Predisposing factors to lateral patellar dislocation include: excess femoral internal rotation, external rotation of the tibia, lateral femoral condyle hypoplasia, insufficiency of the VMO, an increased Q angle, a tight lateral retinaculum, patella alta, patella tilt, generalized ligamentous laxity, and patellofemoral dysplasia. The common theme throughout this list of predisposing features is an inequality in forces acting medially and laterally on the patella, with resultant higher rates of lateral patellar dislocation.
Dejour et al examined CT scans on 134 patients treated for patellar instability. They identified 4 common factors the unstable symptomatic knees: 1. Trochlear dysplasia (85%), 2. Quadriceps dysplasia (83%), 3. Patella alta (24%), 4. Tibial tuberosity-trochlear groove, pathological when greater than or equal to 20 mm (56%).
Halbrecht JL, Jackson DW: Acute Dislocation of the patella, in Fox, JM Delpizzo W (eds): The patellofemoral joint. 1993, p 123.
Dejour H, Walch G, Nove-Josserand L, Guier C. Factors of patellar instability: an anatomic radiographic study. Knee Surg Sports Traumatol Arthrosc. 1994;2(1):19-26.
PMID:7584171 (Link to Abstract)