This is a tutorial on the bones of the pelvis. So we’ve got a few bones that make up the pelvic skeleton. You’ve got these two large hip bones on either side, the sacrum and you’ve got the coccyx.
These two hip bones are actually the os coxae. This comes from Latin. ‘Os’ means bone and ‘coxae’ means of the hip, so it’s the ‘bone of the hip’, the hip bone. We’ve got two hip bones, a sacrum and a coccyx.
True and False Pelvis (Lesser and Greater Pelvis)
The pelvis is separated into two regions. You’ve got the upper region, the superior part of the pelvic bone, which is called the false pelvis. It’s also called the greater pelvis. And you’ve got the lower part, the inferior part of the pelvis, which is called the true pelvis or the lesser pelvis.
The upper part of the pelvis is called the ‘false pelvis’ because it’s’ often regarded as part of the abdominal cavity rather than the pelvic cavity. The lower part is the true pelvis because that contains the actual pelvic cavity.
The lesser pelvis begins at the pelvic inlet, which is formed by this rim here. You’ve got the body of S1 and then this rim, which is formed down here which I’m showing you with the mouse. This is the pelvic inlet. Below this, you’ve got the pelvic cavity. This is housed in the lesser pelvis or the true pelvis.
If I just bring in the abdominal organs, you can see that the upper part of the pelvis actually sits with the abdominal organs whereas the lesser pelvis – you can see the pelvic inlet here at this rim. You’ve got the bladder anteriorly, the reproductive tract behind it and most posteriorly, you’ve got the rectum. These all lie in the pelvic cavity within the true pelvis.
I’ll do a separate tutorial on the pelvic cavity and I’ll talk about inlet, the outlet, the walls and the floors in that. But this tutorial, I’ll just talk you through the features of the pelvic bones.
The hip bone, the os coxae, there’s three parts to it. At birth, the three bones which are joined by cartilaginous joints at the acetabula fossa – this is the acetabula area and this is where the head of the femur articulates with the hip. This is the hip joint. At birth, you’ve got three bones. You’ve got the ilium superiorly, the ischium, posteroinferiorly and the pubis, anteroinferiorly. Just at this region here, they’re all joined by cartilaginous joints and these are fused in the adult. It’s actually one large bone in the adult.
We’re just looking laterally at the hip here. I’ll just show you where the ischium lies. This is the ischium posteroinferiorly. This is the ischium. This is posterior, this side here and this is anterior. Anteriorly, you’ve got the pubis and above, you’ve got the ilium.
If I just rotate the model around, again, we can look at it medially. I’ll just remove this and we can look at the medial view.
Here anteriorly, we’ve got the pubis. That’s this portion here. And then posteriorly, we’ve got the ischium in this region. And above it, the ilium.
I’ll just talk you through some of the features of the ilium, the ischium and the pubis. I’ll just remove the femurs. Here, you can see the head of the femur sitting in the acetabulum. This is the acetabulum here. We’ll just remove here so we can see the hip bones a bit more clearly and move these as well.
Starting superiorly, we’ve got the iliac crest. This is a flattened crest at the top of the ilium. You’ve got muscles which attach here. I won’t talk about them in this tutorial. That’s the iliac crest. This lies at the level of L4.
This is the L4 lumbar vertebra and this is also the area where the aorta bifurcates. If I just bring that in, you can see the bifurcation of the aorta at the level L4.
The iliac crest is a useful landmark to know because it marks the point of L4 and you know that the level of L4 is well below the end of the spinal cord. It’s a useful landmark for lumbar punctures.
If I just bring in the nervous system, you can see the end of the spinal cord well above the level of the iliac crest. You’ve got the iliac crest here and the end of the spinal cord is up here. It’s a useful landmark for lumbar punctures.
If we follow the iliac crest forward, it comes to this spine. This is the anterior superior iliac spine. This is also a useful landmark and this can be easily palpated.
And attaching to this, we’ve got the inguinal ligament. This runs from the anterior superior iliac spine, which is known as ACIS because it’s a lot easier to say than ‘anterior superior iliac spine’. The inguinal ligament runs from ACIS to pubic tubercle.
