Welcome to Vista Lab. I want to thank Bob and Justin for this opportunity to chat with you about sagittal balance. We're sort of halfway through this morning's session. Fair warning, I may have a case at the end of this talk. I may call on a volunteer and I may randomly pick a volunteer. I may not, but I may. So I have some disclosures. I have worked with DePuy on some posterior instrumentation and some other disclosures here. So if we talk about a normal lumbar curve, and Bob talked about some of this stuff, and you'll have some repetition here, but usually it can vary anywhere from 20 to 60 degrees. You may say to yourself, why do some patients have 20 degrees of lumbar lordosis seem to be perfectly fine, and why do others need 60 degrees of lumbar lordosis when somebody else can get away with 20? And this was not a concept that was well thought about when I was resident in the 90s. We thought if you had about 30, 40 degrees of lordosis and you were fusing somebody, as long as you kept that amount of lordosis, it was all good. Well, that amount of lordosis in patient A was just as good as that amount of lordosis in patient B. That's not exactly true. And these kinds of concepts of how much lordosis someone needs have been pretty well developed over the past 5 to 10 years. And so if we look at how to even measure this lordosis, and Dan showed some of this, but basically you have to go, and Bob did too, you have to go to the end plates of L1 to S1, and then you can get your Cobb angle and then figure out what your lordosis is. And typically what happens is thoracic to lumbar lordosis is a ratio of 2 to 1. So you need about twice the lumbar lordosis that you have your thoracic kyphosis in order to stand up straight. That's one parameter that you have to think about. And another parameter you have to think about is the overall SVA and the pelvic incidence. And, again, this is going to cut it a little bit into somebody else's sacral pelvic talk, but, again, repetition is not a bad thing. So what is this SVA business? The Z axis, if you look at a standing lateral 36-inch radiograph, you take a plumb line, you drop it down from the middle of the C7 body, that plumb line should be within 2.5 centimeters of the posterior superior aspect of the sacrum. So that would be in a normal, healthy, maybe 30-, 40-year-old adult, somebody like you. So this is where you want your Z axis to ideally fall in someone who's well balanced. And so 2.5 centimeters or so from the back of the S1 end plate should line up with a plumb line dropped from C7. If that happens, then basically your head is going to be sitting on top of your tailbone. You're going to be relatively well balanced. You're not going to be walking around kyphotic. So that is the Z axis. That's the SVA. That's what we talk about. That's the thing that you need to know. Probably one of the most important parameters when you're planning any kind of spinal fusion is to have that concept in mind. So how do you measure this again? So you take your 36-inch long cassette film, and you drop a plumb line here, and the plumb line will hopefully land over your tailbone at some point. And then you find the back posterior corner of the S1 end plate, and then you can find out what is your SVA from that maneuver. So that little measurement there should be ideally within 2.5 centimeters. So I told you some people need maybe 20 degrees of lordosis. Some people need 60 degrees of lordosis. So who needs what? Well, that's really determined by your pelvic incidence. And your pelvic incidence is independent of the patient's position parameter. So it doesn't matter if they're lying down. It doesn't matter if they're standing up. Basically what happens is if you take this angle from the hip axis to the sacral end plate and perpendicular from it, that's going to be the same whether you're lying or you're standing. And that's something that's built in. It's dialed in. You can't change it. And what ends up happening is that your lumbar lordosis seems to be within 10 degrees of this number. And it's a very important formula. I think by the time all of you get rotated and have taken the written and oral boards, this kind of stuff may show up, I would say, on the written boards, likely to show up in the short term for those of you who haven't taken that yet. So this is how you measure it, from the center of the femoral head to the center of the sacral end plate. And you take a perpendicular from the center of the sacral end plate, and that is the pelvic incidence. Fixed parameter does not change if you're lying down or you're standing up or sitting. And your lumbar lordosis needs to be within 10 degrees of that angle. What does this pelvic incidence mean to you in daily life? And these are random numbers that people just