Video: Bony pelvis

All right everyone, let's begin with some hip warm-ups. Now hands on your hips. Looking great everyone! And give it everything you've got. Aww! Somebody call me an ambulance. There goes my hip again. Hey, to pass the time while we wait for that ambulance, why don't we brush up on our knowledge on the pelvic bones with a tutorial on the anatomy of the bony pelvis.

In today's tutorial, we're going to learn about the bones which make up the pelvis. We'll take a look at the three bones which form the hip bone and explore their associated landmarks, joints, and foramina. We'll also examine the divisions of the bony pelvis and the associated clinical application to help consolidate our knowledge. So sit your pelvis down, let's dive right in!

Before we get started, let's just take a quick look at the main functions of the pelvis. The bony pelvis is a very important structure as it carries the entire weight of the upper body. It acts as a conduit by transmitting this weight from the upper body to the lower limbs. It also supports vital abdominopelvic organs in conjunction with the muscular pelvic floor and is an important attachment point for muscles of the abdomen, pelvis, and thighs. Finally, in females, the bony pelvis supports the fetus during pregnancy and houses the birth canal through which the baby exits during childbirth. So let's find out a little bit more about this extraordinary structure.

The bony pelvis, which we can see here from an anterior perspective and highlighted in green, is formed posteriorly by the sacrum and coccyx and laterally and anteriorly by a pair of hip bones or pelvic bones. Together, they form a ring which is why the pelvis is often referred to as the pelvic girdle. Let's take a look at the hip bones first.

As I mentioned, the hip bone is a paired bone so you can have a left hip bone and a right hip bone. We can see them both here highlighted in green. Each hip bone is actually composed of three separate bones that fuse together towards the end of puberty. These parts include the ilium which is the largest and most superior of the three bones, the ischium which is located posteroinferiorly, and the pubic bone which forms the most anterior portion of each hip bone. These three bones meet together to form a socket-like structure known as the acetabulum.

In order to view the acetabulum more clearly, we've changed our perspective slightly. What we're looking at now is the left hip bone from a lateral perspective. So here's the ilium superiorly, the ischium posteroinferiorly, and the pubis anteriorly.

The acetabulum is a cavity formed by the fusion of these three bones. The acetabulum is especially important in the formation of the hip joint which you can see here from a lateral perspective. The hip joint is a synovial ball-and-socket articulation between the hip bone and femur. The acetabulum forms the socket component of the hip joint while the ball is formed by the head of the femur as we can see in our image here.

Before we move on, let's take a look at some of the anatomical landmarks of the acetabulum. The first one is this moon-shaped structure that is conveniently known as the lunate surface. This curved surface articulates with the head of the femur. Since it is an articular surface, it is covered by articular cartilage. The acetabular fossa is the non-articular portion of the acetabulum and can be found centrally. This fossa houses the ligamentum teres and is continuous with the acetabular notch which we'll meet in just a second.

The next structure we're going to look at is the margin of the acetabulum. As you can see, this margin is bordering the acetabulum and is interrupted here by this notch which is known as the acetabular notch. The acetabular notch is continuous with the acetabular fossa and is bridged by the transverse acetabular ligament thus forming the acetabular foramen.

The last structure of the acetabulum we'll look at today is the acetabular labrum or the acetabular lip. This structure can be confused with the margin of the acetabulum; however, they are not the same thing. The acetabular labrum is a fibrocartilaginous collar on the rim of the acetabulum, as you can see here, that is attached to the margin.

So let's move on to talk about the three bones which contributes to the formation of the hip bone, beginning with the ilium.

The ilium itself is divided into two main parts – the wing or ala and the body. The wing is this expanded portion you see here and the body is the central part of the ilium that you see here, contributing to the formation of the acetabulum. The ilium has some important landmarks that we'll take a look at now. The first one we'll talk about is the arcuate line. We've changed perspectives again so let's orientate ourselves.

We're now looking at the left hip bone from a medial perspective. We can see here that it is a smooth rounded border on the internal surface of the ilium and that it marks the transition between the body and the wing of the ilium. When we join the two hip bones together, we can see that it also forms part of the border of the pelvic inlet which we will meet a little later on.

