So how much and how does the sacroiliac joint move? Well, we can’t look at the joint in isolation we need to look at the whole pelvis and that means also looking at the pubic symphysis which is the joint at the front. After all it is a closed ring so if one joint moves then it must be compensated for elsewhere.
A study looking at people in the flamingo stance showed that one side of the pubic symphysis moved 1.4 mm relative to the other in men, 1.6mm in nulliparous women and 3.1mm in multiparous women.
One of the newer developments in research has been the use of Roentgen stereogrammetry. This uses tiny metal balls embedded in the bone either side of the joint and 2 x-ray devices to create a stereo image that can be used to calculate movement of the markers relative to each other in all planes. This is a more accurate method than others which have been prone to error. The sacroiliac joints do move…but not very much at all. The main movement is the nutation (nodding forward) of the sacrum between the ilia and that is typically of the order of a couple of degrees (although it does vary according to position) and is in fact 40% less in men than in women.
Kibsgård et al looked at women with confirmed SIJ mediated pelvic girdle pain and used tiny metal balls inserted into the bones either side of the joints to accurately measure movement of one side relative to the other. They found very little sacroiliac joint movement despite significant movement of one side of the pubic symphysis compared to the other.
So, what is happening? How can the pubic symphysis move significantly without the sacroiliac joint following suit? Kibsgård et al suggested that this unexpected finding was likely to be either down to setup error or plasticity of the bones of the innominate bones, the latter of which would concur with a study by Paul Goudzwaard et al who showed that the innominate bones could deform up to 3.5mm whilst the pubic symphysis moves mainly in the transverse plane (rather than rotating).
One final piece of the puzzle, Adhia et al discovered that although there wasn’t significant difference in the degree of SIJ movement there was increased asymmetry of iliac rotational motion in patients with SIJ pain. So they didn’t rotate more, or less but they moved differently.
To sum up, the studies using the most accurate techniques currently available indicate that the sacroiliac joints don’t usually move very much and that sacroiliac pain and reduced movement don’t appear to be linked but there is a difference in how the ilia move in affected patients and it seems that the innominates are deformable to account for pubic symphysis movement