Abstract
BACKGROUND: Prosthesis selection for total hip replacement is determined by geometrical matching with femoral medullary cavity of patients and the optimal biodynamics. It is of great significance for elevating outcomes of total hip replacement.
OBJECTIVE: To evaluate the biomechanics of four groups of femoral stem prostheses matched with a femur and to get a stem whose mechanics distribution is similar to the normal femur.
DESIGN: Controlled observation.
SETTING: Department of Orthopaedics of Oriental Hospital Affiliated to Tongji University, Department of Orthopaedics of Changhai Hospital of Second Military Medical University of Chinese PLA, and Department of Mechanical and Power Engineering of Shanghai Jiao Tong University.
MATERIALS: Experiments were performed at the Laboratory of Life Quality and Machinery Engineering of Department of Mechanical and Power Engineering of Shanghai Jiao Tong University from December 2004 to October 2005. A male volunteer aged 40 years with the normal proximal femur (175 cm height and 78 kg weight), free from hip disease, were selected. X-ray image of the eutopic femur was shot and wrote into memory in the format of DICOM. The volunteer signed an informed consent. The experiment was approved by Hospital Ethical Committee. No. Ⅰ prosthesis, Zimmer versys Fiber Metal Taper11#, No. Ⅱ prosthesis, Plus APL 2#, No. Ⅲ prosthesis, Welink Ribbed system cementless01#, and No. Ⅳ prosthesis, Lima F2L 1# were used in this study.
METHODS: The ezDICOM software was used to read files with DICOM format of femoral X-ray image, which was converted into files with bmp format. The image files with bmp format of the proximal femur X ray were introduced with Matlab software after regulation, and the two-dimensional contour data of femur were extracted. Prosthesis matched with the template was set in PhotoShop 7.0 software. The two-dimensional contour data of prosthetic femur were extracted in MATLAB software. The ANSYS software was used to establish the model of geometrical and two-dimensional nonlinearity finite element including femur and femur-femoral stem. Stress distribution in the proximal femur was analyzed and compared by loading.
MAIN OUTCOME MEASURES: Stress distribution of the proximal femur.
RESULTS: Stress value and distribution of No. Ⅰ prosthesis and No. Ⅳ prosthesis in proximal femur were similar to the normal femur. Moreover, No. Ⅰ prosthesis was better than No. Ⅳ prosthesis.
CONCLUSION: The biomechanics of femoral stem prostheses has been evaluated by analyzing and comparing the two-dimensional biomechanics of the femoral stem prostheses based on X-ray and template, which can offer support in optimal prosthesis selection.

INTRODUCTION

Preoperative prosthesis selection is important to obtain good outcomes of total hip replacement. Standard prostheses made by different companies are available for physicians. Some templates from different companies are well matched with the femoral medullary cavity. To choose a proper prosthesis requires general consideration, particularly biodynamics function [1]. Of these prostheses, there must be a prosthesis processing similar biodynamics characteristics to the normal hip. This prosthesis has geometrical matching with the femoral medullary cavity of patients and the optimal biodynamics function. It is of great significance for elevating outcomes of total hip replacement.
Therefore, to establish a rapid evaluation system for estimating biodynamics function of first-selected prostheses can offer supports for physicians to select a proper prosthesis.

MATERIALS AND METHODS

Materials
Experiments were performed at the Laboratory of Life Quality and Machinery Engineering of Department of Mechanical and Power Engineering of Shanghai Jiao Tong University from December 2004 to October 2005.

X-ray image of eutopic femur
A male volunteer aged 40 years with normal proximal femur (175 cm height and 78 kg weight), free from hip disease, were selected. X-ray image of the eutopic femur was shot and wrote into memory in the format of DICOM. The volunteer signed an informed consent. The experiment was approved by Hospital Ethical Committee.

Templates
Commonly used prosthetic templates included Lima, Zimmer, Welink, Aesculap, Plus, Depuy, Strykers and Jinghang. Four of them were employed in this study. Prostheses were considered to be No. Ⅰ, No. Ⅱ, No. Ⅲand No. Ⅳ. Prosthetic data were shown in Table 1.

Equipments
Hardware
PC: CPU 1.4 G, memory 256 MB, VRAM 128 MB was used in this study.

Software
There were ezDICOM (image read and conversion), Matlab6.5 (image measure and calculation), PhotoShop7.0 (image editing), surfacer 10.0 (format conversion) and ANSYS8.0 software (finite element analysis).