Another structure we can see from a medial perspective is the iliac tuberosity. The iliac tuberosity is a roughened elevated area found posterior to the iliac fossa and serves as an attachment point for the posterior sacroiliac ligament. Inferior to the iliac tuberosity, we can see the auricular surface which as its name implies has an ear-shaped appearance and is covered by fibrocartilage. This surface articulates with the sacrum forming the sacroiliac joint.

In this image, we are looking at the pelvis from a posterior perspective and we can see both sacroiliac joints. These joints connect the hip bones to the sacrum posteriorly while the pubic symphysis which we will meet later connects both hip bones anteriorly.

Contributing to most of the internal surface of the wing of the ilium is the iliac fossa. As you can see, it is a large smooth concave surface found on the medial aspect of the hip bone. The iliac fossa is bordered superiorly by the iliac crest and inferiorly by the arcuate line.

If we flip our hip bone over to view its lateral surface, we can see some more bony structures. The first one we'll look at is the gluteal surface. This surface contributes towards most of the external surface of the wing of the ilium and provides various points of attachment for the gluteal muscles. It contains three bony lines – the anterior, the posterior, and the inferior gluteal lines. Let's take a closer look at these lines from a posterior perspective.

The anterior gluteal line is the longest gluteal line. It begins at the iliac crest just posterior to the iliac tubercle and travels in a posteroinferior direction to end at the greater sciatic notch. The posterior gluteal line can be found just posteromedial to the anterior gluteal line and is the shortest of the three gluteal lines. It runs from the dorsal region of the iliac crest, traveling in a posteroinferior direction to reach the greater sciatic notch. The space between the anterior and posterior gluteal line is the place where the gluteus medius muscle originates.

The inferior gluteal line is the least distinct of the three and can be tricky to find sometimes. It begins at the notch between the anterior superior and anterior inferior iliac spines traveling in a posteroinferior direction to reach the greater sciatic notch as we can see here.

Sitting just superior to the acetabulum is the supraacetabular groove of the ilium. This groove is formed by the margin of the acetabulum and the body of the ilium. This is the attachment point for the reflected head of the rectus femoris muscle.

Now we're going to talk about the iliac crest which as I previously mentioned forms the superior border of the iliac fossa and wing of the ilium. Let's take a look at the iliac crest from an anterior view. As we can see, it extends from the anterior superior iliac spine here to the posterior superior iliac spine here. This crest serves as an attachment point for a large muscle of the body known as the latissimus dorsi muscle.

Lastly, we're going to look at the spines of the ilium. The ilium has four different spines and the first one we're going to talk about is one I already mentioned – the anterior superior iliac spine. As I said earlier, the anterior superior iliac spine is a bony projection that marks the anterior limit of the iliac crest and is the origin of the sartorius muscle.

Now we have the anterior inferior iliac spine which is the bony process found at the anterior margin of the ilium inferior to the anterior superior iliac spine. This spine serves the origin of the straight head of the rectus femoris muscle.

Let's change to a posterior view where we find the next spine known as the posterior superior iliac spine which as we already know marks the posterior limit of the iliac crest. It's shorter than its anterior counterpart and serves as the attachment points for the oblique portion of the posterior sacroiliac ligament and for the multifidus muscle.

Still in this posterior view, we can find the last spine of the ilium known as the posterior inferior iliac spine. It is found on the superior edge of this really important structure here known as the greater sciatic notch.

So that's us finished with the ilium. Let's move on to talk about another bone that contributes to the hip bone – the pubic bone – which is also known as the pubis. This bone is divided into three main parts – the body, the superior ramus, and the inferior ramus. The body of the pubis is the wide medial and flat area we can see here which unites with its counterpart at the pubic symphysis. It's worth noting, however, that some texts alternatively describe the body of the pubis as being the component that fuses with the ischium and ilium resulting in the formation of the acetabulum.

There's nothing I love more than a good debate; however, for the purposes of today's tutorial, we will stick with the majority and classify the body of the pubis as the small quadrangular-shaped area at the anterior portion of the pelvic girdle.

