Influence of posterior lateral femoral condyle geometry on patellar dislocation

Patellar instability is a condition with multifactorial aetiology, potentially involving soft tissue characteristics, the bony anatomy of the patella, femur and tibia, and alignment of the lower limb. The shape of the distal femur and patellofemoral joint has been frequently studied using plain orthogonal and skyline radiographs. We investigated a possible contribution of hypoplasia of the lateral femoral condyle in the axial plane to patellar instability. The geometry of the distal femur and alignment of the lower limb on plain radiographs and MRI scans in 25 young adult patients with patellar instability was measured, and compared to a control group of 75 age-matched patients. Measurements were validated by intra-observer and inter-observer reliability studies, and multivariate analysis was used to compare the groups. Cases with and without high Beighton score or knee hyperextension were also compared. The anatomical posterior condylar angle, anterior condylar angle and sulcus angle on axial MRI scans showed insignificant differences between groups. The Blackburne-Peel ratio, anatomical femoro-tibial angle and femoral joint angle showed significant differences between groups, but not the tibial plateau angle. There was a significant correlation between posterior condylar angle and valgus knee alignment. In cases with joint hypermobility, femoral joint angle was significantly increased and posterior condylar angle was significantly decreased. Multiplanar hypoplasia of the lateral femoral condyle resulting in a valgus knee is a risk factor for patellar instability in young patients without osteoarthritis or joint hypermobility. Isolated posterior lateral condyle hypoplasia appears to be unrelated to patellar instability.