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The Sacroiliac Nutation Syndrome, Part 1: Form and Function
Two basic motions
Left figure is counternutation pattern (CS flexion), & inhalation.
Middle figure is neutral.
Right figure is nutation pattern (CS extension), & exhalation.
Two and three dimensional views of SIJ surfaces. 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.
SIJ Ligaments. Ligaments limiting nutation: sacrospinous, sacrotuberous, anterior SIJ ligament, lower two bands of the iliolumbar ligament, interosseous ligament, & short posterior ligaments.
SIJ in weight transfer and as a sensor
The sacroiliac joints hold and transfer the weight of the entire upper body from the sacrum to the hips and legs. They are also important sensors of large force streams between the trunk and legs in which the largest muscles of the body are involved...
Shock Absorption a) As the pelvis rotates posteriorly, the sacrum rotates anteriorly. b) The feet pronate, the fibulae drop, and the knees twist as the lower leg rotates internally more than the upper leg. c) The curves increase as the spine compresses.
Facets wrap around edge of 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...
Nutation and counternutation as it appears in two dimensions
In nutation, the sacral base appears to slide downward and backward. In counternutation, the sacrum appears to slide upward and forward.
The sacrum is 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 ...
Sacral motion in three dimensions
When the sacral base rocks posteriorly on the right, it moves upward on the right, and the apex moves toward the right. At the same time, the sacral base rocks anteriorly and downward on the left.
The gravitational line
The gravitational line passes anterior to the sacral pivot and posterior to the innominate pivot, resulting in anterior rotation of the sacrum and posterior rotation of the ilium. When the force is greater than the restraining ligaments can withstand, they tear, resulting in the nutation lesion.
