Biologic Reverse Total Shoulder Replacement: Improving Results by Honoring Original Principles.
Shoulders are the third most commonly replaced joints in the United States, and are he fastest growing joint replacement, in part due to the US approval of the Reverse Total Shoulder in 2004. Depuy and Tornier share nearly half of the global shoulder repair market. This rapid rate of growth and success has attracted the attention of many other orthopedic manufacturers and the market is quickly expanding to include offerings from many others.
Although the Reverse Total Shoulder is the result of over 20 years of research and development culminating with the current design by Professor Paul Grammont in France in 1991, its rapid success has garnered a lot of attention and tweaking, which may or may not result in improved results.
The Reverse Total Shoulder grew out of the frustration and limitations observed with the more traditional options (anatomical total shoulder and partial shoulder replacement) for the treatment of patients with rotator cuff arthropathy (massive rotator cuff tear and arthritis). To gain a better understanding of this we have to remember that the shoulder, like a golf ball on a tee, has a very shallow bony socket and relies almost entirely on its surrounding tissues and muscles to maintain a stable center of rotation. A stable center of rotation is important because in order for muscles to work optimally, they must be maintained at an optimal level of stretch, so when they contract, they are able to provide the most efficient level of power and function. Conversely, when an unstable center of rotation exists the risk of adversely promoting wear, tear and arthritis of the shoulder joint greatly increases, like driving your car out of alignment significantly speeds up the wear on your tires from too much stress on just one part of the tire.
Although there are many possible ways to end up with an unstable center of rotation and arthritis, resulting in debilitating shoulder pain, the most commonly observed indication is because of a large or massive rotator cuff tear resulting in so called rotator cuff arthropathy. In these patients, because the rotator cuff can no longer help provide a stable center of rotation, surgeries like partial replacements or the standard anatomical total shoulder replacements have little hope of providing a durable solution, because they do not address the unstable center of rotation, similar to just replacing the tires on your car without correcting the alignment problem.
The Reverse Total Shoulder Replacement as conceived by Professor Grammont, addressed these shortcomings, by defining three principles for success:
1. Move the center of rotation down and medial in order to provide a better lever (moment arm) for the deltoid (large muscle on outside of shoulder) to help lift the arm.
2. Use a large ball (hemisphere) without a neck so the center of rotation remains within the bone. All previous designs with a center of rotation that was not in bone eventually failed. Keeping the center of rotation in bone prevents destructive shear forces at the glenoid (cup).
3. Use a higher angle for the shaft (humerus) component from 135 degrees to 155 degrees, so the shaft would be lower on the ball component (glenosphere), resulting in better tensioning of the deltoid and a greater available range-of-motion.
Reverse Total Shoulder replacement systems produced that honor these three original principles have, in experienced hands, resulted in remarkable results for patients that did not previously have any other solution.
As more experience has been gained with the original design, we learned that there are some new challenges and complications. One of the most talked about problems that emerged with this Reverse Total Shoulder design has been the problem of scapular notching. Because the Reverse Total Shoulder design does not have a neck, as the arm moves around the ball (glenosphere) it abuts or impinges onto the scapular neck, especially at the bottom when the arm is at the side. This results in an erosion of the bone below the ball creating a notch. As you can imagine, a small notch is not a problem, but as the notch becomes larger, and the bone erodes to the point that the stability and fixation of the glenosphere is placed at risk, failure or loosening of this component may occur. As you can imagine, a lot of intense research and work has been done to avoid scapular notching. Several research papers have elegantly demonstrated that simply altering the placement of the glenosphere to produce a small inferior overhang, can avoid the scapular notching problem. Others have chosen to take a different approach-move the center of rotation outside the bone by placing more than a hemisphere for the glenosphere component. Unfortunately, by placing the center of rotation outside bone, the stresses placed on the glenoid component are again greater as seen in historically similar designs with higher failures. Having had the benefit of learning from the experience of our colleagues in Europe, for years our preferred method is to maintain the center of rotation within bone and place the glenoid component more inferiorly to avoid the problem of scapular notching.
The second potential issue that has been recognized as a result of moving the center of rotation medial is that while it helps the deltoid work easier, it does remove some of the tension on the remaining rotator cuff possibly weakening the remaining rotational strength and increasing the risk for dislocation and instability.
Along with others, we conceived a unique solution to this problem while still honoring the three original principles of the Reverse Total Shoulder. The Biologic Reverse Total Shoulder concept involves utilizing bone from the humerus (ball) that would typically be sacrificed during the procedure and creating a custom graft to correct any bony deformity on the glenoid, as well as move the center of rotation slightly back out towards its anatomic position, while still maintaining it in bone. This technique creates a bony neck for the glenosphere that helps tension the remaining rotator cuff and provides a more anatomic contour to the shoulder. For those patients with severe rotator cuff loss, we have also been pleased with the results of combining tendon transfers with the Revere Total Shoulder Replacement to help provide more rotational ability.
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