The os coxae, also known as the hip bone or innominate bone, is a large, irregularly shaped bone forming part of the pelvic girdle. It connects the axial skeleton (spine) to the lower appendicular skeleton (legs). It supports locomotion and posture, providing a foundation for the upper body. Before adulthood, it consists of three distinct components.
The Three Constituent Bones
The os coxae is formed from the fusion of three individual bones: the ilium, the ischium, and the pubis. In early life, they remain separate, joined by cartilage before gradually uniting. Each bone contributes a specific region and features to the hip bone.
The ilium is the largest and most superior, characterized by its broad, fan-like shape. It forms the upper section of the hip bone, with its curved superior border known as the iliac crest. It also contributes to the acetabulum, a cup-shaped socket for the femur.
The ischium is the lower, posterior part of the hip bone. Its notable feature is the ischial tuberosity, a roughened prominence often called the “sit bone” because it bears body weight when seated. It also contributes to the acetabulum and the obturator foramen.
The pubis is the smallest and most anterior of the three bones. It consists of a body and two rami (branches). The body unites with the corresponding pubic bone at the pubic symphysis, forming the anterior pelvis. This bone also contributes to the acetabulum and the obturator foramen.
How These Bones Fuse
During childhood and adolescence, the ilium, ischium, and pubis gradually unite to form a single bone. This fusion centers around the acetabulum, the hip joint socket, where the three bones meet. In younger individuals, these bones are joined by a Y-shaped cartilaginous growth plate called the triradiate cartilage.
Ossification, bone formation, occurs within this cartilage, progressively replacing it with bone. Fusion typically begins around puberty, completing by the late teens or early twenties (15-25 years). The triradiate cartilage is important for the development and shaping of the acetabulum, allowing continuous hip socket growth. Once fused, the single os coxae provides increased stability and strength to the pelvic region.
Overall Importance of the Os Coxae
The fully formed os coxae serves multiple functions. It connects the vertebral column (axial skeleton) to the lower limbs, transferring body weight from the upper body to the legs. It also supports the upper body, especially when sitting or standing.
Beyond weight bearing and locomotion, the os coxae forms a protective basin for pelvic organs, including digestive, urinary, and reproductive systems. Its surfaces and projections serve as attachment points for muscles and ligaments. These attachments are important for movements of the trunk and lower limbs, contributing to overall stability and mobility.