Direct in vitro determination of the patellofemoral contact force for normal knees.

Results of the direct in vitro measurement of the full three-dimensional representation of the patellofemoral contact force and the point of application on the patella of the resultant contact force for eleven normal knees are presented. The applied knee moment versus flexion angle pattern was similar to that experienced when rising from a chair. There was a wide variability of the details of the patellofemoral force interaction among the specimens tested. The magnitude of the resultant contact force increased approximately linearly with flexion angle for some knees while in others the force leveled off or decreased at higher flexion angles. The change in direction of the resultant contact force with respect to the patella was relatively small compared to the angular rotation of the patella. The medial-lateral component of the contact force exhibited substantial variability among knees. The direction of this force (medially or laterally directed) varied among knees and, in some knees, changed direction as a function of flexion angle. The point of application on the patella of the resultant contact force migrated superiorly from 20 to 90 deg flexion. About 90 deg flexion this point tended to migrate inferiorly. The only significant and consistent effect of varying the direction of the quadriceps extension force was a change in the medial-lateral component of the contact force. In all cases, the tendency to sublux laterally increased when the extensor force was rotated 10 deg laterally and decreased when the extensor force was rotated 10 deg medially.