Modelling the longer term effects of spinal burst fracture repair using vertebroplasty

Burst fractures account for 15% of all spinal fractures and generally occur in the younger, more active generations. Traditional treatments of burst fractures can be highly invasive and may involve spinal fixation. Favourable short term results have been obtained when vertebroplasty has been used to repair osteoporotic compression fractures. However, there have been limited studies into the use of vertebroplasty for burst fractures. The aim of this study was to develop in vitro and computational models that could be used to investigate the longer term effects of spinal burst fracture repair using vertebroplasty. An experimental technique was established to create fractured porcine vertebrae, of a known severity grade, which were subjected to multiple-cycle loading in order to determine the post-fracture behaviour over time and to compare between augmentation materials. Finite element (FE) models were created using micro-computed tomography (μCT) images of the fractured porcine vertebrae and compared to the experimental results. Methods to represent the plastic deformation of the vertebrae were investigated. The models were used to investigate the effect of fracture dispersion and the level of cement augmentation on post-fracture behaviour. The multiple-cycle loading regime captured the post-fracture behaviour for the majority of the specimens with some propagation of damage but without the complete failure of the specimens. The FE models were best able to predict post-fracture behaviour when there was a lower level of fracture and cement dispersion. The method used to simulate the plastic deformation of the vertebrae captured the displacement of the specimens but not their change in stiffness. The computational results showed that there was little difference between the ability of the polymethylmethacrylate and calcium phosphate cements to restore vertebral stiffness. The results of the study indicate that with current cements, fractures with a severity grade greater than 10.5 should not be augmented without some other form of fixation such as posterior instrumentation. Further work is necessary to develop computational material models that provide better predictions of the fractured bone over multiple cycles.