Morphologic Changes of the Hip Capsule on MRI: A Novel Grading System and Its Associated Factors

Background: Capsule plays a crucial role in stabilizing the hip joint. Current research has found that the joint capsule significantly influences the prognosis after hip arthroscopy. Therefore, assessing the morphological structure of the hip joint capsule is of great importance. Purpose: To systematically characterize the morphological changes of hip capsule abnormalities on Magnetic Resonance Imaging (MRI), and to identify potential factors influencing these morphological changes. Methods: This study enrolled patients who underwent unilateral hip arthroscopy for labral injuries at our institution between January 2022 and December 2022. Demographic information—including sex, age, body mass index (BMI), and Beighton scores—was collected for all participants. Radiographic parameters, specifically the lateral center edge angle (LCEA), Tönnis angle, neck-shaft angle (NSA), and alpha angle, were also documented. The morphological features of the anterior hip capsule were assessed on oblique sagittal MRI and classified according to our novel grading system as follows: Normal (continuous, uniform low signal); Grade 1 (focal/punctate hyperintensity); Grade 2 (linear hyperintensity); Grade 3 (full-thickness edema); Grade 4 (defect/full-thickness tear). The incidence of abnormal capsular findings was analyzed, and potential factors associated with these morphological changes were further examined using multivariable logistic regression analysis. Results: The study cohort comprised 354 patients (157 males, 197 females) with a mean age of 36.6±11.2 years (range: 12-68 years). MRI evaluation of capsular morphology revealed normal findings in 117 patients (33.1%), while the distribution of pathological grades was as follows: Grade 1 (n=29, 8.2%), Grade 2 (n=79, 22.3%), Grade 3 (n=112, 31.6%), and Grade 4 (n=17, 4.8%). Statistical analysis demonstrated significant associations between capsular abnormalities and both gender and age. Female patients exhibited a higher prevalence of abnormal capsular findings (P<0.001), and advanced capsular pathology correlated with increased patient age (P=0.003). Multivariate logistic regression identified female gender as an independent risk factor for higher-grade capsular abnormalities(P=0.016). Conclusion: This study reveals that capsular abnormalities are prevalent (66.9%) in patients with labral injuries, with females demonstrating significantly higher rates than males. Moreover, being female, aging, and a smaller α Angle (<50°) are significant associated factors for higher-grade capsular changes Level of Evidence: IV.