Year : 2011 | Volume
: 22 | Issue : 1 | Page : 140--143
Zirconium for esthetic rehabilitation: An overview
Anjana Raut1, P Laxman Rao2, T Ravindranath3,
1 Department of Prosthodontics, P.G Student, Army College of Dental Sciences, Secunderabad, Andhra Pradesh, India
2 Department of Prosthodontics, Professor, Army College of Dental Sciences, Secunderabad, Andhra Pradesh, India
3 Department of Prosthodontics, Professor and HOD, Principal, Army College of Dental Sciences, Secunderabad, Andhra Pradesh, India
Department of Prosthodontics, P.G Student, Army College of Dental Sciences, Secunderabad, Andhra Pradesh
The demand for esthetic restorations has resulted in an increased use of dental ceramics for anterior and posterior restorations. A few decades ago, all-ceramic restorations were restricted to treatment in the anterior region, but now all-ceramic restorations can be made anywhere in the dentition. The properties of traditional ceramic materials, however, have limited their use to single crowns; larger restorations have been inadvisable because of insufficient strength. In attempts to meet the requirements for dental materials and improve strength and toughness, several new ceramic materials and techniques have been developed during the past few decades The paper reviews the current literature on dental zirconia with respect to survival, properties, marginal fit, cementation, esthetics and suggests clinical recommendations for their use.
|How to cite this article:|
Raut A, Rao P L, Ravindranath T. Zirconium for esthetic rehabilitation: An overview.Indian J Dent Res 2011;22:140-143
|How to cite this URL:|
Raut A, Rao P L, Ravindranath T. Zirconium for esthetic rehabilitation: An overview. Indian J Dent Res [serial online] 2011 [cited 2022 Oct 4 ];22:140-143
Available from: https://www.ijdr.in/text.asp?2011/22/1/140/79979
Dental ceramic materials exhibit many desirable material properties, including biocompatibility, esthetics, diminished plaque accumulation, low thermal conductivity, abrasion resistance, and color stability.
The popularity of metal-ceramic restorations is largely due to predictable strength achieved with reasonable esthetics. The drawback of such restorations is increased light reflectivity because of the opaque porcelain needed to mask the metal substrate.  All ceramic materials offer an esthetic advantage.  Kelly et al identified core translucency as one of the primary factors in controlling esthetics and a critical consideration in the selection of materials. 
The demand for esthetic restorations has resulted in an increased use of dental ceramics for anterior and posterior restorations. A few decades ago, all-ceramic restorations were restricted to treatment in the anterior region, but now all-ceramic restorations can be made anywhere in the dentition. The properties of traditional ceramic materials, however, have limited their use to single crowns; larger restorations have been inadvisable because of insufficient strength.
In the field of restorative dentistry, zirconia has been used for root canal posts since 1989, for implant abutments since 1995, and for all-ceramic posterior FPDs since  1998. Its versatility and material properties seem to be adequate for use in restorative dentistry. However, no clinical long-term studies on either of the above-mentioned prosthetic indications are available so far.
In attempts to meet the requirements for dental materials and improve strength and toughness, several new ceramic materials and techniques have been developed during the past few decades. The alumina based all ceramic is an example of such a metal-free restorative alternative that has been widely researched. The core is fabricated using a split casting technique from which a porous, partially sintered alumina structure results. Low-viscosity glass is then infiltrated through the porous network of sintered alumina particles.
This results in a high-strength, interpenetrating phase composite structure. Alumina has proven to be an acceptable treatment alternative for single crowns as well as anterior fixed partial dentures (FPD). However, it is not recommended for posterior FPD restorations.
With the introduction of Zirconia, posterior FPDs are feasible. Zirconia is a high-strength ceramic, and it is used as an orthopedic material. Zirconia is reported to have higher flexural strength than alumina. The use of the zirconium-oxide all-ceramic material provides several advantages, including a high flexural strength (1000 MPa) and desirable optical properties, such as shading adaptation to the basic shades and a reduction in the layer thickness (compared to conventional ceramics) of the veneer ceramic required to achieve the desired color. ,
At a 1996 international symposium in Munich, zirconium dioxide, which has long been used in the field of orthopedics for hip transplantation, was considered to fulfill all the criteria for an ideal restorative material in dentistry. 
Zirconium is found in igneous rocks such as schist, gneiss, syenite, and granite. It occurs as oxide baddeleyite, which always has a small amount of hafnium oxide present and as the compound oxide with silica, (Zircon-ZrO 2 .SiO 2 ). Zircon is the most common and widely distributed of the commercial minerals. Important commercial deposits are mined in Australia, India, South Africa, and USA.
