Introducing the Impingement Index: A study of the combined effect of alpha angle and femoral version on outcomes after hip arthroscopy for femoral acetabular impingement

OBJECTIVES: Many studies within the recent literature have sought to identify the effect of femoral version and other morphologic characteristics on outcomes after primary hip arthroscopy for femoral acetabular impingement (FAI). In addition to femoral version and combined version, our group has recently begun to use the impingement index to stratify patients undergoing this procedure. We define the impingement index as the alpha angle minus the femoral version. The purpose of the current study is to determine the impact of femoral version, combined version and impingement index on patient reported outcomes after primary hip arthroscopy for FAI. METHODS: A retrospective chart review of a prospectively collected data was conducted from 2010-2016 to identify consecutive patients who underwent primary hip arthroscopy for treatment of FAI. Inclusion criteria are pre-operative CT scan, RESULTS: A total of 456 hips (200 males, 256 females) met the inclusion criteria. The mean age at time of surgery was 28.2 (+/- 10). Average follow-up was 2.6 years (range 23-59 months). The cohort experienced clinical improvement (p<0.001) in all patient-reported outcome measures. The mean improvement was 21.4 points for HHS, 17.4 for HOS ADL, 29.5 for HOS Sport, and 34.3 for QOL. There was no significant difference in outcomes scores when stratified by femoral version (<5°, 5-20°, >20°), including at the extremes of femoral version (<-5°, >30°). There was also no significant difference in outcomes scores when patients were stratified by Mckibbin Index (<25°, 25-40, >40°). The net change in mean HOS Sport for impingement index >75 was significantly lower than any other category (14.5 (>75°) vs. 28.1 (45°-75°) vs. 25.7 (<45°). CONCLUSIONS: Clinically significant improvements can be expected for all femoral version values when a thoughtful algorithm is employed for indicating patients for hip arthroscopy with version abnormalities. However, patients with significant femoral retroversion and large cam lesions may experience less overall improvement compared with normal or increased version. The impingement index may be a valuable tool for predicting outcomes of primary hip arthroscopy for FAI. An elevated impingement index in the setting of femoral retroversion may portend less improvement in patient reported outcomes. This finding warrants further investigation and informs the preoperative decision-making process when indicating a patient for hip arthroscopy for FAI.