Content Reviewers:Viviana Popa, MD
The main functions 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 abdominopelvic structures. So let’s start with the hip bones.
Each one of these hip bones develop from the fusion of three bones, the ilium, ischium, and pubis. Now, let’s start with the upper part of the hip bone, the ilium. You can think of the ilium as a fan.
It has an upper expanded part representing the spread of the fan called the ala, and a lower narrow part representing the handle of the fan called the body.
Now, there are 4 bumps coming off the ilium called spines. Let’s talk about them!
If we look anteriorly to the ilium, 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) + (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 ilium 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.
The iliac crest runs between the anterior superior iliac spine and the posterior superior iliac spine.
Now, when looking at the anteromedial surface of the ilium, we can see a fossa called the iliac fossa. This fossa is covered by a muscle called the iliacus. From a posteromedial view, we can see the auricular surface of the ilium.
Posteriorly, the ilium presents the gluteal surface. This surface is covered by three muscles called gluteus muscles.
Finally, the posterior gluteal line forms part of the origin of the gluteus maximus, which is the largest of the gluteal muscles.
On the infero-posterior part of the ischium, there’s a large, roughened area called the ischial tuberosity which supports our weight while sitting, and where the inferior gemellus muscle and the hamstrings attach.
At the junction of the body and ramus there’s a pointed projection called the ischial spine where the sacrospinous ligament and superior gemellus muscle originate.
Below the ischial spine, specifically between it and the ischial tuberosity, there’s an arch 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 ilium.
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 pecten pubis is an oblique ridge located on the lateral part of the superior pubic ramus, and it’s where the pectineus 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 inter-pubic 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 inter-pubic disc, 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 sub pubic 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. This fossa is formed by the body of the ilium, the body of the ischium, and the superior ramus of the pubis.
The small part of the obturator foramen that is not covered by the membrane is called the obturator canal, through which the obturator nerve and obturator blood vessels pass to supply structures in the medial compartment of the thigh.
Alright, now let’s switch gears and talk about the sacrum.
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