A pedicle screw with a topologically optimized grid lattice structure

Abstract

A pedicle screw with a topologically optimized grid lattice structure relates to the field of orthopedic implants. The effective design part of the spinal screw is the screw rod part. The outer side of the screw rod has a variable pitch thread, and the inside of the screw rod is filled with a topologically optimized grid lattice structure, which reduces the elastic modulus while increasing its compressive strength, effectively reduces stress shielding, and promotes bone growth. In addition, the design of the topological lattice structure allows bone cement to be evenly and fully filled and distributed between the prosthesis and the bone.

Description

Technical Field

[0001] The invention relates to the field of orthopedic implants, and in particular to a pedicle screw with a topologically optimized structure manufactured by SLM. Background Art

[0002] As an advanced metal manufacturing technology, selective laser melting (SLM) has great advantages in the preparation of bone implants.

[0003] Medical metal implants are often used to repair bone tissue in the load-bearing part of the human body, but due to the "stress shielding" phenomenon caused by its high elastic modulus, it is designed into a mesh structure. On the one hand, the mesh implant can provide the necessary living space for the adhesion, migration and proliferation of bone cells in the body, and on the other hand, it can also provide a transmission channel for nutrients and metabolites for cell reproduction and metabolism. Therefore, mesh implants play an important role in bone tissue repair and reconstruction. In recent decades, with the rapid development of rapid prototyping technology, the ability to manufacture complex parts has been greatly improved, but the mechanical strength and other properties of the mesh structure cannot give satisfactory results. With the rapid development of optimization algorithms and computer technology, structural optimization, especially the research and application of structural topology optimization methods, has made great progress. Topology optimization design aims to achieve the optimal performance of the specified target by regulating the porosity, pore size, interpore connectivity and other characteristics of the unit structure, while meeting the strength, stiffness, stability and other one or more design requirements.

[0004] In modern orthopedic surgery, pedicle screw fixation technology is increasingly used in spinal surgery. Due to its good biological stability, safety and reliability, spinal pedicle screw fixation technology has been widely used in the treatment of lumbar disc herniation, spinal fractures, spinal tumors and other diseases. With the popularization of treatment methods, many clinical adverse symptoms have also emerged, such as loosening of implanted screws, changes in the distance between connecting rods, and screw withdrawal after surgery, so there are higher requirements for the stability of pedicle screws. Summary of the invention

[0005] To solve the above problems, the present invention provides a pedicle screw with a topologically optimized structure, wherein the effective design part of the screw is the screw rod part, the screw rod has a variable pitch thread on the outside, and the inside of the screw rod is filled with a topologically optimized grid lattice structure, which increases its compressive strength while reducing the elastic modulus, effectively reducing stress shielding. At the same time, a special surface structure is used in the cancellous bone area to effectively promote bone growth. In addition, the design of the grid lattice structure allows bone cement to be evenly and fully filled and distributed between the prosthesis and the bone.

[0006] The technical solution adopted by the present invention is: a method for designing grid lattice structure primitives based on topological optimization theory, under the stress condition of human bones, the porosity of the grid structure is constrained to complete the maximum stiffness design, laying the foundation for the mechanical properties of the lattice implant.

[0007] The present application provides a grid lattice structure screw after gradient topology optimization for laser selective melting, comprising:

[0008] Its basic structural unit is composed of eight topologically optimized structures with the same morphology but different combination directions. The design of each structure strictly complies with the density cloud map results after topological software optimization.

[0009] The specific topology optimization process is as follows:

[0010] The constraint function is the desired porosity value, and the objective function is to obtain the corresponding optimal material distribution model under the condition of a fixed porosity value, that is, the maximum stiffness of the structure.

[0011] In order to simplify the construction of a basic structure suitable for the human skeleton, the primitives that constitute the human skeleton are identified as a set of small cubes. The topological optimization process can be simplified to an n×n×n (n is greater than or equal to 1) cube. In order to make the designed grid structure have a multi-surface structure inside, the force points and constraint points of the cube are set as opposite points of the body diagonal. The upper right corner (n,n,n) is subjected to a concentrated load force, and the magnitude of the concentrated load force is the force on the human bone. The vertex at the lower left corner (0,0,0) is the fixed end.

