By Elizabeth A. Murray, Mount St. Joseph University
The pelvis is a complicated structure composed of the two hip bones of the appendicular skeleton, as well as the sacrum and coccyx of the axial skeleton. A single hip bone can be called a pelvic bone, but also goes by the Latin name of coxae, and occasionally the term “innominate”, Latin for “no name” or “unnamed”, as it didn’t look like anything in particular. But a single pelvic bone cannot be called a pelvis.
The Three Parts of a Hip Bone
Each hip bone is composed of three major parts: the ilium is the more bladelike upright portion, the ischium is the more inferior and posterior region that we sit on, and the pubis is the more anterior region.
During much of childhood, the ilium, ischium, and pubis are three separate bony elements that are united within the hip socket into a single functional element joined by cartilaginous growth margins. This complexity of each hip bone and its growth region allow the pelvis to grow in height, width, and depth during childhood. Once the cartilage areas ossify to bone, the adult ilium, ischium, and pubis are simply considered the three major regions of each hip bone.
In the pelvis of both kids and adults—the two pubic bones are linked anteriorly at a cartilaginous joint known as the pubic symphysis. This is an amphiarthrotic joint; a pad of fibrocartilage links the two pubic bones, allowing just a bit of movement between the two hip bones.
Hip Bone: Ilium
The ilium of each hip bone is joined to the sacrum at a sacroiliac joint. The two sacroiliac joints are diarthrotic synovial joints in which a capsule contains a joint cavity lined with synovial membrane. The articular cartilage of these joints doesn’t unite each ilium with the sacrum. Rather, a capsule of connective tissue surrounds and unites the adjacent bones, allowing greater freedom of movement in the sacroiliac joints than in the pubic symphysis—but still not nearly as much movement as many other synovial joints.
On the ilium, there’s a feature called the auricular surface. The auricular surface of the ilium is on the medial side of the posterior aspect of the bone. It forms a joint with the corresponding auricular surface of the sacrum. Together, these paired auricular surfaces form each sacroiliac joint, supported by anterior and posterior sacroiliac ligaments. These joints are how the two hip bones and the sacrum form the ring-like structure of the pelvis that protects and houses organs of the lower abdomen and the pelvis.
This article comes directly from content in the video series How We Move: The Gross Anatomy of Motion. Watch it now, on Wondrium.
Hip Bone: Ischium
Inferior to the ilium is the ischium. The bony prominences on which we sit are known as the ischial tuberosities. They are also the major origins for the hamstring muscles of the posterior thigh. The ischial tuberosities are often called the “sit bones” in yoga and other exercise classes. Because they are subject to pressures when sitting, the ischial tuberosities are also sites where those who are bed-bound or use a wheelchair can develop decubitus ulcers, commonly known as bedsores, especially if their positions aren’t periodically adjusted.
The ischium has a pointed posterior projection known as the ischial spine. Above the ischial spine is a feature called the greater sciatic notch, and the lesser sciatic notch is below the ischial spine.
A pair of ligaments, the Sacro tuberous and sacrospinous ligaments, link the sacrum to the ischial tuberosity and the ischial spine, respectively. These ligaments turn the greater and lesser sciatic notches into the greater sciatic foramen and lesser sciatic foramen. These ligaments and the foramina they form are important to the integrity of the pelvis and relate the pelvis to the gluteal region and from there, to the perineum.
Hip Bone: Pubis
Extending anteriorly from the ischial tuberosity is the ramus of the ischium. So, the ischial ramus is a branchlike projection from the anterior surface of the ischial tuberosity that links with the inferior pubic ramus. Anterior to the acetabulum, the body of the pubic bone has both a superior pubic ramus and an inferior pubic ramus.
Together, the superior pubic ramus, body of the pubic bone, inferior pubic ramus, and ramus of the ischium all help to surround and form the largest foramen in the body, the obturator foramen.
Despite its large size in the skeleton, the obturator foramen is nearly completely covered by a membrane of connective tissue that is sandwiched by an obturator internus muscle on its deep side and obturator externus muscle on its superficial side. This leaves only a small portion of this large foramen open for passage of the obturator nerve, artery, and vein.
Some scholars think the large obturator foramen—also present in nonhuman mammals—may be similar to the holes in many dinosaur skulls, functioning to lighten the weight of such a large bone.
The body of the pubic bone has a prominence known as the pubic tubercle that is often more pronounced in biological males. The pubic tubercle and anterior superior iliac spine are the attachments for the structure known as the inguinal ligament. Anatomically, the inguinal ligament is where the torso ends and the lower limb begins.
Common Questions about the Complex Structure of the Pelvis
The pelvis is a complicated structure composed of the two hip bones of the appendicular skeleton, as well as the sacrum and coccyx of the axial skeleton. A single hip bone can be called a pelvic bone, but also goes by the Latin name of coxae, and occasionally the term “innominate“, Latin for “no name” or “unnamed“, as it didn’t look like anything in particular. But a single pelvic bone cannot be called a pelvis.
Each hip bone is composed of three major parts: the ilium is the more bladelike upright portion, the ischium is the more inferior and posterior region that we sit on, and the pubis is the more anterior region.
In the pelvis of both kids and adults—the two pubic bones are linked anteriorly at a cartilaginous joint known as the pubic symphysis. This is an amphiarthrotic joint; a pad of fibrocartilage links the two pubic bones, allowing just a bit of movement between the two hip bones.
