Anatomic and experimental basis for the insertion of a screw at the first sacral vertebra.

The authors present the anatomic and experimental basis of an original technique for screwing at the first sacral level employed in lumbosacral fusion. The anatomic studies were based on specimens from the anatomy museum, frozen sections of the sacrum and CT examinations with three-dimensional reconstruction and assessment of the density of the different structures of S1 in Hounsfield units (HU). The findings were that the ala and lateral portions of S1 contain yellow marrow forming what amounts to a fatty sphere bounded by the cortical bone of the sacroiliac joint, the linea terminalis and the spongy bone of the pedicles and of the body of S1. The experimental study was made by avulsion of sacral screws (system of Cotrel Dubousset), each of 7mm diameter. No screw perforated the sacral cortex. Three directions were tested. The insertion of a screw through the pedicle and body of S1 is advised, with the point of insertion below and lateral to the articular process of S1 and an oblique course forward and inward at an angle of 10 degrees to the sagittal plane. This internal obliquity is limited by the posterior prominence of the iliac ala.