Most of us experience the pelvis, most of the time, as a stable, solid piece of bone. The pelvis doesn't articulate like the spine, and we can't sense its individual bones the way we can see how many ribs make up the ribcage.
But the pelvis is actually a group of different bones, and it has several joints – not just where it is joined to the legs and the lower back, but also within itself.
Bones of the Pelvis
The right and left halves of the pelvis are called the os coxae, or innominate bones. They are shaped a little like an ossified pair of granny panties that has been cut in half down the middle, had a piece the approximate size of a saucer removed from the centre back, and had a hole punched in the bottom front of each half.
The os coxae themselves each consist of three bones. The ilium is the top portion of the pelvis, where hands rest on hips; the ischium is further down, and includes the knobbly sitz-bones, or ischial tuberosities; and the pubis is at the front base of the pelvis.
The sacrum is a fused set of spinal vertebrae that looks a little like a big leaf. It forms the back of the pelvis.
Joints Bordering the Pelvis
The hip socket, or acetabulum, is part of the pelvis (all three coxal bones – ilium, ischium, and pubis – make up the acetabulum); the thighbone, or femur, sits in it to form the hip joint.
Above the sacrum, which forms the centre back of the pelvis, the lower back vertebra called L5 meets the sacrum at three joints – an intervertebral disc articulation and two facet joints.
Joints Within the Pelvis
The three pairs of coxal bones are separate at birth, but by the age of ten, they have become fused together; there is no movement between the ilium, ischium and pubis.
The sacrum, also fused, articulates with the innominates at two joints called the right and left sacroiliac joints, or SIJs. These joints have a small amount of movement.
The innominates are joined at the bottom front of the pelvis to the symphysis pubis, a piece of tough cartilage. The right and left pubic symphysis joints are also called the PSJs. There is very limited movement at these joints, though they become more flexible in women during pregnancy and childbirth.
The sacrum and coccyx (or tailbone) also have a joint, called the sacrococcygeal joint, with a small range of movement. This joint is important in the functioning of the muscles of the pelvic floor and spine.
The Pelvis is a Complex Structure
Though it's still true that the pelvis is generally a stable, solid part of the body, being aware of its articulations and bones can help students of anatomy to understand the muscles and movement of this vital area.
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