Finite elemental analysis of outer and inner surfaces of the proximal half of an intact femur.
Abstract: Finite element (FE) simulation has become increasingly important to analyze the behavior of the femoral bone when it is subjected to external loads. The aim of the present study was to validate FE simulation with experimental work using surface strain at the outer surface of the femur, and to subsequently, evaluate von Mises strain for its inner surface. A standardized femur from the bone repository in the Biomechanics European Laboratory (BEL) in Italy was used for the analysis as a base for finite element modeling. The material model used for the analysis was assumed to be isotropic and linearly elastic, and the external loads applied comprised of a head load and an abductor load. The values of stress and strain in the anterior, posterior, medial, and lateral positions of the femur have been presented in this report. The results from the present study show that strain distribution is not uniform across the section of the femur, suggesting the occurrence of circumferential bending. Thus, better understanding of von Mises strain on the inner surface of the femur is important for total hip replacements (THRs).
Article Type: Report
Subject: Finite element method (Research)
Femur (Models)
Femur (Properties)
Computer-generated environments (Research)
Computer simulation (Research)
Authors: Shuib, Solehuddin
Sahari, Barkawi B.
Voon, Wong Shaw
Arumugam, Manohar
Pub Date: 04/01/2012
Publication: Name: Trends in Biomaterials and Artificial Organs Publisher: Society for Biomaterials and Artificial Organs Audience: Academic Format: Magazine/Journal Subject: Health Copyright: COPYRIGHT 2012 Society for Biomaterials and Artificial Organs ISSN: 0971-1198
Issue: Date: April, 2012 Source Volume: 26 Source Issue: 2
Topic: Event Code: 310 Science & research
Geographic: Geographic Scope: Malaysia Geographic Code: 9MALA Malaysia
Accession Number: 304842708
Full Text: Introduction

The femur is one of the most highly loaded bones in the human body. Along with experimental work designed to verify the authenticity of the finite element (FE) model, the FE method has been employed to obtain comprehensive information about the states of stress and strain in the intact femur [1-4]. Most of the experimental and simulation studies performed, to date, have assumed that the intact femur was being subjected to simple forces, together with the reaction force and the abductor load [5,6].

The finite element method (FEM) demands meticulous attention during the modeling of the intact femur and while creating the FE mesh [7]. In general, there are two basic ways to generate an FE model from computed tomography (CT) scans: geometry-based and voxel-based [8].

To the best of the authors' knowledge, there are no literature reports on simulation work on the inner surface of the intact femur. Therefore, this study has sought to verify the FE simulation work on the outer femoral surface with experimental work [7] by using von Mises strain, and then using the verified model to determine the strain on the inner surface of the intact femur.

Material and Methods

In the present study, the model of an adolescent -sized femur (length 420 mm, canal diameter of 13.0 mm) was selected for which the widely used standard Initial Graphics Exchange Specification (IGES) format file was used [7].

The material used for the femoral bone was a glass fiber with a Young's Modulus of 14,200 MPa and a Poisson's ratio of 0.3 [7]. The model was assumed to be homogeneous, isotropic, and linearly elastic. The FE mesh of the model consisted of a total 16692 elements and 32024 nodes. The femoral bone was fixed at the distal end; the loading condition is illustrated in Figure 1. A head load (HL) and an abductor load (AL) were considered for the loading conditions.

The components of the loads in x, y, and z directions acting on the femur are given as follows [7]:

For the head Load (HL):

x = 927.68 N, y = 0, z = -1744.7 N--1976 N

For the abductor load (AL):

x = -797.06 N, y = 0, z = 949.89 N--1240 N

[FIGURE 1 OMITTED]

[FIGURE 2a OMITTED]

[FIGURE 2b OMITTED]

The methodology of the analysis involved two steps. Firstly, strain values were obtained at the medial (M), lateral (L), anterior (A), and posterior (P) positions on the outer surface of the intact femur. These locations were denoted as M1, M2, M3, M4, M5, L1, L2, L3, L4, L5, A1, A2, A3, A4, A5, P1, P2, P3, P4, and P5, as shown in Figures 2(a) and 2(b) based on their respective distance from the datum. Secondly, the strain values for the inner lateral (L), inner medial (M), inner anterior (A), and inner posterior (P) surfaces were included in the results. The results for the outer and inner strain values were then plotted and have been shown in Figures 3 and 4(a)^4(d). Finally, the results were analyzed and discussed.

Results and Discussion

Outer surface strain

Figure 3 shows the comparison of the data obtained from ANSYS V11.0 simulation for experimental and simulation results for the axial strain along the intact femur [7]. There was good agreement between the experimental and simulation results, which indicated that the model developed was acceptable in terms of geometry, boundary conditions, and loading.

Figures 4(a), (b), (c), and (d) show the von Mises strain results obtained for the medial (M), lateral (L), anterior (A), and posterior (P) locations in the outer and inner surfaces of the intact femur.

