Biomechanical effects of supraspinatus repair on the glenohumeral joint.

We repaired full-thickness rotator cuff tears in human cadaveric shoulder specimens. The purpose of this study was to determine whether a repaired supraspinatus will result in a change in joint forces, contact pressures and area, and position of the humerus relative to the glenoid compared with the pathologic and simulated complete-tear specimens. Force couples exist in the coronal plane between the deltoid and the inferior portion of the rotator cuff and in the transverse plane between the anterior cuff (subscapularis) and posterior cuff (infraspinatus and teres minor). This has served as a model for shoulder and rotator cuff research. Our model differs from previous studies in that the pectoralis major and latissimus dorsi/teres major were included because they are important contributors to shoulder function. Muscle force simulation was performed through a clamp, cable, and pneumatic system for the tendons of the rotator cuff, pectoralis major, and teres major/latissimus dorsi. Each specimen was tested in its native state of a full-thickness supraspinatus tear in 10 degrees and 60 degrees of abduction with neutral humeral rotation. In each position there were 2 loading conditions: (1) all muscles loaded with 60 N and (2) deltoid loaded with 90 N and 60 N for all others (3:2 ratio between the deltoid and supraspinatus). Pathologic tear, repaired, and simulated complete-tear conditions were subjected to the same testing sequence. After repair, there was an increase in percent inferior force in the 10 degrees abduction, 60-N loaded condition ( P = .02). The increase in percent inferior force may represent greater concavity-compression and spacer effect, which are both important functions of the supraspinatus. Contact pressure decreased in both the pathologic and simulated complete-tear conditions at 10 degrees abduction with 90-N deltoid loading ( P = .01). A corresponding increase in area was observed at this position for the pathologic tear condition only ( P = .01). This could represent greater concavity-compression, indicating that strengthening may provide a biomechanical benefit.