Summary

This study used cadaveric shoulders to investigate how increasing the size of a rotator cuff tear alters strain distribution in the tendons by simulating loading in the anterior and posterior regions of the supraspinatus muscle, showing that as tear size increased, strain from anterior loading concentrated in the anterior tendon stump, while strain from posterior loading was more evenly distribute

Abstract

Introduction

Rotator cuff (RC) tears are common injuries. The supraspinatus (SSP) muscle is divided into anterior and posterior regions, with the anterior region contributing most of the muscle force output (Roh, 2000). However, it remains unclear how force generated by these subregions is distributed in the RC tendon stump as tear size progresses. Therefore, we investigated tendon strain distribution resulting from various SSP muscle subregion loadings during tear size progression to understand force couple balance and guide surgical interventions.

Methods

Eight male fresh-frozen cadaveric shoulders (mean 63 years) were used after IRB approval. All soft tissues were removed except for the SSP, infraspinatus (ISP), and subscapularis (SSC) muscles and tendons, and joint capsule and anatomical subregions were defined (Ward, 2006). The humerus was mounted in a custom fixture, positioning the SSP muscle belly at 135° relative to the long axis of the humerus. The entire SSP muscle, and the anterior and posterior regions, were individually loaded using an MTS system (80N, 60N, and 20N, respectively) (Yuri, 2022). Tendinous insertions of the superior, middle, and inferior regions of the ISP and SSC were loaded with weights (10N, 40N, and 10 or 20N respectively). Tear progression models were based on footprint anatomy (Mochizuki, 2008): Tear I: anterior half of the superior facet; Tear II: superior facet; Tear III: superior and anterior 1/3 of the middle facets. 3D-digital Image Correlation measured major strains in the RC tendons across three tendon stump regions (intertubercular groove, superior, and middle facets). Freidman test followed by Bonferroni correction compared strains within these regions, as well as between intact and tear progression models under each loading condition.

Results

Strains from loading the anterior SSP showed no significant differences between three tendon stump regions in the intact and Tear I models. Strains in the intertubercular groove were significantly higher than the superior facet in Tear II and III (p < .05). Tear III strains in the intertubercular groove were significantly higher than in the intact and Tear I (p < .05). Posterior SSP region loading resulted in significantly higher strains in the middle facet compared to the intertubercular groove and superior facet in the intact and Tear I models, and higher than the superior facet in Tear II. In Tear III, intertubercular groove and middle facet strains were significantly higher than in the superior facet (p < .05). Tear III strains in the intertubercular groove were significantly higher than in the intact model (p < .001).

Discussion

In Tear III, where the tear extends beyond the superior facet, strains from anterior muscle loading concentrated in the anterior tendon stump, with high strains in the anterior tendon. This strain distribution alteration can potentially disrupt the anterior-posterior force couple balance. Although no significant differences were observed with isolated SSP tears, the presence of an ISP tear led to changes in strain magnitude and distribution, suggesting that compensatory mechanisms by other RC muscles or surgical repairs might be necessary to restore normal force couple balance.