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A review on zirconium dioxide used in dentistry.


In 2004 Kelly stated ceramics as "composite," means a composition of two or more distinct entities. He proposed the most simplified way of organizing the panorama of all ceramic systems as predominantly glassy materials, particle filled glasses, polycrystalline ceramics. Zirconium oxide comes under polycrystalline ceramics. These ceramics contain materials with densely packed particles and no glossy components [1]. Polycrystalline ceramic cores are glass free, the lower the glass content the better the mechanical properties. E.g. flexural strength and higher fracture toughness. In contrast the optical properties, e.g. translucency of these materials are diminished [2].

Forms of zirconia used in dentistry

Glass infiltrated alumina with partially stabilized zirconia (GIAZ): the inceram system combines the use of glass infiltrated alumina with 35% partially stabilized zirconia as the core material for posterior crowns and fixed partial dentures (fpd's). The flexural strength of the core material ranges from 421Mpa to 800 Mpa and the fracture toughness ranges from 6 Mpa/[m.sup.1/2] and 8 Mpa/[m.sup.1/2] slip casting technique or cad cam technology may be used the infrastructure fabrication. However in terms of translucency, the GIAZ core demonstrated high opacity. Y-TZP cores are used to successfully mask underlying discolored abutments, metallic cores,metal alloy implant abutments. According to sintering fully sintered, partially sintered, green state zirconia are in use [2]. Soft-machining of partially sintered zirconia ceramic blocs by CAD/CAM technology, to produce dental restorations was proposed in 2001 after intensive research work [3,4].


As opposed to metal-ceramics, ceramics containing Zirconia contain a significantly greater amount of crystalline phase, from about 35 to about 99 vol %. This higher level of crystallinity is responsible for an improvement in mechanical properties through various mechanisms, such as crystalline reinforcement or stress-induced transformation. Unfortunately, higher crystallinity is also associated with higher opacity, which is not always desirable for dental ceramics. As an example, zirconia ceramics such as 3Y-TZP (3 mol % Yttria-stabilized Tetragonal Zirconia Polycrystals) offer unsurpassed mechanical properties but are also the most opaque of all-ceramic materials currently available [5].

However, crystallinity is only one of many intrinsic factors contributing to materials performance. Other factors such as crystal size and geometry, modulus of elasticity, phase transformation and thermal expansion mismatch between crystal and glassy phase play a crucial role in determining the final mechanical response of the zirconia [6].

It should also be kept in mind that when it comes to zirconia, extrinsic factors such as working conditions play a major role in the long-term performance of the material. The oral environment assembles a set of challenging working conditions that include humidity, acidic or basic pH, cyclic loading and peak loads that can reach extremely high levels when hard objects are accidentally encountered during mastication. A humid environment is susceptible to lead to stress corrosion and catastrophic failure in ceramic materials including a glassy phase [7]. The same is true for some highly crystalline materials such as 3Y-TZP, which has been shown to undergo microstructural degradation in a humid environment at relatively low temperatures [8-10]. It is therefore generally accepted that tests performed in a humid environment and under cyclic loading are needed to provide valuable information on the longterm performance of dental ceramics [11]. Y-TZP (yttrium stabilized tetragonal zirconia polycrystals) a type of stabilized zirconia, core material flexural strength is 900-1200 Mpa and fracture toughness is 9-10 Mpa/[m.sup.1/2] [12].

Transformation toughening

The remarkable mechanical properties of zirconia, already exploited in several medical and engineering applications, are mainly due to the tetragonal to monoclinic (tm) phase transformation. The tm transformation, which can be induced by external stresses, such as grinding, cooling and impact, results in a 4% increase of volume that causes compressive stresses. These stresses may develop on a ground surface or in the vicinity of a crack tip. It is this clamping constraint about the crack tip that must be overcome by the crack in order to propagate, explaining the increased fracture toughness of zirconia compared to other ceramics. Transformation toughening can occur when the zirconia particles are in the metastable tetragonal form, and on the verge of transformation. The metastability of the transformation is dependent on the composition, size, shape of the zirconia particles, the type and amount of the stabilizing oxides, the interaction of zirconia with other phases and the processing [13]. Micro crack toughening, contact shielding and crack deflection, can also contribute, to a different degree, to the toughening of the ceramic [14-19].

