Session VIII - Pelvis
The Effect of Sacral Malreduction on the Safe Placement of Iliosacral Screws
Mark Cameron Reilly, MD; Christopher M. Bono, MD; Behrang Litkouhi, BS; Michael S. Sirkin, MD; Fred F. Behrens, MD, Orthopaedic Trauma Service, New Jersey Medical School, Newark, NJ
Purpose: To perform a 3-dimensional analysis of the space available for safe iliosacral screw placement in the 1st sacral segment and to determine to what extent malreduction of zone 2 sacral fractures decreases the safety of iliosacral screw placement.
Methods: Six cadaveric pelves were disarticulated from the vertebral column and hips. All soft tissues except for the sacroiliac, sacrospinous and sacrotuberous ligaments were removed. A unilateral zone 2 sacral fracture was created with a microsagittal saw. The specimens were placed in a jig allowing controlled cranial displacement of the hemipelvis through the osteotomy and the symphysis pubis (61-C1.3,a2). Sagittal helical CT scans of the ilium and sacrum were obtained at 1-mm intervals for each specimen with 5, 10, 15 and 20 mm of cranial displacement. The space available for iliosacral screw placement was determined on each image and scanned in to computer-aided-design software. A three-dimensional solid object was defined for each specimen at each magnitude of displacement, and the total volume and percent volume change were calculated. The smallest cross- sectional area of each specimen was determined and defined as the limiting area of each solid. The limiting cross-sectional area was then calculated for each specimen at each magnitude of displacement, and the total number of 7.0-mm cannulated screws that could be simultaneously placed was determined.
Results: The limiting cross sectional area was determined to be at the image containing the center of the S1 foramen in all specimens. This area was decreased by 30%, 56%, 81% and 90% for 5, 10, 15 and 20 mm of displacement. The volume available for iliosacral screw placement was decreased by 18%, 40%, 47% and 51% for 5, 10, 15 and 20 mm of displacement. In 50% of the specimens with 15-mm displacement and 66% of the specimens with 20-mm displacement, it was no longer mathematically possible to insert 2 simultaneous iliosacral screws without exiting the confines of the bony sacrum. In 17% of the specimens with 15-mm displacement and 50% of the specimens with 20-mm displacement, it was no longer mathematically possible to insert a single iliosacral screw.
Discussion: Iliosacral screw placement has become the standard means of internal fixation for transforaminal (Denis II) sacral fractures. Acceptable fracture reduction has been described as being less than 1.5 cm of residual cranial displacement. Although residual displacement leads to leg length inequality and sitting imbalance, it is not thought to compromise fracture healing due to the large opposing surfaces of cancellous bone. Malreduction of transforaminal sacral fractures leads to a substantial decrease in the space available for placement of iliosacral screws as measured by both area and volumetric determinations.
Conclusion: Although this is a preliminary study and more specimens will be measured, at cranial displacements greater than 1 cm, the space available for safe iliosacral screw placement is substantially compromised. At 1.5 cm, several specimens did not have space available for 2 cannulated screws to stay within the bony confines of the sacrum and at 2cm, iliosacral screw placement was frequently not possible. As intraoperative fluoroscopy may underestimate the true magnitude of residual displacement, this anatomic study lends support to the need for accurate reduction of the zone 2 sacral fracture. Fracture malreduction of greater than 1 cm in the cranial direction cannot safely be accepted if iliosacral screws are to be utilized.