To better understand SI joint injury, it helps to understand the anatomy of the lower back and pelvis.

The pelvis supports the spinal column from above. From below, it provides the hip joints for both legs.

The sacrum is part of the pelvis, while the coccyx is the lowest part of the spine. The sacroiliac joint (SI joint) refers to the semi-rigid joint between the sacrum and the ileum or bony pelvis. This joint, like others, is stabilized by ligaments and muscles. Unlike most other joints, it has almost no motion.

Anatomy examples:

• Bones and ligaments of the pelvis
• Sacrum and coccyx
• Sacroiliac joint of the pelvis
• Major blood vessels of the pelvis: