Patellofemoral Instability

Lateral dislocation is the near-universal direction in patellofemoral instability, and the condition is strongly associated with anatomical abnormalities.

Who. The incidence is 7-77 per 100,000 a year, with a peak at 15-19 years. Females outnumber males in some studies, and dislocation often happens during sport or pivoting activities.

Mechanism. A twist on a planted foot with the knee flexed, or a valgus force with external rotation. A direct blow to the medial patella is a rare cause. In a dysplastic knee, dislocation may occur with minimal trauma.

Predicting Recurrence: Instability Risk Scores

The first-dislocation figure is an average. Validated composite scores stratify the individual risk and help decide who to stabilise early, rather than relying on the dislocation count alone.

Patellar Instability Severity Score (Balcarek). A points-based score combining skeletal immaturity or young age, bilateral instability, trochlear dysplasia grade (Dejour), patellar height (Caton-Deschamps) and TT-TG distance. Higher totals predict markedly higher redislocation risk and support earlier stabilisation in first-time dislocators with high-risk anatomy.

Recurrent Instability of the Patella (RIP) score (Hevesi/Krych). It stratifies first-time dislocators using skeletal immaturity (open physes), trochlear dysplasia (sulcus depth) and tubercle lateralisation / patellar height. A high-risk profile carries a substantially elevated recurrence rate.

Both scores make the same point: it is the bundle of anatomical risk factors (dysplasia, alta, a raised TT-TG, immaturity), not simply the number of dislocations, that drives recurrence.

The MPFL. The medial patellofemoral ligament is the primary soft-tissue restraint to lateral patellar translation, providing 50-60% of the restraint in the first 30° of flexion. It matters most in that early arc, before the patella has entered the trochlea. It ruptures in almost all acute dislocations and is the target of surgical reconstruction.

Its attachments. On the femur it attaches between the adductor tubercle and the medial epicondyle, just distal and anterior to the adductor tubercle: the Schoettle point, the landmark for the femoral tunnel. On the patella it attaches to the proximal two-thirds of the medial border and blends with the VMO. It is about 55mm long.

The trochlea. The normal trochlea is a concave groove that accepts the patella during flexion and engages it by 20-30°. A dysplastic trochlea is flat or convex, and the patella then has no bony constraint.

Alignment. Patella alta, a high-riding patella, delays that engagement. The Q angle is the quadriceps vector; valgus and femoral anteversion increase it, and femoral anteversion, an internal rotation of the femur, increases the lateral patellar vector. The TT-TG distance, from the tibial tubercle to the trochlear groove on CT or MRI, measures the lateral offset of the tubercle.

Every factor counts. Trochlear dysplasia, patella alta, a raised TT-TG, femoral anteversion, genu valgum and rotational malalignment must all be assessed. Failure to address them leads to recurrence.

Trochlear dysplasia. Dejour grades it into four types from the radiograph and CT, by the shape of the trochlea and the signs that accompany it.

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The crossing sign, where the trochlear groove line crosses the anterior femoral cortex on the lateral radiograph, is the most sensitive finding and is present in every type.

Patellar height. Both indices are ratios measured on the lateral knee.

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Each measurement has a threshold to recall:

• TT-TG distance, on axial CT or MRI - normal less than 20mm; greater than 20mm is a pathological lateral offset
• Insall-Salvati ratio - greater than 1.2 is alta, greater than 1.4 severe
• Caton-Deschamps index - greater than 1.2 is alta
• Sulcus angle, the angle between the trochlear facets - normal less than 145°; greater than 145° indicates dysplasia
• Lateral trochlear inclination, the angle of the lateral facet - less than 11° indicates dysplasia

MPFL tear pattern. Tears are most common on the femoral side (50-65%), often as a bony avulsion. Patellar-side tears are less common and may be an avulsion from the proximal patella; mid-substance tears are the least common. More than one site can tear, and MRI determines the pattern for surgical planning.

History. Patients often describe the patella moving laterally, and it usually reduces as the knee extends. An immediate haemarthrosis is common. Ask about previous dislocations.

