CN104318009B - A design method of personalized intervertebral fusion device - Google Patents

Abstract

The invention relates to a method for designing a personalized intervertebral fusion device. In the method, the vertebral body is scanned by CT, and then the CT continuous tomographic image data is reconstructed into a three-dimensional model, and the vertebral body model is measured, including measuring the vertebral body sagittal diameter, vertebral body transverse diameter, intervertebral space height, etc. On the basis of the measured parameters, carry out the personalized design and implantation of the intervertebral fusion device, complete the establishment of the finite element model through mesh division and smoothing, material assignment and boundary condition setting, etc., and finally solve and analyze the finite element model , provide a basis for the optimization and improvement of intervertebral fusion, and finally realize the design of personalized intervertebral fusion considering both geometric matching and functional matching.

Description

A design method of personalized intervertebral fusion device

technical field

The invention relates to a method for designing a personalized intervertebral fusion device, and more specifically relates to a method for individually designing an intervertebral fusion device based on CT image three-dimensional reconstruction and measurement technology and finite element analysis technology.

Background technique

In clinical interbody fusion, the implantation method, size, and spatial shape of the intervertebral cage are closely related to nerve damage, implantation stability, and intervertebral bone graft volume. However, due to the large differences in the physiological parameters of patients and the individual characteristics of the internal environment, it is difficult to optimize the interaction between standardized, mass-produced, and serialized intervertebral fusion devices and the human body, resulting in some intervertebral fusion devices entering the body. Excessive wear, fatigue failure, etc. Therefore, the design of intervertebral fusion cages must be individualized. The intervertebral fusion cage faces a complex stress environment in the body. These stresses will not only affect the reliability of the intervertebral fusion cage itself, but also affect the normal function of the intervertebral fusion cage and the reconstruction of the surrounding host tissue. One of the key factors for the success or failure of the device. Therefore, the performance of intervertebral fusion should not only consider its material selection, preparation and surface modification, but also fully consider its configuration design as a medical device and its interaction with surrounding host tissues. At present, the design of most intervertebral fusion cages is based on geometric matching requirements, and the design based on functional matching has not been realized.

Contents of the invention

In order to overcome the problem that the design of most current intervertebral fusion devices is based on geometric matching requirements and does not realize the design based on functional matching, the present invention discloses a method for designing personalized intervertebral fusion devices. The method uses CT three-dimensional reconstruction Technology, restore the anatomical structure features of the spine in a digital virtual environment and perform three-dimensional measurement, on this basis, carry out the personalized design and implantation of the intervertebral fusion device, and use the personalized finite element analysis technology to analyze the designed intervertebral fusion Analyze and improve.

According to actual needs, the present invention imports image data such as CT/MRI into the developed software CageDesigner, automatically divides the vertebral body of the spine, establishes the geometric model of the spine, automatically divides the grid of the model, and can simulate the vertebral body of the spine. Intervertebral fusion surgery process and finite element analysis, to provide reference for the personalized design of intervertebral fusion device.

The invention discloses a method for designing a personalized intervertebral fusion device, which specifically comprises the following steps:

1) Image acquisition and preprocessing: The vertebrae are scanned by spiral CT, and then the raw data of the vertebrae obtained by the CT scanner are imported into the software, and the images are automatically sorted and judged according to the number of the images. Then the imported CT images were processed by median filtering, Gaussian filtering and binarization.

2) Segmentation of vertebral body image and establishment of 3D model: firstly, manually select an initial slice in the horizontal plane view area of the software to start the segmentation process, manually draw the outline of the vertebral body in the initial slice, and then use the level set method to iteratively obtain the initial slice The precise contour of the vertebral bodies of the layers. The subsequent segmentation process will start from the reference layer and perform segmentation operations on all slices from the upper and lower directions. Since the change range of the target in the adjacent slices of the vertebral CT image is small, the contours obtained from the segmentation results of the processed slices can be used as the initial contours of the adjacent slices, and then the level set method is used to obtain the accurate contours of all vertebral bodies . Finally, manual methods can be used to correct the acquired contours. For the segmentation results, the volume rendering method is used to draw and display in the window.

