The relationship between patella alta and patellar subluxation may, in
part, reflect the relationship between the height of the patella abov
e the tibiofemoral joint line and the patellofemoral contact force. Th
is study reports on the direct in vitro measurement of the patellofemo
ral contact force, and its point of application on the patella, as a f
unction of patellar height. The contact force was measured by a specia
lly designed six-degree-of-freedom force transducer under loading simu
lating rising from a chair. The magnitude of the resultant contact for
ce 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 co
ntinues to increase with increasing flexion angle. Early onset of tend
ofemoral contact associated with a low-riding patella results in a con
comitant 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 wi
th 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 migrate
d superiorly with inferior displacement of the patella.