This technique relies on a crystal structure change under stress to absorb energy from cracks. It involves the incorporation of a crystalline material that is capable of undergoing a change in crystal structure when placed under stress. The crystalline material usually used was zirconium oxide. At sintering temperature zirconia is a tetragonal form, and at room temperature it will be in monoclinic form. The monoclinic form occupies about 4.4 vol % more than the tetragonal form. This monoclinic phase is unstable at room temperature. Stabilization can be achieved by adding a small amount of (3-8 mass %) of calcium and yttrium. When the stress is localized, any area on this material is sufficient to transform the grains in the vicinity to a monoclinic stage. The volume increase of 4.4% squeezes the crack closed. These are the potential substructure material for posterior crowns and FPDs [12]. The design compensates for the volume shrinkage that will later occur during sintering of the zirconia blocs (about 25%). The partially sintered blocs are easy to mill, which leads to substantial savings in time and tool wear. The type of zirconia used in this technology is biomedical grade tetragonal zirconia stabilized with 3 mol % yttria (3YTZP) [20]. Unalloyed zirconia is monoclinic at room temperature and tetragonal above 1170[degrees]C [21].

The tetragonal to monoclinic transformation (tm) is associated with a substantial volume increase (~4.5%). The high temperature tetragonal form can be stabilized at room temperature by addition of various oxides, including yttria, ceria, calcia or magnesia [22-23]. Of interest is the fact that the stability and therefore the mechanical properties of 3Y-TZP strongly depend on its grain size [24,25]. Above a critical grain size, 3YTZP is less stable and more susceptible to spontaneous transformation while smaller grain sizes are associated with a lower transformation rate [26,27].

Grain size is determined by the sintering conditions and particularly the sintering temperature and duration. Higher temperatures and longer durations lead to larger grain sizes. Currently available 3Y-TZP ceramics for soft machining of dental restorations require sintering temperatures varying from 1350 to 1550[degrees]C and durations from 2 to 6 hours, depending on the manufacturer. These differences could account for slight differences observed in the mechanical properties of the final product. Nevertheless, 3Y-TZP ceramics for dental restorations offer to date the best mechanical properties of all-ceramic core materials currently available. It should be noted, however, that problems such as crazing or cracking at the interface between veneering porcelain and core material have been reported clinically [28-31].

Mechanisms involving destabilization of the tetragonal phase at the interface with the veneering porcelain have been proposed [32]. Studies evaluating the three-year and five-year performance of 3Y-TZP fixed partial prostheses in vivo have recently been published [33-35]. These studies point out an excellent success rate but a lower survival rate due to complications such as secondary caries and chipping of the veneering ceramic [33].

However, the overall excellent performance of 3Y-TZP restorations processed by the soft machining technique, followed by sintering, has led to its extension to alumina-based ceramics and its combination with other processing techniques such glass-infiltration and heat-pressing.

Some all-ceramic systems can provide superior esthetic results compared with metal-ceramic restorations. Zirconia-cored crowns are the strongest all-ceramic system and may provide improved esthetic results compared with metal-ceramic crowns. No all-ceramic restoration has been shown to have a life span equivalent to that of metal-ceramic restorations. Further clinical trials are needed [36].

When considering use of a zirconia-cored restoration, clinicians should understand that all zirconia materials are not the same. A system that mills the zirconia core in the softer "green" state and then sinters it is superior to one that mills the core in the sintered state. This is because the latter requires a robust milling machine and high-temperature milling that will result in nearsurface damage and defect formation, which will significantly shorten the anticipated life span of the restoration. Milling in the green state followed by sintering allows lower-temperature milling, and the sintering "heals" any milling-induced defects. [36,37].

Factors to be considered when planning for a zirconium Restoration

Esthetic Factors: Space requirements for workability and maximum esthetics: 1.2-mm minimum working thickness and 1.5 mm ideal if masking.