3) Measurement of the vertebral body model: the vertebral body sagittal diameter and the vertebral body transverse diameter are acquired on the upper endplate plane of the vertebral body model. Using the sectioning function of the software, the three-dimensional vertebral body model was sectioned to obtain the upper endplate plane of the vertebral body model. Find the anterior-posterior axis bisecting the vertebral body model into left and right halves on the endplate plane, and measure the distance between the anterior-posterior axis and the edge of the vertebral body model, which is the sagittal diameter of the vertebral body. Measure the distance between two intersection points on the edge of the straight line vertebral body model perpendicular to the anterior-posterior axis, and the one with the largest distance is the transverse diameter of the vertebral body. The height of the intervertebral space was measured in the midsagittal plane of the vertebral model. The anterior and posterior edge points of the vertebral endplates and the center point of the endplates were manually obtained on the midsagittal plane. The distance between the anterior edge of the upper and lower vertebral body endplates is the height of the anterior edge of the intervertebral space, the distance between the posterior edge of the two vertebral body endplates is the height of the posterior edge of the intervertebral space, and the distance between the center points of the two endplates is the height of the center of the intervertebral space.

4) Design and implantation of the intervertebral fusion cage: in the existing intervertebral fusion cage model library, according to the anatomical shape (curve, platform) of the upper edge of the vertebral body, select the geometric shape (curvature) of the fusion cage, and according to the The height of the gap determines the height of the intervertebral fusion device, and the length and width of the intervertebral fusion device are determined according to the transverse diameter of the vertebral body and the sagittal diameter of the vertebral body. The design principle is to make the intervertebral fusion fit the anatomical structure, achieve a larger contact area, meet the mechanical strength, and promote fusion. For double intervertebral fusion cages, they are implanted symmetrically in the posterior half of the vertebral body. For a single intervertebral fusion cage, place it obliquely in the center of the vertebral body.

5) Finite element meshing and smoothing: the vertebral body is meshed using a voxel-based meshing method. First, the pixels of each part are represented by different codes; then the 8 adjacent pixels in the upper and lower layers are connected as nodes to form a voxel, redundant voxels are removed, and the remaining voxels are used as an eight-node hexahedral unit. The pixel codes of the eight nodes of the unit at the surface of the model and at the interface of different materials are different. At this time, the method of mirror division is used to decompose each boundary hexahedron unit into 5 tetrahedron units, and then by moving these tetrahedrons The nodes of the unit are used to smooth the interface; in order to reduce the amount of calculation, some units with the same material will be merged as needed. Using the mapping method, the finite element mesh of the standard intervertebral fusion model in the existing intervertebral fusion model library is mapped to the designed personalized intervertebral fusion to realize the mesh division of the intervertebral fusion.

6) Material assignment and boundary condition setting: Calculate the elastic modulus of the unit by using the gray value of the unit node, set the thickness of the model cortical bone and the friction coefficient of the facet joint, assuming that the cancellous bone material is isotropic, the intervertebral cage and the vertebra Surface-to-surface contact between bodies. All degrees of freedom of the lower endplate of the lower vertebral body are fixed, and the center point R of the upper endplate of the upper vertebral body is selected, and downward pressure is applied on R to simulate vertebral body load-bearing, and torque is applied to simulate forward flexion, backward extension, and lateral bending , to reverse. Through the above steps, the establishment of the finite element model of the vertebral body implanted with the intervertebral fusion device is completed.

7) Solving and analysis of the finite element model: use the voxel finite element model solving algorithm to solve the finite element model of the vertebral body to obtain the maximum principal stress, equivalent stress, subsidence and displacement, which are used to evaluate the vertebral body and intervertebral fusion The stress distribution and displacement of the device, so as to determine whether the design is reasonable. For unreasonable designs, return to step 4 to modify the design, and finally realize the design of a personalized intervertebral fusion cage that considers both geometric matching and functional matching. The final designed personalized intervertebral fusion cage is exported in STL format.

