Effect of body mass index on postoperative mechanical alignment and long-term outcomes after total knee arthroplasty: a retrospective cohort study of 671 knees.

Background: A high body mass index (BMI) is associated with increased rates of complications after total knee arthroplasty (TKA). However, no study has examined the effect of BMI on lower limb alignment using the World Health Organization's (WHO) BMI classification. We believe that the WHO's BMI classification allows a uniform standard worldwide. We sought to investigate the potential association between a high BMI and the incidence of postoperative misalignment. We also evaluated whether a higher BMI is associated with worse clinical function.

Methods: We retrospectively reviewed the data of 671 patients who underwent primary TKA for varus osteoarthritis between January 2010 and December 2015. The patients were divided into the following 5 groups based on their BMI: normal weight (<25.0 kg/m), overweight (25.0-29.9 kg/m), class I obese (30.0-34.9 kg/m), class II obese (35-39.9 kg/m), and class III obese (>40 kg/m). Both weight and height were measured by nurses on admission. Patients' preoperative HKA, gender, age, and side of surgery were collected as baseline. All the patients underwent standing, weight-bearing, full-length radiography before and after surgery to measure the mechanical hip-knee-ankle angle (HKA). We followed up patients by telephone. Among the BMI subgroups, we compared the knee function scores, including the Western Ontario and McMaster Universities Osteoarthritis Index (WOMAC) score, Knee Society-Knee Score (KS-KS), Knee Society-Function Score (KS-FS), Forgotten Joint Score (FJS), and range of motion (ROM). A multivariate linear regression analysis and a logistic regression was conducted to examine the outcomes.

Results: The study had a mean follow-up period of 8.16 years. The multivariate and logistic regression analyses revealed that preoperative alignment (P=0.002) and a higher BMI (P=0.015) were associated with a higher risk of postoperative misalignment. The WOMAC scores were higher in the normal and overweight groups than the other groups (P=0.022). The FJS and KS-KS gradually decreased as BMI increased.

Conclusions: A higher BMI is associated with a greater risk of misalignment and worse long-term clinical outcome after TKA. When treating patients with high BMI, we should pay more attention to the adjustment of lower limb alignment intraoperatively.