Effect of femoral component rotation and patellar design on patellar forces.

A force transducer attached to the patella was used to measure patellar contact forces after total knee arthroplasty for neutral femoral component position and for 10 degrees internal and external femoral component rotations. Knees were cycled continuously under load from 10 degrees to 110 degrees flexion. Two designs of knee replacement with differing degrees of intrinsic tibiofemoral constraint were studied. Additionally, contact forces for a dome shaped patella and a modified dome shaped patella (a patellar component with a central projection surrounded by a peripheral flat region) were compared. When using the design with relatively high intrinsic tibiofemoral constraint, there were no significant changes in patellar contact forces after axial rotation of the femoral component. When using the design with relatively low intrinsic tibiofemoral constraint and averaging over the range of flexion angles tested, the mediolateral component of the patellar contact force increased approximately 17% after 10 degrees internal rotation compared with neutral rotation, and the normal component of the patellar contact force decreased approximately 8% after a 10 degrees external rotation compared with 10 degrees internal rotation of the femoral component. The inferosuperior component of the patellar shear force, when using the modified dome shaped patellar component, was significantly lower than for the dome shaped patella.