The advantages of the personalized intervertebral fusion device design method of the present invention include:

1. In the design, the geometric matching and functional matching are fully considered, and the personalized finite element analysis technology is introduced into the design of the intervertebral fusion cage.

2. Can visually display the shape of the vertebrae and comprehensively measure the parameters that reflect the structural characteristics of the vertebral body, such as the transverse diameter of the vertebral body, the sagittal diameter of the vertebral body, the height of the intervertebral space, etc.;

3. The design method is simple, the operation time is short, and it is easy to promote clinically.

Description of drawings

Fig. 1 is the specific implementation flowchart of the design of individualized intervertebral fusion device according to an embodiment of the present invention;

Fig. 2 is a schematic diagram of parameter measurement at the endplate plane on the vertebral body according to an embodiment of the present invention;

Fig. 3 is a schematic diagram of parameter measurement in the mid-sagittal plane of the vertebral body according to an embodiment of the present invention;

Fig. 4 is a schematic diagram of the implantation position of the double intervertebral fusion device of the present invention;

Fig. 5 is a schematic diagram of the implantation position of a single intervertebral fusion device of the present invention;

Fig. 6 is the result figure of the pyramidal mesh division of an embodiment of the present invention;

Fig. 7 is a personalized intervertebral fusion device designed by an embodiment of the present invention.

detailed description

As shown in Figure 1, it is a specific implementation flow chart of the design of a personalized intervertebral fusion device according to an embodiment of the present invention, including the following steps:

Step 101: Image acquisition and preprocessing.

Perform a spiral CT scan on the vertebrae to obtain vertebral CT images. Import the acquired vertebral original image data into the software and automatically sort and judge the images according to the image numbers. Then the imported CT images were processed by median filtering, Gaussian filtering and binarization.

Step 102: segmenting the vertebral body image and establishing a three-dimensional model.

Firstly, an initial slice was manually selected in the horizontal plane view area of the software to start the segmentation process, and the approximate outline of the cone of the initial slice was manually drawn, and then the precise outline of the cone of the initial slice was obtained through iteration using the level set method. The subsequent segmentation process will start from the reference layer and perform segmentation operations on all slices from the upper and lower directions. Since the change range of the target in the adjacent slices of the vertebral CT image is small, the contours obtained from the segmentation results of the processed slices can be used as the initial contours of the adjacent slices, and then the level set method is used to obtain the accurate contours of all vertebral bodies . Finally, manual methods can be used to correct the acquired contours. For the segmentation results, the volume rendering method is used to draw and display in the window.

Step 103: measuring the vertebral body model.

The sagittal and transverse diameters of the vertebral body were acquired on the upper endplate plane of the vertebral body model. Using the sectioning function of the software, the three-dimensional vertebral body model was sectioned to obtain the plane of the upper endplate of the vertebral body (Fig. 2). Find the anterior-posterior axis L that bisects the vertebral body model into left and right halves on the endplate plane (Fig. 2), and measure the distance between the anterior-posterior axis L and the two intersection points C and D (Fig. 2) on the edge of the vertebral body model. The distance is the sagittal diameter of the vertebral body. Measure the distance between the straight line perpendicular to the anterior-posterior axis L and the two intersection points on the edge of the vertebral body model, and the distance between the two points A and B (Fig. 2) with the largest distance is the transverse diameter of the vertebral body. The height of the intervertebral space was measured in the midsagittal plane of the vertebral model (Fig. 3). On the midsagittal plane, the anterior and posterior edge points E, F, G, and H of the vertebral endplate (Fig. 3) and the center points of the endplate I, J (Fig. 3) were obtained manually. Measure the distance between the front edge points F and H of the upper and lower vertebral body endplates (Fig. 3), which is the height of the anterior edge of the intervertebral space, and the distance between the posterior edge points E and G of the two vertebral body endplates (Fig. 3) is the height of the posterior edge of the intervertebral space. The distance between the center points I and J of the two endplates (Fig. 3) is the center height of the intervertebral space.

