Introduction: Previously, in vivo kinematic studies have determined that axial rotation patterns are quite variable between implant type and specific subjects. Previously, kinematic studies have determined that subjects having a mobile bearing TKA experience axial rotation, but it was unknown as to whether the bearing was rotating. Therefore, the objective of this present study was to analyze the in vivo kinematics for subjects having a mobile bearing prosthesis to determine if the polyethylene rotates relative to the femoral and/or the tibial components.
Methods: Femorotibial contact positions for ten subjects having a mobile bearing TKA, implanted by a single surgeon, were analyzed using video fluoroscopy. Each subject, while under fluoroscopic surveillance, performed a weight-bearing deep knee bend to maximum flexion. Video images were downloaded to a workstation computer and analyzed at varying degrees of knee flexion. Each polyethylene component had four metallic beads, inserted at known positions. Using a 3D model-fitting process, the femoral, tibial and polyethylene insert components were overlaid onto the fluoroscopic images. Initially, the polyethylene insert was made transparent, but the computer would overlay the four metal beads. Then, the polyethylene insert was made viewable and analyzed relative to the metal femoral and tibial components.
Results: All of the subjects experienced polyethylene bearing rotation relative to the metal tibial component and minimal rotation relative to the metal femoral component. On average, relative to the metal tibial component, the subjects experienced 4.7° (2.1 to 7.9°) of polyethylene bearing rotation. The subjects experienced a similar amount of metal femoral component rotation, relative to the metal tibial component. On average, the subjects experienced 4.0° (−0.7 to 10.0°) of rotation of the metal femoral component relative to the metal tibial component. Therefore, on average, subjects experienced only 0.7° of rotation for the metal femoral component relative to the polyethylene bearing. Also, on average, from full extension to 90° of knee flexion the subjects experienced −2.9 mm of posterior femoral rollback of their lateral condyle and –0.4 mm of their medial condyle.
Discussion: This is the firs study to determine the in vivo rotation of the polyethylene bearing for subjects having a mobile bearing TKA. The results from this study determined that the polyethylene bearing is rotating relative to the metal tibial component, but not relative to the metal femoral component. Therefore, as the metal femoral component axially rotates the polyethylene bearing is rotating a similar amount in the same direction. Since bearing rotation does occur under in vivo conditions, subjects implanted with a mobile bearing prosthesis may be subjected to lesser amounts of contact stresses, which may be beneficial to them.
