This study aims to evaluate the biomechanical mechanism of fixation systems in the most frequent T-shaped acetabular fracture using finite element method. The treatment of acetabular fractures was based on extensive clinical experience. Three commonly accepted rigid fixation methods (double column reconstruction plates (P × 2), anterior column plate combined with posterior column screws (P + PS), and anterior column plate combined with quadrilateral area screws (P + QS)) were chosen for evaluation. On the basis of the finite element model, the biomechanics of these fixation systems were assessed through effective stiffness levels, stress distributions, force transfers, and displacements along the fracture lines. All three fixation systems can be used to obtain effective functional outcomes. The third fixation system (P + QS) was the optimal method for T-shaped acetabular fracture. This fixation system may reduce many of the risks and limitations associated with other fixation systems.
Acetabular fractures are frequently associated with high-impact trauma, particularly trauma incurred from road traffic accidents. Pelvic fractures frequently involve injury to organs contained within the bony pelvis because of the impact of forces involved. Furthermore, trauma to extrapelvic organs is common and pelvic fractures are often associated with severe hemorrhage because of the extensive blood supply to the region. The mortality rate of patients with pelvic fractures is between 10% and 16%. This type of injury often causes enormous damage to society and families. Therefore, a precise diagnosis and a well-executed treatment plan are important in achieving functional and durable results [
The displaced T-shaped fracture is complicated to manage and is frequently encountered (commonly from motor vehicle accidents, cycling accidents, or fall from significant heights). Open reduction and internal fixation with interfragmentary screws and reconstruction plates are the treatment of choice in the displaced posterior wall and posterior column fractures of the acetabulum [
Studies on the biomechanical mechanism, practicality, and effectiveness of internal fixation systems are rare because of the complexity of the pelvic fracture and its fixation systems. The finite element method (FEM) has the intrinsic advantages of restricting individual differences without the need for equipment and environmental variations. In biomechanical analysis by using the FE model, any combination of models and any change of external loads are possible. The analysis of the FE model can provide the local reaction mechanism, investigate the stress-strain status in any surface and inner regions of the model, and avoid the restrictions and deviations caused by the use of cadaver tests [
This study aims to create an FE model and evaluate the biomechanical mechanism of three methods of fixation systems for T-shaped fracture, one of the most common fractures with complicated procedures. The biomechanics of these fixation systems were assessed on the basis of effective stiffness levels, stress distributions, force transfers, and displacements along the fracture lines.
The hospital ethics committee licensed this study. A 3D FE model of the pelvis was created on the basis of the computed tomography (CT) scan images with a slice width of 0.5 mm of a Chinese male (40 years old, 175 cm tall, and 65 kg weight). The point cloud was converted to the surface of the pelvic. Mesh division was conducted on each part of the tissue by a combined manual and automatic division method. Eight nodes with nonlinear solid hexahedron elements (C3D8), with an average thickness of 1.5 mm, were offset from the cancellous bone to represent cortical bone. The soft tissues (i.e., endplates, cartilage, and pubic symphysis) between pelvic bone were meshed into hexahedron elements. The bones, cartilages, and endplates were all represented by hexahedral mesh. To make the simulation realistic, ligaments including sacroiliac ligament ring, sacrospinous, sacrotuberous, inguinal, superior pubic, and arcuate pubic represented by truss elements were added to the FE model. The main pelvic ligaments were modeled as truss elements with a length of 2 mm. In addition, the material properties of the model were assumed to be homogeneous and isotropic. The total numbers of elements and nodes were 102506 and 147458, respectively. Furthermore, mesh sensitivity studies revealed that further refinement did not significantly improve calculation accuracy. The FE model of the pelvis is shown in Figure
The material properties of the pelvic tissues [
Material |
|
|
Thickness (mm) | |
---|---|---|---|---|
Bones | Cortical bone | 17000 | 0.3 | 1.50 |
Cancellous bone | 150 | 0.2 | ||
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||||
Soft tissues | End plate (sacrum) | 24 | 0.4 | 0.23 |
Cartilage (sacrum) | 54 | 0.4 | 3.00 | |
Cartilage (ilium) | 54 | 0.4 | 1.00 | |
End plate (ilium) | 24 | 0.4 | 0.36 | |
Pubic symphysis | 5 | 0.495 | ||
|
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Ligamenta | Sacroiliac ligament ring | 350 | 0.3 | |
Sacrospinous | 29 | 0.3 | ||
Sacrotuberous | 33 | 0.3 | ||
Inguinal | 2.6 | 0.3 | ||
Superior pubic | 19 | 0.3 | ||
Arcuate pubic | 20 | 0.3 | ||
|
||||
Fixations | screws | 110000 | 0.3 | |
Plates | 110000 | 0.3 |
FE model of the pelvis.
