Capsular laxity of the hip: findings at magnetic resonance arthrography.

PURPOSE: The 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.
METHODS: After 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.
RESULTS: Of 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.
CONCLUSIONS: On 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.