The Evolution of Dental Ceramics
Porcelain has been prized for its beauty for centuries, but the material’s relative fragility was long considered a barrier to its utility. In fact, it wasn’t until the modern era that science and technology allowed the creation of ceramics with the strength necessary to make their use in dentistry not only feasible but highly desirable.
All dental practitioners have their preferred techniques and materials, and ceramics provide no exception to this rule. However, with new materials and technology being developed every year—and with the preference for all-ceramic restorations on the rise—dental ceramics are worth another look, whether one’s practice is brand new or well established.
From Fragile Beginnings
Dentistry first applied ceramics in the 18th century, when Alexis Duchateau created a complete set of dentures using porcelain. Because the porcelain would not degrade in the wearer’s mouth like dentures made from human or animal teeth,1 this innovation led to exploring different applications for ceramics as a restorative material.
The evolution of dental ceramics continued into the late 1800s, when the Richmond crown, a single-piece crown consisting of a porcelain facing fused to a metal post,2 was introduced. This was closely followed by the development of the all-porcelain jacket crown by Charles Land in 1889.3 These innovations allowed the use of ceramics for restoring single teeth.
In the late 1950s, Abraham Weinstein invented a porcelain-fused-to-metal (PFM) crown. The metal substructure resolved inherent strength problems but lessened the esthetic qualities of these restorations, as the crowns were quite opaque.4 Since then, advancements in dental ceramic restorations have encompassed the material’s chemical composition, manufacturing processes, strength, esthetic properties, and indications for use—particularly where they can be predictably placed in the mouth.
“Since 1965, we’ve seen an increase in strength and fracture toughness largely due to an increase in the crystalline content of ceramic materials,” explains J. Robert Kelly, DDS, DMedSc, professor in the department of reconstructive sciences at the University of Connecticut Health Center. “From the earliest high-strength core materials that were about 55% aluminum oxide to those that were about 70% lithium disilicate, and then to those that contained 100% polycrystalline ceramics, there’s a definite period where dentistry transitioned to advanced engineering ceramics due to enhancements in powder technology and processing.”
Better Materials Through Chemistry
Ceramics are man-made, inorganic materials formed by heating raw minerals at high temperatures.5 Dental ceramic materials can be classified into four types by their chemical composition: glass-based, glass-based with crystalline fillers, crystalline-based with glass fillers, and polycrystalline solids. The processing methods used with each respective ceramic composition enable that material to be used in the anterior or posterior regions—or both—to restore everything from small caries and discolored teeth to larger structural defects, depending on the material’s specific indications and material properties.
“The availability of different dental ceramics helps the profession by providing us with materials for every type of restorative problem in esthetic dentistry, from conservative to aggressive,” explains Edward A. McLaren, DDS, MDS, professor, founder, and director of the UCLA Post Graduate Esthetics Program. “In some ways it can also be a hindrance to have too many choices, so it becomes essential for dentists to know the materials and their characteristics.”
Feldspars are naturally occurring crystalline minerals formed of aluminum silicate with potassium, sodium, barium, or calcium.6 Feldspars are combined with other minerals, such as quartz and kaolin, for improved esthetics and plasticity. Metals such as zinc, titanium, copper, iron, cobalt, nickel, and manganese are added for color; tin, titanium, or zirconium are added as needed for opaqueness.7
The minerals are mixed to a fine powder, then blended with either deionized water or special modeling liquid to create feldspathic porcelain paste. The resultant paste is layered and fired repeatedly to create either fully feldspathic porcelain veneers and crowns or veneers with metal, alumina, or zirconium substructures.8 Feldspathic porcelain is very brittle, and even with a strong core is only recommended for the anterior region.
“If I’m in the anterior part of the mouth with mostly enamel to bond to, that’s when I’ll use feldspathic porcelain,” says McLaren.
These ceramic systems typically have a core fabricated from alumina, spinel, or zirconia that is layered with a veneering ceramic, such as feldspathic porcelain, for esthetics.9 High-alumina ceramics are extremely strong, containing a minimum of 95% pure aluminum oxide. Highly pure zirconia (99.9%) contracts at a rate of 15% to 20% during sintering, delivering a predictable final fit and form.10-12
“Today, ceramics are applied to everything from implant abutments to posterior bridges, from full-arch cases to single anterior cases,” says Ryan Johnson, chief technology officer for Dental Arts Laboratories. “The production methods and materials have improved, so we’re seeing much better performance.”
Leucite is a potassium-aluminum silicate mineral added to feldspathic porcelain through one of two processes.7 The first, incongruent melting, involves melting one material (potasic feldspar) to a different crystalline material (leucite).10 The second “low temperature” process involves adding synthetic leucite powder to feldspathic porcelain paste.
