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Advancements in the physical and mechanical characteristics of dental materials today are unquestionably due to various modifications that have been made to those already in existence. As a result, many materials may be limited to use for specific clinical applications and contraindicated for others. With little time to wade through studies and promotional publications enumerating their use, many dentists are left desperately sorting out the practicalities and ideal indications of each ceramic material. Additionally, there exists ample inaccurate information about the factors affecting their clinical use.1
Dental ceramics are an inorganic structure consisting of metallic and semi-metallic elements and oxygen compounds.2 B ecause ceramics can contain an almost limitless variety of materials, it is difficult to attribute an inclusive definition of elements to this dental restorative. Because of their varied composition, they are suitable for an array of applications.2 I nherently, ceramic materials are brittle and exhibit high compressive strength and low tensile strength, rendering them prone to fracture3 y et abrasive to antagonist teeth and their supporting structure.4 O f their many advantages, including dimensional stability and high wear resistance,4 t he esthetic properties of ceramic materials have been relied upon for years. Ceramic compositions exhibit a similarity to natural dentition. Ceramic materials that most closely resemble natural teeth consist of high glass content and a small quantity of filler particles to control color and other optical effects, including opalescence and opacity.5
Dental ceramics have undergone numerous modifications over the years. Initially, the low fracture toughness of ceramic materials was a major disadvantage. However, with advances in processing techniques, mechanical properties, bonding procedures, and restoration methods, the strength and fracture toughness of dental ceramics has been largely improved.3,6 W hile at one time ceramics had limited use in the posterior segment, today’s restorative indications have been expanded to include crowns, veneers, implant-supported restorations, implant abutments, and fixed partial dentures.6,7 W hen choosing a ceramic, such factors as material characteristics, mechanical properties, bonding technique or luting cement, and the amount of tooth preparation must be considered.8
There are several dental ceramics each characterized by different material properties and all suited to different indications. The various classifications of ceramic materials contribute to the confusion surrounding their characteristics and appropriate use. Ceramics can be classified by microstructure, processing technique, or clinical application.9 F or the purpose of this article, they will be divided into four categories: powder/liquid ceramics, glass-based pressed and machinable materials, high-strength crystalline ceramics, and metal-ceramics.
Powder/Liquid Ceramics
Powder/liquid feldspathic porcelains tend to be the weakest ceramics but are the most translucent and can be used for minimally invasive treatments.9 D emonstrating less than a 1% fracture rate, this ceramic has proven successful when distinct parameters are met.10 D ue to the flexibility of dentin, using a ceramic with low fracture resistance is not recommended for restorations with less than 70% enamel. Flexure risk increases when powder/liquid materials are bonded to dentin,1 a nd the material’s restorative strength decreases particularly with bruxing, overbite or overlapping, bonding to composite or dentin substrates due to their flexibility, and lack of ceramic support.1
Conventional powder/liquid ceramics are a combination of glass and crystal, or are all glass and used for veneering alumina-based cores, for porcelain veneers, or formulated into machinable blocks.9,11 T echnique- and parameter-sensitive when used for posterior restorations, in most cases this material is generally recommended for anterior teeth only. To maintain esthetics, shade changes require 0.2 mm to 0.3 mm of space per shade.1 R isk of bond failure is generally low, but long-term maintenance is required to sustain a successful bond.1 T he same materials in machinable blocks tend to outperform those in powder/liquid form.9,12,13
Glass-Based Pressed and Machinable Materials
Composed of silica or quartz, glass-based ceramic systems contain alumino silicates. Feldspars, (ie, sodium and potassium) found within the alumino silicates in various quantities, are modified to satisfy dental material requirements.9 A lso available in synthetic form, glass-based materials demonstrate enhanced strength and are suitable for inlays, onlays, anterior crowns, and veneers that have demonstrated lower flexure, shear, and tensile risks.1 T he stronger polychromatic machinable versions can be used for full-contour posterior crowns and should be bonded with resin cement.1,3,14 P ressed or machinable glass-ceramics require bonding in clinical situations where the bond seal can withstand long-term use.1 A lthough ideal for minimally invasive restorations due to the ability to maintain its strength at 0.3 mm, fabricating lasting restorations with less than 0.8 mm of tooth material can be challenging.1 R equiring 0.2 mm to 0.3 mm per shade change and 0.8 mm of minimum working thickness, adequate space is provided with glass-ceramic materials to develop esthetically pleasing restorations.1 F lexure, tensile, and shear stress risk varies from medium to high depending upon the material in question, as does bond seal risk.1 G lass-based systems are etchable and, therefore, adhesively bondable.9
