The presence of a rotator cuff tear interferes with age-dependent muscle atrophy of intact shoulder muscles. An MRI study with 3 years' follow-up

Rotator cuff muscle atrophy is frequently studied, but it is unknown whether redistribution of mechanical load in the presence of a rotator cuff tear influence muscle atrophy that is observed in patients. We hypothesized that in the presence of a supraspinatus tear, redistribution of mechanical load towards teres minor and deltoid slows down atrophy of these muscles over time. In this retrospective observational study of 129 patients, we measured the cross-sectional surface-areas on MRI of shoulder muscles in an intact rotator cuff (n = 92) and in a supraspinatus-tear group (n = 37) with a mean follow-up of 3 ± 1.8 years. Mixed models were applied to evaluate changes in surface-area of the rotator cuff and deltoid with adjustments for age, sex and follow-up time. In patients with an intact rotator cuff, the mean surface-area of the teres minor decreased 6 mm²/year (95% CI 0.7-11.1, P = 0.026) and the mean deltoid surface-area decreased 75 mm²/year (95% CI 24.5-124.8, P = 0.004). The presence of a rotator cuff tear was associated with less reduction of teres minor and deltoid surface-area in patients <50 years, with an effect of a tear of 22 mm²/year (95% CI 1.7-41.7, P = 0.034) and 250 mm²/year (95% CI 75.8-424.3, P = 0.006), respectively. Whereas the surface-area of teres minor and deltoid decrease over time in patient with an intact rotator cuff, the decline in surface-area of these muscles was substantially less in the presence of a rotator cuff tear. Our findings indicate that atrophy may be reduced if an increase in mechanical load is exerted onto the muscle.