Description. The sacroiliac joint (simply called the SI joint) is the joint connection between the spine and the pelvis. It is a large diarthrodial joint made up of the sacrum and the two innominates of the pelvis.Each innominate is formed by the fusion of the three bones of the pelvis: the ilium, ischium, and pubic bone.escription. The sacroiliac joint (simply called the SI joint) is the joint connection between the spine and the pelvis. It is a large diarthrodial joint made up of the sacrum and the two innominates of the pelvis.