Title: Finite Element Analysis of Fixation Device for Femoral Neck Fracture: Dynamic Hip Screw
Author(s): Hamed Reza Seyyed Hosseinzadeh MD1*, Mohammad Qoreishi MD2, Ali Sina Shahi MD3
Affiliattion(s): 1 Amirkabir University of Technology, Tehran, Iran 2 Shahid Beheshti University of Medical Sciences, Tehran, Iran 3 Rothman Institute, Philadelphia, PA, USA
* Corresponding Author
Vol 1, Num 2, October 2014
Femoral neck fracture is one of the most common fractures in the elderly patients. Beside its commonness, treatment of these fractures make great challenges to orthopedic surgeons. One of the methods for treatment of femur neck fractures is fixation with dynamic hip screw (DHS). Dynamic hip screw is a device for fixing the fractures of the proximal femur, esp, intertrochantric fractures. Since there is great challenge in fixing these fractures using cannulated screws, DHS is now being used more frequently in treating these fractures. In this research, stress distribution in this device was reproduced by computer simulation. In addition, overall performance of this device is also explained and tried to disclose parts of this device carrying more stress during application in human body. According to the calculated results, it was confirmed that mean stress in dynamic hip screw (lower than 70 MPa) and femur (lower than 48 MPa) in reasonably less than yield point of bone and implant grade stainless steel and also fixation device carries more stresses than femur. In addition, overall displacement of femoral neck fracture fixed by dynamic hip screw is as order of micrometer.
Keywords: Dynamic Hip Screw, Finite element Analysis, Femoral Neck Fracture, Fixation Device, Orthopedic Surgery
One of most common fractures in the elderly patients is femoral neck fracture. Treatment of these types of fractures make great challenges to orthopedic surgeons. According to demographic estimation, it seems that femoral neck fracture incidence is continuously increasing(1,2). A rise of 74% in prevalence of proximal femoral fractures is predicted to occur until the year 2020, in Germany(3). Femoral neck fractures characterize a significant health care problem especially in the aging patients. It has massive effects on health insurance costs.(3,4,6)
The mechanism of injury can reflect the mineral density of bone; from fractures in elderly population with low-energy falls to young population with high-energy trauma(7). Generally, patients under the age of 65 years old are described as young population, in contrast to population older than 75 years old described as elderly(8).
According to the literature there is general consensus that anatomical reduction and stable internal fixation is the treatment goal in these fractures (9,10). Different techniques are used for treatment of femur neck fractures such as arthroplasty (unipolar hemiarthroplasty, bipolar hemiarthroplasty and total hip arthroplasty) and internal fixation with cannulated screw (CS) or dynamic hip screw (DHS)(11-14). However, the optimal selection for treatment and fixation of displaced femoral neck fractures remains debated(15,16). Although new devices for femur neck fracture have been developed recently, their usage is limited in clinical settings because of their weakness against bending and rotational forces(17-19).
Although, DHS can involve more soft tissue dissection and extensive surgical exposure(21), biomechanical studies showed DHS had advantages compared to CS(20).
The finite element method (FEM) is an advanced computer simulation technique for evaluating structural stress. In 1972, this method was developed in engineering mechanics and introduced to orthopedic biomechanics. First, it was used for evaluation of human bones stresses. Nowadays, this method is used for bone-prosthesis structures, fracture fixation devices and various types of tissues other than bone, additionally(22). In this study, FEM was used to evaluate the biomechanical characteristics of DHS.
Materials and Methods
To perform this simulation, all computer models were sketched by Solidworks™ 2011 and these designed models were numerically analyzed in ANSYS 2011 software after assembling models of femur bone and fixation device.
In first step, the 3D computer model of femur (figure 1) was obtained by digitizing a composite anatomical model of a femur in actual size. Then, this computer model of femur was used in the finite element simulation.
The modeling of implant was performed in Solidworks™ 2011 software using actual one obtained from the clinic (Figure 2-a). The 3D map of the implant was prepared accordingly (Figure 2-b). The position of implant on the bone was considered as actual conditions in orthopedic surgery. In addition, threaded end of the screws are not modeled and considered as cylindrical objects with complete adhesion to bone.
