Effects of patella alta and patella infera on patellofemoral contact forces.

The relationship between patella alta and patellar subluxation may, in part, reflect the relationship between the height of the patella above the tibiofemoral joint line and the patellofemoral contact force. This study reports on the direct in vitro measurement of the patellofemoral contact force, and its point of application on the patella, as a function of patellar height. The contact force was measured by a specially designed six-degree-of-freedom force transducer under loading simulating rising from a chair. The magnitude of the resultant contact force increased significantly with superior displacement of the patella. The magnitude of the resultant contact force changed as much as 3% per millimeter of change in patellar height. The details of this response varied among the knees tested and depended on the original anatomical patellar height. For a high-riding patella, the onset of tendofemoral contact is delayed and the magnitude of the patellofemoral contact continues to increase with increasing flexion angle. Early onset of tendofemoral contact associated with a low-riding patella results in a concomitant reduction in the magnitude of the contact force. The medially directed component of the contact force acting on the patella resists lateral subluxation of the patella. This force component increased with superior displacement of the patella. This may explain in part the tendency for a high-riding patella to sublux. For all seven specimens tested the point of application of the resultant contact force migrated superiorly with inferior displacement of the patella.