Zirconia in dentistry characteristics, uses, benefits and drawbacks

Zirconia as a Dental Biomaterial

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Ceramics are very important in the science of dental biomaterials. Among all dental ceramics, zirconia is in evidence as a dental biomaterial and it is the material of choice in contemporary restorative dentistry. Zirconia has been applied as structural material for dental bridges, crowns, inserts, and implants, mostly because of its biocompatibility, high fracture toughness, and radiopacity. However, the clinical success of restorative dentistry has to consider the adhesion to different substrates, which has offered a great challenge to dental zirconia research and development. 

The most popular dental ceramic systems are silica-, leucite-, lithium disilicate-, alumina-, and zirconia-based materials. Currently, zirconia-based ceramics are the most studied, challenging researches for different reasons.

Characteristics of Zirconia

Zirconia (zirconium dioxide, ZrO2), also named as “ceramic steel”, has optimum properties for dental use: superior toughness, strength, and fatigue resistance, in addition to excellent wear properties and biocompatibility.

Zirconium (Zr) is a very strong metal with similar chemical and physical properties to titanium (Ti). Incidentally, Zr and Ti are two metals commonly used in implant dentistry, mostly because they do not inhibit the bone forming cells (osteoblasts), which are essential for osseointegration .

Dental zirconia is, most often, a modified yttria (Y2O3) tetragonal zirconia polycrystal (Y-TZP). Yttria is added to stabilize the crystal structure transformation during firing at an elevated temperature and improve the physical properties of zirconia.

 Upon heating, the monoclinic phase of zirconia starts transforming to the tetragonal phase at 1187 °C, peaks at 1197 °C, and finishes at 1206 °C. On cooling, the transformation from the tetragonal to the monoclinic phase starts at 1052 °C, peaks at 1048 °C, and finishes at 1020 °C, exhibiting a hysteresis behavior. The zirconia tetragonal-to-monoclinic phase transformation is known to be a martensitic transformation . 

 Zirconia Structures for Dentistry

Zirconia structures used for dental purposes are fabricated using CAD-CAM (computer-aided design and computer-aided manufacturing) technology in two possible ways. One method mills the fully sintered block of zirconia with no distortion (shrinkage) to the final structure. The disadvantages are the great wear of the grinding tools (burs) and the population of flaws produced during the machining that may lower the mechanical reliability of the structure . In the other method, the zirconia structure is milled from a pre-sintered block, reaching its final mechanical properties after sintered, which produces structural shrinkage that can be partly compensated at the designing stage, and the fit of the zirconia restoration will be warranted . Both CAD-CAM processes have three main steps: acquisition of digital data, computer processing and designing, and fabrication of the zirconia structure . Most importantly, the CAD-CAM technique has the ability to produce zirconia restorations with sufficient precision for dental use .

Traditionally, zirconia is dull white in color and its opacity can mask the underneath structure. Most dental zirconia systems indicate structural dyeing (coloring) to enhance the esthetic . Currently, full-contoured (anatomical-shaped) monolithic zirconia dental restorations are offered , which could abbreviate or extinguish the dental laboratory work on zirconia based restorations. 

 the most popular zirconia-based restorations have a zirconia infrastructure that is porcelain veneered to adequate anatomic contour and esthetic. process . In addition, primers and liners have been suggested to improve wetting and bonding to zirconia 

The mismatch in some mechanical and thermal properties such as fracture toughness, flexural strength, coefficient of thermal expansion, and elastic modulus affect the bonding between porcelain and zirconia hasized the effect of strength misfit on the development of delamination, showing that the mode of failure changes according to the porcelain strength and suggesting the use of veneering material with a high flexural strength (over 300 MPa) to improve the reliability of zirconia-based restorations .

Significant differences on the coefficient of thermal expansion between the zirconia and porcelain influence in the residual stress distribution during the cooling process affecting the reliability of zirconia-based restorations . The veneering porcelain will experience a change from a viscoelastic state to a solid form when its temperature is reduced and when it passes through the glass transition temperature (Tg from 480 °C to 610 °C) . During cooling after sintering, residual stresses might be 

The initiation and propagation of delamination was also reported to be related to the misfit in elastic moduli and fracture toughness of porcelain and zirconia .

Pressable veneering porcelains were thought to improve bonding between porcelain and zirconia, but studies are controversial on this matter and most of them showed no significant difference between the traditional and pressing methods . New veneering methods using CAD-CAM technology seem to improve wetting and bond strength between zirconia and porcelain. 

Resin Bonding to Zirconia

At first, one could imagine that an all-ceramic restoration would not withstand the intra-oral service. It could be true if the restoration would not be bonded to the tooth structure or remaining restorative materials (e.g., composites and metals), working as an integrated system where diverse stresses, from chewing to parafunctional habits (e.g., bruxism), are distributed throughout the system due to appropriate bonding  This rationale is supported by the ISO 6872:2010  standard that classifies the ceramics according to the intended clinical use and made the distinction between adhesively and non-adhesively cemented restorations.

Today, glass ionomer (GIC) and resin-based cements are the primary choices for bonding ceramic restorations to the remaining tooth structure. GIC and resin-modified GIC (RMGIC) are often used to cement acid-resistant ceramics, mostly because these cements are very easy to use. However, the most popular and effective cements for all types of ceramic restorations are the resin-based composites, including the systems containing the 10-methacryloyloxydecyl-dihydrogen-phosphate (MDP) monomer .

