Ilium- posterior superior iliac spine

Medial view (deep surface), right side

Medial view (deep surface), left side

The posterior superior iliac spine is the most posterior point of the iliac crest. This is at the level of the spinous process of sacral vertebra 2, and it marks the inferior extent of the subarachnoid space. This also marks the middle of the sacroiliac joint. It is normally two to three fingers' width lateral to the midline. It is of medical significance in that one centimeter inferolateral to this is where a physician would insert a needle to obtain material for a bone marrow biopsy. Such a sample could be used to determine if the person had leukemia or other blood disorders. This landmark is recognizable as a "dimple" on the posterior side of the pelvis. I have been told by a number of female students that is particularly easily seen on Mel Gibson in a scene from "Lethal Weapon". Wow.