Introduction: Mobile bearing total knee arthroplasty (TKA) has been developed to theoretically provide a better, more physiological function of the knee and produce less polyethylene (PE) wear. The theoretical superiority of mobile bearing TKA’s over fixed bearing devices has not yet been proven in clinical studies.

The objective of the present study was to analyze in vivo the knee joint kinematics in the sagittal plane in a patient population that had received either a fixed or a mobile TKA in a prospective, randomized, patient- and observer-blinded, clinical study.

Methods: 31 patients were evaluated by means of fluoroscopy during unloaded flexion and extension against gravity, as well as during step-up and step-down with full weight bearing. In these 31 patients, 22 fixed bearing TKAs, 16 mobile-bearing TKAs and 19 natural knee joints were included.

All patients had been operated in a prospective, randomized, patient- and observer-blinded, clinical study, and had received either fixed or a mobile bearing, cruciate retaining Genesis II TKA for primary osteoarthritis.

Fluoroscopic radiographs were evaluated by measuring the „patella tendon angle” as a measure of antero-posterior translation as well as the “kinematic index” as a measure of reproducibility.

Results: During unloaded movement, fluoroscopic analysis did not show a significant difference between both types of prosthesis designs and the natural knee. In the weight-bearing movement, both types of TKA designs did not show the typically arched but a more linear patellar tendon angle curve, with a greater angle in extension and in flexion than the natural knees. This means that the femur glides anteriorly under load near extension and does not show the natural roll-back in flexion. In the mobile-bearing group, inter-individual deviations from the mean during weight-bearing movements were significantly less than in the fixed-bearing group.

Conclusions: In the present study, no functional advantage of mobile bearing TKA over fixed bearing devices could be found. Both TKA designs showed the typical kinematics of an anterior instability. Long-term follow-ups are necessary to elucidate the possible influence of lower PE wear on the incidence of aseptic loosenings.