Fig 2-3 Composition of dental ceramics based on the concept of them being composites consisting of a “matrix” and “fillers.” (a) Composition of dental ceramics used to veneer substructures. (b) Composition of stronger, nonveneering and structural dental ceramics. CAD/CAM, computer-aided design/computer-assisted manufacturing. (See Link 3 for more information.)
In Fig 2-3, we can see that many different dental ceramics, even those from different manufacturers, are really quite similar. For example, Vita Mark II (now Sirona Vitablocs) computer-aided design/computer-assisted manufacturing (CAD/CAM) blocks (Vita) consist of approximately 40% filler particles (crystalline nephaline and albite, and higher-melting glass particles) in a sea of feldspathic glass, while IPS EmpressCAD (formerly Empress, Ivoclar Vivadent) consists of 40% leucite (the crystalline mineral found in metal-ceramic porcelains) again in a sea of feldspathic glass. Similarly, some ceramics differ only in how they are processed; for example, e.maxCAD and e.maxPress (Ivoclar Vivadent) both contain about 70% lithium disilicate crystals in a sea of residual glass (residual from the ceraming step covered in chapter 5), but the former is processed in the laboratory by firing after CAD/CAM machining, whereas the latter is pressed. In general, higher-volume percentages of crystalline filler provide higher strength and fracture toughness. Therefore, the lithium disilicate (Ivoclar Vivadent) and In-Ceram ceramics (Vita) containing approximately 70% filler are listed as structural ceramics and have broadened clinical indications over Vita Mark II and Empress Esthetic ceramics (see chapter 3).
Slide-by-slide lecture discussing Fig 2-3.
By viewing these materials as composites, it is possible to identify similarities in properties; for example, both Vita Mark II and Empress Esthetic are easily acid etched and have remarkably similar clinical indications and behaviors (see chapter 3). Therefore, new ceramics can be more easily understood when introduced by examining their components (ie, their microstructure) and comparing them to that of known materials with similar properties. (Fracture toughness also tends to correlate with clinical indications.)
Predominantly Glassy Ceramics
Dental ceramics that best mimic the optical properties of enamel and dentin are predominantly glassy materials. Glasses are three-dimensional (3D) networks of atoms having no regular pattern to the spacing (distance and angle) between adjacent or next-neighboring atoms; thus, their structure is amorphous, or without form. Predominantly glassy ceramics, also called porcelains, derived from European porcelains in which the original flux—calcium carbonate or chalk (as in the white cliffs of Dover)—was replaced by feldspathic glass.1 Feldspar is a silicate mineral belonging to a family called aluminosilicate glasses. Glasses based on feldspar are resistant to crystallization (ie, devitrification) during firing, have long firing ranges (ie, they resist slumping if temperatures rise above the optimal), and are extremely biocompatible. In feldspathic glasses, the 3D network of bridges formed by Si-O-Si bonds is broken up occasionally by modifying cations such as sodium and potassium that provide charge balance to nonbridging oxygen atoms (see Fig 2-2). Modifying cations alter important properties of the glass, for example, by lowering firing temperatures or increasing thermal expansion/contraction behavior. The low density of these weaker bonds makes the materials themselves weak.
When crystalline feldspar rock is melted and then cooled quickly, it does not recrystallize but remains as an amorphous glass at room temperature. Link 4 illustrates this melting and rapid cooling step (fritting) during the manufacture of a dental porcelain. The modifying cations in the feldspar lower the firing temperature of the porcelain, and this is how manufacturers have lowered firing temperatures over the years. However, with too much modification, the glass becomes soluble, prompting experts at the International Standards Organization to develop a solubility test for the international ceramics standard (ISO 6872).
Mined mineral feldspars in a Vita factory being melted and quenched.
Lead in Dental Porcelains
Another modifying cation causing much recent consternation is naturally occurring lead. In 2008, lead levels around 210 ppm (parts per million) were found in a fixed dental prosthesis fabricated in China for a dental office in Columbus, Ohio, causing all sorts of negative and alarmist media attention.2 But let’s put this amount into perspective. Lead is found as a trace element in virtually everything that comes into contact with the earth. Plants grown in soil as well as surface water and groundwater all extract tiny amounts of lead from the earth. In 2005, a comprehensive study was published that analyzed the French diet based on 41 different categories of food (including regional, seasonal, and national sources).3 It turns out that the average French person eats 18.4 micrograms of lead each day. This amount may seem alarming, but 1 microgram is 1 millionth of a gram. In monetary terms, with gold at $960 an ounce, 18.4 micrograms of gold would fetch only 6.25 one-hundredths of a penny (0.00625¢). That’s how tiny 18.4 micrograms is.
So back to the lead-laden prosthesis from China, assuming that all of the porcelain from one crown were to be eaten in 10 years (which we know is absurd), the daily increase in lead in the body would be 0.078 micrograms, a whopping increase of 0.000004%. If this crown were eaten over only 1 year, that’s still only an increase of 0.00004%. Clearly, dental porcelains are a really, really lousy source of dietary lead. Far better sources include drinking water, bread, fruits, and soups. These foods will provide you with thousands of times more bioavailable lead each day!
Particle-Filled Glasses
When first produced, feldspathic glass is relatively colorless and transparent, so manufacturers add filler particles to adjust the color, opacity, and opalescence. These fillers yield the highly esthetic, but weak, veneering materials for use as small restorations and veneers or as the esthetic outer coating on stronger substructures such as metals and polycrystalline ceramics. To work well on metal substructures, the porcelain must contract at the same rate as the metal during cooling after firing. So yet another filler particle is included for metal-ceramics, the crystalline mineral leucite. Using the Edisonian research approach, Weinstein, Weinstein, and Katz arrived at what they called “component number 1”—a porcelain frit that had a thermal expansion coefficient of nearly 20