Patellar Dislocation — Acute

Acute patellar dislocation occurs when the patella displaces laterally out of the trochlear groove, most commonly in adolescents and young adults engaged in sporting activity. It is among the most common knee injuries in young people and carries a significant risk of recurrence — approximately 15–45% of first-time dislocators will experience a further dislocation. Understanding the medial soft tissue restraints, the role of trochlear dysplasia, and the evidence for acute and chronic management is essential for the orthopaedic surgeon.

• Incidence: approximately 5–7 per 100,000 per year; peak incidence in adolescents aged 10–17 years; female predominance; the majority (approximately 94%) dislocate laterally; medial dislocation is rare and usually iatrogenic (over-correction from medial transfer)
• Medial patellofemoral ligament (MPFL): the primary static restraint to lateral patellar displacement; provides approximately 50–60% of the medial restraining force; runs from the medial border of the patella to the medial femoral epicondyle (Scöttle`s point — approximately 1 mm anterior and 2.5 mm distal to the posterior cortex at the adductor tubercle on a true lateral X-ray); the MPFL is torn in virtually 100% of acute patellar dislocations and is the primary structure requiring reconstruction in recurrent dislocation
• Other medial stabilisers: medial retinaculum; vastus medialis obliquus (VMO — dynamic restraint); medial patellotibial ligament; medial patellomeniscal ligament
• Mechanism: typically a non-contact twisting injury (internal femoral rotation on a fixed foot with the knee flexed) or direct blow to the medial patella; the knee is usually in slight flexion at the time of dislocation; spontaneous reduction occurs in most cases before the patient reaches hospital

• History: lateral knee pain and giving way; a pop or clunk at the time of injury; visible deformity if the patella remains dislocated; haemarthrosis (almost universal — blood from the torn MPFL and any osteochondral fracture); the patella usually reduces spontaneously or is self-reduced before hospital presentation
• Examination after acute dislocation: haemarthrosis (large tense effusion); medial patellar tenderness (MPFL tear site — at the femoral attachment or the patellar attachment); apprehension sign (medial to lateral pressure on the patella with the knee in slight flexion provokes anxiety and pain as the patient anticipates re-dislocation); lateral tilt; assess range of motion (limited by pain and effusion)
• Plain radiographs: AP, lateral, and skyline (Merchant) views; assess for: osteochondral fractures (loose bodies — visible as bony fragments in the medial or lateral joint; may also fracture from the medial patellar facet or lateral femoral condyle during dislocation); trochlear morphology (crossing sign on lateral view — the trochlear floor crosses the line of the anterior femoral condyle = dysplastic); patella alta (Caton-Deschamps); patellar tilt or subluxation on the skyline view
• MRI: the investigation of choice after acute dislocation; confirms MPFL tear (typically at the femoral attachment — Scöttle`s point; less commonly mid-substance or patellar attachment); identifies osteochondral injury (bone bruise pattern — lateral femoral condyle + medial patellar facet); characterises associated chondral damage; assesses trochlear morphology
• CT scan: for detailed assessment of trochlear dysplasia (Dejour classification), TT-TG distance measurement, and patellar tilt; planned pre-operatively in recurrent dislocation workup; not needed for the first acute dislocation in most cases

• Reduction: if the patella remains dislocated at presentation, reduce by extending the knee (relaxes the quadriceps) while gently pushing the patella medially; IV analgesia or Entonox is often required; the reduction is usually straightforward
• Non-operative management for first-time dislocation: the majority of first-time dislocators are managed non-operatively; immobilisation in a hinged knee brace or cylinder cast for 2–6 weeks (protection from re-dislocation while the MPFL heals); then supervised physiotherapy — VMO strengthening, quadriceps rehabilitation, proprioceptive training; return to sport at 3–6 months; recurrence rate approximately 15–45% following first-time dislocation managed non-operatively
• Acute surgical indications: a loose osteochondral fragment requiring removal or fixation (the most common surgical indication for a first-time dislocator); the ISAKOS guidelines support arthroscopic removal of small loose bodies and fixation of large osteochondral fragments; primary MPFL repair for first-time dislocation is generally NOT recommended (evidence shows no significant benefit over non-operative management for recurrence prevention after a single dislocation)
• Aspiration of haemarthrosis: large tense haemarthrosis can be aspirated for pain relief and to allow better examination; not mandatory but improves patient comfort and allows clearer radiological and clinical assessment

