Intervertebral discs, located between the vertebral bodies, are subjected to a constant and complex load by weight. This causes the intervertebral disc to break or dislodge. Since a damaged intervertebral disc can not be restored or aligned, surgical methods are needed to relieve pain. Surgery for this purpose is spondylodesis to fix the segment using interbody fusion cage. Interbody fusion cage are made of titanium-based metallic biomaterials or polymeric biomaterials such as PEEK. Titanium material has strength in synostosis and mechanical properties, but clinical phenomena such as subsidence have occurred due to high elastic modulus. On the other hand, PEEK material has a similar elastic modulus as cortical bone, but clinical problem has been raised that synostosis is limited and creep may occur.
In this study, the interbody fusion cage made of CP grade 4 titanium combined with porous and frame structure was designed to alleviate the clinical phenomena occurred by using each material. For this purpose, nine porous structures according to the unit cell type with porosity and four frame based on the load supporting method were devised. The stiffness and allowable load of each designed model were calculated, then the clinical safety and mechanical stability were evaluated.
Conclusions from this study are as follow.
1. We materialized interbody fusion cage with cortical bone-like stiffness and higher allowable load than PEEK material, which was performed by controlling the morphological parameters of the porous structure and the frame.
2. The stiffness similar to that of cortical bone can improve clinical safety by mitigating the stress shielding resulted in metal materials. Also, mechanical stability is expected to be maintained through higher allowable loads than PEEK material mainly used in the clinic.
3. Based on the results of this study, we determined that the material limitations, if titanium and PEEK used in interbody fusion cage, could be alleviated by combining the porous structure and the frame.
4. Depending on the shape of the frame, different efficiencies for allowable load versus stiffness were calculated. Through this, we confirmed the feasibility of the cage that can maintain the stability and improve the safety by accomplishing the shape optimization design for the load supporting method.