This is another useful landmark. You can feel the pubic tubercle as well. You can feel both these landmarks, ACIS and the pubic tubercle. Mid-way between these two landmarks, you can palpate the femoral artery. If I just bring that into view, you can see this artery running halfway between these two points.
Just inferior to ACIS, you’ve got the anterior inferior iliac spine. And if we follow the iliac crest posteriorly, we come to the posterior superior iliac spine and below that, we’ve got the posterior inferior iliac spine. And then if we follow that around, we’ve got this notch in the ilium. This is the greater sciatic notch. And below that, we’ve got the ischial spine. This is actually part of the ischium, which I’ll come on to talk about.
Above the ischial spine, you’ve got the greater sciatic notch. And below the ischial spine, you’ve got the lesser sciatic notch.
This is a lateral view of the pelvic bone, the hip bone and this is where the gluteal muscles attach. You can see these large gluteal muscles. You’ve got the gluteus maximus, medius and minimus. They attach on this lateral surface of the bone.
And you’ve got a few ridges on this bone, which you can’t see here, but I’ll just mention them. You’ve got the posterior gluteal line and anterior gluteal line and an inferior gluteal line, which are roughened ridges where the gluteal muscles attach.
On the medial surface, you’ve got this slight hollowing in the bone. This is called the iliac fossa. You’ve got the iliacus muscle which attaches here. You can see how it sits there. It inserts on the femur. This flexes the hip. It’s one of the hip flexors.
Another quick thing to mention before moving on is the angle of the pelvis. If we just imagine a horizontal line running along here, the angle of the pelvis is actually about 50° to 60° to this horizontal line -- the pelvic inlet. The pelvic inlet is this brim here, this rim. The pelvic inlet is actually angled at about a 50° to this horizontal line and angled like this.
And also about this angle, you can see the pubic tubercle here and the anterior superior iliac spine are in the same plane. This is the same vertical plane here.
Next, we’ve got the ischium. This lies inferiorly and posteriorly. It’s this bit here. I mentioned the ischial spine. We’re looking from a posterior view at the pelvis. You’ve got the sacrum here, I showed you the greater sciatic notch and you’ve got the ischial spine here.
You’ve got two important ligaments to know about here. You’ve got the sacrospinous ligament and you’ve got the sacrotuberous ligament. Right at the bottom part of the ischium, you’ve got this tuberosity. This is the ischial tuberosity. The sacrospinous ligament runs from the sacrum to the ischial spine and the sacrotuberous ligament runs from the sacrum to the ischial tuberosity.
I’ve just brought those ligaments in. You can see the sacrotuberous ligament attaching onto the tuberosity and the sacrospinous ligament attaching onto the spine. It’s actually a bit further up here. Above this, you’ve got the greater sciatic foramen and below, you’ve got the lesser sciatic foramen.
You’ve got the ischial spine, the ischial tuberosity and if we rotate anteriorly, you’ve got the ramus of the ischium. This big hole here is the obturator foramen. This is covered by a membrane. There’s a little gap above the membrane where the obturator vessels and nerve run.
I’ve just brought the membrane into view here. You’ve got this little canal up here, the obturator canal and you’ve got the obturator vessels, artery and vein and the obturator nerve which comes through this.
Going back to the ischium, if you remember the lateral view, the anteroinferior part is the pubis. Half of this bone is part of the pubis and the other half is part of the ischium. Collectively, this is the ischiopubic ramus, but the posterior bit is the ramus of the ischium and the anterior part is the inferior ramus of the pubis. That’s the ischium. It’s this posterior inferior bit.
Next, we’ve got the pubis, this is the last bit. This is the anterior and inferior part. The pubis has a body and it’s got these two arms, these two branches. You’ve got the superior ramus and the inferior ramus. You’ve got the superior pubic ramus, the inferior pubic ramus and the body. And where the two pelvic bones meet, this is called the pubic symphysis. And this little protuberance is called the pubic tubercle. This is palpable.
Those are the features of the pelvic bones, the hip bones, the ossa coxae. At the back, you’ve got the sacro-iliac joints. At the front, you’ve got the pubic symphysis. The sacrum articulates above with the fifth lumbar vertebra and below, this articulates with the coccyx.
Next, I’ll just do a quick tutorial on the sacrum and the coccyx.