Influence of Various Experimental Conditions in Standard Test Protocols on In Vivo Compressive Stiffness of Spinal Cages by Finite Element Analysis

INTRODUCTION Recently, novel polymers including polyetheretherketone (PEEK) and carbon fiber reinforced polymer (CFRP) have been used for spinal cages because of their advantages such as radiolucency, low incidence of artifacts on magnetic resonance imaging scans, less bone subsidence, and the capacity to tailor its mechanical properties, as well as its safety for patients with metal allergies. When a new cage is developed, it is necessary to evaluate its biomechanical performance by standard test protocols before approval of clinical usage. There are conventional mechanical test protocols for spinal cages, e.g., ASTM F2077. In ASTM F2077, a cage is inserted between metal or polyacetal blocks, and then compressive, shear, and torsional tests are performed by applying loads in the form of stainless steel blocks [1]. Because the in vitro experimental test given in ASTM F2077 uses metal blocks to apply loads which have much different material properties from those of bones and polymer cages, it may be difficult to accurately reflect the in vivo situation. Hence, it is necessary to investigate the influence of the various experimental conditions in standard test protocols on the prediction of in vivo mechanical performance of cages. In this study, the compressive stiffnesses of spinal cages were investigated for various structures (open and closed) and materials (PEEK, CFRP, and titanium) under the situations of mechanical experimental tests and in vivo simulations based on finite element analysis.