Design method of variable-density bone porous prosthesis based on bone density distribution

Abstract

The invention discloses a variable-density bone porous prosthesis design method based on bone density distribution, and relates to the technical field of biomedical engineering.The variable-density bone porous prosthesis design method comprises the following steps of S01, parametric modeling of unit cells, S02, pretreatment of thighbone CT data, S03, mechanical testing and mechanical property grading of porous materials, S04, establishment of a porous thighbone prosthesis model, and S05, design of a variable-density bone porous prosthesis model. According to the variable-density bone porous prosthesis design method based on bone mineral density distribution, the real bone mineral density form is simulated based on CT data of bones, parametric design is adopted in the method, quantitative research is facilitated, and micro design and then filling to a macroscopic model are utilized, that is, design is started from a minimum unit cell structure unit, and then design is started from a second unit cell structure unit to a third unit cell structure unit to a fourth unit cell structure unit to a fifth unit cell structure unit; on the basis of the bone density distribution condition of a three-dimensional CT value, the designed unit cell structures with different porosities are filled into three-dimensional bone CT data, the bone prosthesis designed through the method can be matched with the elastic modulus of surrounding bones, and a good bionic microenvironment can be provided for growth of bone tissue.

Description

Technical Field

[0001] The invention relates to the technical field of biomedical engineering, in particular to a design method for a variable-density porous bone prosthesis based on bone density distribution. Background Art

[0002] As the population ages, bone defects have become a common orthopedic disease, caused by osteoporosis, bone tumors, sports injuries, etc., which seriously affect people's lives. Bone transplantation is an effective method to repair large bone defects.

[0003] Although a variety of bone repair materials have been developed, metal implant prostheses are still the most commonly used bone transplant materials because of their excellent mechanical properties, corrosion resistance and biocompatibility. However, traditional metal prostheses are generally made of solid materials and are manufactured by traditional machining methods or casting, forging and other methods. Although this type of metal implant has better mechanical strength and even surface hardness than natural bones, the weight of the prosthesis is generally much greater than the weight of the autologous bone it fills, so it will cause obvious discomfort to the patient. In addition, compared with human bones, the elastic modulus of solid metal prostheses is usually larger, which makes the metal prosthesis implanted in the human body produce less strain than the bone when subjected to force, resulting in the prosthesis and the bone being unable to be in close contact for a long time, which is easy to cause local stress concentration at the junction, that is, the "stress shielding" effect. In the long run, the metal prosthesis will fall off from the human body at the least, and the prosthesis will cause secondary damage to the human body at the worst, and the implantation surgery will fail.

[0004] To this end, the present invention proposes a variable density bone porous prosthesis design method based on bone density distribution. By using porous materials with interconnected pores inside, not only can the elastic modulus be reduced to match the elastic modulus of the bone, but also a good biomimetic environment can be provided for the growth of bone tissue. Summary of the invention

[0005] In view of the deficiencies of the prior art, the present invention provides a design method for a variable-density porous bone prosthesis based on bone density distribution, which solves the problems raised in the background technology.

[0006] To achieve the above objectives, the present invention is implemented through the following technical solutions: a method for designing a variable density bone porous prosthesis based on bone density distribution, the steps of which are as follows:

[0007] S01. Unit cell parametric modeling: Unit cell structure types are mainly divided into bracket type and implicit surface type. When the unit cell structure type is bracket type unit cell structure, the input data are the spatial coordinates of the key points of the unit cell structure, the unit cell topological relationship and geometric parameters (such as Figure 3 If the implicit surface type unit cell structure is used, the input data is the implicit surface equation and geometric parameters (such as Figure 4 shown);

[0008] S02. Femoral CT data preprocessing: Use the medical image algorithm library to read the femoral CT scan data, cut redundant data, and implement the CT data resampling operation so that the voxel size in the CT data is equal to the outer side length of the unit cell; then, extract and save the three-dimensional coordinates and CT value of each voxel data, as follows Figure 5 The comparison of femur before and after resampling is shown;

[0009] In addition, the femur model is reconstructed in three dimensions using medical image processing algorithms, such as threshold segmentation, region growing, and moving cube algorithms, to generate a three-dimensional femur model;

