Capsular Laxity of the Hip: Findings at Magnetic Resonance Arthrography

PurposeThe purpose of this study was to retrospectively investigate magnetic resonance (MR) arthrography imaging findings associated with capsular laxity of the hip joint found at surgery.MethodsAfter institutional review board approval, 27 patients who had arthroscopy reports that described the presence or absence of capsular laxity of the hip joint were identified over a 2-year period. Preoperative MR images were retrospectively reviewed by 2 blinded radiologists. The following observations were recorded: (1) thickness, signal intensity, and defects of the anterior joint capsule; (2) thickness and signal intensity of the zona orbicularis; (3) width of the anterior and posterior joint recesses at the level of the femoral head; (4) presence of synovitis in the anterior joint recess; and (5) volume of intra-articular contrast and degree of hip rotation. Intrarater and inter-rater agreement was assessed.ResultsOf the 27 patients, 17 were positive and 10 were negative for hip joint laxity at arthroscopy. The mean thickness of the anterior hip capsule was significantly different (P = .0043), measuring 2.5 mm (95% confidence interval [CI], 2.3 to 2.8 mm) in those with hip laxity and 3.3 mm (95% CI, 2.8 to 3.8 mm) in those without laxity. The mean width of the anterior joint recess was 5.8 mm (95% CI, 5.4 to 6.3 mm) in those with laxity and 3.6 mm (95% CI, 3.3 to 3.9 mm) in those without laxity and was significantly different (P < .0001). No other variables were considered useful because of a lack of significant differences between the 2 patient groups or low inter-rater agreement.ConclusionsOn the basis of 95% CIs, hip joint laxity at MR arthrography is associated with widening of the anterior hip joint recess (>5 mm) and thinning of the adjacent joint capsule (<3 mm) lateral to the zona orbicularis.Level of EvidenceLevel III, case-control study.