Elmar K. Tschegg

Bone-implant interface strength and osseointegration: Biodegradable magnesium alloy versus standard titanium control.

Previous research on the feasibility of using biodegradable magnesium alloys for bone implant applications mainly focused on biocompatibility and corrosion resistance. However, successful clinical employment of endosseous implants is largely dependent on biological fixation and anchorage in host bone to withstand functional loading. In the present study, we therefore aimed to investigate whether bone-implant interface strength and osseointegration of a novel biodegradable magnesium alloy (Mg-Y-Nd-HRE, based on WE43) is comparable to that of a titanium control (Ti-6Al-7Nb) currently in clinical use. Biomechanical push-out testing, microfocus computed tomography and scanning electron microscopy were performed in 72 Sprague-Dawley rats 4, 12 and 24 weeks after implantation to address this question. Additionally, blood smears were obtained from each rat at sacrifice to detect potential systemic inflammatory reactions. Push-out testing revealed highly significantly greater maximum push-out force, ultimate shear strength and energy absorption to failure in magnesium alloy rods than in titanium controls after each implantation period. Microfocus computed tomography showed significantly higher bone-implant contact and bone volume per tissue volume in magnesium alloy implants as well. Direct bone-implant contact was verified by histological examination. In addition, no systemic inflammatory reactions were observed in any of the animals. We conclude that the tested biodegradable implant is superior to the titanium control with respect to both bone-implant interface strength and osseointegration. These results suggest that the investigated biodegradable magnesium alloy not only achieves enhanced bone response but also excellent interfacial strength and thus fulfils two critical requirements for bone implant applications.

New splitting method for wood fracture characterization

SummaryA testing procedure with a new and simple specimen shape is presented which is appropriate to characterize fracturing of inhomogeneous and complex materials like wood. With this, the fracture energy of spruce wood is determined in the TL and RL direction. The “size effect”, i.e. influences of specimen dimensions on KIC and Gf (specific fracture energy) are investigated. Stress and deformation distribution in the newly developed specimens are analysed with FE methods. The measured load-displacement curves are approximated by bilinear softening diagrams and FE analysis. Based on these results, it is tried to interpret typical deviations from LEFMs behaviour by mechanisms like microcracking, crack branching or crack tip bridging.

Radiation effects on insulators for superconducting fusion magnets

Abstract Fibre-reinforced plastics (FRPs) are candidate materials for the insulation of superconducting magnet coils in future fusion reactors. This paper reports on a test programme performed to assess the mechanical properties of these materials and to obtain information on the damage and fracture mechanisms. Different types of FRPs (epoxies and poly- and bismaleimides as resins; two- and three-dimensional E-, S- or T-glass fabrics as reinforcements) were irradiated at room temperature by 2 MeV electrons and 60 Co-gamma rays up to 1.8 × 10 8 Gy and by different reactor spectra up to a neutron fluence of 1 × 10 23 m −2 ( E > 0.1 MeV ) at room temperature, 80 K or 5 K. Mechanical tests in tension as well as in the intralaminar crack opening and shear mode were carried out on the irradiated samples at 77 K. After low temperature irradiation, half of the samples were subjected to a warm-up cycle to room temperature before testing at 77 K. Results on the influence of different radiation sources and annealing cycles on the mechanical properties of these materials will be discussed.

Influence of test geometry on tensile strength of fibre reinforced plastics at cryogenic temperatures

Abstract In view of future applications of fibre reinforced plastics as insulating materials for the windings of superconducting magnets in fusion reactors, the mechanical properties of these materials have to be tested under conditions which include the appropriate radiation environment expected at the magnet location. Since the established standards, e.g. for the measurement of the ultimate tensile strength, involve sample sizes which are far too large for testing under a radiation environment, scaling experiments have been made to investigate the influence of sample width and thickness on the ultimate tensile strength and the fracture strain at room temperature, 77 and 4.2 K. The data show that variation of sample width does not affect the results for ultimate tensile strength in a systematic way, and that a reduction of sample thickness leads to a slight reduction of the ultimate tensile strength (15−35% at room temperature, 5−15% at low temperatures). Fractographic investigations show no significant dependence of the fracture surfaces on the test geometry.

Adhesive power measurements of bonds between old and new concrete

A simple procedure for measuring the adhesive bond of cement-bonded materials is introduced and tested with an old-new concrete bond. Cubic or cylindrical specimens with a notch and the interface in their middle are split under stable crack growth conditions. The load is recorded as a function of the crack opening displacement. From this curve, the maximum load (notch tensile stress) and the fracture energy (GF) can be determined. The course of the curve characterizes the mechanical behaviour of the material bond in the crack opening mode and is an important basis for a numerical treatment of interface problems. Different pre-treatments of the old concrete surface have an important influence on the adhesion of the material compound. Good adhesive properties have been measured after sand-blast treatment and poorer properties after a dispersion emulsion treatment.

Crack face interaction and mixed mode fracture of wood composites during mode III loading

Abstract A new testing setup to perform and measure mode I, mode III and mixed mode experiments is applied to wood composites with varying particle size. Specimens of particleboard, MDF (medium density fiberboard) and INTRALLAM LSL ® are tested under mode I and mode III loading conditions. As testing procedure and specimen geometry allow steady state crack propagation, full load displacement curves can be recorded until complete separation of the specimen takes place. Using a special specimen geometry, large crack lengths may be obtained. The testing method is sensitive enough to investigate the mode I crack opening induced by fracture surface asperities during mode III loading, without external mode I loading and therefore the amount of arising intrinsic mode mixity.

