There are four bony projections in the ilium, which is a bone of the pelvic girdle, with each one functioning as points of attachment for muscles and ligaments; the term iliac spine refers to any four projections of the ilium. The posterior boundary of the ilium extends from the posterior superior iliac spine to the back of the acetabulum. The upper boundary of a deep notch referred to as the greater sciatic notch formed by the lower part of the posterior border of ilium.
The Four protuberances of ilium are:
- The anterior end of the iliac crest protrudes forwards as the anterior superior iliac spine.
- The posterior end of the crest creates a protrusion called the posterior superior iliac spine.
- The lowest pan anterior border has a protrusion known as the anterior inferior iliac spine.
- A few centimeters below the posterior superior iliac spine the posterior border has another protrusion called the posterior inferior iliac spine.
The lateral aspect of the ilium comprises of its gluteal surface. Three ridges called the anterior, posterior and inferior gluteal lines marked this surface.
- The posterior gluteal line extends from the iliac crest above towards the posterior inferior iliac spine below and it is vertically oriented.
- The anterior gluteal line is convex upwards and backwards. While its posterior end reaches the greater sciatic notch, its anterior end connects with the iliac crest in front of the tubercle.
- The inferior gluteal line is horizontal. Its anterior end is located just superior towards the anterior inferior iliac spine; and its posterior end connects with the greater sciatic notch. The gluteal surface of the ilium produces a protuberant indentation just above the acetabulum.
The iliac crest may be subcategorized into a ventral part that is made up of the anterior two thirds of the crest as well as a dorsal part is made up of the posterior one third:
- The ventral segment has a wide intermediate area that is limited by inner and outer margins. Approximately 5cm posterior towards the anterior superior iliac spine the outer margin of the iliac crest is most protuberant. This protrusion is referred to as the tubercle of the iliac crest.
- The dorsal segment of the iliac crest has a rim divides medial and lateral surfaces.