A biomechanical study on bilateral SAI annular fixation in the treatment of unilateral Denis Ⅱ sacral fractures

To investigate the biomechanical properties of bilateral SAI annular internal fixation in the treatment of unilateral Denis type Ⅱ sacral vertical fracture, so as to provide a reference for clinical practice.

The finite element approach was utilized to simulate the Denis type Ⅱ fracture of the right sacral (the entire fracture through the sacral foraminarum) as well as the fracture of the upper and lower ramus of the right pubic bone, which represents unilateral vertical instability of the pelvic ring. The posterior pelvic ring was fixed using three different methods: lumbopelvic fixation and S1 transsacral screw (LPF-S1), bilateral S1AI annular fixation and S2 transsacral screw (BS1AI-S2), bilateral S2AI annular fixation and S1 transsacral screw (BS2AI-S1), and the anterior pelvic ring was fixed with pubic ramus screws. Six different loading methods are used to simulate six conditions: standing, forward bending, left flexion, right flexion, left rotation and right rotation. The maximum Von Mises stress of the implant, the vertical displacement of the upper surface of the sacral, and the relative intrafragmentary displacement (RID) of the observation point on the anterior surface of the sacral were recorded and analyzed.

The maximum Von Mises stress of the implant in the three fixed models did not exceed the maximum yield stress of the titanium alloy under different motion conditions in the finite element model. The descending order from high to low was LPF-S1, BS1AI-S2, and BS2AI-S1. The RID of each observation point on the anterior surface of the sacral and the vertical displacement of the upper surface of the sacral were both lower in the BS2AI-S1 group than in the LPF-S1 group and the BS1AI-S2 group. In the standing, forward flexion, left flexion, right flexion, and left rotation conditions, the LSD test results indicated a statistically significant difference in the mean RID of observation points between the BS2AI-S1 and BS1AI-S2 groups (p < 0.05), but not in the LPF-S1 group. In the condition of right rotational motion, there was no statistically significant difference in the mean RID between the three groups (p > 0.05).

The biomechanical stability of the fixation of Denis type II sacral fractures was satisfactory in the LPF-S1, BS1AI-S2, and BS2AI-S1 groups, with the BS2AI-S1 group exhibiting the maximum level of stability. Bilateral SAI annular internal fixation is a viable alternative to the fixation of vertical sacral fractures, as it does not impair the lumbar spine's mobility and accomplishes satisfactory biomechanical stability.