Patellar instability - recurrent lateral patellar subluxation


These above features suggest that patellar instability is predominantly due to dysplastic and shallow of femoral trochlea with asymmetric appearance of the femoral trochlear facet (Type-C trochlear dysplasia). The other contributing factor to patellar instability may include relatively lateral location of the tibial tubercle in relation with the deepest point of the femoral trochlear groove. 

The insall-salvati index is just within normal range and patellar alta is unlikely. The injury pattern described above is highly indicative of recent/recurrent lateral patellar dislocation with resulting sprain/injury of the patellar retinaculum and medial patellofemoral ligament which as a result appears to be laxed - this factor may be contributing to the chronic patellar instability