Osteoporotic spinal fractures are a major concern in ageing Western societies. This study develops a multi-scale finite element (FE) model of the osteoporotic lumbar vertebral body to study the mechanics of vertebral compression fracture at both the apparent (whole vertebral body) and micro-structural (internal trabecular bone core) levels. Model predictions were verified against experimental data, and found to provide a reasonably good representation of the mechanics of the osteoporotic vertebral body. This novel modelling methodology will allow detailed investigation of how trabecular bone loss in osteoporosis affects vertebral stiffness and strength in the lumbar spine.

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 Young's modulus of 13GPa and Poisson's 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

Abstract-While the effects of reduced bone density on osteoporotic vertebral strength are well known, the relative roles of cortical shell and trabecular architecture thinning in determining vertebral stiffness and strength are less clear. These are important parameters in investigating the changing biomechanics of the ageing spine, and in assessing the effect of stiffening procedures such as vertebroplasty on neighbouring spinal segments. This work presents the development of a microstructural computer model of the osteoporotic lumbar vertebral body, allowing detailed prediction of the effects of bone micro-architecture on vertebral stiffness and strength.Microstructural finite element models of an L3 human vertebral body were created. The cortex geometry was represented with shell elements and the trabecular network with a lattice of beam elements. Trabecular architecture was varied according to age. Each beam network model was validated against experimental data. Models were generated to represent vertebral bodies of age <50 years, age 50-75y and age >75y respectively. For all models, an initial cortical shell thickness of 0.5mm was used, followed by reductions in the age >75y models to 0.35mm and 0.2mm to represent cortical thinning in late stage osteoporosis. Loads were applied to simulate in vitro biomechanical testing, compressing the vertebra by 20% of its height.Predicted vertebral stiffness and strength reduced with progressive age changes in microarchitecture, demonstrating a 44% reduction in stiffness and a 43% reduction in strength, between the age <50 and age >75 models. Reducing cortical thickness in the age >75 models demonstrated a substantial reduction in stiffness and strength, resulting in a 48% reduction in stiffness and a 62% reduction in strength between the 0.5mm and 0.2mm cortical thickness models. Cortical thinning in late stage osteoporosis may therefore play an even greater role in reducing vertebral stiffness and strength than earlier reductions due to trabecular thinning.