Step 104: Design and implantation of the intervertebral fusion device.

In the existing intervertebral fusion cage model library, the geometric shape (curvature) of the fusion cage is selected according to the anatomical shape (curve, platform) of the upper edge of the vertebral body, and the height of the intervertebral fusion cage is determined according to the height of the intervertebral space. The transverse diameter of the body and the sagittal diameter of the vertebral body determine the length and width of the intervertebral cage. The design principle is to make the intervertebral fusion fit the anatomical structure, achieve a larger contact area, meet the mechanical strength, and promote fusion. For the double intervertebral fusion cage, it is implanted symmetrically in the posterior half of the vertebral body (Figure 4). First, shrink the edge of the vertebral body by one-third to obtain the gray area shown in Figure 4, where the intervertebral fusion cage cannot be implanted. Align one intervertebral fusion cage with point D (Figure 4), move it to the right until it touches the gray area, complete the determination of the implantation position of this intervertebral fusion cage, and then place the other one symmetrically Intervertebral fusion cage. For a single interbody fusion cage, it is implanted obliquely in the center of the vertebral body (Fig. 5). Firstly, place the long axis h of the intervertebral cage at 45 degrees to the sagittal diameter of the vertebral body, align the posterior edge of the intervertebral cage with point D (Fig. 5), and then move the intervertebral cage parallel to one side until the vertebral The long axis h of the intervertebral cage passes through the center O of the vertebral body (Fig. 5) to complete the implantation of a single intervertebral cage.

Step 105: Finite element mesh division and smoothing.

The vertebral body is meshed using a voxel-based meshing method. First, the pixels of each part are represented by different codes; then the 8 adjacent pixels in the upper and lower layers are connected as nodes to form a voxel, redundant voxels are removed, and the remaining voxels are used as an eight-node hexahedral unit. The pixel codes of the eight nodes of the unit at the surface of the model and at the interface of different materials are different. At this time, the method of mirror division is used to decompose each boundary hexahedron unit into 5 tetrahedron units, and then by moving these tetrahedrons The nodes of the unit are used to smooth the interface; in order to reduce the amount of calculation, some units with the same material will be merged as needed. Using the mapping method, the finite element mesh of the standard intervertebral fusion model in the existing intervertebral fusion model library is mapped to the designed personalized intervertebral fusion to realize the mesh division of the intervertebral fusion. An example of the result of finite element meshing is shown in Fig. 6.

Step 106: Assignment of materials and setting of boundary conditions.

The elastic modulus of the unit is calculated by using the gray value of the unit node, and the thickness of the model cortical bone and the friction coefficient of the facet joint are set. It is assumed that the cancellous bone material is isotropic, and the intervertebral fusion cage and the vertebral body adopt surface-to-surface contact. All degrees of freedom of the lower endplate of the lower vertebral body are fixed, and the center point R of the upper endplate of the upper vertebral body is selected, and downward pressure is applied on R to simulate vertebral body load-bearing, and torque is applied to simulate forward flexion, backward extension, and lateral bending , to reverse. Through the above steps, the establishment of the finite element model of the vertebral body implanted with the intervertebral fusion device is completed.

Step 107: Solving and analyzing the finite element model.

Use the voxel finite element model to solve the commonly used algorithm EBE-PCG (element by element precon-ditioned conjugate gradient) algorithm to solve, get the maximum principal stress, equivalent stress, subsidence and displacement, and use it to evaluate vertebral body and intervertebral fusion The stress distribution and displacement of the device, so as to determine whether the design is reasonable. For unreasonable designs, return to step 4 to modify the design, and finally realize the design of a personalized intervertebral fusion cage that considers both geometric matching and functional matching. The final designed personalized intervertebral fusion cage was exported in STL format (Fig. 7).

