# Design and 3D Printing of Interbody Fusion Cage Based on TPMS Porous Structure
## 1 Introduction  
The spinal implant devices manufactured by the existing technology have obvious shortcomings, and the comprehensive mechanical properties of the solid metal implants do not match, resulting in a serious stress shielding effect between the solid metal implants and the host bone [1, 2]. The ideal interbody fusion cage implantation device should have good biocompatibility, can complete the new bone growth in an effective time, and achieve real interbody fusion. In addition, the device located in the load-bearing position needs to have certain comprehensive mechanical properties. When constructing the implant model of the personalized interbody fusion cage, it is necessary to comprehensively consider the biomechanical properties, porosity, pore distribution, connectivity, and other factors of the fusion cage. The microporous structure design of the fusion cage is the key part of the process of spinal repair and replacement. In the design method of the porous structure model, the typical porous structure design method includes a computer aided design porous structure design method, based on the CT scanning image mapping porous structure design method and TPMS porous structure design method. The computer aided design porous structure design method is a combination of computer graphics and CAD program modeling, through the construction of solid geometry, to build a complex model. This method, as an attempt of the early design of the porous scaffolds, only carries out simple Boolean operations and arrays, and the parameters, such as porosityand gradient pores of the scaffolds, are poorly controllable, as shown in Figure 1A [3]. The method of model reconstruction based on medical images is generally through the analysis and processing of CT or MRI medical data images, using related algorithms to extract features of specific areas and 3D model reconstruction in professional software. According to the number of large-scale images of tibia under the DICOM standard, a clear bone microstructure image was obtained by using image binarization, thermal pixel removal, closure operation, and other algorithms, and then the perimeter and area of bone microstructure pores were measured by the straight line fitting method based on the least square method, as shown in Figure 1B [4; 5]. Although the bone tissue 3D model reconstruction algorithm based on medical images can obtain structural and morphological features, and is similar to bone tissue, its implementation process is generally complex and requires high software algorithms, system hardware, and users. The implicit curved surface porous element construction method can effectively parametrically control the pore shape, pore size, specific surface area, porosity, and other parameters of the porous unit, which makes the mechanical and biological properties of the designed porous implant meet the ideal requirements, and has a positive impact on cell migration and tissue inward growth, which has attracted the attention of researchers [6; 7; 8; 9; 10; 11]. The triply periodic minimal surface (TPMS), as a kind of implicit surface, has the advantages of a controllable parameter design and integrity of model expression, and has the advantages of ideal connectivity and controllability, which provides a new design method for the construction of existing porous implants [12; 13; 14]. The TPMS construction method can be used to construct a porous structure implant model with a linear change of pore structure. By regulating the pore structure, the pore structure of the porous implant changes linearly from the surface to the inside, so that the surface elastic modulus of the porous implant, in contact with bone tissue, is close to that of bone tissue, as shown in Figure 1C [15; 16; 17]. The above studies show that the bone tissue microporous structure design based on the TPMS curved surface porous structure design method can optimize the mechanical properties of porous implants and can be used as the characterization of the inner pore structure of ideal bone implants.