In nature, the main sources of zirconium are zirconita (ZrO2-SiO2, ZrSiO4) and baddeleyita (ZrO2), where the latter contains high concentrations of zirconia (96.5%–98.5%) and is recognized for its extreme purity.1 Zirconia, or zirconium dioxide (ZrO2), is a polymorphic ceramic that has atomic spatial arrangements that vary according to temperature changes. From room temperature to 1170°C, the crystal structure is in a monoclinic form. Between 1170°C and 2370°C, this material is presented as tetragonal zirconia, whereas above 2370°C, this material is presented as cubic zirconia.2 Because of the phase transformation related to the increasing temperature, a better crystal organization, good flexural strength, good fracture toughness, high hardness, excellent chemical resistance, and a decrease in volume can be observed. However, it is important to consider that reversible changes (thermodynamic) also may occur.2–4 To obtain the favored mechanical advantages provided by the tetragonal or cubic phases of zirconia, the ceramic must be stabilized by introducing oxides (CaO and MgO) or rare earth elements (Y2O3 and CeO2).4 These stabilizers, also known as dopants, prevent phase transformations that may occur during heating and/or cooling, maintaining the crystal structure of the phase in which the zirconia was stabilized. Thus, it is possible to keep the zirconia in tetragonal or cubic phases, either partially or wholly stabilized depending on the type and amount of stabilizers.1,4
Usually, zirconia used in dentistry to manufacture ceramic blocks for the CAD/CAM systems is tetragonal zirconia that is partially stabilized by yttrium (3Y-TPZ).3,5 The partial stabilization of zirconia is performed to enable milling of the presintered material, which after sintering offers resistance to fracture and wear. Clinically, tetragonal zirconia is structurally favored for its tenacity.2 It is known that ceramic materials are naturally fragile, with no ability for significant deformation; thus, when they suffer stress, they can crack. In the case of zirconia, the addition of oxides allows for stabilization, yielding a ceramic material with a higher fracture resistance, which enhances its application as a material for the construction of prosthetic infrastructures and implant abutments. However, all ceramic materials, at some point, may present cracks because of microstructural defects or external stresses, and despite the great development of zirconia powders and optimization of synthesis routes,6 these failures have been observed in implants and abutments.7–9
The tension generated can induce a reverse transformation (tetragonal to monoclinic [t → m]) at the area of crack propagation. This processing creates a local increase in volume of approximately 3% to 5%. However, a mechanism of transformation by toughening occurs; in other words, a compressive stress is generated around the crack by the remaining tetragonal particles, reducing the potential of additional crack propagation.2,10 Many factors influence the process of toughening by transformation, and although some factors are not clinically controlled (the presence of defects and porosities, particle size and shape, and type and amount of stabilizers), there are controllable factors involved in the handling of this material.2 The main factor that can be controlled is related to variations in temperature that the zirconia infrastructure may undergo when being sintered after machining, before they receive ceramic veneering.11,12 It is important that the technician performs the heating with controlled temperatures through slow heating and cooling and also the structures must remain loose during sintering (on alumina beads or platinum pins) so that no external stresses are placed on them.
Another important aspect is that zirconia will receive a ceramic veneer that has a lower flexural strength and a higher coefficient of thermal expansion. An interface will be created between these 2 materials because of their different mechanical properties, which can create an area of internal tension. Cracks may be generated and propagate within the fragile ceramic (veneer), which can cause it to fracture.13 Thus, mechanical treatments on the surface of zirconia (such as a discrete wear in the form of undulations or microretentions) have been suggested for improving the adhesion between these materials.11,14,15 However, adjustments and finishing of the zirconia surface may introduce a compressive surface tension, which will increase the susceptibility to aging.4,12 Thus, it is essential to use new fine-grained burs and perform an appropriate thermal treatment with slow cooling after the mechanical treatment.
