These are my disclosures. None of them are really directly relevant to what I'll be talking about for the spinal pelvic parameters, but I think it's also important to disclose that I used to be a brain tumor researcher and a brain tumor surgeon. I did all kinds of work in brain tumors. I'm partially responsible for that terrible question you have on the boards about 1p and 19q. That was my thesis, my PhD work, and extended resection and gliomas, and I was destined to be a brain tumor surgeon. And then I found something really charming about a case like this, and the challenge of doing a reconstructive procedure and trying to get someone functional again. You can see how just how gory sometimes these cases can be, and I've often wondered what, you know, what the public and what anesthesia thinks, you know, when they're looking over the curtains and they're seeing us take these patients apart. And I turned to a place where I often turn when I need advice and answers of questions, and I turned to Dilbert, and there's this nice cartoon where Wally's going to Dogbert, the career counselor, and the career counselor's saying, according to your occupational preference test, you like to remove vital organs from helpless people. That narrows the career choices to spine surgeon or serial killer. Do you get along with other people? And Wally says, other people are insignificant insects, to which Dogbert says, we'll have to go to a tiebreaker question. And I think that kind of sums it up for some of those big cases. Adult deformity, when you hear adult deformity, you know, conjures up all kinds of different things. Most often we think about scoliosis, and that's just one part of what adult deformity is. And you can see all of these cases here, most of these are kind of iatrogenic, and I thanked Praveen for sharing some of his post-operative cases here to give us some examples of how to do surgery, but it's quite a number, it's quite a spectrum. So adult deformity is a very general term. One of the key factors, though, in adult deformity, as I think you're getting the sense of from all the previous talks, is spinal alignment is a unifying, important factor. And you can do a wonderful job of doing a neural decompression, making sure they're stable, but if you don't get them aligned in the end, their sagittal and their chronal alignment, long-term that's generally going to give a bad result, and they're going to come back either to you or to someone else, and very likely need additional surgery. So why is global alignment important? It turns out that we're designed to stand within a certain cone of balance, and Jean Dubose has made this cartoon, and it gets used over and over in presentations, and it's very illustrative of how we're designed to stand. If we deviate out of this cone of balance here, our use of muscles and energy utilization goes up substantially, and it leaves it at a mechanical disadvantage and leads to disability and to pain. There are certainly some cases where you can look at them, and you don't need to measure anything to know that something clearly went wrong with this case. At least, I hope you can appreciate there's something that's not quite right here, and I hope that's not what the surgeon was going for when they created this disaster. You can see the patient's sagittally imbalanced, chronally imbalanced, and has an ODI, or an Oswestry Disability Index of 66, and that ranges from 0 to 100. A 66 is quite disabled, and just by realigning this patient, and you see two years out, and it's mostly a realignment procedure for this case, two years out, ODI of 7. The patient's very functional. It turns out we like to stand upright. And another example where you really don't even need to measure anything, it's not subtle, a kyphoscoliosis, significant sagittal and chronal imbalance, an ODI of 74, and realigning the patient primarily, two years out, ODI of 13, just by following alignment principles. Steve Glasman has one of the landmark papers from 2005 in Spine, and many of you have probably seen this paper. It gets shown quite a lot because, again, it was one of the landmark papers where they showed the importance of sagittal alignment, and they looked at quite a large group of patients, over 300 patients, and they were operative and non-operative patients, and they found that as that C7 plumb line drifts more anteriorly, the health status worsens significantly, and they also found in that paper that relative kyphosis of the lumbar spine, or loss of lumbar lordosis, tends to be very poorly tolerated, and this just shows diagrammatically the worsening health status and disability as that C7 plumb line drifts more anterior to the sacrum. And in patients who are sagittally imbalanced, there's a number of ways of compensating, and you can see this patient here with her spine x-ray right next to her here, and you can see some of those compensatory mechanisms. You can see the pelvic retroversion as she's tipping her pelvis back. For milder cases, you can see some hip extension along with it. Typically, in more severe and fixed deformities, you'll