Background. Clinical problems like postoperative infection and increased incidence of pediatric trauma requiring surgical intervention raised the need for temporary medical implants that would resorb after the bone healing is complete. This would decrease high costs associated with repeated surgeries, minimize recovery times, decrease the risk of postoperative infections, and thus promote higher quality of life to the patients. The concept of biodegradation is already known in medical practice: resorbable sutures are successfully used in surgery. However, a bone implant that would resorb after the fracture heals is a new concept. Magnesium was suggested as a suitable material for these purposes because it is biocompatible and stimulates new bone formation. This doctoral Thesis evaluates both in vitro and in vivo behaviour of magnesium-based implants and consists of four parts: degradation, cellular reaction, early biofilm formation and histology study.Materials and Methods. Degradation properties of Mg2Ag, Mg10Gd, WE43 and 99.99 % pure Mg were studied by immersion and gas evolution tests. The corrosion rate (CR), osmolality, pH, Ca2+ concentrations, and surface changes were determined. Biocompatibility was studied by exposing primary human reaming debris-derived cells (HRD) to described magnesium alloys and assesing cell viability, morphology, differentiation to osteoblasts, along with evaluation of pH changes and Ca2+ concentration induced by magnesium. Mg2Ag, Mg10Gd, WE43 and 99.99 % pure Mg were further tested for their ability to resist biofilm formation. S. epidermidis and E. faecalis were allowed to adhere to the magnesium surfaces for 2 hours (h) followed by rinsing and, for S. epidermidis, further incubation of 24, 72 and 168 h was carried out. Furthermore, an animal study was performed and consisted of histological and histomorphometrical analysis of three magnesium-based materials: magnesium-hydroxyapatite (Mg-HA), W4 (96 % magnesium, 4 % yttrium), and pure magnesium (pure Mg). Bone response to magnesium and implants resorption behaviour was studied.Results. Degradation. WE43 showed the highest CR of all materials tested 1.057 mm/year which is almost twice as high as in the other samples. The lowest mean CR was in Mg2Ag group. All alloys made pH more alkaline and decreased concentration of free Ca2¬+ in the solution. Osmolality decreased in all samples after day 7. Pure Mg had the most constant Sa and Sdr of all materials over the observation period. Cellular Reaction. The number of viable cells in presence of all magnesium samples was stable over the observation period of 21 days. The inhibition of ALP content in osteogenic differentiating HRD was caused by pure Mg at day 14 and 28. All other magnesium alloys did not affect the ALP content. Exposure of HRD to magnesium increased the amount of lysosomes and endocytotic vesicles. Early Stages of Biofilm Formation. E. faecalis were significantly more prevalent on all magnesium surfaces compared to S. epidermidis (p = 0.001). Biofilm growth of S. epidermidis was different on various magnesium materials: the amount of bacteria increased up to 72 h but interestingly a significant decrease was seen at 168 h on Mg2Ag and WE43 surfaces. For pure Mg and Mg10Gd the biofilm formation reached plateau at 72 h. No correlation was found between the surface topology and the amount of adherent bacteria. Histology. Mg-HA had the highest mean amount of tartrate-resistant acid phosphatase (TRAP) positive cells at the implantation site of all groups. It had shown the fastest degradation rate already at 6 weeks. New bone was observed in direct contact to pure Mg and W4. The mean gas volume was highest in W4 compared to pure Mg and Mg-HA but this difference was not statistically significant. Conclusions. In vitro. Mg-Ag alloys seem to be the most promising in respect to the cellular reactions, degradation rates, the effect on the surrounding environment and the ability to withstand the biofilm growth after 72 h in vitro. In vivo. Pure Mg and W4 were the most promising material in the in vivo experiment. Mg-HA had too fast resorption probably due to the high levels of HA comprising 20 % of the material s composition. The level of HA should be reduced in future studies.
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