The effect of BMI and surgical approach on acetabular component malpositioning in a cohort of military patients with hip arthroplasty

Background: Previous studies have looked at body mass index (BMI) and its effects on acetabular cup malpositioning with differing results. In the present study, we sought to add to the literature by examining for a correlation between obesity and cup malposition in a military population. Methods: We reviewed the radiographs and clinical notes of 200 patients undergoing primary total hip arthroplasty (THA) from November 2011 to April 2014. Acetabular component position was determined on standing postoperative anteroposterior pelvic radiographs. We then determined correlations between cup anteversion and inclination with BMI and surgical approach. Further, we determined differences in the rate of acetabular component malpositioning outside of an "ideal" position between nonobese (BMI <30) and obese (BMI > 30) patient groups. Results: We found no correlation between cup anteversion or inclination and BMI when examined independently (r = 0.02, P = 0.79 and r = 0.08, P=0.29, respectively). However, there was a difference in the proportion of cups outside of the "ideal" position between nonobese and obese groups (53% vs. 69%; P = 0.029). Without regard for BMI, the lateral approach had lower anteversion (15.4 vs. 26.6, P< 0.001) and higher cup inclination angles (48.4 vs. 45.3, P = 0.002) compared to the posterior approach. Conclusions: Although obesity may not correlate in a linear relationship with anteversion or inclination, it is a risk factor for acetabular component malposition outside of an "ideal" zone. This study shows that increased BMI may be a risk factor for cup malpositioning in a younger, military THA population.