Methods
Establishing finite element models
The ezDICOM software was used to read files with DICOM format of femoral X-ray image, which was converted into files with bmp format. The image files with bmp format of the proximal femur X ray were introduced with Matlab software after regulation, and the two-dimensional contour data of femur were extracted. Prosthesis matched with the template was set in PhotoShop 7.0 software. The two-dimensional contour data of prosthetic femur were extracted in MATLAB software. The ANSYS software was used to establish the model of geometrical and two-dimensional nonlinearity finite element including femur and femur-femoral stem. Stress distribution in the proximal femur was analyzed and compared by loading.
Taking No. Ⅰ prosthesis as an example, the femoral stem (size 11) was matched with the proximal medullary cavity of the femur. The ball was 28 mm. The image of the prosthetic template matched with the femur is shown in Figure 1.

On the basis of wireframe model, elastic modulus (2 GPa) and Poisson's ratio (0.29) of cancellous bone, elastic modulus (17.3 GPa) and Poisson's ratio (0.29) of cortical bone [3], and elastic modulus (107 GPa) and Poisson's ratio (0.3) of titanium alloy were input with ANSYS software. An automatic meshing scheme was performed to obtain two-dimensional finite element including femur and femur-femoral stem of 3 292 nodal points and 1 089 elements (femur) and 4 035 nodal points and 1 285 elements (prosthesis) (Figures 2 and 3).

Loading and constraint of hip finite element calculation
Hip stress mainly distributed on coronal plane [4], and static force loading analysis of the hip was considered to be simple coronal plane analysis. Thus, the calculation was simpler, which is difficult to obtain precise hip stress, but we can get a general femoral stress[5]. The method is practical for the study of two-dimension with X-ray image.
Commonly, load transmission in the femoral head was done at 16o (Pauwels). The patient's weight was 534 N, which could load the standing phase of a gait cycle. Resultant of hip joint was about 2.7 times of patient's weight (1 460 N) [6]. Two-dimensional full constraint was performed for results.

RESULTS

Femoral key surface stress distribution
After femoral finite element calculation, equivalent stress of femur lateral and medial cortex at each nodal point was measured. Axial distribution charts of mean equivalent stress of the normal lateral and medial femur were gotten by calculating mean equivalent stress at each nodal point of each transverse section (Figure 4 a and b).
Key surface stress distribution after femur-femoral stem matching
Finite element calculation was performed after femur-femoral stem matching to get equivalent stress of each nodal point of the key surface. Axial distribution charts of mean equivalent stress of femur-prosthesis lateral and medial cortex were obtained by calculating mean equivalent stress at each nodal point of each transverse section (Figure 5 a and b).

In the same way, curve charts of stress distribution of different femur-prosthesis stem lateral and medial cortex were obtained and compared it to the normal femur (Figure 6 a and b).

DISCUSSION

Advantage of finite element analysis and usage of two-dimension in this study
Finite element analysis, as a method having inter-verification and inter-complement with traditional biodynamics, has been used in biomedicine study [7-9]. Femur-prosthesis contact area, stress distribution and interspace size were measured by finite element method [10-11]. These results were different to obtain, unless prosthesis with strain gauge was used [12]. However, it only can be used in a few patients. It is impossible to do a controlled study in a patient receiving different prosthesis. Finite element analysis is characterized by no injury to samples, which can exactly imitate the biodynamics behavior of in vivo tissue. Models can be repetitively used and imitate multi-loading, in which the study is time-saving, rapid, and low cost [13-14].
By two-dimensional finite element analysis, this study analyzed the biodynamics characteristics of femur and a group of prostheses matched with femoral geometry. Based on femoral X-ray and prosthetic template (two-dimensions), we used two-dimensional analysis method, besides, stress factor of hip joint mainly distributed on coronal plane and static force loading of hip joint can be considered to be a simple coronal plane analysis [3]. Stress of femur and femur-prosthesis was analyzed by two-dimensional finite element method, which is simple to apply, practical, and the result can meet the requirement of our study.

Optimizing standards of biodynamics after prosthesis implantation
Because of the differences in design, material and technique of hip prostheses, the stress distribution of the prostheses is different after implantation [15]. According to Wolff principle, changes in femoral mechanics can induce femoral stress shielding and bone re-establishment after implantation. If femur loads low stress, bone atrophy will occur due to stress shielding; If femur loads high stress, hyperosteogeny or destruction is found. Stress value and distribution of the femur is optimal at normal hip weight-bearing.
Results of finite element analysis suggested that stress was high at lateral and medial superior segment of the femur, particularly medial collum femoris where can resist high loading by increasing thickness and density of the sclerotin of normal human.
Biodynamics of prostheses can be highlighted during design and produce in different factories, and the difference in stress value and distribution between prosthesis and normal femur after loading can be reduced [16]. This study aimed to select prostheses with the optimal biodynamics from a group of prostheses matching femoral geometry, which could support orthopedists in preoperative prosthetic selection and the construction of hip prosthesis optimization expert system. It is difficult to summarize a quantitative standard of the optimal biodynamics after prosthesis implantation [17]. However, we can choose a prosthesis that has the closest stress value and distribution with the normal femur. It is of significance for a patient.

