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We report on the long-term effects of degrading magnesium implants on bone tissue in a growing rat skeleton using continuous in vivo micro-Computed Tomography, histological staining ...and Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA-ICP-MS). Two different magnesium alloys—one rapidly degrading (ZX50) and one slowly degrading (WZ21)—were used to evaluate the bone response and distribution of released Mg and Y ions in the femur of male Sprague-Dawley rats. Regardless of whether the alloy degrades rapidly or slowly, we found that bone recovers restitutio ad integrum after complete degradation of the magnesium implant. The degradation of the Mg alloys generates a significant increase in Mg concentration in the cortical bone near the remaining implant parts, but the Mg accumulation disappears after the implant degrades completely. The degradation of the Y-containing alloy WZ21 leads to Y enrichment in adjacent bone tissues and in newly formed bone inside the medullary space. Locally high Y concentrations suggest migration not only of Y ions but also of Y-containing intermetallic particles. However, after the full degradation of the implant the Y-enrichment disappears almost completely. Hydrogen gas formation and ion release during implant degradation did not harm bone regeneration in our samples.
Magnesium is generally considered to be one of the most attractive base materials for biodegradable implants, and many magnesium alloys have been optimized to adjust implant degradation. Delayed degradation, however, generates prolonged presence in the organism with the risk of foreign body reactions. While most studies so far have only ranged from several weeks up to 12months, the present study provides data for complete implant degradation and bone regeneration until 24months, for two magnesium alloys (ZX50, WZ21) with different degradation characteristics. μCT monitoring, histological staining and LA-ICP-MS illustrate the distribution of the elements in the neighboring bony tissues during implant degradation, and reveal in particular high concentrations of the rare-earth element Yttrium.
The formation and growth of interior fatigue cracks have been studied thoroughly during the last decades, due to the high practical importance of accurate predictions about the fatigue lives of ...structures and machine components. This is of special interest in the VHCF regime, for which the ultrasonic fatigue method is especially useful. Fracture mechanical values and lifetimes can be measured at constant and variable amplitudes. Endurance limits and ΔK thresholds can also be determined very quickly (e.g. 1010 cycles within a few days). Three different materials were studied. For chromium steel, internal crack formation and a fracture mechanical analysis of its growth were carried out. For a specially manufactured Mg alloy, microstructural features leading to interior cracks were analysed. Finally, for polycrystalline high‐purity copper, the conditions for interior‐crack formation and growth were determined. The utilization of FE‐SEM microscopy assisted in interpreting the prevailing mechanisms responsible for endurance limits, lifetimes and fatigue‐crack growth thresholds.
High cycle fatigue properties of high-pressure die-cast magnesium alloys AZ91 hp, AM60 hp, AE42 hp, AS21 hp and of similarly produced cast aluminium alloy AlSi9Cu3 have been investigated. Ultrasonic ...fatigue tests up to 10
9 cycles show mean fatigue limits of approx. 38–50 MPa (magnesium alloys) and 75 MPa (AlSi9Cu3) in the tested casting condition. Fatigue cracks initiated at porosity in 98.5% of the samples. Considering porosity as initial cracks, specimens fail, if critical stress intensity amplitude,
K
cr is exceeded.
K
cr of the magnesium alloys range from 0.85±0.05 to 1.05±0.05 MPa√m, and 1.85±0.10 MPa√m was found for AlSi9Cu3. Below
K
cr, fatigue cracks may initiate at porosity, however, do not propagate until failure. Using
K
cr, the statistical distribution of defects is linked to the fracture probability at different stress amplitudes.
•Structural features on all scales give insight into different ways of fatigue response.•Ultrasonic-fatigue is most appropriate for studying structural changes in the VHCF regime.•Polycrystalline ...copper is used to study basic mechanisms of fatigue behavior.•Different SEM techniques are applied and correlated with S-N results.•Material properties play a more important role than loading frequency for its VHCF response.
Predicting life times of structures, machines and their parts as well as very small electronic devices for medical purposes is especially difficult for very low stress amplitudes and very high numbers of cycles. The development of the ultrasonic-fatigue method resulted in a reliable testing technique for VHCF loading. Some of its properties are described in this paper. To offer an explanation for the somewhat controversial ultrasonic-fatigue properties reported in the literature, S-N tests were performed at ∼19kHz and 20Hz on polycrystalline copper – a nominally homogenous and ductile material. Moreover, plastic-strain measurements were performed. In addition, micro-structural features and their changes in the high and very-high cycle regimes are reported.