[0012] Enter the basic material properties, namely the elastic modulus and Poisson's ratio for a solid material without pores.

[0013] The cube model is meshed and numerical calculations are performed using ANSYS software during the topology optimization process. In order to ensure the accuracy of the topology optimization model and not reduce the calculation efficiency, the cube model mesh is divided into 20 layers along the three different directions of xyz (20 is the recommended value), so the elements that make up the cube model are 20. 3 That is 8000.

[0014] After completing the basic material property input and meshing of the cube model, it is necessary to call the Topology Optimization module of the ANSYS software to perform topology optimization on the cube model with a convergence accuracy of 0.001 and 20 iterations.

[0015] After the topology optimization is completed, the final cube density cloud map result will be obtained. In the density cloud map, density filtering is performed to keep the red part with an output density above 0.8.

[0016] The specific design plan is:

[0017] Step 1: Based on the density cloud map results of ANSYS topology optimization, the xy section diagrams at different positions perpendicular to the z-axis direction are intercepted, and the unit mesh model after topology optimization is reconstructed and the surface is smoothed using Solidworks 3D graphics software.

[0018] Step 2: The reconstructed unit grid structure is stacked into a unit grid array structure using Boolean operations in groups of eight, which is used as the basic design unit for the subsequent grid array structure. It is then imported into the Materialise Magics software for further repair of the model, mainly to sew up the bad edges, gaps and holes of the structure, and to delete the reverse triangles, overlapping and intersecting triangles and interfering shells.

[0019] Step 3: Select the unit grid array structure repaired in Materialise Magics in step 2, that is, a topology-optimized unit grid array structure.

[0020] Step 4: Repeat the same steps 1, 2 and 3 to obtain a topologically optimized grid structure with a porosity value of 60%-70%. Take any two grid structures with porosity values ​​within the range of 60%-70%, and copy and stack them in the direction parallel to the force line. Perform a Boolean operation on the copied and stacked grid structure and a cylindrical screw rod with a bottom diameter of 6.0mm-6.5mm to obtain a screw rod filled with a grid structure. The filling height of the grid structure accounts for 2 / 3 of the total length of the screw rod.

[0021] Step 5: Make other parts of the subsequent screws on the model filled with the grid structure obtained in step 4.

[0022] Technical effects of the present invention:

[0023] 1. The gradient topology grid lattice screw adopts a unique gradient lattice design, which can provide sufficient specific surface area inside the screw structure for bone cells to attach and grow, making the combination of the screw and the bone more stable and reducing the probability of displacement and falling off. The specific surface area increases with the increase of porosity. When the porosity reaches more than 60%, the specific surface area of ​​the structure exceeds the grid structure constructed by implicit surface, and has better biocompatibility.

[0024] 2. The porosity of the gradient topology grid lattice screw gradually increases from the bottom to the top. On the one hand, the bottom part with small porosity ensures that the bone cement can be evenly and fully filled and distributed between the prosthesis and the bone, achieving the effect of promoting bone growth in the cancellous bone area; on the other hand, the high-porosity grid filling part greatly improves the stability effect on the denser cortical bone area.

[0025] 3. Gradient topology grid lattice screws are a unique type of pedicle screws that can flexibly adjust the size and shape of the pores, and then quantitatively adjust the elastic modulus. They can match and adjust the elastic modulus according to the different spinal bones of the human body and are widely used. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] Figure 1 It is a physical model of the human skeleton primitive;

[0027] Figure 2 It is a topology optimization process;

[0028] Figure 3 It is the three-dimensional model of the structure and the corresponding surface after topology optimization;

[0029] Figure 4 It is a complete three-dimensional model of a cell;

[0030] Figure 5 is the structural arrangement direction after topology optimization;

[0031] Figure 6 It is the lower end of the screw after filling the gradient mesh structure after topology optimization;

[0032] Figure 7 It is the overall cross-sectional view of the pedicle screw filled with topologically optimized gradient structure; DETAILED DESCRIPTION

[0033] In order to make the present invention more clearly understandable, the pedicle screw of the topologically optimized grid lattice structure of the present invention is further described in detail below through embodiments and in conjunction with the accompanying drawings:

