Innominate Bone Anatomy: Structure and Landmarks

Anatomy of the Innominate Bone

The innominate bone (commonly known as the hip bone) is a flat bone that falls into the irregular bone category because it features spines, ridges, recesses, cavities, and holes. It consists of three primitive bones:

• Ilium
• Ischium
• Pubis

The Posterior Border and Iliac Crest

Taking the area located behind the ischial tuberosity and down the bone as a starting point, we continue upward to find the lesser sciatic notch, which ends in a sharp projection: the ischial spine. This spine marks the beginning of the greater sciatic notch, which is completed by another acute ledge: the posterior inferior iliac spine. These two projections are located in the ilioischial area.

Following upward, there is a small depression called the posterior iliac notch, followed by another projection: the posterior superior iliac spine, which marks the beginning of the iliac crest. The iliac crest goes forward, gradually thickening, and describes an upper convex curve that rises gently and descends abruptly in front.

When viewed from above, the iliac crest is an elongated S-shape. Following its route forward, the end of the iliac crest is marked by the anterior superior iliac spine (ASIS). The ASIS is subcutaneous, palpable, and serves perfectly as an anatomical landmark for anthropometric measurements, in addition to being a muscle insertion point.

The Anterior Border and Pubic Region

In the front of the bone, just below the anterior superior iliac spine, we find the anterior iliac notch, which culminates in the anterior inferior iliac spine. After this spine, we enter the iliopubic area, located below the superior pubic ramus.

The superior pubic ramus is oriented toward the sagittal midplane. The upper face of this branch is traversed longitudinally by a thin, sharp projection called the pectineal crest. At the end of this branch, the body of the pubis is visible, and on the medial side, we find a facet that forms part of the pubic symphysis.

From there, we return to the bottom of the bone, where we first find the inferior pubic ramus. This ramus is continuous, without a defined boundary, with the ischial ramus, which is directed toward an area called the body of the ischium. The lower part of this body is thickened to form the ischial tuberosity, the anatomical landmark where we began this description.