A shoulder MRI often captures only part of the shoulder blade. This study finds that partial images can still reveal meaningful three-dimensional changes in the muscles that help lift and stabilize the arm. Rotator cuff tears remain a challenging clinical problem, causing pain, limited arm movement, and weakness.

Repairs are common, yet outcomes remain highly variable, particularly for larger, pulled-back, and long-standing tears. The study set out to measure the size and fat within all four rotator cuff muscles from ordinary MRI scans, even when those scans captured only a limited side portion of the shoulder blade.

The study used MRI-based measurements to examine how rotator cuff muscle size and infiltrated fat differed with age and between sexes. It also established results in a healthy control population of varying demographics for eventual application to patient or athlete populations.

The central idea is like judging the size of a whole loaf from a slice: first estimate the shoulder blade’s full length from the part that appears in the scan. That lets the captured region be described as a percentage of the whole, so scans with different coverage can be compared.

The partial muscle measurements were then compared with complete measurements to test whether the slice carried enough information. The comparison found that coverage of at least thirty percent produced a strong relationship with measurements from the complete shoulder blade.

The same pattern held for normalized muscle volume and for each muscle’s share of the total, and the relationship generally became stronger as coverage increased. Looking at only the outermost thirty percent of the shoulder blade still closely tracks the muscle measurements from the full scan, including each muscle’s size and share of the total.

That makes a much smaller scan area potentially useful without losing the overall picture. Across the muscles, there was a trend toward decreasing size with age. The age effect was significant for the supraspinatus, teres minor, and subscapularis muscles.

More specifically, the supraspinatus, teres minor, and subscapularis became smaller with age, while the supraspinatus made up less of the muscle total and the infraspinatus made up more. Fat infiltration in the infraspinatus was higher in females than in males.

The method accounts for differences in scan coverage and patient size, and the shoulder blade’s outer features can accurately predict its total length. Applied to healthy people of different ages and sexes, it found smaller muscles with age and increased fat infiltration in females.

In the future, patient scans could be compared with this healthy reference database to provide an expected range for muscle size, muscle contribution, and fat infiltration. Age changes the size and balance of several shoulder muscles, while some fat changes differ by sex.

A standardized reading of ordinary MRI scans could eventually help clinicians compare a patient with healthy expectations.