[0034] Embodiment 1:

[0035] A unit grid structure is first established by creating a 20×20×20 cube model in the simulation software ANASY. Figure 1 As shown in the figure, the upper right corner (20,20,20) is subjected to a concentrated load, the lower left corner (0,0,0) and its surroundings are fixed ends, the elastic modulus and Poisson's ratio of the solid material itself without pores and the desired porosity value of 60% are input, and the mesh is divided into 20 layers. Figure 2As shown in Figure 1, the topology optimization control process (ACCUR (accuracy) = 0.001, ITER (iteration number) = 20) is set and the final density cloud map is obtained by solving. In the modeling process, the density cloud map is divided into 20-30 sections along the z-axis direction by using the application software SolidWorks. The images with a density exceeding 0.8 in the section density information are located and recorded by the drawing software. Finally, the lofting is connected in series until the modeling is completed. Figure 3 As shown. Eight basic structural units with the same appearance but different combination directions, namely Figure 3 The structural units in the structure are combined. The specific combination method is: take surface B passing through the force direction as the reference plane, perform a mirror processing to obtain a 1 / 4 cell model of the complete cell structure, then perform a mirror processing on the obtained 1 / 4 cell model with surface A as the reference plane to obtain a 1 / 2 cell model of the complete cell structure, and then perform a mirror processing on the 1 / 2 cell model with surface C as the reference plane, and finally present a complete unit grid structure. Figure 4 .

[0036] Embodiment 2:

[0037] A gradient topology optimized grid lattice structure screw for laser selective melting. Its additive manufacturing equipment uses the laser selective melting equipment SLM280 from SLM-SOLUTION of Germany. HL The materials used are titanium and medical titanium alloy or cobalt-chromium alloy.

[0038] Based on the density cloud map results of ANSYS topology optimization, the xy section diagrams at different positions perpendicular to the z-axis direction were intercepted, and the topology optimization unit mesh model was reconstructed and the surface was smoothed using Solidworks 3D graphics software.

[0039] The reconstructed unit grid structure is grouped into eight units, and the unit grid structures are stacked into a unit grid array structure using Boolean operations, which serves as the basic design unit of the grid array structure. The specific arrangement is: using surface B passing through the force direction as the reference plane, a mirror processing is performed to obtain a 1 / 4 cell model of the complete cell structure, and then the obtained 1 / 4 cell model is mirrored with surface A as the reference plane to obtain a 1 / 2 cell model of the complete cell structure, and then the 1 / 2 cell model is mirrored with surface C as the reference plane, and finally a complete cell structure is presented.

[0040] The complete unit grid array structure is imported into the Materialise Magics software for further repair of the model. The main repair objects are to stitch the bad edges, gaps and holes of the structure, and to delete the reverse triangles, overlapping and crossing triangles and interfering shells.

[0041] Select the cell grid array structure repaired in Materialise Magics, that is, a topology-optimized cell grid array structure.

[0042] By repeating the same steps, we can obtain a topologically optimized mesh structure with a porosity value of 60%-70%. Take any two mesh structures with porosity values ​​of 60% and 70%, such as Figure 5 As shown, replication and stacking are performed in a direction parallel to the force line, and Boolean operations are performed on the grid structure after replication and stacking and the cylindrical screw rod with a bottom diameter of 6.0 mm-6.5 mm to obtain the screw rod filled with the grid structure, as shown in Figure 6 As shown, the filling height of the grid structure accounts for 2 / 3 of the total length of the screw rod.

[0043] The rest of the screw parts are subsequently manufactured on the model filled with the grid structure.

[0044] The outside of the screw shank has a variable pitch thread, such as Figure 7 As shown, the distance between threads increases continuously from the bottom end of the screw to the top end of the screw, with the minimum pitch being 1.5 mm and the maximum pitch being 3.5 mm.

[0045] The middle of the screw rod is provided with a central hole which runs through the entire screw.

[0046] After production is completed, the complete model is repaired and stored, and the model is converted from a solid to an STL file format.

[0047] Import the model in STL file format into Materialise Magics to add support and slice and layer it. After processing, import it into the additive manufacturing device for printing.

