Mobile Bearing Total Knee Arthroplasty Is Associated With an Increased Risk of Mechanical Loosening: A Follow-Up Analysis of the American Joint Replacement Registry.

Abstract

Mobile-bearing (MB) total knee arthroplasty (TKA) designs have proposed biomechanical benefits, including improved patellar tracking and reduction in polyethylene wear. Yet, prior work using the American Joint Replacement Registry (AJRR) has demonstrated an increased risk of all-cause revision with MB-TKA. We therefore utilized the AJRR to further characterize reasons for the increased risk of revision with MB-TKA compared to fixed-bearing (FB) TKA designs. All primary TKAs reported to AJRR from January 2012 to March 2020 in patients 65 years and older who had a minimum 2-year follow-up were analyzed. The TKAs were categorized as MB or FB and as posterior stabilized (PS) or cruciate retaining (CR) femoral designs. Inverse probability weighted cause-specific Cox models were used to determine the risk of all-cause revision, revision for mechanical loosening, and all-cause revision, excluding mechanical loosening, accounting for gender, age, Charlson Comorbidity Index, and year of procedure. A total of 445,701 TKAs were identified; 203,224 were CR (193,662 FB, 9,662 MB), and 242,477 were PS (212,801 FB, 29,676 MB) TKAs. Compared to FB-TKA, those who had PS MB-TKA were associated with an increased risk for all-cause revision (hazard ratio [HR]: 1.2, 95% confidence interval [CI] 1.1 to 1.2, P < 0.001) and revision for mechanical loosening (HR: 2.1, 95% CI 1.8 to 2.5, P < 0.001). Compared to FB-TKA, those with CR MB-TKA were not associated with increased risk for all-cause revision (HR: 1.1, 95% CI 0.9 to 1.5, P = 0.27), but were associated with increased risk of revision for mechanical loosening (HR: 1.8, 1.3 to 2.5, P < 0.001). Excluding mechanical loosening, there was no difference in the risk of revision for PS (HR: 1.0, 95% CI 1.0 to 1.1, P = 0.30) or CR (HR: 1.0, 95% CI 0.9 to 1.2, P = 0.81). When clarifying the reasons for the increased risk of revision associated with MB-TKA, the hazard appears to be significantly increased specifically for mechanical loosening in both PS and CR MB-TKA designs. Additional studies are needed to examine what unique design features associated with MB-TKA are leading to this increased risk of mechanical loosening. Therapeutic level III.