Mobile bearing knee kinematics change over time. A fluoroscopic study in rheumatoid arthritis patients.

BACKGROUND In a previous fluoroscopy study the motion of a mobile bearing total knee prosthesis was evaluated. That study showed that the axial rotation of the insert was limited. Three possible explanations are given for the limited rotation: low conformity between the femoral component and insert, the fixed anterior position of the insert-tibia pivot point leading to impingement and fibrous tissue formation. While the effect of the conformity on the axial rotation will not change over time, the effect of impingement and fibrous tissue is likely to increase, and thereby further decreasing the axial rotation. METHODS In order to accurately assess changes in axial rotation over time in a mobile bearing total knee prosthesis rheumatoid arthritis patient group, patients were evaluated 8 months and 3 years postoperatively using fluoroscopy. FINDINGS In comparison with the 8 months evaluation, the rotation of the femoral component (range: -10.8 degrees to 2.8 degrees) and the insert (range: -5.9 degrees to 1.4 degrees) were further limited at 3 years (respectively, -5.9 degrees to 4.9 degrees and -2.8 degrees to 5.4 degrees). Patterns of axial rotation for the femoral component and insert varied considerably between the trials within patients while at the 8 months evaluation no significant difference within patients was observed. INTERPRETATION This study shows the importance of re-evaluating knee kinematics over time. The axial rotation of both the femoral component as the insert decreased over time, indicating a kinematic change caused by intrinsic factors. The decline in rotation of the insert could be explained by increased impingement and the formation of fibrous tissue.