@misc{GeithSwidergalSchratzenstalleretal., author = {Geith, Markus A. and Swidergal, Krzysztof and Schratzenstaller, Thomas and Holzapfel, Gerhard A. and Wagner, Marcus}, title = {Numerical analysis of stent delivery systems during pre- and intraoperative processes}, series = {15. Deutsches LS-DYNA Forum, 15.-17.10.2018, Bamberg}, journal = {15. Deutsches LS-DYNA Forum, 15.-17.10.2018, Bamberg}, language = {en} } @inproceedings{FranzDreherPrinzenetal., author = {Franz, Daniela and Dreher, Maria and Prinzen, Martin and Teßmann, Matthias and Palm, Christoph and Katzky, Uwe and Perret, Jerome and Hofer, Mathias and Wittenberg, Thomas}, title = {CT-basiertes virtuelles Fr{\"a}sen am Felsenbein}, series = {Bildverarbeitung f{\"u}r die Medizin 2018; Algorithmen - Systeme - Anwendungen. Proceedings des Workshops vom 11. bis 13. M{\"a}rz 2018 in Erlangen}, booktitle = {Bildverarbeitung f{\"u}r die Medizin 2018; Algorithmen - Systeme - Anwendungen. Proceedings des Workshops vom 11. bis 13. M{\"a}rz 2018 in Erlangen}, publisher = {Springer}, address = {Berlin}, isbn = {978-3-662-56537-7}, doi = {10.1007/978-3-662-56537-7_51}, pages = {176 -- 181}, abstract = {Im Rahmen der Entwicklung eines haptisch-visuellen Trainingssystems f{\"u}r das Fr{\"a}sen am Felsenbein werden ein Haptikarm und ein autostereoskopischer 3D-Monitor genutzt, um Chirurgen die virtuelle Manipulation von kn{\"o}chernen Strukturen im Kontext eines sog. Serious Game zu erm{\"o}glichen. Unter anderem sollen Assistenz{\"a}rzte im Rahmen ihrer Ausbildung das Fr{\"a}sen am Felsenbein f{\"u}r das chirurgische Einsetzen eines Cochlea-Implantats {\"u}ben k{\"o}nnen. Die Visualisierung des virtuellen Fr{\"a}sens muss daf{\"u}r in Echtzeit und m{\"o}glichst realistisch modelliert, implementiert und evaluiert werden. Wir verwenden verschiedene Raycasting Methoden mit linearer und Nearest Neighbor Interpolation und vergleichen die visuelle Qualit{\"a}t und die Bildwiederholfrequenzen der Methoden. Alle verglichenen Verfahren sind sind echtzeitf{\"a}hig, unterscheiden sich aber in ihrer visuellen Qualit{\"a}t.}, subject = {Felsenbein}, language = {de} } @inproceedings{EixelbergerWittenbergPerretetal., author = {Eixelberger, Thomas and Wittenberg, Thomas and Perret, Jerome and Katzky, Uwe and Simon, Martina and Schmitt-R{\"u}th, Stephanie and Hofer, Mathias and Sorge, M. and Jacob, R. and Engel, Felix B. and Gostian, A. and Palm, Christoph and Franz, Daniela}, title = {A haptic model for virtual petrosal bone milling}, series = {17. Jahrestagung der Deutschen Gesellschaft f{\"u}r Computer- und Roboterassistierte Chirurgie (CURAC2018), Tagungsband, 2018, Leipzig, 13.-15. September}, volume = {17}, booktitle = {17. Jahrestagung der Deutschen Gesellschaft f{\"u}r Computer- und Roboterassistierte Chirurgie (CURAC2018), Tagungsband, 2018, Leipzig, 13.-15. September}, pages = {214 -- 219}, abstract = {Virtual training of bone milling requires realtime and realistic haptics of the interaction between the "virtual mill" and a "virtual bone". We propose an exponential abrasion model between a virtual one and the mill bit and combine it with a coarse representation of the virtual bone and the mill shaft for collision detection using the Bullet Physics Engine. We compare our exponential abrasion model to a widely used linear abrasion model and evaluate it quantitatively and qualitatively. The evaluation results show, that we can provide virtual milling in real-time, with an abrasion behavior similar to that proposed in the literature and with a realistic feeling of five different surgeons.}, subject = {Osteosynthese}, language = {en} } @inproceedings{BirkenmaierSteigerPhilippetal., author = {Birkenmaier, Clemens and Steiger, Tamara and Philipp, Alois and Lehle, Karla and Krenkel, Lars}, title = {Flow-induced accumulations of von Willebrand factor inside oxygenators during extracorporeal life support therapy}, series = {Proceedings of 12th International Conference BIOMDLORE 2018, June 28-30, 2018, Białystok, Poland}, booktitle = {Proceedings of 12th International Conference BIOMDLORE 2018, June 28-30, 2018, Białystok, Poland}, publisher = {IEEE}, address = {Piscataway, NJ}, isbn = {978-1-5386-2396-1}, doi = {10.1109/BIOMDLORE.2018.8467205}, pages = {3}, abstract = {BACKGROUND: Shear-induced conformational changes of von Willebrand factor (vWF) may be responsible for coagulation disorder and clot formation inside membrane oxygenators (MOs) during extracorporeal membrane oxygenation (ECMO) therapy. OBJECTIVE: The aim was to identify vWF structures inside clinically used MOs and employ computational fluid dynamics to verify the corresponding flow conditions. METHODS: Samples from gas exchange membranes (GEM) from MOs were analysed for accumulations of vWF and P-selectin-positive platelets using immunofluorescence techniques. Streamlines and shear rates of the flow around GEMs were computed using a laminar steady Reynolds-Averaged-Navier-Stokes approach. RESULTS: Most samples