In vivo kinematic patterns were determined for subjects (patients participating in the study), having either a fixed-bearing posterior-stabilized or posterior cruciate-retaining total knee arthroplasty. While under fluoroscopic surveillance, subjects did normal gait and a deep knee bend. Video images were downloaded to a workstation computer and analyzed in three dimensions using an iterative model-fitting approach. Femorotibial contact paths for the medial and lateral condyles, axial rotation, and condylar lift-off were determined. During a deep knee bend, subjects having a posterior-stabilized total knee arthroplasty routinely experienced posterior femoral rollback of their lateral condyle and normal axial rotational patterns, whereas random subjects having a posterior cruciate-retaining total knee arthroplasty experienced paradoxical anterior sliding and opposite axial rotational patterns. During gait, posterior-stabilized and posterior cruciate-retaining total knee arthroplasties experienced similar kinematic patterns, with the presence of paradoxical sliding and opposite axial rotational patterns. Subjects having posterior-stabilized and posterior cruciate-retaining total knee arthroplasties experienced condylar lift-off. Subjects having a posterior cruciate-retaining total knee arthroplasty predominantly experienced lateral condylar lift-off whereas subjects with posterior-stabilized total knee arthroplasties experienced either medial or lateral condylar lift-off. Subjects having a posterior-stabilized total knee arthroplasty experienced significantly greater weightbearing range of motion.