Effect of side holes in cervical fusion cages: a finite element analysis study

The objective of this study was to investigate the effect of side holes on the predicted von Mises stress levels in cervical spinal fusion cages subjected to compressive loading. Models with between zero and ten side holes were developed. Finite element analysis (FEA) was used to simulate compression of the cage, made from the polymer PEEK (polyetheretherketone), between two adjacent vertebrae. The analyses were validated by experimental tests. In all of the models, the von Mises stress was highest at the cage-vertebrae interface with peak stresses of between 14 and 18 MPa. Increasing the Young's modulus of the vertebrae from 12 to 30 GPa increased the peak stress on average by 29 per cent. The stresses in the models were lower than the compressive strength of PEEK (118 MPa), and are well within the PEEK fatigue strength reported (60 MPa at 10 million cycles). This study suggests that the number of side holes had a negligible effect on the stress distribution within the cage; the stress magnitudes were fairly constant across all of the models and did not change substantially with the number of holes. Hence, a cervical cage with side holes is unlikely to fail in compression.