A rib implant structure with gradient mechanical transition, design method and application

Abstract

A rib implant structure with gradient mechanical transition, design method and application thereof, belonging to the technical field of artificial prostheses; the rib implant structure is a spring-like rib structure, which sequentially includes a costal cartilage part, a gradient rib part and a rib part along the length direction. The outer end of the costal cartilage part is connected to the sternum implant, and the outer end of the rib part is connected to the missing end of the human rib; the gradient rib part is connected between the costal cartilage part and the rib part to realize the mechanical gradient transition of the implanted rib from a low modulus to a high modulus structure. By calculation, a reasonable range of structural parameters conforming to the biomechanics of natural ribs is obtained; according to the different elastic moduli of ribs and costal cartilages, a gradient transition is adopted to design a gradient mechanical spring-like rib implant for the 2nd to 6th ribs, which plays a major role in respiratory function, and has the functions of both the costal cartilage part and the rib part at the same time. The invention solves the problem that the elastic moduli of rib implants, costal cartilages and ribs are inconsistent and cannot meet the normal breathing of patients after surgery.

Description

Technical Field

[0001] The present invention belongs to the technical field of artificial prostheses, and particularly relates to a rib implant structure with gradient mechanical transition, a design method and an application thereof. Background Art

[0002] After surgery, patients with chest wall tumors often present with large-area chest rib defects, and chest wall reconstruction is the only effective treatment method. In recent years, the application of additive manufacturing technology in medicine has gradually deepened. Laser additive manufacturing of TC4 rib implants can greatly improve chest wall reconstruction technology due to its personalized adaptation ability. At present, the initial clinical implantation work of laser additive manufacturing TC4 rib implants has been carried out, and its existing limitations and deficiencies mainly focus on the following two aspects: First, the existing rib implants only meet the purpose of repairing the anatomical chest wall appearance, and the rigid reconstruction of simple TC4 rib implants fails to better reproduce the functions of physiological structures such as costal cartilage directly related to respiratory function due to their too high modulus, resulting in restrictive pulmonary ventilation dysfunction in patients after surgery, and even inducing respiratory failure in severe cases; Second, due to the limited bioactivity of titanium alloy, large-area replacement will lead to delayed healing of the surrounding soft tissues of the implant, wound infection, and reduced postoperative recovery ability.

[0003] In response to the early stress shielding phenomenon of rib implants, the design of implants has undergone continuous improvement in structures such as traditional plate-shaped rib implants, rod-shaped / filamentous rib implants, porous rib implants, and "Greek wave" structure rib implants. Compared with the previous static reconstruction and rigid structures without physiological flexibility, the "Greek wave" structure implants may have better elasticity. However, at present, these structures are all in the initial trial stage, lacking relevant basic data on structural mechanical properties, and unable to accurately evaluate their matching degree with natural ribs and the degree of restoration of biological functions.

[0004] In the prior art, a rod-shaped implant with an elastic structure is disclosed as a fixing element connecting at least two bone segments, which realizes the lightweight of sternum, rib, and costal cartilage implants in clinical applications and has certain elastic functions. However, the design of this structure does not consider the influence of parameters such as the elastic modulus of the rib itself on the function, and its structural mechanical properties and the matching with the natural ribs of the human body have not been reported, and it is still impossible to judge the impact on the human respiratory function after implantation.

[0005] In the prior art, a costal cartilage prosthesis implant with an elastic structure is disclosed. The middle part is an elastic structure, and the two ends are fixed structures. By changing the parameters of the middle elastic structure, the elastic modulus of the costal cartilage prosthesis implant can be correspondingly changed according to different implantation sites. However, this technology only focuses on the design of the costal cartilage part and does not provide any description of the fixed structures at both ends. If conventional structures are simply used, stress concentration will occur, and the human ribs cannot be truly simulated. The document mentions that the printing materials can be PEEK and titanium alloy, but does not mention the specific printing parameters of titanium alloy. Moreover, even for the same structure, the material properties of PEEK and titanium alloy are too different to achieve the same performance.

[0006] Table 1 shows the mechanical properties of ribs reported in the literature. The bending elastic modulus of the rib cortical bone is about 11.5 GPa. The bending elastic modulus of the ribs with telescopic function is very low, between 8.7 - 12.6 MPa. The elastic modulus of the rib implants in the prior art cannot reach the range of costal cartilage and cannot simultaneously meet the elastic moduli of ribs and costal cartilage. Therefore, the bionic rib implants in the prior art still cause restrictive pulmonary ventilation dysfunction in patients after surgery, resulting in the inhibition of the recovery of the patient's respiratory function.

[0007] Table 1 Mechanical properties of ribs

[0008] Summary of the Invention

[0009] Technical problems to be solved:

[0010] In order to avoid the deficiencies of the prior art, the present invention provides a rib implant structure, design method and application with gradient mechanical transition. The inventor conducts finite element simulation mechanical analysis on the rib implant model, obtains the influence mechanism of structural parameters on the change of elastic modulus, and calculates a reasonable range of structural parameters that conform to the biomechanics of natural ribs. At the same time, according to the different elastic moduli of human ribs and costal cartilage, a gradient transition is adopted to design a gradient mechanical variable cross-section rib implant for the 2nd to 6th ribs that play a major role in respiratory function, enabling it to have the functions of both the costal cartilage part and the rib part. The present invention solves the problem that the elastic moduli of rib implants, costal cartilage, and ribs are inconsistent and cannot meet the normal breathing of patients after surgery.

[0011] The technical solution of the present invention is: A rib implant structure with gradient mechanical transition, the rib implant structure is an elastic structure with a gradient change in elastic modulus, and the mechanical level along the length direction sequentially meets the mechanical properties of the human costal cartilage part and the rib part. A mechanical gradient transition structure is adopted between the costal cartilage part and the rib part. The outer end of the costal cartilage part is connected to the sternum implant, and the outer end of the rib part is connected to the missing end of the human rib;

[0012] The gradient transition structure is connected between the costal cartilage part and the rib part, realizing the structural mechanics gradient transition of the implanted rib from low modulus to high modulus.

[0013] A further technical solution of the present invention is that the outer envelope elliptical dimensions of the rib implant structure are set to be the same as the external dimensions of the natural human rib. When the major semi-axis A = 3.4 mm and the minor semi-axis B = 3.0 mm, the bending elastic modulus of the costal cartilage part is 396 MPa, and the bending elastic modulus of the rib part is 10.9 GPa; when the major semi-axis A = 5.0 mm and the minor semi-axis B = 3.0 mm, the bending elastic modulus of the costal cartilage part is 170 MPa, and the bending elastic modulus of the rib part is 3.6 GPa.

[0014] A further technical solution of the present invention is that the rib implant structure is a rib structure similar to a spring; the gradient rib part is made of TC4 titanium alloy, and includes three gradient changes from the costal cartilage part to the rib part. The major semi-axes a of the cross-sections of the spring wires of the three gradients are 1.6 - 2.4 mm, 2.4 - 3.2 mm, and 3.2 - 4.8 mm respectively, the minor semi-axes b of the three gradients are all 1.0 mm, and the pitch t of the spring wires of the three gradients are 5.5, 7.0, and 10.0 mm respectively.

[0015] A further technical solution of the present invention is that the costal cartilage part is a spring-like structure made of TC4 titanium alloy. The major semi-axis A of the outer envelope elliptical dimension of the spring-like structure is 3.4 - 5.0 mm, the minor semi-axis B is 3.0 mm. The major semi-axis a of the cross-section of the spring wire is 1.3 - 1.5 mm, the minor semi-axis b is 0.8 - 1.0 mm, the pitch t of the spring wire is 4.0 - 6.3 mm, and its bending elastic modulus is 170 - 396 MPa.

[0016] The rib part is a spring-like structure made of TC4 titanium alloy. The major semi-axis A of the outer envelope elliptical dimension of the spring-like structure is 3.4 - 5.0 mm, the minor semi-axis B is 3.0 mm. The major semi-axis a of the cross-section of the spring wire is 4.8 mm, the minor semi-axis b is 1.2 mm, the pitch t of the spring wire is 10.0 mm, and its bending elastic modulus is 3.6 - 10.9 GPa.

[0017] A further technical solution of the present invention is that the outer end of the costal cartilage part is connected to the sternum implant by a reinforcement structure;

[0018] The top cross-section of the reinforcement structure is consistent with the outer end cross-section of the costal cartilage part and is smoothly connected. Its bottom end is smoothly connected to the side wall of the sternum implant, and the radial cross-sectional area of the reinforcement structure increases along the axis from the top to the bottom.

[0019] A design method for a rib implant structure with gradient mechanical transition, the specific steps are as follows:

[0020] Step 1: Construct a finite element stress-strain analysis model of the elastic structure, and combine experiments to verify the influence of the structural parameters of the spring-like rib implant structure on the mechanical properties;

[0021] Step 2: Construct a fitting relationship model between the structural parameters and the bending elastic modulus E;

[0022] Step 3: According to the bending elastic moduli of the rib and costal cartilage, use the fitting relationship model in Step 2 to calculate the structural parameter values of the rib part and the costal cartilage part in the rib implant structure, and determine the structures of the rib part and the costal cartilage part;

[0023] Step 4: According to the structural parameters of the rib part and the costal cartilage part, determine the gradient change between the two and establish a connection to obtain a gradient transition structure;

[0024] Step 5: Use the selective laser melting method to prepare an integrated structure of the rib part, the gradient transition structure, and the costal cartilage part, that is, obtain the rib implant structure.

