股骨核心减压材料植入体生物力学性能的有限元分析

doi:10.3969/j.issn.2095-4344.2015.35.024

摘要/Abstract

摘要：

背景：借助计算机辅助分析技术进行股骨坏死后的手术治疗和患者康复的预测分析，实现科学施行手术和更快的术后康复，是目前国际相关研究领域的热点研究问题。
目的：通过观察不同的核心减压材料植入体对股骨头的受力变形和应力分布、骨折风险以及在交界面处的生物力学性能的影响情况，为选择合适的核心减压材料植入体提供依据。
方法：基于6例患者的股骨坏死区域的核磁共振扫描图片，建立3D股骨几何模型和有限元分析模型，并设定生理负荷量和4种不同种类的材料植入体Actifuse，BioOss，Osteoset，Prodense，应用有限元分析软件AnsysWorkbench对股骨性能进行分析计算。
结果与结论：实验得到了股骨在工作过程中的最大应力分布规律，随着4种填充材料杨氏模量数值的升高，股骨坏死区域的最大正应力的数值下降，但同时交接面处的生物力学性能如滑动距离、摩擦应力和压力数值升高。文章在综合考虑骨折危险和交接面处的生物力学性能的基础上，对比了4种材料的力学性能后，推荐了合适的核心减压材料为Osteoset。

中国组织工程研究杂志出版内容重点：人工关节；骨植入物；脊柱；骨折；内固定；数字化骨科；组织工程

关键词: 植入物, 骨植入物, 数字化医学, 股骨, 股骨头坏死, 核心减压, 高级核心减压, 股骨骨折的风险, 交界面的生物力学性能, 植入体材料, 有限元法

Abstract:

BACKGROUND: With the help of computer aided analysis technique, the prediction and analysis of the surgical treatment and rehabilitation of patients with femoral necrosis, which can realize the scientific operation and recovery of the patients after surgery, is a hot research topic in the field of international related research.
OBJECTIVE: By studying the force deformation and stress distribution, fracture risk, and Interface’s biomechanical properties of the femoral head in different core decompression materials, we can provide the basis for choosing the appropriate core decompression material.
METHODS: Based on nuclear magnetic resonance imaging images of six patients with different ages, the three-dimensional model and finite element analysis model were established, and the physiological load and four different types of materials (Actifuse, BioOss, Osteoset, Prodense) were set up. Finite element analysis software AnsysWorkbench was used to analyze the performance of the femur.
RESULTS AND CONCLUSION: The maximum stress distribution law of the femur in the work process was obtained. With the increase of the Young’s modulus of the material, the maximum normal stress of the femoral necrosis region was decreased, but the biomechanical properties of the joint surface, such as sliding distance, friction stress and pressure, are increased. On the basis of comprehensive consideration of the biomechanical properties of the fracture risk and the junction, a suitable core decompression material (Osteoset) is recommended.

中国组织工程研究杂志出版内容重点：人工关节；骨植入物；脊柱；骨折；内固定；数字化骨科；组织工程

Key words: Tissue Engineering, Femur Head, Femur Head Necrosis, Biomechanics

中图分类号:

R318

武彪. 股骨核心减压材料植入体生物力学性能的有限元分析[J]. 中国组织工程研究, 2015, 19(35): 5705-5711.

Biao Wu . Finite element analysis of biomechanical properties of femoral core decompression materials[J]. Chinese Journal of Tissue Engineering Research, 2015, 19(35): 5705-5711.

图/表（结果） 14

Fracture risk
In the study, the bone failure criterion based on principal stresses was adopted to estimate fracture risk. Therefore, the maximum values of the first principal stress of bone were calculated and summarized in Table 4 for all models. For normal walking, the maximum principal stress in all models under complete osseointegration with implant was larger than untreated model. For the postoperative state, the data of each material implant are similar; it has little or no change (Table 5). However, the most positions of maximum principal stress in each model under the complete osseointegration and postoperative state are near (Figure 5).

Biomechanical behaviors at interface
Under postoperative state, the biomechanical behavior at interface including in the frictional stress, sliding distance and pressure were indicated in Tables 6-8. The positions for each models are also near, which direct to the normal walking force in the necrotic domain surface (Figure 6).

Results of statistical analysis
The standard deviation of maximum principal stress of six pairs of femur models was indicated in Figure 7. The mean value of the maximum principal stress was given in Table 9.

As displayed in Figure 7 and Table 9, the maximum principal stress with material implants was larger (nearly 30%) than untreated femur model, and the positions of maximum principal stress were almost in the beginning area of the drilling hole. It illustrated that the maximum principal stress would decrease with the increased Young’s modulus value, in contrast the difference between complete osseointegration (bonded) and postoperative state (frictional) would increase with the increased Young’s modulus value.

The statistical analysis of the biomechanical behaviors at interface was shown in Figures 8-10. The mean value of data was given in Table 10. As displayed in Table 10, the data of the femur model with material implants were quite different from the untreated model, based on the position of the biomechanical behaviors direct to the normal walking force in the necrotic domain surface and compared with four material implants, the frictional stress, pressure and sliding distance would almost increase with the increased Young’s modulus (besides the sliding distance of material implant Prodense data).