Both processes increase crack propagation strength in restorations,11 but incongruent melting forms tetragonal leucite crystals that some research suggests provide more internal structural strength than room temperature cubic leucite crystals.13 Tetragonal leucite formation is also the recommended reinforcement method for metal-ceramic restorations due to its ability to form a thermal bond with both materials,14 making the entire restoration more stable.
Lithium-Disilicate Glass Ceramics
Lithium disilicate is composed of silica, lithium dioxide, alumina, potassium oxide, and phosphorous pentoxide, which is melted together and then cooled. The glass is heated at specific temperatures to produce crystalline growth. When optimal crystalline growth has occurred to maximize the material’s strength, the ceramic is pulverized into powder. The powder can be pressed into ingots or processed using other techniques.15 These processes result in raw materials that are either CAD/CAM milled or heat-pressed to reach a strong, monolithic, final restoration.
“If I’m bonding to dentin or there are issues of flexural effects on bond strength, my first thought is to use lithium disilicate and be as conservative as the preparation will allow,” McLaren says.
The press-to-metal technique was developed using second-generation low-fusion, high-expansion leucite-reinforced ceramics pressed onto precious metal (usually gold or palladium) alloys. These ceramics also can be pressed to zirconium dioxide substrates with matching thermal expansion coefficients16 to create an esthetically acceptable anatomical tooth structure.
A study in 2005 reported that more than 50% of all indirect restorations were metal ceramics.17 This was likely due to the ease with which these restorations are manufactured.18
According to Michael DiTolla, DDS, FAGD, director of clinical education and research at Glidewell Laboratories, dentists are shifting gears, however. They are voting with their prescriptions by requesting all-ceramic restorations instead of the PFM restorations that served dentistry well for the past 50 years. This is despite the fact that all-ceramic restorations were traditionally the ones that fractured the most, were the most technique sensitive when being adhesively bonded into place, and that dentists were the most reluctant to try for fear of fracture or failure, he elaborates.
Zirconia occurs as a natural mineral known as baddeleyite. The mineral comprises 80% to 90% zirconium oxide, with primary inclusions of titanium dioxide, silicon dioxide, and iron oxide. Zirconia exists in three separate crystalline phases: the monoclinic phase at room temperature, the tetragonal phase at approximately 1,200˚C, and the cubic crystalline phase at around 2,370˚C. Attempts were made to process this material similarly to other ceramics, but with limited success, because crystal transformation during the cooling process led to cracks. Engineers discovered that zirconia could be stabilized with small amounts of calcium, yttrium, or cerium, however. This advancement led to tetragonal zirconia, which was metastable at room temperature.19,20
The partially stable zirconia, or “greenware,” is then CAD/CAM milled to oversized restorations and single or multi-unit substructures for other applications,21 which are designed to compensate for the 30% volumetric shrinkage that occurs upon sintering. The requisite sintering also creates a more dense, stable, and wear-resistant restoration.22 Zirconia is becoming a standard material for use when fabricating posterior dental restorations. CAD/CAM technology provides the ability to process durable restorations at a fraction of the time, cost, and technique sensitivity of previous methods.
“When ceramics like zirconia evolved, they were processed to produce high strength, but machining and shaping restorations was very difficult, so you could not conveniently use them,” explains Van Thompson, DDS, PhD, professor and chair of the department of biomaterials and biomimetics at New York University College of Dentistry. “With CAD/CAM technology and the ability to use the materials in a partially fired (sintered) state, we now have a wonderful material that can be used in a routine and cost-effective way.”
Technology and Technique
Technology is a driving force behind advances in ceramics. A broader range of dental ceramic restorations and processing techniques are available thanks to technological advances from other industries, as can be seen with the impact of newer computers and high-speed processors on CAD/CAM, Johnson observes. The demand for biocompatibility, esthetics, durability, and less labor-intensive processing has always been present throughout the profession (eg, eliminating hand-layering, refractory die, and foil techniques), but today’s technologies have made achieving those objectives possible, he says.
“Many years ago, dentists were limited to using all-ceramic crowns for anterior restorations,” Thompson recalls. “These had to be fabricated by hand and in layers, which was a relatively expensive process. Today we have the ability to use ceramics in the posterior that are sufficiently strong so we can get away from metal altogether.”
Add esthetics and cost to the equation, and today’s materials for all-ceramic restorations become even more attractive. John O. Burgess, DDS, MS, assistant dean for clinical research at the University of Alabama at Birmingham, says that as the costs associated with metal, gold, and high noble alloys increased, so did the use of lower cost, high-strength all-ceramic materials that demonstrated better esthetics (eg, eliminating the metal band around the gingival margins). Further, the ability to fabricate all-ceramic restorations in a monolithic form—rather than layering—yields restorations with lower chipping rates than layered high-strength core materials, he adds.
