Shoulder Arthroplasty
Total shoulder replacement is indicated for two main diagnostic groups:
> Rheumatoid arthritis
> Osteoarthritis (primary and secondary)
Description
Among the causes of secondary osteoarthritis are the effects of old trauma, postdisloca-tion arthropathy, cuff tear arthropathy, congenital dysplasia, the effects of old sepsis, and osteonecrosis. More rarely, surgery is indicated for tumor and failed surgery (including revision of previous prosthesis). Whatever the cause of the destructive process on the articular surface, the net result remains the same, that is increasing pain and stiffness. The overwhelming indication for surgery is uncontrolled pain 'with loss of movement and consequent loss of function. The expectations for shoulder replacement are higher than for lower limb joint replacements. If in the hip and knee, for instance, a stable pain-free joint with two-thirds range of motion is achieved, then this is seen as an excellent result. At the shoulder, however, being stable and pain free is not enough. It is the most mobile joint in the body, and hence a much greater range of mobility is expected.
The results of shoulder replacement depend almost entirely on the adequacy of soft tissues and musculature and restoration of the anatomic geometry of the joint. This cannot be so easily achieved at the shoulder as it can at either hip or knee. When assessing a shoulder before replacement, it is often thought that there is a complete loss of rotator cuff. There may indeed be loss of rotator cuff function, but this does not imply discontinuity. Often, because of gross erosion of joint surfaces, the center of rotation has been medialized and hence the rotator cuff defunction.
If the rotator cuff has lost its action, then the unopposed deltoid has its resolution of forces directed superiorly, and hence upward subluxation of the humoral head results. If this process can be reversed, that is, by lateralization of the center of rotation by retensioning both cuff and deltoid, then restoration of function can be expected. Unfortunately, in the most severely affected joints, long-term adaptive changes have occurred, that is, shortening of rotator cuff muscles; therefore, complete restoration of geometry by lateralization of the center of rotation is not possible, because the soft tissues cannot be elongated adequately to the anatomic position.
In the more severe cases, loss of the rotator cuff may indeed be true and complete. In this situation, it may be wiser to consider hemiarthroplasty alone because the incidence of glenoid loosening in the face of complete rotator cuff loss is greater. This can be surmised on a mechanical basis alone. If the rotator cuff is not present, the humeral head is subluxed superiorly. If the glenoid replacement is then placed in the anatomic position, a rocking or toggle movement increases the chance of loosening. Interestingly, the results of hemiarthroplasty in relation to total replacement are not too dissimilar. However, it has been shown that hemiarthroplasty alone may not give the complete relief of pain experienced by the total replacement.
All shoulder replacements of whatever design require good bone stock for fixation of the glenoid component. If medial erosion of the glenoid has occurred such that the erosion is to the base of the coracoid, then it is unlikely that adequate seating of the glenoid component ¦will be achieved, and it may be better in this situation to accept the hemiarthroplasty alone.
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