Influence of Heat Treatment on the Degradation Behaviour of Degradable Magnesium Based Implants

Aim of the study was to characterise the influence of heat treatment on the degradation behaviour and stability of degradable magnesium based implants. For this purpose two groups (untreated/ heat treated) of LAE442 pins were separately analysed in an in vitro and in vivo study. The corrosion behaviour was evaluated during 8 weeks degradation in SBF (in vitro) and 48 weeks degradation intramedullary in the rabbit tibia (in vivo). The analyses were made by using μ-computed tomography and three-point-bending tests. Heat treatment led to altered mechanical and corrosion properties of LAE442.While the initial stability declined significantly a reduction of the degradation rate over either in vitro and in vivo evaluation period is determined. If these alterations are still reasonable for osteosynthesis implants remains to be investigated in further projects.DFG/SFB/59

Finite element study of the accommodation behaviour of the crystalline lens after fs-laser treatment

With aging the ability of the crystalline lens to adapt to different viewing distances decreases. Until now there is no satisfying treatment available. It is possible to influence the deformability of the crystalline lens by inducing cuts using femtosecond (fs)-laser. To test the influence of several different cutting geometries is expensive and a huge amount of crystalline lenses is needed. Finit Element Method offers the possibility to test the influence of different cutting geometries on the flexibility. Therefore, it is necessary to develop a method to simulate the cuts in an adequate way. In first simulations cuts were assumed to be isotropic inter layers. Later on, effective material properties for the inter layer were calculated with a homogenization technique. The results confirm the influence of the cuts on the flexibility.DFG/BE3990-

Axial forces and bending moments in the loaded rabbit tibia in vivo

<p>Abstract</p> <p>Background</p> <p>Different animal models are used as fracture models in orthopaedic research prior to implant use in humans, although biomechanical forces can differ to a great extend between species due to variable anatomic conditions, particularly with regard to the gait. The rabbit is an often used fracture model, but biomechanical data are very rare. The objective of the present study was to measure axial forces, bending moments, and bending axis directly in the rabbit tibia <it>in vivo</it>. The following hypothesis was tested: Axial forces and bending moments in the mid-diaphysis of rabbit tibia differ from other experimental animals or indirectly calculated data.</p> <p>Methods</p> <p>A minifixateur system with 4 force sensors was developed and attached to rabbit tibia (<it>n </it>= 4), which were subsequently ostectomised. Axial forces, bending moments and bending angles were calculated telemetrically during weight bearing in motion between 6 and 42 days post operation.</p> <p>Results</p> <p>Highest single values were 201% body weight [% bw] for axial forces and 409% bw cm for bending moments. Whereas there was a continous decrease in axial forces over time after day 10 (<it>P </it>= 0.03 on day 15), a decrease in bending moments was inconsistent (<it>P </it>= 0.03 on day 27). High values for bending moments were frequently, but not consistently, associated with high values for axial forces.</p> <p>Conclusion</p> <p>Axial forces in rabbit tibia exceeded axial forces in sheep, and differed from indirectly calculated data. The rabbit is an appropriate fracture model because axial loads and bending moments in rabbit tibia were more closely to human conditions than in sheep tibia as an animal model.</p

Konstruktion eines innovativen 3D Modells des humanen Schallleitungsapparates basierend auf histologischen Schliffbildern

Zielsetzung: Die anatomisch genaue Darstellung der Gehörknöchelchenkette im 3D Modell ist für die Simulation mit dem Fokus der Optimierung von Mittelohrprothesen unerlässlich.Methodik: Das Felsenbein eines 56jährigen Mannes wurde in ungefärbtes Epoxidharz eingebettet und mit der Feinschliffmethode mit einem Abtrag von ca. 35 µm pro Schleifgang bearbeitet. Die polierte Oberfläche der Probe wurde mit der modifizierten Färbemethode nach Mann-Dominici gefärbt, mikroskopiert und durch 20.996 Fotos in verschiedenen Vergrößerungen dokumentiert. Durch die histologische Untersuchung konnte mit dem Programm Rhinoceros® 5 (64-bit; McNeel) ein dreidimensionales Modell der Gehörknöchelchenkette inklusive Ligamente und Muskeln erstellt werden.Zusätzlich wurde die Probe zum Vergleich vor dem Beginn des Schleifens einer µCT-Untersuchung unterzogen und ein 3D Model aller kalzifizierten Areale der Gehörknöchelchenkette erstellt.Ergebnisse: Aus den histologischen Daten können im 3D Modell der Gehörknöchelchen alle inneren Strukturen, wie ein Blutgefäßsystem und Knorpelanteile differenziert und detailliert dargestellt werden. Zusätzlich lassen sich Weichgewebe wie Ligamente und Muskeln, sowie das Trommelfell wiedergeben.Das aus den µCT-Daten gewonnene Modell der kalzifizierten Strukturen der Gehörknöchelchen erlaubt durch das geringere Auflösungsvermögen nicht die Darstellung des feinen Blutgefäßsystems.Schlussfolgerung: Für die Herstellung eines optimierten Simulationsmodells der menschlichen Gehörknöchelchenkette eignet sich besonders die Feinschliffmethode, da so alle Arten von Gewebe differenziert in einem computergestützten, dreidimensionalen Modell dargestellt werden können.Unterstützt durch: Unterstützt durch den Sonderforschungsbereich 599 "Zukunftsfähige bioresorbierbare und permanente Implantate aus metallischen und keramischen Werkstoffen" im Teilprojekt D1 "Funktionalisierte Mittelohrprothesen", gefördert durch die Deutsche Forschungsgemeinschaft (DFG).Der Erstautor gibt keinen Interessenkonflikt an

From the experimental determination of stress-strain full fields during a bulge test thanks to 3D-DIC technique to the characterization of anisotropic Mullins effect

International audienceThe bulge test is usually mostly used to analyze equibiaxial tensile stress state at the pole of inflated isotropic membranes. Three-dimensional digital image correlation (3D-DIC) technique allows the determination of three-dimensional surface displacements and strain fields. A method is proposed to calculate from these experimental data the membrane curvature tensor at each surface point of the bulge specimen. Curvature tensor fields are then used to investigate axisymmetry of the test; in the axisymmetric case, membrane stress tensor fields are determined from meridional and circumferential curvatures combined with the measurement of the inflating pressure. Stress strain state is then known at any surface point which enriches greatly experimental data deduced from bulge tests. This method is then used to treat an experimental bulge test on a filled silicone rubber membrane. The results highlight that a global membrane with a very heterogeneous strain history is obtained , from equibiaxial behavior at the center of the membrane until a planar (pure) shear state at the periphery of the bulge. Next, different small tensile specimens are cut from the pre-stretched silicone membrane. Identical cyclic tensile tests are realized on all these specimens. The curves are compared and highlight the difference of the stress-softening according to the place of the cut specimen and according to its orientation with respect to the circumferential or meridional direction

Cosserat and Cauchy materials as continuum models of brick masonry

Continuum modeling for masonry-like material accounting for bricks or blocks texture is discussed. The constitutive functions for the contact actions - expressed in terms of size, shape and arrangement of the block assembly - are derived within the framework of the linear elastic Cosserat and Cauchy theories. By varying some important geometrical parameters: the scale factor between the wall and the blocks size, the shape of the bricks and their arrangement, micropolar materials with particular internal constraints are obtained. In a few situations the constrained continuum behaves as a Cauchy continuum. In general, the Cauchy continuum does not provide a proper description of the brick masonry behaviour while the structured continuum model, accounting for the mutual blocks rotation, gives satisfactory results