Fixed Restorations. Irena Sailer
manufacturer should be documented in the patient’s record.
Clinical studies on monolithic zirconia restorations are scarce, and the observation periods rather short. More research with longer observation periods is needed to elaborate the indications and limitations and the effect on the stomatognathic system of this recent type of all-ceramic restorations.
As a side note, it may be hypothesized that the clinical success of zirconia has stimulated the rapid development of the digital technologies and CAD/CAM procedures.
Metal-ceramics
Metal frameworks veneered with feldspathic ceramics are a long existing, well-documented material combination for single- and multiple-unit fixed dental prostheses on teeth and implants19,32,33. The composition of the veneering ceramics is very near to the veneering ceramics for zirconia, based on natural or synthetic feldspathic raw materials. However, the coefficient of thermal expansion has to match that of the underlying metal. It has been evaluated empirically that the coefficient of thermal expansion of the veneering ceramic should be one unit below that of the metal. In that case the metal is shrinking a little more during cooling and puts the ceramic under pressure. Thereby, detrimental tensile stress is avoided in the ceramic area.
Metals provide elasticity. Thus, the layered veneer is protected against tensile stress from underneath during mastication. The success story of metal-ceramic restorations is based on this phenomenon. In the beginning of the 1960s it was the first time that esthetic fixed restorations were achievable by veneering a metal framework with a tailored ceramic. From then on metal-ceramics were the gold standard for fixed restorations. However, the importance of this technique significantly decreased with the progress in all-ceramic restorations using zirconia instead of alloys as framework material. Due to the increasing demand for esthetic, biocompatible, and metal-free restorations by patients, all-ceramic and composite materials are increasingly used and will replace metal-ceramic restorations in the near future.
The metal substructures of metal-ceramic restorations are fabricated from different alloys by casting, milling, or selective laser melting. While casting is possible with all types of alloys, milling and laser melting is only economical with base metal alloys. The advantage of metals is their plastic behavior under stress. While in high-strength composite and ceramics cracks might grow under tensile load due to stress concentration at the crack tip, in metals a crack tip is rounded under stress due to plastic deformation (cf. Fig 1-1-1). Thus the stress intensity is reduced. This is why metals have a much higher fracture toughness compared to ceramics or high-strength resins.
The starting point for the metal-ceramic technique was a high-gold alloy, based on the binary system gold-platinum with a gold content of approximately 70–80% by weight. Over the years, as gold and platinum prices rose, different types of precious metal alloys were developed for economic reasons. These were precious metal alloys mainly based on a considerable amount of palladium, replacing gold as well as alloys based on the binary systems palladium-copper or palladium-silver with only low or even no gold and no platinum content. Further, base metal alloys such as cobalt-chromium alloys and chromium-nickel alloys were developed.
The traditional way to process precious and base metal alloys is casting, applying the lost-wax technique. A wax model of the framework is modeled manually, embedded in a refractory embedding compound, and burnt out, resulting in a hollow shape according to the desired framework. Molten alloy is cast into the hollow. After solidifying of the alloy, the casting object is divested, cleaned, and further processed.
Base metal alloys, such as cobalt-chromium alloys, have recently become a valid alternative to the gold-reduced and palladium-based varieties. They suffered from some technical disadvantages in the past, as casting of these metals is difficult. Their indications in daily clinical practice were very limited for this reason. Yet, CAD/CAM technology enabled the processing of the base metal alloys by allowing for computer-aided milling of industrially fabricated blanks, as well as additive manufacturing by selective laser melting technology.
With all types of metal-ceramics, the dark color of the metals has to be esthetically improved with veneering ceramics, adapted to the material properties of the respective metal alloy. Until today, the veneering procedure for the metal-based types of restorations is mostly performed by manual layering of veneering ceramic34,35. Some veneering ceramics can also be applied by the pressing technique, a veneering process that is not widely used, however.
It may be very challenging to achieve perfect esthetics with metal-ceramics, since the underlying framework is dark and the space for transforming its color into a natural tooth-resembling appearance with veneering ceramic is limited. Dental technicians need to develop pronounced skills and high experience levels for excellent esthetics with metal-ceramics.
Material properties determine the indications for the respective materials. Metal-ceramics will increasingly be replaced by composites and all-ceramic solutions. Composites play a certain role in single tooth restorations. The trend today is toward all-ceramic restorations due to their high esthetics and biocompatibility. For multiple-unit restorations, the material selection portfolio is rather limited. Of all-ceramic options, only zirconia demonstrates sufficient mechanical stability for this indication.
For the practitioner it is important to choose the right material. Table 1-1-1 gives an overview of selected non-metallic material options, their indications, and recommended cementation protocols to facilitate the choice.
Table 1-1-1 Classification, indications, and cementation protocols for selected metal-free restorative materials according to the manufacturers’ instructions
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