The effect of trabecular micro-architecture on vertebra biomechanics: A finite element investigation

Abstract

700,000 vertebral fractures occur each year in the United States alone, 85 of which are associated with osteoporosis. This study presents the development of a microstructural model of an entire lumbar vertebral body to investigate the effects of osteoporotic changes in bone micro-architecture on vertebral biomechanics, specifically, the change in stiffness, stresses and load sharing capacity of the core and cortex. A finite element model of a trabecular microstructure was created using a lattice of 3D beam elements with age representative thickness and separation. The vertebral shell was created around the trabecular microarchitecture using 3D shell elements. Three trabecular microstructures were investigated: (i) age less than 50, (ii) age 50-75 and (iii) age over 75 years. A Youngs modulus of 13GPa and Poissons ratio of 0.3 were applied for parent bone material. Two loading cases were investigated; (i) a uniform pressure of 1MPa applied to the upper endplate, and (ii) an applied displacement of -1mm for the entire upper endplate in the vertical direction. Model results showed that microstructural variations seen with aging decreased predicted vertebral stiffness from 23kN/mm (age 50) to 0.7kN/mm (age 75), increased maximum principal endplate stress from 5.4MPa (age 50) to 73MPa (age 75, for 1MPa applied pressure), and reduced the proportion of total load carried by the trabecular core from 52 (age 50) to 6 (age 75). The model exhibits realistic behaviour compared with experimental studies, and will be used in the future to explore the mechanics of vertebral compression fractures and treatments such as vertebroplasty.