You have two SI jointsone on either side of thesacrum.
These two hip bones (ilium, as mentioned above) are technically called the innominates of the pelvis.
The back of the SI joint is not as moveable as the front.
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The sacrum is a triangularly shaped bone located just below thelumbar spine.
On either side, the ilium provides a kind of buttressing support to the sacrum.
The sacrum locks in between the two ilia (or innominates) by acting as a wedge.
The bones are further held in place by ligaments.
In front, the anterior ligament is really just a thickened extension of the capsule surrounding the SI joint.
The anterior ligament is small, especially in comparison to theligaments in the back.
More simply put, the joint is shaped much like an ear or a kidney bean.
In front, it is a synovial joint (discussed above as diarthrodial).
(Your shoulder and hips are synovial jointsthink of the big movements they can make.)
In back, the SI joint is classified as asyndesmosis.
This simply means the two bones (sacrum and ilium) are held together by ligaments.
We talked about those ligaments earlier.
(These are located in front only.)
SI joint integrity is maintained solely by this interlocking mechanism and the ligaments.
As far as muscles go, portions of the gluteus maximus and the piriformis muscles cross the SI joint.
Function
The SI joint bears the weight of the spine.
(This is called the push off phase of gait.)
At this point, there is minimal space between the two bones of the SI joint.
This is called joint congruency or the close pack position of the joint.
2023;243(3):475-485. doi:10.1111/joa.13857
