Purpose: Femoroacetabular impingement (FAI) syndrome is a clinical disorder of the hip associated with pain and dysfunction, thought to be a precursor to hip osteoarthritis (OA). Several interacting anatomical parameters have been linked to the pathomechanism of FAI, including the alpha angle (αA), beta angle (βA), lateral centre edge angle (LCEA), acetabular version, femoral version, and femoral neck shaft angle (FNSA). Previous investigations of anatomical parameters and hip symptoms in FAI cohorts have primarily used plain radiographs, which permit only single-planar representation of the complex morphology of the femoral head and acetabulum, limiting analysis to the sub-region profiled. MRI provides multiplanar morphologic assessment without the use of ionizing radiation, with potential for post-processing of volumetric sequences to provide comprehensive, multi-planar consideration of αA, βA, femoral and acetabular version. We sought to study the relationship between symptom severity, morphological parameters and patient-level factors in FAI.

Methods: Eighty-nine participants were diagnosed with FAI syndrome in a clinical setting by an orthopaedic surgeon. Age, gender, body mass index, nature of onset (gradual vs sudden) and duration of symptoms were recorded. Participants completed the international Hip Outcome Tool-33 (iHOT-33), a measure of hip health-related quality of life (QOL) (0 = worst possible QOL, 100 = best possible QOL), and the 10-point modified UCLA activity score (1 = “regular participation in impact sports”, 10 = “wholly inactive and dependent on others”). Participants received standardized plain radiographs and hip MRI scans on one of two 3T scanners (Siemens Prisma & Skyra). On a subset of 60 participant scans (41 cam-type, 3 pincer-type, 16 mixed-type FAI), αA was measured in four reconstructed radial planes at 30-degree intervals from superior to anterior (Fig. 1). The βA, defined as the angle formed by the αA vector and a line joining the centre of the femoral head to the acetabular rim, was also measured in these four planes. Femoral version was measured using axial hip and knee MRI sequences. Acetabular version was measured at 1cm inferior to the acetabular sourcil, using axial hip MRI sequences with the bi-ischial line as a reference point. LCEA and FNSA were measured on an AP pelvis X-ray. Statistical analyses included Pearson’s linear regressions and students' t-tests. Results: The 89 participants (32.9 ± 10.7 yrs, 40% female) had a mean iHOT-33 score of 41.0 ± 18.3. Modified UCLA activity scores and iHOT-33 scores were significantly associated (r = −0.351, P = 0.001), suggesting that more activity lends itself to better hip health-related QOL. Higher participant activity levels were also associated with a shorter duration of symptoms (r = 0.361, P = 0.001). No other patient factors were related to iHOT-33 score. The only anatomical parameter significantly associated with iHOT-33 score was the αA in the anterosuperior radial plane (r = −0.297, P = 0.025) (Table 1). Those with larger αAs (>75 degrees, n = 13) had significantly lower iHOT-33 scores than those with smaller αAs (<75 degrees, n = 44) (P = 0.017). There was no significant difference in iHOT-33 score between those with larger αAs (>75 degrees) and smaller αAs (<75 degrees) in any other radial plane, including for the maximum αA measured for each hip. Conclusions: Alpha angle in the anterosuperior radial plane was associated with hip health-related QOL, whereas other anatomical parameters, including αA measured in other radial planes, were not. Clinically, this finding supports the need for αA measurement at the anterosuperior radial plane in FAI diagnosis, and potentially highlights the importance of bony surgical correction at this region. Previous research has identified the anterosuperior region as particularly susceptible to chondrolabral damage in FAI; our study's findings link this structural damage to the presence of symptoms in FAI.