The effect of BMI on the mid-term clinical outcomes of mobile-bearing unicompartmental knee arthroplasty

Objective: To evaluate the impact of body mass index (BMI) on the mid-term clinical outcomes and survival in patients receiving a mobile-bearing unicompartmental knee arthroplasty (UKA). Methods: We retrospectively collected data from 355 patients who underwent UKA from June 2006 to June 2015, with a mean follow-up of 106.5 +/- 22.5 months. Patients were assigned into four groups based on their BMI before surgery: normal weight group (BMI 18.5 similar to 22.9 kg/m(2)), overweight group (23 similar to 24.9 kg/m(2)), obesity group (25 similar to 29.9 kg/m(2)), and severe obesity group (>= 30 kg/m(2)). The knee society score (KSS), knee society function score (KSFS), hospital for special surgery score (HSS), and range of motion (ROM) were assessed before the operation and at the last followup. The femorotibial angle (FTA) was assessed after the operation immediately and at the last follow-up. Kaplan-Meier survival analysis was performed among the four groups. Results: The KSS, KSFS, and HSS in all groups were markedly improved compared with the preoperative values (p<0.001), but the ROM score was not significantly different (p>0.05). There were significant differences in KSS (p<0.001) and HSS (p= 0.004) across the four BMI groups, and these differences were due to the severe obesity group. All groups exhibited an inclination of knee varus deformity at the last follow-up (p < 0.05). Moreover, no marked difference in the implant survival rate was found among the different groups (p= 0.248), or in the survival curves (p = 0.593). Conclusions: BMI does not influence the implant survival rate. The postoperative functional and quality-of-life scores were significantly improved in all groups. Obese (BMI >= 30 kg/m(2)) individuals should not be excluded from UKA.