kind of throw up or draw all kinds of lines on the board. What does this mean to you clinically? What it means to you is the average person usually has a pelvic incidence somewhere around 50. When you have really pathological states like a spondyloptosis, this pelvic incidence is usually really quite high, up in the 70 to 80 degree range. So that means that somebody who has a pathological pelvic incidence needs a lot more lumbar lordosis than someone who has a normal pelvic incidence. So what does it mean? A low pelvic incidence means that you don't have a lot of shear stress at the lumbosacral junction. So if you have a spondylolisthesis, and it's a grade 1 or grade 2 spondylolisthesis, it's unlikely this is going to progress to a grade 3 or grade 4 spondylolisthesis because the shear force pulling the L5 off the sacrum with a low pelvic incidence is low. You don't need a lot of lumbar lordosis if you have a low pelvic incidence. You're in pretty much a relatively conservative, well-defined state where you don't need a lot of energy to stand up. However, if you're one of these people who's born with a very high pelvic incidence, you have a lot of shear stress at the lumbosacral junction. So if you have a grade 1 or grade 2 spondylolisthesis, it's likely to progress to a grade 3 or grade 4 spondylolisthesis because the shear force pulling L5 off of S1 is quite high. So we're going to ask this gentleman who's taking a little nap here what we think the pelvic incidence should be in terms of lumbar lordosis. Someone tap him on the shoulder there. How are you doing? So the pelvic incidence compared to lumbar lordosis should be how many degrees apart? Very good. Excellent. He's got his ears going on, those eyes are closed. It's always good. All right. So here is a visual that I borrowed from Schaffrey. Basically, you see the image on the left side has a lumbar lordosis which is relatively small, but that person needs a lumbar lordosis that is relatively small because their pelvic incidence is small. So small pelvic incidence, flattened lordosis is just fine. If that person has spondylolisthesis, it's unlikely it's going to progress because the force is not pulling L5 off of S1. This person on the right side there has a very big pelvic incidence, so it needs a lot more lumbar lordosis to stand up. And because of that, if they have a spondylolisthesis, it's likely to progress because you can just see how gravity will be pulling L5 off of S1 in this situation. And this, again, is the key thing that you should take away from this talk. The lumbar lordosis should be within 10 degrees of the pelvic incidence. So if you have a mismatch between your lordosis and your pelvic incidence and you're young, usually you can compensate a bit. You can bend your knees, you can sort of pull on your back muscles, you can change your pelvic tilt, you can try to pull it back into alignment. But what ends up happening as you get to the age of 70 is that your knees wear out and you get a lot of muscular back pain because the muscles aren't very strong. You can't really compensate very well, and then you start seeing patients get knee replacements and play on muscular back pain. First of all, they're losing lordosis every year that goes by because the lumbar discs dehydrate, so they're flattening their lordosis and their knees are going bad. They can't compensate anymore. And these are when these folks start showing up in the clinic. And it used to be, when they showed up in my clinic at the age of 70, when I started my practice in the early 2000s, I would say, oh, you're a bit too old for me to want to do some kind of big surgery. And now patients live until 85, they live until 90, and they come and show up at 70 and 75, and they're still active, and they live independently, and they're driving around. And I can't tell them to not have it fixed when it's really causing a big problem for them. So now the age group that I'm treating for these kinds of patients is going up. But what happens is life is a progressive kyphosing event, and Dave Pauly told me that. And basically, as we age, these lumbar discs lose their height, they lose their water content, and as they lose their height and their water content, the spine flattens, and the pelvic instance, as you know, is constant. So now you're going to end up with a mismatch in patients who were originally matched or who used to compensate at least when their knees worked and now can't, and their hips worked and they can't. So this is someone who had a normal lumbar lordosis at the age of 50, and then over time what happened is that the lumbar lordosis was gone because the patient had progressive loss of disc space height in the lumbar spine, and then she started using a walker, and then she started getting kyphosis