Substrate: It is generally understood and accepted that predictable and high bond strengths are achieved when restorations are bonded to enamel, given the fact that the stiffness of enamel supports and resists the stresses placed on the materials in function. It is equally understood that bonding to dentin surfaces as well as to composite substrates is less predictable given the variability and flexibility of these substrates. The more stress placed on the bonds between dentin and composite. Substrates and the restoration, the more damage is likely to occur to the restoration and underlying tooth structure. Therefore, because enamel is significantly stiffer than either dentin or composite and much more predictable for bonding, it is the ideal substrate for bonded porcelain restorations. Substrate is not critical because the high-strength core supports the veneering material.

Flexure risk assessment: High or below. For high-risk situations, the core design and structural support for porcelain become more critical. Each tooth and existing restoration is evaluated for signs of past overt tooth flexure. Signs of excessive tooth flexure can be excessive enamel crazing, tooth and restoration wear, tooth and restoration fracture, microleakage at restoration margins, recession and abfraction lesions.

Tensile and shear stress risk assessment: High or below. Note: For high-risk situations, the core design and structural support for porcelain become more critical. Preparations should allow for a 0.5- mm core plus 1 mm of porcelain to ensure the best esthetic results. There should not be more than 2 mm of unsupported occlusal or incisal porcelain; the restoration core should be built out to support marginal ridges. For higher-risk molar regions, it is more ideal to use zirconia cores vs. alumina cores, rovided the current firing parameters are followed. Fullcontour zirconia restorations (E.G. Prettau Zirconia, Zirkozahn, BruxZir) have been recommended for high-risk molar situations. Failure would not be an issue; some preliminary concern involves wear of the opposing dentition with full-contour zirconia. No clinical data could be found to confirm or refute this. Clinically, only full-contour zirconia against full-contour zirconia in the molar region should be considered when no other clinical option is viable. If a high-stress field is anticipated, stronger and tougher ceramics are needed; if porcelain is used as the esthetic material, the restoration design should be engineered with such support (usually a high-strength core system) that it will redirect shear and tensile stress patterns to compression. To achieve that, the substructure should reinforce the veneering porcelain by using the reinforced porcelain system such as high strength ceramics or metal ceramic system.

Bond/seal maintenance risk assessment: If the risk of obtaining or losing the bond or seal is high, then zirconia is the ideal allceramic to use. High-strength ceramics (specifically zirconia) is indicated when significant tooth structure is missing, an unfavorable risk for flexure and stress distribution is present, and it is impossible to obtain and maintain the bond and seal (E.G. most posterior full-crown situations with sub gingival margins) [38].

If a patient present with compromised interocclusal distance and inadequate resistance and retention form the use of a metal ceramic restoration with metal on the occlusolingual surfaces should be considered. When patients present with parafunctional habits the use of zirconia restorations must be carefully evaluated. If patient insists on being restored with a metal free restoration the highest strength core material should be selected with optimal preparation and core design. An Occlusal guard is recommended for that patient [2].

Zirconia as an implant abutment

Zirconia can be used as an implant abutment material [39]. Although zirconia abutments presented values of fracture strength not as good as conventional titanium abutments they are indicated in aesthetically compromised areas. On the other hand these abutments revealed a good adjustment in the interface with dental implants, excellent biocompatibility and good aesthetical appearance, especially in patients with unitary rehabilitations over implants with a thin gingival biotype [40].

Success and failure rates of zirconia prostheses

In general, restoration success is defined as the demonstrated ability of a restoration (including a prosthesis) to perform as expected. Restoration failure may be defined as any condition that leads to replacement of prosthesis. Conditions that constitute restoration failure include secondary caries, irreversible pulpitis, excessive wear of opposing tooth surfaces, excessive erosion and roughening of the ceramic surface, ditching of the cement margin, unacceptable esthetics, cracking, chipping fracture, and bulk fracture. How should we define clinical success and why is it important to define clinical success from a dental materials perspective? Success may be defined as the intact survival of a prosthesis with acceptable surface quality, anatomic contour, and function, and where applicable, with acceptable esthetics. The CDA system was set up to evaluate restoration quality, but it also covered 14 other components of dental care including history and clinical examination, radiographic examination, diagnosis, treatment planning, and all other aspects of clinical dentistry. The success rate of the zirconia frameworks was 97.8%; however, the survival rate was 73.9% because of other complications. Secondary caries was associated with 21.7% of the FPDs, and chipping of the veneering ceramic occurred in 15.2% of the prostheses. Surprisingly, the authors concluded that zirconia offers "sufficient stability as a framework material" for 3- and 4-unit posterior FPDs in spite of chipping fractures in 15% of the FDPs over the five-year period. The three grades of chipping fractures proposed by Heintze and Rousson represent a simple and practical way to express the severity of these fractures. However, unless specific criteria are proposed to determine when a fracture surface should be polished or when it should be repaired, significant variability will occur. Thus, the following criteria for replacement (Grade 3 fracture) are proposed.

