Frequency of Subspine Impingement in Patients With Femoroacetabular Impingement Evaluated With a 3-Dimensional Dynamic Study

Purpose: (1) To estimate the frequency of subspine impingement (SSI) morphology in patients with a diagnosis of femoroacetabular impingement (FAI) and (2) to describe the performance of the alpha angle, range of motion, and femoral and acetabular anteversion for the identification of cases with and without SSI morphology. Methods: We performed a retrospective observational study of patients with symptomatic FAI evaluated by computed tomography between February 2015 and June 2017. SSI morphology was identified using a 3-dimensional dynamic study with Move Forward software. A case was considered positive if a contact area of the anterior inferior iliac spine with the femoral neck was evidenced. Measurements of acetabular anteversion, femoral anteversion, the lateral center-edge angle, the alpha angle, and the neck-shaft angle, as well as range-of-mobility data, were collected. Results: The study included 135 patients (194 hips), with a mean age of 39.1 ± 13.9 years; 65.2% were women. SSI morphology was found in 23.7% of hips (46 hips) (95% confidence interval, 18.3%-30.2%). Of the hips identified with SSI, 52.2% had a type I anterior inferior iliac spine, 41.3% had type II, and 6.5% had type III. In hips with SSI, median femoral anteversion was 5.6° (interquartile range, 2.1°-7.5°) and values of less than 8° would increase the suspected SSI morphology (81.8% sensitivity, 70.5% specificity). Conclusions: SSI morphology is a frequent finding in patients with symptomatic FAI through a 3-dimensional dynamic study. A decrease in femoral anteversion could be considered a useful criterion to suspect SSI morphology. Level of Evidence: Level IV, case series.