A New Method for Predicting the Porosity of an Interbody Fusion Cage by the Equivalent Material Method

Abstract

Purpose

The interbody fusion cage will cause stress shielding problems due to its material characteristics. This paper aims to find out the change in biomechanical characteristics of porous interbody fusion cages under different conditions and provide a theoretical basis for solving the stress shielding problem.

Methods

The properties of microscopic cells with different porosities are obtained by conducting virtual experiments to demonstrate the material strength of the macroscopic model. Based on the obtained equivalent material properties, the mechanical properties of the porous Ti6Al4V interbody fusion cage in the spine were investigated, and the stress reduction rate under different porosities was analyzed by changing the shape of the fusion cage.

Results

The elastic modulus of the porous fusion cage can be approximately expressed as “E ≈ E₀ (1 − 1.62P − 1.41P² + 4.22P³ − 2.22P⁴).” When P = 90%, the Von Mises stress is reduced by more than 70%, but it approaches the yield strength (85 MPa), and the compressive stress approaches 45 MPa. The two stress reduction rates on the fusion cage with 55% < P < 90% can be approximately expressed in the form of “A + Bx + Cx² + Dx³.”

Conclusion

The relationship between elastic modulus and porosity of equivalent materials is obtained, which provides a theoretical basis for predicting the porosity of fusion cages. Under the osteotomy scheme of this model, two expressions of “P–μ” are obtained, and the applicability of the formulas is verified, which lays a theoretical foundation for further research on the stress problem of the fusion cage.