## 2 Materials and Methods  
### Verification of Porosity Size, Structural Design, and Structural Coherence  
It is known that the pore size of human cancellous bone ranges from 500 to 600 nm [6]. Thus, to find similar pore sizes, a total of five samples were fabricated by varying the pore size (1.0 mmto 1.8 mm, 0.2 mm spacing) using preprocessed commercial software MAGICS RP (Materialize NV, Leuven, Belgium). Scanning electron microscope (SEM) images for all of the faces of the five specimens were photographed at \(\times\)30 ratio. The results were measured, and the average value was calculated after measuring the pores of each face one at a time (Figure 1). Next, an animal experiment was conducted to confirm the degree of bone fusion of each type of porous structure. The purpose of this experiment was to test the strength of separation from the bone after six weeks, when implanting the specimens into the bones of rabbits. We take the high strength result of push down testing to mean that the structure is well-coupled with the bone and through this, it is possible to select the structure that can enhance the higher integration rate, which is the main purpose of spinal cages. We developed a spinal cage made of the above structure that also gives a higher integration rate along with enhanced merging of bone. Rabbit femur was chosen because it was not suitable to transplant the specimen to the spine of a small animal, and the femur was suitable for cutting and preparing the specimen for a second push down test. Additionally, the femur is made up of cortical bone on the inside and cancellous bone on the outside, providing an environment similar to the vertebrate. The five samples of different porous structure and solid structure were designed with a diameter of 2 mm and a length of 6 mm according to the international standard test method (ISO10993-6: 2007). The samples were printed using electron beam melting (EBM) within an ARCAM A1 (ARCAM AB, Molnycke, Sweden). (Figure 2) During the EBM process, the electron beam melts metal powder layer-by-layer to build the implant. The vacuum environment in the EBM machine maintains the chemical composition of the material and provides an excellent environment for building parts in reactive materials such as titanium alloys. The high power of the electron beam ensures a high rate of deposition and an even temperature distribution within the part. The results trigger full melting of the metal powder and high strength properties of the material. The EBM machine titanium Ti6 Al4V ELI (Grade 23) is a gas-atomized powder with a particle size between 45 and 100 microns. This limit on the minimum particle size ensures safe handling of the powder. Ti6 Al4V-ELI has numerous applications in the medical industry. The biocompatibility of Ti6 Al4V-ELI is excellent, especially when direct contact with tissue or bone is required. A powder remover system machine (PRS Machine, ARCAM AB, Molnycke, Sweden) was used to remove any remaining titanium powder on the completed samples. All products are made of titanium alloy (Ti6 Al4V-ELI per ASTM-F136). Specimens 1 to 4 were fabricated using 3D printing with titanium powder, and Specim 5 was fabricated using bar material. Each type of structure was inserted into the femur of the New Zealand White Rabbit and the rabbit's femur was collected at 6 weeks after the insertion. The purpose of push down testing is to implant a specific sample into the bone of an animal and to determine the degree of bone fusion after a certain period of time. Usually after six weeks, there is improved strength, and the push down testing is to check the degree of deviation by sliding the specimen into an axial direction by cutting the specimen so that it can be mounted on the mechanical testing machine. The better strength can preserve the bone fusion with stability. The push down test was performed using MTS 858 Bionix (MTS System Corp, Eden Prairie, MN, USA) to measure the mechanical strength at which the specimen is released from the bone.