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.

Decreased posterior tibial slope increases strain in the posterior cruciate ligament following total knee arthroplasty.

The purpose of this study was to measure the strain in the posterior cruciate ligament as a function of knee flexion angle and posterior tibial slope following total knee arthroplasty with a posterior cruciate ligament-retaining design. Posterior cruciate ligament strain was measured in seven fresh-frozen cadavers for posterior tibial slopes of 10 degrees, 8 degrees, and 5 degrees. For all three levels of tibial slope tested, strain in the posterior cruciate ligament increased with increasing flexion angle to approximately 100 degrees and then decreased with increasing flexion. The strain measured at 8 degrees posterior tibial slope was greater than that measured at 10 degrees, but the difference was not significant. The strain measured at 5 degrees posterior slope was significantly greater than strain measured at both 8 degrees and 10 degrees. This increased strain may result in altered knee kinematics and reduced range of motion at higher flexion angles.