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Statement of problems: Computer-aided design/computer-assisted machining systems offer the possibility of fabricating restorations from one machinable ceramic block. Whether multishaded blocks improve esthetic results and are a viable alternative to individually stained ceramics has not been fully determined.
Purpose: The aim of this investigation was to examine the effect of multishaded blocks on the esthetic appearance of all-ceramic CEREC crowns and compare these crowns with single-shade and stained restorations.
Material and methods: Ten subjects were included in this study. For each subject, 6 different crowns were milled with the use of a CEREC machine. One crown was milled from each of the following machinable ceramic materials: CEREC Vitablocs Mark II in classic colors; Vitablocs Mark II in 3D-Master colors; Vitablocs Mark II in either classic or 3D-Master colors, with additional staining; Megadenta Bloxx multishaded; Mark II experimental multilayer; and an experimental multilayer leucite ceramic. Three independent examiners assessed the esthetic appearance of crowns fabricated to match each subject's anterior tooth shade. A scale of 1 to 6 was used to score the shade match and esthetic adaptation of each crown, with 1 representing excellent characteristics and 3.5 serving as the threshold for clinical acceptability. The examiners' scores were averaged, and the mean values were analyzed with the Wilcoxon signed rank test (P<or=.05).
Results: Only individually stained Mark II restorations achieved a mean score below the threshold of 3.5 for all 10 subjects. These restorations were significantly more esthetic (P<or=.05) than restorations fabricated from all other materials except the Mark II experimental multilayer ceramic. The second best results were obtained for crowns made from single-shaded Mark II 3D-Master blocks: 6 out of 10 restorations were scored below 3.5. Two of the layered materials (Mark II experimental and Bloxx) followed with 5 acceptable restorations out of 10.
Conclusion: Within the limitations of this study, the results provide no evidence that multicolored machinable ceramics improve the esthetics of all-ceramic crowns.
Mg and Mg alloys are becoming more and more of interest for several applications. In the case of biomaterial applications, a special interest exists due to the fact that a predictable degradation should be given. Various investigations were made to characterize and predict the corrosion behavior in vitro and in vivo. Mostly, the simple oxidation of Mg to Mg2+ ions connected with adequate hydrogen development is assumed, and the negative difference effect (NDE) is attributed to various mechanisms and electrochemical results. The aim of this paper is to compare the different views on the corrosion pathway of Mg or Mg alloys and to present a neglected pathway based on thermodynamic data as a guideline for possible reactions combined with experimental observations of a delay of visible hydrogen evolution during cyclic voltammetry. Various reaction pathways are considered and discussed to explain these results, like the stability of the Mg+ intermediate state, the stability of MgH2 and the role of hydrogen overpotential. Finally, the impact of MgH2 formation is shown as an appropriate base for the prediction of the degradation behavior and calculation of the corrosion rate of Mg and Mg alloys.
Production of a dental reconstruction comprises: (a) forming a mold (12) from a model (10); (b) using (12) to make an isotropically heat-expandable replica from a ceramic precursor (14); (c) oxidizing (14) to produce an enlarged ceramic replica; (d) forming a silicone mold from this replica; (e) using (d) to make a second ceramic replica; and (f) sintering the second replica with volume reduction to form a replica identical to (10).
A proposal for the classification of dental alloys according to their resistance to corrosion
(2007)
Objectives
The purpose of this study was to establish a method to compare and classify dental alloys in relation to their resistance to corrosion.
Methods
Alloy samples and pure metal samples were prepared and tested in chemical and electrochemical corrosion according to ISO 10271. For electrochemical test, the rest potential versus time and a potentiodynamic scan were recorded. After chemical corrosion test, the ions released were analyzed by ICP (induced coupled plasma) spectroscopy.
Results
High gold alloys had a similar polarization curve than gold. The same effect was observed for Pd–base alloys, their curves were similar to the one of palladium. The ions released during chemical corrosion were non-precious metallic ions. Thereby Ni–Cr alloys were found to release the most ions. Au–Pt alloys showed the highest release of ions compared with other precious alloys but low compared with Ni–Cr. Electrochemical corrosion was more aggressive than chemical corrosion and every type of elements was etched, the higher the precious metal content, the higher the resistance to corrosion of the alloy.
Discussion
Using the recorded data, a classification system for electrochemical corrosion was developed and discussed to judge the results. Hereby were gold and zinc used as reference materials. The applied classification system defines five classes and it is proposed that alloys of class V are not acceptable. For chemical corrosion resistance, three classes were distinguished according to the quantity of metallic ions released and it is proposed that class III (100–1000 μg/cm2 week) is not acceptable. Palladium and Pd–base alloys showed a higher electrochemical and chemical corrosion resistance than gold.
