Research Project C03

Percutaneous vertebroplasty is a medical procedure for treating weakened or fractured vertebrae. In this procedure, bone cement (a form of the polymer polymethyl methacrylate) is injected inside the cancellous part of the vertebra, also called spongy bone due to its porous nature. The bone cement restores the mechanical strength of the vertebra as it cures, thus stabilising the vertebral column. However, complications can arise if the bone cement leaks outside the vertebra during injection. We want to prevent such cases by simulating vertebroplasty using computational models. Such a simulation can be used as a tool by practitioners to decide the operating parameters for each specific patient to achieve optimum filling of bone cement, without resulting into leakages.

In this project, we aim to develop a thermodynamically-consistent multiphase continuum-mechanical model for vertebroplasty. We use the Theory of Porous Media (TPM) as the framework for modelling at the macro-scale. Additionally, specific constitutive models are used for the mechanical behaviour of the materials involved, which are namely the trabecular bone, the bone marrow, and the bone cement. The major challenges are to model the non-Newtonian behaviour of the bone marrow and the bone cement, to model the phase change of the bone cement, and to determine the permeability of the trabecular bone structure. We found that the pressure needed for injection inside the vertebra dips after an initial peak due to the non-Newtonian rheology of the bone cement. Additionally, the rheological models are used to simulate flow through the syringe, using which we found that the pressure loss in the syringe is not negligible.

In the future, we want to include more practical aspects into the model. To do this, we want to include the ability to model fractured vertebrae, since that is where the procedure is most commonly used for treatment. Additionally, we want to include thermal effects in the model to investigate whether the heat generated from bone cement curing poses a risk of tissue necrosis. Another important aspect is to account for the patient-to-patient variations in the material parameters like permeability, for which we will carry out using Uncertainty Quantification. We will also investigate the representability of artificial materials like aluminium foam as a substitute for actual vertebrae for experimental studies.

The group of Oliver Röhrle and the group of Boyko Gueorguiev-Rüegg at the AO Research Institute (ARI) at Davos have had a long-lasting collaboration. In particular in the field of vertebroplasty, this collaborative research has lead to joint publications. Boyko Gueorguiev-Rüegg and his group from the ARI provide assistance in interpreting and validating the proposed computational models, in particular with respect to clinical relevance and significantly contribute with their expertise in publishing experimentally related research. This includes rheological characterisation of medical-standard bone cements and injection experiments to capture the filling behaviour of bone cement using dynamic-CT imaging. The experimental work conducted at the ARI provides the basis for some of the simulations related to the validation of the pore and REV scale models of Research Project C03 as well as laying out future research potential of investigating the injection process into fractured vertebrae.