High-Strength Crystalline Ceramics
Commonly used to replace metal core structures,1 c rystalline ceramics include alumina-based and zirconia-based systems. Alumina systems have proven successful for increased risk, single-unit applications in the molar region, anterior segment, and bicuspid crowns.1 C ore fracture when using conventional cements has been observed at between 8 and 10 years.1 T herefore, a resin cement or a resin-modified glass-ionomer luting cement is recommended for use with alumina-based systems.1 Z irconia core systems are indicated for high-risk molars due to the flexural, tensile, and shear strengths of the material, making it ideal where healthy tooth structure is lacking, a high risk of bond failure exists, and high stress distribution or flexural stress is present. When fabricating a zirconia restoration, esthetic success can be achieved by preparing for a 0.5-mm core with 1 mm of porcelain veneer and no more than 2 mm of unsupported occlusal or incisal porcelain.1
Metal-Ceramic Materials
Metal ceramics consist of a metal substructure most commonly veneered with a leucite-based feldspathic porcelain.15 D ue to the incompatibility of the coefficient of thermal expansion between the metal substructure and the traditional feldspathic glass ceramic, leucite is added to the feldspar glass and adjusted until the coefficient of thermal expansion is comparable6 o r slightly lower than that of the metal substructure to be effective.15 M etal ceramics are indicated for all full-crown applications, particularly those with high risk factors. However, regarding flexural, tensile, and sheer stress risks in high-risk cases, the structural core supporting the porcelain veneer becomes crucial to achieving a successful restoration. Additionally, micro-cracks can develop due to radial tensile and tangential compressive stresses.15 T o achieve esthetics in the anterior region comparable to zirconia/porcelain crowns, an extra thickness of 0.3 mm is required for metal-ceramics.1 D espite its continued use, the material characteristics, technology, and complex esthetics associated with metal-ceramic restorations can present several challenges.6
When deciding upon a restorative material, several clinical factors require consideration. These factors significantly affect clinicians’ decisions when choosing the ideal material for individual treatment plans. Factors affecting the success of all-ceramic restorations include material properties (eg, wear, durability, and esthetics); bond strength; and case specifics (eg, underlying tooth color, load, and preparation).7 M aterials indicated for the anterior region require optimal esthetics, and strength.7 T hose indicated for the posterior region need to be even stronger due to masticatory forces and the possibility of grinding or bruxing, and depending upon the location, esthetics may or may not be a factor.7 L ongevity and wear resistance are important for any restoration.
Wear and Durability
Every dental material has its advantages and disadvantages contingent upon their material composition. The longevity and wear resistance of a material is difficult to establish because it is based on a variety of factors, including the type of restoration, the area in which the restoration will be placed, bond type, and masticatory forces.16 B y choosing an appropriate material, clinicians guard against fractures, chipping, breakage, and erosion of the restoration itself and the adjacent and opposing dentition.
Esthetics
Over the years, the importance of dental esthetics has increased significantly. Dental manufacturers have responded by developing materials with physical characteristics that allow dentists to provide highly esthetic restorations. Esthetics includes color, shape, length and width of the teeth, and the shape of the dental arch. Esthetics is often a matter of cultural and socio-demographic influences, age, gender, education, and individual preference.17-19 W hile some materials are available in several shades and colors, others are monolithic, monochromatic, and require staining and glazing to achieve optimal esthetics.
Preparation and Load
Sharp angles can cause developmental stress in ceramic restorations. Therefore, rounded edges are necessary to decrease the incidence of fracture.20 Masticatory forces can vary according to the oral segment and can include stresses such as bruxing and grinding, which need to be considered. Due to the fact that the compressive strength of ceramic is 10 times higher than its tensile strength, crowns should be placed primarily under compressive stresses, requiring an anatomical preparation form.21,22 B ecause sheer stress was determined to be a decisive factor in ceramic crown failure, optimally, forces should be at a slant.22
Substrate and Underlying Tooth Color
The dental substrate should be one of the first considerations in material selection. Restorative bonds tend to be stronger and more predictable when the restorative material is bonded to enamel due to the stiffness of enamel as opposed to the flexibility of dentin and composite.1 Additionally, the underlying color of the tooth/substrate can significantly affect the esthetics of the overall restoration. According to a recent study, regardless of the shade or color of the ceramic material, the color and shade of the restoration was significantly altered according to the color of the substrate.23 The need to mask underlying tooth structure determines how thick or thin a restoration should be and presents considerations such as the strength of the material at varying thicknesses, its translucency, the color and shade of the bonding agent, and what type of bonding agent is applicable to the material choice.
