Biomechanical Analysis of Lumbar Interbody Fusion Cages With Various Elastic Moduli in Osteoporotic and Non-osteoporotic Lumbar Spine: A Finite Element Analysis

Abstract

Study design: Finite element analysis (FEA).

Objective: This study aimed to explore the effects of cage elastic modulus (Cage-E) on the endplate stress in different bone conditions: osteoporosis (OP) and non-osteoporosis (non-OP). We also explored the correlation between endplate thickness and endplate stress.

Method: The FEA models of L4-L5 with lumbar interbody fusion were designed to access the effects of Cage-E on the endplate stress in different bone conditions. Two groups of the Young's moduli of bony structure were assigned to simulate the conditions of OP and non-OP, and the bony endplates were analyzed in 2 kinds of thicknesses: .5 mm and 1.0 mm, with the insertion of cages with different Young's moduli including .5, 1.5, 3, 5, 10, and 20 GPa. After model validation, an axial compressive load of 400 N and a flexion/extension moment of 7.5Nm was performed on the superior surface of L4 vertebral body in order to analyze the distribution of stress.

Result: The maximum Von Mises stress in the endplates increased by up to 100% in the OP model compared with non-OP model under the same condition of cage-E and endplate thickness. In both OP and non-OP models, the maximum endplate stress decreased as the cage-E decreased, but the maximum stress in the lumbar posterior fixation increased as the cage-E decreased. Thinner endplate thickness was associated with increased endplate stress.

Conclusion: The endplate stress is higher in osteoporotic bone than non-osteoporotic bone, which explains part of the mechanism of OP-related cage subsidence. It is reasonable to reduce the endplate stress by reducing the cage-E, but we should balance the risk of fixation failure. Endplate thickness is also important when evaluating the cage subsidence risk.

Keywords: cage; elastic modulus; endplate; finite element analysis; osteoporosis.