see the hip flexion and the knee flexion as well. It's a very classic picture here. And Jean Duboiset and the French have been very progressive in demonstrating to us the importance of the pelvis, and Jean Duboiset has coined the term the pelvic vertebra in recognition of just how important of a link the pelvis is between the spine above and our standing posture below. And this is an article that I'll pass out at the end of this talk here. It's a very nice summary. Chris Ames, I put together with him and the International Spine Study Group. It was in Journal of Neurosurgery Spine, and a lot of these videos that you're seeing shown in various talks are through this paper online available through JNS, and I have copies of this paper that I'll pass out at the end. And it summarizes these principles very nicely and actually goes into a lot more detail than we are in these talks. How many people here could, without any problem, measure all of the pelvic parameters pretty comfortably? Maybe, yeah, just a handful, yeah. And it's interesting because, you know, the first few times I was introduced to these concepts, I would see them and people would show them, and I would say, wow, those must be important, but I still don't know how to measure them. And so I'm hoping by the end of this course, one of the things that I can impart to you is you can leave here and feel very comfortable measuring them. And there's a session this afternoon where I'll pass out a handout where you'll actually measure them yourselves and draw them out. And I'll go through them here in a little more detail, because it looks very complicated and very daunting when you see this. But in reality, once you do it a few times on your own patients in the clinic or in the operating room, it's very easy, and it has become second nature. One thing I should point out, though, before going through the pelvic parameters is the importance of lumbar lordosis. And again, emphasizing that loss of lumbar lordosis is not well tolerated. People with a relative flat back are not happy. So going through each of these parameters, and again, to emphasize pelvic incidence is the sum of the other two. So there are three key pelvic parameters, the pelvic incidence, the pelvic tilt, and the sacral slope. Those are the three key parameters. And the pelvic incidence, which is a morphologic parameter, is the sum of the other two. So now to go through them individually, first to the pelvic incidence. The pelvic incidence, again, morphologic parameter. It's not affected by how the patient is positioned. It's not affected by their deformity. Once we're skeletally mature, it typically does not change through a lifetime. So it's going to be a fairly fixed parameter. And the way we measure it is we need to have films that show the femoral heads. So if you're getting x-rays and it doesn't show the femoral heads, you can't measure the pelvic parameters. So when you're getting your x-rays, it's important that they also go down low enough that you can see the pelvis. And to measure the pelvic incidence, first what you do is you find the sacral end plate, which is usually pretty easy to see, and you find the middle of it. And you draw a line that goes straight from the middle of the sacral end plate all the way down to the center of the femoral head. Sometimes the femoral heads aren't superimposed. And if they aren't superimposed, what you do is you draw a line that connects the heads of each of the two of them, and then draw a line from the center of that line. And so you draw a line center of the sacral end plate down to the center of the femoral head. And then you just draw a perpendicular line to the sacral end plate. That's it. And that's the angle. That's the pelvic incidence right there. Fairly easy to draw. You can draw it on pretty much any PAC system and see that angle quite quickly. And that, again, is a morphologic parameter. So if you watch this diagram here as the pelvis is being retroverted here, you can see that that angle is not changing as the pelvis retroverts and changes position. It's fixed. And the pelvic incidence is important to look at, as shown in the previous talks. And in a patient who has a relatively flat sacrum or a very steep slope or pelvic incidence, they're going to require a lot of lumbar lordosis to get their spine back up over and their head up over their sacrum in the end. So a patient with a very high pelvic incidence is going to need a lot of lumbar lordosis. And that contrasts with a patient who has a small pelvic incidence and a fairly vertical sacrum. They aren't going to require as much lumbar lordosis. So if you have a patient comes in who is flat back and can't stand up, and you're trying to figure out how much lumbar lordosis do they need in their spine, because not everyone needs the same amount, you can measure their pelvic incidence. Because regardless of what the other surgeons have done to them, infused them flat and done all these things to them, the pelvic incidence remains the same. It's a morphologic parameter. So you can measure that in that individual patient, and