[0010] S03. Mechanical testing and mechanical property grading of porous materials: By designing representative small-sized scaffold structures to test the compressive properties of materials with different porosities, and using CT values ​​to grade bone density, these grades are matched with the diameter of the porous scaffolds to achieve mapping of femoral CT values ​​to the porosity of the porous structure; Figure 6 An example shown is the fitted equivalent elastic modulus and porosity mapping diagram and grade distribution. It is worth noting that Figure 6 The mapping relationship shown is obtained by fitting the LWP unit cell structure. It is only an example and not the mapping relationship of all porous scaffolds.

[0011] In addition, porous prosthesis structures are not limited to LWP structures. In theory, any porous structure can be used. A prosthesis design usually only uses one topological structure. If a topological structure needs to use different porosities, it is only necessary to fill the designed unit cell with a certain topological structure into the three-dimensional CT data. Therefore, this method is not limited to LWP porous structures, but can be used for all possible unit cell structures.

[0012] S04. Establishment of porous femoral prosthesis model: Construct a unit cell parametric modeling class object, and use the data structure in S03 as the member parameter of the function object; traverse all voxel data and execute the unit cell structure modeling function to construct a preliminary model of the femoral prosthesis; perform Boolean intersection operation on the constructed preliminary femoral prosthesis model and the femoral 3D model to construct a flat and porous femoral prosthesis model.

[0013] Preferably, the geometric parameters in S01 include the outer side length of the unit cell (the unit cell shape is an arbitrary rectangular parallelepiped) and the diameter of the support cylinder. The unit cell topological relationship is expressed as a pair of key point numbers. The key points are the support geometric nodes of the unit cell, which are expressed as fractions of the unit cell side length.

[0014] Preferably, in S02, the approximate range of the femoral area is determined by cutting redundant data, data of non-femoral parts in the image are removed, and the femoral area is identified by setting a threshold or using a segmentation algorithm. The resampled CT data is resampled by using a resampling filter in the library to set a target voxel size, and the CT data is resampled so that the voxel size therein is the same as the outer size of the designed porous unit cell, so as to ensure that the unit cell can replace the CT voxel and fill it into the CT data.

[0015] Preferably, the S02 also includes preprocessing the CT image before performing three-dimensional reconstruction of the femur model using a medical image processing algorithm: including denoising, contrast enhancement, and correction to improve image quality and provide a better basis for subsequent processing. Threshold segmentation can be performed after CT image preprocessing to separate bone and non-bone areas; the area growth starts from a seed point and gradually expands to similar pixels; the moving cube algorithm slides a small cube window on the image data and processes the data in the window to extract edge information.

[0016] Preferably, in S03, a quasi-static compression test is performed on porous materials with different porosities, including at least three porosities. Since the material structure is too large, it will cause material waste and produce buckling behavior during the compression process, which interferes with the compression test results. The designed porous material is usually composed of a small number of complete unit cell structures. According to the international compression standard ISO13314, the size of the porous material is usually designed to be 1 cm 3 Or a cylinder with a diameter of 1-2cm.

[0017] Preferably, in S03, the equivalent elastic modulus, yield strength, and compressive strength of the porous material are measured by compression test data, and after obtaining the equivalent elastic model and yield strength test data of porous materials with different porosities through measurement, the optimal curve passing through these data points is fitted in the form of a Gibson-Ashaby mathematical model through the Levenberg-Marquardt nonlinear optimization algorithm;

[0018] The formula for elastic modulus and density is:

[0019]

[0020] The formula for yield strength and density is:

[0021]

[0022] Where: E is the elastic modulus of the material, E0 is the theoretical elastic modulus of the material, σ yis the yield strength of the material, σ0 is the theoretical yield strength of the material, ρ is the density of the material, ρ0 is the theoretical density of the material, n1 and n2 are exponents of the material properties, and C1 and C2 are constants of the material properties.

[0023] Preferably, in S03, the image data is divided into multiple levels according to the CT value range of each voxel, and each level ID is assigned to the corresponding voxel data. The larger the level ID number, the greater the bone density. The voxel data contains coordinate information, CT value and level ID information.