[0025] A further technical solution of the present invention is that in the above Step 1, first, construct a model of the elastic structure, that is, a spring-like structure model; then, perform a finite element stress-strain analysis on the model; after that, verify the finite element simulation; finally, obtain the influence of the structural parameters of the spring-like rib implant structure on the mechanical properties; the structural parameters include the major semi-axis a and the minor semi-axis b of the elliptical size of the spring wire cross-section, the spring wire pitch t, the major semi-axis A and the minor semi-axis B of the outer envelope ellipse size of the spring-like;

[0026] The larger the cross-sectional dimensions a and b of the spring wire, the greater the structural elastic modulus of the spring-like rib implant; the greater the difference between the major and minor semi-axes of the spring wire cross-section, the lower the structural elastic modulus of the spring-like specimen; and under the same conditions, the influence of the cross-sectional area of the elliptical spring wire on the elastic modulus is greater than the influence of the difference between the major and minor semi-axes;

[0027] The larger the spring wire pitch t, the greater the structural elastic modulus of the spring-like rib implant;

[0028] The smaller the outer envelope ellipse size A and B of the spring-like rib implant, the greater the structural elastic modulus of the spring-like rib implant, and the adjustment of its major semi-axis has a more significant influence on the structural elastic modulus than the minor semi-axis.

[0029] The cross-sectional dimensions of the spring wire show a large positive correlation, the outer envelope ellipse size shows a large negative correlation, and both have a greater influence on the bending elastic modulus of the spring-like rib implant. The spring wire pitch shows a medium positive correlation and has a smaller influence.

[0030] A further technical solution of the present invention is that in the step 2, the fitting relationship model of the flexural modulus E is as follows:

[0031]

[0032] An application of a rib implant structure with gradient mechanical transition, using the rib implant structure as a rib replacement product for chest wall reconstruction; the rib replacement product is an integrated structure, including the 2nd rib to the 6th rib, and a sternum implant;

[0033] The parameters of the 2nd rib are: the length of the costal cartilage part is 28 mm, the length of the first stage of the gradient rib part is 11 mm, the length of the second stage of the gradient rib part is 14 mm, the length of the third stage of the gradient rib part is 7 mm, and the length of the rib part is 0;

[0034] The parameters of the 3rd rib are: the length of the costal cartilage part is 28 mm, the length of the first stage of the gradient rib part is 11 mm, the length of the second stage of the gradient rib part is 14 mm, the length of the third stage of the gradient rib part is 7 mm, and the length of the rib part is 0;

[0035] The parameters of the 4th rib are: the length of the costal cartilage part is 40 mm, the length of the first stage of the gradient rib part is 11 mm, the length of the second stage of the gradient rib part is 14 mm, the length of the third stage of the gradient rib part is 7 mm, and the length of the rib part is 0;

[0036] The parameters of the 5th rib are: the length of the costal cartilage part is 56 mm, the length of the first stage of the gradient rib part is 11 mm, the length of the second stage of the gradient rib part is 14 mm, the length of the third stage of the gradient rib part is 7 mm, and the length of the rib part is 0;

[0037] The parameters of the 6th rib are: the length of the costal cartilage part is 60 mm, the length of the first stage of the gradient rib part is 11 mm, the length of the second stage of the gradient rib part is 14 mm, the length of the third stage of the gradient rib part is 7 mm, and the length of the rib part is 0.

[0038] A further technical solution of the present invention is that the rib implant structure is a surface lattice lightweight structure with pore sizes of 400 μm and 500 μm.

[0039] Beneficial effects

[0040] The beneficial effects of the present invention are as follows:

[0041] (1) The present invention constructs a three-point bending finite element stress-strain analysis model for a rib implant structure with gradient mechanical transition, and verifies the accuracy and effectiveness of the simulation through experiments. Based on the study of the influence of structural parameters on the bending elastic modulus of an elastic rib implant, statistical analysis of the experimental data is carried out to obtain the correlation coefficients of each structural parameter, and a fitting data model of the bending elastic modulus of the rib implant structure with gradient mechanical transition and each structural parameter is constructed. It realizes that the gradient mechanics of the rib implant structure can meet the mechanical requirements of both the costal cartilage part and the rib part at the same time, and a gradient structure is used to connect them to achieve a fine mechanical transition. The elastic modulus of the costal cartilage part is 170 - 396 MPa, and the elastic modulus of the rib part is 3.6 - 10.9 GPa, which can effectively restore the patient's postoperative respiratory function.

[0042] (2) Aiming at the problem of respiratory restriction faced by the application of laser additive manufacturing titanium alloy implants in chest wall bony reconstruction, the present invention introduces the idea of "gradient mechanics" of the structure, and at the same time establishes a spring-like structure close to the elastic modulus of the costal cartilage and the rib and adopts gradient mechanical transition to realize the design and preparation of a novel spring-like structure gradient mechanical bionic TC4 rib implant.

[0043] (3) Aiming at the problems of repeated infection of the wound surface and poor recovery ability after large-area chest wall implantation, the present invention constructs a TC4 surface microstructure by using a lattice structure. Based on the analysis of the cell compatibility and cell growth morphology of the porous surface microstructure, it is recommended to preferably adopt a surface lattice lightweight structure with pore sizes of 400 μm and 500 μm.

[0044] (4) In practical applications, the present invention specifically designs bionic rib implants for the 2nd to 6th ribs that have a major impact on respiratory function. Through mechanical property evaluation, the results show that the structure of the present invention realizes the zonal regulation of mechanical properties; and realizes the variable modulus design of the sternum-rib integrated model. Description of the Drawings

[0045] Figure 1 It is a gradient bionic rib implant structure model with an elastic function spring-like structure;

[0046] Figure 2 It is an integrated forming structure diagram of a gradient mechanical spring-like rib implant structure and the sternum; (a) Front view; (b) Side view; (c) Local enlarged view of the connection; (d) TC4 specimen prepared by SLM;

[0047] Figure 3 It is a spring-like rib implant model; (a) Bending models 1 - 3; (b) Straight models 4 - 7;

[0048] Figure 4 It is a schematic diagram of the structural parameters of a spring-like TC4 rib implant;

[0049] Figure 5 The influence of the minor semi-axis b of the cross-section of the spring wire on the mechanical properties under different conditions;

[0050] Figure 6 The influence of the major semi-axis a on the mechanical properties of the spring-like rib implant;

[0051] Figure 7 The change of the bending structural modulus under the conditions of the same cross-sectional area and the same difference between the major and minor semi-axes of the spring-like structure; (a) The difference between the major and minor semi-axes a / b≈1.70; (b) The difference between the major and minor semi-axes a / b≈1.40; (c) The cross-sectional area π ab ≈1.40π; (d) The cross-sectional area π ab ≈1.70π;

[0052] Figure 8 The stress-strain curves of the models with different spring wire pitches t; (a) a = 1.4mm, b = 0.8mm; (b) a = 1.5mm, b = 0.8mm; (c) a = 1.6mm, b = 1.0mm; (d) a = 1.7mm, b = 1.0mm;

[0053] Figure 9 The load change of the model with different spring wire pitches t at a fixed deformation amount when a = 1.6mm and b = 1.0mm;

[0054] Figure 10 The influence of the major semi-axis A and the minor semi-axis B of the outer ring of the elliptical spring-like on the mechanical properties under different conditions; (a) The minor semi-axis B of the outer ring of the spring-like is fixed; (b) The major semi-axis A of the outer ring of the spring-like is fixed;

[0055] Figure 11 The fitting relationship curve of (ab / AB) and E;

[0056] Figure 12 The fitting relationship curve of x and E;

[0057] Figure 13 The design of the costal cartilage part model and the rib part model; (a) The costal cartilage part model; (b) The rib part model;

[0058] Figure 14 The stress and displacement distribution diagrams of the finite element analysis of the costal cartilage and rib implant models; (a) and (b) are the stress and displacement distribution diagrams of the costal cartilage implant model; (c) and (d) are the stress and displacement distribution diagrams of the rib implant model;

[0059] Figure 15 The stress-strain diagrams of the finite element analysis of the spring-like rib implant structure model; (a) The costal cartilage part model; (b) The rib part model;

[0060] Figure 16 "Gradient mechanics" type spring-like rib implant model;

[0061] Figure 17 Stress and displacement distribution diagrams of finite element analysis of the "gradient mechanics" type spring-like rib implant model: (a) and (b) are the stress and displacement distribution diagrams of the second rib model; (c) and (d) are the stress and displacement distribution diagrams of the third rib model; (e) and (f) are the stress and displacement distribution diagrams of the fourth rib model; (g) and (h) are the stress and displacement distribution diagrams of the fifth rib model; (i) and (j) are the stress and displacement distribution diagrams of the sixth rib model;

[0062] Figure 18 Stress-strain curves of three-point bending simulated for the 2-6 rib implant models;

[0063] Figure 19 Five groups of "gradient mechanics" type spring-like rib implant specimens prepared by SLM and three-point bending tests: (a) Specimens of the second to sixth ribs prepared by SLM; (b) Three-point bending test;

[0064] Figure 20 Stress-strain curves of the three-point bending test of the 2-6 rib implant specimens. Specific implementation mode

[0065] The embodiments described below with reference to the accompanying drawings are exemplary and are intended to explain the present invention and should not be construed as limiting the present invention.

