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Smooth Operator

The increasing demand for natural-looking restorations has been a major driver in the development of new materials and techniques that perform well in terms of function and esthetics. Ceramic materials currently used in indirect restorations include feldspathic porcelain, leucite, lithium disilicate, spinel, alumina and zirconia. And when it comes to all-ceramic restorations, hand layering, pressing, casting, and machining via computer-aided design/computer-aided manufacturing (CAD/CAM) are all used to fabricate these lifelike prostheses.

Thanks to these innovations, there are now ways of creating allceramic restorations that totally rock the natural look, while also offering the requisite strength. But regardless of the material used or the technique employed to create them, such prostheses would in no way pass for real teeth without the skilled hand of a ceramist, lab technician or practiced clinician savvy in the art of finishing and polishing.

Finishing and polishing are like the peanut butter and jelly of the restoration milieu. They are usually spoken of in one breath, as if they were one procedure. There is a degree of dovetailing, as both involve the use of rotary handpieces and abrasives, but they are distinct processes.

Finishing generally involves contouring and removing any imperfections resulting from the manufacturing process. “If we press something, there will sometimes be a bit of surface roughness due to the pressing process,” explains Ed McLaren, DDS, MDC, director of the Center for Esthetic Dentistry at the University of California, Los Angeles. As a master ceramist, McLaren notes that although machining has evolved significantly, it doesn’t match the exact surface characteristics of a natural tooth.

Though finishing techniques vary, they are all aimed at producing realistic contours, clean lines, smooth surfaces and an optimum fit. In the case of a pressed or machined restoration, for example, the sprue must be removed, in addition to any other sort of detailing or “cleaning up.”

Even when a restoration is created with the sophisticated technology of a CAD/CAM system, it must still, at least, be finessed to ensure proper fit. And in practices that have in-office design and milling capability, this task sometimes falls to specially trained dental assistants. This adjusting can be done by fitting a restoration onto the die-cast model of the dentition, or onto a prep in the patient’s mouth, and using bite registration tape to pinpoint high spots that need grinding down.

“At the very minimum,” says McLaren, “there’s going to be a need for polishing — and that would be considered finishing, too.”

 

TECHNIQUES FOR ADDING COLOR AND DEPTH

It’s not all about abrasives when it comes to finishing ceramic restorations. As Ed McLaren, DDS, MDC, a master ceramist and director of the Center for Esthetic Dentistry at the University of California, Los Angeles, explains. “A tooth has layers of different translucencies or opacities. With the layering process, we can actually mimic what’s in a tooth.” This is done in a variety of ways — for example, by using stains and enamels to integrate shading, depth of color and translucency into the buildup so it matches neighboring teeth. But though layering is considered the height of esthetics, surface painting with stains can also create a natural look, and is perfectly suited for posterior teeth.

McLaren provides some examples, saying, “With CAD/CAM, you surface paint to create three-dimensional illusions. For posterior restorations, whether monolithic, pressed or machined, you can put a little paint on the surface, or simply glaze and polish it — or just polish it.” On such restorations, stains are subtly layered onto the surface to mimic lifelike characteristics. For instance, blues provide the illusion of translucency, while ochres and yellow add natural chromas. Glaze, a clear mix of glass powders, can be applied to add luster and seal in color. The restoration can then be fired, often in a small, chairside furnace.

I CAN’T BELIEVE IT’S NOT BUTTER

Polishing enhances esthetics through abrasion that, at the microscopic level, transforms light-dissipating, rough surfaces into light-reflective, smooth surfaces. But it offers more than just eye candy. Polishing results in glossy surfaces that help prevent plaque from gaining a foothold.

“A properly glazed and/or polished ceramic material is critical in optimizing patient hygiene,” reports James Neuber, RDT, a master ceramist in optimal dental physiology and cosmetics at Ocean Ceramics, based in Coquitlam, British Columbia. Referencing the influence of material density when creating a smooth surface, he adds that much has to do with the type of ceramic that’s being polished. “Ceramic materials have varying degrees of grain size,” he notes. “A key factor for a successful surface is that ceramics need to be processed with optimum density.” The way to ensure this, says Neuber, is through a combination of proper manufacturing of raw materials supplied to the lab, and proper lab processing of the materials. “For example, lowquality zirconia that isn’t isostatically pressed by the manufacturer is very porous and difficult to glaze,” he explains. “And porous materials are prone to plaque buildup and poor hygiene for the patient, regardless of the level of home care.”

