A finite element analysis of sacroiliac joint displacements and ligament strains in response to three manipulations

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

PURPOSE

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.

METHODS

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.

RESULTS

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).

CONCLUSION

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.

Patient-specific bone material modelling can improve the predicted biomechanical outcomes of sacral fracture fixation techniques: A comparative finite element study

OBJECTIVE

To evaluate and compare the biomechanical efficacy of six iliosacral screw fixation techniques for treating unilateral AO Type B2 (Denis Type II) sacral fractures using literature-based and QCT-based bone material properties in finite element (FE) models.

METHODS

Two FE models of the intact pelvis were constructed: the literature-based model (LBM) with bone material properties taken from the literature, and the patient-specific model (PSM) with QCT-derived bone material properties. Unilateral transforaminal sacral fracture was modelled to assess different fixation techniques: iliosacral screw (ISS) at the first sacral vertebra (S1) (ISS1), ISS at the second sacral vertebra (S2) (ISS2), ISS at S1 and S2 (ISS12), transverse iliosacral screws (TISS) at S1 (TISS1), TISS at S2 (TISS2), and TISS at S1 and S2 (TISS12). A 600 N vertical load with both acetabula fixed was applied. Vertical stiffness (VS), relative interfragmentary displacement (RID), and the von Mises stress values in the screws and fracture interface were analysed.

RESULTS

The lowest and highest normalised VS was given by ISS1 and TISS12 techniques for LBM and PSM, with 137 % and 149 %, and 375 % and 472 %, respectively. In comparison with the LBM, the patient-specific bone modelling increased the maximum screw stress values by 19.3, 16.3, 27.8, 2.3, 24.4 and 7.8 % for ISS1, ISS2, ISS12, TISS1, TISS2 and TISS12, respectively. The maximum RID values were between 0.10 mm and 0.47 mm for all fixation techniques in both models. The maximum von Mises stress results on the fracture interface show a substantial difference between the two models, as PSM (mean ± SD of 15.76 ± 8.26 MPa) gave lower stress values for all fixation techniques than LBM (mean ± SD of 28.95 ± 6.91 MPa).

CONCLUSION

The differences in stress distribution underline the importance of considering locally defined bone material properties when investigating internal mechanical parameters. Based on the results, all techniques demonstrated clinically sufficient stability, with TISS12 being superior from a biomechanical standpoint. Both LBM and PSM models indicated a consistent trend in ranking the fixation techniques based on stability. However, long-term clinical trials are recommended to confirm the findings of the study.

The Influence of Mesh Density on the Results Obtained by Finite Element Analysis of Complex Bodies

Finite element analysis of complex bodies is frequently used in design to determine the size of deformations. Successive iterations, with progressive refinement of mesh densities, are most often required to obtain a sufficiently accurate convergent numerical solution. This process is costly, time consuming, and requires superior hardware and software. The paper presents a quick and effortless way to determine a sufficiently accurate value of the numerical solution. The mentioned solution is obtained by amending the numerical solution resulting for a certain value of the mesh density of the studied body with an adequate proportionality coefficient determined following the deformation study of simple bodies differently subject to external forces. It is assumed that the elastic displacement of the various bodies has a similar evolution as the mesh density increases and that the values of the proportionality coefficients considered are approximately equal for identical mesh densities. Examples presented are related to the reference body of the mechanical press PAI 25.

Effects of manipulations of oblique pulling on the biomechanics of the sacroiliac joint: a cadaveric study

BACKGROUND

There are many reports on the treatment of sacroiliac joint dysfunction by manipulation of oblique pulling (MOP). However, the specific mechanism of MOP on the sacroiliac joint remains unclear. This study aimed to investigate the effect of MOP on the biomechanics of the sacroiliac joint and the effect of the anterior sacroiliac ligament on the stability of the sacroiliac joint.

METHODS

First, MOP-F1 (F: force) and MOP-F2 were applied to nine cadaveric pelvises. Then, segmental resection of the anterior sacroiliac ligament was performed. The range of motion of the sacroiliac joint was observed in all procedures.

RESULTS

Under MOP-F1 and F2, the average total angles were 0.84° ± 0.59° and 1.52° ± 0.83°, and the displacements were 0.61 ± 0.21 mm and 0.98 ± 0.39 mm, respectively. Compared with MOP-F1, MOP-F2 caused greater rotation angles and displacements of the sacroiliac joint (p = 0.00 and p = 0.01, respectively). In addition, the rotation angles and displacements of the sacroiliac joint significantly increased after complete resection of the anterior sacroiliac ligament (p = 0.01 and p = 0.02, respectively). The increase was mainly due to the transection of the upper part of the anterior sacroiliac ligament.

CONCLUSIONS

MOP-F2 caused greater rotation angles and displacements of the sacroiliac joint and was a more effective manipulation. The anterior sacroiliac ligament played an important role in maintaining the stability of the sacroiliac joint; the upper part of the anterior sacroiliac ligament contributed more to the stability of the joint than the lower part.

Manipulations of Oblique Pulling Affect Sacroiliac Joint Displacements and Ligament Strains: A Finite Element Analysis

Objective

Clinical studies have found that manipulation of oblique pulling has a good clinical effect on sacroiliac joint pain. However, there is no uniform standard for manipulation of oblique pulling at present. The purpose of this study was to investigate the effects of four manipulations of oblique pulling on sacroiliac joint and surrounding ligaments.

Methods

A three-dimensional finite element model of the pelvis was established. Four manipulations of oblique pulling were simulated. The stresses and displacements of sacroiliac joint and the strains of surrounding ligaments were analyzed under four manipulations of oblique pulling.

Results

Manipulation of oblique pulling F2 and F3 caused the highest and lowest stress on the pelvis, at 85.0 and 52.6 MPa, respectively. Manipulation of oblique pulling F3 and F1 produced the highest and lowest stress on the left sacroiliac joint, at 6.6 and 5.6 MPa, respectively. The four manipulations of oblique pulling mainly produced anterior-posterior displacement. The maximum value was 1.21 mm, produced by manipulation of oblique pulling F2, while the minimal value was 0.96 mm, produced by manipulation of oblique pulling F3. The four manipulations of oblique pulling could all cause different degrees of ligament strain, and manipulation of oblique pulling F2 produced the greatest ligament strain.

Conclusions

The four manipulations of oblique pulling all produced small displacements of sacroiliac joint. However, they produced different degrees of ligament strain. Manipulation of oblique pulling F2 produced the largest displacement of sacroiliac joint and the greatest ligament strain, which could provide a certain reference for physiotherapists.