1. The fracture surface extends into a functional area and repair is not feasible.

2. Recontouring will result in a significant unacceptable alteration of the anatomic form from the original anatomy.

3. Recontouring will significantly increase the risk of pulp trauma by the generation of heat.

4. Repair with a resin composite will result in esthetic changes that are unacceptable to the patient [41-47].

With the success of zirconia as a dental restorative material, manufactured 3Y-TZP abutments and implants have recently been introduced on the dental market. A recent review on ceramic dental implants concluded that the currently available clinical data is "not sufficient to recommend ceramic implants for routine clinical use" [48]. Concerns with using 3Y-TZP dental abutments and implants rely on the fact that the material is in contact with biological fluids. As mentioned previously, it is well established that zirconia is susceptible to low temperature degradation [49-51]. Low temperature degradation of 3Y-TZP involves microstructural changes such as grain pull-out, micro-cracking and surface roughening [52-54].

The ISO standard for Y-TZP implants recommends that the amount of monoclinic phase after accelerated aging for 5 hours be less or equal to 25%, as determined by X-ray diffraction [55]. It has also been demonstrated that surface finish and residual surface stresses strongly influence the response of 3Y-TZP to low temperature degradation [56]. It was pointed out that a significant amount of surface roughening and damage can occur in Y-TZP, even in materials containing less than 25% monoclinic phase after aging for 5 hours. Careful consideration should be given to the use of zirconia for dental abutments, particularly since dental abutments undergo some degree of loading through tightening of a metal abutment screw. Perhaps, at the very least, the properties of Y-TZP dental implants should be considered in light of the established ISO standard for Y-TZP biomedical implants [55].

Meanwhile, a considerable amount of research is being conducted with the aim of developing ceramics for dental and biomedical applications with improved reliability [56-59]. This effort has led to the successful production of zirconia/alumina ceramic composites, consisting of either zirconia-toughened alumina (ZTA) or alumina-toughened zirconia (ATZ), depending on the proportion of the main component. These advanced composites exploit the transformation toughening capabilities of zirconia while being less susceptible to low temperature degradation in biological fluids. Ceria-stabilized zirconia/alumina nanocomposites for dental applications have been shown to exhibit high flexural strength (1422 [+ or -] 60 MPa), high reliability and an excellent resistance to low temperature degradation [60,61]. Further research is needed to evaluate the long-tem in vivo performance of these composites in the oral environment.

Shade selection of zirconia

During the shade selection of natural tooth, the gingival third and body of the natural tooth are evaluated for the 'opacity'. Heffernan et al suggested that, More opaque, high value teeth could be restored with more opaque substrates such as In-Ceram alumina or In-Ceram Zirconia. Newer high strength oxide based ceramics (e.g., Densely sintered alumina Procera[R]) can produce a core with a different colour ('white' and 'translucent') and thickness (standard 0.6 mm, thin 0.4 mm) with different optical properties. This variation can be helpful to mask the discolored teeth, or to deal with the minimal occlusal clearance, without compromising the strength of the ceramic material. Y-TZP based material, when used as substructure for Crowns or Bridges, can be colored into one of seven shades (Vita-Lumen shade guide) after milling. The ability to control the shade of the core can also eliminate the need to veneer the lingual and gingival aspect of the connectors in cases with interocclusal limited clearance. Furthermore, the palatal aspect of anterior crowns and bridges can be fabricated from core material only.

