The Surgical Detail That Can Make or Break Your Reverse Shoulder Replacement

I'm Patrick Denard from Medford, Oregon. Very sorry that I could not make it to this meeting. I was really looking forward to being with everybody and learning from you all, but hopefully I can at least be with you in spirit. This is my talk on data-driven glenosphere sizing. Historically with glenospheres, we really had limited options. And if you look at prior work, at least initial work, you saw that a 36mm glenosphere was used in almost all of the cases. Now in more modern scenarios, we have multiple glenosphere sizes varying from perhaps 33 up to 42mm in size. With also options of lateralization or eccentric position. And this creates a dilemma in terms of what is the best option. So my conclusions are smaller glenospheres more closely restore the native muscle tendon lengths. Pain is decreased by maximizing the subcoracoid space, and internal rotation is improved also by maximizing the subcoracoid space. Mark Frankel's recent work looked at this. He looked at muscle tendon length relationships in small, medium, and large patients and noticed that The ideal glenosphere was a 30, 36, and actually 30 again, respectively, with 5 to 10 millimeters of lateralization. Mark also showed that when you look at the deltoid, the more— the likelihood of reaching the normal muscle-tendon length relationship actually decreased as glenosphere size increased. And the same thing was found for the rotator cuff, as you can see here. Now I got into this actually initially with this study by the Exactech Group, and they showed essentially in this study using primarily 38 and 42mm glenospheres that average height patients had the most improvement range of motion, but small patients and tall patients didn't have as great of range of motion improvement. And they concluded that we need to adapt to these patients perhaps more appropriately matching glenosphere size. So we've done several pieces of work on this. This is our first paper, the subcoracoid distance is correlated with pain and interrotation after reverse shoulder arthroplasty. We looked at axillary radiographs, 217 patients, and we found here that as the subcoracoid space increased, pain was actually less, perhaps due to less conjoined tendon impingement. We also found that internal rotation at the spinal level and at 90 improved as that subcoracoid distance was maximized. We then took this a step next and looked at scapular Y radiographs. We essentially found the same thing. If you were further from the coracoid by being posterior and inferior, you had improved internal rotation at 90 degrees. The last study that we looked at was just recently, uh, published and is in fact in press. We took 3D plans, those who had PSI, so presumably had appropriate position relative to the plan, and we essentially found the same exact thing looking at these CT scans. Internal rotation behind the spinal level improved as glenosphere diameter dropped. For every 1mm increase, you had a 0.5 spinal level reduction. Said another way, if you go, for instance, to a 36 down to a 39, you have a 3mm decrease. You would theoretically improve 1.5 spinal levels. You can also see again anterior overhang was detrimental. What about instability? If we go back to some of the early work, note that the most important factor in stability is not glenosphere size, it's actually compression and and that's achieved by lateralization. And that's how I'm achieving, um, stability by having a small glenosphere in my patients. We know virtual models show that 135 and somewhere between 5 and 10 millimeters of lateralization is ideal for range of motion. We know clinically that 6 to 8 millimeters of lateralization is ideal for range of motion based on this work in 455 patients that we did here, where we found that internal rotation was improved with 6 to 8 millimeters of lateralization compared to 0 or 2. Now the caveat here is that it bases— it comes back to patient size, much like that early Exactech work I showed you. This is a study where we did scapular size divided into a mean and 2 standard deviations based on several thousand scapula, and essentially what we found here is the internal rotation was maximized with 6 millimeters of lateralization in the mean scapula, 4 millimeters in smaller patients, and over 8 millimeters of lateralization in larger patients. Now, how does lateralization impact your range of motion? The concern by many has been stress fracture. We actually found that, that, that was not a factor in this study. When we looked at progressive amounts of lateralization, we found the inferior glenosphere overhang in particular was what increased the risk of stress fracture. Every 1 millimeter increase led to 19% increase in the risk of stress fracture. So my RSA glenosphere goals are to first size A to P for the reasons I mentioned, maximize the subcoracoid distance, as you can see here. Next, I tend to respect retroversion. We've done other work that has shown that that is also beneficial. And then I want to position on the inferior rim, not to distalize, and that's to avoid scapular spine stress fracture. Here's some examples. A patient here, 33mm glenosphere compared to a 39mm glenosphere, as you can see outlined in the white ring. And you have to remember that it's not just the size here. You have the cup and the polyethylene. So you can see that the distance isn't just that, that you're going to see based on the glenosphere. It decreases quite a lot here. You see we have 12mm versus 23.9mm going from a 39 to a 33mm glenosphere, at least in the system. And this gives you a sense of why that glenosphere sizing from anterior to posterior is very, very important. And this is why in my patients In particular, glenosphere sizing. The most common glenosphere I use is a 33. As you can see here, it's 48% of my cases. I've really dropped down on my larger glenospheres. And the important distinction, which is why I discussed lateralization, is that you need to maximize stability. So my standard is 8 for men. I drop it down if they are in the lower 20th percentile of height. For women, it's 6 millimeters. I drop it down if they are in the, again, the lower 20th percentile of height, or go up if they are tall, the same height as a man. So thank you very much for your attention and hope you all have a great meeting.