were colonized with equally distributed leukocytes, integrated in thin cobweb-like vWF-structures. Only 25 \% of the samples showed extended accumulations of vWF. Computed streamlines showed considerable cross flow between interconnected neighbouring channels. Stagnation points were non-symmetric and contact faces were washed around closely. The occurring maximum shear rates ranged from 2,500 to 3,000 1/s. CONCLUSIONS: If pronounced vWF structures are present, shape and extent match the flow computations well. Computed shear rates bear a critical degree of uncertainty due to the improper viscosity model. If flow conditions inside the MO were sufficient to affect vWF, a more consistent distribution of vWF across the samples should be present.}, language = {en} } @misc{BirkenmaierKrenkelLehle, author = {Birkenmaier, Clemens and Krenkel, Lars and Lehle, Karla}, title = {Linking flow conditions in membrane oxygenators to arrangements of multimeric von-Willebrand-factor as indication for coagulation}, series = {World Congress of Biomechanics 2018, Convention Centre Dublin, 8.-12. Juli 2018}, journal = {World Congress of Biomechanics 2018, Convention Centre Dublin, 8.-12. Juli 2018}, abstract = {Introduction Shear induced multimerisation of von-Willebrand-factor (vWF) is supposed to play an important role in coagulation inside extracorporeal membrane oxygenators. However, there is no proof that links observed vWF structures to computed or measured flow conditions. Methods The structures of multimeric vWF fibers, observed in clinically used membrane oxygenators is examined using immunofluorescence microscopy (IFM) using Carstairs' staining method (positive ethics committee vote). The flow around the membrane fibres inside the oxygenator is investigated in terms of shear rate, wall shear velocity and streamlines by using CFD (RANS, Carreau-Yasuda viscosity, geometry remodelled after high-resolution µCT-scans). By interpreting the histological and numerical results in this common context, indications for shear induced coagulation mechanisms can be identified. Results The fibre structures of multimeric vWF build regular but not exactly symmetric formations around the contact face (CF) between the crosswise stacked oxygenator fibres (OF), see fig.1B, vWF marked red. Annular around the CF arranged, cells are likely to be found, see fig.1B, nuclei marked blue. The computed streamlines around the OF show attached flow around the circular fibres. However, the irregular arrangement of real OF produce considerable cross flow between the interconnected neighbouring channels, in contrast to previous 2D-simulations. Thus, the CF are washed around closely by blood, also from neighbouring channels. The wall shear velocity streamlines form regular, slightly asymmetric shapes around the contact faces. The occurring maximum shear rates are in the range of 1,000 1/s. Discussion The shapes of vWF structures found in clinically used oxygenators match the computational results in terms of wall shear velocity and streamlines well. The accumulation of cells close to the CF can also be explained by fluid mechanics, as there are small shear gradients and slow velocities. However, occurring shear rates between OFs are too low to trigger multimerisation of vWF. That raises the question where in the circuit the actual activation of vWF is started and how, at least partly chained, vWF multimeres are attracted towards the OF surface. A next step will be the investigation of the actual shear rate triggered (or mediated) multimerisation of vWF. Towards this end, microfluidic experiments with shear triggered coagulation will be performed. Also of big interest is the computation of the flow situation in the oxygenator in proximity to chaining threads, which have been ignored in computations so far. However, first a realistic representation of the effective viscosity in computations is needed, which is not available yet.}, language = {en} } @misc{BirkenmaierKrenkel, author = {Birkenmaier, Clemens and Krenkel, Lars}, title = {Towards a realistic model of blood viscosity and coagulation in membrane oxygenators}, series = {6th European Conference on Computational Mechanics (Solids, Structures and Coupled Problems) - ECCM 6; 7th European Conference on Computational Fluid Dynamics - ECFD 7 : Glasgow, Scotland, UK, June 11-15, 2018}, journal = {6th European Conference on Computational Mechanics (Solids, Structures and Coupled Problems) - ECCM 6; 7th European Conference on Computational Fluid Dynamics - ECFD 7 : Glasgow, Scotland, UK, June 11-15, 2018}, abstract = {Modelling blood flow an shear induced coagulation in membraene oxygenators (MO) is challenging. The relevant geometry of oxygenator fibers (OF) and chaining threads is complex and spans several length scales. In relevant scales and regimes blood shows several significant non-Newtonian effects. Existing models are only capable of accounting for some, but not all relevant effects. Additionally, coagulation