Dr. Kevin Yip

Dr Kevin Yip
Orthopaedic Surgeon
MBBS(UK), FRCS(EDIN), FAM(SING), FHKCOS(ORTHO)

Featured on Channel NewsAsia

Function

The rotator cuff muscles centralize the humeral head and permit a single center of rotation while providing stability and strength. During active shoulder elevation, the rotator cuff muscles depress the humeral head, allowing efficient elevation of the extremity while the head remains reduced in the glenoid.

Studies have been performed evaluating the individual rotator cuff muscles and their respective contribution to shoulder strength. The supraspinatus and infraspinatus provide approximately 45% of abduction strength and the infraspinatus contributes nearly 90% of external rotation power. The supraspinatus and deltoid muscles provide balancing forces in the coronal plane of motion.

The importance of balanced force couples cannot be overemphasized, and the goal of surgery, when necessary, should be to properly and anatomically restore these forces
such that their counteraction is maintained. Furthermore, the importance of the transverse force couple and the need for balanced function in this plane was further emphasized by Burkhart. Loss of greater than half of the infraspinatus or loss of subscapularis function leads to superior humeral head migration, a phenomenon often detected on plain films as the acromial-humeral head distance diminishes to less than 7 millimeters .

If the coronal and transverse force couples remain functional and balanced, the end result is a properly centered humeral head and surprisingly good function even if a significant, possibly irreparable tear is present. Burkhart remains a pioneer in helping us recognize that torn rotator cuffs can be very functional and that attempting to “cover the humeral head” utilizing nonanatomic tissue re-approximation is an approach that must be avoided. Burkhart’s concept of partial repairs in an effort to restore force couples has been successfully applied in lieu of tendon transfers that may serve only to weaken the balanced force couple.

Although there has been speculation that the biceps serves as a humeral head depressor and maintains an active role in shoulder function, comparative anatomic studies as well as electromyographic data indicate that the biceps probably does not function as a humeral head depressor, and that the hypertrophy often encountered in the massive rotator cuff tear may be inflammatory in origin as opposed to a reflection of intrinsic strengthening. There is some evidence from EMG studies that the biceps may serve as an additional shoulder stabilizer in those with anterior shoulder instability.

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