Facets Curl Around the Sacrum
The superior portions of the sacral surface face posterolaterally and the inferior portions face anterolaterally [1-4]. Thus, the orientation of the upper and lower articular surfaces denotes a twisting motion. In effect, they curl around the lateral edges of the sacrum forming, in my view, a modified saddle joint. As a pivot point, the axial sacroiliac joint is at the center point where the joint twists from a posterior orientation to an anterior orientation.
The movement pattern of the sacroiliac joint is complicated due to the shape of the sacrum and the orientation of the articular surfaces. This 3D view Illustrates that the sacrum cannot slide linearly along a track composed of the ridge and groove , nor can the sacrum rotate around a transverse axis. Because the posterior facing surface is superior to the anterior facing surface, the sacrum must pivot on an oblique axis , centered at the axial joint.
By curling around the edges of the sacrum, the facets round out the edges of the sacrum, transforming the wedge shape into a cone shape at the articular regions. Since the joint surfaces determine range of motion, the anterior and posterior sacral surfaces may be relatively flat and the sacrum may still function as a cone. For a demonstration video, please see Sacrum Compared to a Cone
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