A lot of dental alloys are available on the market. Among these alloys, there are the conventional alloys, the so called casting alloys used without ceramics, the bonding alloys used with high fusing ceramics and the universal alloys used without or with low fusing ceramics. It is im portant to know the physical and mechanical properties of these materials but also their biocompatibility and their resistance to corrosion. Dental alloys are generally placed in the mouth for many years, they must not induce adverse biological reactions such as gingival swelling and erythema, mucosal pain and lichenoid reactions. Although these troubles are often caused not by the materials itself (1, 2), they can be induced by the metallic ions released during their corrosion. In order to decrease the risks to the health, it is necessary to study the corrosion of the dental alloys. Currently, the ISO 10271 Standard (3), describes 3 different corrosion tests: a static immersion test (chemical corrosion), an electrochemic al test and a tarnish test. However, there are no indications yet about the possible interpretation of test results. In this paper, we propose a method to compare and classify the dental alloys in relation to their chemical and electrochemical corrosion results. METHODS: The material tested are pure metals such as gold, palladium, silver, copper and zinc as well as dental alloys which are commercially sold (see Table 1). 54 different materials have been tested. A minimum of four samples of each material were tested by electrochemic al corrosio n and a minimum of three in chemical test. The samples were cast and prepared as indicated by the manufacturer and by the ISO 10271. For the electrochemical test, the samples are in the form of disks 11 mm in diameter. They are tested with a potentiostat/galvanostat Voltalab Model 21. For the chemical test, the samples are rectangular with the dimensions 35X10X1.7 mm. The solution used and the operating conditions are described in the ISO 10271. The concentration of metallic ions released is measured by Induced Coupled Plasma
In Vitro Analysis of Human Cartilage Infiltrated by Hydrogels and Hydrogel-Encapsulated Chondrocytes
(2023)
Osteoarthritis (OA) is a degenerative joint disease causing loss of articular cartilage and structural damage in all joint tissues. Given the limited regenerative capacity of articular cartilage, methods to support the native structural properties of articular cartilage are highly anticipated. The aim of this study was to infiltrate zwitterionic monomer solutions into human OA-cartilage explants to replace lost proteoglycans. The study included polymerization and deposition of methacryloyloxyethyl-phosphorylcholine- and a novel sulfobetaine-methacrylate-based monomer solution within ex vivo human OA-cartilage explants and the encapsulation of isolated chondrocytes within hydrogels and the corresponding effects on chondrocyte viability. The results demonstrated that zwitterionic cartilage–hydrogel networks are formed by infiltration. In general, cytotoxic effects of the monomer solutions were observed, as was a time-dependent infiltration behavior into the tissue accompanied by increasing cell death and penetration depth. The successful deposition of zwitterionic hydrogels within OA cartilage identifies the infiltration method as a potential future therapeutic option for the repair/replacement of OA-cartilage extracellular suprastructure. Due to the toxic effects of the monomer solutions, the focus should be on sealing the OA-cartilage surface, instead of complete infiltration. An alternative treatment option for focal cartilage defects could be the usage of monomer solutions, especially the novel generated sulfobetaine-methacrylate-based monomer solution, as bionic for cell-based 3D bioprintable hydrogels.
Although magnesium and its alloys are promising candidates as biodegradable implant materials, the tendency for localized corrosion mechanism in physiological environment limit their biomedical application. Electropolishing is an attractive strategy for improving the corrosion behaviour of metals, but it is still largely unexplored in magnesium materials. In this study, the characterization of electropolished surfaces of AM50 and pure magnesium was performed, focussing on their in vitro degradation behaviour in cell medium. Corrosion rates were evaluated using potentiodynamic polarisation. The surface morphology before and after the onset of corrosion was investigated by scanning electron microscopy and confocal laser scanning microscopy. The presented electropolishing process led to improved surface performances, observable by significantly lower corrosion rates (0.08 mm·year-1 in Dulbecco's modified Eagle's medium), lower arithmetical mean height (0.05 µm), lower water contact angle (25-35°) and lower micro hardness (35-50 HV 0.1) compared to mechanically and chemically treated surfaces. MgO/Mg(OH)2 could be detected on electropolished surfaces. The localized corrosion mode could be reduced, but not entirely prevented. Electropolishing shows great potential as post-treatment of magnesium-based components, but detailed tests of the long-term corrosion behaviour are an important area of future research.
The corrosion behaviour of WE43 magnesium alloys using the mini cell system was studied. Voltammetry and impedance spectroscopy were applied to study on the one hand the effect of microstructure of the working electrode and on the other hand the effect of proteins in the electrolyte. Two types of alloy samples were produced (i) by permanent mould casting and (ii) by gas atomization followed by extrusion. The results showed that the microstructure was strongly influenced by the production process. The extruded samples showed an improved homogeneity of phase distribution compared with cast samples as it was aimed for. Due to increased homogeneity it was expected to find higher corrosion resistance. However, the electrochemical results are contradictory and suggest an additional phase in the extruded microstructure. Using energy dispersive X-ray spectroscopy (EDX) the secondary magnesium rare earths (RE) phase of extruded samples showed differing composition than of cast samples as well as additional oxide phases. After the samples were electrochemically investigated in cell medium with and without fetal calf serum (FCS), an impact of FCS was detected in voltammetry due to the length of the polarisation curve. As the tip of the mini cell in contact with the working electrode is small, developing gases tend to spread on the working electrode and break the contact of liquid to the counter electrode; which results in disrupting the current flow. This effect was more pronounced when rising the voltage and was found reduced when using electrolytes with FCS. Impedance spectra were slightly deformed by FCS, seen as a kinetic effect but not as a basic differing corrosion reaction. The insight into the effects of FCS was provided by the mini cell system as this system enables to collect entire series of measurements. In contrast of two single measurements, those series reflected the slight difference caused by FCS. The focus of the electrochemical corrosion study was set on the first half hour of immersion.
This review comprehensively covers research carried out in the field of degradable coatings on Mg and Mg alloys for biomedical applications. Several coating methods are discussed, which can be divided, based on the specific processing techniques used, into conversion and deposition coatings. The literature review revealed that in most cases coatings increase the corrosion resistance of Mg and Mg alloys. The critical factors determining coating performance, such as corrosion rate, surface chemistry, adhesion and coating morphology, are identified and discussed. The analysis of the literature showed that many studies have focused on calcium phosphate coatings produced either using conversion or deposition methods which were developed for orthopaedic applications. However, the control of phases and the formation of cracks still appear unsatisfactory. More research and development is needed in the case of biodegradable organic based coatings to generate reproducible and relevant data. In addition to biocompatibility, the mechanical properties of the coatings are also relevant, and the development of appropriate methods to study the corrosion process in detail and in the long term remains an important area of research.