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.

The location of the sacroiliac joint on the outer table of the posterior ilium.

Ten cadaveric sacroiliac joints were cleaned of soft tissue and analyzed to determine the average location of the sacroiliac joint on the outer table of the posterior ilium. The superior and inferior limbs of the joint were characterized according to length and width. The longitudinal axis for each limb was determined and located on the outer table of the posterior ilium. The location of the sacroiliac joint was then characterized with respect to established anatomic landmarks. The average lengths of the superior and inferior limbs were 4.4 and 5.6 cm, respectively. The width of each limb averaged 2.0 cm. The average distance from the longitudinal axis of the superior limb to the posterior superior iliac spine was 5.5 cm. The average longitudinal axis of the inferior limb was 1.2 cm superior to the inferior margin of the posterior inferior iliac spine. The angle between the two axes averaged 93 degrees.