welcome back to anatomy on catalyst university my name is kevin tockoff please make sure to like this video and subscribe to my channel for future videos and notifications we discussed a bunch of different ligaments and the structure of the si joint in the previous video so remember we looked at an anterior view saw all of these ligaments here posterior view and then we ended with this cross section and i left you with a thought here we've got this sacrum right here here's the right ilium here's the left ilium how the heck does the sacrum with all the weight on top of it just stay situated in these two s-i joints so here's an anterior view i know we've got all these ligaments here right but consider how much weight the sacrum has to bear all the lumbar vertebrae the thoracic vertebra the cervical vertebra the head all the weight of the torso the abdomen the head the neck the arms everything really is sitting on the sacrum and there's nothing underneath the sacrum there's nothing below it that's just holding the sacrum up literally the right and left ilium right here are just holding it in place so for instance if you do a high jump and you land how the heck does the sacrum support that how does it not just fall through how do these ilia right here left and right just keep the sacrum in position and that's a really important consideration because the surface area of the joints really small and these ligaments yes they're very strong but by themselves they would not be enough to keep the sacrum in place the sacroiliac joints are small in area but they are extremely stable joints and remember we have this phenomenon in anatomy where for joints you usually sacrifice stability for mobility or mobility for stability so the sacroiliac joints have a negligible mobility they do move some as we'll see in a few minutes but their mobility really just should be considered almost zero so these are an extremely stable set of joints and this video is really going to be focusing on how they are that stable especially when the sacrum has to bear all that weight and the size of the joint is really small all right so we're going to be talking about the stability of the si joint and in this picture right here this is just a model a 3d model of the pelvis it begs the question how does that sacrum not just fall through because it looks like you have a vertical joint right how does the sacrum not just slip through there with all the weight that it has to bear well first of all as we mentioned the s-i joint mobility under normal circumstances is extremely limited normally it's only about two degrees if your shoulder joint had two degrees it would you wouldn't even notice two degrees two degrees is negligible if you go on a goniometer and look at two degrees you probably can't even detect two degrees okay when you're looking at your shoulder joint you can detect 90 degrees of movement that's easy okay two degrees you probably can't even detect that so the mobility here is pretty much negligible in the s i joints okay in fact the only movements that really are even worth considering are most likely deformations and slight gliding motions in response to body weight and ground reaction forces no significant movements occur here it's all stability so now for the first reason that we have such a stable joint we have what's called form closure and force closure this right here is a little model here that they use to describe how the s-i joint works okay now the first mechanism that explains why the s-i joint is so stable has to do with form closure and force closure we're going to first look at form closure which is basically the vertical support of the load and we're going to look at this model here that's used to describe that in this picture the black box is the sacrum and this is the right ilium and this is the left ilium vertical load support here implies that each side of the ilium has a little bit of bone underneath the sacrum okay and so you can see here that even though the sacrum is just sitting there it won't fall through due to gravity because there's a little bit of ilium support underneath okay there's not much but there is a little bit form closure alone wouldn't be enough so you also have to have force closure horizontal support of the load directed internally and so this right here is going to be the force closure so here's the sacrum okay and again you have both sides of the ilium on either side and there's mechanisms that force the ilium toward the midline okay and so they're pushing inward and by pushing inward they create some friction there so to speak and it prevents the sacrum from falling through and so if you combine the net effects of form closure and force closure you get a situation that kind of looks like this so even though the sacrum due to gravity and everything above it actually is directed downward there's some vertical force applied by the ilium that prevent it from falling downward and internal horizontal force that force it together and force that closure and prevent it from falling downwards okay so those are the form and force closure effects of the ilium on the sacrum and also in terms of that force closure you have a bunch of muscles that are arranged in a cross-bracing manner and those muscles actually force the ilia closer together which is actually what creates this effect where the force is directed internally and that prevents the sacrum from falling downward so the point is you have a bunch of forces here that are really forcing the iliac closer together and also some vertical support from ilia underneath the sacrum in fact you can kind of see that right here you can see some of the ilium beneath this part of the sacrum which is providing some of that vertical support okay so that's the first reason why the si joint is so stable it has a self locking mechanism the next reason is well legos now what the heck do legos have to do with the si joint well let's think about this we think about legos right on one side they have these things sticking up right and we know intuitively on the bottom side they have little holes and so i can put this blue lego on its bottom it has those holes and theoretically those holes should really just kind of fit into these pegs on the red lego right they should just stick in there like that i think at some point we've all used legos and when they click they just click in like that and they stick it turns out that the articular surfaces of the si joint that is the sacrum and the ilium have the same pattern so here's the ilium here's the surface of the sacrum and actually this picture really doesn't