Patellar Dislocation — MPFL

Patellar dislocation is the most common acute traumatic ligamentous injury of the knee in the paediatric and young adult population. The vast majority are lateral dislocations. Recurrence is the primary clinical challenge, and failure to address the underlying anatomical risk factors leads to a cycle of repeated instability, chondral damage, and premature patellofemoral osteoarthritis.

• Incidence: approximately 5–77 per 100,000 population; peak incidence in adolescents aged 10–17 years
• Recurrence after first dislocation: approximately 15–44% overall; up to 60–70% in adolescents under 16 years
• After second dislocation: recurrence risk rises to approximately 50% without surgical intervention
• Mechanism: knee in slight flexion and valgus with femur internally rotating on a planted foot — indirect; or direct blow to medial patella
• The patella almost always dislocates laterally and spontaneously reduces — many patients present without the patella still dislocated
• Osteochondral injury: occurs in 20–40% of acute dislocations — from impact of medial patella facet against lateral femoral condyle as patella relocates; must be assessed on MRI

Patellar stability is provided by bony constraint and soft tissue restraints. Understanding each component is essential for appropriate surgical planning.

• MPFL (Medial Patellofemoral Ligament): primary soft tissue restraint to lateral patellar translation — provides approximately 50–60% of total medial restraining force; torn in virtually all acute lateral dislocations
• MPFL runs from the medial border of the patella to the medial femoral condyle (saddle point between medial epicondyle and adductor tubercle — Schöttle point)
• Other medial soft tissue restraints: medial patellotibial ligament, medial patellomeniscal ligament, VMO (dynamic medial stabiliser)
• Bony constraint: trochlear depth and morphology — a shallow or dysplastic trochlea fails to engage the patella, allowing lateral translation
• TT-TG distance (tibial tubercle to trochlear groove): measured on axial CT or MRI — represents lateral offset of patellar tendon from trochlear groove; normal <15 mm; 15–20 mm = borderline; >20 mm = significantly increased Q-angle equivalent; indicates need for tibial tubercle osteotomy (medialisaton/anteromedialsation)
• Trochlear dysplasia (Dejour classification): Type A = shallow sulcus; Type B = flat trochlea; Type C = asymmetric facets; Type D = cliff pattern with medial facet hypoplasia — severe dysplasia (B–D) requires trochleoplasty or sulcus-deepening in selected cases
• Patella alta (Caton-Deschamps ratio >1.2): patella positioned too high — engages trochlea late in flexion; increases instability risk; Caton-Deschamps ratio: patella tendon length / patella length on lateral radiograph
• Patellar tilt: assessed on axial (sunrise) view — increased lateral tilt suggests tight lateral retinaculum; TTTG >20 mm

• Clinical examination: medial retinacular tenderness (MPFL injury); apprehension test (positive laterally); J-sign (lateral patellar jump at terminal extension in dysplasia); patellar glide (normal = 2 quadrants laterally; >3 quadrants = laxity); crepitus; effusion
• Plain radiographs: AP, lateral, and axial (Merchant/sunrise) views; assess trochlear morphology (sulcus angle >145° = dysplasia), patella height (Caton-Deschamps, Insall-Salvati), patella tilt and shift on axial view
• Crossing sign on lateral X-ray (trochlea line crosses subchondral line) = trochlear dysplasia — Dejour classification requires lateral X-ray and CT/MRI
• MRI: mandatory after first acute dislocation — confirms MPFL tear (medial patellar attachment most common site), identifies osteochondral injury (bone bruise pattern: lateral trochlea + medial patella = kissing contusions = pathognomonic), and assesses articular cartilage
• CT axial scan: gold standard for TT-TG measurement; also assesses trochlear morphology and patellar tilt; must be obtained with both knees on same scan for accurate measurement

• First-time dislocators in most adults: 3–6 weeks of immobilisation in extension followed by physiotherapy — VMO strengthening, hip abductor and external rotator strengthening, patellar taping/bracing
• Indications to consider acute surgical intervention: osteochondral fracture with a large loose body requiring fixation, first dislocation in a very young athlete with severe instability, documented complete MPFL avulsion with significant proximal retraction
• Isolated first-time dislocation without osteochondral injury: non-operative treatment appropriate with good rehabilitation; surgery for first-time dislocators not consistently superior to physiotherapy in RCT evidence
• Recurrent instability (>2 dislocations) or functional instability despite conservative management: surgical reconstruction indicated
• Patellar bracing: McConnell taping and patellar tracking braces — adjunct to physiotherapy; reduce pain and improve VMO activation; not curative

MPFL reconstruction is the cornerstone of surgical management for recurrent lateral patellar instability. It addresses the primary soft tissue restraint deficiency.

