Session I - Basic Science

The Effect of Sacral Fracture Malreduction on the Initial Strength of Iliosacral Screw Fixation for Transforaminal Sacral Fractures

Kathleen S. Beebe, MD¹; Mark C. Reilly, MD¹; Regis L. Renard¹; Chris T. Sabitino¹; Michael S. Sirkin, MD¹; Michael D. Stover, MD²; Fred F. Behrens, MD¹;

¹UMDNJ- New Jersey Medical School, Newark, New Jersey; USA;

²Loyola University Medical Center, Maywood, Illinois, USA (AO Grant)

Purpose: The definition of acceptable displacement of sacral fractures is not agreed upon. It has been previously demonstrated that the area of fracture interdigitation is significantly affected by sacral fracture malreduction. It is our hypothesis that internal fixation of sacral fractures in a malreduced position will lead to a decrease in the strength of iliosacral screw fixation due to the limitation of fracture bony contact.

Methods: Six cadaveric pelves were cleared of all extraneous soft tissues except for the ligamentous structures stabilizing the sacroiliac joint. Dual energy x-ray absorptiometry (DEXA) was performed on all pelves, which were then divided into two groups of roughly equivalent bone density. The intact stiffness of each pelvis was determined by using a MTS servo hydraulic testing machine (MTS Corp., Eden Prairie, Minnesota). Osteotomies were performed on one side of each pelvis through the sacral foramina, with separation of the pubic symphysis to simulate a transforaminal sacral fracture (type 61-C1.3, c4 [a2]). Fixation with two partially threaded 7.3 cannulated screws was performed with three pelves reduced anatomically and three pelves malreduced cranially by 1 cm. The Medivision Navigation System (Synthes, Paoli, Pennsylvania) was used to ensure uniform screw placement and depth insertion. Sagittal CT scanning was performed to allow for calculation of the cross-sectional overlap at each fracture site. The pelves were retested with the MTS machine, and load-versus-displacement curves were generated and compared with intact stiffness.

Results: The pelves that were anatomically reduced had a 46.7% ± 7.3% return to intact stiffness, whereas the pelves fixed at 1 cm of vertical displacement returned to 28.2% ± 8.9% of the initial intact stiffness (P = 0.0494). As in previous studies, cranial malreduction of the transforaminal sacral fractures resulted in a statistically significant decrease in the area of overlap between the two sacral fragments. Linear regression analysis demonstrated a statistically significant association between cross-sectional area and final stiffness of the internally fixed fractures (R² = 0.920 and P = 0.0024).

Discussion: Internal fixation of sacral fractures has been shown to be anatomically compromised by malreduction. The results of this preliminary cadaveric biomechanical study would indicate that malreduction of a transforaminal sacral fracture may also result in a less mechanically sound fixation construct.

Conclusions: The results of this study demonstrated that cranial malreduction of transforaminal sacral fractures in a cadaveric fracture model results in reduced final stiffness of iliosacral fixation.