Finite Element Analysis of Different Percutaneous Fixation Strategies Used for the Treatment of Tile C1.2 Pelvic Fracture

Abstract


Introduction
Crush injuries and car accident injuries are often accompanied by pelvic fractures.Pelvic fractures are complicated traumas that are mostly caused by high-energy injuries.Half or more associated with different co-morbidities or multiple injuries, the most serious of which are traumatic hemorrhagic shock and pelvic injuries.Each case is unique, providing a challenge for the surgeon to provide the optimal care and xation technique.Anterior pelvic ring fractures are the most common pelvic fractures.Simple pubic branch fractures can often be treated conservatively.In some patients with unstable pelvic fractures with posterior ring injury, the anterior ring also needs to be treated while the posterior ring is xed [1].There are many ways to treat the anterior ring, such as the Stoppa approach, the ilioinguinal approach, and In x technology.However, for some patients with multiple injuries, patients who are unable to tolerate open reduction and internal xation with a plate, or those who are in an acute stage and unwilling to accept plate xation due to social factors, In x technology is often the rst choice, and has been shown to achieve good clinical results.Surgeons have improved their technique in using traditional In x technology in recent years.For example, in the traditional two-nail In x technology, an additional nail is added to the joint nodules of the pubic bone (the healthy side and the affected side), and sometimes four nails are added on both sides.The best clinical outcomes when stabilizing anterior ring injuries in pelvic fractures have not been systematically studied.For this reason, the author created three-dimensional nite element models to simulate the Tile C1.2 pelvic fracture, and calculate the biomechanical effect of different combinations of In x technology on the treatment of Tile C1.2 pelvic fractures.These models can provide some reference for clinical practice.

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.

Research Methods
We simulated Tile C1.2 pelvic fractures, which are characterized by a sacroiliac joint dislocation and pubic fracture.

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.

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.

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.

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).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.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. 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/cm 3 ).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 weightbearing in patients.External rotation was simulated by applying 500 N to the left anterior superior iliac spine.

Results
The Axial Load Analysis

Displacement analysis
The displacement of the nine different models under a 500 N vertical load is shown in Table 5.We  5.The maximum stress distribution of the 10 models at 500 N loading on the left anterior superior iliac spine is shown in Table 6.When the anterior pelvic ring is fixed with Infix, the overall pelvic ring stress of The effect is better when fixed with Infix.There are differences depending on the number of screws on the pubic branch of Infix.We find that 4 screws provide the best results.

Displacement analysis
The values of displacement of the pelvis, the left sacroiliac joint, and the pubic branch fracture of the 10 Models under external rotational loading are shown in Table7.When we compare Infix among fixation types represented by Models 2-5, the overall pelvic displacement and sacroiliac joint fixation with the 4 screw Infix was the lowest.When the pelvis was fixed only with an S1 screw in Model 6, displacement of the overall pelvis, sacroiliac joint and pubic branch fracture site are 3.473 mm, 0.258 mm and 1.007 mm, respectively.Model 10, with an S1 screw and the 4-screw-Infix type, had displacements of the overall pelvis, sacroiliac joint and pubic branch fracture site of 2.412 mm, 0.149 mm and 0.017 mm, respectively.
The results indicate that under external rotational loading, Tile C1.2 fractures can be fixed effectively by simply using sacroiliac screw fixation.

Discussion
It is well known that unstable pelvic ring fractures require surgical treatment, but the selection of the ideal surgical scheme has been the subject of many controversies [2][3][4].Surgeons often focus on the restoration of the stability of the posterior pelvic ring and neglect the repair of structural damage to the anterior pelvic ring [5].Hill et al. [6] showed that although the death rate 1 year after pubic branch fracture injury is lower than that of hip fracture, but the mortality rate after 5 years tends to be the same.This may be related to long-term passive braking and persistent pain.This undesirable situation usually occurs in patients with posterior ring damage that is not detected initially.At present, a general view is that any types of anterior and posterior ring injuries should be xed in order to enable early movement and prevent postoperative displacement [1,7].The results indicate that In x xation strategy, regardless of the combination or distribution of screws on the pubic ramus, does not play a decisive role in stability of the unstable posterior ring.The S1 sacroiliac screw can x effectively the posterior pelvic ring in cases of pelvic fracture, and on some level it improves the anterior pelvic ring.The combination of the S1 screw and the In x system provides strong stability with additional screws to x the anterior pelvic ring, increasing stability.Therefore, this present study shows that 3-screw-In x plus the S1 screw exhibit the best effect.Pelvic fractures like Tile C1.2 are characterized by a unilateral posterior arch through the sacroiliac joint with complete disruption of the posterior arch, and an anterior ring fracture that needs to be xed by the In x system.The iliac screw combined with In x is an effective and minimally invasive alternative to the treatment of Tile C1.2 fractures.According to the principle of trauma minimization, it is recommended to use the iliac screw + 3 nail In x xation on the healthy side.However, this needs further in-vivo study to verify that it will result in positive clinical outcomes.