[0024] Preferably, the mapping of the femoral CT value to the porosity of the porous structure in S03 is achieved by designing multi-level mechanical properties by combining the mechanical property range of human bones (such as elastic modulus and yield strength), the design constraints of the porous prosthesis, and the mechanical characteristics of the porous structure, and converting them into stent diameters of various structural types; the stent diameter is bound to the voxel level ID in order of stent diameter size and saved in the voxel data.

[0025] Preferably, the three-dimensional model of the femur in S02 is obtained by computed tomography (CT) technology, wherein the basic unit of CT data is voxel, and the voxel is a cube in three-dimensional space. The ratio of the side length of the voxel is determined by the resolution of the CT scan. Before performing three-dimensional reconstruction, the CT data needs to be resampled to ensure that the original voxel size becomes the designed unit cell size. It is worth noting that after resampling, the voxel size must be ensured to be the same as the outer size of the designed unit cell structure.

[0026] The present invention provides a design method for a variable-density porous bone prosthesis based on bone density distribution, which has the following beneficial effects:

[0027] The present invention is a design method for a variable-density bone porous prosthesis based on bone density distribution. Based on the CT data of the bone, that is, the bone density distribution, the method can imitate the bone density morphology of the real bone, and the method is a parametric design, which is convenient for quantitative research. The method is a multi-scale design method, which uses micro-design and then fills it into a macro model, that is, the design starts from the smallest unit cell structure unit, and then based on the bone density distribution of the three-dimensional CT value, the designed unit cell structure with different porosity is filled into the three-dimensional bone CT data. The bone prosthesis designed by the method of the present invention can not only match the elastic modulus of the surrounding bones, but also provide a good bionic microenvironment for the growth of bone tissue. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] Figure 1 A flow chart of a method for designing a non-uniform porous femoral prosthesis according to the present invention;

[0029] Figure 2 A diagram of a femur model according to an embodiment of the present invention;

[0030] Figure 3 A schematic diagram of the design process of the unit cell structure of the bracket type of the present invention;

[0031] Figure 4 It is a schematic diagram of the design process of the implicit surface type unit cell structure of the present invention;

[0032] Figure 5 The figure is a comparison diagram before and after the femur resampling of the present invention (a is before resampling, b is after resampling);

[0033] Figure 6 is a mapping diagram of equivalent elastic modulus and porosity of the present invention;

[0034] Figure 7 A three-dimensional model diagram of a normal femur is extracted for the embodiment of the present invention;

[0035] Figure 8 This is a unit cell structure diagram of an embodiment of the present invention;

[0036] Fig. 9 A diagram of a rectangular parallelepiped model composed of small unit cells according to an embodiment of the present invention (a is an external view of the rectangular parallelepiped model, and b is an internal view of the rectangular parallelepiped model). DETAILED DESCRIPTION

[0037] The following will be combined with the drawings in the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0038] See also Figure 1 The embodiment of the present invention provides a technical solution. This embodiment takes the process of designing a femoral prosthesis CAD model and constructing a finite element model as an example:

[0039] Step 1: Extract the femur 3D model

[0040] CT data and medical image processing algorithms are used to extract a three-dimensional model of a normal femur; Figure 7 As shown in the figure, the 3D model of the femur is composed of a large number of small cuboids, each of which is called a voxel. The voxel itself can change size according to the resolution of the CT data. The medical image algorithm library is used to read the CT data, cut redundant data, and implement the resampling operation of the CT data, and finally read and record the voxel coordinates;

[0041] Step 2: Design of porous unit cell structure

[0042] Starting from the smallest unit cell structure, the porous unit cell structure that meets the mechanical properties is designed based on the relationship between the gray value of the CT image and the bone density and bone elastic modulus. Figure 8 The figure shows a design diagram of a porous unit cell structure with a latticeweaire-phelan (LWP) topological structure. The unit cell structure shown in the figure is only used as a design example. Based on the complex mechanical environment of human bones, the influence of porosity on biocompatibility, and the manufacturing limit of 3D printing equipment, the mechanical property level and porosity type are determined as input data, so as to design a reasonable unit cell structure type.

