 Good day everybody. This is Dr. Sanjay Sanyal, Professor, Department Chair. This is going to be a demonstration of the clinical developmental and the functional aspect of the sacrum and the coccyx. Sacrum is called sacrum because it is a sacred bone. It is shaped like a heart, roughly. The reason why it is shaped like this is because the lower portion of the sacrum, rapidly tapering, is because the lateral muscles of the lower pieces of the sacrum, especially S3, S4 and S5, they degenerate during embryonic life. In other words, they do not develop. The reason is because they are not concerned with weight-pairing. Only the first part of the sacrum, the first piece and little bit of the second piece of the sacrum is concerned with weight-pairing, so therefore the lateral muscles here disappear and that's the reason why the sacrum is tapering. If you were to take a look at the torsal aspect of the sacrum, you will find that the sacrum has got multiple ridges on its torsal aspect. And these have been divided into three parts. We have median ridge and that is referred to as the median crest. This is formed by the fusion of the spinous processes of the first, second, third and fourth piece of the sacrum. The fifth spinous process does not fuse and that remains open as a sacral hiatus. Which I shall mention just a little later in more detail. Then on either side of the median crest we have this ridge here. This is called the intermediate crest. This is formed by the fusion of the articular processes of the sacral pieces. Again, the articular process of the fifth one does not fuse and it forms a connoisseur of the sacral hiatus. Laterally, we have these crests here. These are referred as the lateral crest. This is formed by the fusion of the transverse processes of the sacrum, which forms the lateral crest. And of course, we can clearly see the torsal sacrum foramina. Classically, there should be four torsal sacrum foramina and there should be four ventral sacrum foramina. Initially, till the age of about 20 years, the sacral pieces are separate and they are covered by highland cartilage and they are separated by intervertebral discs. Starting from approximately the age of 20 years, going right up to middle life, the sacral pieces they start fusing. And they get completely fused till about middle life. The same thing principle applies to the coccyx also. The first piece of the coccyx fuses with the sacrum and the other pieces of the coccyx, namely the second, third and fourth piece of the coccyx, they also fuse with the first piece and they form the sacro coccyxial piece as one entity as we can see here. Coming to some functional aspects of the sacrum. This is the sacral promontory. This is the anterior projecting portion of the body of S1. This is clinically very important because we use this as a landmark when we are measuring in a female during obstetrics examination to determine the true or the obstetric conjugate, which is the one of the anterior posterior diameters of the female pelvis. These are the transverse process of the first piece of sacrum, which is referred to as the ala of the sacrum. Ala means wing and this is the anterior margin of the ala. The ala of the sacrum, laterally it articulates with the ilium and forms what is known as the sacro iliac joint. So, to understand the sacro iliac joint, we shall take a look at the lateral aspect of the sacrum and see how it works. So, I have shown this another specimen here. So, this is the anterior portion and this is the posterior portion. The anterior portion is literally smooth and this is covered by highland cartilage. This forms the cinnabial part of the sacro iliac joint. This literally smooth portion of the sacrum is referred to as the auricular process. This articulates with the similar auricular process of the ilium to form the cinnabial part of the sacro iliac joint. Posterially, this portion which is rough is the tuberosity of the sacrum, which articulates with the corresponding tuberosity of the ilium to form the syndysmosis or the non-cynabial part of the sacro iliac joint. And bridging this gap between the sacrum and the ilium, we have some very strong ligamentous fibers which are referred to as the interocious sacro iliac ligament. These interocious sacro iliac ligament constitute approximately 10 cm2 of the surface area. And the other ones which constitute the maximum strength of the sacro iliac joint. As a result of this strong sacro iliac joint, there is only very restricted mobility and how does this mobility work? To understand the mobility, let's take a look at the next piece of the sacrum here. When a person is jumping from a height or the person is standing erect, the force is transmitted from the lumbar region to the sacrum at the lumbar sacrum