## 4 Discussion  
In the push down test, all of the specimens produced with additive manufacturing showed better push down strength than the specimens prepared by machining, and Type 3 had the highest push down strength, and therefore has the best bone fusion strength. Specimens made with machining had smoother surfaces, which led to a larger surface area and thus an excellent fusion strength, while those made with additive manufacturing had a rougher surface area. As a result, the Type 3 structure showed the best bone fusion strength.  
The sample coated with a porous coating on PEEK surface showed the following push down testing results: 2.5 times in 0 weeks, 6 times in 12 weeks and 10 times in 24 weeks, which are higher results in comparison to the samples without coating, and which is similar to our research result [16].  
The analysis of the potential sedimentation in FEM method according to the titanium and the PEEK materials studied by [PERSON] show that the amount is 1.6 times higher for flexibility, 1.5 times higher for extension and 1.4 times higher for material bending, so the titanium material is considered more vulnerable than PEEK in all exercises [17]. This is similar result to our findings.  
In the study on compressive strength according to the porosity of titanium material studied by [PERSON], it is reported that as the porosity grows, the elastic rate increases; and if the porosity increases about two times then the elasticity increases 1.7 times [18]. The titanium materials in our porosity can be identified with similar results from our study that have high elasticity to reduce subsidence.  
In the cage prototype test of mechanical performance, the porous composite fusion cage showed better results than the commercial PEEK cage, although it showed lower results than all commercial PEEK cages in static compression, static compression shear and torsion tests. In the subsidence test, it showed better results than the commercial titanium cage and showed similar results to the commercial PEEK cage. In the compression fatigue test, there was no change or breakage in the cage after 6000 N to 5,000,000 cycles. The compressive load threshold value of human Lumbar static FSU (Functional Spinal Unit) are known to be 5000 to 8500 N and the cancellous bone strength is known to be 2000 N [19; 20; 21; 22]. Therefore, the results obtained at 6000 N were similar to those of human static FSU,  
Figure 10: Average result of performance test (left yield load and right stiffness).  
and the composite fusion cage was considered structurally stable, since it sustained 5,000,000 cycles under the load, which is about 3 times higher than the cancellous strength.  
In all static compression, compression shear and torsion test results, the commercial titanium cage showed the highest yield load and stiffness results and the commercial PEEK cage showed the lowest results. However, in the subsidence test, the commercial PEEK cage showed the highest results. These results are thought to be because of the difference of material property elasticity of titanium and PEEK, rather than the difference of design.  
As a result, because the porous composite cage showed better or similar results compared with both commercial cages, that were approved by the Korean Food and Drug Administration and FDA 510 K and which have been used for many years without any problems, it is concluded that the porous composite cage has sufficient performance results for clinical use [23,24].