Fracture characteristics of wood under mode I, mode II and mode III loading

Abstract Wood is a highly optimized cellular orthotropic material. Its structural properties have major influence on the fracture mechanics behaviour of the material. The aim of the presented study is the evaluation and analysis of differences in the fracture process of wood subjected to fundamental fracture modes. The results of fracture mechanics experiments for mode I, mode II and mode III in two crack propagation systems were evaluated. The fracture process is discussed by means of the load-displacement diagrams recorded under stable crack propagation and the calculation of the specific fracture energy. It was found that the typical specific fracture energy is much higher for the mode II and mode III cases than for mode I loading. The differences for wood are explained by different energy-dissipating processes such as the development of larger damage zones for mode II and mode III loading than for mode I loading. The influence of the chosen orientation and its relation to structural parameters of the material are also discussed. The findings are supported by microscopic images of typical fracture surfaces.

Low-temperature interlaminar shear strength of reactor irradiated glass-fibre-reinforced laminates

Abstract Glass-fibre-reinforced plastics (GFRPs) are candidate insulating materials for superconducting magnet coils in future fusion reactors. Therefore, the influence of radiation damage (gamma and fast neutrons) especially on the interlaminar shear behaviour of these materials has to be investigated carefully. Different types of GFRP laminate (two-dimensional E- or S-glass fibre reinforcements, epoxy or polyimide resins) have been irradiated at room temperature in the TRIGA reactor (Vienna, Austria) and at 5 K in the FRM Munich (Garching, Germany) up to a neutron fluence of 5 × 10 22 m −2 ( E > 0.1 MeV ) prior to short-beam-shear (SBS) testing at 77 K. After low-temperature irradiation, half of the samples were subjected to a warm-up cycle to room temperature before testing at 77 K. Results on the influence of different radiation sources, irradiation temperatures and annealing cycles as well as the boron content of some laminates on the interlaminar shear strength (ILSS) are compared and discussed, together with microstructural observations made with a scanning electron microscope.

Comparative biomechanical and radiological characterization of osseointegration of a biodegradable magnesium alloy pin and a copolymeric control for osteosynthesis

Magnesium alloys offer great advantages as degradable implant material for pediatric fracture fixation and hold the potential to overcome certain critical shortcomings inherent to currently used degradable (co)polymers. Besides good biocompatibility and appropriate degradation kinetics, sufficient implant anchorage in host bone is critical to prevent implant failure. Bone-implant anchorage of biodegradable magnesium alloys, however, has not yet been related and compared to that of copolymers, their degradable counterparts currently in clinical use. The aim of this study, therefore, was to comparatively assess bone-implant interface strength and the amount of peri-implant bone of a biodegradable magnesium alloy pin (Mg-Y-Nd-HRE) and a self-reinforced copolymeric control (85/15 poly(l-lactic-co-glycolic acid)). To this purpose, push-out testing, microfocus computed tomography (μCT), histological and scanning electron microscopic examination was performed after 4, 12 and 24 weeks of transcortical implantation in 72 rats. Biomechanical testing revealed significantly higher ultimate shear strength for the magnesium alloy pins than for the copolymeric controls at all 3 timepoints (P≤0.001 for all comparisons). As evaluated by μCT, the amount of bone present near the interface and in a wider radius (up to 0.5mm) around it was higher in the magnesium alloy implants at 4 weeks, without significant differences at 12 and 24 weeks. Histological examination confirmed direct bone-to-implant contact for both implant types. In vivo degradation of implants did not induce any noticeable local or systemic inflammation. This data suggests that the investigated degradable magnesium alloy rod exhibits markedly superior bone-implant interface strength and a greater amount of peri-implant bone than a self-reinforced copolymeric control currently in use; thus it fulfills a crucial prerequisite for its successful clinical deployment as an alternative degradable orthopedic implant material. Further studies, however, are warranted to evaluate the long-term degradation behavior and biocompatibility of the investigated degradable magnesium-based alloy.

PHB, crystalline and amorphous magnesium alloys: Promising candidates for bioresorbable osteosynthesis implants?

In this study various biodegradable materials were tested for their suitability for use in osteosynthesis implants, in particular as elastically stable intramedullary nails for fracture treatment in paediatric orthopaedics. The materials investigated comprise polyhydroxybutyrate (PHB), which belongs to the polyester family and is produced by microorganisms, with additions of ZrO2 and a bone graft substitute; two crystalline magnesium alloys with significantly different degradation rates ZX50 (MgZnCa, fast) and WZ21 (MgYZnCa, slow); and MgZnCa bulk metallic glasses (BMG). Push-out tests were conducted after various implantation times in rat femur meta-diaphysis to evaluate the shear forces between the implant material and the bone. The most promising materials are WZ21 and BMG, which exhibit high shear forces and push-out energies. The degradation rate of ZX50 is too fast and thus the alloy does not maintain its mechanical stability long enough during the fracture-healing period. PHB exhibits insufficient mechanical properties: it degrades very slowly and the respective low shear forces and push-out energy levels are unsatisfactory.