Although helping to reduce plaque adherence is important, many clinicians feel the most important thing about polishing is that it makes restorations less abrasive and easier on opposing dentition. “Lithium disilicate and zirconia are very hard, for example, and there was concern that these types of restorations would be very abrasive,” McLaren notes. “But if they are polished well, they are less abrasive than conventional porcelain. And, actually, if you put a layer of glaze over a very smooth zirconia or lithium disilicate, it actually creates a more abrasive surface. If you use a glaze layer, it is important to polish it, or microscopically it will be as rough as sandpaper.”

McLaren explains this is because most glazes and ceramics are twophase materials: part glass and part crystalline. “Some have a very small crystal size, and the edges of those crystals can be sharp microscopically. Yet, as long as they are polished smooth after glazing, abrasiveness is not a problem in these harder materials,” he says, adding, “Whether you glaze or not, however, you have to go back and polish.”

Jack Ringer, DDS, a general practitioner based in Anaheim Hills, Calif., agrees that ceramic restorations must be glazed and/or polished in order to tame abrasiveness. Ringer, who will be installed as president of the American Academy of Cosmetic Dentistry on April 27, adds that there’s another dimension to consider, as well. “The most important role of finishing is to create a sealed surface for the porcelain,” he says. “If porcelain is left unglazed or unpolished, it will absorb moisture, causing it to stain in the oral environment.” Not a good thing in an esthetic restoration.

 

POINT of SALE

 

FINISHING AND POLISHING ALL-CERAMICS

  • Finishing is necessary to ensure the proper final shape and fit of an indirect restoration.
  • Polishing an esthetic restoration is critical in creating a smooth surface that properly reflects light, while discouraging the formation of plaque.
  • Polishing may even take the place of glazing in sealing restorations and taming abrasive characteristics that could potentially harm opposing dentition.
  • Advise your clients that it’s wise to invest in an armamentarium containing a wide range of finishing burs and polishing tools that can address the variety of ceramic restorations available.
  • For practices equipped with in-house CAD/CAM capability, savvy dentists will want to have a furnace and supply of stains available in order to efficiently deliver top-quality all-ceramic restorations.

SEAL IN FRESHNESS

Unlike polishing, glazing is not necessarily a foregone conclusion for all-ceramic restorations. In a restoration machine-milled from a bisquefired, sintered blank, for example, the glazing process may be skipped. Current thought is mixed on this, with some studies supporting polishing in place of glazing,1,2 while others suggest that abrasiveness may be higher in polished ceramic that’s left unglazed.3,4 Despite the mixed opinions, there are still many clinicians and technicians who advocate both. “All ceramic materials require a glaze to be properly matured at the appropriate temperature, and then polished with a pumice/diamond compound whether in the lab or in the mouth,” opines Neuber.

“All materials can have some porosity and microcracks,” explains McLaren. “The process of glazing will seal the surface, can add a little color and luster, and heal those microcracks, so you actually have a tougher surface that’s harder to crack.” McLaren, who glazes his restorations between a prepolish and final polish stage, again cautions, “After glazing, you still need to polish for the roughness, because glaze alone has proved to be the worst surface of all in every study.”

SPIN DOCTORS

Sales professionals will want to get themselves up to speed on what constitutes a well-stocked finishing and polishing armamentarium. This way they can be prepared to offer the latest instruments to clients who are either already well-versed in all-ceramic materials, or who are just venturing into this realm.

Finishing and polishing instruments generally comprise a head, neck and shank designed to fit onto a rotary handpiece. Some come in the form of attachment heads that snap onto a mandrel. They are constructed according to the material on which they will be used — meaning, instruments suited for ceramics are different than those used for other types of restorations. Instruments can also vary within the ceramic family itself.

