What is the SI Joint?
The Sacroiliac (SI) joint is an important joint that connects the sacrum to the ilium bone. The SI joint is a weight-bearing joint that plays a vital role in stabilizing the spine, as well as transferring weight between the upper body and spine to the lower extremities. It is synovial joint that contains fluid to lubricate and reduce friction between the articulating bones that form the joint.
What makes up the SI Joint?
The sacrum is a triangular-shaped bone made up of five fused vertebrae, situated at the base of the spine. The sacrum connects the two ilium (pelvic) bones on each side, forming the two SI joints (one on either side of the body), as well as the pelvic bowl. Sacral movement in the sagittal plane of motion (flexion and extension movements) is referred to as nutation and counternutation. In optimal function, the sacrum moves in opposition to the pelvis during sagittal plane activities such as walking and running.
Coles Notes — Functions of the SI Joint
Transfers weight from upper body to lower body
Connects the sacrum (part of the spine) with the two ilium (pelvic) bones on either side
Plays a role in pregnancy and childbirth by becoming a more lax (mobile) joint
Acts as a shock absorber for the rest of the spine above
Serves as a muscle attachment site for the deep gluteal muscles (such as the piriformis) and some of the pelvic floor muscles