Title: Finite Element Analysis of Fixation Device for Femoral Neck Fracture: Dynamic Hip Screw versus Cannulated Screws
Author(s): Hamed Reza Seyyed Hosseinzadeh PhD 1*, Alisina Shahi MD 2, Touraj Shafaghi MD 3
1 Amirkabir University of Technology, Tehran, Iran 2 Rothman Institute, Philadelphia, USA 3 Shahid Beheshti Medical University, Tehran, Iran
* Corresponding Author
Vol 2, Num 2, April 2015
Introduction:Femoral neck fracture usually occur in elderly patients and it is one of the common injuries. Dynamic hip screw (DHS) and cannulated screws are well-known techniques for treatment of femoral neck fractures. Dynamic hip screw is a device to fix fractured femoral neck and in comparison to cannulated screws has more engineered design.
Materials and Methods:In this research, stress distribution in these devices were examined by computer simulation. In addition, overall performance of these devices were also explained and tried to disclose parts of this device carrying more stress during loading in human body.
Results and Conclusion: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) is reasonably less than yield point of bone and implant grade stainless steel and fixation device carry more stresses than the femur itself. It was also confirmed that mean stress in cannulated screws (lower than 140 MPa) and femur (lower than 42 MPa) in reasonably less than yield point of bone and implant grade stainless. Simulation results also showed that dynamic hip screw is dissipating more applied external stress and therefore, less stress is transmitted to bone in comparison to fixation by cannulated screws.
Keyword:Dynamic Hip Screw, Cannulated Screw, Finite Element Analysis, Femoral Neck Fracture, Fixation Device, Orthopaedics, A.6, J.1, J.2
Femoral neck fracture is one of the most common skeletal injuries and still is a great challenges to orthopedic surgeons. Unfortunately, according to population studies, its incidence is increasing sharply.[1,2] A rise of 74% in prevalence of proximal femoral fractures was predicted until the year 2020, in Germany. Moreover, an 8.6% in-hospital mortality was described in patients with over 85 years. This kind of fractures also causes high morbidity.[3-5] Femoral neck fractures characterize as 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 bone mineral density of proximal femur; from fractures in elderly population with low-energy falls to young population with high-energy trauma. 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.
According to the literature there is general consensus about the anatomical reduction and stable internal fixation as the treatment goal in these fractures.[9,10] Different techniques are used for treatment of femur neck fractures such as arthroplasty (hemiarthoplasty, 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 choice for treatment of displaced femoral neck fractures remains debated.[15,16] Although the devices used in femoral neck fracture were improved recently, their usage is limited in surgical processes because of their weakness against bending and rotational forces.[17-19]
Biomechanical studies showed DHS had advantages compared to CS. According to the results of these studies, canncellous screws had lower mean peak force and lower ultimate load to failure compared to the DHS.
Since, DHS can result in more soft tissue dissection and extensive surgical exposure should be performed to use this device , CS are often suggested for fixation of both undisplaced and displaced femoral neck fractures in young patients with good bone stock.
The finite element method (FEM) is an advanced computer technique for evaluation of 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.
The aim of this study is to evaluate and compare the biomechanical characteristics of CS and DHS by finite element method.
Materials and Methods
In the 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 each device was performed in Solidworks™ 2011 software using actual one obtained from the clinic (Figure 2-a). The 3D map of the device was prepared accordingly (Figure 2-b). The position of dynamic hip screw and cannulated screws on the bone were considered as actual conditions in orthopedic surgery (Figure 3). For simplicity, threaded end of the screws are not modeled and considered as cylindrical objects with complete adhesion to bone.
Finite Element Models
Assembled and discretized finite element of the femur with DHS device and cannulated screws are shown in the figure 4-a and 4-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.