that gives you an idea. Are they someone who needs a lot of lumbar lordosis, or is it someone who doesn't need as much? And that can really affect what you do as far as whether you go down the path of a pedicle subtraction osteotomy, or whether you try to do some Smith-Petes and try to restore their physiologic lumbar lordosis. And again, there's that estimate of lumbar lordosis should be within about 10 degrees of pelvic incidence. And this is an evolving formula. And as we were talking about before, you don't want to overcorrect people. And in some people, if they have a high pelvic incidence, someone who has, say, an 80 degree pelvic incidence, that may be someone you want to aim for a lumbar lordosis of 70 rather than 80 or 90. If it's someone who has a pelvic incidence of, say, 40, maybe that's someone you want to be doing 40 or 50, not 30. So there is some play in that as far as the formulas go, because if you overcorrect them, that does put them at some risk of getting PJK. And this is a case that illustrates the concept of the pelvic parameters and the pelvic incidence matching with the lumbar lordosis. This is a 64-year-old who came in, had had four previous lumbar procedures. And you can see lots of instrumentation in there and lots of disruption. And he came in in minimal pain when he's sitting or he's supine. But when he stands or walks any distance, he has severe pain. And the referring physician says, well, this guy is totally decompressed. I can't understand why he can't walk. Do you think he's just malingering? And you can look at this and you can say, well, sure, he looks nice and decompressed. Why should he have any pain? But if you bother to get some upright x-rays, and especially if you get full-length upright x-rays or the 36-inch x-rays, you can see that there's something that's not quite right here. You can see that the C7 plumb line is sitting far in front of the sacrum. He's sagittally imbalanced. You can see his pelvic incidence, which is 62, and his lumbar lordosis, which is 32. He has a mismatch of 30 degrees. He does not have enough lumbar lordosis. He has a flat back syndrome, and he can't stand up. Therefore, when he tries to walk or stand, he's compensating with his pelvis. He's compensating with bending his knees, and he's becoming fatigued and having pain. There are few people who have a pelvic incidence of zero. So you don't usually need a lumbar lordosis of zero. And so this was revised osteotomies and correction, and much happier patient. A lot more metal, but a lot happier. The second parameter is the pelvic tilt. And in contrast to the pelvic incidence, the pelvic tilt is a compensatory mechanism. It does change based on the alignment. And the way we measure that is, again, we draw that same line from the center of the sacral end plate down to the head of the femur. But instead of drawing a perpendicular to the sacral end plate, we reference it to a vertical line, a vertical reference line. And so the pelvic tilt is just the angle between those two lines. And you can see that as the pelvis retroverts and changes position, that angle changes. In contrast, this is a similar patient here. They started to retrovert their pelvis. The pelvic tilt is increasing, but their plumb line, their C7SVA is decreasing. And you can see here, this is an individual, for example, who has a pretty dramatic retroversion of the pelvis and a high pelvic tilt, and they're masking their alignment here. So you could see this patient in clinic, and you'd say, well, their C7 plumb line sits right over their sacrum. They must be globally aligned. But if you measured their pelvic tilt, you would see how they're globally aligned. It's by retroverting their pelvis. And it turns out, well, you could say, well, thickness retrovert their pelvis and stand upright. Why do we need to do anything with their spine? Why do we need to do a big PSO or a realignment procedure? But it turns out that retroverting the pelvis is not well tolerated. Imagine walking with your pelvis retroverted back, and that's how you function through life. It's not well tolerated. And Virginie LaFage has shown this very nicely in a series of papers. One of the key papers was this one from Spine, in which she showed that patients who have a higher pelvic tilt have greater disability. And this is independent of SVA or global alignment. Patients turn out they don't like to walk with their pelvis retroverted. And it's especially the case for walking. Someone can stand with their pelvis retroverted and have a diminished disability index. But when they start walking with retroverted pelvis, it has even greater impact. And the third parameter, the sacral slope, is the most intuitive to measure. It's simply the slope of the end plate of the sacrum. So again, you draw a line along the end plate of the sacrum, and then it's a horizontal reference line. It's literally the slope of the sacral end plate. And that does change as the patient retroverts the pelvis. The sacral slope is going to decrease to the same degree that the pelvic tilt increases, because the two always have to sum