Ahhhh, the good ole sacroilliac joints. The ones everybody makes fun of.
The pelvis is comprised of three bones, right and left Illium and the Sacrum. The sacrum is the one in the middle at the end of your spine. That's the bone L5 sits on and of course the rest of your spine.
The illiums attach on each side of the sacrum and connect in the front at the os pubis. Normally it takes quite a trauma to injure the sacroilliacs. I see it with downhill skiing injuries. Also when pregnant the pelvis will loosen up for the delivery. Which is one reason women have more problems than men do in that area.
It's impossible to have one sacroilliac joint out of place without the other one being affected. To determin which way the illium is out of place is easy. Which leg is short? The legs are attached toward the front of the illiums. We measure which way the illium is out by that bump on each side of the sacrum that literally IS the sacroilliac joint. That's called the posterior superior spine (PSS). If it's gone back and down compared to the sacrum that's the posterior side and will draw the leg on that side up because the illiums rotate around the sacrum. Thus the short leg is the posterior illium.
The opposite side will be relatively anterior. The posterior illium is usually the traumatized side. But even though that side has had the trauma the other side will jam up and be a problem also.
To determin sacroilliac correctness measure the legs while in the prone (face down) position. Whichever leg is short is the poterior illium. Very easy to do. Just lay the patient down and look at the heels. It's usually very obvious.
The spine can adapt to as much as an inch and a half leg deficiency. That is; a leg that has been injured and the leg itself is 1 1/2 inches shorter than the other. One of the illiums out of place cannot give you that large of a descrepancy in leg lengths.
Either way it's easy to ascertain which illium is posterior and out of its normal allignment with the sacrum. Whichever leg is short in the prone position is the posterior illium. Then it's just a matter of adjusting accordingly.
If the legs are the same length and you still feel pain and discomfort in the sacroilliac area there are several other things that need to be checked. First; 50 percent of all low back pain, which would include the sacroilliacs, comes for the upper cervical spine. So if there's no leg deficeincy check the neck. Second; the nerve supply to that area of the body comes from the lower thoracic spine. Third there might be an old knee, ankle, or foot injury. Plus, there are other "reflex" factors that come into play when the organs are involved.
Having the correct pelvis allignment is very important to the stability of the entire body. It's the foundation everything above sits on. There are two things us chiropractors consider when correcting structural problems. Of course they are mechanics and nerve supply. The mechanics are from the pelvis up and the nerve supply is from the upper cervical down. Both absolutely necessary to permenently correct any injury. Of course that's simplifying it a great deal but I hope that gives you an idea of things.
The good ole sacroilliacs - literally a pain in the butt.