Newswise — Athough most artificial joints resemble the shape and structure of the joint they're designed to replace, one new prosthesis—the reverse shoulder—takes a different approach, reversing the position of the normal ball-and-socket design. The purpose of this anatomical flip-flop is to give you a shoulder that is stable enough to let you raise your arm, even if your rotator cuff is torn beyond repair, according to Cleveland Clinic's Arthritis Advisor.
"When we replace a shoulder joint, it's normally with a traditional, anatomically shaped design," says Joseph P. Iannotti, M.D., Ph.D., chairman of the department of orthopaedic surgery at Cleveland Clinic. "But this design only makes sense if the tears in your rotator cuff are repairable."
A working rotator cuff is the key to normal shoulder function since it's this group of four tendons encircling the shoulder that keeps the joint stable, holding the head of the humerous firmly against the curve of the scapula (glenoid cavity). Only when this head is stable can it act as an effective fulcrum, allowing the pull of your rotator cuff and deltoid muscles to raise your arm.
Loss of function"As you age, it's not uncommon to develop a large tear in your rotator cuff," says Dr. Iannotti. "We often see tears in people in their 60s and 70s that may have been there for months or even years."
At some point, when a tear develops, the rotator cuff is unable to hold the humeral head within the socket, allowing it to slip out of place, hindering shoulder function. "Such tears can lead to pseudo or functional paralysis," says Dr. Iannotti. "Though your nerves are fully functional, you can no longer raise your arm to shoulder level."
If the tear is repairable, the shoulder muscles not too atrophied, and the shoulder has severe arthritis, your surgeon will likely recommend a traditional shoulder prosthesis whose design mimics the natural position of the humeral head (ball) and scapular depression (socket). This anatomic design is an effective solution as long as your repaired rotator cuff can provide adequate stability to the new joint.
"Some 20 to 30 percent of patients with significant rotator cuff tears and arthritis still have enough residual function to raise their arms to shoulder level before surgery, and are still good candidates for an anatomic prosthesis," says Dr. Iannotti.
Reverse-shoulder prerequisites However, if your tears are irreparable, the pain from arthritis is significant and, most importantly, you are unable to lift your arm to 90 degrees, then you may be a candidate for a reverse-shoulder procedure. Though the reverse shoulder prosthesis (made by DePuy, Tornier, Encore, and Zimmer) has been used clinically in Europe for more than two decades, it only received approval from the U.S. Food and Drug Administration in November 2004. "Though I mostly recommend reverse shoulders for those over age 70, it can also make sense for younger patients who have a previous shoulder implant that has failed," says Dr. Iannotti, who has performed more than 120 of the procedures. Candidates for a reverse shoulder must meet two other criteria—good deltoid muscle function and a healthy glenoid bone. The importance of these is best explained by looking at the design of the reverse shoulder prosthesis.
Stable by designAs the name implies, the reverse shoulder flip-flops the normal position of the shoulder's ball and socket, putting a metal (titanium) stem topped with a plastic cup where the head of your humerus was and, on the other side, putting a metal plate and partial sphere (glenosphere) where the depression (glenoid cavity) on your scapula was. According to design engineers, this reversal changes the center of rotation within the joint, making the new head of the prosthesis inherently stable, regardless of the health of your rotator cuff.
With this design, good glenoid bone quality is a must because the bone must hold the screws which anchor the plate that holds the metal glenosphere. A functioning deltoid muscle is important since the new prosthesis depends upon this muscle for its arm-lifting power.
"As long as you have about 75 percent deltoid function, you'll be eligible for a reverse shoulder," says Dr. Iannotti.
Measured expectationsThe range of motion you regain with a reverse shoulder will depend, in part, on how much function remains in your rotator cuff. "At a minimum, even with a little or no cuff function, you should be able to raise your arm to shoulder level," says Dr. Iannotti. "And those who retain at least partial function, especially in the posterior rotator cuff tendons, will often obtain 120 to 140 degrees of shoulder elevation."
Of course, getting to that point means a serious commitment to rehabilitation. The rehab program with a reverse shoulder is similar to that with an anatomic shoulder design, but it often can begin a bit sooner, since the shoulder's more innately stable design is less dependent on the health and healing of surrounding tissues.
Imperfect solutionThough a reverse shoulder can be a big help to certain people, it's far from an ideal solution. "The nature of the design puts a higher than normal load on the screws that are anchored into the glenoid process and, with repeated stress, can become loose and cause premature failure," explains Dr. Iannotti, who still views the reverse shoulder as a last-resort salvage solution in very selected patients.
"The reverse shoulder remains a complex procedure, for which there is not enough data to know long-term—greater than 10-year— results," says Dr. Iannotti. "What we do know is that, for a select group, it may be the best hope for more normal shoulder function."
Are You a Candidate?If you experience or have one or more of the following, you may be eligible for a reverse shoulder procedure:"¢ Severe shoulder arthritis with ongoing pain"¢ Inability to raise extended arm to shoulder height"¢ Irreperable large or massive rotator cuff tears"¢ Healthy bone stock in scapula clavicle (glenoid cavity)"¢ Functioning deltoid muscle"¢ At least 65 years old, but younger if you've experienced failure of a previous shoulder replacement
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