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/m(2) (n = 253); >25.0-30.0 kg/m(2) (n = 559); >30.0235.0 kg/m(2) (n = 373); >35.0 kg/m(2) (n = 182). Results: Despite lower pre-operative, 1 and 3 year WOMAC and SF-36 scores patients with the highest BMIs >35.0 kg/m(2) experienced similar improvements to patients with a 'normal' BMI (18.5-25.0 kg/m(2)) 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/m(2) group were lower than for any other BMI group (>35.0 kg/m(2) = 84.6% satisfied versus 18.5-5.0 kg/m(2) = 93.3% satisfied, p = 0.01) as was the proportion of patients who stated they would have the operation again (>35.0 kg/m(2) = 69.6% versus 18.5-25.0 kg/m(2) = 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.