Part 1 Illustrations

Bilateral Nutation

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...

Figure 2 – Reciprocating Unilateral Motion

Here is the more common pattern of reciprocating side-to-side motion. When one side is in nutation, the other side is in counternutation.

Figure 3 – 3D Views of the Sacroiliac Joint

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.

Figure 4 – Sacrum as a Hub

"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.)

Figure 5 – Sacral Movement Induced by Innominates

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.)

Figure 6 – Facets Curl Around the Sacrum

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...

Figure 7 – Piriformis Sacrospinous up to D

Pelvic tensegrity in normal motion

Figure 8 – Airplane Pitch Roll & Yaw

The combined movement around these three axis each centered in the bottom of the sacrum, represent sacral motion

Figure 9 – Sacrum Compared to a Cone

The sacrum is compared to a cone, with the articular surfaces wrapping around it. This configuration allows the large anterior-posterior ...

Figure 10 – Sacral Motion in 3 Dimensions

During counternutation, the sacral base pivots posteriorly and superiorly, as the sacral apex moves toward that side. At the same time, nutation occurs contralaterally, as the sacral base pivots anteriorly and inferiorly