Anatomy of the pelvic girdle

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Introduction0:00–0:37

The pelvic girdle, or the bony pelvis is a bony ring formed by the left and right hip bones and the sacrum. And it surrounds the pelvic cavity and connects the vertebral column to the lower limbs.
The main function of the pelvic girdle are to transfer the weight of the upper body to the lower limbs when sitting or standing and provide attachment points for muscles that help with locomotion and posture.
It also provides support and protection for the abdominal pelvic structures. So let's start with the hip bones.

Hip bones0:37–0:57

The right and left hip bones are irregular shaped bones located at the sides of the pelvic cavity which bound and form the lateral walls of the pelvis.
Each one of these hip bones developed from the fusion of three bones the ileum, ischium, and pubis. Now let's start with the upper part of the hip bone, the ileum.

Ilium0:57–4:09

You can think of the ileum as a fan. It has an upper expanded part representing the spread of the fan, called the ala, and the lower narrow part representing the handle of the fan called the body.
Now, there are 4 bumps coming off the ileum called spines. Let's talk about them.
If we look anteriorly to the ileum, we can see two of the spines. One of them is superior and the other one is inferior.
The naming is easy because it follows the rule of anterior or posterior plus superior or inferior. The superior one is called the anterior superior iliac spine, and the inferior one is called, you guessed it, the anterior inferior iliac spine.
Now, looking at the ileum from a posterior view, we can see the remaining two spines, one superior and, again, one inferior.
If we follow the rule just mentioned, we can name them easily. The superior one is called the posterior superior iliac spine.
And the inferior one is called, wait for it, posterior inferior iliac spine. The main function of the spines is to provide attachment points for various muscles.
For example, the longest muscle in the body is called the sartorius, and it attaches to the anterior superior iliac spine.
Now, let's move to the next part of the ileum, the iliac crest, which is the rough, curved uppermost part of the ileum, representing the rim of the fan.
The iliac crest runs between the anterior superior iliac spine and the posterior superior iliac spine. Now, when looking at the anterior medial surface of the ileum, we can see a fossa called the iliac fossa.
This fossa is covered by a muscle called the iliacus. From a posterior medial view, we can see the auricular surface of the ileum.
Right and left auricular surfaces of both ilia articulate with the sacrum, forming two sacroiliac joints. Above the auricular surface there is an even rougher bit of bone called the iliac tuberosity which has synovial and syndosmotic articulation with the reciprocal surface of the sacrum.
Posteriorly, the ileum presents the gluteal surface. This surface is covered by three muscles called gluteus muscles.
These muscles originate from lines on the gluteal surface of the ileum called posterior, anterior, and inferior gluteal lines.
The gluteus minimus originates between the anterior and inferior gluteal lines. The gluteus medius originates between the anterior and posterior gluteal lines.
And finally, the posterior gluteal line forms part of the origin of the gluteus maximus, which is the largest of the gluteal muscles.
Moving on to the next part of the hip bone, the ischium. The ischium has two main parts, a body and a ramus.

Ischium4:09–5:26

The body is the superior expanded part of the ischium that connects to the ileum superiorly and to the pubis anteriorly and helps to form the acetabulum.
The ramus is the lower narrow part of the ischium that connects anteriorly to the lower part of the pubis and helps form part of the oberiator foramen.
On the inferior posterior part of the ischium there's a large roughened area called the ischial tuberosity which supports our weight while sitting.
And where the inferiorgamellus muscle and the hamstrings attach at the junction of the body and ramus, there is a pointed projection called the ischial spine where the sacrospinous ligament and superiorgamellus muscle originate.
Below the ischial spine, specifically between it and the ischial tuberosity, there's an arc called the lesser sciatic notch, and above the ischial spine is the greater sciatic notch which is formed by parts of both the ischium and the ileum.
Alright, now let's move on to the 3rd and final part of the hip bone, the pubis. The pubis has 3 parts, a body, superior ramus, and inferior ramus.

Pubis5:26–6:48

The body of the pubic bone is the thickest, most anterior part of the pubic bone. The superior ramus of the pubis connects to the ileum forming the ileop pubic ramus.
The inferior pubic ramus connects to the inferior ramus of the ischium, forming the ischiopubic ramus. The angle at which both of these converge is called the pubic arch.
Next is the pubic crest, which is a thickening on the anterior part of the pubis where the abdominal muscles attach. It has a swelling on its lateral side called the pubic tubercle, which is the distal attachment point of the inguinal ligament.
The pectineal line or pectin pubis is an oblique ridge located on the lateral part of the superior pubic ramus. And it's where the pectinous muscle originates from.
Anteriorly, the bodies of the right and left pubic bones are joined together in the median plane through a fibrocartilage called the interpubic disc.
This connection forms a joint called the pubic symphysis. The pubic symphysis is a secondary cartilaginous joint that is supported superiorly by the superior pubic ligament, which connects the pubic bodies to the interpubic disc.

Pubic symphysis 6:48–7:27

And inferiorly by the inferior pubic ligament which forms the apex of the pubic arch. When the bodies of the two pubic bones meet, they form an angle inferiorly called the subpubic angle.
The pubic symphysis is important in stabilizing the pelvic girdle and transferring weight from the upper body to the lower body.
And it undergoes important changes during childbirth to allow the fetus to pass through the birth canal. Now looking at the hip bone from a lateral view, we can see a deep cup-shaped fossa called the acetabulum.

Acetabulum and obturator foramen 7:27–8:25

This fossa is formed by the body of the ileum, the body of the ischium, and the superior ramus of the pubis. The head of the femur fits into this fossa, forming the hip joint.
And right below the acetabulum, there's the obterator foramen. The obterator foramen is bound by the ramus of the ischium and the inferior ramus of the pubis, and it's partially covered by the obterator membrane.
The small part of the obterator foramen that's not covered by the membrane is called the obterator canal, through which the obterator nerve and obterator blood vessels pass to supply structures in the medial compartment of the thigh.
Also, the obterator membrane gives origin to two muscles called the obterator internus and externus muscles. Now, let's take a quick break, pause the video and see if you can recall some of the most important features of the ileum, ischium, and pubis.

Quiz8:25–8:48

All right, now let's switch gears and talk about the sacrum. The sacrum is a wedge-shaped bone composed of five fused sacral vertebrae.

Sacrum8:48–9:44

The sacrum forms the posterior superior wall of the pelvic cavity, and it has a superior base and an inferior apex. The base of the sacrum is formed by the superior surface of the S1 vertebra.
Now, in its uppermost portion, the S1 vertebra has an anterior lip called the sacral promontory, which arches a bit towards the front, and on both sides, the sacral vertebrae extend laterally to form the sacral ali.
The apex of the sacrum, its tapering inferior end, articulates inferiorly with the coccyx, forming the sacrococcygeal joint.
This joint is a secondary cartilaginous joint and is supported by the anterior and posterior sacrococcygeal ligaments. All right, now the sacroiliac joints connect the sacrum to the hip bones, and they actually consist of two joints.

Sacro-iliac joint9:44–10:20

An anterior synovial joint which is formed by the articulation between the sacrum and the auricular surfaces of the ileum.
And a posterior syndosmosis joint formed by the articulation between the tuberosities found on the posterior surfaces of the ileum and the sacrum.
The synovial portion is unlike other synovial joints as it has very limited mobility, but that's because their main role is to transmit the weight of the body into the hip bones.
The sacroiliac joint is supported by multiple ligaments, including the anterior and posterior sacroiliac ligaments, which is synonymous with the fibrous synovial capsule.

Ligaments of the sacroiliac joint10:20–12:06

And they support the joint anteriorly and posteriorly. A third sacroiliac ligament, the interosseous sacroiliac ligament, is a primary structure involved in transferring weight and lies between the sacral and iliac tuberosities in the anterior synovial joint.
There's also the sacro tuberous ligament which goes from the posterior surface of the ileum and the lateral surfaces of the sacrum and the coccyx to the ischial tuberosity.
And that's why it's called the sacro, referring to the sacrum, and tuberous, referring to the ischial tuberosity ligament.
See, a lot of these ligaments are named based on their origin and insertion points, making them easier to remember. A similar ligament called the sacrospinous ligament passes from the lateral surface of the sacrum and the coccyx to the ischial spine.
These two ligaments convert the greater and lesser sciatic notches into the greater and lesser sciatic foramina as shown here.
There are multiple important structures passing through these two foramina. For example, the sciatic nerve passes through the greater sciatic foramen to enter the posterior thigh.
This nerve and its branches supply structures in the back of the thigh and the leg. Finally, the iolumbar ligament binds the 5th lumbar vertebrae to the iliac crest of the ileum.
The sacro tuberous, sacrospinous, and iolumbar ligaments are called the vertebro pelvic ligaments because they bind parts of the vertebral column to the hip bones.
All right, I know that was a lot. Take a deep breath.

