Michael Roland

An automated workflow for the biomechanical simulation of a tibia with implant using computed tomography and the finite element method

In this study, a fully automated workflow is presented for the biomechanical simulation of bone-implant systems using the example of a fractured tibia. The workflow is based on routinely acquired tomographic data and consists of an automatic segmentation and material assignment, followed by a mesh generation step and, finally, a mechanical simulation using the finite element method (FEM). Because of the high computational costs of the FEM simulations, an adaptive mesh refinement scheme was developed that limits the highest resolution to materials that can take large amounts of mechanical stress. The scheme was analyzed and it was shown that it has no relevant impact on the simulation precision. Thus, a fully automatic, reliable and computationally feasible method to simulate mechanical properties of bone-implant systems was presented, which can be used for numerous applications, ranging from the design of patient-specific implants to surgery preparation and post-surgery implant verification.

An optimization algorithm for individualized biomechanical analysis and simulation of tibia fractures.

An algorithmic strategy to determine the minimal fusion area of a tibia pseudarthrosis to achieve mechanical stability is presented. For this purpose, a workflow capable for implementation into clinical routine workup of tibia pseudarthrosis was developed using visual computing algorithms for image segmentation, that is a coarsening protocol to reduce computational effort resulting in an individualized volume-mesh based on computed tomography data. An algorithm detecting the minimal amount of fracture union necessary to allow physiological loading without subjecting the implant to stresses and strains that might result in implant failure is developed. The feasibility of the algorithm in terms of computational effort is demonstrated. Numerical finite element simulations show that the minimal fusion area of a tibia pseudarthrosis can be less than 90% of the full circumferential area given a defined maximal von Mises stress in the implant of 80% of the total stress arising in a complete pseudarthrosis of the tibia.

Less than full circumferential fusion of a tibial nonunion is sufficient to achieve mechanically valid fusion - Proof of concept using a finite element modeling approach

BackgroundAlthough minimally invasive approaches are widely used in many areas of orthopedic surgery nonunion therapy remains a domain of open surgery. Some attempts have been made to introduce minimally invasive procedures into nonunion therapy. However, these proof of concept studies showed fusion rates comparable to open approaches never gaining wider acceptance in the clinical community. We hypothesize that knowledge of mechanically relevant regions of a nonunion might reduce the complexity of percutaneous procedures, especially in complex fracture patterns, and further reduce the amount of cancellous bone that needs to be transplanted. The aim of this investigation is to provide a proof of concept concerning the hypothesis that mechanically stable fusion of a nonunion can be achieved with less than full circumferential fusion.MethodsCT data of an artificial tibia with a complex fracture pattern and anatomical LCP are converted into a finite element mesh. The nonunion area is segmented. The finite element mesh is assigned mechanical properties according to data from the literature. An optimization algorithm is developed that reduces the number of voxels in the non union area until the scaled von Mises stress in the implant reaches 20% of the maximum stress in the implant/bone system that occurs with no fusion in the nonunion area at all.ResultsAfter six iterations of the optimization algorithm the number of voxels in the nonunion area is reduced by 96.4%, i.e. only 3.6% of voxels in the non union area are relevant for load transfer such that the von Mises stress in the implant/bone system does not exceed 20% of the maximal scaled von Mises stress occurring in the system with no fusion in the non union area at all.ConclusionsThe hypothesis that less than full circumferential fusion is necessary for mechanical stability of a nonunion is confirmed. As the model provides only qualitative information the observed reduction of fusion area may not be taken literally but needs to be calibrated in future experiments. However this proof of concept provides the mechanical foundation for further development of minimally invasive approaches to delayed union and nonunion therapy.

Simulations of 3D/4D Precipitation Processes in a Turbulent Flow Field

Precipitation processes are modeled by population balance systems. An expensive part of their simulation is the solution of the equation for the particle size distribution (PSD) since this equation is defined in a higher-dimensional domain than the other equations in the system. This paper studies different approaches for the solution of this equation: two finite difference upwind schemes and a linear finite element flux-corrected transport method. It is shown that the different schemes lead to qualitatively different solutions for an output of interest.

Personalized Orthopedic Trauma Surgery by Applied Clinical Mechanics

In this study, the concept of applied clinical mechanics is used to present first steps in the direction of personalized orthopedic trauma surgery. As example process, a complex distal tibia fracture treated with an implant is chosen. Based on an automated workflow, routinely acquired tomographic data is segmented, assigned with material parameters and extended to an adaptive volume-mesh with hanging nodes. For the finite element simulations, this bone-implant system is equipped with realistic axial loading conditions. An optimization algorithm is then used to analyze the amount of fracture healing that will provide a full weight bearing capacity of the injured extremity in combination with the implant.