The functional topography of the sacroiliac joint.

This work analyzes the motion permitted by the topography of the sacroiliac joint surface. Gross contour profiles of the joint were recorded in the frontal and sagittal planes. Best-fit axes of rotation for each contour profile were obtained by statistical methods and were compared with those reported in the literature. There was considerable scatter of the locations of the most likely centers, or axes, of rotation in the frontal and sagittal planes. Were motion to occur about these axes, the articular surfaces would first separate by a distance sufficient to allow them to move over one another. Energy would be needed to separate the joint surfaces being pulled together by the surrounding ligaments. Thus, the sacroiliac joint may function as a shock- absorbing structure by virtue of energy absorbed in the ligamentous tissue.