Pelvis12:06–13:11

And let's move on to the pelvis itself. The pelvis has a superior opening called the pelvic inlet and an inferior opening called the pelvic outlet.
Both the sacrum and the hip bones participate in forming these openings. The bony edge defining the pelvic inlet is called the pelvic brim.
Posteriorly there is a sacral promontory and the aa of the sacrum. Laterally there's a linea terminalis which is composed of the arcuate line of the ileum and pectin pubis.
Anteriorly, the pelvic brim is formed by the pubic symphysis. On the other hand, the pelvic outlet is bounded by the tip of the coccyx posteriorly, the inferior margin of the sacred tuberous ligament posterior laterally, and ischial tuberosities laterally.
And the pubic arch anteriorly. Now the pelvic inlet divides the pelvic cavity into an upper larger area called the greater or false pelvis and a smaller lower area called the lesser or true pelvis.

True and false pelvis13:11–13:54

Each part of the pelvis has its own boundaries. Let's start with the greater or false pelvis.
The greater pelvis is superior to the pelvic inlet and defined by the aa of the ileum posterior laterally and by the first sacral vertebra posteriorly.
It contains the ileum and the sigmoid colon. The lesser pelvis is between the pelvic inlet and outlet and is bounded by the hip bones, sacrum, and coccyx and contains organs of the urinary and genital systems.
Now in anatomical position, the pelvis is tilted in the anterior posterior plane in a way that makes the right and left anterior superior iliac spines in the same plane as the pubic symphysis.

Anatomical position of the pelvis13:54–14:46

This anterior pelvic tilt makes the pubic bones appear to be located inferiorly instead of anteriorly, making the pubic bones and pubic symphysis the main weight bearing contributors.
This also means the pelvic inlet is more vertical than horizontal. Also, this orientation of the pelvis makes the superior surface of the sacrum visible when looking at the pelvis from an anterior view.
Furthermore, the acetabulum faces inferior laterally and articulates with the head of the femur, and the operator foramen is inferior medial to the acetabulum.
Finally, there are differences between male and female pelvis. These differences are mainly due to the heavier build and larger muscles of men compared with the female pelvis, which is adapted for childbirth.

Differences in male and female pelves14:46–15:58

The male pelvis is thick and heavy compared to the female pelvis, which is thin and light. The lesser or true pelvis is narrow and deep in males, but wide and shallow in females.
Also, in males, the pelvic inlet is heart shaped, while in females it's oval shaped. The pelvic outlet is comparatively small in males but large in females to give the fetus enough room to come out of the pelvis.
Finally, there's a subpubic angle which is below 70 degrees in males but is more than 80 degrees in females. But these differences are not solely the interest of obstetrics and gynecology specialists.
Other medical specialties such as forensic medicine use these facts to determine the gender of skeletal remains. However, remember that even though there are some differences between the male and female pelvis, the pelvis of any person may have some features of the opposite sex.
All right, as a quick recap, the pelvic girdle is composed of 3 bones, the 2 hip bones and the sacrum, and it connects the vertebral column to the lower limbs.

Review15:58–17:15

Each hip bone is composed of three parts the ileum, ischium, and pubis. Also, the two hip bones are connected posteriorly to the sacrum and anteriorly to each other through the pubic symphysis.
The sacrum is connected to the ilia through the sacroiliac joints. These joints are supported by the sacrospinous and sacro tuberous ligaments which help form the boundaries of the greater and lesser sciatic foramina.
The pelvic cavity has a superior entrance called the pelvic inlet and an inferior exit called the pelvic outlet. The pelvic inlet splits the pelvis into the greater and lesser pelvis.
Now in an anatomical position, the pelvic girdle is tilted in a way that brings the anterior superior iliac spine and the pubic symphysis line in the same plane.
Finally, the male and female pelvis are different because males have a larger muscle mass, so their pelvis is thicker and heavier, while the female pelvis is adapted to childbirth.
So has a more spacious, lesser pelvis and a larger pelvic outlet.