Dental Ceramics 101
Ceramics are widely used in professional dental products for restorative dentistry, orthodontics, periodontics and endodontics.
Dentistry and ceramics are natural partners. The inherent non-reactivity of most ceramics is perfect for biological uses (indications) in the body, especially dentistry. Ceramics are widely used in professional dental products for restorative dentistry, orthodontics, periodontics and endodontics. Ceramic powders are also essential for many home care dental products.
Our teeth are uniquely formed with a hard ceramic exterior of hydroxyapatite, which is a ceramic crystal. When decay (caries) breaches that hard coating and infiltrates the underlying dentin (hydroxyapatite with collagen), repair in the form of restorative dentistry is needed. In the early 1900s, man-made ceramic restorations (fillers for broken/decayed teeth) were “silicates,” a restorative based on mixing a glass powder with phosphoric acid cement. A sodium aluminate glass with added calcium fluoride was mixed with 50% buffered phosphoric acid. The mixtures set in situ to rebuild/replace the tooth structure, creating an esthetic restoration and an alternative to dental amalgam for the front (anterior) teeth. However, these silicate fillings were not durable enough and tended to stain.
Later in the 20th century, glass ionomers were invented that relied on glass formulas containing fluoride and calcium to react with polyalkenoic acids. These fluoride-releasing restorations (or cements) are excellent for patients where decay is anticipated to recur.
Decayed teeth are now commonly restored with dental composite (composite resin material). Dental composites contain various organic compounds and very fine glass powder (60-80% by weight), and may also include fumed silica or quartz particles. The glasses in dental composite materials are formulated to have the same refractive index as the resin matrix into which they are blended, and to have higher atomic number elements in the glass. The glass powders are coated with silane to bond to the resin matrix. Without the unique glass formulas, the dental filling (restoration) would not be durable or visible on a dental X-ray (radiopaque).
Many composite products are marketed around the world, varying in percentage of handling and curing time (convenience to dentist), ceramic fill, opacity, radiopacity, and color (shade) to satisfy the patient. Some composite restorative materials are more durable for the posterior vs. the anterior teeth. The restorations must blend with the surrounding teeth in color (shade) and form. To be esthetic, the materials must also be fluorescent in UV light, ensuring that the appearance of the restorative material is like that of natural teeth. UV fluorescence is often created by adding other ceramic powders, especially the lanthanum-series oxides such as terbia, erbia, gadolinium oxide or neodymium oxide.
Related adjunctive products include the bonding systems for composites. Dental sealers for younger patients’ teeth are also in this category. All these materials contain the ceramic powders used in composites, but in lesser amounts. Combination products have also been created from dental composites and glass ionomers for certain dental restorations or for cementing items into or onto teeth.
Crowns and Bridges
Sometimes a glass ionomer or dental composite restoration will not be sufficient to replace significant loss of the natural tooth structure for teeth with major decay or fracture. In these instances, a crown or bridge (fixed prosthodontics) is required. Composite materials are sometimes used for these applications on top of a metal substructure; however, composite is not as durable or stain resistant as dental porcelain enamel. Porcelain-fused-to-metal crowns and bridges last much longer than composite restorations and are much more protective of the underlying tooth than other partial-coverage repairs described previously. Bridges substitute for missing teeth, eliminating the need for a partial (removable) denture.
Millions of crowns and bridges have been made since the 1950s by enameling dental porcelain onto metal alloys. Dental porcelain is special because of its high thermal expansion, which matches the thermal expansion of the underlying metal structure. It was the serendipitous discovery by Dr. A. Weinstein of the high thermal expansion of the leucite ceramic crystal (KAlSi2O6) made from potassium feldspar that enabled the reliable manufacture of porcelain enamel for the high-gold alloys favored in dentistry. These porcelain systems are also called feldspathic porcelains because they are commonly made by heat-treating potassium feldspar to precipitate leucite crystals in a glass matrix.
A dental laboratory fabricates these porcelain-enameled crowns and bridges using a porcelain system with many components that vary in opacity, shade and firing temperature. The technician may use 30 ceramic powders to create the matching, natural restorations. Usually four firings are required in a special dental furnace that can draw a low vacuum. The vacuum during firing reduces the pores, which increases the density and strength and improves the transparency for better esthetics. Dental technicians who make these crowns and bridges are masters of miniature art, creating restorations that beautifully blend into the adjacent teeth for the dentist to cement in place.
Dental porcelain is used for ceramic veneers on teeth. Veneers are thin ceramic restorations cemented onto the anterior teeth to correct problems such as permanent stains, fractures, malformed or misaligned teeth, or wear of the biting (incisal) edges. The leucite crystals in the dental porcelain provide enough strength (80-100 MPa) for such uses, and also for some anterior crowns.