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Examination. In an acute injury the effusion is a haemarthrosis. Beyond that, examine for the factors that predispose to instability:

• Alignment - valgus, femoral anteversion, tibial torsion; an increased Q angle suggests malalignment
• Tracking - the J-sign (lateral tracking in terminal extension), lateral tilt, subluxation
• Laxity - the Beighton score for generalised hypermobility
• The other knee - the predisposition is often bilateral

The apprehension test is the most reliable clinical sign of patellofemoral instability. With the knee in 30° of flexion, apply a lateral force to the patella. The test is positive if the patient resists or shows apprehension, a fear of dislocation.

Radiographs. The standard views:

• AP weight-bearing - valgus alignment
• Lateral - trochlear dysplasia (crossing sign, supratrochlear spur) and patella alta (Insall-Salvati)
• Skyline/Merchant - patellar tilt, subluxation and trochlear morphology
• Long-leg alignment - if an osteotomy for valgus is being considered

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MRI locates the MPFL tear and its extent, shows osteochondral injury to the medial patella and lateral femoral condyle and any loose cartilage or bone fragments in the joint, and assesses trochlear morphology.

CT is the most accurate test for the TT-TG, measured on axial images from the deepest point of the trochlear groove to the tibial tubercle. The TT-PCL distance is an alternative less affected by trochlear dysplasia. CT also gives the rotational profile (femoral anteversion, tibial torsion), and 3D reconstruction of the trochlea helps surgical planning.

The decision. Recurrent instability is an indication for surgery, and the operation is built around the anatomy: MPFL reconstruction for the soft tissue, with bony procedures added for the abnormalities the measurements reveal.

First Dislocation

The acute knee. A tense effusion is aspirated. Radiographs rule out a fracture or loose body, and MRI assesses the MPFL and any osteochondral injury.

Non-operative management is appropriate for most first-time dislocators without a loose body: a brace in extension for 2-4 weeks, then progressive range of motion and quadriceps strengthening. Full recovery takes 6-12 weeks.

Early surgery. An osteochondral loose body needs fixation or removal. With a loose body, a large chondral or MPFL avulsion, high-risk anatomy or a professional athlete, early MPFL repair or reconstruction may be warranted.

Recurrent Instability

MPFL reconstruction is the standard operation. It may be sufficient alone if the anatomy is favourable, and lateral retinacular tightness is addressed if present. Before surgery, identify every anatomical risk factor: CT for the TT-TG, MRI for the soft tissue and radiographs for dysplasia.

Adding bony procedures. The measurements decide what goes with the MPFL reconstruction:

• TT-TG less than 20mm, no dysplasia - MPFL reconstruction alone
• TT-TG greater than 20mm - tibial tubercle medialisation
• Patella alta - tibial tubercle distalisation, or combined with medialisation
• High-grade dysplasia (Dejour B-D) - consider trochleoplasty, which is controversial and technically demanding
• Combined abnormalities - may need MPFL reconstruction, TTO and other procedures

The 20mm threshold is a guide. Borderline values (15-20mm) are weighed together with the TT-PCL, patellar height and dysplasia (see Controversies).

Good prognosis goes with an isolated MPFL rupture without dysplasia, a normal TT-TG distance, a first or second dislocation, compliance with rehabilitation and correct surgical technique.

Poor prognosis goes with severe trochlear dysplasia (Dejour C-D), multiple dislocations with cartilage damage, uncorrected anatomical abnormalities, generalised ligamentous laxity and tunnel malposition.

Global Epidemiology

• Incidence of first-time patellar dislocation approximately 5.8-7 per 100,000 overall, rising to 29-77 per 100,000 in adolescents aged 10-17.
• Peak incidence 15-19 years; sex distribution roughly equal, with the highest single-group risk in females aged 10-17 (Fithian cohort).
• The majority of patients carry 2 or more anatomical risk factors; trochlear dysplasia is present in around 85% of recurrent cases.

Society Guidance, Side by Side

The unifying global theme is an "a-la-carte" or menu-based philosophy (popularised by the Lyon/Dejour school and endorsed across ESSKA): identify every contributing factor and correct each one, rather than applying a single operation to all patients.