Disclosures are here, which you saw yesterday, haven't changed in the last 24 hours. So when do we fixate to the pelvis? So this is from an article that Lou Timmey-Allen, who was a resident some years ago when I was at Emory, helped me with. And basically the indications for extension of fixation to the pelvis is a high grade listhesis like grade 3 or grade 4 spinal listhesis, long segment fusions, L2 or above to the sacrum, which can happen in trauma, osteomyelitis, cancer, or deformity, and treatment of L5-S1 pseudoarthrosis. Now why in these kinds of situations do you want to extend fixation to the pelvis? I need a volunteer to tell me. What are we trying to prevent? Yeah? Protect your S1 screw. Yeah, you're trying to protect your S1 screw. That's one thing you're trying to do. What's the other thing you're trying to do? That's right. You don't want to have a sacral insufficiency fracture. So basically you get a long segment construct that goes, you know, thoracic spine to S1, and then what can happen, especially people who have a high pelvic incidence, is that basically they can fracture S1, S2, and it's very painful. And so basically it cracks, it fractures forward, it gives in kyphosis, and then they come back with very unusual complaints of tailbone pain, L5-S1 distribution pain, they could even get a cauda equina from that if they, depending on where they break it. So you're trying to prevent that complication. That's why you fixate to the ilium in order to back up the bottom of the construct. So these are some examples of doing that, long segment fixation for deformity. This is high grade listhesis. If you look at the force of gravity here on that film, the picture in number C there, you can see how L5 is trying to be pulled off of S1. So if you have a short segment L5-S1, you can see how if you're trying to resist that force, S1 might crack right through the screw or just below the S1 screw from the shear force that happens if you have a high sacral slope, a high pelvic incidence, and a high grade listhesis. One of the interesting things, if you go back and you look at patients who have high grade listhesis, is people who usually progress to a grade 3 or grade 4 listhesis, they usually have a very high sacral slope. It's not the people who have a flat sacrum compared to the ground when they do the 36-inch long cassette x-rays. It's the people whose sacrum is tilted deep. And the reason why they're progressing their listhesis is because gravity keeps pulling L5 off of S1. How many of you get long cassette x-rays routinely at your institution, 36-inch x-rays? How many of you don't? So at our institution, any time we get a fusion now, pretty much we're getting a 36-inch x-ray. And we want to see what is the sacral slope, we want to see what is the pelvic incidence, we want to see what is the lumbar lordosis, we want to match the lumbar lordosis to within how many degrees of the pelvic incidence? Anyone? 10 degrees. That's right. So you've got to think about some of the biomechanical parameters that perhaps as neurosurgeons we're not taught, but you've got to think about them. So this is a demonstration of a sacral insufficiency fracture. You have somebody with a very high sacral slope here, the sacrum is more than 45 degrees to the ground. You have a high grade listhesis, more than grade 2. Here's the fracture. Very painful. We don't want that. So this is one where we treated it with iliac fixation because we had to grab below the S1 screws. You know, S2 screws are not all that strong. Here's another example of when you might want to do it. This is a sacrectomy for chordoma. Now there's no more lumbosacral junction because we cut through the SI joints. So now we have to reconstruct lumbar spine into pelvis. And when you fixate the pelvis, there are zones of fixation. This again is important for board style examinations coming down the road sometime in your future. The fixation zones in the sacral pelvic unit are zone 1, which is for proximal sacrum and S1 screws. Zone 2, you can get the alar wings and distal sacrum. And zone 3, where you go into the pelvis. So important to know the three zones of fixation. And the important things also to understand here is that there is an instantaneous axis of rotation at L5-S1. It is located in the posterior portion of the disc space. And if you have screw fixation, which is anterior to the instantaneous axis of rotation, then you're going to have better fixation and you're going to resist flexion motions. So an S1 screw, you can see here, is anterior to the instantaneous axis of rotation by a bit. But look at the zone 3 fixation with an iliac fixation. How much more anterior it is to that pivot point there, and how much more resistance to flexion you're going to get. So this is a slide that I borrowed from Chris Schaffrey. This is the sacrum of someone who is 60 years old and a woman. And you can see that you can basically see through the bone. So the bone layer under S1 has high bone marrow density. And this is why, if you're going to put a screw in, you want to put your screw in. Here's cortical fixation point number 1. Here's cortical fixation point number 2, if you're bicortical. But if you reach this promontory of the sacrum here, that's three cortices you're grabbing. So you're grabbing the cortex, anterior cortex, posterior cortex, tricortical, so-called fixation. And even if you have this kind of very weak bone, you could achieve some reasonable insertional torque of your screw. And it's important to know that if you put a short little screw that goes from here and just sits halfway into the sacrum, that's not going to give you very strong fixation on someone