It can be seen that for the medial (M) and lateral (L) positions in the outer surface of the femur, the highest peak occurs at the M3 and L3 locations, and the smallest peak occurs at the Ml and L1 locations. For the medial (M) and lateral (L) positions on the inner surface, the highest peak occurs at the M2 and L2 locations, and the smallest peak occurs at the M1 and L3 locations.

For the anterior (A) and posterior (P) positions on the outer surface of the femur, the highest peak occurs at the A1 and P2 locations, and the smallest peak occurs at the A5 and P4 locations. For the anterior (A) and posterior (P) positions on the inner femoral surface, the highest peak occurs at the A2 and P1 locations, and the smallest peak occurs at the A5 and P4 locations.

Figure 5(a) shows the minimal and the maximal von Mises strain values while Figure 5(b) shows the minimal and the maximal von Mises stress values for the intact femur. From these figures, it can be seen that the strain values at the outer and inner surfaces were not the same. This indicates that the stresses on the femur were not uniform, which gave rise to circumferential bending. This phenomenon was due to the fact that the load was applied on the femoral head, and because the line of action of the load did not coincide with the axis of symmetry of the femur. This offset caused bending of the femur. Therefore, strain measurement at the outer femoral surface alone is not sufficient to predict the inner femoral surface strain

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[FIGURE 5a OMITTED]

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Inner surface strain

During the benchmarking stage, it was found that the present simulation results for strain at the outer femoral surface were very close to the experimental results (Figure3) [7].

For medial (M) and lateral (L) positions on the outer femoral surface, the highest peak occurs at the M3 location and the smallest peak occurs at the M1 and L1 locations. For the anterior (A) and posterior (P) positions on the outer femoral surface, the highest peak occurs at the A1 and P2 locations while the smallest peak occurs at the A5 location. All these results are similar to those reported by McNamara et al. (1997) [2]. However, the smallest peak which occurred at the P4 location was not in agreement with results reported by McNamara et al. (1997) [2] according to which the smallest peak occurred at the P1 location [2].

For the medial (M) and lateral (L) positions on the inner femoral surface, the highest peak occurred at the M2 and L2 locations while the smallest peak occurred at the M1 and L3 locations. For the anterior (A) and posterior (P) positions on the inner femoral surface, the highest peak occurred at the A2 and P1 locations while the smallest peak occurred at the A5 and P4 locations.

Thus, by knowing the location and values of the highest peak for the inner surface of an intact femur, better understanding can be gained about various problems related to THRs, such as i) the method to reduce fixation failure [9], ii) and he means to improve the longevity of THRs [10].

As for the minimal and the maximal von Mises stress and strain values for the intact femur shown in Figures 5(a) and 5(b), the greatest amount of stress or strain would be expected to occur at the greater trochanter [11]. This location is in the region where femoral neck fractures could happen, and is also, in the vicinity of the implant rim [4].

Conclusions

Strain distribution in the femur indicated that the strain was not uniform on the outer and inner surfaces of the femur, resulting in circumferential bending due to load offset.

References

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[5.] J.A. Simoes, M.A. Vaz, S. Blatcher, M.Taylor, Influence of head constraint and muscle forces on the strain distribution within the intact femur, Medical Engineering & Physics, (22):453-9(2000).

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[7.] M. Viceconti, L. Bellingeri, L. Cristofolini, A. Tonia, A comparative study on different methods of automatic mesh generation of human femurs, Medical Engineering & Physics, (20):1-10(1998).

[8.] M. Lengsfeld, J. Schmitt, P. Altera, K. Kaminsky, R. Leppek. Comparison of geometry-based and CT voxel-based finite element modelling and experimental validation. Medical Engineering & Physics, (20):515-22(1998).

[9.] L. Cristofolini, P.Erani, A.S. Teutonico, F.Traina, M.Viceconti, Partially cemented hip stems do not fail in simulated active patients, Clinical Biomechanics, 22 (2):191-202(2007).

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[11.] J.A. Szivek, J.B. Benjamin, P.L. Anderson, An experimental method for the application of lateral muscle loading and its effect on femoral strain distributions, Med Eng Phys, 22 (2):109-16(2000).

Solehuddin Shuib (1), Barkawi B. Sahari (2), Wong Shaw Voon (2), Manohar Arumugam (2)

(1) School of Mechanical Engineering, Engineering Campus, Universiti Sains Malaysia, 14300 Nibong Tebal, Seberang Perai, Malaysia

(2) Department of Mechanical Engineering and Manufacturing, Institute of Advanced Technology (ITMA) --

(3) Department of Orthopaedics, Faculty of Medicine & Health Sciences, Universiti Putra Malaysia, 43400 UPM, Serdang, Selangor Darul Ehsan, Malaysia

Received 22 October 2011; Accepted 18 March 2012; Available online 27 April 2012
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