innominatum

The innominatum, also known as the os coxae or hip bone, is a large, flattened, irregular bone that forms the pelvic girdle. In adults, the left and right innominata fuse together at the pubic symphysis anteriorly and articulate with the sacrum posteriorly to form the pelvis. Each innominatum is composed of three distinct bones that are fused together in adults: the ilium, the ischium, and the pubis.

The ilium is the largest and uppermost part of the innominatum, forming the prominent hip bone. It has a broad, wing-like superior portion called the ala, which articulates with the sacrum at the sacroiliac joint. The ischium is the posteroinferior part of the innominatum. It bears the weight of the body when sitting and has a large projection called the ischial tuberosity. The pubis is the anteroinferior part of the innominatum. The three bones meet at the acetabulum, a deep, cup-shaped socket that articulates with the head of the femur to form the hip joint. The innominatum plays a crucial role in weight-bearing, locomotion, and protection of pelvic organs.