whose bone looks like this. And a lot of people at the age of 60, their bone looks like this. And you can look at their x-ray, you can see right through their bone. So this is a demonstration of a case from the OR. You can see how I try to put the S1 screws right up into the promontory. If you get the promontory with the S1 screw, you're going to increase the insertional torque by as much as 99%. This is work from Ron Lehman, where basically he shows if you grab the posterior cortex, how much insertional torque you get. You get the anterior cortex, you get more, but you get that sacral promontory, you really get a whole lot more. So what about the L5-S1 inner body cage? Why put that in? Well, that decreases the S1 screw strain by 30 to 40%. So you get a long construct, you want to put an inner body at the base of the construct in order to fuse the 5-1 disc, because otherwise the S1 screws can come loose. Now if you're going to put a cage into L5-S1, should your cage be in the back part of the disc, middle part of the disc, or front part of the disc? Time mechanically, which is the strongest of those three? I will ask Dr. Andrew Yu. That's correct. So this is work by Polly. Basically you can see that if you have your cage way up to the front, far away from the rod, that gives you 18 times more stiffness than if you have your cage toward the back of the disc, spaced near the rod. So it is important where you put that inner body cage. Zone 2 fixation is not so strong. That's basically screws into S2 or into the sacral ala. It gives you some additional biomechanical benefit, but not a whole lot. So these are a demonstration here of S1 screw and sacral alar screw. So here's S1. Here's the sacral alar screws, and I borrowed this one I think from Dr. Chopin in France when I went to visit him, but basically he puts those, and then he also puts an iliac fixation screw. So he does three screws at the sacral pelvic junction, and he doesn't want that to loosen. So this is a demonstration of the sacral alar screw going from medial to lateral, as opposed to the S1 screw which goes from lateral to medial. S2 pedicle screws are fairly short. Zone 3 fixation, which is really what we're here to talk about, is fixation of the ilium. Significant biomechanical advantage from that. Here's the maximum moment of failure biomechanically from this, where you can see that with an iliac screw you get a whole lot more maximum moment of failure than if you don't have it. This is a drawing of it, and there are some surrounding structures you have to be careful of. You don't want to strike the iliac vessels or the colon. You don't want to come into the greater sciatic notch and hit the sciatic nerve. In the greater sciatic notch, if you violate that, placing your iliac screw, there's a superior gluteal artery, and if you tear that, what happens is that vessel will retract up into the pelvis, and you can't get it to stop bleeding. And then, I think Zia had a case report that he had to get an angio and an embo of the vessel because it was bleeding inside the pelvis when he violated it one time. So you want to stay out of that greater sciatic notch and don't want to get into that kind of trouble. You can put more than one iliac screw if you have no ability to put a sacral screw. This was a case where we ended up with four iliac screws because this patient pulled the S1 screws completely out, and we couldn't salvage them. So sometimes you need more than two. What is the entry point for these? Well, the fixation starts one centimeter superior and medial to the PSIS. So if you look at, here's the iliac crest. This is the posterior superior iliac spine, so starting point is one centimeter superior and a centimeter medial. In days when I trained, and I spent some time with Andra too, we used to just basically knock this whole bone off and put the screw there. Well, the problem with doing that is then the patients would feel the screw head when they sat down, especially if they were thin. So now what I do is I recess that screw underneath the PSIS so they don't feel the screw head, otherwise you end up coming back and taking it out for pain. So that's an important point to remember. You put these screws in. The most common place where you're going to violate is you're going to come out too superficially on the lateral pelvis area. You can use that operator outlet to make sure that you stay in the bone, and you don't want to come into the greater sciatic notch here because, again, you're going to strike potentially what, Priyash Patel? Yes, you do not want to strike the superior gluteal vessel. So here's an example of a case that I revised where somebody left the screw a bit proud, and this patient is very thin, doesn't have a whole lot of tissue back here, and the patient came into my clinic and said, it really feels pokey back there, and we took a look, and sure enough, I can see the set screw and the screw head through the skin. So that's a messy situation. So I had to go revise that and put deeper screws in. These things were, of course, completely loose and infected. You don't want to have these screws too prominent. We already talked about the operator outlet view. Mike showed you pictures. This is what it looks like on the lateral view. The sacrum is in red, greater sciatic notch is in yellow, and the hip is in green. So you can see how if you line up a screw from the PSIS, and you aim it toward the anterior inferior iliac spine, you can get a hundred-something millimeter screw in there. Usually these are about 80 millimeters, and this is a video showing how you do it. You can put more than one screw if you need to. I call this a butterfly pattern, my