Mimicking the natural features of the teeth has led companies to make huge investments that generate incomes of about US$ 2 billion per year in dental crowns, 20% represented by pure ceramic crowns. The increasing concern about esthetics leads the dentists to indicate materials that have a greater number of similarities with the natural structures.
The dental ceramic have very interesting characteristics and, therefore, they became very popular. The translucency and color of these materials contribute to the achievement of a highly satisfactory esthetic outcome; and its hardness and wear resistance contribute to its ability to withstand high chewing efforts. The combination of these qualities enables the preparation of partial restorations (veneers, inlays, onlays and overlays), total unitary restorations and fixed restorations up to three elements in pure ceramic, as long as there is care on planning the case and in the choice of material to be used. The clinical results achieved can be very pleasant and functional, since the systems are ceramic materials that are close to the characteristics of natural teeth.
Ceramics and its processes
To select the most appropriate type of ceramics and processing for a given clinical application, the professional should be familiar with the peculiarities of each different system. In spite of various systems available, the dental market held a natural selection, leaving today three major types of ceramics and their respective processes: the traditional feldspathic porcelain that allow stratification by means of the application of powder and liquid and with that achieving an incomparable esthetic result; the lithium disilicate that can be used in a pressed or a computed aided system (CAD/CAM), they are the more versatile materials in the market by allowing the use as a monolithic structure or an infrastructure for the porcelain. It has excellent esthetics and resistance to fracture, has been a material of choice for various cases; and zirconia that is processed with a computed aided system (CAD/CAM), has become an extremely popular infrastructure due to its extreme resistance to fracture. Its mechanical properties present the highest values among all dental ceramic systems, allowing the creation of structures of ceramics before impossible. In addition, it has large opacity what makes it difficult to mimic the dental structures, but allows the easier masking of structures or darkened metal.
Why do ceramics fail?
The ceramics are fragile and susceptible to fracture due to its sensitivity to external or internal microcracking, which functions as the hub of tensions and cause their spread to the fracture. Therefore, any failure in ceramics can become the beginning of a fracture, from a failure in the processing step to one on the clinical stages (internal, proximal or occlusal adjustments). Thus, any adjustment must be performed with caution and adequate instruments such as specific horns and rubbers for ceramics. Another way to improve the resistance of ceramics is the adhesive cementation, even if it is allowed the use of other cements, resin cements add to the ceramics greater resistance to fracture.
Why zirconia does not fracture?
Seeking to compensate the limitations of feldspathic ceramics, mainly those related to its fragility, various types of oxides were incorporated into it, that act limiting the cracks among them, as the zirconium oxide (zirconia). Besides, the yttrium oxide is added to the pure zirconia, which stabilizes the zircons at the room temperature and generates a multiphase material known as partially stabilized zirconia (Y-TZP). The high initial resistance and tenacity of Y-TZP results from the physical properties of this stabilization.
When stabilized with yttrium, zirconia stays in a tetragonal structure (t) at room temperature. External conditions, such as tension of traction, acting on the end of the crack, can transform the metastable tetragonal phase (t) into the monoclinic phase (m), which is more stable. This transformation is associated with a local increase of 3-5% in volume, which results in compressive stress located around and in the edges of the crack, acting against the tensions of traction that lead to fracture.
This physical property is known as transformation toughening. By means of this phenomenon, the polycrystalline diamonds showed flexural resistance of 900 - 1.200 Mpa and toughness of 9 - 10 MPa·m1/2, which is two times greater than that of alumina and three times higher compared to the lithium disilicate. However, the zirconia, though less commonly, may fracture. They degrade by aging at low temperatures, a phenomenon that occurs in its surface and degrades its excellent mechanical properties.
Compared to other ceramic materials, zirconia offers superior infrastructure stability and displays a combination of high flexural strength and high resistance to fracture, associated with phase transformation during processing (from tetragonal to monoclinic), besides it has a lower elasticity modulus. These properties, altogether, can explain the excellent survival rates for zirconia infrastructures. However, the main cause of mechanical complications in zirconia prostheses relates to the cracks of the coating ceramics. This may be due to a deficient form (design) of the infrastructure, which does not promote adequate support to the coating ceramics and may be associated with the concentration of tension during the application of this layer.
Advances in composition and processing of dental ceramics indicate that the mechanical strength of these materials can be improved. During the process of increasing the resistance of ceramics, there is usually the esthetic qualities tend to worsen. The challenge, then, is to develop ceramic structures with excellent esthetics and resistance.
With the emergence and evolution of 3D printers, a new form of production of ceramics can become a reality. Once the CAD-3D model is created, a printing sequence to build the layers on top of each other is performed. This strategy has been used to build a variety of structures with applications ranging from electronics to the scaffold for bone graft.
The potential advantages over the CAD/CAM systems include the ability to spatially composition, degree of porosity to meet specific projects and the non-necessity of prior impression. In addition, this manufacturing technology allows you to control more precisely the internal morphology, shape and distribution. Another benefit of this system is its ability to print with a variety of materials at the same time, as well as create graduated structures ( "graded" ), which leads us to another innovation.
Graduate materials are those that present, in a single structure, different types of mixed materials. Structures composed of gradations of feldspathic ceramic, alumina or zirconia and, again, feldspathic ceramic are being studied in the laboratory and encouraging results have been demonstrated.
In Dentistry, a graduated material would be a crown formed inside by feldspathic ceramic, in the middle by zirconia, or disilicate, or any other ceramic and the outer surface of feldspathic ceramic once again. In this way, the material would combine the esthetics and the adhesion properties present in feldspathic ceramic with the resistance of the oxide-based ceramic.
After all, what ceramics should I use?
The clinical decision is the very essence of the work of the dentist and cannot be made only on the instructions of book, with clear rules to follow. Clinical decisions should be made in the context of the circumstances of the patient. The decision to use a specific type of material must be based in(s): expectations of treatment; resistance; esthetics; ease to manufacture; long-term predictability. This is particularly important when the chosen material is different from the one that present clinical success for many decades to come, as the metal-ceramic crowns.
The collaboration between the dentist and the laboratory technician before the treatment of the patient allows the qualified ceramist to mimic the hue and chroma, as well as estimate the value and the characterizations found in natural dentition, regardless of the material used.
The decision of the material of choice must be in agreement with the ceramist's preference, because it is better a metal-ceramic crown well done by a technician that master the system than a pure ceramic of a technician who is not familiar with the material.
With the diversity of materials and clinical situations that exist, the constant updating of the professional is important, to know and understand the variety of materials available and the factors that will contribute to the success or failure of the treatment. The tables below show some possibilities to guide the choice of the material.
My purpose is to help you gain autonomy through knowledge, facilitating the dentistry and communication. AND how do I do this?
I put myself in your shoed, I identify what is preventing you from being happy with your results and untie the knots by means of a simple and accessible language! I graduated from UNESP | Araçatuba; did my Master and PhD in Operative Dentistry at UNESP Araraquara | I am a Professor of Operative Dentistry at the Federal University of Pelotas | RS.
Jul, 27th 2015