[0043] Step 3: The porous unit cell structure replaces the voxel to obtain the porous femoral structure model

[0044] Based on the relationship between the porous structure size parameters, equivalent elastic modulus and equivalent strength, the elastic modulus distribution of the prosthesis is optimized. According to step 1, all voxels are indexed according to the spatial coordinates and CT values, and the unit cell structures with different porosities replace a voxel in CT. In this way, all small unit cells will be stacked into a three-dimensional model with a rectangular shape but a porous structure inside, as shown below Fig. 9 As shown;

[0045] Based on the mapping relationship between the unit cell size parameters of the topological structure and the equivalent elastic modulus and equivalent strength, a porous unit cell structure with appropriate porosity is selected to replace the voxels in the femoral CT data. Then, the three-dimensional model of the femur is subjected to Boolean intersection operation with the generated rectangular porous structure to obtain a porous femoral model with satisfactory mechanical properties. Figure 2 As shown;

[0046] In summary, the porous material with interconnected pores inside the model can not only reduce the elastic modulus to match the elastic modulus of bones, but also provide a good bionic microenvironment for the growth of bone tissue. For example, the interconnected pores not only help the reproduction and migration of bone cells, but also provide a transport channel for the transmission of nutrients, oxygen, etc. The porosity distribution inside the model is consistent with the bone density distribution. The greater the bone density, the smaller the porosity. After obtaining the model, the model can be directly printed out according to a certain proportion through 3D printing technology, and the porous structure parameters (such as porosity, pore size, etc.) can be precisely controlled.

[0047] It is worth noting that although this embodiment is a method for establishing a porous femoral model, this method is applicable to the establishment of all types of porous bone prostheses.

[0048] The above shows and describes the basic principles and main features of the present invention and the advantages of the present invention. It is obvious to those skilled in the art that the present invention is not limited to the details of the above exemplary embodiments, and the present invention can be implemented in other specific forms without departing from the spirit or basic features of the present invention. Therefore, no matter from which point of view, the embodiments should be regarded as exemplary and non-restrictive. The scope of the present invention is defined by the attached claims rather than the above description, and it is intended that all changes falling within the meaning and scope of the equivalent elements of the claims are included in the present invention. Any figure mark in the claims should not be regarded as limiting the claims involved.

[0049] In addition, it should be understood that although the present specification is described according to implementation modes, not every implementation mode contains only one independent technical solution. This description of the specification is only for the sake of clarity. Those skilled in the art should regard the specification as a whole. The technical solutions in each embodiment may also be appropriately combined to form other implementation modes that can be understood by those skilled in the art.

Claims

1. A method for designing a variable density porous bone prosthesis based on bone density distribution, characterized in that: The steps are as follows: S01. Unit cell parametric modeling: Unit cell structure types are mainly divided into bracket type and implicit surface type. When the unit cell structure type is bracket type unit cell structure, the input data are the spatial coordinates of the key points of the unit cell structure, the unit cell topological relationship and geometric parameters. If the implicit surface type unit cell structure is used, the input data are the implicit surface equations and geometric parameters. S02. Femoral CT data preprocessing: using the medical image algorithm library to read the femoral CT scan data, cut redundant data, and implement the CT data resampling operation so that the voxel size in the CT data is equal to the outer side length of the unit cell; then, extract and save the three-dimensional coordinates and CT value of each voxel data; In addition, the femur model is reconstructed in three dimensions using medical image processing algorithms, such as threshold segmentation, region growing, and moving cube algorithms, to generate a three-dimensional femur model; S03. Mechanical testing and mechanical property grading of porous materials: By designing representative small-scale scaffold structures to test the compressive properties of materials with different porosities, and using CT values ​​to grade bone density, these grades are matched with the diameter of the porous scaffolds to achieve mapping of femoral CT values ​​to the porosity of the porous structure; S04. Establishment of porous femoral prosthesis model: Construct a unit cell parametric modeling class object, and use the data structure in S03 as the member parameter of the function object; traverse all voxel data and execute the unit cell structure modeling function to construct a preliminary model of the femoral prosthesis; perform Boolean intersection operation on the constructed preliminary femoral prosthesis model and the femoral 3D model to construct a flat and porous femoral prosthesis model.

