Patellar Dislocation / Instability

Patellar dislocation is displacement of the patella out of the trochlear groove, almost always laterally, usually in adolescents and young adults after a pivoting injury.

Recurrence is common: in a meta-analysis of first-time dislocations (Huntington 2020) about 34% recurred, rising to over 70% when 3 anatomical risk factors were present.

Associated chondral or osteochondral injury is common and must be actively excluded. Long term, dislocation is linked with patellofemoral osteoarthritis, particularly after osteochondral injury.

The patella is held by static restraints (the medial patellofemoral ligament or MPFL, trochlear groove shape and patellar height) and dynamic restraints (the quadriceps, mainly vastus medialis obliquus). The MPFL is the main restraint to lateral translation in the first 0° to 30° of flexion, before the trochlea engages.

A lateral dislocation almost always tears the MPFL. In adults the tear is most often at the femoral attachment; children and adolescents frequently tear or avulse it from the patella, sometimes with a bony sleeve. As the patella relocates, the medial facet can shear against the lateral femoral condyle and shed an osteochondral fragment.

Anatomical risk factors:

• •Trochlear dysplasia, the strongest single risk factor in a meta-analysis
• •Patella alta
• •Increased tibial tubercle to trochlear groove (TT-TG) distance
• •Patellar tilt

Younger age and open physes also raise recurrence risk. Genu valgum, femoral anteversion and generalised joint hypermobility contribute.