[0066] In this embodiment, a rib implant structure with gradient mechanics transition is provided. According to the different elastic moduli of human ribs and costal cartilages, a gradient transition is adopted to design a "gradient mechanics" type spring-like rib implant for the second to sixth ribs that plays a major role in respiratory function, enabling it to have the functions of both the costal cartilage part and the rib part. First, the problem that the elastic moduli of rib implants, costal cartilages, and ribs are inconsistent and cannot meet the normal breathing of patients after surgery is solved; at the same time, the problem that due to the limited bioactivity of titanium alloy, large-area replacement will lead to delayed healing of the soft tissues around the implant, wound infection, and reduced postoperative recovery ability is solved. The specific research results are as follows:

[0067] 1. A three-point bending finite element stress-strain analysis model of a spring-like structure was constructed, and the accuracy and effectiveness of the simulation were verified by experiments. By adjusting the combination of structural parameters such as the size of the ellipse of the spring wire cross-section of the "gradient mechanics" type spring-like rib implant, the pitch of the spring wire, and the size of the ellipse of the outer ring of the spring-like structure, the structural bending elastic modulus of the implant model can be accurately adjusted.

[0068] (1) The larger the cross-sectional dimensions (a, b) of the spring wire, the greater the structural elastic modulus of the spring-like rib implant; the greater the difference between the major and minor semi-axes of the cross-section of the spring wire, the lower the structural elastic modulus of the spring-like specimen; and under the same conditions, the influence of the cross-sectional area of the elliptical spring wire on the elastic modulus is greater than the influence of the difference between the major and minor semi-axes.

[0069] (2) The larger the pitch t of the spring wire, the greater the structural elastic modulus of the spring-like rib implant.

[0070] (3) The smaller the outer envelope ellipse dimensions (A, B) of the spring-like rib implant, the greater the structural elastic modulus of the spring-like rib implant, and the adjustment of its major semi-axis has a more significant influence on the structural elastic modulus than the minor semi-axis.

[0071] (4) The cross-sectional dimensions of the spring wire show a large positive correlation, the outer envelope ellipse dimensions show a large negative correlation, both have a greater impact on the flexural elastic modulus of the spring-like rib implant, the pitch of the spring wire shows a medium positive correlation, and the impact is smaller.

[0072] (5) A fitting relationship model between the structural parameters a, b, A, B, t and the flexural elastic modulus E was established, and the goodness of fit R2 was 0.8607. This fitting relationship model has a certain fitting accuracy and prediction ability.

[0073] 2. The "gradient mechanics" structural design of the spring-like rib implant was realized, and the elastic modulus can simultaneously meet the elastic modulus requirements of costal cartilage and ribs. Based on this, a thoracic rib integrated specimen with a spring-like rib implant model with a structural gradient mechanics was prepared by selective laser melting forming technology. Further, a surface microstructure was constructed using a lattice structure, and through cell culture and characterization analysis, the effect of the microstructure characteristics on the cell adhesion and proliferation ability of the material surface was clarified.

[0074] (1) The structural gradient mechanics of the spring-like rib implant was realized to simultaneously meet the mechanical requirements of the costal cartilage part and the rib part, and a gradient structure was used to connect them to achieve a fine mechanical transition, which can effectively restore the patient's postoperative respiratory function.

[0075] (2) A surface microstructure of TC4 was constructed using a lattice structure. It was found that as the pore size increases, the surface roughness increases, and the surface roughness of the surface microstructure specimen with pores can be increased by 7 - 10 times compared with the solid specimen. At the same time, when the pore size is above 300 μm, the porosity exceeds 50%.

[0076] (3) The results of cell experiments show that the surface microstructure specimens with pore sizes of 400 μm and 500 μm exhibit excellent bioactivity. Cells spread better on their surfaces, with the cells being elongated and having a large number of pseudopodia. This result indicates that the surface microstructure specimens with larger pore sizes can provide more attachment areas and deeper implantation depths for cells, which is more conducive to cell growth and improving the postoperative bone integration ability.

[0077] Example:

[0078] The preparation method of a rib implant structure with a gradient mechanical transition in this example is as follows:

[0079] Step 1: Construct a finite element stress-strain analysis model of a spring-like structure, and combine experiments to verify the influence of the structural parameters of the spring-like rib implant structure on its mechanical properties;

[0080] Step 1.1 Construction of the CAD model of the spring-like rib implant

[0081] According to the physical properties of natural ribs, the natural ribs of the human body have a certain curvature. Their length ranges from approximately 100.0 - 200.0 mm, the width ranges from 8.5 - 17.0 mm, and the thickness ranges from 3.0 - 9.4 mm. According to its anatomical structure, it is found that the cross-sectional profile of the middle part of the rib is approximately elliptical, with the front and rear ends being slightly thicker and flat. The initial design structure of the spring-like structure is as Figure 3 shown, and the design parameters are shown in Table 3-1. The cross-section of the outermost contour line (referred to as the outer envelope) of the spring-like structure is an ellipse, and its major and minor semi-axes are set as A and B respectively, as Figure 3 (b) shows, and the initial design size is set as A = 5 mm, B = 3 mm.

[0082] The cross-sectional shape perpendicular to the spring-like rib implant is also similar to the cross-section of the natural rib of the human body, being elliptical. The design data of the major and minor semi-axes of the ellipse are shown in Table 2, where the major semi-axis a is set as 1.5 mm and 1.6 mm, and the minor semi-axis b is 0.8 mm and 1.0 mm. The length of the straight specimen is 75 mm, and the length of the curved specimen is based on the curvature of the human rib (7.69×10 -3 mm -1 ≤K≤12.50×10 -3 mm -1 ), approximately a section of an arc of a circle with a radius of 130 mm (K = 7.69×10 -3 mm -1 ). Since the radius of the arc is large, the length of the curved specimen is taken as 110 mm.

[0083] Table 2 Initial design parameters of the spring-like rib implant model

[0084]

[0085] Step 1.2 Material Property Setting of the Model

[0086] Select 7 groups of spring-like rib implant models with different structural parameters in Table 3-1, and use Abaqus finite element analysis software to simulate the three-point bending test to obtain the bending elastic modulus of the spring-like rib implant models under different structural parameters. When simulating, the three-dimensional model of the part is modeled by UG and then imported into Abaqus software for simulation analysis. The material properties of TC4 used in the finite element analysis are shown in Table 3. Both the indenter and the base are set as rigid bodies, that is, under the action of any force, the volume and shape do not change.

[0087] Table 3 Material Properties of TC4 Rib Implants

[0088]

[0089] Step 1.3 Boundary Condition Setting and Mesh Generation of the Model

[0090] Step 1.4 Finite Element Simulation Mechanical Results and Analysis of the Model

[0091] Through the comparative analysis of the simulation results, it can be concluded that the mechanical properties of the spring-like rib implant will be affected by the structural parameters. Therefore, the stiffness coefficient and elastic modulus of the spring-like rib implant can be reduced by changing the structural parameters, so as to promote the better recovery of the patient's respiratory function after surgery.

[0092] Step 1.5 Analysis of the Influence of Rib Implant Structural Parameters on Mechanical Properties

[0093] From the above analysis, the design parameters of the spring-like rib implant are: the major semi-axis a and minor semi-axis b of the cross-sectional ellipse of the spring wire, the pitch t of the spring wire, and the major semi-axis A and minor semi-axis B of the outer envelope ellipse of the spring-like. These three groups of parameters jointly determine the mechanical properties of the model, and different parameter combinations will present different mechanical property levels. The length of the spring-like rib implant model is 75mm. In order to explore the influence of different structural parameters on the mechanical properties, this study uses the method of controlling variables to analyze the influence law of a single parameter in turn.

[0094] Step 1.5.1 Influence of the Cross-Sectional Ellipse Size of the Spring Wire on Mechanical Properties

[0095] The law of the change of the minor axis b was explored under four conditions where the pitch t of the spring wire was 6.3 mm and the major semi-axis a of the ellipse of the spring wire cross-section was 1.4 mm, 1.5 mm, 1.6 mm, and 1.7 mm respectively. The values of b were 0.8 mm, 1.0 mm, and 1.2 mm respectively. The specific experimental combinations are shown in Table 4. In addition, corresponding to the elastic deformation stage of the spring-like rib implant specimen, the specimen produced elastic reversible deformation under the action of an external force. Then, as the strain increased, it entered the plastic deformation stage from the elastic deformation stage, and the specimen underwent irreversible deformation, and the stress rapidly decreased. However, in order to solve the problem of the matching of the elastic modulus of the rib implant, the following research and analysis mainly focused on the structural elastic modulus of the specimen in the elastic deformation stage, and the plastic deformation stage was still not discussed.