Although zirconia can display good mechanical properties, it is susceptible to degradation at low temperatures, a phenomenon that decreases its length of use as a biomaterial.16,17 Aging is a progressive and spontaneous phenomenon that occurs through a slow surface transformation (t → m), a process that increases in the presence of water or moisture. This phenomenon spreads, leading to an increase in volume that places stress on the neighboring particles and results in subcritical crack growth. Surface degradation, microcracks, decreased resistance in the medium, and long-term and catastrophic fractures may occur due to the aging of zirconia.18,19 The susceptibility to aging depends on the size, quantity, distribution of grains, and the type of stabilizer, as well as the presence of residual stress. Again, clinical and laboratory care must be taken to maintain the integrity of the zirconia surface, remembering that, when subjected to mechanical treatment, the ceramic must be subsequently thermally treated to ensure that internal tension areas are not formed.11
Recently, different vitrification processes of zirconia have been described in the literature.20,21 Initially, vitrification was indicated to create a surface more prone to acid etching and silanization.22,23 However, authors have suggested modifying the outer surface of the zirconia through vitrification, to improve its chipping resistance, minimizing aging, and enabling its exposure to the oral environment.23 The aim of this article was to present a clinical case where zirconia ceramic infrastructures were used for a prosthetic rehabilitation involving teeth and implants. Special clinical protocols, like the use of the provisional restorations as personalized transfers to duplicate the gingival tissue architecture, and laboratory procedures involving the mechanical and thermal treatment of the surface that will contact the veneering ceramic and the external surface vitrification of the zirconia are presented, emphasizing the appropriate care that needs to be taken to take advantage of the good mechanical properties of the material and to control its aging.
A female patient, unhappy with her smile, sought treatment to replace some lost teeth (#14, #15, #24, and #26). Tooth #27 had a Class III furcation lesion with an indication of extraction. Teeth #12, #11, and #21 presented old and nonfitting metaloceramic crowns. After a detailed planning involving diagnostic wax-up, the crowns were removed and immediate provisional restorations were placed. Edentulous areas were reconstructed with autogenous grafts. After 6 months, the implants were placed (Cone Morse; Neodent, Curitiba, Brazil) aided by the surgical guides deriving from the wax-up. Two implants were installed in the middle right maxilla (Titamax CM: 3.5 × 9.0 mm and 3.5 × 11 mm) and 3 implants were installed in the middle posterior left maxilla (Titamax CM: 3.5 × 11.0 mm, 5.0 × 9.0 mm, and 5.0 × 9.0 mm). The implants stayed submerged for 6 months, and during this period, a cosmetic periodontal surgery was performed at the anterior segment to align the gingival margin from #13 to #23. After osseointegration and surgical opening, the metallic prefabricated abutments were installed (implants #14 and #15: Micro Pilar CM; implants #24, #26, and #27: Mini Pilar CM; Neodent). A set of acrylic-pressed provisional restorations were installed, restoring adequate shape and contour while also serving as a reference for the preparation of future prostheses. The preparations were made with a deep chamfer cervical margin, controlled depth, and internal rounded angles, with the future aim of placing ceramic crowns with the zirconia infrastructures. This decision was made considering the presence of a thin periodontal biotype and substrates of different colors.
The 1-step impression technique was used (Express XT; 3M-ESPE, St Paul, MN), with 2 retraction cords (Ultrapack #000 and #00; Ultradent Products, South Jordan, UT), around the prepared teeth (Fig. 1). The periimplant soft tissue contour was molded with the aid of the provisional restorations. They were placed with long screws (Neodent), and an open custom tray was used (Fig. 2). In the impression, the analogs (Neodent) were connected to the provisional restorations using long screws, and a working cast was obtained (Fig. 3). This approach was taken to ensure that the appropriate emergence profile could be reproduced in the working cast, eliminating the need for transfer customization.
