Female Patients and Decreased Hip Capsular Thickness on Magnetic Resonance Imaging Associated With Increased Axial Distraction Distance on Examination Under Anesthesia: An In Vivo Study

Purpose: To investigate the relation of hip capsular thickness as measured on preoperative magnetic resonance imaging (MRI) and intraoperative hip joint axial distraction distance on an examination under anesthesia.

Methods: A retrospective review of primary arthroscopic hip procedures performed between November 2018 and June 2021 was conducted. The inclusion criteria included a diagnosis of femoroacetabular impingement syndrome and preoperative radiographic imaging and MRI. Fluoroscopic images were obtained at 0 lb and 100 lb of axial traction force. Total distraction distance was calculated by comparing the initial joint space with the total joint space at 100 lb. Hip capsular thickness was measured on MRI. Analysis was conducted using multiple linear regression, independent-samples t tests, and Mann-Whitney U tests.

Results: Eighty patients were included. Bivariable regression showed an association between an increased distraction distance and female sex (β = 4.303, R = 0.561, P < .001), as well as decreased anterior axial (β = -1.291, R = 0.365, P < .001) and superior coronal (β = -1.433, R = 0.501, P < .001) capsular thickness. Multivariable regression (R = 0.645) showed an association between an increased distraction distance and female sex (β = 3.175, P < .001), as well as decreased superior coronal capsular thickness (β = -0.764, P = .022). Independent-samples t tests showed that female patients had significantly decreased superior coronal capsular thickness (2.92 ± 1.14 mm vs 3.99 ± 1.15 mm, P < .001).

Conclusions: Female sex and decreased hip capsular thickness in the superior aspect of the coronal plane on magnetic resonance scans were found to be predictors of increased hip joint axial distraction distance on examination under anesthesia prior to hip arthroscopy, with anterior axial capsular thickness being a moderate predictor. Poor predictors of distraction distance were posterior axial and inferior coronal capsular thickness, age, body mass index, and lateral center-edge angle. Female patients were also found to have thinner hip capsules in the superior region, which may explain the association between female patients and increased distraction. These findings further characterize the relation between capsular thickness and hip laxity.

Level of evidence: Level IV, case series.