Ceramics in Dentistry: From Analog to Digital
Ceramics and ceramic-filled dental composites have evolved to restore teeth with a natural and lifelike appearance.
The search is continual for materials that can successfully replace damaged parts of the human body. Ceramics have been successfully used for many skeletal components, including bone and teeth.
As shown in Figure 1, teeth have three main layers, with the outermost (enamel) being composed of the mineral hydroxyapatite (Ca10(PO4)6(OH)2). Underneath is dentin, which lies on top of the living inner structure called the pulp. The visible portion of the tooth that projects into the mouth is called the crown. Human enamel is the hardest substance in the human body, but it can be eroded and decalcified by acids or abraded and fractured during mastication. Each tooth also has its own unique anatomy and visual appearance.
Ceramics are ideal candidates to replace damaged dentition due to their hardness, biocompatability and appearance. This challenging application requires strong, inert materials that can be fabricated with the desired anatomy and coloration, resulting in restorations that appear natural.
Tooth-Shaded Dental Composites
Silver-mercury dental amalgams have traditionally been used when a small portion of tooth structure has become compromised. Amalgam is not esthetically pleasing, has questionable biocompatibility, and does not adhere to dentin or enamel. Clinicians must therefore remove healthy tissue to cut features, such as undercuts, to provide for mechanical retention. Over time, the remaining tooth structure can fatigue and lead to failure of the crown, which would then need to be replaced.
Biocompatible, tooth-colored, self-adhesive restorative materials known as glass ionomer cements are based on an ionic polymer backbone, typically polyacrylic acid, which becomes crosslinked by polyvalent ions that are extracted from fluoroaluminosilicate glass fillers engineered for this special application. The glass is milled to a powder with a controlled surface area, which is mixed with the water-based ionic polymer just prior to use. The ratio of Al:Si is adjusted to make the glass slightly soluble in the ionic polymer.
Due to its ionic size, Al3+ can replace Si4+ in the glass network; however, the ionic charge difference results in sites that are susceptible to acid attack. Polyvalent ions, such as Ca2+, Sr2+ and Al3+, are liberated and subsequently crosslink the ionic polymer, causing it to set. Some of the acid groups on the polymer can also bind to the calcium on the surface of the prepared dentin or enamel. While lacking the appearance, strength and abrasion resistance required for many applications, these materials continue to find utility due to their ease of use, tolerance to moisture contamination during the restoration process and ability to release fluoride ions long term in the oral cavity. Fluoride can exchange with the hydroxyls in the apatite mineral, improving its resistance to chemical attack.
In the 1980s, adhesives were developed that had an affinity for both dentin and enamel to provide for the bonding of composite resin-based restorative materials. Clinicians only needed to remove the damaged area and were then able to restore the tooth to near its original strength. Dental composites are typically based on filled methacrylate resins, which can be free radically polymerized through the application of actinic radiation, such as blue light. Glass and ceramic fillers are added to the resins to control rheology and sculptability, minimize polymerization shrinkage, increase wear resistance, and create the desired optical properties (e.g., coloration, opalescence and fluorescence). Composites initially lacked the desired esthetics, translucency was poor, and the strength and abrasion resistance were inferior to enamel.
In 1987, the first esthetic dental composites were introduced that used a sol-gel-derived zirconia-silica nano-structured synthetic filler system that demonstrated clinical performance approaching natural enamel. Zirconium was incorporated to help control the optical properties of filler particles to match the resin system, provide X-ray opacity for diagnostic purposes, and provide mechanical strength and abrasion resistance.1
Dental composites have continued to evolve, with their highly engineered glass and ceramic filler systems, and are now used to restore teeth with a very high esthetic result (see Figure 2). Clinical studies in excess of 20 years have proven their efficacy as a biocompatible repair material for damaged dentition.2
Analog Crown and Bridge Materials
Dental composites are typically used when the restoration can be made directly in the patient’s mouth. When the size or complexity of the repair requires the fabrication of a prosthesis, then materials such as glass-ceramics, alumina and transformation toughened zirconia (TTZ) have found utility. ISO Standard 6872:2008 “Dentistry–Ceramic Materials” gives guidance for the flexural strength and fracture toughness requirements for different types of dental restorations (see Figure 3).
Prosthodontists and dental technicians, who have knowledge of dental anatomy, centric occlusion, arc of closure and smile design, have traditionally used wax as a sculpting material to create a wax-up of the dentition to be replaced. This is performed on a plaster model, which is prepared by pouring wet plaster in a mold made of the patient’s dental anatomy using impression materials. The wax-up can be used to prepare a metal casting or glass pressing following the lost wax technique.
Glass pressings can be crystallized to further improve strength, with leucite and lithium disilicate glass-ceramic systems finding the most acceptance. These metal or ceramic materials can serve as substructures, which are artistically layered with dental porcelains to create an appearance that is almost indistinguishable from natural teeth.
Restorations can now be designed in the virtual world using a digital workflow. Digital impressions are obtained rapidly with very accurate, hand-held 3D scanners (see Figure 4). The manual wax-up is replaced by manipulation of a virtual model, starting with digitized tooth forms. These are transformed into the desired physical shape using CNC milling technology (see Figure 5). Glass-ceramics, along with sintered alumina and zirconia, can be wet milled and processed directly into the finished restoration chairside.
TTZ is the fastest-growing esthetic material and the only ceramic material that can meet the strength and toughness requirements for all restoration types (e.g., long-span bridges). A key to this material’s success, in addition to superior mechanical properties, has been the ability to incorporate ions into the zirconia lattice, which will impart coloration similar to natural dentition.3 The ions are introduced by soaking or painting porous pre-sintered forms of the dental zirconia with a dyeing liquid containing the coloring ions. Viscosity control and other additives are incorporated to fixate the ions in place during the drying process.
Combinations of dyeing liquids can be applied to create a natural gradient shading effect that eliminates the need to layer porcelain to obtain a lifelike appearance. Final sintering of the zirconia maturates the color. Incorporating coloring ions directly into the crystal lattice, as opposed to adding pigments, enables monolithic TTZ restorations to achieve the required strength, translucency and esthetic properties of natural teeth (see Figure 6). Advanced ceramic powder processing, pressing and pre-sintering process controls are applied to ensure the sintered restoration consolidates in a predictable manner, with consistent and uniform shrinkage. This results in the final fired dimensions meeting those specified by the clinician in the original digital model.
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1. Morris, G.P., “Chemical Ceramic Expertise Applied to Development of 3M Dental Restorative Materials,” Northwest Dentistry, 1992, 4: 29-31.
2. Da Rosa Rodolpho, P.A., Donassollo, T.A., Cenci, M.S., Loguercio, A.D., Moras, R.R., Bronkhorst, E.M., Opdam, N.J., Demarco, F.F., “22-Year Clinical Evaluation of the Performance of Two Posterior Composites with Different Filler Characteristics,” Dental Materials, 2011, 27: 955-963.
3. U.S. Patent 6,709,694.