Primary Total Knee Arthroplasty in Patients with BMI of ≥50 kg/m2: A Cohort Study with Long-Term Follow-up.

BACKGROUND The obesity epidemic has given rise to an orthopaedic patient subgroup with a body mass index (BMI) of ≥50 kg/m2. Without sound evidential guidance, arthroplasty surgeons and anesthesiologists do not know whether they can push the limits of the surgical feasibility of total knee arthroplasty (TKA) without risks of harm. METHODS In a retrospective cohort study of patients who had undergone primary TKA for degenerative arthritis at our academic center (n = 10,389; 6,821 women, 4,070 men, and 38 unknown), we compared the outcomes between patients with a BMI of ≥50 kg/m2 (n = 627) and patients in other weight classes. The average patient follow-up was 8.6 years. We used Cox proportional hazards models to estimate the association between BMI and revision risk, using overweight patients (BMI = 25 to 29.99 kg/m2) as the reference group while adjusting for patient age and sex. Patient satisfaction, pain scores on a visual analogue scale (VAS), and the Oxford Knee Score (OKS) were compared among groups preoperatively and at 1, 5, and 10 years postoperatively. RESULTS In the first year after surgery, the adjusted hazard ratio (HR) for revision TKA for patients with a BMI of ≥50 kg/m2 was 3.7 (95% confidence interval [CI] = 1.9 to 7.2), with overweight patients as the reference. There was virtually no difference between patients with a BMI of 35 to 39.99 kg/m2 and those with a BMI of 40 to 49.99 kg/m2. After the first year, the HR was 1.2 (95% CI = 0.7 to 2.4) for revision TKA for patients with a BMI of ≥50 kg/m2. Those patients reported worse preoperative function of the knee, with a median OKS of 15 versus 23 for overweight patients. For obese patients, each additional unit of BMI corresponded with an additional OKS improvement of 0.07 point (95% CI = 0.04 to 0.10) at 1 year. CONCLUSIONS Our study confirms the increased risk of failure of TKA in patients with a BMI of ≥50 kg/m2 in the first year after surgery, but we found no evidence of worse outcomes in the 40 to 49.99 kg/m2 group compared with the 35 to 39.99 kg/m2 group. The increase in the revision risk in the ≥50 kg/m2 group was found only in the first postoperative year and plateaued afterwards. Despite worse function and higher failure rates, patients with a BMI of ≥50 kg/m2 reported benefits and high satisfaction with TKA. LEVEL OF EVIDENCE Therapeutic Level III. See Instructions for Authors for a complete description of levels of evidence.