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Analysis on Biomechanical Characteristics of Shape
Memory Alloy Fixation Grip for Femoral Fractures
Taemin Byun1, Tae Soo Bae2 and Kwon Hee Kim1
1 Department of mechanical engineering, Korea University, Seoul, Korea 2 Department of biomedical engineering, Jungwon University, Goesan, Korea
1 Taemin Byun, ll3179ll@korea.ac.kr 2 Tae Soo Bae, bmebae@jwu.ac.kr
1 Kwon Hee Kim, kwonhkim@korea.ac.kr
Abstract. As life the expectancy has increased with the advancement of
medical technology, the frequency of femur fractures has also increased. The
conventional treatment utilizes a bone fixation plate and screw system.
However, because the screws are inserted into directly into the bones, it can
cause bones to get crushed and leave holes during removal of the screws. For
the reasons above, secondary fractures can occur. In this study, the feasibility of
a fixation grip which is composed of a shape memory alloy was investigated to
resolve the disadvantages of bone fixation plates and screws by compressive
and torsional analysis.
Keywords: Shape memory alloy, Fixation grip, Femur fracture, Finite Element
Analysis, Biomechanical Characteristics
1 Introduction
South Korean society is aging rapidly. It has been reported that elderly population is
expected to be approximately 5.4 million in 2010 [1]. The population is expected to
become an aged society by 2026. As the elderly population rate is increasing, so are
the femur fracture rates. [2].
Femur fractures include simple and complex fractures. Commonly, an expert
surgeon considers the fracture`s characteristics for operation. In some cases, fixation
nails are inserted into the bone marrow cavity. In other cases, bone fixation plates and
screws are tightened upon the surface of the fractured region. However, fixing screws
on the bones directly, may cause bone deformation or may crush the bones. Therefore,
after treatment, the screws are left in the bones. Thus, the remaining holes may cause
secondary fractures [3]. For the reasons above, the bone fixation plate and screw
system is difficult use to patients who suffer from osteoporosis or are advanced in age.
However, for patients who cannot undergo operations, bone restoration requires a
longer time and potential complications may lead to the patients’ death [4].
The first alternative to overcome the limitations of the bone fixation plate and
screw system, is to use smaller components to reduce the size of holes upon the bones
[5]. This method improves treatment quality, but holes are left after removing the
Advanced Science and Technology Letters Vol.141 (GST 2016), pp.19-26
http://dx.doi.org/10.14257/astl.2016.141.05
ISSN: 2287-1233 ASTL Copyright © 2016 SERSC
screws. Thus it is not appropriate for elderly patients. To overcome these problems, a
shape memory alloy has been employed to fix the fractured region instead of fixation
plate and screws and clinical reports have been documented [6][7][8].
In this study, we have investigated the feasibility of a shape-memory-alloy grip for
bone fixation by using biomechanical analysis. To analyze the shape-memory-alloy,
the finite element analysis was employed for compressive and torsional analysis of
the bone fixation grip which consists of shape-memory-alloy. The alloy was analyzed
by finite element analysis for compressive and torsional analysis
2 Method
2.1 Design of Shape-memory-alloy Fixation Grip for Femoral Fracture
Brojan`s study [9] and Hassan`s study [10] suggested the concept of fixation grips
made of shape memory alloy (Fig 1). The fixation grip surrounds the fracture instead
of directly on the fractured area. In this study, the concept of the two studies was
referred to, while performing bone fixation grip modeling (Fig 2).
Fig. 1 Concept of Shape-memory-alloy fixation grip [9][10]
Fig. 2. 3D Modeling of Shape-memory-alloy fixation grip
2.2. Design of Femoral Fracture Model
To collect basic information concerning designs of femur fracture fixation plate
Advanced Science and Technology Letters Vol.141 (GST 2016)
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models, 3-dimensional femur models were reconstructed using commercial software
(Mimics 16.0, Materialise) based on CT (Computed Tomography) images (Pixel Size:
0.832 mm, Slice Increment: 1.0 mm / provided by KISTI (Korea Institute of Science
and Technology Information)) of 10 Korean cadavers (Fig 3). The cross section of the
femoral shafts were measured. The average length was found to be 31mm and cortical
bone thickness was found to be 7mm [11]. Femur fracture models were formed in a
cylinder shape following the figures (Fig 4).
