The pelvis is a bony structure that can be found in both male and female skeletons. The exception with this compound structure when compared to all other bones is that it has differences that are classified by sex, both for functional and general developmental reasons. The rest of the human skeleton differs only in size, which is genetically determined and is usually slightly larger in males than in females. The structure of the pelvis is designed to give females the ability to undergo pregnancy and childbirth, while males are able to hold larger and heavier muscles upon their frame.
Therefore it is heavier in men and has more muscle attachments, a narrower pubic arch, subpubic angle and space between the ischial tuberosities which in turn makes the pelvic outlet smaller. The ilia in women are comparatively more flared than in men which makes their greater pelvis more shallow. The shape of the pelvic inlet and the obturator foramen is oval in women and heart shaped and round in men respectively. In general, the pelvis is broader in women so that there is ample space for the fetus to exit its mother's body.
The pelvis is formed by four bones which include a pair of hip bones otherwise known as innominate bones, the sacrum, which comes with the five lower sacral bones that are fused together and the coccyx which has four fused and a single individual terminal vertebra. The pelvic girdle consists of the hip bones and the sacrum and its function is to transmit the weight from the upper body to the lower limbs, while allowing the body to stay balanced. Meanwhile the ilium, the ischium and the pubis fuse together at puberty to form the innominate bones and are joined by the cartilage found in the acetabulum.
The pelvis as a compound structure contains four different joints, one of which is paired. The lumbosacral joints are compound joints that are stabilised by the iliolumbar ligaments and are made up of two posterior zygapophyseal joints and an intervertebral joint via an intervertebral disc between L4 and S1.
The sacrococcygeal joint is a secondary cartilaginous joint that connects the sacrum to the coccyx. It is reinforced via the anterior and posterior sacrococcygeal ligaments.
The sacroiliac joint is an atypical synovial joint which contains fibrocartilage and has a very limited range of movement. Its articulatory surfaces are between the sacrum and the ilium. It is stabilised by the interosseous ligaments and by the anterior and posterior sacroiliac ligaments.
Lastly, the pubic symphysis is the joint that unites the pubic bones in a secondary cartilaginous articulation. It contains a fibrocartilaginous interpubic disc and is reinforced by the superior and inferior pubic ligaments.
The two main ligaments of the pelvis are the sacrotuberous and sacrospinous ligaments, which enclose the greater and lesser sciatic notches, forming the greater and lesser sciatic foramina. Due to natural forces, when in an orthostatic or upright stance, the weight of the upper body which is relayed via the spine rotates the sacrum and tips it backwards. However, this movement is prevented by the pelvic ligaments.
The sacrotuberous ligament extends from the dorsum of the lateral border of the sacrum as well as the posterior surface of the ilium to the ischial tuberosity and is the larger of the two ligaments. The sacrospinous ligament stretches between the lateral border of the sacrum to the ischial spine.
These foramina are created by the positioning of bony notches and ligaments. The greater sciatic foramen is made up of the greater sciatic notch and the fibers of the sacrospinous and sacrotuberous ligaments. It contains:
- the piriformis muscle
- the sciatic nerve
- the inferior gluteal nerve and artery
- the inferior pudendal nerve, artery and vein
- the nerve that supplies the obturator internus muscle
- the nerve to the quadratus femoris
- the posterior cutaneous nerve of the thigh
The lesser sciatic foramen is made up of the same ligaments as the greater foramina, however they come into contact with the bony lesser sciatic notch. It contains:
- the tendon of the obturator internus
- the nerve of the obturator internus
- the pudendal nerve
- the internal pudendal artery