After the invention of leucite porcelains, many biocompatible alloys have been developed for dental porcelain systems based on gold, palladium, nickel and chromium alloys, creating unique properties and a range of affordable options. However, as we age, people become “long in the tooth” (recession). This reveals the edge of the underlying metal of enameled crowns, which is a less esthetic outcome than desired. Hence, the industry has searched for strong ceramics to replace the metal substructures that are enameled.
Stronger all-ceramic restorations have been developed to overcome the esthetic problems of metals. However, not all ceramic formulas have shown the durability or esthetics required for anterior teeth. The “aluminous jacket crown,” a glass-bonded alumina, was used in the early 1900s with some success. In the 1950s, the Dicor® crown system was introduced by Corning Inc. and Dentsply based on a glass that could be “cerammed” and form tetrasilicic fluoromica crystals. Dicor crowns were made in dental laboratories by casting the glass using the lost-wax casting technique; the crowns were then partially crystallized to develop their strength. The exterior of the crown had to have a thin glass coating (glaze) to create the right color. These crowns were stronger than dental porcelain but were restricted to anterior use. Resin-based dental cement was required, which was just becoming common. Without resin cement, the crowns were prone to fracture.
Greater success in esthetics and durability of all-ceramic restorations has been obtained since the 1990s, when Ivoclar Corp. introduced lithium disilicate all-ceramic restorations. The lithium disilicate crystal-glass combination was stronger and more esthetic that its predecessors. Equipment was designed to allow the ingots of lithium disilicate and glass to be uniaxially hot-pressed into a tooth shape using a ceramic investment formulated for the lithium disilicate material. The pressing reduced the pores for better strength and esthetic results. A unique fluoroapatite-containing porcelain is used for the outermost coating and shaping of the crown or bridge on the lithium disilicate substructure.
Also in the 1990s, the In-Ceram® system was introduced by Vita Corp. using a modified slip-casting technique to make a green ceramic tooth substructure of spinel (MgAlO4), alumina or zirconia. The partially fired slip-cast material has the exact shape and size required for the crown or bridge. The green crown or bridge form is infiltrated by a powdered glass, colored to match the tooth in a second heat treatment. These innovative infiltration glasses contain a high proportion of lanthanum oxide to reduce the surface tension and fill the pores of the green ceramic when fired at 1,500°C. The result is a very strong restoration that also precisely fits the tooth with an inherent coloration to match the surrounding teeth. A special porcelain, designed to match the thermal expansion of the strong underlying ceramic substructure, is fired as a veneer to create the final anatomical form and esthetic result.
Since 2000, CAD-CAM systems for making ceramic crowns and bridges have burgeoned for machining high-purity alumina and tetragonal zirconia. These machined materials are strong enough to span as many as five teeth in a bridge. Many systems are available; some manufacturers machine green ceramics, while others machine fully fired alumina or zirconia. The crowns machined green are oversized but their shrinkage is known precisely, enabling the fired restoration to fit precisely on the tooth. The zirconia CAD-CAM systems create crowns and bridges suitable for our posterior teeth, where the biting forces are the highest. Such restorations require veneering with porcelains that have the thermal expansion matched to the substructure and to superficially create the color of the surrounding teeth.
Each crown or bridge must be manufactured to precisely fit our teeth. This custom fit is achieved through impression materials that are used after the dentist has drilled and shaped (prepared) our teeth for the restoration. The impression materials are set in the mouth and then removed. Before dentures are created, a sodium alginate impression material is required; this material is filled with silica diatoms, creating a relatively low-cost material. Other polymer-based impression materials are needed for high-accuracy impressions for crowns and bridges and have fillers including titania or silica. Even lead oxide is used in polysulfide impression materials for the setting reaction.
A gypsum model is usually made from an impression, representing another important ceramic in dentistry. The gypsum models are used for study models and for forming the restorative device. Notably, with the advance of digital dentistry, the use of impression and model materials is forecast to decline.
Crowns and bridges require a cement to affix them to the teeth. The earliest cements were based on zinc oxide (ZnO) powder mixed with phosphoric acid, eugenol or polycarboxylic acids to cause setting. Today, glass ionomer cements with fine glass particles are now used because the cement will release fluoride in a limited way to prevent decay recurrence. Resin cements containing ceramic fillers are frequently used. These less-viscous forms of composite can create the < 20 µm cement layer needed. Combinations of glass ionomers and resins are also used to create cements referred to as resin-modified glass ionomers or compomer cements.
