Video: Sacrum and coccyx

Hello everyone and welcome to our new anatomy tutorial about the sacrum and the coccyx. Throughout our tutorial, we will look at these bones from several aspects -- anterior, posterior, lateral, and midsagittal, which is basically a cross-section through the vertical axis of these bones, observed from the right side.

In this tutorial, we will focus on the parts, surfaces, and landmarks of these two bones, as well as the joints they form with each other and the other bones around them. Let's begin now with the first bone in this tutorial which is the sacrum.

The sacrum is a triangular bone formed by the fusion of five sacral vertebrae. Its anterior surface, also known as the pelvic surface, is concave. This helps to increase the volume of the pelvic cavity of which the sacrum forms the posterior wall. From the back, the posterior, or dorsal surface of the sacrum, is convex and exhibits several well-defined landmarks.

Cranially, or from the top, the sacrum articulates with the lowermost lumbar vertebra, vertebra L5, forming the lumbosacral joint. From the sides, it articulates with the ilium of the hip bone, forming the sacroiliac joint, while caudally, or from the bottom, it articulates with the coccyx, or the tailbone, via the sacrococcygeal joint.

Let's now take a look at the parts of the sacrum.

The sacrum consists of a base, apex, and two lateral parts. The broad upper region that we see here is the base of the sacrum and it contains three landmarks. The first is the promontory, which is this notable forward projection on the anterior border of the base. This is an important landmark in obstetrics for measuring the pelvic inlet.

The next part is the wings, or the ala of the sacrum, which are these wing-like expansions on either side of the base. The third and final part of the base is the superior articular processes. Projecting upward or superiorly from the base, these processes meet the inferior articular facets of the fifth lumbar vertebra, helping to form part of the lumbosacral joint from the posterior side.

Next, we'll examine the lateral parts of the sacrum, consisting of two landmarks. First is the auricular surface, which we can see on the lateral sides of the wings. This is an articular surface through which the sacrum joins the hip bone, or more precisely, the ilium, to form the sacroiliac joint. The second component of the lateral part is the sacral tuberosity, which is found just behind the auricular surface. It is a roughened area which serves as an attachment site for the strong interosseous sacroiliac ligaments, which help stabilize the sacroiliac joint.

Let's look at the sacrum from an anterior view and analyze the landmarks on the pelvic surface. Let's begin with these four horizontal ridges that we can see here called the transverse ridges. They mark the positions of the intervertebral joints between the sacral vertebrae in the immature bone.

Next, we see four pairs of openings called the anterior sacral foramina. These allow the anterior rami of the sacral spinal nerves to exit the sacral canal, supplying parts of the pelvis and lower limbs.

Moving on to the dorsal surface, there's three crests that we can identify, two of which are paired. Each is a representation of the fusion of different vertebral processes of sacral vertebrae. Let's start from the midline, where we can see the median sacral crest. This is the only one of the three that isn't paired. The crest is formed by the fused spinous processes of sacral vertebrae S1 to S4, and as such, bears four small tubercles along its length.

Moving laterally, on either side of the median crest, we have the intermediate sacral crest, which is a row of tubercles representing the fused articular processes of the sacral vertebrae. And moving again a bit more laterally, we see another set of paired ridges known as the lateral sacral crests. These represent the fused transverse processes of the sacral vertebrae.

If we go back to the intermediate sacral crests for a moment, specifically to their inferior or caudal ends, we can see that they are continuous with another paired landmark known as the sacral horns. These are in fact the inferior articular processes of vertebra S5 and articulate with the coccygeal horns. They form the lateral borders of another structure called the sacral hiatus. This is an opening which is formed due to the non-formation of the laminae and spinous process of vertebra S5.

And if we look at the sacrum in this midsagittal section, we can see that the sacral hiatus forms the caudal opening of this structure here, which is the sacral canal. It transmits the sacral spinal nerves and filum terminale. This is the continuation of the vertebral canal.

Alright, let's get back to the dorsal surface of the sacrum to look at some more landmarks we can see in this area.

Between the intermediate and lateral sacral crests are the four pairs of posterior sacral foramina through which the dorsal, or the posterior rami of the sacral spinal nerves exit, similarly to the anterior sacral foramina and the anterior rami that we mentioned before.

Finally, let's now look at the apex of the sacrum. This is the tapered inferior end of the sacrum. The inferior end of the sacrum has an oval facet which articulates with the base of the coccyx, forming the sacrococcygeal joint, which we can see here highlighted in green.

And with that, we've covered the sacrum. Let's now have a look at the coccyx.

The coccyx, or the tailbone, lies below the sacrum. Small and triangular in shape, it usually consists of four fused coccygeal vertebrae, though three to five is also possible. The coccyx angles downward and slightly forward, but this orientation can vary between individuals and may have some mobility at the sacrococcygeal joint.

The coccyx consists of three parts, which are the base, horns, and apex. The base of the coccyx is actually its upper, or cranial surface. It has an oval facet which articulates with the apex of the sacrum to form the sacrococcygeal joint.

The coccygeal horns project upward from the base. They align with the sacral horns, reinforcing the connection between the coccyx and the sacrum. And finally, this is the apex of the coccyx, which is essentially the last coccygeal vertebra.

And with that, we've reached the end of this tutorial. Thanks for watching and see you next time!