CT-based analysis of muscle volume and degeneration of gluteus medius in patients with unilateral hip osteoarthritis

BACKGROUND The gluteus medius (GMED) affects hip function as an abductor. We evaluated muscle volume and degeneration of the GMED by using CT-based analysis and assessed factors that affect hip abductor strength in patients with unilateral hip osteoarthritis (OA). METHODS We examined clinical and imaging findings associated with hip abductor strength in consecutive 50 patients with unilateral hip OA. Hip abductor muscle strength and Harris hip score (HHS) were assessed. Leg length discrepancy (LLD) and femoral offset were assessed using X-ray; CT assessment was employed for volumetric and qualitative GMED analysis. Volumetric analysis involved measurement of cross sectional area (CSA) and three-dimensional (3D) muscle volume. CT density was measured for the qualitative assessment of GMED degeneration with or without adjustment using a bone mineral reference phantom. RESULTS Hip abductor muscle strength on the affected side was significantly lower than that on the contralateral healthy side and positively correlated with overall score and score for limping of gait of HHS, demonstrating the importance of hip abductor strength for normal hip function. A significant correlation was found between CSA and 3D muscle volume, unadjusted CT density and adjusted CT density, and hip abductor strength and these CT measurements. Multiple linear regression analysis demonstrated that 3D muscle volume, adjusted CT density, and LLD are independent factors affecting hip abduction. CONCLUSIONS 3D measurement of muscle volume and adjusted CT density more accurately reflect quantity and the GMED quality than do conventional assessments. Increase in muscle volume, recovery of muscle degeneration, and correction of LLD are important for improving limping in patients with hip OA.