Age-related and sex-related differences in rotator cuff and deltoid muscle: a 3-dimensional segmentation analysis in healthy shoulders

Muscle volume and fatty infiltration of the rotator cuff and deltoid may influence shoulder function and affect outcomes following rotator cuff repair and shoulder arthroplasty. However, age-specific and sex-specific changes and reference values for these parameters in healthy individuals remain limited. Establishing such benchmarks may help differentiate physiological muscle changes from pathological degeneration and improve surgical decision-making and outcome predictions. This study aimed to quantify age-related and sex-related differences in rotator cuff and deltoid muscle characteristics using quantitative 3-dimensional computed tomography-based muscle segmentation in healthy shoulders. Computed tomography scans of 96 healthy right shoulders (51 females, 45 males, age range: 18-79 years) were retrospectively analyzed. Subjects were stratified into 6 age groups by decade (18-29, 30-39, 40-49, 50-59, 60-69, and 70-79 years). The 4 rotator cuff muscles and the deltoid were manually segmented, and their muscle fiber volume, intramuscular fat volume, and 3-dimensional fatty infiltration (3DFI) were measured. The normalized volume of each muscle was calculated by dividing muscle or fat volume to the patient's scapular bone volume (normalized muscle fiber volume [NV] and normalized intramuscular fat volume [NV], respectively). Sex-related and age-related comparisons, correlations, and combined effects of age and sex using 2-way analysis of variance were evaluated. The mean age was 49.2 &#xb1; 15.4 years with participants evenly distributed across the 6 age decades. Males exhibited significantly greater NVin the subscapularis, infraspinatus, and deltoid, while females generally demonstrated higher NVand 3DFI. Increasing age was associated with decreased NVin the supraspinatus (&#x3c1; = -0.43), subscapularis (&#x3c1; = -0.37), infraspinatus (&#x3c1; = -0.32), teres minor (&#x3c1; = -0.28), and deltoid (&#x3c1; = -0.31). Age was also associated with increased NVand 3DFI, particularly in the supraspinatus and infraspinatus (P < .01). Two-way analysis of variance demonstrated significant independent effects of age on NVacross all muscles. The age-related decline in NVappeared more pronounced in males, whereas age-related differences in females were primarily observed in fat-related parameters. Significant age-related and sex-related differences exist in rotator cuff and deltoid muscle volume and fatty infiltration in healthy shoulders. The presented sex-specific and age-specific reference values may help contextualize and distinguish physiological from pathological muscle changes.