Replacement of joints with artificial implants is one of the major breakthroughs of Orthopedic Surgery. Before the availability of joint replacements, many individuals around the globe felt crippled due to intense joint pain and limited mobility. Replacement of the hip joint was developed first and shoulder joint replacement followed soon after. Shoulder replacements have become one of the best surgeries to improve pain and quality of life for so many people!
If you gather some information about shoulder replacement for yourself, a friend or a family member, you may run into the terms anatomic and reverse joint replacement. This topic can be confusing; so what is the difference between the two?
It helps to review the basic anatomy of the shoulder joint, which is essentially formed by a ball and a socket. The ball portion of the shoulder joint is called the humeral head, and the socket is called the glenoid. If you analyze the radiograph below, you may appreciate that the ball is relatively larger than the socket because this is the reason why the shoulder has such an ample range of motion. The rotator cuff is a group of muscles and tendons essential for the good function of the shoulder joint. One of the roles of the rotator cuff is to maintain the position of the humeral head (ball), centered on the glenoid (socket). Without a rotator cuff, the ball slips all over the socket, and coordinated motion is lost.
For years, the most successful artificial parts to replace the shoulder joint mimicked our anatomy: a round artificial part replaces the upper part of the bone in the arm and the artificial part that replaces the socket is concave. Since the parts mimic anatomy, implantation of these parts is called anatomic shoulder replacement. This procedure is extremely successful, providing reliable pain relief and restoration of motion, but it does not work in the absence of a functional rotator cuff.
It took orthopedic surgeons some time to come up with a solution for patients who needed a replacement and had a poor rotator cuff, but nowadays we do have such a solution, and it is called reverse shoulder replacement. As you can see in the example below, the anatomy is reversed (image on the right): the artificial ball is placed on the socket side, whereas the upper portion of the artificial part for the humerus is concave. This results in (1) a more stable joint that does not depend on the rotator cuff to stay centered, and (2) optimal deltoid tension. This provides enough power for shoulder mobility, despite the absence of a rotator cuff.
Reverse shoulder replacement was initially introduced for a condition called cuff-tear arthropathy, where long-standing rotator cuff tears lead to progressive joint degeneration. Its success has prompted the use of reverse shoulder replacements in many other circumstances, such as really bad fractures, unfixable cuff tears (even if there is no arthritis), osteoarthritis when the quality of the socket bone is compromised, and salvage of failed replacements, amongst others.
So which one is best, anatomic or reverse? Well, it depends on the reason for the operation. Anatomic replacement restores motion in every plane and provides a more natural feel to the joint. However, it will not work in patients with a damaged rotator cuff or a really bad fracture. Reverse replacement compensates for a poor rotator cuff and reliably gives patients the ability to raise their arm, but most individuals have a hard time getting their hands behind their backs. In addition, in some individuals the increased tension of the deltoid muscle after a reverse replacement can lead to a crack in the acromion, the bone in the top of the shoulder where the deltoid takes off. When that happens, pain may become chronic, and raising of the arm becomes limited to some extent.
In the end, discussing the advantages and disadvantages of these two kinds of replacement with a shoulder surgeon is best. Patients are recommended one or the other based on their underlying condition. Both are good, but one is better than the other depending on the circumstances. If you want to learn more, you may want to watch the following video