Principles for Selection of Dental Ceramics PDF

Principles for Selection of Dental Ceramics Prof Muawia Qudeimat Dr Maryam AlNaser Dental ceramics have become a reference material in modern dentistry. Used for various types of dental restorations, including crowns, bridges, veneers, and inlays, ceramic provides patients with a high-quality a...

Principles for Selection of Dental Ceramics Prof Muawia Qudeimat Dr Maryam AlNaser Dental ceramics have become a reference material in modern dentistry. Used for various types of dental restorations, including crowns, bridges, veneers, and inlays, ceramic provides patients with a high-quality aesthetic solution that perfectly mimics the natural appearance of teeth. Advantages of Ceramics Compressive strength Ceramics can withstand the intense forces generated during chewing, especially in the case of posterior teeth, where the pressure is higher. Biocompatibility Do not contain toxic substances or allergens, which reduces the risk of side effects. Ceramic is chemically inert, meaning it does not react with other substances in the oral cavity. Color stability Review of Terms All-ceramic: refers to a dental restoration made entirely of ceramic material. Monolithic or uni-layer: restorations consist of a single ceramic material. Bilayered Restorations: Formed by a ceramic core covered with a ceramic veneer, where the core supports the restoration, and the veneer provides its final shape, shade, and appearance. What are advantages and disadvantages of monolithic and bilayer ceramics? Types of Ceramic Restorations Porcelain High-temperature porcelain: comprises a mixture of quartz, clay, and feldspar powders. Used for: prosthetic teeth. Feldspathic dental porcelain: contains powders of potassium feldspar and glass. Used for: ceramometallic restorations and fabricating porcelain inlays and veneers. Aluminous porcelain: comprises a mixture, the composition of which is similar to that of feldspathic porcelain, but with aluminum oxide fillers. Alumina Alumina, also known as aluminum oxide, has been used as an implant material in the biomedical field for orthopedic and dental reparations since 1964 Early clinical applications showed a fracture rate as high as 13%. It was found that material failure was because ceramic restorations could not be sintered to full final density. However, even though alumina has a very high resilience to fracturing, fractures are still frequent in clinical use. Since the introduction of third-generation alumina, there have been significant advances in the mechanical properties and manufacturing methods of alumina-based ceramic materials. Zirconium Dioxide Advantages: Excellent biocompatibility, appearance and wear properties, superior toughness, fatigue resistance, and strength. 1. Monolithic zirconium dioxide restorations Commonly used for areas that are less aesthetically important, such as for the posterior teeth, due to their tendency to be opaque, monochromatic, and the fact that they have a lack of translucency and fluorescence. They are excellent for masking discolored dental preparations, for example, in those that have darkened due to chronic diseases or previous dental treatments. Zirconium Dioxide 2. High-translucency zirconium dioxide Restorations are more aesthetically pleasing than the monolithic type due to their natural and vibrant translucency and opalescent characteristics, which make them suitable for anterior teeth. Despite high-translucency zirconium dioxide being weaker than its monolithic counterpart, the biaxial flexural strength of the former is still high and ranges from 590 MPa to 720 MPa and above. Zirconium Dioxide 3. Porcelain-layered zirconium dioxide Restorations are designed to have the appearance of porcelain and the structural strength of zirconium dioxide. They are fabricated using a porcelain veneer over the ceramic coping. The aesthetic quality of these types of restorations is similar to that observed in high-translucency zirconium dioxide Lithium Disilicate In lithium disilicate, the lithium oxide crystals (Li₂O) are dispersed in a glassy matrix of silica (SiO₂), providing flexural strength values of up to 440MPa and inhibiting the propagation of cracks due to their interlocking orientation However, it has been established that the failure rate of these restorations can reach 3.3%, as they can show damage such as ceramic fracturing, chipping, and rupturing of the core ceramic Due to their excellent aesthetic properties, restorations based on lithium disilicate are used to repair anterior teeth Methods for Fabrication of All-Ceramic Restorations Conventional Technique The conventional technique is the most common fabrication method for the veneer ceramic in ceramometallic restorations. Hot Pressing Technique Hot Pressing Technique Materials that can be used for this technique are: leucite (IPS Empress; Ivoclar, Liechtenstein), lithium disilicate (IPS Eris; Ivoclar, Liechtenstein), spinel (Alceram; Innotek Dental Corp, Lakewood, CA, USA), and ceramics based on them (for instance, IPS Empress ceramics). The main disadvantage of this method is: the high cost of the equipment, due to the necessity to use a specially automated pressing furnace. Dry Pressing Method This is one of the methods which enables polycrystalline (alumina or zirconium dioxide) restorations to be fabricated. First, a life-sized natural model of the restoration to be created is obtained. Then, this model is scanned to obtain an enlarged (oversized) 3D model in order to fabricate a mold with oversized dimensions. Afterwards, ceramic powder is pressed into the mold, obtaining a green body, which is sintered. During sintering, the green body shrinks to the required dimensions. The dye model is oversized by around 12–20% to compensate for the shrinkage that occurs during sintering. Examples of ceramic materials produced with this method are: Procera ® Alumina and Procera ® Zirconia by Vita Slip-casting and Glass Infiltration Advantage: high strength Disadvantages: high opacity and long processing times. CAD/CAM CAD/CAM The advantages of these methods include: a reduction in clinical time and cross-infections between the clinic and the laboratory, and a reduction in patient discomfort, particularly when intraoral scanning is used. Disadvantages of CAD/CAM systems include: the cost of equipment and its maintenance, which makes this manufacturing technique more expensive than others. Category 1 (porcelains)—are the most esthetic, especially in thin sections and thus can be used the most conservatively but are the weakest. Category 2 (glass-ceramics) also can be very translucent but requires slightly thicker dimensions for workability and esthetics than Category 1. Although demonstrating progressively higher fracture resistance, Categories 3 and 4 are more opaque and, therefore, require additional tooth reduction that produces a less conservative alternative. Five Clinical Parameters to Evaluate for Choosing a Material 1. Space Required and Color Change for Esthetics In general with porcelains, the dentist needs a porcelain thickness of 0.2 mm to 0.3 mm for each shade change (A2 to A1) Glass-ceramics need the same space requirements as porcelain for effective shade change; High-strength all ceramic crowns require a thickness of 1.2 mm to 1.5 mm, depending on the substrate color; metal-ceramics need a thickness of at least 1.5 mm to create lifelike esthetics. 2. Substrate 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. Bonding to dentin surfaces—as well as to composite substrates—is less predictable given the variability and flexibility of these substrates. 3. Flexure Risk Assessment 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 3. Flexure Risk Assessment Low Risk: there is low wear, minimal-to-no fractures or lesions in the mouth, and a reasonably healthy oral condition. Medium Risk: signs of occlusal trauma are present; mild- to- moderate gingival recession exists, along with inflammation; bonding mostly to enamel is still possible; and there are no excessive fractures. High Risk: Occlusal trauma from parafunction is evident, more than 50% dentin exposure exists, there is significant loss of enamel due to wear of 50% or more, and porcelain must be built up more than 2 mm. 4. Excessive Shear and Tensile Stress Risk Assessment All types of ceramics are weak in tensile and shear stresses. The same parameters are evaluated, similar to flexure risk, eg, deep overbites and potentially large areas where the ceramic would be cantilevered. 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. 5. Risk of Bond Failure A good bond in combination with a stiffer tooth substructure (eg, enamel) is essential to reinforce the restoration. If the bond and seal cannot be maintained, then high- strength ceramics or metal-ceramics are the most suitable because these materials can be placed using conventional cementation techniques. 5. Risk of Bond Failure Clinical situations in which the risk is higher for bond failure are: 1. Moisture control problems 2. Higher shear and tensile stresses on bonded interfaces 3. Variable bonding interfaces (eg, different types of dentin) 4. Material and technique selection of bonding agents (ie, as dictated by such clinical situations as the inability to achieve proper isolation for moisture control to enable the use of adhesive technology) 5. Experience of the operator Category 1: Powder/Liquid Porcelains Deal as the most conservative choice but are the weakest materials and require specific clinical parameters to be successful. Space requirements for shade change: 0.2 mm to 0.3 mm for each shade change. Substrate: A rate of 50% or more remaining enamel is on the tooth, 50% or more of the bonded substrate is enamel, and 70% or more of the margin is in enamel Shear/tensile risk: Low-to-low/moderate risk. Large areas of unsupported porcelain, deep overbite or overlap