Relative Roles of Cortical and Trabecular Thinning in Reducing Osteoporotic Vertebral Body Stiffness: A Modeling Study

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.