## 1 Introduction  
The incidence of lumbar degenerative diseases such as spinal stenosis and degenerative spondylolisthesis is increasing due to the aging population, and when there is no response to conservative treatments, then posterior lumbar interbody fusion (PLIF) is widely used [1]. The PLIF with cage insertion can achieve a firm interbody fusion and achieve stable coronal and sagittal alignment during the fusion. It is also considered a good method because it has an indirect nerve decompression effect by increasing the disc space that is commonly accompanied by degenerative lumbar diseases, thereby widening the nerve root [2; 3].  
Commonly used materials for spinal fusion cages are titanium alloy and PEEK. The titanium alloy material has a high modulus of elasticity and is reported to be highly susceptible to subsidence of the spinal fusion cage, due to the difference in elastic modulus of the cortical bone, and has the characteristic of providing excellent bone fusion [4] In contrast, the recently introduced PEEK spine fusion cage has a similar elastic modulus to the cortical bone of the vertebrae, and is favorable for load and stress dispersion. Therefore, it is known to show lower frequency in subsidence than the titanium alloy cage. However, the fusion rate is low and the probability of non-fusion is higher than that of the titanium alloy spinal fusion cage [5]  
The main purpose of the spinal cage during spinal fusion is to fuse two verteba bodies and to keep the height between them. However, the elastic modules of the spinal cage and the verteba body cause subsidence, which means that the spinal cage enter the verteba body and reduce the height between verteba bodies. As a result, we might witness another disease such as pedicle stenosis and so on.  
In order to solve the problem of the metal material, a measure was attempted to reduce the elastic modulus by manufacturing it in a porous structure. [PERSON] et al. produced a titanium porous body via freeze forming (molding) method that has an elastic modulus of 4 GPa and a compressive strength of 150 MPa with a porosity of 60% or more [6]. However, there was a problem with subsidence difference and the radiopacity of the metal itself, along with the MRI (Magnetic Resonance Imaging) artifact.  
In order to solve the problem of the low synostosis rate that cannot directly combine with Peek-based bone, [PERSON] et al. used ion plasma solvents to coat titanium less than 100 nm, and reported a 30% increase in the integration of bone cells, as compared to general titanium material and uncoated PEEK material [7; 8]. [PERSON] conducted cellular and animal testing by coating titanium 1 \(\upmu\)m thick with E-beam deposition [9]. As a result, cell integration, multiplication, differentiation, etc., were improved in comparison to general PEEK, and bone integration in the femur of rabbit was also reported to be improved by more than 50% [10]. However, there are still problems and limitations with this coating procedure.  
Therefore, in this study, the body of the spinal fusion cage is made of PEEK material, which reduces the likelihood of subsidence of the spinal fusion cage, due to the difference of the elastic modulus with the spinal body cortical bone. For the vertebral end plate, which is related to the fusion rate, we aim to design a new composite vertebral cage by combining a cover of porous structure, using the additive manufacturing of titanium alloy material, to fabricate a prototype and then to biomechanically verify it.