Hip bone

At a glance

English

Hip bone

Latin (TA2)

Os coxae

System

Bone

Region

Pelvis

Side

Paired, left and right

Muscles on it

External abdominal oblique and Internal abdominal oblique

About the hip bone

Each hip bone is a broad, irregular plate that is narrow at its middle and flares out at its upper and lower ends. Also called the innominate or coxal bone, it forms one side of the bony pelvis. At the front the right and left bones meet at the pubic symphysis; at the back each joins the sacrum at the sacroiliac joint. Along with the sacrum and coccyx, they make up the pelvic girdle, the ring that encloses the pelvic cavity and links the lower limb to the axial skeleton.

In a child the bone exists as three separate pieces held together by hyaline cartilage: the ilium above, the ischium below and behind, and the pubis below and in front. All three meet at a Y-shaped plate of cartilage in the floor of the acetabulum, the deep cup on the outer surface that receives the head of the femur. The pieces unite around puberty, and the last secondary centres, such as those for the iliac crest and ischial tuberosity, have fused by the early to mid twenties.

The ilium is the largest part. Its wing is capped by the iliac crest, an S-shaped rim that can be felt under the skin along the waist, and its body meets the sacrum. The ischium is the sturdiest part and carries the ischial tuberosity, the bone you sit on; the gluteus maximus covers it when standing but slides off it when seated. The pubis has a flat body that meets its partner at the symphysis across a fibrocartilage disc, and two rami that, with the ischium, frame the obturator foramen. A continuous ridge, the pelvic brim, separates the false pelvis above from the true pelvis below.

Many muscles anchor here: the abdominal wall muscles on the crest and pubis, all three gluteal muscles on the outer ilium, the deep lateral rotators of the hip around the sciatic notch and obturator foramen, the hamstrings on the ischial tuberosity, and the sartorius and rectus femoris on the iliac spines at the front. A break in this bone is a pelvic fracture, which is a different injury from a so-called hip fracture of the upper femur.

Function and supply

Function

Carries the weight of the trunk to the lower limbs through the sacroiliac and hip joints, supports the pelvic organs and gives attachment to the muscles of the trunk, hip and thigh.