Influence of Body Mass Index (BMI) on Functional Improvements at 3 Years Following Total Knee Replacement: A Retrospective Cohort Study

Background The number of patients presenting for total knee replacement who are classified as obese is increasing. The functional benefits of performing TKR in these patients are unclear. Aim To assess the influence pre-operative body mass index has upon knee specific function, general health status and patient satisfaction at 3 years following total knee replacement. Design Retrospective comparative cohort study using prospectively collected data from an institutional arthroplasty register. Methods 1367 patients were assessed using the Western Ontario and McMaster University Osteoarthritis Index (WOMAC) and Medical Outcomes Trust Short Form-36 (SF-36) scores supplemented by a validated measure of satisfaction pre-operatively and subsequently at 1,2 and 3 year post-operatively. Comparisons were made by dividing the cohort into 4 groups based on body mass index (BMI) 18.5–25.0 kg/m2 (n = 253);>25.0–30.0 kg/m2 (n = 559);>30.0−35.0 kg/m2 (n = 373);>35.0 kg/m2 (n = 182). Results Despite lower pre-operative, 1 and 3 year WOMAC and SF-36 scores patients with the highest BMIs >35.0 kg/m2 experienced similar improvements to patients with a ‘normal‘ BMI (18.5–25.0 kg/m2) at 1 year (Difference in WOMAC improvement = 0.0 (95%CI −5.2 to 5.2), p = 1.00) and this improvement was sustained at up to 3 years (Difference in 1 year to 3 year improvement = 2.2 (95%CI: −2.1 to 6.5), p = 1.00). This effect was also observed for the SF-36 mental and physical component scores. Despite equivalent functional improvements levels of satisfaction in the >35.0 kg/m2 group were lower than for any other BMI group (>35.0 kg/m2 = 84.6% satisfied versus 18.5–5.0 kg/m2 = 93.3% satisfied,p = 0.01) as was the proportion of patients who stated they would have the operation again (>35.0 kg/m2 = 69.6% versus 18.5–25.0 kg/m2 = 82.2%,p = 0.01). Conclusion Obese and morbidly obese patients gain as much functional benefit from total knee replacement as patients with lesser body mass indexes. This benefit is maintained for up to 3 years following surgery. However, these patients are less satisfied with their knee replacement and almost a third would not have the operation again.