Zirconia-based ceramics are composed of zirconium dioxide, partially stabilized with yttrium oxide (ZrO 2 -TZP; 3 mol% Y 2 O 3 ), and exhibit a polymorphic structure with monoclinic, tetragonal, and cubic crystalline phases. The monoclinic is the predominant phase at room temperature and is stable up to 1170ºC, when it transforms into the tetragonal phase. Similar to metal transformations, a tetragonal to monoclinic transformation (t/m) is also likely to occur in zirconia crystals and is called a martensitic-like transformation. This martensitic-like transformation can be induced by externally applied stress and is associated with a volumetric expansion (3% to 5%) of the crystalline structure. As a result, a propagating crack due to tensile stress in the ceramic material will develop a corresponding compressive stress at the tip of the crack, thus preventing continued crack growth.  The yttria partially stabilized tetragonal zirconia polycrystalline ceramics (Y-TZP) have an elastic modulus of approximately 200 MPa, and a flexural strength (FS) of 820MPa. These properties are far higher than those exhibited by other high-strength ceramics. 
External treatments such as grinding or airborne-particle abrasion can induce an external t/m, which, in turn, is likely to increase the mean FS of zirconia-based ceramics. Severe grinding, on the other hand, may introduce deep surface flaws that act as stress concentrators and may decrease the mean FS . ,,
Color and Esthetics
Ceramics have been advocated as the material of choice for matching the natural dentition.  The ability to blend a porcelain crown with its natural counterpart involves consideration of size, shape, surface texture, translucency, and color.  The translucency of dental porcelain is largely dependent on light scattering.  If the majority of light passing through a ceramic is intensely scattered and diffusely reflected, the material will appear opaque. If only part of the light is scattered and most is diffusely transmitted, the material will appear translucent.  The amount of light that is absorbed, reflected, and transmitted depends on the amount of crystals within the core matrix, their chemical nature, and the size of the particles compared to the incident light wavelength. 
For maximal scattering and opacity, a dispersed particle slightly greater in size than the wavelength of light, and with a different refractive index to the matrix, is required. This effect is seen with zirconium oxide, which has a maximal opaquing effect. 
Zirconia-Fixed Partial Dentures
While success has been more promising with 35% partially stabilized zirconia, the opaque core precludes its use for the anterior sextant. Yttrium-stabilized tetragonal zirconiapolycrystal-based materials offer the most versatility because of their mechanical, esthetic, biocompatible, and metal-like radiopaque properties, although only short-term data are available. Furthermore, an emphasis on careful patient selection and operating technique appears to be paramount for success. The system is questionable for bruxers, periodontally involved teeth exhibiting increased mobility, and cantilever prostheses. The primary mode of failure is fracture, usually located in the area between the retainer and pontic, emanating from the gingival surface of the connectors under high tensile stress, resulting in catastrophic loss. An in vitro test evaluating moduli of rupture with a three-point bending test suggests that placing zirconium on the intaglio surface of the pontic and connector area instead of veneering porcelain may increase the load bearing capacity of the FPD up to 10 times. A minimum connector girth of 9 mm 2 has been recommended for three-unit FPDs. 
Design and Manufacture
Zirconium dioxide ceramics are used in dentistry as framework materials for the fabrication of crowns and posterior FPDs. The framework is primarily fabricated by the help of a CAD/CAM system and special software provided by the manufacturers. Cercon smart ceramic system utilizes conventional waxing method for designing the infrastructure for crowns and bridges with specific thickness. A special laser scanner scans the wax pattern and the data are transferred into the computer aided manufacturing unit which is then utilized for milling the framework.
Over the last few years, dental rehabilitation with osseointegrated dental implants has become a well-accepted treatment modality. A serious effort is made to create implants that are more "patient friendly," maintaining at the same time the characteristics giving them high success rates.
It may be speculated that by using zirconia instead of alumina as an abutment material, adverse technical events such as abutment fractures are reduced.
In contrast to alumina, zirconia allows radiographic visualization of the abutment because of its higher radiopacity. On the other hand, depending on the restorative procedures and mucogingival embrasure, the white color inherent to zirconia can result in a too-bright appearance of the final reconstruction. In such cases, the surface can be colored with a corresponding veneering ceramic material to match the natural dentition.
Since the expectations regarding esthetics in dentistry are growing, research in the field of all-ceramic materials for restoration of the natural dentition and dental implants was intensified. Although the crown that abuts the implant may be esthetically optimal, the possibility exists that the grayish color of the titanium implant shines through the thin periimplant mucosa, thus impairing the entire esthetic result.
Today, dental implants and abutments usually are fabricated out of commercially pure titanium because of its well-documented biocompatibility and mechanical properties. However, from an esthetic point of view, titanium abutments may cause an unnatural bluish appearance to the soft tissue.
