Summary

This classification system offers a simple way to grade fatty atrophy in hip abductor muscles and proved to correlate with various hip pathologies.

Abstract

Background

No standardized classification system exists for fatty infiltration of hip abductors, though preoperative fatty infiltration correlates with postoperative outcomes in abductor tendon repairs.

Objective

We aimed to create a simple, reliable, and reproducible MRI classification system for fatty infiltration specific to hip abductors and examine the correlation between fatty infiltration grades and hip pathology.

Methods

We retrospectively identified 100 patients who underwent bony pelvic MRI with no infection, fracture, tumor, or prior history of hip surgery. Using these images, fatty infiltration of the gluteus medius (GMed) and gluteus minimus (GMin) of both hips were graded according to the new system we created. Grades consisted of a whole number (-2 to 2) corresponding to muscle thickness with respect to average, a decimal number (0-0.4) representing the ratio of fat-to-muscle cross-sectional area, and a letter (A, P, or B) indicating location of fat within the muscle. Demographic information and details of hip pathology were collected for each patient, and descriptive data was compiled. Wilcoxon rank sums were used to determine if grade values were significantly different for those who did and did not have certain hip pathologies.

Results

Interquartile ranges for muscle thickness and fatty atrophy were [-1,0] and [0.1,0.2] for GMin and [-1,0] and [0,0.1] for GMed, respectively. Fat was most often localized to the anterior portion of the muscle in both the GMed and GMin. Fat located in the posterior portion or throughout the GMin was ubiquitously associated with pathology. Higher grades for fatty atrophy in the GMin and GMed correlated with tendon tear, tendinitis, and intra-articular hip cartilage loss (GMin: p<0.0001, p=0.0017, p<0.0001; GMed: p=0.0248, p=0.0001, p=0.005). Higher fatty atrophy grades in the GMin were separately associated with labral tear (p=0.0251), while the same relationship was found between the GMed and greater trochanteric bursitis (p=0.0154). Analysis of muscle thickness displayed a positive association between width of the GMed and incidence of tendon tear, tendinitis, and cartilage loss (p=0.0055, p=0.0064, p=0.0473).

Conclusion

This classification system offers a simple way to grade fatty atrophy in hip abductor muscles and proved to correlate with various hip pathologies.