joint, which incidentally the angle is approximately 130 to 160 degrees. And the force is then transmitted laterally through the ale of the sacrum to the ilium. And from there, when the person is standing, the force is transmitted through the femur, and when the person is sitting, the force is transmitted through the ischial tuberosity. The weight of the body, it tends to rotate the sacrum approximately through an axis somewhere in the middle of the first piece of the sacrum and tends to push the first part of the sacrum downwards and forwards. But in this movement, it is restricted by two strong ligaments. One of the ligament is on the lateral aspect, that is the sacro tuberous ligament, and the other is the sacrospinus ligament, which restricts the downward and the forward movement. And so therefore, the sacro iliac joint provides extreme strength, stability, and restricted mobility during standing, jumping, and moving. And it also allows a little bit of mobility during pregnancy and childbirth to allow the fetal head to descend down, which of course is mediated by the female hormones. To continue with the sacro iliac joint, the sacro iliac joint can be likened to that of a suspension bridge. In a suspension bridge, basically, we have three structures. One, we have the bridge itself, then we have the suspension cables, and then we have the pillars of the pylons. So just think of a suspension bridge when you think of the sacro iliac joint. The sacrum itself is like the bridge itself. The pylons will be the two ilium, one on either side, and connecting the sacrum, especially the tuberosity of the sacrum with the tuberosity of the ilium, as I mentioned just now, we have the strong, intrusive sacro iliac joint, sacro iliac ligaments, they constitute the suspension cables. So that is the good analogy to understand the functional dynamics of the sacro iliac joint. Coming to some clinical correlations pertaining to the sacrum. Let's now focus on the dorsal aspect, what I referred earlier to as the sacral hiatus. What exactly is the sacral hiatus? Sacral hiatus is this inverted v-shaped opening on the dorsal aspect of the sacrum, which connects with the sacral canal. As we know the sacral canal, we have the remnants of the cauda equina. How is the sacral hiatus produced? It is formed by the non-fusion or the absence of the spinous process of S5, and therefore there is an opening here. The lateral margins of the sacral hiatus are referred to as the sacral corno, which incidentally are continuous with the corno of the coccyx. The lateral margins of the sacral hiatus, the so-called sacral corno, are formed by the inferior articular processes of the S5 sacral vertebra. The opening of the sacral hiatus is produced not only by the absence of the spinous process of S5, but it is also produced by the absence of the lamina of the S5. There is a very important clinical correlation pertaining to the sacral hiatus. The sacral hiatus in life is covered by a membrane called the sacrocoxygen ligament, and this sacral hiatus is located at the lower angle of the sacral triangle. The sacral triangle can be seen on a living person on the back. If you draw a line, horizontal line connecting the two posterior superior iliac spines, and then bring those two lines down at the upper end of the gluteal cleft, just at the place where the gluteal cleft starts is the location of the sacral hiatus. And this is the location that we can use to give caudal epidural analgesia for painless childbirth or obstetric analgesia by injecting the dye into the sacral epidural space by injecting the anesthetic agent through the sacral epidural space. So this is a very useful route of giving caudal epidural anesthesia. Likewise, we can also give anesthetic agents through the dorsal sacral foramina, and that is referred to as the trans sacral epidural analgesia. To continue with the clinical correlations of the sacrum, especially the coccyx is so-called tail bone is actually derived from the caudal eminence in embryonic life. The word coccyx literally means cuckoo because it is shaped like the beak of a bird. That's why it is called coccyx. When a person is sitting, this coccyx can come in contact with the surface or with the person sitting. And if the person falls hard on his buttock, sometimes there can be injury to the coccyx. It can even be fractured and can produce a painful condition known as coccydinea. The coccyx gives attachment to the coccyxia muscle, the gluteus maximus, and the anal coccyxia ligament. These are all the points which I wanted to let you know about the sacrum and the coccyx. Thank you very much for watching. Dr. Sanjay Sanyal signing out. If you have any questions or comments, please put them in the comment section below. Have a nice day.