Bones of pelvis (bony pelvis)
The bony pelvis is composed of four main elements - paired hip bones, sacrum, and coccyx. The sacrum and coccyx are parts of the axial skeleton, and they are formed by variably fused vertebrae.
Some authors suggest dividing the bony pelvis into two parts - anterior and posterior. The anterior part is made up of both hip bones and is referred to as the pelvic girdle, while the posterior part is known as the pelvic spine and includes the sacrum and coccyx.
- The hip bones are the largest, irregular-shaped bones of the pelvic skeleton. They are composed of three fused bones - ilium, ischium, and pubis.
- The sacrum is a triangular-shaped bone located at the base of the spine. It is formed by usually 5 fused sacral vertebrae.
- The coccyx is also known as the tailbone. It is the final bony segment of the spine. Like the sacrum, it is also composed of fused vertebrae. The coccyx usually consists of 3 to 4 coccygeal vertebrae.
The size and shape of the male and female pelvis differ. The female pelvis is typically larger and broader than the male pelvis. However, the male pelvis appears more massive.
The pelvic inlet is wide and rounded in the female pelvis, while it is more heart-shaped (or oval-shaped) in the male. The true pelvis in females is broader and shallower.
Additionally, the iliac crests of the male pelvis are positioned higher, and that causes the male pelvis to look more narrower and taller. Finally, the angle of the pubic arch is less than 90 degrees in males and more than 90 degrees in females.
Joints of pelvis
Several joints are formed between the bones of the pelvis. Overall, the pelvic bones form the following four joints:
- Sacroiliac joints - paired articulations between the auricular surface of the ilium and the auricular surface of the sacrum; they are the strongest joints in the body; they are transition sites between the pelvis and the spine;
- Sacrococcygeal symphysis - found posteriorly; fibrocartilaginous joint formed between the apex of the sacrum and the base of the coccyx; slightly mobile;
- Pubic symphysis - found anteriorly; cartilaginous joint type located between both bodies of the pubic bones (their symphyseal surfaces) in the anterior midline; usually, there are no movements, except during the pregnancy; the joint provides little mobility during labor when the pelvic diameters increase in size.
All these joints are stabilized and strengthened by several ligaments. To read more about each joint and their ligaments, please visit our article on ligaments of the pelvis.