processes are influencing fluid properties and geometry significantly. Due to the enormous size of the discretised geometries highly detailed viscosity and coagulation properties of blodd flow in MOs. First step is to find a gemoetry dependent viscosity representation on basis of parametric micro channel experiments with anti-coagulated blood. Next step is a statistic coagulation model, based on micro channel experiments with human (re-calcified citrated) whole blood an evaluation of clinically used osygenators. Since shear rate dependent (i.e. viscosity dependet) coagulation in return influences the viscosity, a combined model with suitable implementation in a RANS framework is necessary. Towards this end, micro channel experiments with new and used single OFs triggering coagulation are performed. Structures of multimeric von Willebrand fibers (vWF), as indicator for shear induced coagulation, are compared to computed and measured flow conditions, using immunofluorescence microscopy, RANS-computations and µPIV, respectively. Preliminary examinations in clinically used MOs show good agreement between occurring structures of vWF, cell depositions and computed flow patterns (geometry form µCT-Scans). However, computed shear rates might be to low to actually trigger activation of vWF. The complex geometry of MOs results in huge meshes, which makes RANS with statistical modelling of viscosity and coagulation a reasonable approach. Towards this end, experimental data on micro channel level with evaluation on real application level is crucial. Especially regarding clotting processes, micro fluidic experiments are powerful research tool.}, language = {en} } @article{BenditzAuerSpoerreretal., author = {Benditz, Achim and Auer, Simon and Sp{\"o}rrer, J.F. and Wolkerstorfer, S. and Grifka, Joachim and S{\"u}ß, Franz and Dendorfer, Sebastian}, title = {Regarding loads after spinal fusion, every level should be seen separately: a musculoskeletal analysis}, series = {European Spine Journal}, volume = {27}, journal = {European Spine Journal}, number = {8}, publisher = {Springer-Verlag}, doi = {10.1007/s00586-018-5476-5}, pages = {1905 -- 1910}, abstract = {The number of spinal fusion surgeries is steadily increasing and biomechanical consequences are still in debate. The aim of this study is to provide biomechanical insights into the sagittal balance of the spine and to compare spinal load before and after spinal fusion. METHOD: The joint reaction forces of 52 patients were analyzed in proximo-distal and antero-posterior direction from the levels T12-L1 to L5-S1 using musculoskeletal simulations. RESULTS: In 104 simulations, pre-surgical forces were equal to post-surgical. The levels L4-L5 and T12-L1, however, showed increased spinal forces compression forces with higher sagittal displacement. Improved restauration of sagittal balance was accompanied by lower spinal load. AP shear stress, interestingly decreased with sagittal imbalance. CONCLUSION: Imbalanced spines have a risk of increased compression forces at Th12-L1. L4-L5 always has increased spinal loads. These slides can be retrieved under Electronic Supplementary Material.}, subject = {Biomechanische Analyse}, language = {en} } @inproceedings{AurbachJungtaeublSpickaetal., author = {Aurbach, Maximilian and Jungt{\"a}ubl, Dominik and Spicka, Jan and Dendorfer, Sebastian}, title = {EMG-based validation of musculoskeletal models considering crosstalk}, series = {World Congress Biomechanics, 28-30 June 2018, Dublin}, booktitle = {World Congress Biomechanics, 28-30 June 2018, Dublin}, doi = {10.1109/BIOMDLORE.2018.8467211}, abstract = {BACKGROUND: Validation and verification of multibody musculoskeletal models sEMG is a difficult process because of the reliability of sEMG data and the complex relationship of muscle force and sEMG. OBJECTIVE: This work aims at comparing experimentally recorded and simulated muscle activities considering a numerical model for crosstalk. METHODS: For providing an experimentally derived reference data set, subjects were performing elevations of the arm, where the activities of the contemplated muscle groups were measured by sEMG sensors. Computed muscle activities were further processed and transformed into an artificial electromyographical signal, which includes a numerical crosstalk model. In order to determine whether the crosstalk model provides a better agreement with the measured muscle activities, the Pearson correlation coefficient has been computed as a qualitative way of assessing the curve progression of the data sets. RESULTS: The results show an improvement in the correlation coefficient between the experimental data and the simulated muscle activities when taking crosstalk into account. CONCLUSIONS: Although the correlation coefficient increased when the crosstalk model was utilized, it is questionable if the discretization of both, the crosstalk and the musculoskeletal model, is accurate enough.}, language = {en} }