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A Novel MRI Classification System for Hip Abductor Muscle Atrophy: The Wisconsin Hip Abductor (WHAb) Classification
Megan N Baughman1, Donna G Blankenbaker2, Samuel J Mosiman1
1Department of Orthopedic Surgery, University of Wisconsin-Madison, Madison, Wisconsin, USA.
Background:
Fatty atrophy of hip abductors is negatively associated with surgical outcomes in abductor repairs; however, there is no standardized system for its classification.
Purpose:
To (1) create a simple, reliable magnetic resonance imaging (MRI) classification system for fatty infiltration specific to hip abductors and (2) examine the correlation between fatty infiltration grades and hip pathology.
Study Design:
Cross-sectional study; Level of evidence, 3.
Methods:
We retrospectively identified 200 hips from 100 patients who underwent bony pelvic MRI with no infection, fracture, tumor, or history of hip surgery. Fatty infiltration of the gluteus medius (GMed) and gluteus minimus (GMin) of both hips was graded on MRI. Grades consisted of a whole number (-2 to +2) corresponding to muscle thickness compared with mean values; a decimal number (0 to 0.4) representing various ratios of fat-to-muscle area; and a letter A (anterior), P (posterior), B (both), or N (none) indicating fat localization. Demographic information and details of MRI-identified hip pathology were collected for each patient. Kappa coefficients were calculated to determine intra- and interrater reliability. Descriptive data were compiled, and Wilcoxon rank sums were used to determine significant differences in grades based on pathology.
Results:
For both GMed and GMin, median muscle thickness was ≤1 SD of the reference value. In most cases, fat content fell between 0% and 50% in GMin and 0% and 25% in GMed. Fat was predominantly localized to the anterior portion of both muscles. Intra- and interrater reliability was moderate to substantial in all grading categories. Higher grades for fat content in the GMin and GMed correlated with tendon tear, tendinosis, and intra-articular cartilage loss (GMin: P < .001, P = .002, P < .001; GMed: P = .03, P = .001, P = .005, respectively). Higher fat content grades in the GMin were also associated with labral tear (P = .03), and in the GMed with greater trochanteric bursitis (P = .02). There was a positive association between width of the GMed and incidence of tendon tear, tendinosis, and intra-articular cartilage loss (P = .006, P = .006, P = .047, respectively).
Conclusion:
This newly described classification system offers a straightforward method of grading fatty atrophy in hip abductor muscles and demonstrates correlation with various hip pathologies.
