The John Insall Award: no functional advantage of a mobile bearing posterior stabilized TKA

Abstract

Background: Mobile bearing (MB) total knee design has been advocated as a means to enhance the functional characteristics and decrease the wear rates of condylar total knee arthroplasty (TKA). However, it is unclear if these designs achieve these goals.

Questions/purposes: We asked whether function of patients or survivorship would be greater or complications would be lesser in groups of patients with MB compared with fixed bearing (FB) TKA. We also sought to describe retrieval findings.

Methods: We randomized 507 primary TKAs in 416 eligible patients to receive MB (n = 252) or FB (n = 255) devices from November 2001 to August 2007 (Investigational Device Exemption G000180, ClinicalTrials.gov registration number NCT00946075). Patients were blinded to treatment allocation. WOMAC Index, SF-12 Health Survey, knee range of motion, and Knee Society scores were collected and compared preoperatively and at 6, 12, and 24 months postoperatively. We recorded device failures and complications until October 2009. Kaplan-Meier survivorship was compared using the log rank test. Twelve retrieved MB devices underwent pathologic analysis. The minimum postoperative time was 2.2 years (mean, 5.9 years; range, 2.2-7.9 years).

Results: We found no differences in mean clinical assessment scores or mean score changes from baseline at any postoperative interval through 2 postoperative years. Nineteen of the 252 MB and 13 of the 255 FB knees had undergone revision of any component. Estimated survival at 6 postoperative years was similar for the two devices: 90.1% (95% confidence interval [CI], 84.1-93.9) for MB and 94.2% (95% CI, 90.1-96.6) for FB. Two MB and no FB tibial components were revised for loosening. There was one case of MB insert dislocation. Retrieved MB devices demonstrated no unexpected wear or mechanical device failures.

Conclusion: We found no evidence of functional advantage of the MB design. Survivorship was similar, although the study is limited by short duration of followup.

Fig. 1

Treatment allocation and patient followup to the primary analysis end point are shown by flow diagram.

Fig. 2A–C

Mean WOMAC pain (A), functional impairment (B), and stiffness (C) domain scores at pre- and postoperative evaluation intervals. Mean scores were similar (p > 0.05) at all intervals.

Fig. 3A–B

Mean SF-12 physical (A) and mental (B) component normed scores at pre- and postoperative evaluation intervals. Mean scores were similar (p > 0.05) at all intervals.

Fig. 4A–B

Mean Knee Society clinical (A) and functional (B) scores at pre- and postoperative evaluation intervals. Mean scores were similar (p > 0.05) at all intervals.

Fig. 5A–B

Mean active knee extension (A) and flexion (B) at pre- and postoperative evaluation intervals. Mean differences of flexion between groups at preoperative, 6-month, 1-year, and 2-year intervals were 2.6° (p = 0.041), 3.5° (p = 0.008), 2.6° (p = 0.027), and 1.1° (p = 0.303), respectively.

Fig. 6

Kaplan-Meier survival curves are shown for fixed and mobile bearing groups with numbers at risk shown at yearly intervals. End point for the analysis was any component no longer in situ for any reason. Estimated survival probabilities at 6 postoperative years were 94.2% (95% confidence interval [CI], 90.1–96.6) for fixed bearing devices and 90.1% (95% CI, 89.1–93.9) for mobile bearing devices with 56 and 53 at-risk subjects remaining, respectively. Survival curves were similar (p = 0.351).