Let's take a look at the pubic bone from a lateral view now for a clearer picture of the superior pubic ramus. The superior pubic ramus is this portion right here that is located just above the obturator foramen. The pecten pubis, also known as the pectineal line of the pubis, is a small ridge which can be found here on the superior pubic ramus. It is a continuation of the arcuate line of the ilium and contributes to the formation of the pelvic brim.

From this lateral view of the left hip bone, we can identify the inferior pubic ramus. The inferior pubic ramus lies here between the pubic symphysis and the inferior ramus of the ischium. Again, the pubic bone possesses several bony landmarks and we're going to talk about the ones we can see from a medial perspective starting with the iliopubic eminence.

The iliopubic eminence is a flat prominence located at the superior pubic ramus. It marks the point of union between the ilium and pubis. Another structure found on the medial surface of the pubic bone is the symphyseal surface. This surface unites with its counterpart through a midline cartilaginous joint called the pubic symphysis which you can now see highlighted in green from an anterior perspective. As previously mentioned, this joint connects the two pubic bones together forming the anterior connection of the hip bones.

Sitting just beneath the pubic symphysis is a small archway known as the pubic arch. The pubic arch is formed by the inferior rami of the pubis and ischium on each side collectively known as the ischiopubic rami. The angle created by the pubic arch is known as the subpubic angle. This angle usually differs between the male and female pelvis. The male pelvis tends to have a lesser subpubic angle usually less than 70 degrees while female pelvises have a greater subpubic angle usually greater than 80 degrees.

Next, we'll have a look at the structure we can now see highlighted in green which is the pubic tubercle. We can see that it is located on the anteromedial portion of the body of the pubis, just lateral to the symphyseal surface. The pubic tubercle is a continuation of the pecten pubis and serves as an attachment point for the inguinal ligament.

Okay, so now let's take a look at the obturator crest. The obturator crest extends from the pubic tubercle here to the acetabulum here. It provides a point of origin for the pubofemoral ligament. On the inferior aspect of the superior pubic ramus, we can find the obturator groove. It's a little bit easier to see the obturator groove if we take a look at the hip bone from a lateral view. The obturator groove lies just beneath the obturator crest as we can see here. We can also identify the obturator groove from a medial perspective. This deep groove provides a passageway for the obturator nerve and vessels.

Inferior to the obturator crest and groove, we find the obturator foramen which is the large opening you can see here enclosed by the pubis and ischium. Most of the obturator foramen is covered by a connective tissue membrane called the obturator membrane. A narrow obturator canal remains open between the membrane and adjacent bone. The obturator nerve, artery, and vein pass through this canal while passing between the pelvic cavity and the medial compartment of the thigh.

Now that we've covered the bony elements of the pubic bone, it's time for us to move on to the third and final component of the hip bone which is the ischium. The ischium is the most posterior and inferior part of the hip bone. It is divided into two main parts – the body and the ramus. The body of the ischium is this larger portion right here located just behind the obturator foramen. It projects superiorly and joins the ilium and superior pubic ramus.

The ramus of the ischium is found here just below the obturator foramen. Let's look at the ramus of the ischium in a bit more detail. Here we're looking at it from a medial perspective. We can see that the ramus of the ischium fuses with the inferior ramus of the pubic bone. So this is a meeting point between the ischium and the pubic bone and is known as the ischiopubic ramus.

The ischiopubic ramus forms the inferior boundary of the obturator foramen and functions as an attachment site for the obturator internus and externus muscles.

Sticking with the posterior perspective, we can see another key structure of the ischium which is the ischial spine. The ischial spine is a bony prominence found between the greater sciatic notch and the lesser sciatic notch. It serves as an attachment point for some important muscles such as the coccygeus muscle.

Let's move on to talk about the greater sciatic notch in more detail.

This notch lies between the posterior inferior iliac spine here and the ischial spine here that we just talked about. When we add these ligaments here, the sacrospinous and the sacrotuberous ligaments, this notch becomes what is known as the greater sciatic foramen.