[0096] Table 4 Design of the cross-sectional dimension parameters of the spring wire

[0097]

[0098]

[0099] The finite element analysis and simulation of the specimens with the above different structural parameters were carried out under the same conditions, and the results are as Figure 5 shown. The elastic moduli of the spring-like rib implant specimens with different elliptical spring wire cross-sectional dimensions obtained from the finite element analysis and simulation are shown in Table 5. From Figure 5It can be seen that as the strain gradually increases, the spring-like rib implant specimens all show the same change trend, that is, the stress also shows an upward trend. At the same time, in Figures (a), (b), (c), and (d), when the major semi-axis a is 1.4 mm, 1.5 mm, 1.6 mm, and 1.7 mm, the slope of the elastic stage in the stress-strain diagram, that is, the elastic modulus, increases from 130.87 MPa to 169.30 MPa, from 139.58 MPa to 319.26 MPa, from 175.65 MPa to 411.81 MPa, and from 141.46 MPa to 535.92 MPa respectively. That is, the structural bending elastic modulus of the specimens all shows an increase with the increase of the minor semi-axis b of the elliptical spring wire. This is determined by the gradual increase of the force-bearing area π·a·b. For example, when a = 1.4 mm, as b increases from 0.8 mm to 1.2 mm, the force-bearing area of a single spring wire increases from 1.12π to 1.68π. Therefore, the increase in the deformation resistance leads to the increase in the elastic modulus. This law is simply referred to as the "area law". In addition, it can also be found that as the major semi-axis a increases, this difference becomes more obvious, which is mainly determined by the difference between the major and minor semi-axes, that is, the increase in a / b. For example, when a = 1.4 mm and b = 1.2 mm, a / b = 1.17, and when a = 1.7 mm and b = 0.8 mm, a / b = 2.13. The difference between the major and minor semi-axes reduces the concentration and symmetry of the force distribution per unit cross-section of the ellipse. Therefore, the greater the difference, the lower the bending structural elastic modulus of the spring-like rib implant specimens. This law can be simply referred to as the "law of the difference between the major and minor semi-axes". And Figure 5 in (a) when the major semi-axis a = 1.4 mm and the minor semi-axis b = 1.2 mm, the corresponding curve is close to the curve corresponding to b = 1.0 mm, which is also because in these two cases, the sizes of a and b are close, and the cross-sectional morphology of the elliptical spring wire is approximately circular, so the bending structural moduli presented by the two are very close; when a = 1.4 mm and b = 0.8 mm, that is, a / b = 1.75, when the difference between the major and minor axes is more obvious, the presented law is the same as that of the other three groups.

[0100] Table 5 Elastic moduli of spring-like rib implant models with different minor semi-axes b

[0101]

[0102]

[0103] As Figure 6 shown are the stress-strain curves presented when the major semi-axis a of the ellipse in the cross-section of the spring-like structure gradually changes and under the conditions of a specific spring wire pitch and minor semi-axis b. Figure 6 All three figures show that under the condition of a fixed minor semi-axis b, the greater the major semi-axis a, the greater the elastic modulus. For example, Figure 6In (c), when a increases from 1.4 mm to 1.7 mm, the bending structural modulus increases from 169.30 MPa to 535.92 MPa. The main reason is still that the increase in the cross-sectional area of the spring-like structure improves the deformation resistance of the structure, thereby increasing the bending elastic modulus of the structure, which still conforms to the "area law". This law can also be reflected in comparing the ranges of the bending elastic moduli of groups (a), (b), and (c) in Figure 6: from (a) to (c), the gradual increase in the b value increases the cross-sectional area of the structure. Therefore, the values of the bending structural elastic moduli from (a) to (c) also gradually increase. In addition, it should be particularly noted that for Figure 6 in (a), the value of a / b is greater than that in (b) and (c) in the figure. Therefore, the difference between the major semi-axis a and the minor semi-axis b is the largest. Therefore, referring to Figure 5 the confirmed "law of the difference between the major and minor semi-axes", it can be known that the deformation resistance of this group is the smallest. Therefore, although it generally conforms to the "area law" basically, the differences between the curves are not obvious. This situation is still reflected in Figure (b), which is still dominated by the "law of the difference between the major and minor semi-axes". In Figure (c), since the value of the minor semi-axis has increased to 1.2 mm, significantly reducing the difference between the major and minor semi-axes, and the value of a / b is between 1.17 and 1.42. Therefore, the density and symmetry of the force distribution on the cross-section are both good. Therefore, the difference in the structural elastic modulus of this group is the most obvious, gradually increasing from 169.30 MPa to 535.92 MPa, with a 216.55% increase.

[0104] Figure 7 (a), (b), (c), and (d) in the figure show the changes in the bending structural modulus presented under the conditions of basically the same difference between the major and minor semi-axes and the cross-sectional area of the spring-like structure. Analyzing Figure 7 (a) and (b) in it, under the condition of approximately the same difference between the major and minor semi-axes, due to different cross-sectional areas, the bending structural moduli are also different. For example, in Figure (a), the difference between the major and minor semi-axes a / b is about 1.7. When a = 1.4 mm and b = 0.8 mm, its cross-sectional area is 1.12π, and the elastic modulus is 130.87 MPa. When a = 1.7 mm and b = 1.0 mm, its cross-sectional area is 1.70π, and the elastic modulus is 303.37 MPa. The bending modulus of the spring-like structure with a larger cross-sectional area is larger. In Figure (b), the difference between the major and minor semi-axes a / b is about 1.4. When a = 1.4 mm and b = 1.0 mm, its cross-sectional area is 1.40π, and the elastic modulus is 163.54 MPa. When a = 1.7 mm and b = 1.2 mm, its cross-sectional area is 2.04π, and the elastic modulus is 535.92 MPa. The bending modulus of the spring-like structure with a larger cross-sectional area is larger. It shows that under approximately the same difference between the major and minor semi-axes, the bending modulus of the spring-like structure with a larger cross-sectional area is larger.

[0105] AnalyzingFigure 7 As can be seen from (c) and (d), under the condition of similar cross-sectional areas, due to different differences in the major and minor semi-axes, the bending structure modulus is also different. For example, Figure 7 in (c), the cross-sectional area is approximately 1.40π. When a = 1.4 mm and b = 1.0 mm, the difference in the major and minor semi-axes is 1.4, and the elastic modulus is 163.54 MPa. When a = 1.7 mm and b = 0.8 mm, the difference in the major and minor semi-axes is 2.125, and the elastic modulus is 141.46 MPa. The bending modulus of the spring-like structure with a larger difference in the major and minor semi-axes is smaller; Figure 7 in (d), the cross-sectional area is approximately 1.70π. When a = 1.4 mm and b = 1.2 mm, the difference in the major and minor semi-axes is 1.17, and the elastic modulus is 169.30 MPa. When a = 1.7 mm and b = 1.0 mm, the difference in the major and minor semi-axes is 1.7, and the elastic modulus is 303.37 MPa. The bending modulus of the spring-like structure with a larger difference in the major and minor semi-axes is actually larger, which does not conform to the "law of the difference in the major and minor semi-axes". A more precise analysis reveals that at this time, the cross-sectional areas corresponding to the differences in the major and minor semi-axes of 1.17 and 1.7 are 1.68π and 1.7π respectively, that is, the latter has a larger difference in the major and minor semi-axes and also a larger cross-sectional area. Based on the result that its elastic modulus is also larger, it can be inferred that the influence of the "area law" on the elastic modulus is greater than that of the "law of the difference in the major and minor semi-axes", that is, the cross-sectional area has a more significant influence on the elastic modulus.

[0106] 1.5.2 Influence of Spring Wire Pitch on Mechanical Properties

[0107] To explore the influence of the spring wire pitch t on the mechanical properties of the spring-like TC4 rib implant model, under the condition that the spring wire dimensions (a, b) and the outer envelope dimensions of the spring-like structure (A = 5 mm, B = 3 mm) are specific values, the spring wire pitch t is changed to 5.0 mm, 6.0 mm, and 7.0 mm for law exploration. The specific experimental combinations are shown in Table 6.

[0108] Table 6 Test Parameters of Spring Wire Pitch t

[0109]

[0110] The finite element stress-strain simulations of the specimens with the above different parameters were carried out under the same conditions. Figure 8 The stress-strain curves of the spring-like TC4 rib implant models with different spring wire pitches t under four groups of different spring wire sizes are shown. From Figure 8As can be seen from (a) and (b), under the conditions that the major semi-axis a and minor semi-axis b of the elliptical spring wire are a = 1.4 mm, b = 0.8 mm and a = 1.5 mm, b = 0.8 mm respectively, the linear part of the stress-strain curve corresponding to the change range of the spring wire pitch t from 5.0 mm to 7.0 mm does not change significantly; while Figure 8 In (c) and (d), under the conditions that a = 1.6 mm, b = 1.0 mm and a = 1.7 mm, b = 1.0 mm, it can be clearly found that as the spring wire pitch t increases from 5.0 mm to 7.0 mm, the linear part of the stress-strain curve gradually becomes steeper. As can be seen from Table 7, its elastic modulus increases from 195.78 MPa to 379.35 MPa and from 251.95 MPa to 386.30 MPa respectively. The modulus of the spring-like bending structure is proportional to the load P at the compressed part. As the spring wire pitch t increases, at a certain fixed deformation amount, the change of P is as Figure 9 shown, showing an increasing trend. This is because the larger the spring wire pitch t is, the fewer the number of spring wire turns within the span L, and the more difficult it is to generate the same deformation amount, so P increases. Therefore, as t increases, the bending structure modulus increases.