Finite Element Models
Assembled and finite element discretized of the femur with DHS implants are shown in the figure 3-a and 3-b, respectively. ANSYS™ 2011 software was utilized to discretize the model. Note that sliding surfaces in this simulation has their own meshes for estimating contact issues.
Constant and Material Properties
Mechanical simulation process was applied after discretizing the model and applying material properties to assembled 3D model. 316L austenitic stainless steel (316L ASS) material properties were assigned to the implant. The mechanical properties of the femur in individuals with mean age of 45 years and mean weight of 60 Kg were applied in model. Non-Linear elastic anisotropic material properties were assigned to all materials involved in the model.
The friction coefficient between elements of model was considered as 0.8. The contact between the femoral implants and screw was considered as coupling type. Stress distribution in standing and fixed individual femur (with mean weight of 60 Kg and mean age of 45 years) was considered for biomechanical analysis. In this condition, the force on the femur cap of standing position is 1,500 N. If the diameter of femur cap is about 5 cm, stress distortion on the femur cap will calculate as bellow (formula 1):
Results and Discussion
The main biomechanical topics are displacement and stress distribution under applied load. These two subjects were examined and reported in our simulation. According to calculated results, displacement of head of fractured femur which is detached completely from rest of femur and fixed by device was 1 millimeter parallel to the applied load (Y direction in figure 4) and 2 millimeters perpendicular to the applied load (X direction in figure 5). This displacement is compatible to the gap we considered where femur is fractured. In addition, other part of bone deformed in range of micrometer. Generally speaking, when gap of fractured region is filled under pressure, overall displacement of fixed fractured femur by dynamic hip screw is as order of micrometer.
Important aim of studying the performance of fixation device under applied load is understanding stress distribution and figuring out how stress is sharing between bone and fixation device. Strictly speaking, the optimal condition would be no critical stress (high stress near compression and tensile strength of bone) load on the bone. In addition, fixation device serves for bearing critical stress. In our case, stress distribution (von misses stress) in fixed femur by dynamic screw device under 60 kg load has been shown in figure 6 and 7. For more clarity, stress distribution of fixation device and femur have been shown separately.
Simulated stress distribution showed that femur carries 5 to 10 MPa. This value of stress is much less than 190 MPa of compression strength of bone. Although most of the femur bears low stress under 60 Kg load, stress rises to 16 to 48 MPa around the screws used to connect device to femur. These level of stress are in safe range in femur as well and it could be concluded that there is no critical stress condition in femur.
Stress distribution in dynamic hip screw device is shown in figure 7. In most parts of the device, stress level is less than 70 MPa which is reasonably lower than 200 MPa yield point of implant grade stainless steel, except in two regions of the DHS. These two regions are shown in figure 7. Generally, stress in these two regions raises to 130 MPa. Stress at corners in these two regions reached very high stress and caused plastic deformation. This is because of neglecting curves (for simplicity) at corners to reduce stress concentration. If correct curves were applied to this region, stress would obviously be reduced to lower values.
According to calculated results of stress distribution, the mean stress in dynamic hip screw and femur is reasonably less than yield point of bone and implant grade stainless steel. The results also show that fixation device carries more stresses than femur.
In this research, finite element simulation was performed to understand stress distribution in fractured femoral neck bone and dynamic hip screw device. According to the calculated results, it could be concluded that fixation device carries more stresses than femur and mean stress in dynamic hip screw and femur in reasonably less than yield point of bone (lower than 48 MPa) and implant grade stainless steel (lower than 70 Mpa). In addition, overall displacement of fractured femoral neck fixed by dynamic hip screw is at the order of micrometer.
Hamed Reza Seyyed Hosseinzadeh MD Computational materials science and engineering scientist/researcher, Department of Mining and Metallurgical Engineering, Amirkabir University of Technology, Tehran, Iran Corresponding author firstname.lastname@example.org
Ali Sina Shahi MD Postdoctrate research fellow, Rothman Institute, Philadelphia, PA, USA email@example.com
Mohammad Qoreishi MD Orthopaedic surgeon, Assistant professor, Shahid Beheshti Medical University, Tehran, Iran firstname.lastname@example.org
Acknowledgements: None declared.
Financial disclosure: None declared.
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