To explain the true motion pattern of the sacrum and sacroiliac joints, an analogy can be made by using a cone representing the sacrum, two sticks representing the innominates, and strings representing the ligaments.

As the right side of the sacral base goes into nutation, the sacrum moves anteriorly and inferiorly on the right as it rotates to the left along an oblique axis [1]. The sacral apex moves to the left. The right ilium moves posteriorly while the right ischium moves anteriorly.

At the same time, the left side of the sacral base goes into counternutation, the sacrum moves posteriorly and superiorly on the left as it rotates to the left on an oblique axis. The sacral apex moves to the left. The left ilium moves anteriorly while the left ischium moves posteriorly.

These actions reciprocate left and right during normal movement patterns. For a more comprehensive explanation, please see the link Reciprocating Unilateral Motion. However, in the dysfunctional movement pattern of the Sacroiliac Nutation Lesion, the side in lesion will go through less nutation due to compensatory muscular actions. Please see the link The Sacroiliac Nutation Lesion.

When the sacrum reaches its endpoint of range of motion, it is stopped by the restraining ligaments and, by extension, the connective tissue and muscles throughout the trunk, pelvis, and lower extremities, at which time movement is reversed.

You are invited to compare the similar movement patterns of the cone above with the sacrum in the link 3D Sacral Motion.

For this sacral rotation to happen, the ligaments must maintain contact around the body of the cone as it rotates, so they can pull from both the anterior and posterior aspects of the sacrum. By curling around the edges of the sacrum [2-5], the facets round out the edges of the sacrum, transforming the wedge shape into a cone shape at the particular regions. Since the joint surfaces determine the range of motion, the anterior and posterior sacral surfaces may be relatively flat and the sacrum may still function as a cone. Please see the link Facets Curl Around the Sacrum.

With the sacrum, contact is made by the ligaments as they pull the sacrum through its range of motion; this can only be accomplished if the facets curl around the edge of the sacrum. As such, it can be seen that the sacrum does not slide linearly on the ilia, nor does it rotate on a transverse axis relative to the ilia; these movements only appear to occur when visualized two-dimensionally. Instead, the rotation of the sacrum changes its position relative to the ilia as it rotates on an oblique axis from nutation to counternutation, as is apparent when viewed three-dimensionally. This concept is consistent with Gracovetsky’s [6, 7] hypothesis on the role of the spine in transferring motion to the lower extremities. In this video, I suggest a logical step toward describing how the torque described by Gracovetsky is transferred through the sacroiliac joint by articular surfaces that cause the sacrum to move as a cone. When the sacrum reaches its endpoint of range of motion, it is stopped by the restraining ligaments and, by extension, the connective tissue and muscles throughout the trunk, pelvis, and lower extremities, at which time movement is reversed.

The concept that the sacrum functions as a cone is fundamental in understanding how the musculoskeletal system integrates into a functionally complete model. This concept directly counters the current concept that the sacrum is a keystone wedged between the ilia. For more on the difference between the cone and wedge concepts, please see: Keystone Form & Function vs. Sacral Suspension

References:

1. Mitchell, F.L., Jr. and P.K.G. Mitchell, The Muscle Energy Manual. Vol. 3. 1999, East Lansing: MET Press.
2. Solonen, K.A., The sacroiliac joint in the light of anatomical, roentgenological and clinical studies. Acta Orthopaedica Scandinavica. Supplementum, 1957. 27(Suppl 27): p. 1-127.
3. Dijkstra, P.F., A. Vleeming, and R. Stoeckart. Complex Motion Tomography of the Sacroiliac Joint and an Anatomical and Roentgenological Study. in First Intedisciplinary World Congress on Low Back Pain and Its Relation to the Sacroiliac Joint. 1992. San Diego, CA.
4. Snijders, C.J., Transfer of Lumbosacral Load to Iliac Bones and Legs: Part 1 – Biomechanics of Self-Bracing of the Sacroiliac Joints and its Significance for Treatment and Exercise. Clinical Biomechanics, 1993a. 8: p. 285-294.
5. Vleeming, A., et al., The role of the sacroiliac joints in coupling between spine, pelvis, legs and arms., in Movement, Stability, and Low Back Pain, A. Vleeming, et al., Editors. 1997, Churchill Livingstone. p. 53-71.
6. Gracovetsky, S., An hypothesis for the role of the spine in human locomotion: a challenge to current thinking. Journal of Biomedical Engineering, 1985. 7(3): p. 205-16.
7. Gracovetsky, S.A. and S. Iacono, Energy transfers in the spinal engine. Journal of Biomedical Engineering, 1987. 9(2): p. 99-114.