[0111] Table 7 Elastic modulus of spring-like models with different spring wire pitches t

[0112]

[0113] In summary, by comparing the changes of the spring wire pitch t under four conditions where the major semi-axis a and minor semi-axis b of the spring wire are a = 1.4 mm, b = 0.8 mm, a = 1.5 mm, b = 0.8 mm, a = 1.6 mm, b = 1.0 mm and a = 1.7 mm, b = 1.0 mm respectively, it can be found that the influence of the spring wire pitch t on the mechanical properties of the spring-like rib implant model has the following rule, that is, as the spring wire pitch t increases, the slope of the stress-strain curve of the specimen gradually increases, which means that the elastic modulus of the specimen gradually increases.

[0114] 1.5.3 Influence of the outer envelope size of the spring-like structure on the mechanical properties

[0115] The outer envelope size of the spring-like TC4 rib implant is determined by the path of the cross-section sweeping process of the spring wire. The schematic diagram of the outer envelope size is as Figure 4 shown in (b). The influence of the major semi-axis A and minor semi-axis B of the outer envelope size of the spring-like structure on the mechanical properties of the spring-like TC4 rib implant model will be analyzed. Under the condition that the major semi-axis a and minor semi-axis b of the spring wire and the spring wire pitch t are certain, that is, the major semi-axis a = 1.4 mm, the minor semi-axis b = 0.8 mm, and the spring wire pitch t = 5.0 mm, three groups of data are tested by changing the major semi-axis A and minor semi-axis B of the outer envelope size. The specific experimental combinations are shown in Table 8.

[0116] Experimental parameters of the outer envelope dimensions of Class 8 spring structures

[0117]

[0118]

[0119] Table 9 shows the flexural elastic moduli of each model under different outer envelope dimensions of springs. It can be seen that as the strain gradually increases, the stress also rises within a certain range. Figure 10 What is presented in (a) is the stress-strain curve when the major axis A changes while the minor axis B of the fixed outer envelope dimension is 2.6 mm. As the major axis A increases, the slope of the stress-strain curve decreases, that is, the flexural elastic modulus decreases. It can also be seen from Table 9 that when A = 3.1 mm and B = 2.6 mm, E = 675.7 MPa, and when A = 4.6 mm and B = 2.6 mm, E = 220.5 MPa. This indicates that under the condition of the same spring wire size, the larger the outer ring size of the Class 8 spring structure, the larger the inner hollow part, the lower the proportion of the solid part of the Class 8 spring-shaped TC4 rib implant, and the lower the elastic modulus of the specimen.

[0120] Similarly, Figure 10 What is presented in (b) also shows the same rule. Under the condition of the same spring wire size, when the major axis A of the elliptical Class 8 spring outer envelope is fixed, as the minor axis length B increases, the elastic modulus of the model gradually becomes smaller. It can also be seen from Table 3-14 that when A = 4.6 mm and B = 2.6 mm, E = 220.5 MPa, and when A = 4.6 mm and B = 3.6 mm, E = 107.1 MPa. By comparing Figure 10 What is presented in (a) and (b), it can also be found that the decrease in the flexural elastic modulus caused by the change of the major axis A from 3.1 mm to 4.6 mm (an increase of about 48.4%) is 67.4%, while the decrease in the flexural elastic modulus caused by the change of the minor axis B from 2.6 mm to 3.6 mm (an increase of about 38.5%) is 51.4%. Therefore, the change of the major axis A has a greater impact on the change of the flexural elastic modulus.

[0121] Table 9 Elastic moduli of models under different outer ring dimensions of elliptical Class 8 springs

[0122]

[0123] In summary, by comparing the stress-strain diagrams of the outer envelope dimensions of different types of springs under the condition that the spring wire size and the spring wire pitch t are constant, it can be found that the following rules exist for the influence of the major semi-axis A and the minor semi-axis B of the outer envelope dimensions of the elliptical spring-like shape on the mechanical properties of the spring-like TC4 rib implant model, that is, as the outer envelope dimensions of the spring-like shape increase, the elastic modulus of the spring-like TC4 rib implant decreases.

[0124] 1.5.4 Analysis of the Correlation Behavior of Structural Parameters

[0125] In order to quantitatively represent the relationship between the bending elastic modulus of the spring-like rib implant model and each structural parameter, the correlation coefficient is characterized for it. Due to the different research objects, there are various definitions of the correlation coefficient. The more commonly used ones are the Pearson correlation coefficient, the Spearman correlation coefficient, and the Kendall correlation coefficient. These three correlation coefficients are also collectively referred to as the "three major statistical correlation coefficients". The Pearson correlation coefficient, also known as the Pearson product-moment correlation coefficient (PPMCC), was proposed by Karl Pearson in the 1880s and is used to measure the linear correlation degree between two variables X and Y. It is usually represented by the lowercase English letter r, and its definition is the quotient of the covariance and the standard deviation between the two variables. The formula can be expressed as Equation (1):

[0126]

[0127] where and are the average values of X i and Y i respectively.

[0128] The value range of the Pearson correlation coefficient r is [-1, 1]. When r = 0, it means that the two groups of variables do not have a linear correlation. When r is in the range of (-1, 0), it means that the two groups of variables have a negative correlation. The smaller the value, the greater the negative correlation. When r = -1, it means that the two groups of variables show a completely negative linear correlation relationship. When r is in the range of (0, 1), it means that the two groups of variables have a positive correlation. The larger the value, the greater the positive correlation. When r = 1, it means that the two groups of variables show a completely positive linear correlation relationship.

[108] . The judgment of the relationship strength is shown in Table 10 below:

[0129] Table 10 Judgment of the Strength of the Correlation Coefficient

[0130]

[0131] Take the bending elastic moduli of the spring-like rib implants corresponding to the three structural parameters studied above as the Y values of each structural parameter, and take the three structural parameters: the elliptical size (a·b) of the spring wire cross-section, the spring wire pitch t, and the elliptical size (A·B) of the spring-like outer envelope as the X values. Calculate the Pearson correlation coefficient r between them. The results are shown in Table 11, where a correlation greater than 0.8 represents a high correlation, and a correlation less than 0.8 represents a medium correlation.

[0132] As can be seen from Table 3-16, the elliptical size (a·b) of the cross-section and the elliptical size (A·B) of the spring-like outer envelope show a high correlation, and their correlation coefficients reach 0.899 and -0.937 respectively. Among them, the elliptical size (A·B) of the spring-like outer envelope shows an important correlation, which proves that the spring wire cross-sectional area and the spring-like outer envelope elliptical area have a significant impact on the bending elastic modulus of the spring-like rib implant. At the same time, the cross-sectional area shows a large positive correlation, indicating that the larger the cross-sectional area, the greater the bending elastic modulus of the spring-like rib implant model, which is consistent with the conclusion of the "area law"; the spring-like outer envelope elliptical area shows a large negative correlation, indicating that the smaller the spring-like outer envelope elliptical area, the greater the bending elastic modulus of the spring-like rib implant model, which is consistent with the conclusion obtained from the previous analysis. The correlation coefficient of the spring wire pitch t is 0.765, showing a medium correlation, which indicates that under the same conditions, the spring wire pitch t has a certain impact on the bending elastic modulus of the spring-like rib implant model, and t shows a positive correlation, indicating that the larger t is, the greater the bending elastic modulus of the spring-like rib implant model, which is consistent with the conclusion obtained from the previous analysis. The correlation coefficient of the difference in major and minor semi-axes a / b is -0.47, showing a negative correlation, indicating that the greater the difference in major and minor semi-axes, the smaller the bending elastic modulus of the spring-like rib implant model, which conforms to the "difference in major and minor semi-axes law", but its significance level is much lower than the "area law" and is also completely consistent with the previous conclusion.

[0133] Table 11 Pearson correlation coefficients between the bending elastic modulus of the spring-like rib implant model and each structural parameter

[0134]

[0135] In summary, the elliptical area parameter (a·b) of the spring wire cross-section, the elliptical area parameter (A·B) of the spring-like outer envelope, and the spring wire pitch t have a more significant impact on the mechanical properties of the spring-like rib implant. Subsequently, a fitting model of mechanical properties will be constructed with these as key parameters.

[0136] Step 2: Construct a fitting relationship model between the structural parameters and the bending elastic modulus E;

[0137] From the previous analysis of the influence law of the structural parameters of the spring-like rib implant on the mechanical properties, it can be found that the change process of the bending elastic modulus of the spring-like rib implant is a complex change under the comprehensive influence of various factors such as the elliptical area parameter (a·b) of the spring wire cross-section, the spring wire pitch t, and the elliptical area parameter (A·B) of the spring-like outer envelope. In order to provide a basis for predicting the bending elastic modulus of the spring-like rib implant and designing the structural parameters, it is necessary to establish a mathematical model for the trend term of the change amount of the spring-like rib implant affected by the structural parameters. In order to systematically establish a fitting data model, it is necessary to statistically analyze the bending elastic modulus data of the spring-like rib implants with different structural parameters. Table 12 shows the structural parameters and bending elastic modulus values of the spring-like rib implant models with different structures designed in the previous text.

