Biomechanical Modeling of Options for Internal Fixation of Unilateral Fractures of the Sacrum

Abstract

Relevance. Currently, the stability of various options for the fixation of sacral fractures by the finite element method has not been sufficiently studied.Purpose — the biomechanical characteristics of two variants of internal fixation of unilateral sacral fractures by various implants and the localization of the line of its fracture with respect to the articular facet of the L5-S1 vertebrae were studied.Materials and Methods. Using the finite element method, we studied the biomechanical characteristics of two options for fixing a one-sided longitudinal fracture of the sacrum with different localization of the line of its fracture: outside, inside and directly on the joint facet L5-S1. Two fixation options are considered: cannulated sacroiliac screws and a similar option in combination with a bilateral lumbar-pelvic transpedicular system.Results. The stresses in implants and bone under compression load and torso forward or backward are almost the same in all models. In the model of fixation with a sacroiliac screw of a one-sided longitudinal sacral fracture, the line of which passes through the articular process S1 of the vertebra (Isler II type), the greatest stress in the screws under compression load and bending moment was 619.7 MPa, which exceeds the yield strength of the titanium alloy and can damage the implants. In all models where the transpedicular system additionally acted as fixing structures, a decrease of 42–77% of maximum displacements was noted, by 28–79% of equivalent stresses in implants under all types of loads, while the equivalent stresses in the bone structures did not differ significantly. In models where the transpedicular system was additionally applied, a decrease of 42–77% of maximum displacements was noted, by 28-79% of stresses in implants under all types of loads, while the stresses in the bones did not differ much.Conclusion. In all cases of localization of the line of unilateral fracture of the sacrum, the use of a transpedicular system in combination with sacroiliac screws is more stable from the point of view of biomechanics. The most unstable is a one-sided longitudinal fracture of the sacrum passing through the facet L5-S1.