So here we have the greater sciatic foramen. We can see the sacrospinous ligament here anterior to the sacrotuberous ligament. Several important structures pass through this foramen including the piriformis muscle and the sciatic nerve.

We mentioned another notch earlier – the lesser sciatic notch. This notch lies between the ischial spine here and the ischial tuberosity. Similarly, when we add to the sacrospinous and the sacrotuberous ligaments here, this notch becomes what is known as the lesser sciatic foramen. The lesser sciatic foramen has many important structures passing through it including the obturator internus muscle, the nerve that supplies this muscle, the internal pudendal vessels, and the pudendal nerve.

The last structure of the ischium that we'll talk about today is the ischial tuberosity. This rough, bony projection can be found at the junction between the lower end of the body of the ischium and the ischial ramus. Its function is to bear the weight of the body in a seated position, therefore, we can also call it the sitting bone. Can you feel your ischial tuberosities against the chair? You are literally seated on them.

Now that we've covered the hip bones, let's take a look at the bones which form the posterior boundary of the pelvis and make up the pelvic spine.

The sacrum is a group of five fused vertebrae at the base of the spine. It forms the link between the spine and the hip bones and is important in force transfer between these two structures. The sacrum articulates with the fifth lumbar vertebra of the spine at the lumbosacral joint and with the hip bones at the sacroiliac joints which we've already mentioned earlier in this tutorial.

Let's look at the lumbosacral joints in a little more detail with this posterior view where we can see the superior articular process and the body of the sacrum articulating with the inferior articular process and body of the fifth lumbar vertebra.

This joint allows for flexion, extension, and minimal lateral flexion of the lower vertebral column. The coccyx is formed by four fused vertebrae and can be found just distal to the sacrum. It is the terminal part of the vertebral column. The apex of the sacrum and the base of the coccyx articulate together to form the sacrococcygeal joint. This joint is a symphysis with its articular surfaces being lined by hyaline cartilage. It is slightly mobile and only allows limited degrees of flexion or extension. Movement at this joint is vitally important in increasing the anteroposterior diameter of the pelvis during labor.

Now that we've taken a look at the bony components of the pelvis, let's piece them all together and take a look at the skeletal pelvis as a whole.

The pelvis can be divided into two segments – the greater or false pelvis and the lesser or true pelvis. The greater pelvis lies superior to the pelvic opening or inlet which we'll meet in just a second and is marked by the ala of the ilium laterally, the fourth to fifth lumbar vertebrae and the base of the sacrum posteriorly, and the inferior part of the anterior abdominal wall anteriorly.

The greater pelvis houses the lower abdominal organs and communicates with the abdominal cavity superiorly. It is sometimes referred to as the false pelvis as it lies above the pelvic brim superior to the pelvic inlet and forms part of the larger abdominopelvic cavity. The lesser pelvis lies just inferior to the greater pelvis and is bounded by the pubic symphysis, body of the pubis, and the superior ramus of the pubis anteriorly; parts of the ilium and ischium laterally, and by the sacrococcygeal complex posteriorly.

The lesser pelvis is often known as the true pelvis as it encloses the pelvic cavity. The cavity of the lesser pelvis, the pelvic cavity proper, contains parts of the reproductive organs, bladder, and rectum. As we've mentioned before, separating the greater pelvis from the lesser pelvis is an oblique plane known as the pelvic inlet. The pelvic inlet is the superior aperture of the pelvis and it defines the boundary between the abdominal and pelvic cavities. The pelvic inlet is defined by the sacral promontory and margin of ala posteriorly and the linea terminalis elsewhere.

Each linea terminalis is made up by the arcuate line, pecten pubis, and pubic crest. The outer bony edges of the pelvic inlet are known as the pelvic brim. The pelvic outlet is the inferior aperture of the pelvis and marks the inferior limit of the pelvic cavity. It is bounded by the tip of the coccyx posteriorly, the inferior border of the sacrotuberous ligament and ischial tuberosity laterally, and the pubic arch anteriorly.