A T-shaped fracture combines a transverse component and vertical component that separates the lower ischiopubic segment into the anterior and posterior columns. This fracture can be categorized as a combined fracture. In this paper, a converging line was developed to represent the fracture line. The converging line originated from the anterior inferior iliac spine or groove on the upper brim of the acetabulum and runs along the center of the acetabulum to separate to two branches: one to the anterior side of the acetabulum and the other reaching the inferior side of the acetabulum. To simulate the instability of the pelvis, the inferior ramus was also ruptured as shown in Figure
Finite model of three fixation systems; (a) fracture model without fixation systems; (b) double column reconstruction plates (P × 2); (c) an anterior column plate combined with posterior column screws (P + PS); (d) an anterior column plate combined with quadrilateral area screws (P + QS).
T-shaped fractures separate the pelvis into three parts. The upper part of the pelvis cannot be kept stable because of the lower parts of the pelvic that serve little or even no effect on support of the body weight. The pelvis suffers lesser stiffness compared with the intact bone. In this case, the properties (i.e., density and elastic modulus) of the mesh along the converging line were weakened to one-tenth of the normal mesh [
The type and nature of the acetabular fracture substantially influence the approach used [
This fixation system was the most traditional and widely used method compared with the other two systems, particularly in impacted and osteochondral fragments. This fixation system consisted of two reconstruction plates and its set screws [
Longer operation time, infection, greater blood loss, abductor weakness, and heterotopic ossification should be avoided during the surgery. Thus, a minimally invasive approach with one reconstruction plate was applied for the complex fracture. The anterior column plate (P) and its set screws were the same as the former (P × 2) system. Furthermore, two lag screws were incorporated from the outer surface of the quadrilateral area superior to the arcuate line and into the ischial spine under screw fixation. The two lag screws should be completely immersed in the iliac bone and not through the acetabulum cartilage to avoid extra damage to the patient (see Figure
The position attaching the reconstruction plate in this fixation system was almost the same as the two former (P × 2 and P + PS) systems. The quadrilateral area screws (QS) were inserted from the outer surface along the arcuate line into the ischial spine. The QS were fixed by the reconstruction plates and the quadrilateral area cortical bone where they were inserted (see Figure
The purpose of this study was to evaluate the validation of the fixation systems of the T-shaped fracture in double-limb stance. This stance was similar to the existing models, as previously described by Sawaguchi et al. [
The pelvic model experiences a vertical force loaded on the upper surface of the sacral bone. The von Mises stress and displacement distribution in the iliac bone are shown in Figure
Stress and displacement distribution in the cortical bone of iliac bone; (a) stress distribution in the cortical bone of iliac bone; (b) displacement distribution in the cortical bone of iliac bone.
The rigidity of five different configurations (nonfractured model, fracture model, and three fixation systems models), which were obtained from simulated results, was compared with each other (Table
Effective stiffness levels of the fixation systems.
Displacement (mm) | Total stiffness (N/mm) | Max von Mises stress (MPa) | |
---|---|---|---|
Nonfractured model | 2.590 | 231.66 | 27.9 |
Fracture model | 2.702 | 222.06 | 64.0 |
P × 2 | 2.616 | 229.36 | 28.7 |
P + PS | 2.645 | 226.84 | 35.2 |
P + QS | 2.607 | 230.15 | 37.8 |
The stress distributions in the iliac bone are shown in Figure
Stress distribution in the iliac bone under different conditions; (a) stress distribution in the iliac bone in the nonfractured model; (b) stress distribution in the iliac bone in the fracture model; (c) stress distribution in the iliac bone in the first fixation system (P × 2); (d) stress distribution in the iliac bone in the second fixation system (P + PS); (e) stress distribution in the iliac bone in the third fixation system (P + QS).
The stress distribution patterns in each fixation system under the standing stance are shown in Figure
Stress distribution in different fixation systems; (a) stress distribution in the first fixation system (P × 2); (b) stress distribution in the second fixation system (P + PS); (c) stress distribution in the third fixation system (P + QS).