A ceramic implant could solve these esthetic problems, especially in the anterior region. The ceramic implants used so far (Crystalline Bone Screw 36, Sandhaus, and the Tübingen implant, Friadent) are too brittle and prone to fracture. Their clinical long-term results are questionable. The high-performance ceramic zirconia was evaluated regarding its use as a dental implant material. Zirconium dioxide is very stable and highly biocompatible. YPSZ implants have stress distribution similar to cpTi implants and may be a viable alternative, especially for esthetic regions.
Ceramic transgingival components have been introduced by many manufacturers to provide clinicians with more esthetic abutments than those fabricated from titanium (Ti). However, since these products are generally made of very stiff material such as alumina, they are often affected by unpleasant technologic problems because of their low resistance to bending forces.  Much attention has been recently focused on other ceramic materials commonly used in orthopedics, such as zirconia ceramic, which combines biocompatibility, pleasant esthetics, and impressive resistance to fractures.  Studies have proved that zirconia ceramic may be a suitable material for manufacturing implant abutments, with a low bacterial colonization potential. 
Posts made of tooth-colored material, such as glass fibers or zirconia ceramics, have become popular because they increase the transmission of light within the root and overlying gingival tissues. In addition, the restoration of endodontically treated teeth with metal-free materials eliminates the potential hazards of corrosion and allergic hypersensitivity. 
Zirconia post and core systems were introduced by Meyeberg et al. In vitro studies demonstrated that their strength greatly exceeds that reported for other all ceramic post and cores. In addition, a short-term clinical study has reported a high success rate.
Before cementation, the ZrO 2 post and core often must be reshaped to achieve accurate fit between the material and the prepared root canal. Chairside grinding with diamond burs may affect the mechanical properties of the post. Grinding and airborne-particle abrasion exert a counteracting effect on mechanical properties and reliability of zirconia posts. Composite can be adapted and prepared to the definitive core shape. The chemical bonding of bis-GMA-based composite to zirconia is difficult to achieve, and, therefore, the bond must rely on macro retention only. Improved results in vitro have been reported with glass ceramics as a core material. In this situation, the lost wax technique is used, and glass ceramic (IPS Empress Cosmo) is heat pressed over the zirconia post. The glass ceramic contains 15 wt% ZrO 2 , and good adhesion between glass ceramics and ZrO 2 posts has been reported.
Cementation and Bonding
The ceramics with high crystalline content (aluminum and/or zirconium oxides) have demonstrated better clinical results than feldspar-, leucite-, and lithium disilicate-based ceramics. In fact, increasing the mechanical strength, by increasing the crystalline content and decreasing the glass content, results in an acid-resistant ceramic whereby any type of acid treatment produces insufficient surface changes for adequate bonding to resin. Hydrofluoric acid attacks the glass phase producing a retentive surface for micromechanical bonding, and the silane coupling agent promotes a chemical bond between the silica of these ceramics and the methacrylate groups of the resin. The silica coating process is based on similar adhesive principles and appears to be a promising method for treating high crystalline ceramics.
Surface treatments including a tribochemical silica coating process (Rocatec; 3M ESPE), airborne-particle-abrasion with either 250 μm or 50 μm aluminum oxide, airborne particle abrasion with 50 μm aluminum oxide combined with 38% hydrofluoric acid etching or diamond abrasion with a rotary cutting instrument were reported to have only a minor influence on bond strength to zirconia ceramic. Zirconia-based restorations do not require an adhesive interface for retention. Along with the strength of the material, the cementation technique is also important to the clinical success of a restoration. Due to their high fracture resistance, zirconium-oxide crowns and FPDs can be cemented using conventional methods recommended by the manufacturers.
However, resin bonding between a tooth and the restoration is advocated for improving the retention, marginal adaptation, and fracture resistance of restorations. ,
In comparison to Al 2 O 3 ceramics, ZrO 2 ceramics usually have twice the flexural strength and fracture toughness. The main reason for the superior fracture strength of ZrO 2 ceramic lies in the metastable tetragonal crystalline structure at room temperature. This structure represents an efficient mechanism against flaw propagation. In contrast, ZrO 2 ceramic exhibits thermal conductivity values 1 to 10 times lower than Al 2 O 3 ceramic. Therefore, ZrO 2 abutments should be more at risk to fail than Al 2 O 3 by heat-producing surface treatments that produce high-temperature spots because of the very low heat dissipation of the material. In addition, grinding ZrO 2 ceramic induces surface flaws and micro cracks, which influence the fracture resistance of the material.
The strength and fracture toughness have often been the first parameters investigated to understand the clinical potentiality and limits of any dental ceramic. Therefore, zirconia has been developed with the aim of providing a stronger and tougher material to the dental speciality. This paper recommends that zirconia can be successfully selected as a core material for fixed prosthodontic restoration of both anterior and posterior teeth as it is well supported by documented clinical evaluation.
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