Recently, novel polymers including polyetheretherketone (PEEK) and carbon fiber reinforced polymer (CFRP) have been used for spinal implants. Because the in vitro experimental test uses metal blocks with different material properties from those of polymer cages in standard test protocols for prediction of the mechanical performance, it is necessary to analyze the influence of various experimental conditions, such as the material of the blocks. In this study, the compressive stiffness of spinal cages was estimated for different materials (PEEK, CFRP, and titanium) and shapes (open and closed) under simulations of the mechanical experimental tests and the in vivo situation using finite element analysis. The stiffness percentages of the open cage, when the stiffness in Case 1 was assumed 100%, were changed from 37%, 57% and 67% to 301%, 440% and 499% in the PEEK, CFRP and titanium materials, respectively by loading conditions. The stiffness was affected by shapes of cage as well as experimental conditions such as the load application method or fixation block. Hence, it may be necessary to consider the experimental conditions during in vitro mechanical tests for the stiffness estimation of spinal cages made of novel polymers to obtain results relevant for an in vivo situation.

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