Hip capsular thickness as a marker for inflammatory conditions (capsulitis): an MRI-based case-control study.

Objectives: To determine whether MRI can differentiate hips with nonspecific inflammatory conditions from controls based on measurements of the capsule and to identify the most significant measurements.

Methods: In this retrospective and case-control study, we reviewed hip MRIs containing the term "capsulitis" in the reports. Cases with other known diagnoses or confounding joint pathologies were excluded. Controls were asymptomatic hips with normal laboratory results. Three musculoskeletal radiologists independently reviewed the studies, assessing for qualitative (edema, enhancement, effusion, synovitis, and subjective thickening) and quantitative (capsular thickness in standardized planes) parameters. Interobserver agreement, group comparisons, and optimal cutoffs were analyzed.

Results: The final study group comprised 38 cases, and the control group included 51 matched subjects. Inter-reader agreement was moderate to excellent. For capsular thickness, ICC ranged from 0.49 (posterior sagittal) to 0.87 (anterior sagittal). For qualitative findings, kappa ranged from 0.59 (capsular enhancement) to 0.84 (joint effusion), the greatest being capsular thickening and effusion. All planes showed significantly larger capsular thickness in cases vs. controls (p < 0.05). Sensitivity reached 85.2% for anterior sagittal measurement (mean, 6.27 vs. 4.58 mm), and specificity reached 84% for lateral coronal (6.53 vs. 4.13 mm). Among cases, capsular signal abnormality was observed in 89.2% of cases, pericapsular edema in 91.9%, capsular enhancement in 97.3%, pericapsular enhancement in 94.6%, joint effusion in 81.1%, synovitis in 89.2%, and capsular thickening in 100% of cases.

Conclusions: Our study reveals increased hip capsular thickness in nonspecific inflammatory conditions of the hip (capsulitis), both for qualitative and quantitative assessments.