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引言

In recent years, there are 10 000-20 000 patients of osteonecrosis of the femoral head in the United States per year^[1-20]. This disease usually affects the health of patients in their late 30 and early 40 years old^{[8, 10, 20]}. Due to the increased incidence and high morbidity, femoral head necrosis is attracting more and more concern.

Core decompression is an established technique for treatment of early osteonecrosis^{[12, 15, 18]}. The procedure is designed to decrease pressure within the bone by restoring blood flow to the bone by using drilling one or more small channels into the dead bone (necroticlesion) from the lateral subtrochanteric region of femur^{[1-4, 6, 11, 18]}. After operation, the patients are normally requested to be partial weight bearing for several weeks, normally six weeks due to the risk of fracture^{[3, 5, 6, 14, 21-25]}. Using the theory of finite element method, the femur model can be created with the computer and the simulation can be calculated by the ANSYS software; this can predict the patient’s postoperative recovery situation^{[10, 13, 19, 26]}. Thus, with the help of computer aided analysis technology, research of core decompression in femurs and the scientific implementation of operation treatment are of great significance to improve the case’s cure rate undoubtedly^{[1, 7, 16, 27]}.

The aim of this study was to solve the following problems: (1) what is the stress distribution of femur as well as the biomechanical behaviors at interface? (2) Is there an optimal implant material for which the fracture risk would be lowest under complete osseo-intergration and postoperative state? (3) Which material implant has good biomechanical behaviors at the interface?

The basic research idea of this study is: (1) building the three dimensional (3D) femur model derived from the reconstruction of CT or magnetic resonance imaging (MRI) images; (2) ascertaining the structural nature, loading condition as well as boundary constraint conditions of the femur^{[25, 27-30]}; (3) building 3D

finite element mode, and analyzing the influences of core decompression on the mechanics of the femur.

中国组织工程研究杂志出版内容重点：人工关节；骨植入物；脊柱；骨折；内固定；数字化骨科；组织工程

材料方法

Design
Computer simulation experiment.

Time and setting
This study was conducted in the following laboratories at the University of Duisburg-Essen, Germany. (1) July 2014 to January 2015, Department of Orthopaedics, University Hospital Essen, University of Duisburg-Essen, Essen, Germany. (2) July 2014 to January 2015, Department of Anatomy, University Hospital Essen, University of Duisburg-Essen, Essen, Germany. (3) January 2015 to May 2015, Department of Mechanics and Robotics, University of Duisburg-Essen, Duisburg, Germany.

Materials
In this study, six patients’ MRI images were selected as the study object. The femoral model of the patients was divided into two groups corresponding to postoperative and rehabilitation respectively, then a series of simulation analysis were carried out, and the stress situation of each model by implanting different materials was compared, finally, the optimal implant materials were chosen.

Mimics simulation
A 3D computer aided design model of six pairs of full femur was generated from MRI scan data set of approximately 350 slices using the software Mimics. The procedure included: first, a segmentation process that assigns the bone region to pixels of the image (Figure 1). Then, atriangular surface model was extracted from segmentation results based on surface reconstruction techniques. The last step was the generation of the model volume enclosed by the surface (Figure 2).

Finite element simulation
After each femur datum was input into Ansys Workbench, the femoral head necrosisdomain should be defined base on the real operation. A drilling channel of 9 mm diameter was created from an entrance point on the lateral sub-trochanteric region penetrating through the necrotic domain (Figure 3). The necrotic domain of simulation should be larger so that the head necrosis can be excised.

Finite element meshes were generated using 4-node tetrahedron element. A mesh refinement was performed in the excision region to better capture the mechanical behavior of the excision domain. The element numbers for 10-node tetrahedron element meshes were kept unchanged as those for 4-node meshes. As no significant differences in the total strain energy and maximum stress were found for the six pairs of different meshes, the mesh with maximum element length of 3 mm and 4-node tetrahedron element was chosen as the base for all models (Figure 4). The numbers of nodes and elements for each femur model were given in Table 1.

After the 3D model was established, the material properties were set for each femur based on the empirical formula of clinical medicine research.Density: ρ=1.131+0.001067×HU (g/cm3); Young’s modulus: E=6950×ρ1.49 (MPa); Poisson ratio: v=0.3.

The four kinds of implant material properties were given in Table 2. To estimate the fracture risk, the maximum. Principal stress under complete osseointegration (bonded) and postoperative state (friction coefficient is 0.2) were considered as the main index. The biomechanical behaviors at interface should be considered approximately sliding distance, frictional stress and pressure. Peak load of hip joint force representing daily activities was investigated for normal walking. The load was extracted from the patient’s data and details of the load components were given in Table 3. In each analysis, the applied load was distributed over nodes on the femoral head within a radius of 10 mm to avoid stress concentration. The surface which is at the half of the femur was fully constrained (Figure 4).

Main outcome measures
There were the value of the maximum principle stress, frictional stress, sliding distance, and pressure.

中国组织工程研究杂志出版内容重点：人工关节；骨植入物；脊柱；骨折；内固定；数字化骨科；组织工程