Abduction causes increased strain gradient compared to forward flexion: Evidence from a cadaver model of simultaneous strain study of the rotator cuff tendons.

Various strain studies of the supraspinatus have been done in isolation. Given that rotator cuff muscles function as a group, it may be physiologically representative to measure strain behaviour with the glenohumeral joint intact. Here, we explored a novel method in measuring simultaneous strain behaviour of the rotator cuff tendons and investigated the effect of full-thickness anterior tear of the supraspinatus on the infraspinatus and subscapularis tendons. Nine cadaveric shoulders were evaluated on a customized rig. Using linear differential variable transducers to measure strain, each shoulder was subjected to up to 60° of elevation in the coronal, scapular, and sagittal planes. We also assessed 30° of external rotation and up to 60° of internal rotation of the humerus. Full-thickness anterior tear of the supraspinatus was then made before re-assessing strain patterns in the scapular plane. Findings: Strain measurements of the intact tendons revealed a significant strain gradient between the articular and bursal sides of the supraspinatus during increasing degrees of elevation in the coronal and scapular planes. Full thickness anterior tear of the supraspinatus is localised to the tendon and does not affect the surrounding cuff musculature, with a potential shielding effect of the infraspinatus during early glenohumeral abduction. Significant strain gradient exists between the articular and bursal sides of the supraspinatus during abduction but not during forward flexion in an intact glenohumeral joint. Rehabilitation exercises for anterior supraspinatus tears can be appropriately targeted on the remaining intact rotator cuff musculature. • Significant strain gradient of the supraspinatus exists during abduction. • No significant strain gradient of the supraspinatus during forward flexion. • Internal and external rotation caused minimal cuff strain changes. • Full thickness anterior tear of the supraspinatus is localised to the tendon. [ABSTRACT FROM AUTHOR]