2. The method for designing a variable density porous bone prosthesis based on bone density distribution according to claim 1, characterized in that: The geometric parameters in S01 include the outer side length of the unit cell (the unit cell shape is an arbitrary rectangular parallelepiped) and the diameter of the support cylinder. The unit cell topological relationship is expressed as a key point number pair. The key point is the support geometric node of the unit cell, which is expressed as a fraction of the unit cell side length.

3. The method for designing a variable density porous bone prosthesis based on bone density distribution according to claim 1, characterized in that: In the S02, the approximate range of the femoral area is determined by cutting redundant data, the data of the non-femoral part in the image is removed, and the femoral area is identified by setting a threshold or using a segmentation algorithm. The resampled CT data is resampled by using a resampling filter in the library to set a target voxel size, and the CT data is resampled so that the voxel size therein is the same as the outer size of the designed porous unit cell, so as to ensure that the unit cell can replace the CT voxel and fill in the CT data.

4. The method for designing a variable density porous bone prosthesis based on bone density distribution according to claim 1, characterized in that: The S02 also includes preprocessing the CT image before the three-dimensional reconstruction of the femur model using the medical image processing algorithm: including denoising, contrast enhancement, and correction to improve image quality and provide a better basis for subsequent processing. Threshold segmentation can be performed after CT image preprocessing to separate bone and non-bone areas; the area growth starts from the seed point and gradually expands to similar pixels; the moving cube algorithm slides a small cube window on the image data and processes the data in the window to extract edge information.

5. The method for designing a variable density porous bone prosthesis based on bone density distribution according to claim 1, characterized in that: In the S03, a quasi-static compression test is performed on porous materials with different porosities, including at least three porosities. Since the material structure is too large, material waste will be caused, and buckling behavior will occur during the compression process, which will interfere with the compression test results.

6. The method for designing a variable density porous bone prosthesis based on bone density distribution according to claim 1, characterized in that: In the S03, the equivalent elastic modulus, yield strength, and compressive strength of the porous material are measured by compression test data. After obtaining the equivalent elastic model and yield strength test data of porous materials with different porosities through measurement, the optimal curve passing through these data points is fitted in the form of a Gibson-Ashaby mathematical model through the Levenberg-Marquardt nonlinear optimization algorithm; The formula for elastic modulus and density is: The formula for yield strength and density is: Where: E is the elastic modulus of the material, E0 is the theoretical elastic modulus of the material, σ y is the yield strength of the material, σ0 is the theoretical yield strength of the material, ρ is the density of the material, ρ0 is the theoretical density of the material, n1 and n2 are exponents of the material properties, and C1 and C2 are constants of the material properties.

7. The method for designing a variable density porous bone prosthesis based on bone density distribution according to claim 1, characterized in that: In S03, the image data is divided into multiple levels according to the CT value range of each voxel, and each level ID is assigned to the corresponding voxel data. The larger the level ID number, the greater the bone density. The voxel data contains spatial coordinate information, CT value and level ID information.

8. The method for designing a variable density porous bone prosthesis based on bone density distribution according to claim 1, characterized in that: The mapping of the femoral CT value to the porosity of the porous structure in S03 is achieved by designing multi-level mechanical properties in combination with the mechanical property range of human bones (such as elastic modulus and yield strength), the design constraints of the porous prosthesis, and the mechanical characteristics of the porous structure, and converting them into stent diameters of various structural types; the stent diameter is bound to the voxel level ID in the order of stent diameter size and saved in the voxel data.

9. The method for designing a variable density porous bone prosthesis based on bone density distribution according to claim 1, characterized in that: The femur three-dimensional model in S02 is obtained by computed tomography (CT) technology, wherein the basic unit of CT data is voxel, and the voxel is a cube in three-dimensional space. The ratio of the side length of the voxel is determined by the resolution of the CT scan. Before three-dimensional reconstruction, the CT data needs to be resampled to ensure that the original voxel size is equal to the designed unit cell outer size.

Images