Regressive waxing of the infrastructure was performed to allow the double scanning technique to be performed while also ensuring a uniform amount of veneering ceramic (Fig. 4). At the time of waxing, the anatomy of the palatal aspect of the maxillary anterior teeth was maintained so that this area could remain in zirconia because there was little space in this area, due to the occlusal pattern of the patient. The crown preparations, the implant analogs, and the emergence profile were scanned in the working cast (Scanner Lava; 3M-ESPE), generating a detailed first digital image for each area. The regressive wax-ups were also scanned, and a second digital image was superimposed on the first image (Fig. 5). The superposition between these 2 images resulted in a digital design of the infrastructure with ideal shape and thickness (Fig. 6). These designs, besides allowing a uniform and standardized injection of the veneering ceramic, were used as a reference for the milling of the stabilized zirconia blocks (Lava Blocks; 3M-ESPE) that were fabricated on a 5-axis milling machine (Lava CNC 500; 3M-ESPE). The pieces were fabricated approximately 25% greater than the original to compensate for the subsequent shrinkage that occurs during sintering (Figs. 7 and 8). The infrastructures were clinically tried-in to confirm adequate clinical adaptation (Fig. 9).
The infrastructures were mechanically and thermally treated in the laboratory before applying the veneering ceramic. During the mechanical treatment, the external surfaces of the infrastructures were prepared with a ceramic rotatory instrument (JOTA AG Rotary Instruments, Rüthi, Switzerland). Ripple-shaped microretentions were made to improve the adherence between the veneering ceramic and zirconia. Care was taken to maintain uniformity of the surface by making the smallest ripples possible. On the palatal surfaces, the ripples were not performed at the middle cervical third, as this region was not covered by ceramic. The infrastructure was then brought to the furnace, heated to 1050°C at a speed of 50°C per minute, kept at this temperature for 15 minutes, and then slowly cooled to 680°C. Thus, with the heat treatment, internal stresses arising from the mechanical treatment were released, allowing for possible microcracks to be contained by the t → m transformation.11,15
On the metallic abutments, pressed ceramic (VITA PM9; VITA Zahnfabrik, Bad Säckingen, Germany) was used as the overlaying material. The zirconia infrastructure was waxed to the final shape of the teeth. The polished pieces were embedded in a phosphate coating and subsequently pressed with ceramic pellets. After divestment, they were adjusted to the working cast. The press technique was selected for the posterior teeth because it is a faster laboratory process and increases the flexural strength of the elements when compared with the conventional technique. In the anterior teeth, the ceramic (VITA VM9; VITA Zahnfabrik) was applied traditionally. Because there is no sufficient wetting between the zirconia and the veneering ceramic, there was a need to create an interface, applying a thin ceramic layer (process known as wash dentin) to serve as a junction between the infrastructure and the subsequent ceramic masses. Furthermore, it is important that the ceramic is enriched by fluorescent agents. The application of this ceramic was carried out in an irregular manner to favor the dispersion of light, whereas the presence of fluorescence allowed for increased ceramic vitality/brightness in areas of high chromatization (middle, cervical, and interproximal areas). The heating was performed at a higher temperature (∼50° more) than the normal veneering ceramic heating. Dentinal ceramics were applied and a first burning was performed. Then, a translucent ceramic layer was applied over the dentinal ceramics until the final form was obtained. After this last heating cycle, the final shape definition (macrosurface and microsurface morphology) was performed.
After the prosthesis was clinically tried and properly adjusted, the makeup technique (applying pigments with high fluorescence protected by 2 layers of fine ceramics) was performed in the posterior prosthesis and glazing was performed in the anterior region. As the lingual faces of the upper anterior teeth remained in zirconia, without veneering ceramic, the superficial vitrification procedure was accomplished with 2 layers of glaze (VITA Akzent; VITA Zahnfabrik), enabling zirconia to be exposed to the oral environment.22,23 The fixed crowns were cemented with adhesive cement (RelyX U100; 3M-ESPE), and the screw-retained crowns were placed with 10N (Fig. 10).
