Space Available for Sacroiliac Screws in the S1 Vertebral Body

Abstract

Objectives: To define the maximal sagittal cross sectional dimensions of the S1 vertebral body and quantify the area available for safe placement of transverse sacroiliac (SI) screws. Methods: A laboratory investigation was performed using twelve fresh, frozen, non-preserved cadaveric pelvic specimens (six males, six females). The sacrum was dissected and removed from each pelvis. After the gross height and widths were measured, the sacra were split in the midsagittal plane and the dimensions of the first sacral segment were recorded. Then, starting from the center, the vertebral bodies were sequentially reamed in a medial to lateral direction in increasing one millimeter increments until a cortical breech occurred in the narrowest portion of the ala at the first sacral neural foramina. Using the diameter of the largest reamer, the cross-sectional area of space available for a transverse sacroiliac screw in the S1 body was calculated. Results: The cross-sectional area of space in the sagittal plane, corresponding to a transverse sacroiliac screw trajectory, of the first sacral vertebral body averaged 204 mm2 (range, 153 mm2 to 226 mm2 ), corresponding to a mean maximum reamer diameter of 16.1 mm (SD 1.08). The male and female sacra did not differ significantly with respect to overall size (mean height, 15.6 cm, SD 1.02; mean width 10.7 cm; SD 0.75) and the dimensions of the S1 body (mean height 28.41 mm, SD 2.23, mean depth 27.17 mm, SD 3.69). Conclusions: Our results improve upon our understanding of the surgical anatomic parameters of the upper sacral segment with respect to placement of transverse sacroiliac screw(s) by quantifying the cross-sectional area in the sagittal plane.