Anatomic consideration in the anterior approach to the sacro-iliac joint.

Anatomical study of L4 nerve branch, L5 nerve root, and bony stock in the sacroiliac region was performed in cadaveric specimens to provide anatomical references for anterior fixation of the sacroiliac joint. At 1 cm intervals along the sacro-iliac joint, the distance to the lateral border of the L4 nerve branch (to the lumbosacral trunk) and the L5 nerve root was measured. Computed tomography scans and cadaveric sectioning of the sacro-iliac region were performed to evaluate the bony stock available for fixation and the optimal screw direction to prevent neurologic damage. At 4 cm above the pelvic brim, the L4 nerve branch was a mean distance of 23 mm medial to the sacro-iliac joint, whereas the L5 nerve root was 26 mm. At the pelvic brim, those distances decreased to 10 mm for both structures. Four centimeters cephalad to the pelvic brim, the AP dimension of Sl complex was decreased. Screws that were 1 cm medial to the Sl joint and directed posteriorly and parallel to the joint did not endanger neurologic structures. At four centimeters above the pelvic brim, there is less danger of neurologic injury; however, a decrease in sacral bony stock may affect fixation.