Computer-Simulated Model Guides Reverse Shoulder ReplacementShoulder replacement or arthroplasty is not unusual anymore. Regular replacements of the ball and socket joint are often done for patients with painful and limited range of motion. But the standard shoulder replacement isn't the best choice for some people. Some patients need a reverse shoulder arthroplasty (RSA). This is mostly used for people who have injuries of the rotator cuff and shoulder arthritis that leave the shoulder unstable.
Reverse replacements put the ball of the joint just off the shoulder blade with the socket off the upper arm. This placement is different from the usual ball at the top of the humerus (upper arm bone) and the socket on the shoulder blade.
The reverse shoulder joint isn't a perfect solution. It comes with problems of its own. For example, some patients end up with pain, loss of motion, and a problem called impingement. Impingement results in an inability to put the arm all the way down at the side. The implant design, location, and angle result in the two parts of the implant bumping up against each other, preventing full motion.
Specifically, the medial (inside) edge of the socket (now located at the top of the humerus) bumps up against the lateral outside edge of the scapula (shoulder blade) where the new round ball (glenosphere) is located. This creates a problem called scapular notching. Depending on the location of the glenosphere, motion can range from zero degrees (no impingement) up to 38 degrees of impingement (in other words, the arm is 38 degrees away from the body).
When reverse shoulder replacements were first introduced, the glenosphere was placed in the middle of the glenoid (anatomic shoulder socket). But problems developed and surgeons recognized the benefit of changing their surgical technique to avoid those complications. Despite changes made, scapular notching was still a problem.
In this study, investigators at the Florida Orthopaedic Institute Research Foundation study ways to place the implant to minimize the chances of developing abduction impingement. They did this using a computer-simulated program. The program allowed them to try various angles and positions of the glenosphere and then check to see if impingement occurred and by how much.
The authors divide the factors affecting impingement into two groups. The first group was surgical factors. These included location and tilt of the glenosphere (where and how it is implanted on the scapula). The second group was implant-factors such as size, center-of-rotation, and humeral neck-to-shaft angle. Placing the glenosphere further out of the bone changed the center-of-rotation. This is called offsetting.
The authors were able to run hundreds of simulated combinations of these factors. They used three different center-of-rotation offsets, three glenosphere sizes, and three different humeral neck-shaft angles. They ran all of the tests with two different glenosphere positions: superior (implanted up higher in the scapula) and inferior (placed down lower in the scapula). They were able to measure how much impingement changed with small changes in conditions.
In this way, they could figure out which factor or variable had the most effect on impingement and by how much (measured by degrees of motion). They found that an inferior placement of the glenosphere with a lateral offset gave the best result with minimal (or no) impingement. The greatest amount of impingement (38 degrees) occurred with a superior position of the glenosphere and no offset.
By playing with the amount of offset (zero, five millimeters, or 10 millimeters), they were able to find that motion was best with a five- or 10-millimeter offset and using an inferior position of the glenosphere. Glenosphere tilt, humeral neck-shaft angle, and implant size had less of an effect on impingement.
Prosthetic size seemed to have the least effect on impingement. Motion was greatest when the largest implant size was used. Abduction (moving the arm away from the body) and adduction motion (bringing the arm next to the side) was best with a 150-degree neck-shaft angle.
The authors set out to find ways to improve the surgical technique and implant design for the reverse shoulder replacement procedure. The goal was to improve range-of-motion and limit impingement. Ideally, they wanted to eliminate the scapular notching that prevents patients from resting their involved arm down alongside the body.
The surgeon won't always be able to follow the guidelines set out here because of individual patient factors. Poor bone quality, frayed soft tissues, and muscle strength or muscle imbalances can force the surgeon to make other choices than what is recommended here. Variations in normal anatomy may also redirect the surgeon. Whenever there is a choice, these changes in surgical technique can be used to provide positive patient outcomes with improved motion, reduced pain, and increased function.
Sergio GutiÃ©rrez, MS, et al. Range of Impingement-Free Abduction and Adduction Deficit After Reverse Shoulder Arthroplasty. In The Journal of Bone & Joint Surgery. December 2008. Vol. 90-A. No. 12. Pp. 2606-2615.
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