in her thoracic spine, and now she's completely bent over. So that makes a big difference over a 20-year period of time. And someone's done laminectomies for her for her stenosis here, and then she had a lot of back pain and they put in a spinal cord stimulator. Well, none of this really helped her because her real problem is here. She has insufficient lumbar lordosis for a very large pelvic incidence and got a very big SVA here. So these are the kinds of things that you have to think about when you get these patients coming into the clinic. So when the spine is in balance, it's the least amount of workload for the posterior spinal muscles. The spine is not in balance. Then you get a lot of muscle fatigue and a lot of muscle pain, and this is why these people come in hurting, not only just because they have degenerative facet arthritis, but also because they have muscle fatigue and pain. So what happens is, as your plumb line shifts anteriorly, as that SVA gets higher and higher, what happens is that the disability scores get worse and worse. So there's been a number of papers that have looked at this, but in essence, the more you lean forward, the less you're able to stand up straight and walk, the worse off all your standardized outcome measures are. Now, I told you that some people can temporarily adjust their pelvic tilt in order to try to lean back, and they use their lumbar muscles and their pelvic girdle muscles to try to do this, and this is how you measure pelvic tilt. And I'm sure this is going to be gone over again by whoever has the secret pelvic talk, but you can do this for a while. You can retrovert on your hips. It's not sustainable, and patients end up getting their hip replaced. So this is what happens in someone who is unable to compensate. Large SVA, no pelvic tilt. But you can do some pelvic tilting in order to correct for your SVA if you have good muscles for a short period of time, but that compensation is not going to go on forever. So Bob talked about this earlier. The SVA in someone who's young should be about 2 centimeters. In someone who's 70, 5 centimeters is probably okay. So what happens is you put them back at 2 centimeters, and they kyphos over the top of your construct. Some of the basics of that are still not worked out. We can't really predict who's going to kyphos and who can't just yet perfectly. So how much lordosis you have to dial in is probably a little bit less in someone who's over age 70 than in someone who's under age 70. So this is where I said I might call on a volunteer. So let me just take a volunteer before I randomly pick one to come talk to us about this case example. Not everybody all at once. Not Justin. Well, Rajiv is one of our residents, so why don't we have Rajiv come up. Yeah, yeah, come on up. I'll learn some more names by this afternoon's talk, so it won't just be Rajiv all the time. Here you go, Rajiv. So this is a 50-year-old man with mid and low back pain. He's had a lumbar fusion. In fact, he's had several lumbar fusions, but he still has lots of back pain. So the question is, why does he have back pain? And so he's seen three neurosurgeons before he gets to me. He had an L5-S1 fusion. He had an L4-5 fusion. He still had back pain, so they took the hardware out. And then they had an L3-4 fusion. And you see his decompression there. You can see a laminectomy defect. And he still has a ton of pain. And this is his lateral X-ray. They told him, your fusion is solid now, L3-S1. Hardware looks fine. Would you get any other tests for him? Yeah, I think he needs a 36-inch standing scalene. Okay, and any other tests besides that? He has a bit of buttock pain too, a little bit of posterior thigh pain. MRI could be, I would get an MRI. So he actually ended up getting a myelogram. What do you think of his myelogram there? It looks pretty wide open. I don't see any signs of lumbar stenosis. It would be interesting to look at the hair roots to see if there's any foraminal stenosis that may be related to his buttock pain. But from that perspective, it looks okay. He has what looks like a grade 1 L5-S1 spondy. And it was kind of amazing. Just in terms of bone, he has a loss of lumbar load. He has a bit of a flat back there. So he saw several neurosurgeons. They told him, there's nothing compressing your nerves. Your decompression looks good. We know there's a little bit of slip at 5-on-1, but posterior laterally, there's a solid fusion there. They said, you don't need anything. Ultimately, he kind of came to my office, and then he got these films. So what do we see? He's never gotten these films before. He's been to three neurosurgeons. So most importantly on the right, he has very significant positive sagittal balances. SVA is way over 5 centimeters, probably in the order of 10 