Bonding and Cementation
The ability to secure a lasting bond is important to the success of indirect restorations. Two factors that determine the choice between adhesive and conventional cements are the underlying substrate and the restorative material. While some ceramic materials can be etched with hydrofluoric acid and adhesively bonded using a resin cement, high-strength ceramics such as alumina and zirconia cannot be etched and bonded in this fashion.24 Alumina and zirconia can be successfully bonded using a resin-modified glass-ionomer cement.24 Optimal bonds eliminate microleakage, result in long-lasting restorations, and can decrease the likelihood of fracture.
A New Ceramic for Indirect Restorations
A new ceramic material for indirect restorations has been introduced that eliminates many of the conventional drawbacks associated with ceramic materials, including a high fracture rate due to their brittle nature and low tensile strength. Uniquely defined as a resin nano-ceramic (Lava™ Ultimate, 3M ESPE, www.3mespe.com), the material contains 80% ceramic and 20% composite resin with nano-technology.25,26
Resin nano-ceramic (RNC) is neither a resin, composite, nor a pure ceramic, but a combination of all three. Although consisting primarily of ceramic, the non-brittle and fracture-resistant nature of the material results from the addition of mono-dispersed, non-aggregated, non-agglomerated, and synthesized 20-nm diameter silica and 4-nm to 11-nm diameter zirconia nanomers, producing zirconia-silica nanocluster particles.25,26 The structural integrity of the nanocluster particles allows for the incorporation of a high proportion of ceramic filler.
Treated with a silane coupling agent, these engineered nanoparticles chemically bond to the nano-ceramic surface and the resin matrix during the manufacturing process (Figure 1).25,26 When nanomer particles are combined with nanocluster fillers, the interstitial spacing of the filler particles is reduced, providing a nano-ceramic content by weight of approximately 80%. This produces a reinforced matrix that is stronger, harder, and more wear-resistant than resin.25,26
To develop the resin nano-ceramic, ceramic material is precisely manipulated using nanotechnology, then joined with resin technology and subjected to a specialized high-heat controlled, proprietary manufacturing process for multiple hours. This eliminates the need for firing the material after milling. However, the controlled process ensures a strong, esthetic material incomparable to feldspathic porcelain or composite blocks. Due to the incorporation of nanoparticles, the resin nano-ceramic demonstrates enhanced wear resistance, yet the same optical properties, polish retention, and lasting esthetics of a glass ceramic.25,26
Clinical Applications
The new resin nano-ceramic (Lava™ Ultimate) is designed for chairside milling or milling in a dental laboratory and is indicated for inlays, onlays, veneers, and full crowns. Because the resin nano-ceramic material demonstrates an elastic modulus similar to dentin, masticatory forces are absorbed via shock absorption characteristics, so restorative stress is reduced. Additionally, the flexural strength of 200 MPa provides excellent resiliency. These combined characteristics decrease the opportunity for fracture or chipping, rendering the material an ideal option for implant supported crowns.25,26
Compared to other CAD/CAM materials, the resin nano-ceramic functions differently, so milling, polishing, and adjustments are easier, the extra firing step is eliminated, and the procedure is faster.25,26 T eeth should be prepared with a 5˚ to 6˚ taper, with rounded edges and a shoulder margin. Resin nano-ceramic restorations require bonding with an adhesive resin cement.25,26
Conclusion
Today’s advances in science and technology have lent numerous and varied improvements to ceramic materials, expanding their applications from limited use to the posterior segment, to anterior and posterior indications including crowns, veneers, implant-supported restorations, implant abutments, and fixed partial dentures.6,7 W hen choosing an indirect dental material, there are several factors to weigh regarding the individual treatment, and several more regarding material choice, rendering it a confusing task for dentists. The availability of a new resin nano-ceramic presents dentists with an alternative that resolves the challenges previously encountered with all-ceramic restorations. This article outlined the clinical aspects to consider when choosing a ceramic material and reviewed today’s current ceramic alternatives available for indirect restorative dentistry.
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