[0138] Table 12 Structural Parameters and Bending Elastic Modulus of Spring-Like Rib Implant Models

[0139]

[0140]

[0141] From the analysis of the correlation behavior of the structural parameters, it can be seen that the elliptical size (a·b) of the spring wire cross-section has a large positive correlation with the bending elastic modulus of the spring-like rib implant, and the elliptical size (A·B) of the spring-like outer envelope has a large negative correlation with the bending elastic modulus of the spring-like rib implant. The correlation of the spring wire pitch t is relatively small, showing a medium correlation. Therefore, when establishing a mathematical model for the trend term of the change amount of the spring-like rib implant affected by the structural parameters, the elliptical size (a·b) of the spring wire cross-section needs to be directly proportional to the result, and the elliptical size (A·B) of the spring-like outer envelope needs to be inversely proportional to the result. The two have a major influence, and the spring wire pitch t has a small influence. Therefore, first establish the fitting relationship model between (ab / AB) and E, as Figure 11 The figure shows the fitting relationship curve between the ratio (ab / AB) of the elliptical size (a·b) of the spring wire cross-section to the elliptical size (A·B) of the spring-like outer envelope and E, and the following fitting relationship formula (2) between (ab / AB) and E is obtained:

[0142]

[0143] Conduct an effectiveness test on the fitting relationship model between (ab / AB) and E, and obtain the goodness of fit R 2 value of 0.8607. The goodness of fit R 2 is also called the coefficient of determination. This value is an index of the fitting degree of the trend line. Its numerical size can reflect the fitting degree between the estimated value of the trend line and the corresponding actual data. The higher the fitting degree, the higher the reliability of the trend line. The goodness of fit R2 is a value within the range of 0 to 1. When the goodness of fit R of the trend line 2 equals 1 or is close to 1, its reliability is the highest; otherwise, the reliability is lower. Therefore, the fitting relationship model between (ab / AB) and E has a high effectiveness.

[0144] After establishing the fitting relationship model between (ab / AB) and E, substitute the value of (ab / AB) into formula (2), and introduce (2a / t) to establish the following relationship model, obtaining formula (3):

[0145]

[0146] The values of x corresponding to different structural parameters a, b, A, B, and t obtained through formula 3 are shown in Table 13.

[0147] Table 13 Values of x corresponding to different structural parameters of the spring-like rib implant model

[0148]

[0149]

[0150]

[0151] Figure 12 is the fitting relationship curve between x and E, obtaining the following fitting relationship formula (4) between x and E:

[0152] E = 0.9844·x 1.0025 (4)

[0153] Conduct an effectiveness test on the fitting relationship model between x and E, and obtain the goodness of fit R of this fitting relationship model 2 is 0.8607, showing a relatively high level. Therefore, this fitting relationship model has a high reliability. Combining formula (3) and formula (4), the fitting relationship formula (5) between the structural parameters a, b, A, B, t and the flexural modulus of elasticity E can be obtained:

[0154]

[0155] This fitting relationship model has a high fitting accuracy and prediction ability, can better grasp the influence law of the structural parameters of the spring-like rib implant model on the mechanical properties as a whole, and can provide relatively reliable basic data for the evaluation of the flexural modulus of elasticity of the spring-like rib implant and the design of structural parameters.

[0156] Step 3: According to the bending elastic modulus of the rib and costal cartilage, use the fitting relationship model in Step 2 to calculate the structural parameter values of the rib part and costal cartilage part in the rib implant structure, and determine the structures of the rib part and costal cartilage part.

[0157] Based on the influence law of the above-mentioned spring-like rib structure parameters on the mechanical properties, in order to make the target spring-like structure model meet the ideal requirements, for the costal cartilage part model, it is necessary to reduce the elliptical size of the cross-section of its spring wire and at the same time reduce its spring wire pitch t; while the rib part has a larger elastic modulus, the elliptical size of the cross-section of its spring wire should be adjusted to a larger level, and at the same time its spring wire pitch t should be increased. Considering the integrity and coordination of the implant, the spring-like outer envelope elliptical sizes of the costal cartilage part and the rib part are set to the same size, with the major semi-axis A = 4.0 mm and the minor semi-axis B = 3.0 mm, which is basically the same as the external shape size of the natural rib of the human body. The designed costal cartilage part model and rib part model are as Figure 13 shown, and the specific structural parameters are shown in 14.

[0158] Table 14 Design parameters of the costal cartilage part and rib part models

[0159]

[0160] Step 4: According to the structural parameters of the rib part and the costal cartilage part, establish a connection based on the gradient change between the two to obtain the gradient rib part.

[0161] Step 4.1 Stress and displacement distribution analysis of the costal cartilage part and rib part

[0162] Figure 14 The stress and displacement distribution results of the costal cartilage part and rib part of the spring-like rib implant model are shown as follows. When a displacement of 15 mm is applied and the action time is 1 s, the maximum Mises stress of the costal cartilage part model reaches 1000 MPa, and the maximum Mises stress of the rib part model reaches 1758 MPa. If the Mises stress reaches the yield stress of the material, the material will yield. Given that the yield strength of the TC4 material is 970 MPa, this indicates that irreversible plastic deformation has occurred in the compressed parts of the costal cartilage part model and the rib part model when the displacement reaches the maximum. At the same time, the stress of the costal cartilage part model and the rib part model is mainly concentrated at the middle force-bearing position in contact with the rigid indenter, and the stress gradually decreases from the middle force-bearing position to both ends until the stress value at the end decreases to the lowest. From the displacement of the costal cartilage part model and the rib part model, the maximum displacement values of the two groups of models are mainly concentrated at the force-bearing position and the upturned parts at both ends.

[0163] From Figure 14(a) The rib cartilage model shows that the spring wire is thinner, the spring wire pitch t is smaller, 4.0mm, and the load it can bear when under pressure is smaller; the rib model shows that the semi-long axis a of the spring wire cross section is larger, reaching 4.8mm, making the whole shape flat and long, and the spring wire pitch t is larger, 10.0mm. The gap between adjacent spring wires is smaller because the distance of 10.0mm of the spring wire pitch t is occupied by the adjacent elliptical spring wire with a semi-long axis a of 4.8mm. Therefore, the rib model has more solid parts and can bear a larger load when under pressure. The bending elastic modulus of the costal cartilage and rib models were calculated. The elastic modulus of the costal cartilage model was 372MPa, and the elastic modulus of the rib model was 4.017GPa. This result met the target requirements. The elastic modulus of the costal cartilage was increased by dozens of times compared with that of the natural human rib, making it safe and elastic. The elastic modulus of the rib part can effectively alleviate the "stress shielding" effect while ensuring safety.

[0164] Figure 15 The figure shows the stress-strain curves of the three-point bending simulation of the rib cartilage and rib parts of the spring-like rib implant model. Figure 15 (a) is the stress-strain curve of the three-point bending simulation of the costal cartilage model. It can be seen from the figure that the costal cartilage model is in the elastic stage when the strain is in the range of 0-0.20. The displacement of the costal cartilage model in the elastic deformation stage is 12.028mm, which is greater than the maximum amplitude of the thorax movement of 10mm. At the same time, when the strain is 0-0.10, the maximum stress reached by the costal cartilage model is only 33MPa. Compared with the stress-strain curve of the three-point bending simulation of the rib model in Figure (b), it can be seen from the figure that when the strain is 0- When the value of rib model is 0.10, the rib model is in the elastic stage. In this elastic deformation stage, the displacement of the rib model is 6.014 mm. Although the floating range of the chest surface is 5-10 mm during normal breathing, this floating range mainly acts on the connection between the front end of the rib and the sternum, and the rear end of the rib remains basically unchanged. Therefore, the elastic displacement of the rib model can also meet the requirements. Within this range, the maximum stress reached by the rib model is close to 300 MPa, which is quite different from the maximum stress reached by the costal cartilage model within this range, which is in line with expectations.

[0165] After analyzing the structural design and mechanical properties of the costal cartilage and rib models, in order to make the complete implant have the functions of both, a gradient transition of structure and mechanical properties needs to be achieved between the two. At the same time, in order to make the transition between structure and function more refined, this study designed three gradient models in sequence to connect them, thus forming a spring-like structure gradient mechanical bionic rib implant model with the elastic function of costal cartilage, such as Figure 1As shown. The specific structural parameters of each gradient connection part are shown in Table 15.

[0166] Table 15 Design parameters of each gradient connection part

[0167]

[0168]

[0169] In chest wall reconstruction surgery, according to the different lengths of rib resection and the scope of chest wall bone resection, chest wall reconstruction can be divided into local and global reconstruction. When more than half of the natural ribs of the human body are resected, global chest wall reconstruction should be performed at this time; when less than half of the natural ribs of the human body are resected, local chest wall reconstruction should be performed at this time. Since global chest wall reconstruction is relatively rare, this study mainly focuses on the rib implant model in local chest reconstruction. In order to avoid the "stress shielding" effect after the implant is implanted, during the design process, the modulus of the elastic rib model should be slightly lower than that of the natural ribs of the human body to promote the growth of the rib stump and its combination with the implant. In the local rib implant model, considering that the elastic modulus of the natural human ribs decreases from the first rib to the twelfth rib in turn, and the undulation of the human chest mainly concentrates on the 2nd to 6th ribs of the human body, combining the strength and respiratory function of the natural human chest, the 2nd to 6th ribs of the rib implant model are designed as a gradient mechanics-like spring-shaped structure bionic rib implant, and the elastic modulus decreases from the 2nd to the 6th in turn, and the length of the elastic structure increases in turn, as shown in Figure 16 . The specific design parameters of each part of the 2nd to 6th ribs are shown in Table 16.