The two paths were generated along the fracture line to access the validation of the fixation systems. One path is below the fracture line and the other above the fracture line. The displacement along two paths was shown in Figures
Magnitude of displacement and stress distribution along two paths; (a) the path below the fracture line; (b) the path above the fracture line; (c) magnitude of displacement along the first path; (d) magnitude of displacement along the second path; (e) stress the along first path; (f) stress along the second path.
This study aims to simulate the mechanical behavior of the T-shaped fracture and assess the biomechanical mechanism of the fixation systems recommended for fracture stabilization. The biomechanical mechanism was evaluated by the effective stiffness, stress distribution, and force transformation of the three models.
A number of approaches have been used to predict stress patterns in biomechanical applications, for example, experimental techniques such as strain gauging and photoelastic analysis and numerical procedures such as FEM, to obtain comprehensive information on the pelvis biomechanical mechanism. The versatile features of FEM analysis are its potential for evaluating stresses/strains throughout the pelvis for all materials concerned and for parametric analysis. Material properties and loading and boundary conditions are easily varied to investigate their influences. Thus, FEM has been selected as a tool to investigate the effects of different fixation systems on complex pelvic T-shaped fractures.
The treatment of pelvic fractures was based on extensive clinical experience and theories that formed the procedures and guidelines for the treatment of fractures. Considering the geometry and structure of the pelvis, one of the most popular systems in surgery is anterior column fixation, which involves the inner surface of the ilium and the superior surface of the superior pubic ramus. The single reconstruction plate is applicable to almost all pelvic fractures, whereas the anterior reconstruction plate alone cannot achieve an acceptable clinical or radiological outcome [
Erkmen et al. conducted an FE analysis to estimate the complex stress fields in the pelvic bone, fixation screws, and plates and to evaluate the function of the fixation screws and plates [
The bone in the quadrilateral area is very thin and presents an almost all cortical bone feature. The bone is also proximal to the joint. This particular bone has a high incidence of all types of pelvic fracture. Traditional clinical operations address these fractures without involving the bone in the quadrilateral area to prevent introducing unnecessary risks, whereas extensive clinical experience suggests that the fixation system involved in this area can provide great function outcomes [
The stress and displacement distributions changed when the fracture occurred. The stress level in most parts of the iliac bone in the fracture models were lower than that in the nonfractured model. The stress levels increased significantly in all of the positioned plates attached or screws inserted. Therefore, fixation systems can increase the stiffness of the entire pelvis. The function of the fixations can be explained from the stress distribution pattern: higher stress in the fixation system component corresponds to the greater role played by this component. The maximum stress was observed in the reconstruction plate or the QS; thus, the plate and QS played a dominant role in keeping the stability of the fracture model. The stress level in the fixed screws was lower than in the PS and QS perhaps because the lag screws (PS and QS) penetrated through the fracture line, which fit closely to the irregular surfaces to overcome the resistance generated by shear and torsion. Therefore, the reconstruction plates combined with the lag screws can produce excellent functional outcomes for complex pelvic fractures [
Compared with the other two fixation systems (P × 2 and P + PS), the third fixation system (P + QS) can increase the total stiffness and decrease the maximum displacement. This fixation system can transfer more body weight from the upper separated part to the downward separated part through the plate than the other two systems. These findings may be explained by the fact that the elastic modulus of the fixation system was considerably greater than that on the cortical bone (110 GPa versus 17 GPa). Therefore, this fixation (P + QS) is the optimization method for T-shaped fracture in terms of the total pelvic stiffness, stress distribution, and screws role.
The results in this paper were based on the FE model. A few points should be noted. First, the geometry and structure of the pelvis was complex. The pelvis included many sharp corners and small clearance spaces, which cannot be simulated in the FE model. Previous studies have shown that the total pelvic mechanism was insensitive to these detailed features [
This study aims to assess the biomechanical mechanism of the fixation systems for T-shaped fracture. Three fixation systems selected in this study are powerful on increasing the approximate biomechanical stability. The third fixation system (P + QS) is the optimal method for T-shaped fracture in terms of total pelvic stiffness, stress distribution, and screw function. Furthermore, this fixation system entails a short operation time, minor cuts, and low possibility of infection during the surgery. Further clinical studies are needed to validate the observations of the current FE study.
Double column reconstruction plates
Anterior column plate combined with posterior column screws
Anterior column plate combined with quadrilateral area screws.
The authors declare that there is no conflict of interests regarding the publication of this paper.
This work is supported by the Top Young Academic Leaders of Shanxi and the Outstanding Innovative Teams of Higher Learning Institutions of Shanxi. The financial contributions are gratefully acknowledged.