even do it justice i really couldn't find a very good one that was high resolution but you can see here that the ilium surface is up and down up and down kind of like the legos and then the sacrum has a complementary fit okay and again this picture doesn't really do it justice it actually underestimates the degree of that spiking in and out okay but the point is is when the sacrum sits in on the ilium and those interlocking surfaces which are complementary sit next to each other it makes it very difficult for the sacrum to drop through okay if you took these legos and turned them horizontally by 90 degrees they wouldn't fall they would stay connected because they kind of just click into one another that's kind of the way the ilium and the sacrum fit together in the si joint on either side okay so hopefully that makes sense and so this interlocking mechanism like this and then this self-locking mechanism of form closure and force closure these two things combined really create some massive stability of the si joint okay and you don't want a lot of movement here because if there were a lot of movement then you might actually dislocate the si joint there are other structures here that we're going to talk about that actually restrict further movement but before we do that we need to talk about two potential movements mutation and counter mutation all right here's really the more neutral appearance of the lumbar spine the pelvis and then here's of course the sacrum let's suppose we were to rotate the sacrum we'd also have to get some rotation of the lumbar spine but the point is if we take the superior surface of the sacrum and rotate it anteriorly okay that also means take the inferior surface of the sacrum and rotate it posteriorly so imagine taking the sacrum and rotating it like these arrows show that movement is nutation and you get something like this so right here this is in the mutated state okay notice that in addition to movements of the lumbar spine the superior part of the sacrum has been rotated forward or anteriorly and the inferior part of the sacrum and coccyx have been rotated posteriorly we're now in the mutated state and so from going to this position up here down here that is nutation okay now it turns out that restriction of mutation is very important for stability of the si joint and the major structure that restricts mutation or sacral mutation is the sacral tuberous ligament now as i mentioned in the previous video the sacrospinous ligament also assists with that but again the sacrotuberous ligament is the major structure that restricts sacral mutation so we don't get anywhere near this degree of mutation this is not a normal movement if we obliterated the sacrotuberous ligament we might get this okay but again this movement is restricted via the sacrotuberous ligament we can also further restrict that notation range of motion also by the action of biceps femoris and that's because it originates on the ischial tuberosity so biceps femoris remember that's one of the hamstring muscles the lateral one that can also restrict mutation okay additionally there is a little bit of sacral mutation maybe about two degrees that occurs but the amplitude of that sacral mutation even within that range can be controlled and modulated by co-activation of some of the pelvic floor muscles and the sacral multifidus but the point is generally what you would need to know here is what mutation is and also that it's restricted mainly by the sacrotuberous ligament okay now for the opposite movement counter notation so here we're in a mutated state down here we need to get back to the neutral state that movement is counter notation so it's the opposite so we have initially our coccyx and inferior sacrum they're already rotated out posteriorly so they need to rotate in the opposite direction they need to rotate back anteriorly you can see they've done that up here and then the superior part of the sacrum would need to be rotated back posteriorly you can see that's happened here and so when you go from the mutated state back to the neutral state that is counter mutation okay and in general we can say that it's posterior tilt of the superior sacrum so it tilts back posteriorly gets back to its original position but also the coccyx rotates anteriorly along with the inferior sacrum now in terms of sacral counter nutation it's restricted mainly by the long dorsal sacroiliac ligament remember this is one of the two ligaments that was part of the posterior sacroiliac ligament right here this is the long dorsal sacral iliac ligament okay that ligament actually restrains counter mutation and that ligament's actually shown right here remember that it actually connects the most posterior part of the iliac crest really with the sacrum and then also partially blends with this ligament right here which is the sacrotuberous ligament remember that the sacrotuberous ligament which is right here restricts sacral mutation long dorsal sacroiliac ligament restrains sacral counternutation and we can also get further restriction of counter irritation range of motion by co-activation of latissimus dorsi actually through its attachment at the thoracolumbar fascia but the major things to know here would be what counternutation is and also the major ligament that restricts that range of motion that is long dorsal sacroiliac ligament and the main takeaways from this video are really just to understand what creates this massive stability of the si joint and really understanding that its mobility is extremely low mobility is about two degrees you won't even be able to detect two degrees go on a goniometer and measure out two degrees if you tried that motion you may not even be able to detect that at all this is certainly a lot less than what we would expect even for something like e version of the ankle which already has a low range of motion but certainly negligible compared to motion of the shoulder joint or the knee joint or the hip joint you shouldn't even be able to detect this okay so very very very very stable very very very very immobile and you don't want this mobility because if it had mobility you might get dislocations here and along the same lines of these two slides if you did have a disruption of the sacrotuberous ligament or the long dorsal sacroiliac ligament you might actually allow some extra mobility there and that in that extra mobility can impair load transfer through the si joint creating si joint dysfunction hopefully this video made sense to you please make sure to like this video and subscribe to my channel for future videos and notifications thank you