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Choices in the Zirconia Marketplace. A Short History Leading to Zirconia

November 1, 2006
by James L. Soltys, DDS


It comes as no surprise that in today’s “esthetic age” of dentistry that dentists continue to seek a material that can take the place of metal, in both strength and longevity, but with an overall “white” color. Many believe that Zirconium is the answer to the long search. This article is designed to inform the reader what this new material is, how it is processed, and what choices the dentist and lab has to use zirconium.

The quest for a modern all ceramic material for tooth restoration started in the 1960s with McClean’s porcelain jacket crown.1 Initially made of feldspathic porcelain, and later strengthened by an aluminous porcelain core, the material was limited to anterior crowns only due to a low compressive strength. Bonding, resin cements, and silanization had not yet been invented. In 1963,2 Weinstein, Katz, and Weinstein brought us the discovery of the metal oxide-ceramic bond that became the standard of strength in ceramic crowns, the Porcelain-Fused-to-Metal crown (PFM.).2 To this day, it remains the standard to which all materials are compared for physical properties. Although the PFM is strong, the metal, or masking of the metal, definitely poses a problem in esthetics.

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The 1970s brought little in change to the marketplace; it wasn’t until the 1980s that an explosion of new products hit the marketplace. In 1983 both Dicor (Corning Glass) and Cerestore (Coors Biomedical) were introduced. Both of these materials were short lived, but introduced ceramic technology that is still used today. Dicor was the first cast glass using the lost wax technique, then surface stained for color.3 This technique, which is still used in current pressed ceramics, such as Empress (Ivoclar) or Finesse (Dentsply). Cerestore also used a lost wax technique to press an enlarged magnesium oxide core that was then fired and shrunk prior to placing subsequent veneering porcelain.4 This was the precursor to Empress2 (Ivoclar), and InCeram Spinell (Vident). Dicor was initially cemented with zinc phosphate and had a low survival rate for posterior crowns. Malamet discovered by bonding the crowns in place, a much higher success rate was achievable, but again best survival was on anterior teeth.5,6

The 1990s brought rapid change to the ceramic marketplace. Three different technologies were released almost at the same time: InCeram (Vident), Empress (Ivoclar), and Cerec/ Celay (Siemens). InCeram introduced “slip casting” where an all ceramic core was first fabricated, then a glass was baked into this sponge-like core to fill the spaces and strengthen the core.7 A veneer layer was then fired on top of this core, as in a PFM. The result was a, then unheard of, very hard core. Empress crowns were made by investing a fully waxed crown, burning out the wax in an oven, then pressing a molton ingot of a leucite based glass into the mold.8 This material could be etched and silinated, then bonded to the tooth. Of significance, veneers, inlays/onlays, and full contour crowns could be fabricated using the same process. The resulting restoration could be surface stained, or cut back and layered for even better esthetics. Empress2 followed using the same press casting to make a core out of a new ceramic, lithium disilicate.9 The original Cerec/Celay machine was a copy milling machine, not unlike a key cutting machine, to duplicate a resin inlay into a solid block of ceramic, which could then be bonded into place.10

Why is there a need for a strong all ceramic material, when the PFM is such a strong restoration? The answer, of course, is esthetics. In our hyper bleached “natural” tooth shaded society, the presence of metal doesn’t look as good. There is a legitimate need for the low percentage of patients with a metal allergy. The quest for strength is most important for posterior restorations and fixed bridgework; the quest for ultimate esthetics is for anterior teeth.

All ceramic crowns look better than a metal based crown because the color of the metal must be “opaqued” entirely before a natural looking ceramic is layered on top of the coping. This not only adds thickness to the crown, but the opaque layer entirely blocks the passage of light into the body of the tooth. This bounces light back from the crown and makes the crown too high in value, or brightness, at the thinnest part of the crown, which is the gingival margin. The resulting appearance is that of obvious crown work present in the patient’s mouth. In contrast, an all ceramic crown has a core that may be transluscent, as in castable ceramics, that actually radiates the natural color of the tooth, especially in thin sections. Or an all ceramic crown may have a dense white or dentin colored core that does not let light pass through, as in metal, but the white core can be covered with a veneering porcelain that still looks better than the dense metal core. Although the core is not translucent, it still has better light refractory properties.11

Later in the 1990s, Procera (Nobel Biocare) was introduced.12 Procera used a process of pressing an aluminum oxide core made on a computer generated enlarged die (to compensate for firing shrinkage), then “sintering” or refiring the ceramic core at a high temperature under pressure, to form a hard and dense aluminous oxide core to which a veneered porcelain was added. There is no glass phase filling in the matrix gaps as in InCeram. The sintering process reduces spaces within the ceramic and thus is accompanied with overall shrinkage. To this point, sintered alumina oxide was the hardest and strongest nonmetal crown material. Computer generated CAD CAM lab work had started. (CAD = Computer Aided Design CAM = Computer Aided Manufacturing.)

