The relationship between tear severity, fatty infiltration, and muscle atrophy in the supraspinatus

Background: Fatty infiltration and muscle atrophy have been described as interrelated characteristic changes that occur within the muscles of the rotator cuff after cuff tears, and both are independently associated with poor outcomes after surgical repair. We hypothesize that fatty infiltration and muscle atrophy are two distinct processes independently associated with supraspinatus tears. Materials and methods: A retrospective review of 377 patients who underwent shoulder magnetic resonance imaging at one institution was performed. Multivariate analysis was performed based on parameters including age, sex, rotator cuff tear severity, fatty infiltration grade, and muscle atrophy. Results: A total of 116 patients (30.8%) had full-thickness tears of the supraspinatus, 153 (40.6%) had partial thickness tears, and 108 (28.7%) had no evidence of tear. With increasing tear severity, the prevalence of substantial fatty infiltration (grade >= 2) increased: 6.5% of patients with no tears vs 41.4% for complete tears (P < .001). Similarly, the prevalence of supraspinatus atrophy increased with worsening tear severity: 36.1% of no tears vs 77.6% of complete tears (P < .001). Multivariate analysis demonstrated a significant independent association between fatty infiltration and muscle atrophy when taking into account sex, age, and tear severity. Conclusions: Fatty infiltration and muscle atrophy are independently associated processes. Fatty infiltration is also related to increasing age, muscle tear severity, and sex, whereas muscle atrophy is related to increasing age but not tear severity. In patients without rotator cuff tears, fatty infiltration and atrophy prevalence increased independently with increasing age.