Declarations
Data Availability datasets used and/or analyzed during the current study are available from the corresponding author on reasonable request.

Consent
This study obtained consent to publish from the participants to report individual patient data (CT image data).

2. 1 von
Mises stress distribution.The maximum stress distribution of the 10 models with 500 N axial loading is shown in Table4.With different combinations of Infix fixation, the stress on the whole pelvic ring changed slightly.The Iinfix combination with 4 nails changed the most (124.145MPa) .When the posterior pelvic ring is fixed with the iliac screw, the overall pelvic ring stress is significantly reduced, but there is no significant difference with the combination of different forms of Infix fixation.Infix fixation without the left sacroiliac screw can also play a role in which the minimum stress can be reduced to 15.498 MPa (Model 4).Model 6 with a simple sacroiliac screw shows better results (stress reduction to 11.621 MPa).The improvement is more significant when combined with Infix technology (reduction to 9.428MPa).The stresses from the fractures also are reduced when the pelvis is fixed with Infix plus a pubic ramus screw: Model 1 (7.959MPa) > Model 2 (5.316MPa) > Model 3 (3.685MPa) > Model 4 (2.545MPa) > Model 5 (2.374 MPa).A sacroiliac screw also disperses some stress leading to lower stresses: Model 1 (7.959MPa) > Model 6 (5.132 MPa).But the stress reduction is larger when using the 4-screw-Infix system and an S1 sacroiliac screw (Model 10),leading to a stress of only 1.743 MPa on the fracture segments.The results indicate that the sacroiliac screw disperses some stresses that are transferred among fragments.Simple fixation of the anterior pelvic ring does not stabilize pelvic fracture.When the posterior pelvic ring is fixed by the S1 sacroiliac screw, the facture model is stable.However, maximum stress on the fracture segments indicates that Model 8 (2.711 MPa) > Model 9 (1.892MPa) > Model 10 (1.743 MPa).Stresses between the 3-screw-Infix model and the 4-screw-Infix model are not very large, but differences between Model 9 and Model 8 are greater.
found that total displacement, vertical displacement and horizontal displacement are reduced on the pelvis, left sacroiliac joint and fracture site displacement compared to Model 1.When the posterior pelvic ring is fixed with the sacroiliac screw, displacement is reduced by a large amount.With the combination of Infix and a sacroiliac screw, displacement reduction is not very large.Of all the Infix alone models, parameters of total displacement, vertical displacement and horizontal displacement are in the order of Model 5 < Model 4 < Model 3 < Model 2. With addition of the sacroiliac screw, displacement parameters are in the order of Model 10 < Model 9 < Model 8 < Model 7. The fixation model with 4-screw-Infix in addition to a sacroiliac screw shows the least displacement.The detailed values for displacement in each model are shown in Table

Models 2 -
5 is in the range of 149.602-158.914MPa, which is similar to Model 1 (162.581MPa).Model 6 with S1 sacroiliac screw fixation alone has a lower stress of 120.521MPa.With the combination of the S1 screw and Infix, the overall pelvic ring stress of Models 7-10 is in the range of 99.471-113.354MPa.Model 10 shows the least stress.Similarly, the stress on the left sacroiliac joint is more than 30 MPa when the anterior ring is fixed.While the posterior ring is fixed by the S1 screw, stress on the left sacroiliac is reduced to 26.431MPa.The combination between Infix and the S1 screw reduces the stress further.The results indicate that when the pelvis is subjected to external rotation stress, none of the Infix fixations effectively fix the unstable Tile C1.2 fracture.But the fixation of the posterior ring may provide enough stability.The use of simple sacroiliac screws improves the overall pelvic ring and sacroiliac joints.

Figures
Figures

Figure 1 CT
Figure 1

Table 2 :
Elements and nodes of the models in this study

Table 4 :
The von Mises stress of different parts of all models under loading.

Table 5 :
The peak (mm different part of all under loading.

Table 6 :
The peak von Mises stress of different parts of all models under external rotational loading.

Table 7 :
peak (mm of different parts of all models under external rotational loading.
[10]]n pelvic fracture xation is divided into internal xation and external xation, but current research shows that compared with more invasive internal xation, minimally invasive reduction may have some bene ts for patients[8,9].The amount of bleeding, soft tissue injury and the incidence of nerve injury have improved.On the other hand, various internal xation methods are more challenging for surgeons.In clinical practice, most of the minimally invasive techniques choose In x technology for xation.A variety of combinations of In x technology have been produced, but there are not many biomechanical studies on unstable pelvic fractures.Due to the source of the cadaver specimen and its inherent variability, we performed a three-dimensional nite element analysis and d the stability of different percutaneous xation methods for the treatment of Tile C1.2 fractures.Finite element models have many potential limitations, but they do provide the advantages of repeatable and comparable biomechanical tests[10].