Mechanical simulation process was performed after discretizing the model and applying material properties to assembled 3D model. 316L austenitic stainless steel (316L ASS) material properties were assigned to the used devices and screws. The mechanical properties of the femur in individuals with mean age of 45 years and mean weight of 72 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
Main important biomechanical considerations are displacement and stress distribution under applied load. These two subjects were examined and reported in our simulation. According to calculated results in our previous research, 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 and 2 millimeters perpendicular to the applied load. In addition, other part of bone deformed in range of micrometer. Generally speaking, when the gap of fracture region is filled under pressure, overall displacement of fracture fixed by dynamic hip screw is at the order of micrometer.
Important point of studying the performance of fixation device under applied load is understanding stress distribution and figuring out how stress is shared with bone and fixation device. Generally speaking, best condition would be no critical stress (high stress near compression and tensile strength of bone) condition on bone. In addition, fixation device must bear all critical stress on it. In our case, stress distribution (von misses stress) in femur fixed by dynamic hip screw and cannulated screws under 60 kg load have been shown in figure 5a  and 5b, respectively. For more clarity, stress distribution of fixation device and femur have been shown, separately.
According to our previous published results , fractures fixed by dynamic hip screw carries 5 to 10 MPa under 60 Kg load. This value of stress is much less than 190 MPa of compression strength of bone. Although in this case, most parts of the femur bear low stress under 60 Kg load, in the area adjacent to screws used to connect the dynamic hip screw device to femur, stress rises to 16 to 48 MPa. These levels of stress are in the safe range for femur too. So, it could be concluded that there is no area of critical stress condition in femur when dynamic hip screw is used.
In case of cannulated screws, fracture fixed by CS carries 5 to 30 MPa under 60 Kg load which is spread in a wide area of femur below the screws. This value of stress is much less than 190 MPa of compression strength of bone. Comparing these results with that of the DHS shows that, in the same condition of applied stress, the femur bears less stress if dynamic hip screws device is used.
Previously it was confirmed that in most part of dynamic hip screw device , stress level is less than 70 MPa which is reasonably lower than 200 MPa yield point of implant grade stainless steel except for two regions. Generally, stress in these two regions is 130 Mpa.
Simulated stress distribution in case of cannulated screws has been shown in figure 6. It is obvious that the highest stress was produced at the fracture site (region 1 in figure 6), which is at the range of 120 MPa. This level of stress is comparable to maximum stress in dynamic hip screw device which was 130 MPa. Another important point in cannulated screws under external load is different stress distribution for each screw. The maximum stress was detected in the uppermost screw (first screw from top) in the femoral neck. Whereas, the screw at lower position is bearing the lowest stress (53 MPa) in comparison to the other screws at the facture site. But, surprisingly, the tip of this lowest screw is bearing the highest stress (17 MPa) in comparison to other screws (region 2 in figure 6).
According to calculated results of stress distribution, it could conclude that mean stress in both dynamic hip screw and cannulated screws in reasonably less than yield point of implant grade stainless steel. In addition, in both fixation methods, that is, the stress in bone is also in elastic deformation range. So, the bone bears stress much lower than its full strength. This study shows that DHS can dissipate stress in the device better than CS. So, in fractures fixed by CS, the bone bears more stresses.
In this research, finite element simulation is used to understand the stress distribution in femoral neck fracture fixed whether by dynamic hip screw device or cannulated screws. According to the simulated/calculated results, it is confirmed that DHS can dissipate stress better than CS, that is to say, femur is bearing lower level of stress in case of using DHS than CS. In both DHS and CS, the maximum stress in the devices are 130 and 120 MPa, respectively.
Hamed Reza Seyyed Hosseinzadeh PhD Computational materials science and engineering scientist/researcher, Department of Mining and Metallurgical Engineering Amirkabir University of Technology, Tehran, Iran Corresponding author email@example.com
Ali Sina Shahi MD Postdoctrate research fellow, Rothman Institute, Philadelphia, PA, USA firstname.lastname@example.org
Touraj Shafaghi MD Assistant Professor, Orthopaedic Surgeon, Shahid Beheshti Medical University, Tehran, Iran email@example.com
Acknowledgements: None declared.
Financial disclosure: None declared.
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