Infraspinatus muscle atrophy is sometimes observed in rotator cuff tears that appear to involve only the supraspinatus tendon. The goal of this study was to evaluate infraspinatus muscle atrophy as a function of the sagittal extent of tears, using magnetic resonance imaging. Forty-three patients (45 shoulders) were enrolled, including 23 men and 20 women with a mean age of 60 years. Patients were divided into 4 groups reflecting the sagittal extent of their tears: 16 shoulders with intact cuff (group A), 8 tears localized to the anterior half of the superior facet (group B), 10 tears extending to the posterior half of the superior facet (group C), and 11 tears extending to the middle facet (group D). The infraspinatus muscle was divided into 4 compartments using proximal muscular slices in oblique sagittal images; muscle atrophy in each compartment was graded on a scale of I to IV. Within each compartment, differences in degree of atrophy between the 4 groups were statistically tested. No shoulders in groups A and B had more than grade II atrophy in all compartments. However, grade III and IV atrophy was observed in group C in the anterosuperior compartment and in group D in all compartments. Considering the new anatomical finding that the infraspinatus inserts into the posterior portion of the superior facet as well as the middle facet, infraspinatus muscle atrophy in group C patients may have resulted from infraspinatus tendon involvement in this type of tear.
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