There is often an assumption by patients that weight loss will occur once their knee pain is relieved by total knee arthroplasty (TKA). This study aims to evaluate (1) the change in patients' body mass index (BMI) after TKA; (2) if postoperative change in BMI influences functional outcome and survival rate of TKA; and (3) the predictive factors associated with change in BMI.
Seven thousand seven hundred thirty-three patients who underwent a primary TKA between 2001 and 2010 were included in this study. Functional outcome scores collected at 2 years after surgery include the Oxford Knee Score, Knee Society Function Score, Knee Society Knee Score, Physical Component Score, and Mental Component Score of short form 36.
Among these patients, 1067 (14%) had reduction in BMI, 5045 (65%) maintained their BMI, and 1621 (21%) had gain in BMI. The differences in improvement in Oxford Knee Score, Knee Society Function Score, Knee Society Knee Score, and Physical Component Score among the 3 groups of patients were less than the known minimal clinically important difference of these scores. The 10-year survival rate of TKA was not influenced by patients' change in BMI after surgery (P = .435). Obese class I, II, and III patients were more likely to reduce their BMI after TKA than those with normal preoperative BMI (P = .002, P = .012, and P = .004, respectively), while older patients were less likely to have gain in BMI after surgery (P = .001).
A change in BMI after TKA did not influence the functional outcome clinically or the survival rate of the TKA implant.