[0048] Although the present invention has been described herein in conjunction with specific embodiments, many other variations, modifications and applications will be apparent to those skilled in the art. Therefore, the present invention should not be limited by the specific disclosure herein, but should be limited by the appended claims.

Claims

1. A pedicle screw with a topologically optimized grid lattice structure, characterized in that: include: The inside of the screw rod is filled with a topologically optimized grid lattice structure, which is obtained by arraying a single cell. A single cell is composed of eight topologically optimized structures that have the same appearance but different combination directions. The steps of topology optimization are as follows: Step 1: Based on the density cloud map results after ANSYS topology optimization, the xy section diagrams at different positions perpendicular to the z-axis direction are intercepted, and the unit mesh model after topology optimization is reconstructed and the surface is smoothed using Solidworks 3D graphics software; Step 2: The reconstructed unit grid structures are grouped into eight units, and the unit grid structures are stacked into a unit grid array structure using Boolean operations, which serves as the basic design unit of the subsequent grid array structure; Import the cell grid array structure into Materialise Magics software to further repair the model. The repair targets are to stitch the bad edges, gaps and holes of the structure, and delete the reverse triangles, overlapping and crossing triangles and interfering shells. Step 3: Select the unit grid array structure repaired in Materialise Magics in step 2, that is, a topology-optimized unit grid array structure; Step 4: Repeat steps 1, 2 and 3 to obtain a topologically optimized mesh structure with a porosity value of 60%-70%, take any two mesh structures with porosity values ​​within the range of 60%-70%, and replicate and stack them in a direction parallel to the force line, perform a Boolean operation on the replicated and stacked mesh structure and a cylindrical screw rod with a bottom diameter of 6.0 mm-6.5 mm, and obtain a screw rod filled with a mesh structure, and the filling height of the mesh structure accounts for 2 / 3 of the total length of the screw rod; Step 5: Make other parts of the subsequent screws on the model filled with the grid structure obtained in step 4; The specific combination method of a single cell is: a topology optimized structural model is mirrored with surface B passing through the force direction as the reference plane to obtain a 1 / 4 cell model of the complete cell structure, and then the obtained 1 / 4 cell model is mirrored with surface A as the reference plane to obtain a 1 / 2 cell model of the complete cell structure, and then the 1 / 2 cell model is mirrored with surface C as the reference plane, and finally a complete cell structure is presented; The center hole is located in the center of the screw shaft and runs through the entire screw; The basic structural unit of the grid lattice structure is composed of eight topologically optimized structures with the same morphology but different combination directions; The specific topology optimization process is as follows: The primitives that make up the human skeleton are identified as a set of small cubes. The topological optimization process can be simplified to an n×n×n cube, where n is greater than or equal to 1. The force points and constraint points of the cube are set as the opposite points of the body diagonal. The upper right corner (n,n,n) is subjected to a concentrated load force, the magnitude of which is the force on the human bones. The vertex at the lower left corner (0,0,0) is the fixed end. Input basic material properties, i.e. elastic modulus and Poisson's ratio for solid material without pores. The cube model is meshed. ANSYS software is used for numerical calculation in the topology optimization process. The mesh of the cube model is divided into 20 layers along the three different directions of x, y and z. Therefore, the elements that make up the cube model are 20. 3 pcs, that is, 8000; After completing the basic material property input and cube model meshing, it is necessary to call the Topology Optimization module of ANSYS software to perform topology optimization on the cube model with a convergence accuracy of 0.001 and 20 iterations. After the topology optimization is completed, the final cube density cloud map result will be obtained. In the density cloud map, density filtering is performed to keep the part with output density above 0.

8.

2. The pedicle screw with a topologically optimized grid lattice structure according to claim 1, characterized in that: One-piece molding using additive manufacturing technology.

3. The pedicle screw with a topologically optimized grid lattice structure according to claim 1, characterized in that: The materials used are titanium, medical titanium alloy or cobalt-chromium alloy.

4. The pedicle screw with a topologically optimized grid lattice structure according to claim 1, characterized in that: The outer side of the screw rod has a variable pitch thread, and the distance between the threads increases continuously from the bottom end of the screw to the top end of the screw, wherein the minimum pitch is 1.5 mm and the maximum pitch is 3.5 mm.

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