PROPERTIES OF ZIRCONIUM

In the 2000s various companies introduced CAD CAM systems that milled a new material — Zirconium. This material also shrinks upon sintering, but can be milled in the softer partially sintered state with ease.

Zirconium is a naturally occurring element, found in the Periodic Table of Elements with atomic number 40, and is classified as a transitional metal. Zirconium occurs in nature as the mineral ZIRCON (ZrSiO4), which is then purified to Zirconium. Zirconium is a silvery metal, very corrosion resistant, and is similar to titanium in properties. When zirconium is combined with oxygen, it becomes zirconium oxide, also known as ZIRCONIA, a very biocompatible and strong ceramic. This has been used in the biotechnology industry since 1976 in the form of replacement hip and bone replacement prosthesis. When it is combined with 3% Yittria, it becomes an even stronger oxide, ZrO2Y2O3.13 This is the form used in dentistry, but is still commonly referred to as just “zirconia”. The oxide powder is hardened by sintering under heat and pressure. Partially sintered zirconia, also known as partially stabilized zirconia (PSZ), is an easily milled solid, similar to PVC plastic in feel. Upon further sintering, the material shrinks more, but becomes significantly stronger, and is called tetragonal zirconium polycrystal (TZP).

Dental ceramics can be classified into two categories: those that contain glass interspersed in a crystalline matrix, or dense polycrystalline oxides with no glass component (Table 1). Those with glass are weaker materials because the glass undergoes degradation in the presence of water; cracks form, and eventually the material fails. The polycrystalline materials are sintered, degrade little in water, and are more difficult to initiate a crack. Unlike the glass containing ceramics, the polycrystalline ceramics cannot be acid etched, silinated and bonded to the tooth.

The strength of a material is the ability to resist stress, and equals the load per unit area. The unit of measurement in ceramics is Mpa, or megapascal. In Table 1, the strength as measured in a three point flexural test is presented. Notice the difference in strengths between the glass containing ceramics and those that are sintered polycrystalline ceramics. It is obvious that the zirconia products are quite an improvement in str
ength.14,15

All ceramics eventually fail by catastrophic failure through a crack that propagates through the entire thickness of the ceramic. Different components are added to the glass matrix to require more energy to go around the particle and thus interfere with the crack propagation. This is also true of the polycrystalline ceramics, and it takes significantly more energy to propagate the crack through the sintered crystals.

Zirconia is different. When a crack is initiated in the zirconia, the molecular structure of the zirconia changes from the Tetragonal phase to the Monoclinic phase. As this occurs, a 4% volume increase occurs that “squeezes” the crack shut.15 Thus, it is claimed, the zirconia coping can stop crack propagation from occurring.

Along with the increased strength of the fully sintered zirconia comes the ability to use this material as a framework for a fixed partial denture. A drawback to other forms of ceramic for bridge frameworks was the required surface area of the connector between the pontic and the bridge retainer coping. In metal ceramics, the connector can be as little as 5sq millimeters. With earlier ceramics such as Empress2 (Ivoclar) and InCeram (Vident), the connector needed to be 12-16mm. With zirconium, a more reasonable 7-9mm is obtainable for anterior and posterior bridges.16

SINTERED VS. UNSINTERED ZIRCONIUM

The procedure to work with zirconium is through a milling machine. This is where the lab industry has turned to CAD CAM technology. This stands for Computer Aided Design Computer Aided Manufacturing. The human element comes from designing the product on a computer screen (the CAD), then a computer controlled milling machine shapes the block of zirconia (the CAM) according to the computer design. This high technology system is a marvel to watch; the marginal accuracy is incredible. The milling process uses two forms of zirconium. The softer partially sintered form is easier to mill, takes less time to mill, and allows the milling burs to last longer.

This form has the drawback that the final sintering process shrinks the zirconia another 20-23%. The computer design module contains algorithms that account for this shrinkage in order for the final coping to precisely fit the original die. Of interest, the internal surface of the coping can be milled for a precise layer of cement thickness separate from the accurate coping fit to the crown margin.

Some machines mill the fully sintered form of zirconia. This is a significantly harder material to mill. The machine must use a liquid cooled closed system with diamond cutting burs. The milling time is significantly longer –hours instead of minutes, although there is no additional sintering time. Milling the surface of the harder zirconia may lead to surface imperfections causing potential crack initiation.