Example Design of personalized lumbar intervertebral fusion device

The L3-L4 lumbar spine was scanned with spiral CT. Scanning parameters: slice thickness 0.63mm, slice distance 0.63mm, tube voltage 120kV, current 225mAs, resolution 512*512pxl, and 150 lumbar spine CT images were obtained. The 150 CT data were imported into the software for median filtering, Gaussian filtering and binarization.

Firstly, manually select a picture with vertebral body and vertebral arch in the horizontal plane view area of the software as the initial slice to start the segmentation process, manually draw the outline of the vertebral body in the initial slice, and then use the level set method to iteratively obtain the vertebral body of the initial slice Precise contours of the body. The subsequent segmentation process will start from the reference layer and perform segmentation operations on all slices from the upper and lower directions.

Using the sectioning function of the software, the three-dimensional vertebral body model was sectioned to obtain the plane of the upper endplate of the vertebral body (Fig. 2). Find the anterior-posterior axis L that bisects the vertebral body model into left and right halves on the endplate plane (Fig. 2), measure the distance between the anterior-posterior axis L and the two intersection points C and D (Fig. 2) of the vertebral body model edge, and obtain Vertebral sagittal diameter. Measure the distance between the straight line perpendicular to the anterior-posterior axis L and the two intersection points of the vertebral body edge, and the distance between the two points A and B (Fig. 2) with the largest distance is the transverse diameter of the vertebral body. On the midsagittal plane, the anterior and posterior edge points E, F, G, and H of the vertebral endplate (Fig. 3) and the center points of the endplate I, J (Fig. 3) were obtained manually. Measure the distance between the anterior edge points F and H of the upper and lower vertebral body endplates (Figure 3) to obtain the height of the anterior edge of the intervertebral space, and the distance between the posterior edge points E and G of the two vertebral body endplates (Figure 3) to obtain the height of the posterior edge of the intervertebral space. The distance between the plate center points I and J (Fig. 3) gives the center height of the intervertebral space.

Select JAGUAR ^TM LUMBAR I/F CAGE (dimensions: height = 9mm, width = 9mm, length = 25mm, elastic modulus: 3600MPa, Poisson's ratio: 0.25) as a reference model, according to the measured vertebral transverse diameter Design the length of the new intervertebral fusion device, and set the height of the new intervertebral fusion device according to the height of the intervertebral space.

A double intervertebral fusion cage was implanted in the posterior half of the vertebral body. Mesh and smooth the model. Set the facet joint friction coefficient to 0.1, assume that the cancellous bone material is isotropic, and adopt surface-to-surface contact between the intervertebral fusion cage and the vertebral body. All degrees of freedom of the lower endplate of the L4 vertebral body were fixed, and the center point R of the upper endplate of the L3 vertebral body was selected. A downward pressure of 400N was applied on R to simulate the load-bearing of the vertebral body, and a torque of 10N-m was applied to simulate forward flexion and backward flexion. Stretch, bend, twist.

Use the voxel finite element model to solve the commonly used algorithm EBE-PCG (element by element precon-ditioned conjugate gradient) algorithm to solve, get the maximum principal stress, equivalent stress, subsidence and displacement, and use it to evaluate vertebral body and intervertebral fusion The stress distribution and displacement of the device, so as to determine whether the design is reasonable. For unreasonable designs, return to step 4 to modify the design, and finally realize the design of a personalized intervertebral fusion cage that considers both geometric matching and functional matching. The final designed personalized intervertebral fusion cage was exported in STL format (Fig. 7).

It should be understood that the description of the present invention in the foregoing description and description is only illustrative and not limiting, and that the above-described embodiments may be modified without departing from the present invention as defined in the appended claims. Various changes, deformations, and/or corrections.

