THE EFFECTS OF OBESITY ON FUNCTIONAL OUTCOMES AFTER TOTAL KNEE ARTHROPLASTY: A PROSPECTIVE COHORT STUDY

The prevalence of obesity has been steadily increasing over the last two decades1. Total knee arthroplasty (TKA) in the setting of obesity demonstrates increased risk of medical perioperative complications, however there is no conclusive evidence on the impact obesity has on TKA outcomes2,3. The purpose of this study was to investigate the effects of obesity on functional outcomes after TKA in a prospective cohort analysis. Patients undergoing unilateral TKA due to osteoarthritic changes were recruited. Functionality was measured at pre-op, 6 and 12 months post-op using the Forgotten Joint Score-12 (FJS-12) and Western Ontario and McMaster University Osteoarthritis Index (WOMAC). Two linear-fixed effects models with contrasts were performed examining the impact of obesity (pre-op BMI, treated as a continuous variable) on return to function (FJS-12; WOMAC). Contrasts of estimated marginal means (EMMs) were used to examine functional change over time between various BMI levels (18, 21.75, 27.5, 32.5, 37.5, 42.5) representative of the World Health Organization's BMI classifications (underweight, normal, pre-obesity, obesity class I, II, and III) with pre-obesity serving as the comparison level. Both models controlled for potential effects of age, sex, and two comorbid conditions (diabetes, cardiovascular) that were associated with elevated pre-op BMI. Of the 351 enrolled patients, 296 completed one year follow-up and were included in the analysis. Approximately 61% (n=180) of participants were female, the mean (SD) age was 66.04 (7.64) years, and mean BMI (SD; range) at pre-op was 34.04 (6.93; 18.3, 53.0). Overall mean (SD) FJS-12 scores at pre-op, 6, and 12 months were 11.9 (11.8), 62.7 (27.5), 64.9 (27.9) respectively. Overall mean (SD) WOMAC scores at pre-op, 6, and 12 months were 51 (15.8), 14.1 (15.0), 12.3 (13.9) respectively. No differences were found in length of stay (LOS), readmissions, and ED visits among BMI categories. Compared to the pre-operative score, significant differences by BMI categories were observed for the 6-month FJS-12 (β = −0.66, p = .007) and 12-month WOMAC (β = −0.34, p = .021) scores. For example, at 6 months a greater change in FJS-12 score was observed for those with lower levels of BMI compared to those with higher levels of BMI. However, by 12 months, all patients appeared to return to functional levels regardless of BMI level. Despite a slower return to function, patients with elevated BMI are able to return to similar levels of function to those with a lower BMI by 12 months. We did not find any significant differences in readmission, ED visits, or LOS. This similar return to function justifies candidacy for surgery.