analysis on biomechanical characteristics of shape memory alloy...

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, [email protected] 2 Tae Soo Bae, [email protected]

1 Kwon Hee Kim, [email protected]

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)

20 Copyright © 2016 SERSC

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


Copyright © 2016 SERSC 21

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).



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



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



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

1. Statistics Korea, 2006; 2007 (In Korean)

2. Health insurance review & assessment service, “Status of femur fracture in age”, 2015 (In

Korean)

3. Wool, SL.: Less rigid internal fixation plate: historical perspectives and new concept,

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living in older adults after femur fracture surgery, Vol.22, No. 1, pp 1-12, 2015, Korean

Society of Muscle and Joint Health (In Korean)



5. Jakob, M.: Fracture of the distal radius treated by internal fixation and early function, J

Bone Joint Surg, Apr;82(3):340-344, 2000

6. Kang, S.-B.: Fracture Treatment using TiNi shape memory alloy bone fixater (BRM-SH

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and Applications, 2002

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periprosthetic femoral fractures. 36.3: 619-626, International orthopaedics, 2012

9. Brojan, M.: Shape Memory Alloys in medicine, RMZ-Materials and environment, Vol. 55,

No. 2, pp. 173-189, 2008

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680, pp. 119-122, Applied Mechanics and Materials, 2014

11. Croker, S. L., Clement, J. G., Donlon, D.: A comparison of cortical bone thickness in the

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