Finishing and polishing: in zirconia-based ceramics, grinding can increase the strength of ceramics to achieve the proper fit and occlusion [62]. In reality, removal of the glazed surface of ceramics can cause the unfavorable secondary impact on opposing teeth. Glazing or reglazing is the most accepted method of sealing surface roughness. But recent studies have suggested that a polished surface can also seal the rough surface of ceramic and control the surface luster. A review of ceramic polishing concludes that an adjusted surface can be reglazed or sequential finishing and polishing using Shofu porcelain veneer kit [63,64].

Four stages of finishing: Hybrid points with fine grade-15 micron Dura-white stones Ceramiste silicone rubber points Ceramiste silicone rubber cups with fine diamond polishing paste - Westone Diglaze) [65].

Cementation: Polycrystalline ceramics cannot be etched due to lack of glass in their microstructure. They are cemented using conventional and adhesive cementation techniques other than the one used for glass ceramics is optional. For cement retained implant supported restorations, either with or without a ceramic abutment, high strength ceramic are the material of choice. Definitive cementation should be done [2].

Zirconium oxide ceramics don't contain a specific group to bond to siliniziation agents. Therefore, zirconia has to be sandblasted or coated with particles. Through this treatment with tribochemical reaction, the surface of zirconia is coated with a small particle of "Silicium oxide". This can bind well to silinization agents and establishes chemical bonding to the adhesive resin cement. Kern et al. showed phosphate-modified resin cement had good bond strength to Zirconium oxide ceramic after airborne particle abrasion [66-68].


One problem that has plagued almost all trials of zirconia-cored restorations has been a relatively high rate of chipping of the veneering ceramic. Laboratory data suggest that restorations are more prone to chipping if the thickness of the veneer is inappropriate in relation to the core thickness especially if the veneer material in the cusp area is unsupported by the core. Sandblasting of zirconia can weaken the core material by inducing cracks.

Zirconia based FDP's exhibited more chipping of the veneer than PFM restorations. The frequency of chipping is independent of veneering material. The frequency of chipping is more in FPD's with more than 3 units than in FPD's with 3 units. Chipping is high in FPD's with unsupported cusps [69].

The fracture rate of the veneering ceramic has ranged from 8 to 50 percent at one to two years in these trials, while the reported rate of veneer fracture with metal-ceramic restorations has been between 4 and 10 percent after 10 years. The cause of this chipping is not known, and both core flexure and bond failure have been suggested. Another possible cause of chipping is the lack of uniform support of the veneering ceramic by the core. A well-established principle for metal-ceramic restorations is that the metal core should support a uniform thickness of veneering ceramic and that there should be a maximum of 2 mm of unsupported porcelain. This is accomplished with an anatomical-contour wax-up and controlled cutback [70-75].


The technological evolution of dental ceramics has been remarkable over the past four decades. From feldspathic porcelains to zirconia-based all-ceramics, tremendous progress has been made in terms of mechanical performance, with a tenfold increase in flexural strength and fracture toughness. Common important characteristics of all-ceramic systems, such as the proportion of glassy phase and amount of porosity, both influence optical and mechanical properties. Residual stress states between crystalline phases and glassy matrix, as well as microcracking also play a key role in the development high strength ceramics. The two most recently introduced all-ceramic systems (hard machined lithium disilicate and soft machined 3Y-TZP) are excellent examples of successful material development to match specific requirements of dental restorations.


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Sriharsha Babu Vadapalli (1), Sunil Chandra Tripuraneni (1), Kaleswararao Atluri (1), N. Suman Kumar (2)

(1) Department of Prosthodontics, Drs. Sudha and Nageshwara Rao Siddhartha Institute of Dental Sciences, Vijayawada, Andhra Pradesh, India

(2) Department of Prosthodontics, CKS Teja Institute of Dental Sciences, Tirupati, Andhra Pradesh, India

Received 24 May 2016; Accepted 6 July 2016; Published online 14 September 2016

* Coresponding author: Dr. Sriharsha Babu Vadapalli

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Author:Vadapalli, Sriharsha Babu; Tripuraneni, Sunil Chandra; Atluri, Kaleswararao; Kumar, N. Suman
Publication:Trends in Biomaterials and Artificial Organs
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Date:Apr 1, 2016
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