Oh, do you hear that? I think that's the siren. The ambulance must be close. But just before the ambulance arrives, I'm going to give you a little pelvis fun fact. Did you know that the male and female pelvis may differ structurally in a number of ways? The fundamental differences in the pelvis of the two sexes are based on the size, shape, and thickness of the pelvis as a whole as well as a number of proportional differences between various parts and landmarks.

The female pelvis tends to have a relatively shallow greater pelvis with a wider more circular inlet. The lesser pelvis is wider than that generally seen in males due to longer superior pubic rami and we also generally see a wider and more curved subpubic angle between the ischiopubic rami coupled with shorter, blunted ischial spines.

The sciatic notches are also generally broader in the female pelvis. The sacrum and coccyx of the female pelvis also differ in both shape and size. The female sacrum and coccyx tend to be shorter, wider, and curved posteriorly with the less pronounced promontory. All of these features provide for greater accommodation of the fetus as it passes through the pelvic cavity during childbirth.

The male pelvis, on the other hand, is significantly heavier and thicker than the female pelvis and is characterized by a heart-shaped inlet with a narrower subpubic angle between the ischiopubic rami as well as conical medially projecting ischial spines that would be more likely to cause obstruction during childbirth. The sacrum and coccyx of the male pelvis tend to be long and narrow with a more prominent sacral promontory when compared to the female pelvic girdle.

So what do you think? Think you can tell a male and female pelvis apart? How did you do? Oh, hey, I think that ambulance has finally arrived! Let's explore some clinical application now by taking a closer look at pelvic fractures.

Pelvic fractures are caused by both high and low impact injuries and are relatively uncommon in adults. However, in the elderly, pelvic fractures can frequently occur from falling from a standing height increasing their level of occurrence. Pelvic fractures may present with pain and swelling, and depending on the location of the fracture and severity of injury, there may also be obvious deformity.

A diagnosis of a fractured pelvic girdle is usually determined by x-ray, and in some cases, CT scan. Pelvic fractures can be described as stable and unstable. In a stable fracture, there is usually only one break and the fracture is typically located along the iliac wing, sacrum, or pubic rami. Unstable fractures are characterized by two or more breaks in the pelvic ring and usually lead to displacement. Unstable pelvic fractures may cause extensive internal bleeding and shock.

Depending on the level of severity, treatment of pelvic fractures may require surgical stabilization and fixation. Non-surgical treatment involves rest, walking aids to reduce load force on the pelvic girdle, and medications to manage pain and inflammation.

We have reached the end of the tutorial on the anatomy of the pelvis. Let's just go over a quick summary to remind us of what we learned today.

First we looked at the hip bone and its bony landmarks. We saw that each hip bone is actually composed of three parts that fuse together towards the end of puberty including the ilium, the ischium, and the pubic bone. We examined each bone individually beginning with the ilium. We identified that the ilium comprises the superior portion of the acetabulum and is located superior to the hip joint. We explored the bony landmarks of the ilium and its importance to the pelvic girdle.

Next we explored the pubic bone which is also known as the pubis. This bone forms the anteroinferior third of the acetabulum and articulates with its counterpart at the pubic symphysis. We explored the bony landmarks and joints of this bone of the pelvic girdle.

Finally, we examined the ischium – the final component of the hip bone. This bone can be located at the posteroinferior aspect of the hip joint making up the posteroinferior third of the acetabulum. We explored its associated bony landmarks and functions before moving on to examine the posterior aspect of the pelvis.

We briefly examined the bones of the pelvic spine, the sacrum, and the coccyx before moving on to explore the skeletal pelvis as a whole. We learnt that the pelvis can be divided into greater and lesser segments by the pelvic inlet. The lesser pelvis or true pelvis contains the pelvic cavity, a space that is delimited by the pelvic inlet and the pelvic outlet.

Next we had a quick look at the differences between the male and female pelvis and identified how they can differ based on the size, shape, and thickness of the pelvis as well as proportional differences between certain parts and landmarks. Finally, we took some time to learn about the cause, diagnosis, and management of pelvic fractures.

That brings us to the end of this tutorial. I hope you enjoyed learning about the pelvis. See you next time and happy studying!