Establishing Normative Sonographic Muscle Ratios for the Rotator Cuff in a Military Beneficiary Population

Rotator cuff (RTC) muscle size is predictive of shoulder function and potential pathology following injuries. Ultrasound (US) offers a non-invasive method to differentiate normative from pathologic RTC musculature, which can guide the diagnosis and treatment of shoulder pathology for efficient, patient-centered care. This study's objective was to establish normative sonographic measurements and ratios for RTC muscles within a military population.

In this IRB-approved cross-sectional observational study (WRNMMC-2020-0288) at 3 military treatment facilities, military beneficiaries underwent a standardized US scanning protocol to measure the dimensions of the upper trapezius (UT), supraspinatus (SS), infraspinatus (IS), and teres minor (TM) muscles. Ratios were calculated by comparing to adjacent muscle measurements. Statistical analyses included measures of distribution to produce normative ranges, Z-scores to compare subgroups and evaluate specific subgroup differences, and Pearson's correlations to evaluate the strength of relationships between measures.

One hundred twenty-eight participants (52 female, 88.2% active duty, 32.9 ± 7.5 years; 231 shoulders) were enrolled, excluding shoulders with a history of significant injury or surgery. Mean and standard deviation (SD) for UT and SS thickness (mm) were 8.8 ± 2.2 and 22.2 ± 4.1, respectively. Mean and SD for SS, IS, and TM cross-sectional area (CSA; cm2) were 6.48 ± 1.82, 8.65 ± 2.79, and 4.19 ± 1.45, respectively. Ratios (mean ± SD) for UT: SS thickness, SS: IS CSA, and IS: TM CSA were as follows: 0.41 ± 0.13, 0.77 ± 0.19, and 2.17 ± 0.73. Although dominant-sided RTC muscle measurements were significantly larger in biological males than females (P < .01), corresponding ratios were not significantly different.

This study established a robust reference for normative sonographic measurements and ratios for RTC muscles in an active, healthy, military population that accounts for sex-based differences in absolute muscle size. US offers a cost-effective, efficient, and validated alternative to magnetic resonance imaging for assessing RTC muscle size. The introduction of sonographic muscle ratios as a diagnostic tool has the potential to enhance personalized shoulder health assessment, enabling timely, accurate diagnoses and patient-centered treatment plans that can optimize warfighter readiness. Future research should determine if RTC muscle ratios are more predictive of shoulder pathology in symptomatic personnel than absolute measurements.