# Biomechanical analysis of iliosacral and transiliac–transsacral screw combinations for fixation of undisplaced Denis II vertical shear fractures in dysmorphic sacrum

## TL;DR

This study compares different screw combinations for stabilizing a specific type of sacral fracture, finding that a combination of S1 and S2 screws offers the best stability.

## Contribution

The study introduces a biomechanical comparison of ISS and TTS screw combinations for dysmorphic sacral fractures using finite element analysis.

## Key findings

- The maximum stress in all internal fixation groups was below titanium alloy's yield strength, indicating no implant failure risk.
- The anterior S1-ISS combined with S2-TTS showed the least vertical and angular displacement in all motion states.
- Screw deformation was lowest in the anterior S1-ISS and S2-TTS combination for both S1 and S2 segments.

## Abstract

Percutaneous iliosacral screws (ISS) and transiliac-transsacral screws (TTS) are effective for treating posterior pelvic ring instability. However, the biomechanical stability of undisplaced sacral dysmorphism fractures remains underexplored. This study evaluated various ISS and TTS combinations to provide a clinical reference for fixing such fractures.

A finite element model of a complete Denis type II dysmorphic sacral fracture (extending through the sacral foramen) was developed. The stability of the posterior pelvic ring was evaluated using seven fixation techniques: S1-ISS (Group 1), S2-ISS (Group 2), S1-ISS + S2-ISS (Group 3), S2-TTS (Group 4), anterior S1-ISS + S2-TTS (Group 5), middle S1-ISS + S2-TTS (Group 6), and posterior S1-ISS + S2-TTS (Group 7). In all models, the anterior pelvic ring was fixed with pubic ramus screws. The upper sacral surface was subjected to six loading modes to simulate physiological states: standing, forward flexion, left flexion, right flexion, left rotation, and right rotation. The following parameters were recorded and analyzed: vertical displacement and sagittal angular displacement of the upper sacral surface, relative displacement of five pairs of observation points on the anterior fracture line, and maximum Von Mises stress and deformation of the S1 and S2 screws.

The finite element analysis revealed that the maximum stress in all internal fixation groups across the six loading modes was below the yield strength of titanium alloy, indicating no risk of implant failure. Screw deformation was highest in G1 and lowest in G5 for the S1 segment, and highest in G2 and lowest in G 5 for the S2 segment. G5 also exhibited the minimum vertical and angular displacements of the sacral upper surface in all motion states.

This study demonstrates that S1-ISS combined with S2-TTS fixation provides excellent biomechanical stability for undisplaced vertical fractures of the sacral dysmorphism, particularly the combination of anterior S1-ISS and S2-TTS. This fixation method offers a promising clinical treatment option.