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The sacrum is a large triangular bone made up of five fused vertebrae below the lumbar
spine. The sacrum is wedged between the two ilium bones of the pelvis. The sacroiliac
joint is the joint between the sacrum and the ilium. This joint transmits forces from
the upper limbs and spine, to the hip joints and lower limbs and vice versa. It also acts
as a shock-absorber. Several muscles influence the movement and the stability of the sacroiliac
joint, either through attachments to the sacrum or the ilium or via fascial or ligamentous
attachments to the strong anterior or posterior sacroiliac joint ligaments. Due to the bony
position and the strong ligaments, the sacroiliac joint is very stable. The joint consists of
a postero-superior ligamentous section (two thirds of the joint) and an antero-inferior
synovial component (one third of the joint). In the synovial portion, the smooth hyaline
cartilage on the sacral side is twice as thick as the fibrocartilage on the iliac side. Degenerative
and inflammatory changes are often seen later on the thicker sacral side. The sacroiliac
joint surfaces develop many irregular ridges and depressions during adolescence. These
surfaces lock into place during the push-off phase in walking to increase joint stability.
The sacroiliac joint only slides 2-4 mm and rotates 1-3 degrees via ligament stretching
during weight-bearing and forward bending. Movements are a combination of sliding, tilting
and rotation. The movements decrease with aging and the joint may undergo a fibrous
fusion in later life. Although normal sacroiliac joint movement is small, it is essential for
normal, pain-free lower back and pelvis function. A loss of this movement is commonly seen in
people with low back and pelvic pain.