inadequate addressing of anatomy); infection; stiffness; patellar fracture (through patellar anchor tunnel — avoid tunnels >3.5 mm in diameter); incorrect femoral tunnel position (most common technical error)

• Graft over-tightening and medial compartment OA: the most serious long-term consequence of technical error in MPFL reconstruction; an over-tightened graft medialises the patella excessively, increasing medial patellar facet contact stress and medial compartment loading; this accelerates medial patellofemoral OA; clinical signs include medial patellar tenderness, restricted flexion, and medial joint line pain after surgery; prevention is the key — correct graft tensioning at 30° flexion with 1 cm of patellar mobility as a check; revision is difficult and outcomes are poor
• MPFL reconstruction in skeletally immature patients: the femoral tunnel must avoid the distal femoral physis in growing patients; the physis lies immediately adjacent to Scöttle`s point; alternative techniques include a physeal-sparing approach using periosteal fixation at the femoral attachment or soft tissue fixation to avoid a bone tunnel; MRI is helpful to map the physis pre-operatively; the MPFL repair/reconstruction in children should minimally disrupt the physis to avoid growth disturbance
• Two-stage approach for trochleoplasty + MPFL reconstruction: some surgeons perform trochleoplasty as a first procedure, allow 6 months for healing and remodelling of the trochlea, then perform MPFL reconstruction as a second procedure; this staged approach allows independent assessment of trochlear correction adequacy before committing to soft tissue reconstruction; others perform both simultaneously in experienced centres; the staged approach reduces the risk of over-constraining the reconstruction relative to the newly deepened trochlea
• Rehabilitation after MPFL reconstruction: partial weight-bearing for 2 weeks, progressing to full weight-bearing at 4–6 weeks; hinged knee brace for 6 weeks; early range of motion exercises; VMO and quadriceps strengthening commences at 2–4 weeks; return to sport at 6–9 months; proprioceptive training and sport-specific exercises in the later rehabilitation phase

• MPFL: primary medial restraint (50–60% of medial force); torn in virtually 100% of dislocations; reconstruction = gold standard for recurrent instability
• Scöttle`s point: 1 mm anterior to posterior cortex + 2.5 mm distal to posterior medial condyle on true lateral X-ray; femoral tunnel target; MUST be confirmed fluoroscopically intraoperatively
• Femoral tunnel too proximal: graft tightens in flexion → restricted ROM + medial overload; too distal: graft tightens in extension → extension block; isometric placement is the goal
• Graft tensioning: at 30° flexion; 1 cm lateral patellar translation = correct tension; over-tightened = medial OA; under-tensioned = continued instability
• TT-TG >20 mm: MPFL reconstruction alone insufficient; must add TTO (Fulkerson AMZ or Elmslie-Trillat); addressing all anatomical contributors is essential for low recurrence
• Recurrence rate with MPFL reconstruction: 2–5% in well-selected patients; much higher if anatomy not addressed (TT-TG uncorrected, severe dysplasia without trochleoplasty)
• Kujala score: primary PROM for patellofemoral instability; 0–100 (100 = best); improves significantly after MPFL reconstruction
• Graft over-tightening: most serious technical error; medial patellar OA; restricted flexion; medial joint pain; prevention — correct tensioning and isometric placement
• Skeletally immature patients: avoid femoral tunnel through distal femoral physis; physeal-sparing techniques; pre-op MRI to map physis
• Gracilis autograft: most commonly used; appropriate stiffness and length; mild flexion weakness donor site; semitendinosus may be too bulky