• Graft options: gracilis autograft (most common); semitendinosus; quadriceps tendon; synthetic — gracilis gives good length and diameter; minimal donor site morbidity
• Patellar fixation: two parallel tunnels or two suture anchors in medial patella — graft passes through tunnels or attached via anchors; do not tunnel through patella beyond 50% of its depth to avoid fracture; divergent tunnels improve bone bridge strength
• Femoral attachment — Schöttle point: intersection of the posterior cortex line of the femur, the posterior line of Blumensaat, and 1 mm anterior to the posterior femoral cortex on lateral fluoroscopy — precise placement is critical; malposition is the most common cause of failure
• Femoral tunnel position: too distal = graft tightens in flexion (restricts flexion); too proximal = graft tightens in extension; correct position = isometric through full ROM — confirm with fluoroscopy before definitive fixation
• Graft tensioning: knee at 30–45° flexion; patella centralised in trochlea; apply appropriate tension — overfilling restricts flexion; undertensioning leads to recurrence
• Recurrence rate after MPFL reconstruction: approximately 2–10% in well-selected patients without uncorrected bony abnormalities

MPFL reconstruction alone is insufficient when significant bony abnormalities are present. These must be identified and corrected concurrently or as staged procedures.

• Fulkerson anteromedialisaton osteotomy: oblique osteotomy of tibial tubercle — simultaneously medialises (reduces TT-TG) and anteriorises (offloads distal patellofemoral joint); ideal for patients with elevated TT-TG + distal patellar chondrosis
• Lateral retinacular release: no longer performed in isolation — does not address MPFL deficiency and increases risk of medial instability; release only as adjunct when tight lateral retinaculum documented on examination
• In skeletally immature patients (open physes): avoid crossing the tibial physis with osteotomy; defer tubercle osteotomy until skeletal maturity; MPFL reconstruction with soft tissue fixation away from physis preferred

• Multiplanar assessment before surgery is mandatory: TT-TG on CT, Caton-Deschamps on MRI lateral view, Dejour trochlear dysplasia classification on lateral X-ray and axial CT — operating without this information risks addressing the wrong abnormality
• MPFL reconstruction fails most commonly due to incorrect femoral tunnel placement — too distal tightens the graft in flexion and restricts ROM; always confirm Schöttle point with intraoperative fluoroscopy before tunnel drilling
• Osteochondral injury at first dislocation: if a large loose osteochondral fragment is identified on MRI — urgent arthroscopic surgery indicated for fixation (if attached to cartilage-bearing surface) or removal (if from non-bearing surface); 2 mm or larger fragments are surgically relevant
• Combined procedures: MPFL reconstruction + Fulkerson osteotomy for elevated TT-TG; MPFL + distalisation for patella alta; MPFL + trochleoplasty for severe trochlear dysplasia — staging vs single session depends on surgeon experience and patient factors
• TT-TG-TG ratio: recent concept — TT-TG normalised to trochlear groove width; addresses the limitation of using absolute TT-TG values in patients with different knee sizes; more predictive of instability than TT-TG alone in emerging literature

• MPFL provides 50–60% of medial restraining force; torn in virtually all lateral patellar dislocations
• MPFL femoral attachment = Schöttle point (saddle between medial epicondyle and adductor tubercle)
• Femoral tunnel too distal = graft tightens in flexion = restricted ROM; too proximal = tightens in extension
• TT-TG >20 mm = tibial tubercle medialisaton or anteromedialisaton (Fulkerson osteotomy)
• Caton-Deschamps >1.2 = patella alta = consider tibial tubercle distalisation
• Kissing bone bruise (lateral trochlea + medial patella) on MRI = pathognomonic of lateral patellar dislocation
• Crossing sign on lateral X-ray = trochlear dysplasia
• Fulkerson osteotomy = anteromedialisaton = reduces TT-TG AND offloads distal patellofemoral compartment
• Lateral retinacular release in isolation: contraindicated — risk of medial instability
• Recurrence after second dislocation without surgery: approximately 50% — surgical stabilisation strongly indicated