A Biomechanical Analysis of Rotator Cuff Deficiency in a Cadaveric Model

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Abstract
We conducted this cadaveric study to define a biome chanical rationale for rotator cuff function in several deficiency states. A dynamic shoulder testing appara tus 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 re sulted in a significant increase (101 %) in the middle deltoid force required to initiate abduction. This in crease 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 infraspi natus tendon was functional. Global tears resulted in an inability to elevate beyond 25° 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, infraspina tus, and teres minor tendons) remained intact. Signifi cant 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 addi tion to anatomy.

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