Incidence of patella baja before and after primary total knee arthroplasty based on body mass index

In the setting of total knee arthroplasty, patella baja has been associated with decreased range of motion and increased risk of certain extensor mechanism complications. The goal of this study was to determine whether obese patients had an increased prevalence of patella baja before and/or after primary total knee arthroplasty. A multicenter retrospective review of 5089 unilateral total knee arthroplasties performed between 1998 and 2012 for osteoarthritis was conducted. Only total knee arthroplasties with cemented modular, metalbacked constructs and patellar resurfacing were included. An a priori power analysis determined that 500 patients (stratified into 5 World Health Organization body mass index groups, matched by age and sex) were needed to detect a significant Insall-Salvati ratio difference of 0.07. Patella baja was defined as an Insall-Salvati ratio of less than 0.8. Preoperative and postoperative Insall-Salvati ratios were compared between groups and analyzed using linear regression and analysis of variance. Preoperatively, there was a higher prevalence of patella baja in the higher body mass index groups (<25 kg/m ² ) compared with normal weight patients (10% vs 6%; P=.02). Postoperatively, there was no difference in the prevalence of patella baja between the 2 groups (5% vs 5%; P=.91). On comparison of postoperative Insall-Salvati ratio with preoperative Insall-Salvati ratio, the higher body mass index groups had a greater change in Insall-Salvati ratio (Δ 0.10 vs Δ 0.07; P=.01). This is the first study to report an increased prevalence of patella baja in obese patients and to show that the prevalence normalizes to that of a control group after total knee arthroplasty. Obese patients undergoing primary total knee arthroplasty had a higher rate of preoperative patella baja.