The Effect of Body Mass Index on the Relative Revision Rates of Cemented and Cementless Unicompartmental Knee Replacements: An Analysis of Over 10,000 Knee Replacements from National Databases.

BACKGROUND: Unicompartmental knee replacements (UKRs) are being performed in an increasingly overweight population. There are concerns that cemented fixation will not last. Cementless fixation may offer a solution, but the relative performance in different body mass index (BMI) groups remains unknown. METHODS: Ten thousand, four hundred and forty cemented and cementless UKRs were propensity matched. Patients were stratified into 4 BMI groups: underweight (<18.5 kg/m2), normal weight (18.5 to <25 kg/m2), overweight (25 to <30 kg/m2), and obese (≥30 kg/m2). The effect of BMI on the relative performance of UKR fixation groups was studied. Cox regression was used to compare revision and reoperation rates. RESULTS: The revision rate per 100 component-years significantly increased (p < 0.001) with BMI for the cemented UKRs. Revision rates per 100 component-years for the normal, overweight, and obese groups were 0.92 (95% confidence interval [CI], 0.91 to 0.93), 1.15 (95% CI, 1.14 to 1.16), and 1.31 (95% CI, 1.30 to 1.33), respectively. This was not observed for the cementless UKR, with revision rates of 1.09 (95% CI, 1.08 to 1.11), 0.70 (95% CI, 0.68 to 0.71), and 0.96 (95% CI, 0.95 to 0.97), respectively. The 10-year implant survival rates for the matched cemented and cementless UKRs in the normal, overweight, and obese groups were 93.8% (95% CI, 89.1% to 96.5%) and 94.3% (95% CI, 90.9% to 96.5%) (hazard ratio [HR], 1.17; p = 0.63), 88.5% (95% CI, 84.2% to 91.7%) and 93.8% (95% CI, 90.2% to 96.2%) (HR, 0.61; p = 0.005), and 90.7% (95% CI, 88.2% to 92.6%) and 91.8% (95% CI, 88.9% to 94.0%) (HR, 0.74; p = 0.02), respectively. There were insufficient numbers in the underweight group for analysis (n = 13). Obese patients had less than half the rates of aseptic loosening (0.46% vs. 1.31%; p = 0.001) and pain (0.60% vs. 1.20%; p = 0.02) in the cementless group as compared with the cemented group. CONCLUSIONS: Higher-BMI groups had higher revision rates for the cemented UKRs but not for the cementless UKRs. Cementless fixation was associated with reduced long-term revision rates compared with cement fixation in the overweight and obese groups. In the obese group, the rates of aseptic loosening and pain were at least 50% lower in the cementless UKR group. LEVEL OF EVIDENCE: Prognostic Level III. See Instructions for Authors for a complete description of levels of evidence.