Explanation of reference numbers

101. Image acquisition and preprocessing 102. Segmentation of vertebral image and establishment of 3D model

103. Measurement of vertebral body model 104. Design and implantation of intervertebral fusion device

105. Finite element mesh division and smoothing 106. Material assignment and boundary condition setting

107. Solving and analysis of finite element model.

Claims (4)

1. a design method of a personalized intervertebral cage model, characterized in that it comprises the following steps: 1) Image acquisition and preprocessing: perform spiral CT scanning on the vertebrae, and then perform image preprocessing on the raw data of the vertebrae obtained by the CT scanner, so as to obtain the vertebral body image; 2) Segmentation of vertebral body image and establishment of 3D model: Manually select an initial slice to start the segmentation process, manually draw the approximate outline of the vertebral body in the initial slice, and then use the level set method to iteratively obtain the precise outline of the vertebral body in the initial slice , the subsequent segmentation process will use the reference layer as the starting point to segment all slices from the upper and lower directions, and use manual methods to correct the obtained contours. For the segmentation results, use the volume rendering method to draw , and display in the window; 3) Measurement of the three-dimensional model of the vertebral body: cut the three-dimensional vertebral body model to obtain the upper endplate plane of the vertebral body model, find the anterior-posterior axis bisecting the vertebral body model into left and right halves on the endplate plane, and measure the anterior-posterior axis and The distance between two intersection points on the edge of the vertebral body model is the sagittal diameter of the vertebral body. Measure the distance between the two intersection points on the edge of the straight line vertebral body model perpendicular to the anterior-posterior axis, and the one with the largest distance is the transverse diameter of the vertebral body. The height of the intervertebral space was measured on the midsagittal plane of the vertebral body model, and the front and rear edge points of the vertebral body endplate and the center point of the endplate were manually obtained on the midsagittal plane, and the distance between the front edge points of the upper and lower vertebral body endplates was measured as the vertebral body The height of the front edge of the gap, the distance between the posterior edge points of the two vertebral body endplates is the height of the posterior edge of the intervertebral space, and the distance between the center points of the two endplates is the center height of the intervertebral space; 4) Design and implantation of the intervertebral fusion cage model: In the existing intervertebral fusion cage model library, according to the anatomical shape of the upper edge of the three-dimensional model of the vertebral body, select the geometric shape of the fusion cage model, and determine the vertebral fusion cage according to the height of the intervertebral space. The height of the intervertebral fusion cage model, the length and width of the intervertebral fusion cage model are determined according to the vertebral body transverse diameter and vertebral body sagittal diameter, so that the intervertebral fusion cage fits the anatomical structure and achieves a larger contact area. The number of intervertebral cage models should be implanted in different parts of the three-dimensional vertebral body model. For double intervertebral cage models, it should be symmetrically implanted into the second half of the vertebral body three-dimensional model. For a single intervertebral cage model , implant it obliquely into the center of the three-dimensional model of the vertebral body; 5) Finite element meshing and smoothing: Carry out finite element meshing and smoothing on the three-dimensional model of the vertebral body and the intervertebral cage model respectively; 6) Material assignment and boundary condition setting: set the material properties and boundary conditions of each part of the three-dimensional model of the vertebral body, and complete the establishment of the vertebral body finite element model with the intervertebral fusion cage model through the above steps; 7) Solving and analysis of the finite element model: The finite element model of the vertebral body is solved using the voxel finite element model solving algorithm, and the solution results are used to evaluate the stress distribution and displacement of the 3D vertebral body model and the intervertebral fusion cage model. To determine whether the design is reasonable; for an unreasonable design, return to step 4) to modify the design, and finally realize the design of a personalized intervertebral fusion cage model that considers both geometric matching and functional matching. 2. The design method of the individualized intervertebral fusion device model according to claim 1, wherein the image preprocessing described in the 1) step comprises median filtering, Gaussian filtering and binarization. 3. the design method of individualized intervertebral cage model according to claim 1, is characterized in that, the 5th) step described finite element grid division and smooth concrete mode are: adopt voxel-based grid division method The three-dimensional model of the vertebral body was meshed and smoothed, and the meshing method was used to mesh the designed intervertebral cage model. 4. The design method of the individualized intervertebral cage model according to claim 1, characterized in that, the solution result described in the 7th) step specifically includes maximum principal stress, equivalent stress, subsidence and displacement.