“Lithium disilicate—when used properly as a monolithic, full-contour restoration—represents a major and significant change for dentistry,” McLaren believes. “I have no reservations about recommending it as much as I would have recommended gold 30 years ago.”
According to DiTolla, the monolithic concept isn’t new to dentistry. Cast gold was the first monolithic material used in dentistry. Although many dentists might admit that cast gold is an ideal restorative material, it is highly unesthetic, and most patients are averse to having it placed in their mouths.
Considerations for choosing which contemporary dental ceramic to use include time constraints, the patient’s oral habits, material characteristics, and the region in the mouth where the restoration(s) will be placed. Metal-supported ceramics and zirconia restorations have been avoided in the anterior due to their opacity.
However, CAD/CAM, monolithic restorations, and refined layering techniques are providing greater predictability, durability, functionality, and esthetics across available all-ceramic restorations. Additionally, material testing prior to introduction is lending greater insight into how materials will actually perform in the oral environment under different conditions.
“We have laboratory processes that have evolved to be predictive of clinical performance, which is something we didn’t have in the past,” Thompson clarifies. “We can subject ceramics to the conditions of the teeth and oral environment and determine how they will perform. This allows us to screen for failures, resolve issues, and be predictive.”
According to Kelly, this means that ceramic materials are engineered based on what has been learned about material stress and failure, yielding better results. Additionally, processing techniques have been enhanced based on knowledge of how laboratories, clinicians, or machines can impart damage on restorations, he adds.
If dentists remain informed about available ceramic materials, they’ll have more choices for providing esthetic and sufficiently strong restorations for the indication at hand, says Thompson.
“There’s hardly an intracoronal restoration that isn’t now being done with ceramics,” notes Kelly. “Veneers are very robust, and anyone who has a CEREC machine in the office can perform modern, conservative restorations by relying more on bonding than on placing an aggressive crown preparation.”
In the past, new ceramic alternatives were met with questions focused on what would be sacrificed—strength, longevity, or esthetics. Today, practitioners have the option of placing all-ceramic restorations that will stand the test of time and wear—and still satisfy even the most demanding patients.
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Inside Dentistry and our sister publication Compendium offer in-depth, insightful continuing education activities on this and many other topics to keep you current on the latest advances in dentistry. To learn more about the latest developments in dental ceramics and earn continuing education (CE) credit in the process, don’t miss these recent activities:
Why Porcelain Breaks
Chairside Advice from the Experts
Should unsupported glass-ceramic restorations be conventionally cemented?
“I don’t necessarily agree that you should use conventional glass-ionomer cements. The problem is that they are much more soluble than resin-based cements. If, over time, the cement solubilizes and fails, exposing the glass ceramic to saliva, the restoration can undergo hydrolytic degradation, which weakens the material.”
Edward A. McLaren, DDS, MDS
Professor, Founder, and Director, UCLA Post Graduate Esthetics
Director, UCLA Center for Esthetic Dentistry
Founder and Director, UCLA Master Dental Ceramist Program
What is the best practice for using a primer with
“The big controversy among opinion leaders is whether you need to use a zirconia primer every time. What we have found is that if the preparation is retentive, then using a traditional cementation technique is fine. Otherwise, you definitely need to prime and bond the restoration.”
John Powers, PhD
Senior Vice President and Senior Editor, The Dental Advisor
How can I ensure a strong bond between natural tooth and zirconia when a glass-ionomer cement is used?
“The lab can sandblast the interior of the restoration with a small-particle size alumina at low pressure to reduce the risk of the zirconia separating from the tooth. Even though there are some studies that suggest sandblasting may degrade the strength of the zirconia, it does not degrade the bond strength. There is absolutely no bond between the cement and the zirconia unless there is some mechanical retention.”
Russell Giordano, DDS
Assistant Professor and Director of Biomaterials,
Goldman School of Dental Medicine, Boston University
Should I be concerned that full-contour zirconia will wear opposing dentition?
“What we found after 200,000 and 400,000 cycles in our wear machines (7 to 9 years of clinical wear) is that highly polished zirconia produces the least enamel wear; in fact, significantly less than some commonly used feldspathic porcelains. Polished-then-glazed zirconia wears significantly more than polished-only zirconia. It seems as though the glazing material produces most of the enamel wear as far as enamel wear against zirconia is concerned, with even polished-then-glazed zirconia exhibiting aggressive enamel wear.”
John O. Burgess, DDS
Assistant Dean for Clinical Research, University of Alabama
Excerpted from: Johnson P. Next generation materials . Inside Dental Technology. 2012;3(3):24-31.