[0170] Table 16 Design parameters of each part of the 2-6 rib costal cartilage-rib gradient model

[0171]

[0172] Based on Figure 16 the model, a finite element stress-strain analysis was carried out on the spring-like rib implant model with a "gradient mechanics" structure, as shown in Figure 17 . The stress and displacement result distribution diagrams of the three-point bending simulation when the rib implant models of the 2nd to 6th ribs are compressed at the middle position are shown. From Figure 17 (a), (c), (e), (g), (i), it can be seen that the stress of the rib implant models of the 2nd to 6th ribs is mainly concentrated at the middle loading position in contact with the rigid indenter, and the stress decreases gradually from the middle loading position to both ends until the stress value at the end is reduced to the lowest. From Figure 17 (b), (d), (f), (h), (j), the displacement distribution diagrams of the elastic rib implant models of the 2nd to 6th ribs can be seen that the maximum displacement values of the 6 groups of models are mainly concentrated at the loading position of the indenter in the middle of the model and the upturned parts at both ends.

[0173] Figure 18 The stress-strain curve obtained from the three-point bending simulation of the gradient mechanics class spring-like rib implant model from the 2nd rib to the 6th rib at the middle position. Table 17 shows the elastic modulus values of the rib implant models from the 2nd rib to the 6th rib obtained through calculation. Observe Figure 18 , the stress-strain curves of the models from the 2nd rib to the 6th rib are linear within a certain strain value range, corresponding to the specimen being in the elastic deformation stage at this stage. It can be found that the models of the 2nd rib and the 3rd rib are in the elastic deformation stage when the strain value is 0 - 0.1, while the model of the 6th rib is in the elastic deformation stage when the strain value is 0 - 0.2. The model of the 6th rib is significantly easier to maintain elastic deformation compared to the models of the 2nd rib and the 3rd rib. The reason is that the structural design of the 6th rib model is all costal cartilage within a range of 75 mm, while only a small part of the structure of the 2nd rib and the 3rd rib models contains elastic costal cartilage. Therefore, the 6th rib model has an elastic deformation stage nearly twice that of the 2nd rib and the 3rd rib models. At the same time, from Figure 18 it can be found that the stress-strain curves of the 2nd rib model and the 3rd rib model are close, the stress-strain curves of the 4th rib model, the 5th rib model and the 6th rib model are close, and at the same time, the stress-strain curves of the models from the 2nd rib to the 6th rib change sequentially. As can be seen from Table 17, the elastic moduli of the 2nd rib model and the 3rd rib model are close, being 966 MPa and 961 MPa respectively, and the elastic moduli of the 4th rib to the 6th rib models are also close, being 420 MPa, 391 MPa, and 380 MPa respectively. It can be found that the elastic modulus gradually decreases from the 2nd rib model to the 6th rib model, which is consistent with the initial structural design requirements. As can be seen from Table 4 - 3, the lengths of the elastic costal cartilage parts of the 2nd rib and the 3rd rib models are the same and short, both being 28 mm. Therefore, their elastic modulus values are close and large; the lengths of the elastic costal cartilage parts of the 4th rib to the 6th rib models are large and increase slightly, being 40 mm, 56 mm, and 60 mm respectively. Therefore, the elastic modulus values of the 4th rib model to the 6th rib model are relatively small. Thus, it can be seen that the different initial structural designs can indeed affect the elastic modulus of the model. The gradual increase in the elastic structure length of the models from the 2nd rib to the 6th rib results in the elastic modulus of the models from the 2nd rib to the 6th rib gradually decreasing as required in the design.

[0174] Table 17 Elastic Moduli Obtained from the Finite Element Analysis of Different Types of Spring-Like Rib Models

[0175]

[0176] 4.2 Preparation of Specimens of Gradient Mechanics Class Spring-Like Rib Implants and Analysis of Bending Mechanical Properties

[0177] The gradient mechanical spring-like rib implants from the 2nd rib to the 6th rib were prepared by selective laser melting technology. The five groups of specimens prepared are as follows Figure 19 As shown in (a) of, the macroscopic morphology of the specimen meets the expectations and there are no obvious defects. Figure 19 Figure (b) of is the three-point bending test of the gradient mechanical spring-like 6th rib implant specimen.

[0178] Figure 20 Figure is the stress-strain curve obtained from the three-point bending test of the gradient mechanical spring-like rib implants from the 2nd rib to the 6th rib under compression at the middle position. Table 18 shows the elastic modulus of each specimen in the three-point bending test obtained by calculation. From Figure 20 It can be found that the stress-strain curves of the specimens from the 2nd rib to the 6th rib change successively. Moreover, the elastic deformation stage of the specimens from the 4th rib to the 6th rib is significantly longer than that of the specimens from the 2nd rib and the 3rd rib. And it can be seen from Table 18 that the elastic modulus values of the specimens from the 2nd rib to the 6th rib gradually decrease, with the maximum being 955 MPa for the 2nd rib specimen and the minimum being 415 MPa for the 5th rib specimen. This phenomenon is consistent with the law obtained from the above three-point bending simulation, that is, due to the increase in the length of the elastic rib cartilage part of the specimen, the elastic modulus of the specimen decreases successively. The elastic modulus of the 6th rib specimen is 447 MPa, slightly greater than 415 MPa of the 5th rib specimen. Theoretically, the elastic part lengths of the 5th rib and the 6th rib specimens are close, and the elastic modulus of the 5th rib specimen should be slightly greater than that of the 6th rib specimen. However, we found that during the three-point bending test, the pressing position of the indenter of the 5th rib specimen is at the gap between adjacent spring wires, while the pressing position of the indenter of the 6th rib specimen is in contact with the spring wire. Therefore, compared with the 5th rib specimen, when the same displacement is applied, the spring wire at the middle part of the 6th rib specimen in the compressed position will require a greater load to deform, resulting in the slightly larger elastic modulus of the 6th rib specimen than that of the 5th rib specimen.

[0179] Table 18 Elastic modulus obtained from the three-point bending test of the 2-6 rib implant specimens

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[0181] Table 4-6 Elastic modulus of the 2-6 rib gradient mechanical spring-like rib model

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[0183] 4.3 Construction and preparation of the elastic sternum-rib integrated implant model

[0184] After the structural design of the gradient mechanical spring-like rib implants from the 2nd rib to the 6th rib and the mechanical evaluation results meet the required requirements, in order to clearly display the overall gradient mechanical structure optimization design of the 2nd rib to the 6th rib, the model of the gradient mechanical spring-like rib implants from the 2nd rib to the 6th rib with a length of 75 mm and having the elastic function of the costal cartilage part is assembled with the manubrium sterni implant model, and a certain reinforcement is carried out at the connection between the two to obtain Figure 2 the gradient mechanical spring-like elastic sternocostal integrated implant model shown in the figure, where Figure 2 (a) of which is the front view of the gradient mechanical spring-like sternocostal integrated implant model, Figure 2 (b) of which is the side view of the gradient mechanical spring-like sternocostal integrated implant model, Figure 2 (c) of which is the partial enlarged view of the connection between the single gradient mechanical spring-like rib implant model and the manubrium sterni implant model, Figure 2 (d) of which is the TC4 gradient mechanical spring-like sternocostal integrated implant specimen prepared by selective laser melting technology after the model is determined.

[0185] From Figure 4 (a) the front view of the gradient mechanical spring-like sternocostal integrated implant model, it can be seen that the two ends of the 2nd rib to the 6th rib models are symmetric with each other, and it can be clearly seen that the 2nd rib to the 6th rib models have obvious structural differences, and the distance between the 2nd rib and the 3rd rib models is slightly larger than the distance between the remaining adjacent gradient mechanical rib models, which conforms to the real situation of the natural human sternum and ribs. From Figure 4 (b) the side view of the gradient mechanical spring-like sternocostal integrated implant model, the difference in the gradient mechanical structure of the 2nd rib to the 6th rib can be observed more obviously. The elliptical spring wires of the 2nd rib model at the top of the overall structure obviously change from thin to thick from the connection to the end, and the spring wire pitch of the thick part near the end also becomes relatively larger. Similarly, the 3rd rib and the 4th rib models also show a structure almost the same as that of the 2nd rib model, but the elliptical spring wires at the end are slightly thinner than those of the 2nd rib model at the end. This is because the length of the thinner costal cartilage part of the 2nd rib model is smaller, so under the same rib length, the thicker gradient part it occupies is more; at the same time, since the 5th rib and the 6th rib play a major role in the chest wall undulation during breathing, the proportion of the thinner costal cartilage part of the 5th rib and the 6th rib models with gradient mechanics is relatively large.

