The os coxa (also known as the innominate) is a paired bone which articulates with the sacrum to form the pelvis. The os coxae articulate with one another ventrally at the pubic symphisis. The pelvis anchors the leg to the axial skeleton though the articulation between the os coxa and the head of the femur. The articulated pelvis is a bowl shaped structure which cradles abdominal viscera.
Below are complete specimens of the os coxa. More examples of the bones of the Pelvis are available at the Digital Teaching Collection.
The ilium is the superior blade-shaped portion of the os coxa. The ilium is divided into the body and the ala (meaning wing) by the arcuate line. The superior border is termed the iliac crest. In superior view, this crest forms an S-shape as it curves around the iliac fossa. Four named spines decorate the ilium: the anterior-superior iliac spine, the anterior-inferior iliac spine, the posterior-superior iliac spine, and the posterior-inferior iliac spine. These spines provide attachment sites for muscles and ligaments of the hip. The greater sciatic notch is located just inferior to the posterior inferior iliac spine. This notch transmits several nerves for the lower leg. The rough, ear-shaped sacral articular surface of the ilium is termed the auricular surface. Just superior to the auricular surface is a large and irregular iliac tuberosity. Several roughened gluteal lines are visible on the lateral surface of the blade for the attachment of the gluteal muscles.
The ischium is the inferior portion of the of the os coxa. The body of the ischium begins at the dorsally projecting ischial spine, which forms the posterior border of the greater sciatic notch. The body projects inferiorly to end in a robust ischial tuberosity. This tuberosity is the insertion point for many hip extensors. It is also the portion of the os coxa which bears weight when sitting down. The ramus of the ischium sweeps superior-medially towards the pubic symphisis. The obturator foramen is the large hole which is bordered by the ischium and the pubic rami.
The pubis is the ventral portion of the os coxa. It is comprised of the iliopubic ramus, the ischiopubic ramus, and the pubic symphisis. The two rami are named for the portions of the os coxa to which they connect. These rami form the superior and inferior margins of the obturator foramen. The rami converge ventrally, where they meet the pubic symphisis, the roughened surface where the os coxae meet.
Siding the Os Coxa
Complete os coxa are readily sided. The illium is superior, the ischium is inferior, and the pubis is ventral. The acetabulum faces laterally. More fragmentary material can also be sided. Siding highly fragmentary os coxae is best accomplished with comparative material.
Due largely to the demands placed on females during the process of birth, the os coxa is one of the most sexually dimorphic bones in the body. Because females are on average smaller in body size than males, the female os coxa is generally smaller than that of males. However, the female os coxa exhibits several differences compared with males, mostly related to maximizing the dimensions of the birth canal. The superior opening, or aperture, of the articulated pelvis is generally heart-shaped in males, while in females it is larger and more rounded. The female ilium has more laterally flared blades than the male ilium. The greater sciatic notch is much more open in females, while it is more constricted in males.
The morphology of the pubic symphisis has proven to be a relatively reliable age indicator. Several rigorous methods for aging unknown skeletons using unknown skeletons have been developed. As a general overview, the symphisis exhibits a ridged and grooved surface in younger individuals. This surface becomes grooved in somewhat older individuals, and beveled edges become apparent. Older individuals have even more complete beveled edges, the surface looses its ridged texture completely, and the surface takes on a porous appearance.