Part 1 Illustrations
Nutation is anterior/inferior movement of the sacrum with relative posterior/inferior movement of the ilia. As the ilia move backwards, the pelvis narrows and the ilia approach each other posteriorly...
Here is the more common pattern of reciprocating side-to-side motion. When one side is in nutation, the other side is in counternutation.
The articular part appears to be "L" shaped. Weight bearing occurs at the posterior syndesmosis, which is 1.57 times as large as the synovial part.
"The sacrum is viewed as the hub of a tension network: the model provides for stability of the sacrum when it is loaded from any direction and can be used during bipedal or quadrupedal standing and walking or even standing on one's head." (With permission from Stephen M. Levin, M.D.)
Levin accounts for movement of the hub (sacrum) within the wheel (pelvis) by a twisting action of the rim (effect of walking). "Twisting the rim of a bicycle wheel forces the hub to tilt". (With permission from Stephen M. Levin, M.D.)
When horizontal lines are drawn from both sides of the upper articular surfaces of the SIJ, they would converge posterior to the sacrum, and when horizontal lines are drawn from both sides of the lower...
The combined movement around these three axis each centered in the bottom of the sacrum, represent sacral motion
The sacrum is compared to a cone, with the articular surfaces wrapping around it. This configuration allows the large anterior-posterior ...