Rotator Cuff–Deficient Arthritis (Rotator Cuff Tear Arthropathy)
In some cases of arthritis there is severe damage to the rotator cuff along with arthritic changes of the articular surfaces of the joint. This can occur in severe cases of rheumatoid arthritis, in cases of crystalline arthropathy, or in patients with large chronic rotator cuff tears. These diseases result in severe erosive arthritis and superior migration of the humeral head. This often is associated with erosion and bone loss along the superior portion of the joint (see Plate 1-50). When these patients come to surgery, there is often little or no rotator cuff tissue surrounding the humeral head.
Patients with less severe rotator cuff–deficient arthritis have superior migration of the humeral head, yet the humeral head is still stable and contained within an intact coracoacromial arch. When the humeral head is contained within the coracoacromial arch and glenoid surface, along with sufficient remaining rotator cuff tissue to maintain a stable fulcrum for rotation of the humeral head within the intact coracoacromial arch, a humeral hemiarthroplasty can be performed with a sat- isfactory clinical result.
In these patients, the physical examination will generally demonstrate the ability to elevate the arm to at least 90 degrees and the humeral head remains contained under the coracoacromial arch. The clinical findings of superior escape and pseudoparalysis of the shoulder are not found (see Plate 1-48). The shoulder examination is more consistent with part B on Plate 1-40 than the examination seen on part A in Plate 1-40 or the examination seen on Plate 1-48. In patients with rotator cuff–deficient arthritis, a contained humeral head, and active elevation to 90 degrees, hemiarthroplasty provides a more simple surgery than reverse total shoulder replacement. The patients in this category of rotator cuff–deficient arthritis generally achieve good pain relief and moderate improvement in shoulder function above shoulder level. In patients who are good candidates for hemiarthroplasty it is important to preserve all parts of the rotator cuff that are intact at the time of surgery and to provide a postoperative rehabilitation program to strengthen those parts of the rotator cuff and deltoid that remain. It is also important in this group of patients to preserve the mechanical integrity of the coracoacromial arch.
When rotator cuff deficient arthritis is associated with loss of the containment mechanism of the humeral head within the coracoacromial arch, then the patient loses a fulcrum to rotate the humeral head and the result is termed pseudoparalysis of the shoulder. When these conditions occur in the older and less active patient, the best treatment is reverse shoulder arthroplasty. Physical findings, consistent with superior escape of the humeral head and pseudoparalytic shoulder, include a relatively normal contour to the anterior aspect of the shoulder with the arm in the resting position. With attempted active elevation of the shoulder, there is a superior shift of the humeral head and a prominence of the humeral head anteriorly. This is associated with lack of the ability to forward flex the arm as well as lack of active external rotation (loss of the superior and posterior parts of the rotator cuff). This lack of function is consistent with pseudoparalytic shoulder. Classic findings of rotator cuff tear arthropathy are difficulty with active elevation of the shoulder, marked degenerative changes and collapse of the humeral head, superior migration of the humeral head, and associated severe rotator cuff deficiency and subchondral cyst formation in the humeral head.
Reverse total shoulder arthroplasty means that the components are designed to be opposite to the normal anatomy. The convex surface is on the glenoid side, and the concave surface is on the humeral side. This causes a semi-constrained joint, resulting in a fixed center of rotation, thereby substituting for the function of the rotator cuff. When there is loss of function of the coracoacromial arch to contain the superiorly migrated humeral head and there is superior escape of the humeral head and resultant pseudoparalysis of the shoulder, a hemiarthroplasty will not provide any improvement of function. The reverse arthroplasty enforces a fixed fulcrum for rotation of the humeral component around the glenoid component and therefore partially substitutes for the containment function of the rotator cuff with the center of the humeral head within the glenoid. With a reverse total shoulder replacement the center of rotation is shifted from the center of the humeral head in the normal shoulder anatomy to the center of the spherical glenoid component. This results in a large medial shift in the center of rotation. A large medial shift in the center of rotation results in a more than doubling of the moment arm of the deltoid, thereby improving its mechanical advantage and ability to elevate the arm. In addition, medi- alization of the center of rotation to the interface between the glenoid bone and its surface contact with the glenoid component minimizes the moment arm for the forces around the glenoid component, resulting in a decrease in the sheer forces around this component. This results in a very low incidence of component failure owing to the mechanical stresses at this bone-prosthetic interface. The reverse total shoulder replacement does dramatically improve the patient’s ability to elevate the arm as a result of these mechanical features. The reverse total shoulder does not provide better rotational function, and the best results of reverse total shoulder arthroplasty are seen in patients with some posterior rotator cuff function before surgery (e.g., an intact teres minor tendon). Patients with some external rotation function will often achieve near full elevation of the shoulder after reverse total shoulder replacement. Those patients without any external rotation function before surgery will generally get less improvement with reverse replacement but generally will get shoulder-level elevation. Patients with no posterior cuff function may benefit from muscle transfers performed a the time of the reverse total shoulder arthroplasty.