A History of Dental Ceramics
Gregg Helvey, DDS
Ceramics play an integral role in dentistry. Their use in dentistry dates as far back as 1889 when Charles H. Land patented the all-porcelain “jacket” crown.1 This new type of ceramic crown was introduced in 1900s. The procedure consisted of rebuilding the missing tooth with a porcelain covering, or “jacket” as Land called it. The restoration was extensively used after improvements were made by E.B. Spaulding and publicized by W.A. Capon. While not known for its strength due to internal microcracking, the porcelain “jacket” crown (PJC) was used extensively until the 1950s.
To reduce the risk of internal microcracking during the cooling phase of fabrication, the porcelain-fused-to-metal (PFM) crown was developed in the late 1950s by Abraham Weinstein.2 The bond between the metal and porcelain prevented stress cracks from forming. Lost-wax fabricated metal copings also addressed the problem of the marginal fit experienced with traditionally constructed porcelain jacket crowns. While PFM crowns have a decrease in porcelain failures, the addition of a metal block-out opaque layer diminished the esthetics of these restorations. A resurgence of an all-ceramic restoration came in 1965 with the addition of industrial aluminous porcelain (more than 50%) to feldspathic porcelain manufacturing. W. McLean and T.H. Hughes developed this new version of the porcelain jacket crown that had an inner core of aluminous porcelain containing 40% to 50% alumina crystals.3 Although it had twice the strength of the traditional PJC, it still could be used in the anterior region only (due to its lower strength). Its higher opacity was also major drawback.4
Another development in the 1950s by Corning Glass Works led to the creation of the castable Dicor® crown system. Glass was strengthened with various forms of mica. The process involved the use of the lost-wax casting technique, which produced a casted glass restoration. Then, this was heat-treated or “cerammed.” The ceramming process provided a controlled crystallization of the glass that resulted in the formation and even distribution of small crystals. The type of crystal formation depended on the feldspathic formulation used. Examples of different crystalline formations are leucite, fluoromica glass, lithium disilicate, and apatite glass ceramics.5 The crystal formation increased the strength and toughness of the glass ceramic. For the Dicor® material, time and temperature controlled the rate of growth, amount, and size of tetra silicic fluoromica crystals. The resultant monochromatic crown was shaded with an application of a superficial color layer. The processing difficulties and high incidence of fracture were factors that led to the abandonment of this system.6
Leucite was first added to feldspathic porcelains to raise the coefficient of thermal expansion to match the metals to which they were fired. The crystalline leucite phases also helped feldspathic porcelain to slow crack propagation. High leucite-containing ceramics Empress® 1 and optimal pressable glass (OPC) were introduced in the late 1980s and were the first pressable ceramic materials. Although the initial steps for fabrication for Empress and OPC were similar to Dicor and Cerestore in which the restoration was formed in wax, a heated leucite-reinforced ceramic ingot was pressed into the mold using a specially designed pressing furnace, whereas the Dicor crown was created using centrifugal casting.
This process of pressing ceramic ingots became very popular due to the esthetics and ease of use in the laboratory. Despite the increase in strength of leucite-reinforced pressed Empress material, fracture was still possible when used in the posterior region.3
During this time, a glass-infused ceramic core system was developed. Vita used a slip-casting process in which the core achieved an 85% sintered alumina by volume and introduced the In-Ceram® system. This glass-infused alumina core had a flexural strength of 352 MPa.7 To increase the translucency and esthetics, Vita replaced the sintered alumina with spinel (MgAl2O4). The change of infused oxides slightly reduced the flexural strength but produced a restoration more fitting for the anterior region. Vita also added another variation of the infused core by mixing alumina with zirconium oxide crystals, which increased the flexural strength to 700 MPa. It was intended for posterior crowns and bridges.
In the mid 1990s Nobel Biocare introduced the Procera® AllCeram core, which was the first computer-aided design/computer-aided manufactured (CAD/CAM) substructure. This core consisted of 99.9% alumina to which a feldspathic ceramic was layered.
The use of CAD/CAM technology spurred a whole new generation of ceramic substructures consisting of zirconium dioxide. Several manufacturers (Lava, 3M ESPE; Procera Forte, Nobel Biocare; and Cercon, DENTSLY) introduced crown-and-bridge frameworks milled from blocks of presintered yttrium-stabilized zirconium dioxide blocks. The oversized milled frame-works were then sintered for 11 hours at 1500°C providing excellent fit with 900 MPa to 1300 MPa of flexural strength. Other manufacturers (Everest, KaVo, DC-Zirkon, Precident DCS) milled fully sintered zirconium dioxide blocks (because it removed the shrinkage factor), which one study found to have a superior marginal fit.8 Both fabrication methods provide a framework with sufficient flexural strength, allowing them to be used for multi-unit posterior bridges.
In 1998 Ivoclar introduced IPS Empress II, which was a lithium disilicate ceramic material used as a single- and multiple-unit framework indicated for the anterior region. The frame-work was layered with a veneering ceramic specially designed for the lithium disilicate. A 5-year study revealed a 70% success rate when used as a fixed partial denture framework.9
Authentic®, a second-generation, low-fusing, high-expansion, leucite glass-reinforced ceramic material, was introduced into the European market in 1998 by Ceramay GmbH & Co and later that year was introduced to the US market by Microstar. Laboratory technician Brian Lindke experimented with pressing Authentic to specific alloys. Working with Argen, alloys with matching coefficients of thermal expansion were developed that were compatible and hence the introduction of the Press-to-Metal™ technique. Soon this technique was adopted, substituting the metal with zirconium dioxide frameworks. Ceramic pressable ingots with a compatible coefficient of thermal expansion were developed for this technique.
Lithium disilicate re-emerged in 2006 as a pressable ingot and partially crystalized milling block (Cerec® for chairside and inLab® milling units for laboratories). The flexural strength of the material was found to be more than 170% higher than any of the currently used leucite-reinforced ceramics. The ceramic material can be milled or waxed, and then pressed to full contour and subsequently stained. Another option allows for cutting the crown back, followed with layering with different specially designed apatite ceramic glass. The layering ceramic has the same basic components as natural tooth enamel. CAD/CAM milling of a framework (zirconium dioxide or metal), a full-contoured crown (lithium disilicate at chairside or in the laboratory), or an implant abutment has opened the market for digitized restorative dentistry.
Dental material manufacturers seem to be leaning away from metal alloy-containing restoratives and favoring all-ceramic restorative dentistry. Research and development appears to be heading in two directions—improving the strength and esthetics of bilayered zirconium dioxide restorations and achieving a milled monolithic posterior bridge material.
The author would like to thank Ruth Egli, RDH, for her editorial contribution.
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About the Author
Gregg Helvey, DDS