“The same finishing and polishing instruments can be used on lithium disilicate and zirconia due to these materials’ similarities in hardness,” notes Ringer, “whereas ‘softer’ materials, like some of the pressed ceramics, require different finishing materials and techniques.” He adds, however, that while feldspathic porcelain needs a softer material for the initial polishing process, the material used for the final polish can be the same for both feldspathic and strengthened ceramics.

When it comes to finishing and polishing instruments, there’s enough variety to make a clinician’s head spin. But account executives can help by recommending complete systems that make this process a no-brainer. Because there are countless ways to finish and polish, however, it pays for reps to at least know the difference between a finishing bur and a polishing wheel.

 

TESTING, TESTING

OK. So how the heck do you determine just how rough, smooth or hard a surface actually is? When venturing into all-ceramic restoration territory, sales execs are likely to hear terms such as Vickers, Mohs, Rockwell and SEM being bandied about. These are various kinds of tests and ways of assessing the hardness and smoothness of ceramics.

Scanning electron microscopy (SEM) is a primary method used in dentistry to verify the relative smoothness of a surface. It collects information by scanning the surface with a focused electron beam, with the resulting images providing an indication of the material’s condition. Other methods include visual assessment and atomic force microscopy. The last of these examines surface topography in three dimensions.5

There are numerous ways to test material hardness. According to James Neuber, RDT, a master ceramist at Ocean Ceramics, a primary method is the Vickers hardness test. “The quest has always been to have a ceramic material with a Vickers rating similar to natural teeth,” he says. The Vickers test is based on degree of indentation caused by a square-based diamond pyramid. Its values are written as HV. For example, diamond hardness in the Vickers test would be 10,000 HV.

Like the Vickers test, the Rockwell hardness test, often used in metallurgy, also relies on depth of indentation.

The Mohs scale is a relative hardness assessment that’s based on scratch resistance, or the ability of one material to scratch another. It’s a primary test used to determine hardness of minerals, such as diamonds, which check in at a Mohs hardness of 10 — one of the hardest materials known to man.

CUTTING REMARKS

For trimming and finishing, instruments must have the ability to cut. Although abrasive stones may be used when finishing softer materials, such as porcelain (which has a Mohs hardness rating of 6 to 7), other types of ceramic materials can range up to 9. For these, the cutting instrument must be harder. For this reason, diamond-coated burs — which boast a Mohs rating of 10 — are top players when it comes to creating contours and making adjustments. These burs are formed from stainless steel shanks, onto which tiny diamond crystals are electroplated or bonded. They are available in various shapes, sizes and grits, and because they are operated at very high speeds (which generate heat), they’re used with a water spray to keep things cool.

Tungsten carbide ranks just behind diamond in hardness, sporting a Mohs rating between 8.5 and 9. Tungsten carbide burs are often multibladed, with straight or twisted flutes, which may be toothed. These instruments may offer smooth tips, which is why some clinicians prefer them for contouring work along gingival margins.

There are divided opinions on whether diamond or tungsten carbide leaves a smoother finish, paving the way to the next step of polishing. Some clinicians believe that the regularity of carbide burs provides the smoothest surface.

But while he thinks the use of a carbide bur is fine for cutting material to remove a restoration, McLaren emphasizes that he would never use a carbide for post-cementation adjustments because of cracking. He reports that scanning electron microscopic imaging has revealed that carbide burs create more cracking than diamonds. Instead, he recommends a fine diamond in whatever shape is needed for the anatomy of the tooth.

 

TALK THE TALK

 

  • Bisque-Fire: The process of bringing material to a sintered state.
  • Glaze: A colorless mixture of glass powder used to seal ceramic restorations.
  • Mohs Hardness: A scale of 1 to 10 that determines relative material hardness based on one substance’s ability to scratch another.
  • Sintering: To heat materials until they cohere prior to vitrification.
  • Sprue: A nub of excess material left from the process of manufacturing an indirect restoration (it can be likened to an apple’s stem).
  • Vitrification: Ceramic that’s melted into a glassy, nonporous state during heating.

