Effect of humeral rotation on rotator cuff strain, loading and kinematics: an in vitro study.

Background: Despite its main function as abductor, the role of the supraspinatus as stabilizer and rotator cannot be neglected. A supraspinatus tear may not only influence humeral head rotation during abduction but also the strength and loading of the acting (intact) rotator cuff muscles. The purpose of this study was to investigate the effect of constrained humeral rotation and elevation on rotator cuff loading, strain and kinematics with intact and torn cuff conditions.

Methods: Active humeral elevation until 30° was simulated in twelve fresh-frozen cadaver shoulders with free humeral rotation and blocked humeral rotation. The loading protocol was applied to the intact rotator cuff, and after a 50% and 100% wide (full-thickness) crescent-shaped (n = 6) and reverse L-shaped (n = 6) tears were created in the supraspinatus tendon.

Results: Constrained humeral rotation led to an increase in supraspinatus loading force and maximum supraspinatus strain for both tear shapes. Range of motion was significantly reduced in 7 of the 12 specimens due to blocked humeral rotation. In the 100% wide reverse L-shaped tear group, constrained rotation led to an anterior translation of humeral head, in contrast to the posterior translation observed with free rotation.

Conclusions: Blocking humeral head rotation leads to an increase in supraspinatus and infraspinatus strains. According to its function as external rotator of the shoulder, the strain in the infraspinatus was higher at the beginning of abduction. However, small rotator cuff tears might not biomechanically result in increased humeral rotation, possibly because the load on the infraspinatus is compensated by the subscapularis.

Level of evidence: Basic Science Study; Biomechanics.