[0186] From Figure 2From the partial enlarged view of the connection between the (c) gradient mechanics spring-like rib implant model and the manubrium sterni implant model, it can be seen that since the spring wire of the rib cartilage part at the starting end of the single-gradient mechanics spring-like rib implant model is relatively thin, if it is directly connected to the sternum implant model, stress concentration is most likely to occur at this connection when subjected to load, resulting in fracture of the connection part, thus affecting the safety of the implant; therefore, in order to ensure the safety of the implant, the spring wire at the connection is strengthened to a certain extent, and a smooth transition with a certain curvature is used at the connection to connect the gradient mechanics spring-like rib implant model and the sternum implant model, so that the structural function of the gradient mechanics spring-like rib implant can be exerted on the basis of ensuring the safety of the implant; from Figure 2 As can be seen from (d), the macroscopic morphology of the TC4 gradient mechanics spring-like sternum-rib integrated implant specimen prepared by SLM meets the expectations, without obvious defects, and the gradient structure differences between different rib specimens can be more clearly observed in the SLM-prepared spring-like rib implant specimens of the 2nd to 6th ribs shown in the figure. The thickness of the spring wire at the end is obvious, and the overall specimen can simultaneously possess the elastic functions of the rib part and the costal cartilage part, realizing the gradient transition of structure and mechanics, which conforms to the initial design concept.

[0187] Step 5: Use the selective laser melting method to prepare an integrated structure of the rib part, the gradient rib part, and the costal cartilage part, that is, obtain the rib implant structure.

[0188] Selective laser melting (SLM) is a powder bed-based powder forming technology, which is most closely related to the basic idea of rapid prototyping and is generally used for the forming of metal materials. SLM uses a fiber laser source as the energy source, and the whole process is carried out in an inert gas-filled chamber. According to the data model, a product with a specific shape and structure is directly formed by layer-by-layer addition. During the forming process, the powder melts to produce metallurgical bonding. Through this technology, products with good surface quality, complex structures, and good performance can be formed. The SLM technology has relatively broad requirements for materials. In theory, any powder material that can form atomic connections between powder particles after laser heating can be used as the forming raw material for this technology. In the field of biomedical applications, the SLM technology has been used to manufacture orthopedic implants such as zygomatic bones and phalanges.

[0189] Since residual stress will be generated during the process of fabricating rib implant structures by SLM, it is necessary to perform stress relief treatment on the spring-like rib implant specimens fabricated by SLM to prevent component deformation or cracking caused by residual stress. For the titanium alloy specimens fabricated by SLM, stress relief treatment usually adopts a stress relief annealing heat treatment regime. The stress relief annealing heat treatment regime adopted in this paper is to hold at 650 °C for 1 h in an SX-4-10 box-type resistance furnace, and then cool in the furnace to room temperature. The specific stress relief annealing heat treatment regime is shown in Table 19.

[0190] Table 19 Selective laser melting forming heat treatment process

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[0192] The surface microstructure of TC4 was constructed using a lattice structure. The pore sizes were 200 μm, 300 μm, 400 μm, and 500 μm respectively. It was found that with the increase of the pore size, the surface roughness increased, and the surface roughness of the surface microstructure specimens with pores was 7-10 times higher than that of the solid specimens. At the same time, when the pore size was above 300 μm, the porosity exceeded 50%.

[0193] When performing cell experiments on surface microstructure specimens with different pore sizes, it was found that the surface microstructure with a larger pore size could provide more attachment area and deeper implantation depth for cells, which was more conducive to cell growth. The cells spread well on the surface microstructure specimens with pore sizes of 400 μm and 500 μm, with a larger pore size. The cells were elongated, with a large number of pseudopodia, a large span, and a large number of proliferated cells. This indicated that the surface microstructure specimens with pore sizes of 400 μm and 500 μm had higher roughness, providing a physical environment suitable for cell adhesion and growth, so their surfaces had excellent bioactivity and biocompatibility, which was conducive to improving the bone integration ability.

[0194] Although the embodiments of the present invention have been shown and described above, it can be understood that the above embodiments are exemplary and should not be construed as limiting the present invention. Those of ordinary skill in the art can make changes, modifications, substitutions, and variations to the above embodiments within the scope of the present invention without departing from the principles and purposes of the present invention.

Claims

1. A rib implant structure with gradient mechanical transition, characterized in that: The rib implant structure is an elastic structure with a gradient change in elastic modulus, and its mechanical properties along the length direction successively meet the mechanical properties of the human costal cartilage part and the rib part. A mechanical gradient transition structure is adopted between the costal cartilage part and the rib part. The outer end of the costal cartilage part is connected to the sternum implant, and the outer end of the rib part is connected to the missing end of the human rib; The gradient transition structure is connected between the costal cartilage part and the rib part to achieve a mechanical gradient transition of the implanted rib from a low modulus to a high modulus structure; The rib implant structure is a spring-like rib structure; the gradient transition structure is made of TC4 titanium alloy and includes three gradient changes from the costal cartilage part to the rib part. The major semi-axis a of the cross-section of the spring wire in the three gradients is 1.6 - 2.4 mm, 2.4 - 3.2 mm, and 3.2 - 4.8 mm respectively, the minor semi-axis b of the three gradients is 1.0 mm, and the pitch t of the spring wire in the three gradients is 5.5 mm, 7.0 mm, and 10.0 mm respectively; The outer envelope elliptical dimensions of the rib implant structure are set to be the same as the external shape dimensions of the natural human rib. When the major semi-axis A of the outer envelope ellipse of the spring-like structure is 3.4 mm and the minor semi-axis B is 3.0 mm, the bending elastic modulus of the costal cartilage part is 396 MPa, and the bending elastic modulus of the rib part is 10.9 GPa; or, when the major semi-axis A of the outer envelope ellipse of the spring-like structure is 5.0 mm and the minor semi-axis B is 3.0 mm, the bending elastic modulus of the costal cartilage part is 170 MPa, and the bending elastic modulus of the rib part is 3.6 GPa.

2. The rib implant structure with gradient mechanical transition according to claim 1, characterized in that: A reinforcement structure is used to connect the outer end of the costal cartilage part to the sternum implant; The top cross-section of the reinforcement structure is consistent with the outer end cross-section of the costal cartilage part and is smoothly connected, and its bottom end is smoothly connected to the side wall of the sternum implant. Moreover, the radial cross-sectional area of the reinforcement structure increases along the axis from the top to the bottom.

3. The rib implant structure with gradient mechanical transition according to claim 1, characterized in that: The rib implant structure is a surface lattice lightweight structure with pore sizes of 400 μm and 500 μm.

4. A design method for the rib implant structure with gradient mechanical transition according to any one of claims 1 - 3, characterized in that The specific steps are as follows: Step 1: Construct a finite element stress-strain analysis model of the elastic structure, and combine experiments to verify the influence of the structural parameters of the rib implant structure on the mechanical properties; Step 2: Construct a fitting relationship model between the structural parameters and the bending elastic modulus E; Step 3: According to the bending elastic moduli of the rib and the costal cartilage, use the fitting relationship model in Step 2 to calculate the values of the structural parameters of the rib part and the costal cartilage part in the rib implant structure, and determine the structures of the rib part and the costal cartilage part; the structural parameters include the major semi-axis a and the minor semi-axis b of the elliptical size of the cross-section of the spring wire, the pitch t of the spring wire, the major semi-axis A and the minor semi-axis B of the outer envelope ellipse of the spring-like structure; Step 4: According to the structural parameters of the rib part and the costal cartilage part, determine the gradient change between the two and establish a connection to obtain the gradient transition structure; Step 5: Use the selective laser melting method to prepare an integral structure of the rib part, the gradient transition structure, and the costal cartilage part, that is, the rib implant structure is obtained.

5. The design method for the rib implant structure with gradient mechanical transition according to claim 4, characterized in that: In the said step 1, first, a model of the elastic structure, namely a spring-like structure model, is constructed; then, finite element stress-strain analysis is carried out on the model; after that, the finite element simulation is verified; finally, the influence of the structural parameters of the spring-like rib implant structure on the mechanical properties is obtained. Among them, the larger the major semi-axis a and the minor semi-axis b of the cross-sectional dimension of the spring wire, the greater the structural elastic modulus of the spring-like rib implant; the greater the difference between the major and minor semi-axes of the cross-section of the spring wire, the lower the structural elastic modulus of the spring-like specimen; and under the same conditions, the influence of the cross-sectional area of the elliptical spring wire on the elastic modulus is greater than the influence of the difference between the major and minor semi-axes. The larger the pitch t of the spring wire, the greater the structural elastic modulus of the spring-like rib implant. The smaller the major semi-axis A and the minor semi-axis B of the outer envelope ellipse of the spring-like shape, the greater the structural elastic modulus of the spring-like rib implant, and the adjustment of its major semi-axis has a more significant influence on the structural elastic modulus than the minor semi-axis. The cross-sectional dimension of the spring wire shows a large positive correlation, and the outer envelope ellipse dimension shows a large negative correlation. Both have a greater influence on the bending elastic modulus of the spring-like rib implant. The pitch of the spring wire shows a medium positive correlation and has a smaller influence.

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