• After two or more dislocations, or first-time dislocation with significant anatomical risk factors (severe trochlear dysplasia, patella alta, TT-TG >20 mm) and high risk of recurrence, surgical stabilisation should be considered
• MPFL reconstruction: the cornerstone of surgical management for recurrent patellar dislocation; a tendon graft (gracilis or semitendinosus autograft most commonly) is used to reconstruct the MPFL from the medial patellar border to the medial femoral epicondyle at Scöttle`s point; the femoral tunnel position is the most critical technical point — incorrect tunnel placement (too proximal or too distal) causes the graft to be isometric only in a narrow range and tight or lax at other positions, leading to failure or restriction of motion; MPFL reconstruction alone is appropriate when trochlear dysplasia is absent or mild (Dejour A/B) and TT-TG is <20 mm
• Tibial tubercle osteotomy (TTO): indicated when TT-TG >20 mm or significant patella alta is present; the tibial tubercle is osteotomised and transposed — anteromedialization (AMZ — Fulkerson osteotomy) moves the tubercle medially (reduces TT-TG) and anteriorly (reduces PFJ contact pressure); a pure medialisation (Elmslie-Trillat) is used when PFJ cartilage is healthy and anterior offloading is not needed; distalisation of the tubercle reduces patella alta
• Trochleoplasty: for severe trochlear dysplasia (Dejour Type B, C, D) — the dysplastic supratrochlear bump is resected and the trochlear cartilage is deepened or reshaped; technically demanding; reserved for cases where the bony deficiency is the primary driver of instability; usually combined with MPFL reconstruction; significant complication risk including chondrolysis if the cartilage is damaged during sulcus deepening

• Scöttle`s point — femoral tunnel position in MPFL reconstruction: the femoral attachment of the MPFL is at Scöttle`s point; on a true lateral radiograph of the knee, this is located approximately 1 mm anterior to the posterior femoral cortex line and 2.5 mm distal to the posterior aspect of the medial femoral condyle (at the adductor tubercle); correct placement is confirmed fluoroscopically intraoperatively; a tunnel that is too proximal (near the adductor tubercle) tightens in flexion and restricts ROM; a tunnel too distal (near the joint line) tightens in extension; the graft must be isometric through the functional range of motion (0–90°)
• Dejour classification of trochlear dysplasia: Type A — trochlear sulcus angle >145°; crossing sign on X-ray; Type B — flat or convex trochlea; crossing sign + supratrochlear spur; Type C — lateral facet more prominent than medial; double contour sign; Type D — asymmetric trochlea with cliff pattern; Types B, C, D associated with highest dislocation risk; trochleoplasty considered for Types B–D particularly when the supratrochlear spur causes early disengagement of the patella
• Graft tensioning in MPFL reconstruction: the graft must be tensioned with the knee at 30° of flexion; tensioning with the knee in full extension risks over-tightening the graft as the MPFL normally relaxes in flexion; over-tightened MPFL reconstruction causes medial compartment overload and accelerates medial OA; a graft tensioned correctly should allow 1 cm of lateral patellar glide at 20–30° of flexion

• MPFL: primary restraint to lateral dislocation (50–60% of medial restraining force); torn in virtually 100% of dislocations; attaches patella to medial femoral epicondyle (Scöttle`s point)
• Trochlear dysplasia: most important risk factor for recurrence; Dejour Types B–D — highest risk; crossing sign on lateral X-ray = dysplastic trochlea; sulcus angle >145°
• TT-TG distance: normal <15 mm; >20 mm = abnormal; surgical threshold for TTO; measured on CT or MRI; reflects lateralisation of tibial tubercle
• Patella alta: Caton-Deschamps >1.3; Insall-Salvati >1.2; high patella = late trochlear engagement = instability risk
• First-time dislocation: non-operative (brace + physio) for most; surgery for loose osteochondral fragment; primary MPFL repair NOT routinely recommended; recurrence ~15–45%
• MPFL reconstruction: gracilis/semitendinosus graft; femoral tunnel at Scöttle`s point (1 mm anterior to posterior cortex, 2.5 mm distal to medial condyle posterior aspect); isometric placement critical; graft tension at 30° flexion
• TTO (Fulkerson AMZ): for TT-TG >20 mm; anteromedialisation reduces TT-TG + offloads PFJ cartilage; Elmslie-Trillat = pure medialisation (no anterior component)
• Trochleoplasty: for Dejour B/C/D dysplasia when bony deficiency is primary driver; sulcus deepening; combined with MPFL reconstruction; significant complication risk
• MRI findings after dislocation: MPFL tear (femoral attachment most common); bone bruise — lateral femoral condyle + medial patellar facet; osteochondral fracture
• Scöttle`s point tunnel too proximal: graft tightens in flexion → restricted ROM; too distal: graft tightens in extension → extension block