Properties of Zirconia

the most important components of esthetic tooth appearance are: color, fluorescence, opalescence and translucency. One major drawback of full contour zirconia restorations is their opacity .

Translucency

Lately, colored zirconia with improved translucency  has been developed to closely match colors of human teeth. The flexural strength of this new material is 900-1400 MPa and has a fracture resistance of up to 6 MPam 1/2. These conveniences have made it the zirconia used more and more for crowns and bridges inlateral applications . For excellent esthetics, it is important to reproduce the translucency of the natural tooth, as it provides an enchanted natural appearance of the prosthetics. Translucency is the substance property that allows the passage of light and its dispersion, and then the objects will not be seen clearly through the material. This property could be defined as a state between transparency and complete opacity . Translucency can be regulated by controlling the absorption, reflection, and transmission of light through the material. The translucency is higher when the reluctance is low and the transmission is high . In a few studies it has been reported that translucency is affected by the layer’s thickness and by the grain size .

X-ray opacity

The dental restorative materials have different degrees of opacity which provides helpful information for diagnose. The zirconia can be added in dental filling composite materials because it represents an X-ray opaque agent. In a study conducted in order to assess the X-ray opacity of zirconia, four different materials (pure titanium, NANOZR, Y-TZP, alumina plates) with the same thickness (0,2-2mm) and human tooth wermateriad against an X-ray agent. It has been found that Y-TZP and NANOZR showed increased opacity .

Wear behavior

Many studies conclude that there seems to be an agreement that polished full zirconia crowns have the lowest mean weight loss values of the antagonistic human enamel and this strongly related to its very smooth surface that increases its biologic compatibility and lowers its abrasiveness  and results in a decreased level of antagonistic wear than other ceramics . After glazing and polishing and one final glazing zirconia presented important opposing enamel wear, and while the veneering ceramic exhibit even more important wear on the antagonist . However further clinical studies are required to support the results of the in vitro testing.

uses of zirconia in dentistry

Zirconia-Based Dental Posts

The requirement for more esthetic posts, especially under 

all ceramic restorations, has started the development of new post materials . In situations where all-ceramic restorations are used for restoring anterior teeth, metal posts may result in unfavorable esthetic results, such as a grey discoloration of translucent all-ceramic crowns and the surrounding gingival margin . Additionally, corrosive reactions with prefabricated posts may cause complications involving the surrounding tissues and oral environment, including a metallic taste, oral burning, sensitization, oral pain, and other reactions . These concerns have led to the development of white or translucent posts made of zirconia and other ceramic materials.

Zirconia-Based Crown and Bridge

The fabrication of zirconia frameworks of either pre-

sintered or highly isostatic pressed zirconia for crown and bridge has also been employed .

Zirconia frameworks offer new perspectives in metal free fixed partial dentures and single tooth reconstructions because of zirconium’s high flexural strength of more than 900 MPa and showed good first clinical results .

Zirconia-Based Implant Abutments

As a result of utilizing the zirconia ceramics for the fabrication of tooth-supported restorations, this encouraged the clinicians to extend its application for implant-supported restorations .

Utilizing zirconia as implant-supported restorations isdue to the higher toughness and the lower modulus of elasticity of zirconia. In stabilized and transformation-toughened forms, zirconia provides some advantages over alumina in order to solve the problem of alumina brittleness and the consequent potential failure of implants . These abutments are distinguished by their tooth-matched color, their good tissue compatibility, and their lower plaque accumulation

Zirconia dental crown benefits

zirconia crown preperation

Crowns made of zirconia are becoming increasingly common, and they do offer some advantages.

Strength

One of the biggest advantages of zirconia is its strength and durability. Consider how much force your back teeth exert on the food that you chew.

Your crowns need to be made of a strong material, so zirconia may be a good choice for crowns in the back of your mouth. Also, because zirconia is so strong, a dentist won’t have to do as much preparation of your tooth.

Longevity

Zirconia-based crowns fared just as well over the course of 5 years as metal-based crowns, according to a 2017 randomized controlled trial published in the Journal of Dentistry. And crowns made of zirconia, called monolithic zirconia crowns, are especially durable.

Biocompatibility

Zirconia is the choice of many dentists for its biocompatibility, which means it’s less likely to provoke the body into producing a reaction or immunological response like inflammation.

A 2016 in vitro studyTrusted Source confirms this, and it also found only a limited amount of cytotoxicity.

Same-day procedure

Many dentists can make zirconia crowns in their offices rather than sending an impression of your tooth to a lab to have a crown made. Then, they can cement the crown into your mouth in a single visit.

The CEREC, or Chairside Economical Restoration of Esthetic Ceramics, process uses computer-aided design/computer-aided manufacturing (CAD/CAM) technology to speed up this process. The dentist uses a dental milling machine to actually make the crown from a block of zirconia.

This process eliminates the need to stretch the procedure into two visits. However, not every dentist office has this technology in-house or offers zirconia crowns.

Disadvantages of having a zirconia crown

Can be hard to match

One potential disadvantage of a zirconia crown is its opaque appearance, which can make it look less than natural. This is especially true for monolithic zirconia crowns, which are made just from zirconia, although it may be less of an issue for teeth in the back of your mouth.

Potential wear on other teeth

Some dentists have been hesitant to use zirconia crowns in some circumstances for fear that the hardness of the zirconia could cause wear and tear on opposing teeth.

While that may be a concern, a 2012 study in the Journal of Dentistry found that feldspathic porcelain was much more likely than zirconia ceramic to cause wear on the enamel of opposing teeth.