The joint itself may be chronically inflamed but, in and of itself, is structurally normal. The clinical significance of the small motion of the SIJ is till subject to debate. In addition, each SIJ is dependent on the other SIJ and the symphysis pubis. Any change in the characteristics of one joint in the pelvic ring will change the characteristics of the two other joints. Stability of the SIJ is both static and dynamic.
Dynamic stability is muscle dependent.
Although no muscle originates from or inserts onto the SIJ, many have fibrous expansions that blend with the SIJ ligament complex. The SIJ ligaments can therefore be affected by muscle activity. Trauma or hormonal changes, such as those occurring in pregnancy, will allow the SIJ ligaments to become lax and the joint to move beyond its normal range, passing beyond its normal congruity to an area of incongruity. The joint is particularly vulnerable to locking when the trunk is bent forward and lateral flexion or rotation is superimposed. The joint can remain locked in a neutral or subluxed position.