of teeth, bonding to more flexible substrates (eg, dentin and composite), bruxing, and more distally placed restorations increase the risk of exposure to shear and tensile stresses Category 1: Powder/Liquid Porcelains Indications: Generally indicated for anterior teeth Occasional bicuspid use and rare molar use would be acceptable only with all parameters at the least-risk level. Category 2: Glass-based Pressed or Machinable Materials IPS empress® (Ivoclar Vivadent) and Authentic® (Jensen Dental), and the higher-strength IPS e.max® (Ivoclar Vivadent) Monolithic IPS e.max, due to its high strength and fracture toughness, has shown promise as a full-contour, full- crown alternative, even on molars. Category 2: Glass-based Pressed or Machinable Materials Space requirements: 0.8 mm of minimum working thickness 0.2 mm to 0.3 mm for each shade change. Substrate: Less than 50% of the enamel is on the tooth, less than 50% of the bonded substrate is in the enamel, and 30% or more of the margin is in the dentin. Flexure risk assessment: Medium for Empress, Vitablocs Mark ii, and Authentic-type glass-ceramics or layered IPS e.max. Category 2: Glass-based Pressed or Machinable Materials Tensile and shear stress risk assessment: Medium for empress, Vitablocs Mark ii, and Authentic- type glass ceramics or layered IPS e.max. Medium to medium/high for bonded monolithic IPS e.max.4. Bond/seal maintenance risk assessment: Low risk of bond/seal failure for empress, Vitablocs Mark ii, and Authentic type glass- ceramics or layered IPS e.max. Medium for monolithic IPS e.max. Category 2: Glass-based Pressed or Machinable Materials Indications: Pressed or machined glass ceramic material such as empress, Vitablocs Mark ii, and Authentic are indicated for thicker veneers, anterior crowns, and posterior inlays and onlays in which medium or lower flexure risks and shear and tensile stress risks are documented. IPS e.max, which has higher toughness, is also indicated for the same clinical situations as the other glass-ceramics but can be extended for single-tooth use in higher-stress situations (as in molar crowns). This is provided it is used in a full-contour monolithic form and cemented with a resin cement. Category 3: High-Strength Crystalline Ceramic Mostly all-crystalline materials (eg, inCeram®, Vita) are used for core systems to replace metal that would then be veneered with porcelain. Alumina-based systems, eg, in- Ceram, Procera® (Nobel Biocare), were first on the market but are now generally being replaced with zirconia systems. For zirconia core systems (eg, Vita YZ, Vident,; Procera® Zirconia, nobel Biocare,; Lava , 3M ESPE) Alumina systems have been shown to be very clinically successful for single units, with a slightly increased risk in the molar region. They can be recommended for any single- unit anterior or bicuspid crown. Category 3: High-Strength Crystalline Ceramic Space requirements: 1.2-mm minimum working thickness. 1.5 mm ideal if masking. Substrate condition: substrate is not critical because the high- strength core supports the veneering material. Tensile and shear stress risk assessment: High or below. For high-risk situations, the core design and structural support for porcelain become more critical. For higher-risk molar regions, it is more ideal to use zirconia cores vs. alumina cores, provided the current firing parameters are followed. Category 3: High-Strength Crystalline Ceramic Full-contour zirconia restorations (eg, Prettau Zirconia, Zirkozahn; BruxZir®, Glidewell Laboratories,) have been recommended for high-risk molar situations. Bond/seal maintenance risk assessment: if the risk of obtaining or losing the bond or seal is high, then zirconia is the ideal all- ceramic to use. Category 3: High-Strength Crystalline Ceramic Indications: High-strength ceramics (specifically zirconia) is indicated when significant tooth structure is missing, an unfavorable risk for flexure and stress distribution is present. Category 4: Metal-Ceramics Indications: Generally, they have the same indications as Category 3 zirconia- based restorations. With metal-ceramics, manufacturers have eliminated the complications throughout the years; these materials do not have the same thermal firing sensitivity as zirconia does. However, anterior teeth metal-ceramics need to be approximately 0.3 mm thicker to have the same esthetics as properly designed zirconia/porcelain crowns. Category 4: Metal-Ceramics Space requirements: 1.5 mm to 1.7 mm for maximum esthetics. Substrate: the substrate is not as critical because a metal 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. Category 4: Metal-Ceramics Bond/seal maintenance risk assessment: if the risk of obtaining or losing the bond or seal is high, then metal ceramics are an ideal choice for a full crown restoration Indications: Metal-ceramics are indicated in all full-crown situations, especially when all risk factors are high. Clinical Scenarios