The SI Joint is the connection between the spine and the pelvis. The SI Joint is a true diarthrodial joint. The articular surfaces are ear shaped, containing irregular ridges and depressions. Its concave sacral surface is covered with thick hyaline cartilage and its convex iliac surface lined with thin fibrocartilage.
Biomechanically, the SI Joint’s movement is induced by motion occurring at other locations in the body.
This movement is very small, with less than 4 degrees of rotation and less than 1.6 mm of translation.
While the SI Joint is vulnerable to shear during rotation or translation, compression of the joint allows it to resist shear.
Learn more about the sacroiliac joint at http://si-bone.com/