Patient-reported outcome metrics following total knee arthroplasty are influenced differently by patients’ body mass index

PURPOSE: This study investigated the impact of body mass index (BMI) on improvement in patient outcomes (pain, function, joint awareness, general health and satisfaction) following total knee arthroplasty (TKA). METHODS: Data were obtained for primary TKAs performed at a single centre over a 12-month period. Data were collected pre-operatively and 12-month postoperatively with the Oxford Knee Score (OKS) measuring pain and function, the EQ-5D-3L measuring general health status, the Forgotten Joint Score-12 (FJS-12) measuring joint awareness and a single question on treatment satisfaction. Change in scores following surgery was compared across the BMI categories identified by the World Health Organization (< 25.0, 25.0–29.9, 30.0–34.9, 35.0–39.9 and ≥ 40.0). Differences in postoperative improvement between the BMI groups were analysed with an overall Kruskal–Wallis test, with post hoc pairwise comparisons between BMI groups with Mann–Whitney tests. RESULTS: Of 402 patients [mean age 70.7 (SD 9.2); 55.2% women] 15.7% were normal weight (BMI < 25.0), 33.1% were overweight (BMI 25.0–29.9), 28.2% had class I obesity (BMI 30.0–34.9), 16.2% had class II obesity (BMI 35.0–39.9), and 7.0% had class III obesity (BMI ≥ 40.0). Postoperative change in OKS (n.s.) and EQ-5D-3L (n.s.) was not associated with BMI. Higher BMI group was associated with less improvement in FJS-12 scores (p = 0.010), reflecting a greater awareness of the operated joint during activity in the most obese patients. Treatment satisfaction was associated with BMI category (p = 0.029), with obese patients reporting less satisfaction. CONCLUSIONS: In TKA patients, outcome parameters are influenced differently by BMI. Our study showed a negative impact of BMI on postoperative improvement in joint awareness and satisfaction scores, but there was no influence on pain, function or general health scores. This information may be useful in terms of setting expectations expectation in obese patients planning to undergo TKA. LEVEL OF EVIDENCE: Level 1.