In a laboratory study, seven fresh anatomic knee specimens were evaluated to define the three-dimensional motions of the patella before and after total knee arthroplasty (TKA) with the AMK knee. The patella was displaced medially by an average of 4 mm and tilted medially by an average of 4 degrees after standard TKA. Medial translation or internal rotation of the femoral component further displaced and tilted the patella medially, but lateral translation or external rotation of the femoral component produced less predictable changes in patellar tracking. The patterns of patellar tracking after external rotation of the femoral component came closer to reproducing those of the intact knee than any other femoral component position. The high lateral ridge on the femoral component effectively prevents patellar dislocation but may produce abnormally high stresses on the patellar implant, especially if the implant is medially displaced or internally rotated. This could lead to accelerated wear or loosening of the patellar component.