For children’s baby (primary) teeth, non-customized-fit zirconia crowns are available. Children’s teeth may require crowns if caries have attacked the thinner hydroxyapatite coating of the primary teeth, a condition often precipitated by unhealthy oral flora, sugary beverages or lack of a hygiene regimen. Preformed zirconia crowns are an esthetic option for such situations; otherwise, the less expensive stainless steel crowns or composite-faced stainless steel are prescribed. The zirconia crowns are beautifully manufactured to fit the 20 primary teeth in various sizes and usually one or two shades. The fit of the crown is less critical than for the permanent teeth because of the anticipated loss (exfoliation) of the primary teeth. However, maintenance of the primary teeth is critical to the development of the face and mitigation of the need for orthodontics.
For orthodontics, ceramic brackets have been developed that reduce the obviousness of the treatment. Sapphire, polycrystalline high-purity alumina and tetragonal zirconia brackets have been marketed for the purpose of tooth realignment. Some ceramic-coated wires are used because the coating helps blend with the teeth. Again, ceramic-filled cements are used to affix ceramic or metal brackets to the teeth.
Dentures are required when many teeth are extracted. Some denture teeth are made of porcelain; formerly, porcelain was used for the base of the denture in which porcelain teeth were mounted. Today, artificial teeth in partial or full dentures and the denture base for the teeth are usually made of polymethylmethacrylate, but commonly include ceramic powders for increasing their durability and esthetics.
Ideally, we want our teeth to remain alive (vital) with an interior (pulp) that is actively circulating blood and contains sensory nerves. However, trauma and decay sometimes require the pulp be removed because it has suffered an injury or disease from which it cannot recover. In these cases, root canal treatment (endodontic treatment) is useful. The pulp is removed and the roots are filled with materials to seal the tooth, which prevents bacteria from migrating through the root into our jaw where further damage could occur. The usual endodontic treatment includes filling the roots with a sealing cement and rubber points (gutta percha). The gutta percha contains ZnO and other ceramic powders for pigments and radiopacity.
A relatively new cement to dentistry is tricalcium silicate (and dicalcium silicate), which ceramic engineers know as portland cement. Special formulations of tricalcium silicate powder are created for dentistry that are finer, free of toxic metals such as arsenic or lead, and are blended with radiopaque powders. The tri/dicalcium silicate powder is bioactive. That is, in vivo our bodies react to the cement by precipitating hydroxyapatite on the surface of this cement. Because of this surface layer of hydroxyapatite in contact with living tissue, the cement is perceived to be “natural” like dentin or enamel, and the body will react to tricalcium silicate by healing. This ceramic material can be used on the living pulp and within the root after endodontic procedures to assist with healing the pulp and sealing the end of the root. Calcium aluminate cements have similar properties; they are currently used in a dental cement and are being researched for other endodontic and periodontal uses.
Posts are sometimes inserted in tooth roots after endodontic treatment to support the recreation of the coronal part of the tooth prior to the manufacture of a crown. These thin peg-like items may be made of resin-bonded glass fibers or zirconia.
While titanium is the norm, tetragonal zirconia is being used for some tooth implants that are screwed into the bone of the jaw (alveolar) to replace missing teeth. Titanium implants are successful because of their intrinsic titania surface layer, making them more biocompatible that other metals. Hydroxyapatite is used as a coating for some implants.
Granulated hydroxyapatite is a common graft material for augmenting the alveolar bone when periodontal complications occur. Hydroxyapatite granules may also be placed in the socket where a tooth has just been extracted to prevent the collapse of the surrounding bone and make later implant placement easier. Bioactive glasses are similarly used. Many new compositions of bioactive glasses are being developed and tested based on silica, calcia, phosphate, and often sodium oxide. Such ceramic graft materials are designed to be gradually replaced (resorbed) by bone for dental and medical uses.
Our toothpaste or hygienists’ products rely heavily on ceramic fillers to abrade and remove the incipient plaque that forms as we digest food. In addition to salts including sodium bicarbonate, toothpastes or the flavored pastes used by hygienists (prophy paste) may contain varying amounts of ceramic powders including silica, calcium/magnesium carbonate, hydrated alumina, or phosphate salts.
Ceramics to the Rescue
Our teeth are critical for our ability to properly digest food and to speak. They give us confidence to achieve social and professional success, and oral health is increasingly correlated with life satisfaction and systemic health. Luckily, we have ceramics to help us deal with the consequences of a lifetime spent exposing our teeth to bacteria and acids, while stressing, eroding, and abrading these precious vital organs.