The Shoulder

Am J Sports Med 1996 May;24(3):286-292

1 A biomechanical analysis of rotator cuff deficiency in a cadaveric model.

Thompson WO, Debski RE, Boardman ND 3rd, Taskiran E, Warner JJ, Fu FH, Woo SL

Department of Orthopaedic Surgery, University of Pittsburgh, Pennsylvania, USA.

We conducted this cadaveric study to define a biomechanical rationale for rotator cuff function in several deficiency states. A dynamic shoulder testing apparatus was used to examine change in middle deltoid muscle force and humeral translation associated with simulated rotator cuff tendon paralyses and various sizes of rotator cuff tears. Supraspinatus paralysis resulted in a significant increase (101%) in the middle deltoid force required to initiate abduction. This increase diminished to only 12% for full glenohumeral abduction. The glenohumeral joint maintained ball-and-socket kinematics during glenohumeral abduction in the scapular plane with an intact rotator cuff. No significant alterations in humeral translation occurred with a simulated supraspinatus paralysis, nor with 1-, 3-, and 5-cm rotator cuff tears, provided the infraspinatus tendon was functional. Global tears resulted in an inability to elevate beyond 25 degrees of glenohumeral abduction despite a threefold increase in middle deltoid force. These results validated the importance of the supraspinatus tendon during the initiation of abduction. Glenohumeral joint motion was not affected when the "transverse force couple" (subscapularis, infraspinatus, and teres minor tendons) remained intact. Significant changes in glenohumeral joint motion occurred only if paralysis or anatomic deficiency violated this force couple. Finally, this model confirmed that rotator cuff disease treatment must address function in addition to anatomy.

Clin Orthop 1994 Jul;304:84-90

2 Shoulder instability caused by rotator cuff lesions. An in vitro study.

Loehr JF, Helmig P, Sojbjerg JO, Jung A

Department of Orthopaedics, University of Wurzburg, Germany.

Although many clinical and experimental studies have been published on shoulder instability, little is known about the relationship between degenerative changes of the cranial part of the rotator cuff and shoulder instability. The fact that some cuff lesions cause few symptoms and do not progress, while others are not only incapacitating but also progress to massive ruptures and cuff arthropathy, is still not well understood. In this study the motion patterns of the intact joint, and changes induced by a division of the supraspinatus tendon alone or of the supra- and infraspinatus tendons, were investigated in cadaver specimens. The results demonstrate that a one tendon lesion does not influence the motion pattern, whereas a two tendon lesion induces significant changes compatible with instability of the glenohumeral joint.