THE SHINING

Compared to finishing, polishing necessitates a softer touch. Polishing instruments, available in grits ranging from extra-coarse to super-fine, are flexible enough to accommodate contours. Designed for efficient polishing of surfaces ranging from concave occlusals and interproximals to convex buccals, linguals and crowns, these instruments — usually diamond- or aluminum oxide-coated rubber or silicone — are available as cups, flames, wheels, points, discs and brushes.

As with finishers, the selection of polishers depends on the type of ceramic being polished. “While with porcelain you can use almost anything, lithium disilicate and zirconia are too hard for silicone wheels,” McLaren notes, adding that harder rubber instruments impregnated with diamond particles are called for. In addition, he says that polishing with an air-driven instrument doesn’t generate enough torque for these harder materials; he thus recommends electric handpieces for polishing.

Due to the advances in materials and techniques, the elimination of steps — such as pre-polish and glazing — is certainly up for debate. In fact, it is difficult to find consensus on best practices when it comes to these last steps in the all-ceramic restoration process. But one thing is for sure: The material hasn’t been invented yet that can forego the processes of finishing and polishing. So until that happens, smooth operators will continue to be needed to ensure the success of allceramic restorative procedures.

  1. Rosenstiel SF, Baiker MA, Johnston WM. A comparison of glazed and polished dental porcelain. Int J Prosthodont. 1989;2:524–529.
  2. Owen S, Reaney D, Newsome P. Finishing and polishing porcelain surfaces chairside, International Dentistry — Australasian edition. 2011;6(4):68–73.
  3. Al Wahadni A, Martin DM. Glazing and finishing dental porcelain: a literature review. J Can Dent Assoc. 1998;64:580–583.
  4. Tuncdemir AR, Dilber E, Kara HB, Ozturk AN. The effects of porcelain polishing techniques on the color and surface texture of different porcelain systems. Materials Sciences and Applications. 2012;3(5):294–300.
  5. Yilmaz K, Ozkan P. Methods for the generation of smoothness in dental ceramics. Compend Contin Educ Dent. 2010;31:30–38.

BONUS WEB CONTENT : GLASS AND CRYSTAL

The types of materials used in crafting all-ceramic restorations include those composed of various levels of glass, and those composed of crystals. Material and technique selection is, of course, dependent upon desired effect and where the restoration is to be placed, with more esthetic options chosen for anterior teeth, and those offering more strength selected for posterior placement.

Feldspathic porcelain offers the best esthetics in the ceramic family. Due to the material’s high glass content, restorations made from it can exhibit the kind of translucency, opalescence and fluorescence seen in real dentition. These optical characteristics come with a price, however. They typically lack strength, so they are either used in anterior restorations, where they can shine esthetically, or they’re layered over a substructure, which creates a strong restoration, but one with an opacity that doesn’t look real. Fusing the material over an alumina or zirconia (rather than metal) framework can be both strong and relatively esthetic, though lacking in translucency. Such restorations have also shown a tendency to chip.

Another type of ceramic adds crystals of lithium disilicate, alumina, leucite, spinel or almunia/zirconia to feldspathic porcelain in an effort to achieve both strength and beauty. This is called particle-filled glass, and popular members of this group — which can be pressed or machined using CAD/CAM technology — are leucite-reinforced ceramics and lithium disilicate. Even in demanding posterior locations, these harder ceramic materials, which can be used monolithically, are rock solid.

Also in this category is crystalline-based material partially filled with glass. Otherwise known as glass-infiltrated ceramics, these exhibit high flexural strength. This group includes materials such as alumina, zirconia and spinel.

Polycrystalline ceramic is yet another variety. It comprises aluminum oxide and zirconia and is a nonglass formulation engineered to provide strength while limiting the tendency to crack. Unfortunately, it is too opaque to be considered lifelike. These ceramics are often used as core material, though zirconia, stabilized with yttrium oxide, is used as a substrate, but has also been gaining traction as a monolithically constructed restoration.

MENTOR April 2013, 4(4): 34–37.