Body Mass Index Is Not Associated With Risk for Mechanical Loosening Following Primary Total Knee Arthroplasty: An Analysis From the American Joint Replacement Registry.

Abstract

Conflicting evidence leaves surgeons without clarity on the association of elevated body mass index (BMI) with the risk of mechanical loosening after total knee arthroplasty (TKA). This investigation examined the associated risk of mechanical loosening following TKA based on BMI. We analyzed American Joint Replacement Registry data in patients aged 65 years or more from January 2012 to March 2022 who had a minimum of 2-year follow-up linked to supplemental Centers for Medicare and Medicaid data. We identified primary TKAs and excluded those with hybrid or reverse hybrid fixation, missing component data, highly constrained implants, and augments. A secondary analysis evaluated supplemental tibial stem fixation. Patients were stratified into two groups: those who had an elevated BMI more than 35 (n = 50,557) and patients who had a BMI less than 35 (n = 148,573). Cumulative Incident Function curves and cause-specific Cox models evaluated the risk of revision for mechanical loosening, adjusting for sex, age, cruciate-retaining versus posterior-stabilized femoral design, cemented versus cementless fixation, patellar resurfacing, and Charlson Comorbidity Index. Adjusted hazard ratios (HRs) demonstrated no difference in associated risk for mechanical loosening for elevated BMI patients (HR: 0.99; 95% confidence interval [CI]: 0.97 to 1.01; P = 0.32). The Cumulative Incident Function curves showed no difference in the risk of mechanical loosening across all time points. An additional 2,956 cases were identified with supplemental tibial stem fixation; there was no difference in associated risk for mechanical loosening for elevated BMI patients with supplemental fixation (HR: 1.14; 95% CI: 0.07 to 18.21; P = 0.93). Patients who had an elevated BMI had no associated risk for mechanical loosening following TKA with a minimum 2-year follow-up. Additional tibial stem fixation was not associated with the risk of mechanical loosening for elevated BMI patients. Further investigation is essential to discern other potentially modifiable etiologies for the risk of mechanical loosening. Therapeutic Level III.