to be the pelvic incidence. It's just a geometric proof that they always sum to be the pelvic incidence. And so how important are these pelvic parameters in the big picture? And this was a study done from the NYU group, as well as through the International Spine Study Group, where we started from scratch. And we said, let's look at these patients. Look at 125 adult deformity patients. We're going to look at 300 different radiographic parameters. You name it. Every radiographic parameter you can imagine. And we're going to correlate those to a series of health related quality of life measures. The Oswestry Disability Index, the Osteoporosis Index, the Short Form 12, and the Scoliosis Research Society Questionnaire. So these standardized instruments. And which ones are going to sort out? Which ones are going to really matter? Well, it turns out that the three that were most highly correlated with health related quality of life measures, the number one was the mismatch between the pelvic incidence and the lumbar lordosis. So if someone has a relative flat back that has quite a dramatic impact on their functional status. Number two was the SVA. So their C7 plumb line relative to the posterior superior aspect of their sacrum. And number three was the pelvic tilt. And this was among all of the chronal and sagittal parameters. And this was in both operative and non-operative patients. So these parameters really do matter in this patient population. It's not just something to measure and to have a number. And this has resulted in a number of alignment objectives. As we talked about in the question session from Tyler's presentation, these are general goals. And we're fine tuning them because we may not necessarily need to make grandma have an SVA of zero because she may not tolerate it well. And we need to adjust these for age groups. And these are some of the abstracts that we've submitted this year to the SRS and some of the bigger meetings coming up where we've adjusted these measures for different age groups. But the general alignment objectives are to try to get your SVA less than five centimeters, to get your pelvic tilt less than 20, and to have your lumbar lordosis be proportional to your pelvic incidence. And this I just wanted to show, just highlighting here, mechanisms of compensation. And this is a nice picture I borrowed from Chris Schaffrey. And it shows a patient with sagittal imbalance. You can see a number of features. So this individual is compensating with thoracic hypokyphosis, so flattening of the thoracic spine. And that's a goal of trying to keep the head upright and to be able to maintain horizontal gaze. It's important to be able to look forward. Also, you can see the cervical and occipital cervical hyperlordosis. When we're tipped forward in sagittal imbalance, we tend to hyperlordosis our cervical spine in order to maintain horizontal gaze. And we've shown this in a previous publication. After we do PSO and realign the patient's spine, that their hyperlordosis relaxes in their cervical spine and becomes more physiologic. You can also see the pelvic retroversion in this patient. And you can also see his knees bent. So it's a very classic picture of how someone's compensating for their sagittal imbalance. So some of the keys for getting a successful sagittal plane analysis, you need to have good radiographs. It's critical to have good imaging to see what's going on. If you just get regional or focal x-rays, it can be very misleading. You can see this here. If you just saw this picture, you wouldn't recognize that this patient has a very high pelvic tilt in order to be able to try to stand upright. And you might just think, well, it looks fine, it looks solid, it looks fused. Long films are essential. You can look for compensatory mechanisms. You can look to see if they're hyperlordosing their neck, if they're bending their knees, for example. You can see if someone comes into your office and they can barely stand upright, but then you see their x-ray looks like they're standing straight up and down, you might send them back to make sure that they weren't just bending their knees to compensate for it. This is an EOS image, which is another nice feature. If you can see them all at once, you can be certain that they aren't bending their knees and compensating. So in summary, sagittal spinal pelvic malalignment is associated with poor health-related quality of life. If we do a surgery that's very nice, decompress the nerves, get them stabilized, but we leave them with poor alignment, they're not going to have a good outcome for the most part, and they're going to most likely come back and need more surgery down the road. Keep in mind the surgical goals when we're planning surgery in these patients and when we're selecting patients for surgery, and if we can leave someone sagittally aligned, taking into account the spinal and the pelvic parameters, then we're setting them up to have the best outcome that we can. Thank you.

spinal pelvic parameters

adult deformity

surgical outcomes

reconstructive spinal procedures

spinal alignment