RESUMEN
UNLABELLED: 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. STATEMENT OF SIGNIFICANCE: 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.
Asunto(s)
Aleaciones/química , Implantes Experimentales , Magnesio/química , Animales , Calcio/análisis , Masculino , Espectrometría de Masas , Fósforo/análisis , Ratas Sprague-Dawley , Microtomografía por Rayos X , Itrio/análisisRESUMEN
Fractures of the growth plate region were performed with cadaver specimens obtained from the ovine distal femur and proximal tibia. Specimens of 6 different ages, ranging from 1 week to 4 years, were investigated in order to determine changes in the fracture characteristics. Mechanical properties (crack resistance and notch tensile strength), supported by microscopy of the distal tibia (thickness of growth plate and its zones, trabecular bone volume ratio) were determined. The crack propagated through different regions depending on age, which was observed both in microscopy and mechanical tests. In specimens of younger animals the fracture typically went through trabecular bone, often parallel to the growth plate, and only sometimes through the growth plate cartilage. Specimens of older animals fractured directly through the growth plate cartilage, while trabecular bone was not affected at all. Adult specimens had significantly higher mechanical values than the young ones. The results reveal the underlying mechanical properties that induce different fracture patterns of the epiphyseal growth plate at different stages of growth. The possibility of fractures through trabecular bone parallel to the growth plate in newborns and infants should be considered when clinical radiographs of paediatric fractures are analysed and classified.
Asunto(s)
Envejecimiento , Fracturas Óseas/fisiopatología , Placa de Crecimiento/fisiopatología , Articulación de la Rodilla/crecimiento & desarrollo , Fenómenos Mecánicos , Fracturas de Salter-Harris , Ovinos , Animales , Fenómenos Biomecánicos , Fémur/crecimiento & desarrollo , Resistencia a la Tracción , Tibia/crecimiento & desarrollo , Factores de TiempoRESUMEN
Biodegradable Mg alloys are of great interest for osteosynthetic applications because they do not require surgical removal after they have served their purpose. In this study, fast-degrading ZX50 Mg-based implants were surface-treated by micro-arc oxidation (MAO), to alter the initial degradation, and implanted along with untreated ZX50 controls in the femoral legs of 20 male Sprague-Dawley rats. Their degradation was monitored by microfocus computed tomography (µCT) over a total observation period of 24weeks, and histological analysis was performed after 4, 12 and 24weeks. While the MAO-treated samples showed almost no corrosion in the first week, they revealed an accelerated degradation rate after the third week, even faster than that of the untreated ZX50 implants. This increase in degradation rate can be explained by an increase in the surface-area-to-volume ratio of MAO-treated implants, which degrade inhomogeneously via localized corrosion attacks. The histological analyses show that the initially improved corrosion resistance of the MAO implants has a positive effect on bone and tissue response: The reduced hydrogen evolution (due to reduced corrosion) makes possible increased osteoblast apposition from the very beginning, thus generating a stable bone-implant interface. As such, MAO treatment appears to be very interesting for osteosynthetic implant applications, as it delays implant degradation immediately after implantation, enhances fracture stabilization, minimizes the burden on the postoperatively irritated surrounding tissue and generates good bone-implant connections, followed by accelerated degradation in the later stage of bone healing.
Asunto(s)
Implantes Experimentales , Magnesio/farmacología , Microtomografía por Rayos X/métodos , Animales , Procesamiento de Imagen Asistido por Computador , Masculino , Oxidación-Reducción/efectos de los fármacos , Ratas , Ratas Sprague-Dawley , Propiedades de SuperficieRESUMEN
Fracture mechanical parameters (notch tensile strength, specific fracture energy/crack resistance and specific crack initiation energy) of epiphyseal plate cartilage, trabecular bone (metaphysis) and cortical bone (diaphysis) were determined on ovine femur specimens. The fracture behaviour before and after crack initiation was recorded in force-displacement diagrams from wedge splitting tests. Crack propagation was stable both during and after the formation of a principal crack. This is the main advantage of the wedge-splitting method by Tschegg in comparison with tensile tests. Microscopy of the epiphyseal plate during fracture showed fibre elongation and tearing in the crack tip region. The results of this study can help to understand the mechanics of epiphyseal plate injuries and the obtained values can be used for computational simulations and models.