## 4 Discussion  
This study evaluated the behavior of each tissue and implant in the spine, when a porous structure was applied to the metal spinal fixation system, used for surgical treatment of patients with spinal diseases, and examined the stability and applicability of the porous structural implant. Recently, titanium alloys such as Ti-6 Al-4V have been mainly used in spinal implants. Metallic materials can be very safe in terms of strength, but they can cause problems such as stress shielding, damage to adjacent bone and tissues, and excessive restriction of spinal  
Figure 12: Safety factor (equivalent stress/yield stress) of pedicle screw, spinal cage and spinal rod at different porosities (\(p\)) of spinal implants in each spinal motion: (**a**) flexion; (**b**) extension; (**c**) lateral bending; and (**d**) rotation. Figure 13: Equivalent stress (\(\sigma\)) distribution of spinal implants at porosity 50% in extension: (**a**) pedicle screw; (**b**) spinal cage; and (**c**) spinal rod. behavior due to their excessive elastic modulus, compared with that of the bone. To resolve these issues, studies on porous spinal implants have been conducted. Porous spinal cage has been particularly focused on, and it has been reported that an increase in ROM and a decrease in stress are obtained when the porosity of the spinal cage is increased. However, studies that applied porous structures to pedicle screws and spinal rods have been scarce. Particularly, in clinical trials, fracture of the adjacent bone was caused by the high elastic modulus of pedicle screws and spinal rods. In this study, a porous structure was applied to the entire spinal implant system, including pedicle screws and spinal rod [1; 2; 3; 4; 5; 6; 7; 8; 9; 10; 11; 12; 13; 14; 15; 16; 17; 18; 19; 20; 21; 22; 23; 24; 25]. This study was conducted using ANSYS workbench 2019 R1, a commercial FE analysis program, and FE modeling was performed on lumbar 3 to sacrum 1, where spinal diseases occur most frequently. The spinal disease site was assumed to be between lumbar vertebrae 3 and 4, and a pedicle screw, spinal cage, spinal rod, and bone graft were applied to this site. The microstructure of the porous spinal implant was assumed to be a diamond structure, and the same porosity was applied to all the spinal implants. The microstructure and porosity were obtained from the results of a tensile test by [PERSON] et al. Equivalent material properties of porous structures with 15%, 40%, 60%, and 75% porosity were estimated using the Romberg-Osgood formula based on porosities of 0%, 34%, 50%, 73%, and 81%. The spinal implant was evaluated at nine values of porosity, for the behavior of spinal tissues and implants [32; 33; 34; 35; 36; 37; 38; 39; 40; 41]. The behavior of spinal tissues and implants was evaluated by examining the ROM of the spine, intervertebral disc stress per ROM, pedicle screw and vertebral cage adjacent bone stress, and safety factors of each implant. The increase in ROM with increasing porosity indicated that the motor capacity of the spine was improved. However, the stress of adjacent IVDs was reduced, thereby increasing safety. In the evaluation of the adjacent bone stress of the pedicle screw and the spinal cage, the stress of the pedicle screw decreased and increased again with increased porosity, assuming the best performance at a porosity between 40% and 60%. Finally, in the evaluation of the safety rate of implants, the safety factor showed a tendency to decrease rapidly as the porosity decreased, but the safety factor was more than three in all implants with a porosity of 73% or less. Thus, it was estimated that the behavior of spinal tissue and implant was most stable at a porosity between 40 and 60%. This study is original in that the porous structure was applied to all of the spinal implants, but it is limited in terms of microstructure shape and size, porosity diversity, and detailed behavior of the microstructure; the FE model was also limited to some parts of the lumbar spine.