Effect of body mass index on functional outcome in primary total knee arthroplasty - a single institution analysis of 2180 primary total knee replacements

AIM: To evaluate the effect of body mass index (BMI) on short-term functional outcome and complications in primary total knee arthroplasty. METHODS: All patients undergoing primary total knee arthroplasty at a single institution between 2007 and 2013 were identified from a prospective arthroplasty database. 2180 patients were included in the study. Age, gender, BMI, pre- and post-operative functional scores [Western Ontario and McMaster University Arthritis Index (WOMAC) and SF-36], complications and revision rate were recorded. Patients were grouped according to the WHO BMI classification. The functional outcome of the normal weight cohort (BMI < 25) was compared to the overweight and obese (BMI ≥ 25) cohort. A separate sub-group analysis was performed comparing all five WHO BMI groups; Normal weight, overweight, class 1 obese, class 2 obese and class 3 obese. RESULTS: With a mean age of 67.89 (28-92), 2180 primary total knee replacements were included. 64.36% (1403) were female. The mean BMI was 31.86 (18-52). Ninty-three percent of patients were either overweight or obese. Mean follow-up 19.33 mo (6-60 mo). There was no significant difference in pre or post-operative WOMAC score in the normal weight (BMI < 25) cohort compared to patients with a BMI ≥ 25 (P > 0.05). Sub-group analysis revealed significantly worse WOMAC scores in class 2 obese 30.80 compared to overweight 25.80 (P < 0.01) and class 1 obese 25.50 (P < 0.01). Similarly, there were significantly worse SF-36 scores in class 2 obese 58.16 compared to overweight 63.93 (P < 0.01) and class 1 obese 63.65 (P < 0.01) There were 32 (1.47%) superficial infections, 9 (0.41%) deep infections and 19 (0.87%) revisions overall with no complications or revisions in the normal weight cohort (BMI < 25). CONCLUSION: Post-operative functional outcome was not influenced by BMI comparing normal weight individuals with BMI > 25. Patients should not be denied total knee arthroplasty based solely on weight alone.