The Relationship Between Supraspinatus Tendon Moment Arm Length, Shoulder Anatomical Features, and Shoulder Disorders

This study aimed to evaluate the relationship between the moment arm length of the supraspinatus tendon and shoulder pathologies. The muscle-tendon moment arm represents the mechanical advantage of a muscle and significantly influences its role as a stabilizer or prime mover. In 83 patients, the presence of biceps tendinitis, rotator cuff tendinosis and tears, acromioclavicular joint degeneration, and footprint cysts were assessed. A perpendicular line was drawn from the center of the humeral head to the center of joint motion, and its length was measured (B). The distance from the rotator cuff footprint to the center of joint motion was also calculated (A). Subsequently, the ratio of these two lengths was determined (A/B). The mean moment arm length ratio (A/B) was 1.53 ± 0.11, the mean subacromial distance was 8.56 ± 1.35 mm, and the mean coracohumeral distance was 8.51 ± 1.51 mm. No significant correlation was identified between moment arm length and shoulder pathologies that could impact diagnosis, characterization, or treatment planning. A reduction in coracohumeral distance was associated with an increased incidence of tendinosis and tears in the subscapular tendon. Similarly, a decrease in subacromial distance correlated with a higher incidence of rotator cuff tears. Although moment arm length is important in surgical planning following tendon rupture, it does not serve as a diagnostic guide. The association between subacromial and coracohumeral distances and rotator cuff tendon tears may reflect a bidirectional relationship, where anatomical narrowing and tendon injury may influence each other.