Tibiofemoral incongruity in association with patellar instability.

Ninety-three knees with patellar instability were repaired after conservative treatment failed. In 34 there was evidence of tibiofemoral incongruity consisting of an articular depression of the medial femoral condyle by the meniscus. Secondary articular invasion by synovium occurred in 20 knees. Certain clinical phenomena indicate the presence of this impingement lesion: distinct tenderness over the joint line anterior to the medial collateral ligament, pseudo-locking, and increased pain when walking with the tibia rotated externally. These findings are in addition to the usual parapatellar tenderness, apprehension sign, and crepitus encountered in patellar instability. Excessive external tibial rotation is the probable cause of the impingement rather than genu valgum or dysplasia of the vastus medialis. Hypertension may be a factor. The lesion may play a role in the genesis of osteoarthritis subsequent to patellar instability. Transfer of the tibial tubercle medially may be expected to aggravate tibiofemoral incongruity by increasing tibial external rotation. Other procedures are preferred for patellar stabilization.