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HomeMotionStabilityStrengthStuffing and strengthThe deltoidThe rotator cuffSmoothness

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Mechanics of Glenohumeral Arthroplasty.

Last updated Thursday, January 27, 2005

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Figure 25 - Humeral cut made in excessive retroversion
Figure 25 - Humeral cut made in excessive retroversion

Figure 26 - Overstuffing the joint places the cuff under tension when the arm is adducted or rotated
Figure 26 - Overstuffing the joint places the cuff under tension when the arm is adducted or rotated

Strength

The strength of the shoulder after shoulder arthroplasty is dependent on reestablishing the integrity, strength and coordination of the muscles controlling the glenohumeral and scapulothoracic articulations.

Stuffing and strength

The amount of stuffing of the joint sets the resting length of the cuff muscles and to a lesser extent that of the deltoid. If the components are too small, the cuff will be slack at rest and thus place the muscles at the low end of the ideal length-tension relationship. If the joint is overstuffed, the cuff muscles may be at the high end of their length-tension curve. The distance between the effective cuff insertion and the humeral head center establishes the moment arm for the cuff.

The deltoid

The deltoid is the most important motor of the shoulder arthroplasty. The integrity of its origin, insertion, and nerve supply must be maintained. This is most easily accomplished by gently approaching the joint through the deltopectoral interval and by identifying and protecting the axillary nerve both anterior-medially as it crosses the subscapularis and inferior capsule and laterally as it exits the quadrangular space and winds around the tuberosities on the deep surface of the deltoid. Rehabilitation of the deltoid is critical to the active motion following arthroplasty.

The rotator cuff

The rotator cuff mechanism is in jeopardy in shoulder arthroplasty for several reasons. The suprascapular nerve, which supplies the supraspinatus and infraspinatus, is at risk during surgical releases as it courses medial to the coracoid and then down the back of the glenoid 1 cm medial to the glenoid lip. The cuff tendons are at risk during surgery because the humeral cut must come close to their insertion to the tuberosities superiorly and posteriorly. A humeral cut made in excessive retroversion is likely to detach the cuff posteriorly, and a cut made too low on the humerus is likely to detach the cuff superiorly. Overstuffing the joint places the cuff under tension when the arm is adducted or rotated. Most shoulder arthroplasties are performed for older individuals in whom the quality of the cuff tissue may be compromised not only from age-related changes, but also from disuse enforced by chronic glenohumeral roughness. Shoulder arthroplasty may quickly restore motion and smoothness to the joint, placing new and substantial demands on the disused cuff tissue. Thus the rehabilitation program and the patient's activities after arthroplasty must gradually increment the loads on the cuff, allowing the tissue the opportunity to toughen over time.

If a cuff defect exists at the time of the arthroplasty, a cuff repair to bone should be carried out, if the quantity and quality of the cuff tissue are sufficient to allow a durable repair under physiologic tension with the arm at the side. If the tuberosities are nonunited or if a tuberosity osteotomy is performed, secure fixation is required to restore cuff function. Under these circumstances the rehabilitation after arthroplasty is changed dramatically to allow for secure healing of the cuff mechanism to the humerus.


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