Mechanics of Glenohumeral Arthritis.

Figure 43

Strength

The requisites of strength include:

a. A functional deltoid.

b. A functional rotator cuff.

c. Normal length relationships of muscle origin and insertions.

Surgery and stuffing

In the arthritic shoulder, strength can be compromised by cuff deterioration, disuse, previous injury and previous surgery. The surgeon may be able to enhance the strength of the shoulder through muscle balancing, tendon repairs, tuberosity reattachment and effective rehabilitation. (Brems, 1994) It is critical that the procedure not impair the function of the muscle-tendon units (see figure 43).

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.

Jacobson and Mallon have provided a method for measuring the glenohumeral offset ratio (Jacobson and Mallon, 1993) while Hsu et al have reviewed the influence of abductor lever arm changes after shoulder arthroplasty. (Hsu, Wu, Chen, et al., 1993)

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