Bone cement spreading during vertebroplasty

The human skeleton goes through a lifelong process called bone tissue remodelling, in which mature bone tissue is removed and replaced by newly formed tissue. If this process is no longer in equilibrium and bone tissue does not grow in sufficient quantities, it will lead to bone shrinkage and a decrease of bone density. This disease, which often occurs in old age, is called osteoporosis and can lead to massive restrictions in the stability of the entire skeleton and can be associated with a greatly increased risk of bone fractures. In the area of the spine, the weakening of the bone structure can lead to sintered fractures in the vertebral bodies and to serious complaints from those affected. Vertebroplasty is a therapeutic treatment to stabilize osteoporotic vertebral bodies and represents a minimally invasive operation, in which so-called bone cement is injected percutaneously into the vertebral body and stabilizes it again after the previously liquid bone cement has hardened. During the treatment, particular care must be taken to ensure that no injection material emerges from the vertebral body. Possible serious consequences caused by cement leakage are an embolism or a compressing of the spinal cord.

Eine Therapiemaßnahme zur Stabilisierung von osteoporotischen Wirbelkörpern ist die Vertebroplastie. Dies ist eine minimalinvasive Operation, bei welcher sogenannter Knochenzement in den Wirbelkörper perkutan injiziert wird und diesen nach dem Aushärten des zuvor flüssigen Knochenzements wieder festigt. Bei der Operation muss insbesondere darauf geachtet werden, dass kein Injektionsmaterial aus dem Wirbelkörper austritt. Mögliche schwerwiegende Folgen einer solchen Zementleckage sind etwa das Verursachen einer Embolie oder die Kompression des Rückenmarks.

The numerical simulation of vertebroplasty can help to avoid complications during the procedure and to make predictions about the spreading of the bone cement within the vertebral body. The main goal here is to optimize the process of treatment by testing factors such as the position of the injection needle, the choice of bone cement and the necessary injection pressure in preoperative simulations. On the one hand, this gives surgeons the opportunity to assess risks in advance and, on the other hand, to achieve the best possible therapy result. Another goal in simulating vertebroplasty is to gain a deeper understanding of the forces and pressures that arise within the vertebral body during bone cement injection. This enables predictions to be made as to whether the trabecular bone structure is deformed or destroyed by the resulting stresses caused by the injection.

The employed continuum mechanical model is based on the Theory of Porous Media (TPM) and relies on the formulation of constitutive relations for the solid skeleton of the bone, the injected bone cement and the bone marrow, which fills the bone at the beginning. This enables the direct consideration of important parameters such as the bone cement viscosity and the anisotropic permeability of the bone structure in the vertebra. The resulting system of equations is solved in a fully coupled manner and allows the simultaneous investigation of the injection process and the spreading of the bone cement with the associated displacement of the bone marrow and the resulting stresses and deformations within the bone structure.

Credits: University of Stuttgart / Christian Bleiler