Aim of this study was to evaluate the response of bone to novel biodegradable polymeric composite implants in the femora of growing rats. Longitudinal observation of bone reaction at the implant site ...(BV/TV) as well as resorption of the implanted pins were monitored using in vivo micro-focus computed tomography (µCT). After 12, 24 and 36 weeks femora containing the implants were explanted, scanned with high resolution ex vivo µCT, and the surface roughness of the implants was measured to conclude on the ingrowth capability for bone tissue. Scanning electron microscope (SEM) and energy dispersive X-ray spectroscopy (EDX) were used to observe changes on the surface of Polyhydroxybutyrate (PHB) during degradation and cell ingrowth. Four different composites with zirconium dioxide (ZrO2) and Herafill® were compared. After 36 weeks in vivo, none of the implants did show significant degradation. The PHB composite with ZrO2 and a high percentage (30%) of Herafill® as well as the Mg-alloy WZ21 showed the highest values of bone accumulation (increased BV/TV) around the implant. The lowest value was measured in PHB with 3% ZrO2 containing no Herafill®. Roughness measurements as well as EDX and SEM imaging could not reveal any changes on the PHB composites׳ surfaces. Biomechanical parameters, such as the adhesion strength between bone and implant were determined by measuring the shear strength as well as push-out energy of the bone-implant interface. The results showed that improvement of these mechanical properties of the studied PHBs P3Z, P3Z10H and P3Z30H is necessary in order to obtain appropriate load-bearing material. The moduli of elasticity, tensile strength and strain properties of the PHB composites are close to that of bone and thus promising. Compared to clinically used PLGA, PGA and PLA materials, their additional benefit is an unchanged local pH value during degradation, which makes them well tolerated by cells and immune system. They might be used successfully for personalized 3D printed implants or as coatings of rapidly dissolving implants.
•Tested PHBs for biodegradable bone implants do not degrade within 36 weeks in vivo.•No or almost no accumulation of bone tissue and bone–cell ingrowth could be detected.•Low adhesive strength of bone-implant interface and no change over time was found.•Increasing the implant-surface roughness is suggested to stimulate bone–cell adhesion.•Alternative applications of specified PHB composites are proposed.
Position-resolved small-angle X-ray scattering was used to investigate the nanostructure of the wood cell wall in two softwood species (Norwegian spruce and Scots pine) and two hardwood species ...(pedunculate oak and copper beech). The tilt angle of the cellulose fibrils in the wood cell wall versus the longitudinal cell axis (microfibril angle) was systematically studied over a wide range of annual rings in each tree. The measured angles were correlated with the distance from the pith and the results were compared. The microfibril angle was found to decrease from pith to bark in all four trees, but was generally higher in the softwood than in the hardwood. In Norwegian spruce, the microfibril angles were higher in late wood than in early wood; in Scots pine the opposite was observed. In pedunculate oak and copper beech, low angles were found in the major part of the stem, except for the very first annual rings in pedunculate oak. The results are interpreted in terms of mechanical optimization. An attempt was made to give a quantitative estimation for the mechanical constraints imposed on a tree of given dimensions and to establish a model that could explain the general decrease of microfibril angles from pith to bark.
Bioresorbable materials for implants have become increasingly researched over the last years. The bone–implant-interfaces of three different implant materials, namely a new bioresorbable magnesium ...alloy, a new self-reinforced polymer implant and a conventional titanium alloy, were tested using various methods: push-out tests, SEM and EDX analyses as well as surface analyses based on stereoscopic 3D pictures were conducted. The fracture energy is proposed as a very significant reference value for characterizing the mechanical performance of a bone–implant system. By using a video-extensometer system instead of, as is commonly done, tracking the movement of the crosshead in the push-out tests, the accuracy of measurement could be increased.
Mode I fracture characteristics of different wood species (one softwood and three hardwoods) in two crack propagation systems were investigated using the wedge splitting test under loading ...perpendicular to the grain. From the obtained load–displacement curves the initial slope, the critical stress intensity factor and the specific fracture energy were determined. The initial slope and the critical stress intensity factor were shown to depend strongly on density within all species whereas for the specific fracture energy differences between the softwood and the hardwoods were found. Differences between the crack propagation systems could be explained by the higher volume fraction of radial oriented tissue (rays) of the hardwoods.
•High-frequent ultrasonic VA-blocks are superimposed to low-frequent square loads.•Life-times under specified combined loading conditions are shorter than predicted.•Predictions used completely ...reversed S–N data, FCG-rates and a modified Miner rule.•Correlation of life-times with fractographic results allows interpretation of results.•Small crack arrest and micro-plasticity seem to be main life-limiting mechanisms.
Aim of this study is an interpretation of the influence of variable-amplitude (VA) cycles superimposed to low-frequency loads on fatigue life of 7075-T651 Al-alloys. Constant-amplitude (CA) 20kHz stress/strain-life (S–N) and (ε–N)-curves with and without superimposed mean loads serve as basis. For combined fatigue loading, life-time measurements were performed. Life-time estimations based on the S–N results reveal a damaging effect of the superimposed ultrasonic vibrations in the high cycle fatigue (HCF) and the very high cycle fatigue (VHCF) regimes. The CA and VA-life time results are correlated with fractographic observations. An interpretation of fatigue lives under combined low and high-frequency VA-loading is proposed considering small/short-crack propagation and arrest mechanisms.
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.
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