Axial Sacral View

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   "Tongue in Groove" SI Joint. A sign of upper sacral dysmorphism.

   Recessed Sacral Ala secondary to upper sacral dysmorphism may predispose to nerve root injury by giving a false impression that an iliosacral screw is completely intraosseous on lateral sacral xray. Preop CT and well defined inlet views will warn of this possibility.

   "Safe Zones" (green double arrows) indicates the breadth of safe iliosacral screw passage between the sacral foramina and the anterior cortical margin of the sacral ala. The left side of the image (red anterior cortical sacral margin) shows a diminished safe zone seconday to a recessed sacral ala.