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Os coxae

Other Terms: Innominate bone, Coxal bone, Pelvis, Bone of pelvic girdle, Pelvic bone, Os coxae, Os coxal, Hueso de la cadera




These elements are the ilium, ischium, and pubis. In more primitive vertebrates these three elements are distinguished as three distinct bones. Over phylogenetic time they fuse to form the single os coxa. The junction of the bones occurs in the joint socket, the acetabulum. The united os coxae and sacrum form the pelvic girdle, uniting the posterior limbs to the axial skeleton. In general, the bone is composed two plates of compact bone with an internal core of spongy bone. In some thin areas of the bone, the inner spongy bone is absent and there exists only a thin plate of compact bone. In other areas where the bone is thick, dense compact buttresses are formed for additional strength. The details of the bone will be described with each of its elemental parts.


This term is from the Latin os meaning bone and coxa an old Latin term which came down from the Sanskrit word kaksha meaning hip. When Galen described the hip bone he named its three parts but never gave the entire fused element a name. Vesalius, who followed Galen noted this and referred to it as the unnamed bone, the os innominatum. Vesalius, however, referred to it in his work as the os coxa.


The os coxa articulates with three bones: the femur, sacrum, and opposite os coxa. The bowl-like acetabulum articulates with the rounded head of the femur. The medially directed, rough surfaces on the posterior portion of the ilium articulate with the sacrum by forming rough syndesmotic articulations and a synovial surface, the auricular surface of the ilium. Anteriorly, the adjoining medial faces of the pubic bones form the pubic symphysis.


The os coxa ossifies from three primary centers corresponding to each of the elements - ilium, ischium and pubis. The first center arises in the ilium during the eighth prenatal week. This center appears just cranial to what will become the greater sciatic notch. During the fourth month a second center arises in the body of the ischium. Soon after, during the fourth or fifth month, a final center appears in the in the superior ramus of the pubis. At birth a considerable amount of cartilage is yet present, separating the boney centers. Around the seventh year the pubis and ischium join to form an ossified ramus inferior to the obturator foramen. At puberty secondary centers appear in the iliac crest, acetabulum, and ischial tuberosity. These secondary centers fuse with the primary centers anywhere between the fifteenth and twenty-fifth years.


Os coxae


Os coxal


Hueso de la cadera


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