Chronic patellofemoral instability.

The patellofemoral joint is intrinsically unstable because the tibial tubercle lies lateral to the long axis of the femur and the quadriceps muscle, and the patella is therefore subject to a laterally directed force. This apparent ‘design fault’ is minimised by the resistance of the lateral lip of the trochlea to lateral movement of the patella during flexion (Brattström 1964). In full extension the patella lies at the upper end of the trochlea and enters its groove as flexion begins. It is important for stability that it should engage the trochlea correctly. Once engaged, the patella is held in place by two mechanisms. The first is its contact with the lateral edge of the trochlea, which acts as a lateral buffer, and the second is the soft-tissue tension which prevents it slipping laterally. In the normal knee, from 0° to 30° of flexion, the median ridge of the patella lies lateral to the centre of the trochlea; it moves medially to become centred in the trochlear groove at between 30° and 60° of flexion. As flexion proceeds the patella is more deeply engaged in the trochlea and is held firmly by soft-tissue tension. When the knee is flexed beyond 90°, it tilts so that its medial facet articulates with the medial femoral condyle.