daughter named that one, and then this is a snowski pattern. You can put the screws parallel if you don't have the ability to place S1 screws. You can also place the sacroalar screw where you're crossing the SI joint. Sponsors started talking about this in pediatric deformity patients. Why? Because you're basically putting the screw through the sacrum, so it's really low-profile. The kids don't feel it, because they certainly don't have a lot of tissue down there. It's a very strong fixation. This is a comparison of the trajectory and starting point of an iliac screw, which is in blue, with the sacroalar iliac screw, which is in red. You can see how the starting points are different. Here's the iliac starting point here, and here's the sacroalar iliac starting point here, which is just lateral to the S1 foramen, and then goes through the SI joint. This is an older video now from, I don't know, maybe it was 2006 or something. Lou was a resident there. Now he's on the faculty at the BNI. This is my time when I was at Emory, and Lou was one of our residents back in the day. We basically did this video. It's free online at SpineSection.org, as most of the videos that I'm showing you are free online at SpineSection.org, and also, they're in the Red Journal or the White Journal, if you ever want to look at them. Here's the PSIS. In surgery, I use my finger. I feel where is the PSIS, then I know from that PSIS point, I have to go one centimeter superior and one centimeter medial to find the entry point for an iliac screw. There's the entry point in black there. Here again is the iliac crest. Here's the PSIS. Find the entry point. We're going to go find the PSIS, which feels like a little thumb of bone sticking out if you feel it in the cadaver. You can go one centimeter up and one centimeter medial, and then you get your starting point there. That's a good starting point to make sure that your screw head is not proud and will be recessed underneath the bone. Don't knock off that whole PSIS bone and then have a prominent screw head. Here it is in the OR. From PSIS, one centimeter up, one centimeter medial, and then there's the entry point there. Apparently someone is trying to put one in next door. I take just a small little drill and make a small entry hole, then I take a gear shift. This is a greater sciatic notch, which we don't want to enter. Usually I don't take the whole gluteus muscle off. Usually you can take some of the gluteus muscle off with the bovie and then fill it with your finger to guide the screw trajectory, but that's painful for them, so I don't usually take it off. I can do it by feel now, but it's something that you could keep in mind. Using a gear shift is just like cannulating a big pedicle. You don't want to pop out into the gluteus muscle. That's the most common error, is not getting a steep enough trajectory. You could take the gluteus muscle off and feel the slope of the pelvis if you needed to, and you could feel for the greater sciatic notch if you needed to, but again, it's painful for the patients who don't tend to do it. I usually don't use a lot of force, it basically feels like a pedicle to me, so you'll see I just twist it in. And then I take a Baltic feeler probe and then I feel to make sure there's no cortical wall violation. Again, most common error is a cortical wall violation superficially in the gluteus. You could have too steep an angle and make a cortical wall violation into the retroperitoneum, which is not ideal. Measure it out, this is like 75 or something. And then I usually tap with an undersized tap. You don't have to tap. The reason that I tap is that then I can take my Baltic feeler and I can feel all those tap threads to make sure I truly am inside the pelvis and it didn't have a violation. I don't use a lot of fluoro for this now. You could do that obturator outlet view and look down and do all that kind of business, but usually I just take an AP shot just to make sure I'm staying out of the greater sciatic notch. I think that's one of the things that, as Mike said, I pay more attention to now is using less fluoro. And then if you line it up nicely, sometimes you don't need a connector, you can just take your screw through L5 through S1 and drop it into the LAG screw. Notice that I'm burying the head underneath the bum. I'm not leaving that head proud. So now the patient will not feel that screw head. They feel their own bum when they sit down, they don't feel your screw head. And then you can use a little connector or sometimes you can go straight into the rod. So you can place it MIS, Mike was showing you this paper and the pictures here so I won't rehash that. Here's the potential complications we always have to worry about. If you are going to have a greater sciatic artery injury, then that could be a problem for you. You don't want to hit the sciatic nerve, you don't want to fracture the sacrum, you don't want to fracture the iliac crest, you don't want to malposition your screw, and if you leave it prominent, then it can rot through the skin, you get an infection. So those are all the bad things you want to try to avoid. So again, when do you do it? L2 or above to the sacrum, high grade listhesis, lumbosacral fractures or tumors where you have no S1, or salvage of S1 pseudarthrosis. Over time these iliac screws loosen because you cannot fuse that joint, so these are temporary stabilizers until your L5-S1 inner body is solid, solidly fused, and you can place it MIS, you may need to use a connector. I'll stop there and see if there's any questions.

indications for extending fixation

spinal surgeries

pelvis fixation

biomechanical parameters

iliac fixation