The Mobile-bearing TKA: Is There Still a Place?
Résumé
During the 1980s, fixed-bearing total knee arthroplasty (TKA) became a reliable, reproducible procedure with about 95% implant survival after 10 years. However, implant loosening and polyethylene wear became recognized as long-term causes of late failure. If loosening is related to abnormal strains at the bone-implant interface, polyethylene wear is mainly caused by the lack of congruency during implant motion, and thus surgeons tried to improve TKA's kinematics by introducing a second degree of freedom in their implants. Mobile-bearing knee replacements are based on a mobile polyethylene insert that articulates with a metallic femoral component and a metallic tibial tray. Those TKA designs have sought to increase congruency without increasing the stresses on the implant fixation. In vitro kinematic studies have shown reduced polyethylene wear in mobile bearings because of increased implant conformity and reduced polyethylene contact. In vivo kinematic analysis tends to confirm that mobile-bearing implants allow more natural knee movement. Despite several prospective, randomized studies having been performed, there is no clinical evidence supporting mobile bearing superiority over fixed-bearing designs. Studies focusing on gait parameters, clinical scores, or implant survival failed to confirm in vitro substantial advantage of mobile-bearing implants. More, recent improvement in polyethylene materials might solve wear issues: the last generation of cross-linked polyethylene, associated with a more morphometric design will probably provide superior implant survivorship and more physiological knee biomechanics.