centimeters. Coronal balance seems pretty good. He doesn't seem to have any coronal malalignment. And again, standing up, you can see that he has significant flat back loss of lordosis that's contributing to his positive sagittal imbalance. So you may say to yourself, this guy looks like this on his x-ray. How is it that three people didn't pick up that he's leaning into a different time zone? And basically, what I can tell you is, when he comes into the office, he's kind of a big burly guy, and he wears big baggy pants, and he has a cane. And he kind of leans on the cane, and he bends his knees inside those baggy pants, as far back as he possibly can. He can hold it there for a little while, enough to get up on the exam table and for people not to really notice what was really going on. So we had him lock his knees and stand up as straight as he could, took his cane away. This is how he stands up. So this is someone who's had now three fusions for degenerative spine disease, but no one has paid attention to what is his SVA, what is his pelvic incidence, what is his lumbar lordosis. So Rajiv, just point out for us there how we're going to measure his pelvic incidence. So you look for the two febrile heads, which are here and here. Find the midpoint between the two febrile heads, which is about here. We look for sacrum, which is here, the sacral end plate. Find the midpoint of the sacral end plate. Draw a right angle downwards and the angle between the center of the febrile heads and the right angle to the sacral end plate. And how do we measure his lumbar lordosis? The lumbar lordosis, similarly, take sacral end plate. Different folks in literature either use T12 or L1. As you were just saying, you can use the top of L1, although some folks use T12 for that. So angle between top of L1 and sacral end plate. And that should be within what number of degrees of the PI? 10 degrees. Okay, and how do you measure his SVA? His SVA, you would want to find the centroid of C7 and draw a plumb line down from that and then measure the distance from that to the back point of the sacral end plate. So here you can see the center of his two febrile heads denoted by the blue line there. And there's the perpendicular to the sacral end plate. That's his pelvic incidence. He's got clearly insufficient lordosis for that. And the question becomes what are we going to do about him now? That's what we ended up doing. So you can have a seat. But let me show you quickly a little cartoon about that. This is a video that we made for the MOC textbook. But basically this is how this guy looks pre-op. So here's measuring his lumbar lordosis. So from the top end plate of L1 to the top end plate of S1. So we know that this number needs to be within 10 degrees of his pelvic incidence. Here's his plumb line. Now we know what his SVA is. There's the back of the posterior corner of S1. Now we're going to measure his pelvic incidence. Center of the femoral head. We're going to go to the center of the sacrum. And we take a perpendicular from there. So the green angle needs to be about the same as the purple angle. Within 10 degrees of that. So now we know how much more lordosis we need. And so if we're going to correct him surgically with a PSO, we want to have all these things line up. And here's where we want to end up. We want to end up with a lot more lordosis. And this amount of lordosis has to be within, again, 10 degrees of his pelvic incidence. We're going to make sure his plumb line now is within he's only in his 50s, so within 2 to 3 centimeters of his sacral end plate, posterior corner. If he was 70-something, then I might give him 5 centimeters, but he's young. And then we're going to measure his pelvic incidence and make sure that's going to be within 10 degrees of his lordosis. So center of the femoral head. Center of the sacrum. Perpendicular from there. How many of you do this kind of stuff at your institution for any patient that you're going to do a lumbar fusion for? How many of you do it for people who clearly have a kyphosis? How many of you do not get this for any patients? So I'll conclude with this. If someone is over age 70, you want your SVA to be less than 5 centimeters. The pelvic incidence is fixed. Your lordosis should be within 10 degrees of your pelvic incidence. For a while, you can compensate with your hips and your knees and pelvic tilt. That's not going to go on forever. And the more you age, the more the joints will wear out. So then you end up having to correct them. And if you're going to correct them, get the x-rays, do the measurements, plan your surgery so that you can execute in a well-defined fashion. Thanks.

sagittal balance

lumbar lordosis

normal lumbar curve

pelvic incidence

sagittal vertical axis

musculoskeletal issues