Zirconia has been one of the materials of choice for manufacturing conventional or implant-supported prostheses. This is due to its potential to be a framework making it suitable for the indication of prosthetic infrastructures and implant abutments.1,3,5,10 However, its aging in a moist condition, known as the low-temperature degradation, has worried researchers because this phenomenon is directly related to zirconia prosthesis failure.11,17 The aging process starts on the surface of zirconia in a slow, gradual, and spontaneous way. A high rate of monoclinic zirconia is observed, leading to destabilization of the material and hence reducing its strength and hardness.16–19 Some mechanisms have been proposed to explain aging in moist conditions, and the most accepted model was proposed by Guo,24 which describes the following phenomena: (1) initial water chemical adsorption on the particle surface, (2) reaction of water with surface oxygen to form hydroxyl groups, (3) penetration of the hydroxyl groups into the material by diffusion across the grain, (4) formation of defective protons by filling the oxygen voids with hydroxyl ions, and (5) t→m transformation caused by oxygen voids concentration reduction. It is known that a large part of controlling the aging of zirconia is related to the control of its microstructure (size, shape and location of grain, type and amount of stabilizer oxide, the presence of defects, and/or oxygen voids within the matrix) and industrial processing (synthetic route used, temperature, and milled processes).2,3,17 However, if clinical and laboratory care is not taken when handling zirconia parts, the benefits of a reliable processing and microstructure can be reduced, favoring the phenomenon of aging.3,17
Surface treatments of zirconia also have been performed routinely in laboratories to improve the bonding interface between zirconia and the veneering ceramic. When realized, mechanical treatments should be followed by heat treatment because there is an inevitable tension introduction to the surface of zirconia, which can promote the growth of subcritical cracks and water penetration, destabilizing the material.2,12 When zirconia is subjected to a slow heat treatment after its mechanical treatment, tensions are released and a new superficial restructuring occurs.23 It is based on the principle that the process of surface modification of zirconia occurs through vitrification that is a process of sealing zirconia surface defects by using an enameling agent (glaze) subjected to heat treatment at a lower temperature than sintering zirconia.22,23 The glazed surface becomes smoother and more uniform, and there is an improvement in its mechanical properties (strength, wear, and fatigue resistance) due to the infiltration of glass compositions into the surface of zirconia. This ceramic film adhered to zirconia also improves chemical durability by stabilizing against thermal and chemical degradation. Considering aging, this fact is positive because it is the glazed layer that contacts the gingival tissue and the moisture and not zirconia.
Therefore, currently, a major concern of professionals (clinician and laboratory technician) has been maintaining surface integrity of zirconia.2,23 From a clinical perspective, this reinforces the need for excellent preparations with adequate occlusal and axial reduction, internal rounded angles, and a clearly defined cervical margin so that the need for adjustments in the prosthetic infrastructure is minimized. In the presence of implants, the areas of tissue conditioning should be perfectly molded because if the emergence profile of the abutment or prosthesis remains in zirconia, the material surface that is in contact with the tissue, despite the biocompatibility of zirconia,19,25 should be kept as smooth as possible, and adjustments in this area should be avoided.1,5 The impression technique proposed in this article provides a good copy of the conditioned area, using the temporary prosthesis that underwent the conditioning of the shape and profile of the gum tissue and that have been in intimate contact with the prepared tissue.17 Besides, when implant crowns are manufactured in zirconia, the infrastructure design and thickness are very relevant. In this case, screw-retained crows were chosen for their reversibility. But, for this prosthetic design, the digital design has to provide an adequate space for the metallic rings that will stay between the infrastructure and the abutment because the screwing of ceramic structures directly on the abutment can generate tension in this area that will lead to ceramic chipping or complete crack.9 The same thing may happen to zirconia customized abutments, and it is very important to consider that its design is even more critical and misfits between the abutment and the implant may result in a possible failure of the whole prosthetic work.7
Ceramic prostheses on implants, made with zirconia infrastructures, must be designed ensuring proper thickness, proper interface preparation for the ceramic cover, and an area of highly polished tissue contact. By this route, the phenomenon of aging of zirconia could be controlled.
The authors claim to have no financial interest, either directly or indirectly, in the products or information listed in the article.
The authors give special thanks to Anadelia Soares (3M-ESPE/Brazil).
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