Sacrum and coccyx

Hello, everyone! This is Joao from Kenhub, and welcome to another anatomy tutorial where, this time, we're going to be talking about the sacrum and the coccyx. And, in order to do so, we’re going to be looking at these 2 images that you see now on the screen. So, here on the image on the left side, you can clearly see here the ventral view of the sacrum and also here the coccyx. And, on this image of the right side here, you find the dorsal view now of the 2 bones.

So, in this tutorial, we’re going to be talking about two main points. One is going to be the different bones and bony landmarks that we find here and another one will be then the most important ligaments that we find attached to the sacrum. As you can clearly see here on these 2 images, the sacrum is triangular in shape and is made up of 5 initially and fused bones that will fuse between the ages of 30 and 40. Superiorly, the sacrum will be articulating with the last lumbar vertebra - L5 –and inferiorly will articulate with this bone that we’re going to be talking about on this tutorial as well, the coccyx.

First structures we’re going to be highlighting here on these 2 images are the wings of the sacrum. You can clearly see why we call them wings. They do spread on both sides of the bone and look like 2 wings. These are then the structures that will be supporting the psoas major muscle and also the lumbosacral trunk. We also have here some important structures – as you can see these holes that we’re going to be talking about on the next slide – these are known as then the sacral foramina, and they are highlighted here in green. On the sacrum, you’re going to be finding 4 pairs of sacral foramina. We can see here 3 clearly highlighted but notice here, there are 2 other foramina that you find a bit above.

Now, on the anterior aspect, the foramina will be transmitting the ventral primary rami of the sacral nerves and on the posterior aspect, the dorsal primary rami. So, important structures will be passing through the sacral foramina. And here on this image, you can now see them from a posterior view and you can now clearly see here the 4 pairs highlighted on both sides of the sacrum.

The next structure – the next landmark that we’re going to be highlighting here on our bone, the sacrum – is then the base of the sacrum. Now, this is the broad upper end of this bone that is directed upwards and curves forwards. Superiorly, the base will be articulating with the 5th lumbar vertebra and the wings of the sacrum are located on either side of the base as you can clearly see here on the image.

The next structure worth highlighting here is known as a joint. This is the lumbosacral joint which is then formed – you can clearly see here from the name – formed between the 5th lumbar vertebra which you can see here also on this image and then the first sacral vertebra. So all of these happening in your vertebral column.

The next structure that we’re going to be highlighting now – these 2 processes – which are known as the superior articular process of the sacrum, singular, or the superior articular processes of the sacrum. Now, they are found superiorly on the posterior aspect of the sacrum and they will be articulating with the inferior articular process of the 5th lumbar vertebra forming then the joint that we talked about before, the lumbosacral joint. Now, these processes are located on either side of the opening of the sacral canal.

We’re going to move on and talk about the next structure that you now are seeing from the ventral side of the sacrum. Now, this is known as the sacral promontory. Now, this is a forward projecting anterior surface of the body of the first sacral vertebra – as you can see here, this is the first sacral vertebra. This part of the body of the first sacral vertebra also articulates with the last lumbar vertebra – L5 – like we’ve seen when we talked about the lumbosacral joint.

And going then a bit further down, you can see here this highlight which shows then the apex of the sacrum which is the most inferior part of this bone that will be articulating with the coccyx. We can also show you here the apex if we zoom in but now seen from a dorsal view of the sacrum. You can also see how the apex is then articulating with the coccyx.

Next on our list, we’re going to be highlighting this structure – a very important structure of, or landmark, of the sacrum – which is known as the median sacral crest. As you can clearly see here on this image, we’re now looking at the dorsal view of the sacrum. So, you notice here that this structure is a vertically running ridge found on the dorsal part of the sacrum and it is formed by then remnants of the spinous processes of the sacral vertebrae.

The next structure we’re going to be seeing here highlighted in green is known as the sacrococcygeal symphysis. Now, this is a slightly movable joint between the sacrum and the coccyx and it is known as an amphiarthrodial joint. And you can also see it here from a dorsal view.

And, finally, we’re going to move on and talk about this bone that we have been pinpointing throughout this entire tutorial, this is then the coccyx – the very famous tailbone. Now, the coccyx is made up of 4 rudimentary vertebrae which you can also see a bit here on this image split by these lines. It is the most inferior part of the spinal column. We’re going to also turn here to the dorsal side of the coccyx so you can see the bone highlighted in green.

And now we’re ready to move on to the next and last portion of this tutorial where we’re going to talk about a couple of very important ligaments. The first one seen here highlighted in green, now we’re looking at a dorsal view of the sacrum, the coccyx, and now you have a few more bones here, the hip bones as well as the right femur. This ligament is known as the sacrospinous ligament that extends from the lateral margins of the sacrum – as you can see here – and also the coccyx to then the ischial spine.

The next ligament that we’re going to be seeing here still from a dorsal view of the pelvis, we’re going to then see the sacrotuberous ligament. Now, as you can see here on this image, this ligament is located between the sacrum all the way to then the medial margin of the ischial tuberosity. It also attaches to the posterior sacroiliac ligaments, the posterior superior iliac spine – as you can see here on this image – and the coccyx.

Now that you just completed this video tutorial, then it’s time for you to continue your learning experience by testing and also applying your knowledge. There are three ways you can do so here at Kenhub. The first one is by clicking on our “start training” button, the second one is by browsing through our related articles library, and the third one is by checking out our atlas.

Now, good luck everyone, and I will see you next time.