Study of dens fracture in the elderly and the influence of osteoporosis with a finite element model

Sammanfattning: Cervical spine injuries are a serious threat, as they may damage the central nervous system. In the elderly, cervical fractures due to falls are very frequent. The overall weakening of the bony and ligamentous spine decreases the resistance to fractures. Fractures of the dens of the second cervical vertebra (C2) are the most frequent individual fractures in the upper spine. Osteoporosis and impaired conditions play the main role in increasing the fracture risk. Several mechanisms may induce dens fractures: hyper-extension, lateral bending, shear, torsion, but the mechanisms of fractures have not been fully understood. Osteoporosis reduces overall bone strength. Cortical bone thinning occurs in the vertebræ, and there is a general loss of bone mass. Trabecular micro-architecture of bones loses integration, leading an increasing porosity. Mechanical properties worsen, and failure occurs more easily. In this study the role of osteoporosis on the genesis of dens fractures was investigated. An existing finite element (FE) model of the human spine was employed to simulate the effect of parameters associated with osteoporosis on the loading conditions of the dens. A baseline case was first simulated. Then, cortical thickness, cortical and trabecular bulk modulus and shear modulus were decreased in steps. Three impact scenarios were simulated: a lateral fall, a backward fall, and a forward fall. Effects of osteoporotic variations on the ligamentous spine deformations were studied. A mesh convergence analysis was performed to assess the influence of mesh size on the stresses in vertebra C2. The effect of reduced cortical and trabecular bulk moduli alone on stress distribution was not that apparent. In the dens, a reduced cortical thickness, in lateral and backward fall, caused higher maximum stresses than in the baseline. Conversely, in forward fall, reduced cortical thickness caused lower stresses than in the baseline. The effect of reducing trabecular bulk and shear moduli altogether was to decrease the stresses in dens trabecular bone. In lateral and backward fall, by reducing cortical bulk and shear moduli altogether, stresses in dens cortex decreased; whereas, in forward fall, stress decreased in dens neck cortex, and increased in dens apex and waist. It is concluded that cortical thinning, and reduced bulk and shear moduli of bone compartments considerably alter the stress distribution in C2, as well as the ligamentous spine response. The extent of such variations depends also on the impact scenarios. Finally, stresses in the model were found to be sensitive to the mesh size currently used in the human spine FE model.