Instrumented Measurement of Patellar Mobility

To provide an objective analysis of medial and lateral patellofemoral laxity, we examined 94 uninjured athletic subjects and 22 patients with unilateral lateral patellar dislocation. We developed an instrument to measure the compliance of the medial and lateral patellar re straints. The instrument recorded the force- displacement relationship as the patella was pushed medially and laterally. Subtracting the medial displace ment from the lateral displacement at a given force level allowed the tester to assess the peripatellar soft tissue "balance." The results for both the 2.5- and the 5-pound tests were significant. Paired comparisons differenti ated the three groups, with significant differences be tween control and affected ( P = 0.0001), control and contralateral ( P = 0.0036), and affected and contralat eral ( P = 0.0157) knees. The mean result of the lateral minus medial displacement test for our sample popu lation of control subjects was -2.1 mm for the 5-pound test. A negative value in this test indicates that medial displacement exceeds lateral displacement. This find ing was present in 81 % of control subjects. In contrast, the mean result for the patients' affected knees was +3.2 mm for the 5-pound test. Using the value of 0.0 mm as the diagnostic determinant for peripatellar im balance, we found a test sensitivity of 91% and a speci ficity of 81 %.