MRI-based rotator cuff muscle morphometry across isolated supraspinatus tendon tear severity.

OBJECTIVE: To compare rotator cuff muscle morphology and relevant shoulder biomechanical characteristics on MRI among patients with intact, partial-thickness, and full-thickness supraspinatus tendons. MATERIALS AND METHODS: This study included 245 patients aged 45-65 years without tears or substantial abnormalities of the other rotator cuff tendons. Cross-sectional areas of the rotator cuff muscles and supraspinatus fossa were measured. Supraspinatus occupancy ratio, supraspinatus-based muscle ratios, acromiohumeral distance, critical shoulder angle, Goutallier grade, and acromion type were assessed. Multivariable linear regression evaluated associations with morphometric parameters, while ordinal logistic regression evaluated factors associated with higher Goutallier grade. False-discovery-rate correction was applied to exploratory group comparisons and correlations. RESULTS: The intact, partial-thickness, and full-thickness groups included 71, 97, and 77 patients, respectively. After adjustment for age, sex, acromiohumeral distance, critical shoulder angle, and acromion type, full-thickness tears were independently associated with lower supraspinatus muscle area (β = -0.383, p < 0.001), occupancy ratio (β = -0.530, p < 0.001), and supraspinatus-to-infraspinatus, supraspinatus-to-subscapularis, supraspinatus-to-teres-minor, and supraspinatus-to-total-rotator-cuff ratios (all p < 0.001). Partial-thickness tears were not independently associated with these parameters. Partial-thickness and full-thickness tears were associated with higher Goutallier grade than intact tendons, with a stronger association for full-thickness tears. Older age, lower acromiohumeral distance, and female sex were associated with higher Goutallier grade. CONCLUSION: In patients, full-thickness supraspinatus tears were associated with lower supraspinatus morphometric measures relative to the remaining cuff muscles, whereas partial-thickness tears were not independently associated with these measures. These cross-sectional associations do not establish progression or causality.