What are the demographic and radiographic characteristics of patients with symptomatic extraarticular femoroacetabular impingement?

BACKGROUND: Extraarticular femoroacetabular impingement (FAI) can result in symptomatic hip pain, but preoperative demographic, radiographic, and physical examination findings have not been well characterized. QUESTIONS/PURPOSES: The purposes of this study were to (1) define the demographic characteristics of patients with symptomatic extraarticular FAI; and (2) identify relevant radiographic and physical examination findings that are associated with intraoperative locations of extraarticular FAI.
METHODS: For purposes of this study, we defined extraarticular FAI as abnormal contact between the extraarticular regions of the proximal femur (greater trochanter, lesser trochanter, extracapsular femoral neck) and the ilium or ischium. The diagnosis was suspected preoperatively, but it was confirmed at the time of surgery by direct visualization of extraarticular impingement after surgical hip dislocation. A prospective single-center hip preservation registry was used to retrospectively characterize patients presenting between October 2010 and November 2013 with symptomatic hip pain and intraoperative findings of extraarticular FAI (N = 75 patients, 86 hips). Detailed demographic data were recorded. Radiographs, CT, and MRI scans were reviewed for all patients by two of the authors (BFR, ELS). Outcome instruments including modified Harris hip score (mHHS), Hip Outcome Score (HOS), and International Hip Outcome Tool (iHOT-33) were assessed preoperatively. A comparison group of all patients (N = 1690 patients, 1989 hips) undergoing surgery for intraarticular FAI over the study period were included for demographic comparisons. Cases with extraarticular FAI accounted for 4% (75 of 1765 patients) of our cohort over the study time period.
RESULTS: Patients with extraarticular FAI were more likely to be younger (mean ± SD, 24 ± 7 years versus 30 ± 11 years; difference [95% confidence interval {CI}], -7 [-9 to -4]; p < 0.001), female (85% versus 49%; odds ratio [95% CI], 6 [3 to 12]; p < 0.001), to have undergone prior hip surgery (44% versus 10%; odds ratio [95% CI], 9 (6 to 15); p < 0.001), and have lower preoperative outcome scores after adjustment for age, sex, and revision status (mHHS 55 ± 15 versus 63 ± 15; adjusted difference [95% CI], -4 (-8 to -1); p = 0.017; HOS ADL 64 ± 19 versus 73 ± 18; adjusted difference [95% CI], -7 (-11 to -3); p = 0.002) than patients undergoing surgery for intraarticular FAI. Within the extraarticular FAI group, preoperative femoral version on CT was different among patients with anterior versus posterior extraarticular impingement (median [first quartile, third quartile], 8° [2, 18] versus 21° [20, 30], respectively; p = 0.005) and anterior versus complex extraarticular impingement (median [first quartile, third quartile], 8° [2, 18] versus 20° [10, 30], respectively; p = 0.007]. Preoperative external rotation in extension was increased in patients with anterior versus complex extraarticular FAI (median [first quartile, third quartile], 70° [55, 75] versus 40° [20, 60]; p < 0.001).
CONCLUSIONS: Extraarticular FAI is an uncommon source of impingement symptoms. We suspect the diagnosis often is missed, because many of these patients had prior hip surgery before the procedure that diagnosed the extraarticular impingement source. This diagnosis seems more common in younger, female patients. Radiographic and physical examination findings correspond to locations of intraoperative extraarticular impingement. Future studies will need to determine whether surgical treatment of extraarticular impingement pathology improves pain and function in this subset of patients.