MILLING BASICS

Acuracy in the fit of the final coping is dependent on how many axis of rotation through which the block of ceramic and/or the milling burs can three dimensionally move during the production process, five being the most axis available. There are three translational axis: forward-backward, left-right, and up-down; there are two rotational axis: rotating around the holder like an axle, or rotating on a clockwise-counterclockwise spin (Fig. 1). It is also dependent on the minimum radius of the cutting tip of the milling bur; if the line angle is smaller than the bur, the coping will be over milled with a subsequent gap at the axial wall junction (Figs. 2 & 3). This would be most prominent at the depth of a sharp shoulder, box preparation, or oclussal-axial wall junction.

CAD CAM systems fall into two categories: a wholly in-office unit (Cerec InOffice), or a lab based system (all the others). The Cerec InOffice is designed to manufacture the final product from a solid block of ceramic or composite resin. The lab based systems mill a ceramic that will be used as a crown coping or bridge substructure to which a veneering porcelain will be added, the same concept as a porcelain to metal framework. This difference in the manufacturing process defines the available strength in the resulting restorative product.

It is very important to note that for a dentist to adjust any part of a zirconia restoration, it must be done with a diamond bur USING WATER SPRAY.

CURRENT SYSTEMS IN THE NORTH AMERICAN MARKETPLACE

CEREC InHouse (Sirona)

CEREC has been around in different forms for 20 years. It originally was a copy milling machine, like a key cutting machine at your local hardware store. A plastic pattern made in the mouth was copied via milling a piece of solid ceramic or composite. The initial ceramic used was DICOR. The second version added onlay capacity into the machine; the third and current generation has added more complex three dimensional design and milling capabilities.

As mentioned, this in house system produces a final restoration, not a framework. There is an obvious advantage to placing a fixed restoration at the day of preparation, avoiding the need for provisionalization and a separate cementation appointment. This is a four axis milling system with two diamond milling burs; a 1.6mm tapering 45 degree end cone diamond and 1.2mm or 1.6mm straight cylindrical diamond. The milling blocks are a feldspathic porcelain (VitaBlocs by Vident), a leucite based ceramic (ProCAD by Ivoclar) or a composite resin block (Paradigm MZ-100 by 3M). Material strength of a glass based ceramic (or composite resin) has a certain level of strength, and thus expected longevity, as compared to the stronger sintered ceramic products. The milling blocks come in various single shades, and one brand sells a tri-colored block. Final shading or color match is a achieved with surface stains. The machine can make veneers, inlays/onlays, or full coverage crowns. All three materials can be adhesively cemented with a resin cement.

The process starts with taking an optical impression in the mouth, or of a die poured from a traditional impression. The optical wand is connected to a self-contained chairside computer processor and screen. The operator then designs the restoration via software that identifies margins, contact areas, and occlusal morphology. This information is sent to the milling machine and the restoration is made.

Cerec InLab

The Cerec InLab uses the same machinery and software/processor components as the Cerec InOffice. Instead of using an optical scanner to “take an impression” in the mouth, a cast is mounted in the milling unit and scanned with a built in laser. This information is read by the processor, then the operator designs the restoration on the computer screen. The InOffice ceramic blocks can be used by the InLab, or different Vident InCeram ceramic blocks that comes in the choice of Spinell, Alumina, or Zirconia compositions that are glass infiltrated. A recent addition are partially sintered Alumina and Zirconia blocks that must be milled at a larger size then sintered to an accurate smaller size. These can be used for frameworks up to 40mm. With all InCeram blocks, instead of a full contour finished restoration, the coping or framework is made that a veneering porcelain is fired onto. This provides a stronger restoration, but one that cannot be made in 90 minutes, as the InCeram process has a longer time component. A solid wax block has recently been introduced to mill a wax pattern that can then be pressed or cast in the material of the lab’s choice. The lab can work on only one restoration at a time.

Nobel Biocare Procera

Procera is a ceramic product consisting of a highly dense Aluminous Oxide or Zirconium Oxide sintered core. Again, there is no glass phase in this polycrystalline ceramic. The process begins in the lab where the trimmed die is placed in a scanning machine. A stylus probe is placed in contact with the die which is on a spinning turntable. The probe records the data into a computer which is used to design the crown coping. This informa
tion is then transferred via a modem to Nobel Biocare who fabricates the coping at one of two centers, then mails the coping back to the lab. The lab then finishes the crown by firing a compatible ceramic onto the framework. Thus the investment to the lab is for the scanner and computer only, not the milling machine. Procera can also be used to manufacture custom zirconia or titanium implant abutments for various implant manufacturers. A larger scanner is also available for the manufacturing of multi unit frameworks for implant supported bridgework out of solid blocks of titanium.

Long span bridgework for natural teeth or implants using partially sintered zirconium has just recently been added to the Procera line.

