Femoroacetabular impingement (FAI) is a common hip deformity, characterized by either an enlarged, aspherical femoral neck deformity (cam-type) or an over coverage of the acetabular socket (pincer-type), and is a large contributor of early osteoarthritis. As muscle adapts with the progression of symptoms, diffusion-weighted imaging can help assess how muscle microarchitecture is affected by FAI through fractional anisotropy (FA; scale = 0 tissue injury/microdamage; scale = 1 intact tissue for water diffusion)[1]. The purpose was to compare the fractional anisotropy of hip muscles between FAI patients’ symptomatic-affected hip and their contralateral-unaffected hip as well as to healthy controls.

Eight confirmed FAI patients awaiting surgery (n = 8, m:f = 4:4, age = 28 ± 9 years, BMI = 25 ± 5) and eight healthy control participants (n = 8, m:f = 2:6, age = 22 ± 2 years, BMI = 22 ± 3) were included in this study. Each participant underwent 3T MRI to image the lower spine and hips and the hip joint centres were first localized and the muscles on the transverse plane were segmented to determine the centroids. Muscles of interest included: psoas, iliacus, iliocapsularis, sartorius, pectineus, tensor fasciae latae (TFL), gluteus minimus, gluteus medius, gluteus maximus, superior gemellus, obturator internus, and rectus femoris. The diffusion-weighted imaging data were then denoised to produce the FA maps and overlayed with the segmented muscle imaging data to calculate the FA values at each muscle’s centroid. Wilcoxon signed-rank tests (FAI-affected vs. FAI-unaffected) and Mann-Whitney U tests (control vs. each FAI side) were performed to compare the FA values.

The FAI group’s symptomatic-affected side showed higher FA values for their gluteus medius (MD = 0.26; IQR = 0.22–0.30) compared to controls (MD = 0.21, IQR = 0.18–0.24; p = 0.02). Additionally, symptomatic FAI hips showed substantially higher iliacus FA (MD = 0.29; IQR = 0.26–0.33) compared to controls (MD = 0.24 IQR = 0.23–0.25, p = 0.06). There were also noticeably elevated FA values in the FAI group’s symptomatic iliocapsularis compared to the contralateral-unaffected side and control group.

The most important finding was that the FAI group’s symptomatic-affected hips had higher FA for their gluteus medius (primary abductor) and also substantially higher FA for their iliacus (a primary flexor). These microarchitectural differences between FAI patients and controls suggest that fiber alignment and orientation for these muscles change due to the inherent anatomical differences in FAI as well as changes to symptoms and function. The higher FA in the gluteus medius may be indicative of homogeneous fibre orientations due to its anatomical association with smaller femoral neck shaft angles in symptomatic FAI patients or to altered abductor mechanisms compared to the contralateral hip and to healthy controls. The iliacus FA is substantially different between symptomatic FAI and controls, which further supports the functional differences in hip flexion movement and strength in patients with FAI. This research provides insights into muscle tissue changes and allows us to relate anatomical and functional differences of FAI patients to the muscles itself.