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. Malposition of the SIJ leads to imbalance in the muscles, ligaments, and fascia.
Muscles try to substitute for incompetent ligaments but are usually unsuccessful.
The capsule becomes lax, leading to joint inflammation. The joint itself will not show degenerative changes until late in the process. Due to chronically abnormal spine, pelvis, and lower extremity mechanics, a painful disabling condition ensues. By fixing the joint in its normal position, the normal balance is restored. It should be noted that SIJ subluxation is a clinical observation that has never been proven.