The patella index as a guide to the understanding and diagnosis of patellofemoral instability.

An analysis of 1004 X-rays has demonstrated that the medial patella facet is significantly smaller in patients with patellofemoral instability. There is no such significant difference in the width of the whole patella. Therefore the longitudinal ridge separating the smaller medial facet from the larger lateral facet must be closer to the medial patella border. As this ridge articulates with the groove between the femoral condyles, its medial position is accounted for by the patella being more laterally positioned in patients with patellofemoral instability.