Estudio de la biomecánica del procedimiento de vertebroplastia

Resumen

Vertebroplasty (VP) is a surgical procedure used to treat osteoporosis induced vertebral compresion fractures. This procedure involves the injection of bone cement into the porous vertebral body through a cannula. The present thesis has been focused on: a) how to improve the injection and infiltration procedure by numerical simulations and in vitro assays in order to reduce the extravertebral pressure of the injection; and b) to study the influence of trabecular bone microstructure in the intervertebral pressure and characterize the biomechanics of bone tissue infiltrates. In this sense, injectability studies have been performed both analytically and numerically. The simulations and the experimental tests demonstrate that conic cannula requires lower injection pressure than standard cannulas, with constant section. Furthermore, the velocity profiles obtained are optimum to reduce filter-pressing problems of ceramic cements. The results are important because the extravertebral pressure represents more than 95% of the total pressure required in the VP procedure. The study concludes with the realization of four biomedical patented devices that includes a new conic cannula for vertebroplasty and three new mixing/dosing-injection devices. On the other hand, in this thesis, synthetic foams and real trabecular vertebral bone have been characterized and reconstructed in 3D. The histomorphological parameters obtained show that the synthetic foams have different macroporosity as compared to the real bone; in fact, foams are similar to osteoporotic bone. In this sense, 2D and 3D models, obtained from micro tomography images, have been used to characterize the infiltration process of cement-like materials. The evaluation by computational fluid dynamics of the permeability and other fluidic parameters showed that the histomorphometric parameters were highly correlated with the fluidic parameters. This correlation explained the structural differences between the synthetic foams and the real vertebral bone models. A first conclusion is that synthetic foams have similar behavior than osteoporotic vertebrae but are unable to reproduce the anisotropic properties of trabecular bone. In this thesis the method of Voronoi has been used to design novel 3D porous scaffolds to study also the infiltration problems along the new developed structures. The methodology allows the reproduction of the anisotropy of the real trabecular tissue. In this sense, the porosity and the level of trabecular interconnection, as well as the width and trabecular separation, have been studied in relation to both the mechanical and the fluid behavior of the new models. The numerical results confirm the suitability of the new Voronoi scaffolds for both mass transport and mechanical support. In fact, the characteristic high surface area of the new scaffolds should facilitate the adhesion and cell growth; these, to be confirm in future investigations. On the meantime, the results show that the new scaffolds can be used to replace bone defects if fabricated with optimum biomaterials by rapid prototyping techniques. The thesis also presents a study where models with identical porosity have been obtained by two different routes: a) by modifying the trabecular thickness; and b) by eliminating trabeculae units. The results clearly show that the elastic modulus of the porous scaffolds decrease more severely when the "mass bone" loss was related to the loss of trabeculae units as compared to a uniform thinning of the trabeculae units. The thesis highlights the importance of the form adopted by the cement infiltrated inside the porous vertebra in relation to its own mechanical stability. In this sense, the numerical simulations showed that cement vertical infiltration recovers the strength of the vertebra better than cement horizontal infiltration. For this reason, it has been put forward that the use of cannulas with lateral opening and closed distal end is the best option.