1.1 Research Object
One volunteer (48 years, male) was selected for X-ray and B-ultrasound to rule out pelvic abnormalities, injuries, osteoporosis, tumors, and other relevant factors. A routine 64-slice spiral CT scan was performed with a slice thickness of 1 mm. Data were collected in DICOM format.
1.2 Research Methods
We simulated Tile C1.2 pelvic fractures, which are characterized by a sacroiliac joint dislocation and pubic fracture.
1.2.1 Image processing phase
The DICOM data were imported into MIMICS 16.0 software. We defined the coronal plane, sagittal plane and transverse plane in the software, and reconstructed a three-dimensional model of the pelvis using the protocols of region growth (Figure 1). We stored the binary STL file to use in subsequent processing.
1.2.2 Establishment of an unstable pelvic fracture model
A model of the left pubic fracture was created in three dimensions. The dislocation of the sacroiliac joint in the posterior ring was simulated by removing the ligaments.
1.2.3 Mesh modification and ligament reconstruction
The STL file was imported into the 3-MATIC software to modify the mesh and model the anterior and posterior sacroiliac ligaments (Figure 2), the sacrotuberous ligament and the sacrospinous ligament. The final file was exported in STL format.
1.2.4 Model fixation and experimental classification
Implant models were established in UG 8.5 software. The assembly process for internal fixation in the fracture models also was performed with UG software. We established 9 fixation models according to the fixation types (Table 1、Figure 3).
Table 1: model grouping
Model 1
|
blank” group that was not fixed
|
Model 2
|
Infix fixation model
|
Model 3
|
3-screw-Infix model with one screw fixed on the affected pubic ramus
|
Model 4
|
3-screw-Infix model with one screw fixed on the healthy pubic ramus
|
Model 5
|
4-screw-Infix model with two screws fixed bilaterally on the pubic ramus
|
Model 6
|
S1 sacroiliac screw fixation model
|
Model 7
|
combined fixation types from Models 2 and 6
|
Model 8
|
combined fixation types from Models 3 and 6
|
Model 9
|
combined fixation types from Models 4 and 6
|
Model 10
|
combined fixation types from Models 5 and 6
|
1.2.5 Mesh transformation (Table 2)
All the models were meshed using HyperMesh 11.0 software (Altair Engineering, Inc, USA), which was also used to convert from surface mesh to volume mesh.
Table 2: Elements and nodes of the models in this study
|
Element
|
Nodes
|
|
Pelvis
|
Infix
|
S1 screw
|
Pelvis
|
Infix
|
S1 screw
|
Model1
|
1769595
|
|
|
347409
|
|
|
Model2
|
1767251
|
81972
|
|
346904
|
127411
|
|
Model3
|
1771089
|
82477
|
|
348041
|
128674
|
|
Model4
|
1768532
|
82489
|
|
347213
|
128774
|
|
Model5
|
1769577
|
83530
|
|
347358
|
129599
|
|
Model6
|
|
|
1752798
|
|
|
343984
|
Model7
|
1762146
|
81972
|
30332
|
347213
|
127411
|
46317
|
Model8
|
1770021
|
82477
|
30332
|
347212
|
128674
|
46317
|
Model9
|
1762172
|
82489
|
30332
|
346725
|
128774
|
46317
|
Model10
|
1755099
|
83530
|
30332
|
344871
|
129599
|
46317
|
1.2.6 Material properties and boundary conditions (Table 3)
Frictional contact interactions were assumed between the different parts of the models. The interface of the S1 screw and the sacral bone structure was set as equivalent. The interface of the S1 screw polished rod and sacral bone structure was simulated using contact pairs with a friction factor of 0.3. The Infix screw and the stem were set to be equivalent, as was the Infix screw and bone structure. The interface between the fragments was simulated by contact pairs with a friction factor of 0.3.
Table3: Material properties used in the current study.
Material name
|
Young’s modulus(MPa)
|
Poisson’s ratio
|
Anterior sacroiliac ligament
|
208
|
0.2
|
Interosseous ligaments
|
2300
|
0.2
|
Posterior sacroiliac ligament
|
3000
|
0.2
|
Short posterior sacroiliac ligament
|
3000
|
0.2
|
Sacrotuberous ligament
|
46
|
0.2
|
Sacrospinous ligament
|
46
|
0.2
|
Infix-screw
|
110000
|
0.3
|
The bone tissue was modeled as an inhomogeneous, isotropic and elastic-plastic material. The material properties were determined by the apparent density of bone tissue ρ (g/cm3). This density was determined by the gray level from the CT scans. A linear relationship between Hounsfield units and grayscale was assumed, and we chose the following density–elasticity equation to convert apparent density into Young’s modulus (E, in MPa). The Poisson’s ratio of the femur was assigned as 0.3.
All the FE models were subjected to a load of 500 N applied superior to S1 to assume full weight-bearing in patients. External rotation was simulated by applying 500 N to the left anterior superior iliac spine.