Changes in stress after prosthesis implantation
Stress distribution of the femur after prosthesis implantation differs from stress distribution of the normal femur. Following prosthesis implantation, load was transferred through the connection of the prosthesis and medullary cavity. Stress was obviously reduced in the proximal segment of the outer layer cortical bone, and slightly reduced in distal segment, but increased in middle segment. Stress was obviously decreased in the proximal segment of the inter layer cortical bone, but significantly increased in distal segment due to direct contact with the prosthesis. Stress distribution was inequable along femoral axial ray compared to the normal femur. The stress distribution might induce stress shielding and bone atrophy in the proximal femur, and hyperostosis and stress damage in the distal femur [18-19].
The contact compression stress from an ideal prosthesis and nodal point of cortical bone surface should be low and even [20]. Results of finite element calculation confirmed that mean stress value of the prosthesis was twice of the mean stress value of the femur. Therefore, distribution and maximal value of equivalent stress between the prosthesis and nodal point of cortical bone surface can be considered as the parameters to assess prosthetic design.
Rapid evaluation of prosthetic biodynamics
The optimal prosthesis matched femur and possessing close biodynamics with the normal femur can be selected by analyzing and comparing biodynamics of a group of prostheses matched femoral geometry. For a patient receiving total hip replacement, this prosthesis is the first choice. In this study, the stress value and distribution of No. Ⅰ and No. Ⅳ prostheses were similar to normal femur, particularly No. Ⅰ prosthesis, which is the optimal choice.
In summary, biodynamics of hip prosthesis can be rapidly assessed by two-dimensional finite element analysis based on femoral X-ray and femoral prosthetic stem template. That is, from a group of prostheses matched femoral geometry, a prosthesis whose value and distribution of biodynamics are similar to normal femur is selected. This result can support orthopedists in prosthetic selection and in hip prosthesis optimization expert system.

REFERENCES

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股骨柄假体的二维生物力学评价与
优化选择☆

陆晴友1，曲爱丽2，王 芬2，吴岳嵩3，王成焘2
1同济大学附属东方医院骨科，上海市 200120; 2上海交通大学机械与动力工程学院，上海市 200030; 3解放军第二军医大学长海医院骨科，上海市 200433
陆晴友☆，男，1968年生，安徽省芜湖市人，汉族，2005年解放军第二军医大学毕业，博士，主治医师，主要从事关节外科方面的研究。
摘要
背景：人工髋关节置换术前选择假体时，不仅要考虑在几何学上达到与患者股骨髓腔的匹配，而且还要在生物力学性能上达到最优，这对提高全髋关节置换术后疗效，具有较现实的意义。
目的：对4组与股骨匹配的股骨柄假体进行生物力学的评价，寻找与正常股骨力学分布最为接近的股骨柄假体。
设计：对比观察。
单位：同济大学附属东方医院骨科、 解放军第二军医大学长海医院骨科、上海交通大学机械与动力工程学院。

材料：实验于2004-12/2005-10在上海交通大学机械与动力工程学院生命质量与机械工程实验室完成。选用无髋部疾患、股骨近端形态正常志愿者（男性，40岁，身高175 cm，体质量78 kg）的股骨拍摄标准正位X射线片，并以DICOM格式存入存贮器中。志愿者签署知情同意书，同时报医学伦理委员会审批。模板：Ⅰ号假体：Zimmer versys Fiber Metal Taper11#；Ⅱ号假体：Plus APL 2#；Ⅲ号假体：Welink Ribbed system cementless01#；Ⅳ号假体：Lima F2L 1#。
方法：在Matlab中导入股骨近端X射线正位片及4组股骨-股骨柄假体模板的bmp格式的图像文件，分别提取股骨、带假体模板的股骨二维轮廓数据，利用ANSYS软件建立股骨、股骨-股骨柄几何模型及二维非线性有限元模型，负荷加载后对股骨近端的应力分布进行分析、比较。
主要观察指标：股骨近端的应力分布。
结果：Ⅰ，Ⅳ号假体对所研究的股骨而言其应力大小及分布与正常股骨相近，但两者相比Ⅰ号更为接近，应为最佳选择。
结论：通过基于X射线片与模板的股骨柄假体二维生物力学的分析比较，可对股骨柄假体的生物力学性能作出评价，为假体的优化选择提供依据。
关键词：股骨柄假体；有限元分析；生物力学；评价
中图分类号: R318 文献标识码: A 文章编号: 1673-8225(2008)09-01766-05
陆晴友，曲爱丽，王芬，吴岳嵩，王成焘.股骨柄假体的二维生物力学评价与优化选择[J].中国组织工程研究与临床康复，2008，12(9):1766-1770
[www.zglckf.com/zglckf/ejournal/upfiles/08-9/9k-1766(ps).pdf]
（Edited by Lu Y/Qiu Y/Wang L）