Progression of fatty muscle degeneration in atraumatic rotator cuff tears

Background: The purpose of this prospective study was to examine the progression of fatty muscle degeneration over time in asymptomatic shoulders with degenerative rotator cuff tears. Methods: Subjects with an asymptomatic rotator cuff tear in 1 shoulder and pain due to rotator cuff disease in the contralateral shoulder were enrolled in a prospective cohort. Subjects were followed annually with shoulder ultrasonography, which evaluated tear size, location, and fatty muscle degeneration. Tears that were either full-thickness at enrollment or progressed to a full-thickness defect during follow-up were examined. A minimum follow-up of 2 years was necessary for eligibility. Results: One hundred and fifty-six shoulders with full-thickness rotator cuff tears were potentially eligible. Seventy shoulders had measurable fatty muscle degeneration of at least 1 rotator cuff muscle at some time point. Patients with fatty muscle degeneration in the shoulder were older than those without degeneration (mean, 65.8 years [95% confidence interval (CI), 64.0 to 67.6 years] compared with 61.0 years [95% CI, 59.1 to 62.9 years]; p < 0.05), and the median size of the tears at baseline was larger in shoulders with degeneration than in shoulders that did not develop degeneration (13 and 10 mm wide, respectively, and 13 and 10 mm long; p < 0.05). Tears with fatty muscle degeneration were more likely to have enlarged during follow-up than were tears that never developed muscle degeneration (79% compared with 58%; odds ratio, 2.64 [95% CI, 1.29 to 5.39]; p < 0.05). Progression of fatty muscle degeneration occurred more frequently in shoulders with tears that had enlarged (43%; 45 of 105) than in shoulders with tears that had not enlarged (20%; 10 of 51; p < 0.05). Additionally, tears with enlargement and progression of muscle degeneration were more likely to extend into the anterior supraspinatus than were those without progression (53% and 17%, respectively; p < 0.05); however, this relationship was lost when controlling for tear size (p = 0.56). The median time from tear enlargement to progression of fatty muscle degeneration was 1.0 year (range, 22.0 to 6.9 years) for the supraspinatus and 1.1 years (range, -1.8 to 8.5 years) for the infraspinatus muscle (p = 0.98). Conclusions: Progression of fatty muscle degeneration is more common in tears that are larger at baseline, enlarge over time, and undergo a larger magnitude of enlargement. Our study findings also suggest that an often rapid progression of muscle degeneration occurs in relation to a clinically relevant increase in tear size in some degenerative cuff tears. Level of Evidence: Prognostic Level II. See Instructions for Authors for a complete description of levels of evidence.