Fig. 3. Femur model and Measurement of femur cross section
Fig. 4. Simplified femur fracture model
2.3 Material Properties
As for materials used in the analysis, elderly femur property values were applied
based on the previous study (Table 1). Property values of shape memory alloy
components were set in reference to shape memory alloy experiment and analysis
data (Table 2).
Table 1. Mechanical properties of elderly femur [12]
Parameter Value
Young`s modulus 16,700MPa
Poisson`s ratio 0.26
Advanced Science and Technology Letters Vol.141 (GST 2016)
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Table 2. Mechanical properties of shape memory alloy [13]
Parameter Value
Young’s Modulus 75,000 MPa
Passion’s ratio 0.33
Hardening parameter 500 MPa
Reference Temperature 5℃
Elastic Limit 300 MPa
Temperature Scaling Parameter 7.5
Maximum Transformation Strain 0.08
Martensite Modulus 28,000 MPa
Lode Dependency Parameter 0
2.4 Construction of Finite Element Model
To implement the finite element analysis for compression and torsion, the bone
fixation grip model under 2.1 and femur model under 2.2 were constructed as the
finite element model (Fig 5). The solid elements of ANSYS (Soild186, Solid187)
were applied to the finite element models of the cylinder and the bone fixation grip,
and the shape memory effect function of ANSYS 14.5 was utilized (Table 3).
Fig. 5. 3D Finite element models
Table 3. Information of 3D Finite element models
Type Hexahedron (Solid186, Solid187)
Nodes 33,795
Elements 25,134
2.5 Boundary Condition of Compression Analysis
Compressive analysis was conducted by applying the characteristics of shape
recovery of the femur models with the bone fixation grip; fixed support was set to the
femoral distal part; then a force of 750N, the average body weight of Koreans was
applied to the proximal part (Fig 6).
Advanced Science and Technology Letters Vol.141 (GST 2016)
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Fig. 6. Compressive analysis of boundary conditions
2.6 Boundary Condition of Torsional Analysis
The torsional analysis was conducted by applying the characteristics of shape
recovery of the femur models with the bone fixation grip; fixed support was set to the
femoral distal part; then a torque of 12N-m [14], the physiological torque of the
human thigh was applied to the proximal part (Fig 7).
Fig. 7. Torsional analysis of boundary conditions
3 Results
3.1 Results of Compression Analysis
Compressive analysis found 36.56MPa (von-Mises Stress) in the femur (Fig 8), along
with 1.39mm directional displacement to the compression direction (Fig 9).
Displacement due to compression force was analyzed. Additionally, the strain graph
result indicates that the shape memory alloy has the effects of shape memory
Advanced Science and Technology Letters Vol.141 (GST 2016)
Copyright © 2016 SERSC 23
Fig. 8. von-Mises stress of femur in proximal part
Fig. 9. Directional displacement of femur in proximal part
Fig. 10. Results of strain in fixation grip
3.2 Results of Torsional Analysis
The torsional analysis found 37.76MPa (von-Mises Stress) in the femur (Fig 11)
along with 0.42mm total displacement (Fig 12). Displacement was due to the torque
that was analyzed. Additionally, the strain graph result indicates that the shape
memory alloy has the effects of shape memory
Advanced Science and Technology Letters Vol.141 (GST 2016)
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Fig. 11. von-Mises stress of femur in proximal part
Fig. 12. Total displacement of femur in proximal part
4 Conclusion
In this study, the bone fixation grip manufactured by shape memory alloy has been
investigated to overcome the disadvantages of the conventional bone fixation plate
and screw system. Compressive analysis found that the femur model which includes
shape memory alloy grip tolerates the loaded human weight and maintains its location.
In the torsional analysis, the performance indicated that the shape-memory-alloy
fixation grip fulfilled its role when torsion was applied to the femur model with the
bone fixation grip.
In future studies, according to the commercialized fixation and screw system, finite
element analysis will be implemented to analyze the compressive and torsional
analysis. Through the comparison between the fixation plate-screw system and shape
memory alloy grip with finite element analysis, stability of shape-memory alloy
fixation grips will be investigated.
References
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Copyright © 2016 SERSC 25
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