SI JOINT OVERVIEW

Meet the SI
The sacroiliac joints (abbreviated SIJs or SI joints) are found where the spine (sacrum) meets the pelvis (ilium). The joint itself is a boomerang-shaped meeting of two bony surfaces, each covered in cartilage. The bones of the SI are supported with a strong network of ligaments. This structure allows very little movement compared to more familar joints. Though it has a very small range of motion, it plays an important role in almost every physical activity of a functional adult.

Role of the SI
As shown in the illustration, all the forces between the ground and the upper body are transmitted through the SI joints. Movement in the SI coordinates with movement in the spine to distribute these forces across the trunk and abdomen. It is believed that the SI plays a critical role in shock absorption and force distribution.

A Unique Joint
Though it usually doesn’t draw attention to itself, the sacroiliac joint develops with a person throughout their lifetime. In children, the SI joint surfaces are smooth and glistening. In adults, the joint surfaces acquire a bumpy, interlocking pattern, unique to each individual. In men, the joint becomes less and less flexible into adulthood. Elderly men often have developed a complete bridging of the joint, where it fuses into a solid structure. In women, the joint remains more flexible, especially during pregnancy as the hormone relaxin loosens the joints of the pelvis to allow delivery.

These changes , while normal, can contribute to SI joint problems in the future.