Depth insufficiency of the proximal trochlear groove on lateral radiographs of the knee: relation to patellar dislocation.

The depth of the trochlear groove was frequently noted to be insufficient in knees with patellar instability, particularly in the proximal portion of the trochlea. To confirm this observation, the depth of the trochlear groove of the femur was measured on lateral radiographs of 218 knees: 40 knees in 20 asymptomatic subjects, 116 knees in 69 patients undergoing radiography for various symptoms (96 without and 20 with patellar subluxation, determined on axial radiographs), and 62 knees in 34 patients who underwent surgery for recurrent dislocation or subluxation of one or both patellae (40 treated and 22 contralateral knees). In the 40 knees that had been operated on, the proximal trochlear depth (measured 1 cm below the upper limit of the trochlear groove) was 2.74 mm +/- 1.35, in contrast to 5.94 mm +/- 1.74 in the asymptomatic subjects and 5.84 mm +/- 1.53 in the patients with symptoms but no patellar instability. Recognition of depth insufficiency in the proximal portion of the trochlea should prompt a search for patellar instability. Axial views made with 30 degrees of knee flexion and lateral rotation of the leg are particularly helpful.