3M Espe LAVA

LAVA is a Zirconia based ceramic product for single units or bridgework up to 47mm in length. The cast is mounted in a scanning unit that uses three lasers to record the die. The crown or bridge is designed in the computer design module, and this information is relayed to the milling unit. The zirconia is milled in the softer partially sintered form. This has the advantage of saving wear and tear on the milling drills as well as speeding up considerably the milling process. This is a five axis milling machine using multiple sized carbide milling burs. Up to 35 blocks can be fed into the milling unit to process without interruption one after another, thus a lab can run the unit through the night. The disadvantage of the softer zirconia is that the sintering process then shrinks the material another 20-23%. This is accounted for in the computer programming, and an accurate fit is achieved. Sinterred zirconia is one color — bright white. What distinguishes LAVA from other products is the white core is dyed one of seven dentin shades prior to the sintering process, thus the veneering porcelain does not have to mask this high value brightness first. A lab may choose to buy the entire unit to produce their own copings, and/or become a milling center to produce copings for other labs on consignment. As with other zirconia products, an appropriate veneering porcelain is then fired onto the framework.

Dentsply CERCON

CERCON is a similar zirconia product to LAVA. The main difference in this system is that the lab fabricates a conventional waxup on the cast first, then this wax coping is scanned three dimensionally using a laser scanner module. This removes the CAM component. An attached 5 axis milling machine then mills the framework from a block of partially sintered zirconia. Only one block can be programmed to mill at a time, and there are currently only two shades to choose. A lab would buy this machine for their milling purposes only. Bridgework up to 58mm can be processed. As with other zirconia products, an appropriate veneering porcelain is then fired onto the framework.

NEO Complete System by Cynovad

NEO holds similarities to other CAD CAM systems, but is unique in other ways. It can mill a wide range of materials from fully sinterred zirconia to noble, base and titanium metals. Frameworks or full contour crowns can be processed. The scanner uses an optical prism based colored light scanner to record data points. Once the restoration is designed in the lab, the information is sent via modem to a central milling center where it is manufactured and returned to the lab. A unique module the lab may purchase uses the software to design metal frameworks for crowns or bridges; this information is fed to an in-house wax printer module that produces three dimensional wax copings (with sprue) much like an ink jet printer moving across a page.

Cementation of all ceramic crowns

There are really only two questions to ask when choosing a cement for a ceramic product. 1) Is the ceramic glass based or polycrystalline? and 2) Can I achieve absolute moisture contol?

Feldspathic, leucite, or lithium disilicate products have a dominant glass component percentage. Thus, they can be etched with hydrofluoric acid and silinated to form a chemical bond to a resin cement on an adhesively treated tooth. In fact, a resin cement is required for successful cementation of a feldspathic or leucite based ceramic. All InCeram based or sintered polycrystalline alumina or zirconia based crowns cannot be etched and/or silinated and thus cannot be adhesively cemented with a resin cement.17 That is not to say the cement won’t adhere to the tooth, it just won’t chemically adhere to the crown. InCeram products should be cemented with Panavia according to Kern.18

If you cannot isolate the tooth from ALL moisture, you cannot pick a material that requires bonding for cementation, as moisture inhibits set of resin cements. This results in an uncured margin that leaks causing sensitivity, decay, and chronic gingival inflamation (Figs. 4 & 5). Eventually the unsupported porcelain cracks and catastrophically breaks. This includes deep subgingival margins and interproximal boxes.

Zirconia or Alumina based crowns can be cemented with anything from zinc phosphate, zinc oxide, glass ionomers, resin re-enforced glass ionomers, and resin based cements. Again, there is no chemical adhesive bond. It is also not advised to sand blast the inside for increased cement strength. There have been concerns about the early resin re-enforced glass ionomers expanding after setting, but the manufacturers have claimed that this has been corrected.

SUMMARY

You may have noticed as a dentist, that CAD CAM in the dental laboratory industry has brought us a new material — zirconia. The material is white, metal free, biocompatable, and strong enough to make bridge frameworks. As important to the lab industry, which is facing a major manpower shortage and stiff competition from Chinese and other foreign outsourcing, CAD CAM provides an accurate technology to replace the human error of waxing and casting. Future technology is in the works to lay the porcelain onto the coping using multicolor “porcelain printers” connected to digital shade cameras! Yes, we have a new material on the market, and hundreds of thousands of restorations have been placed worldwide. Time will tell if the material has a long term durability and strength, and indeed will be a metal substitute.

Dr. Soltys is a Board Certified Prosthodontist who maintains a private practice in Victor, NY. He is a Clinical Assistant Professor in the Prosthodontics Department at the Eastman Dental Center at the University of Rochester Medical Center in Rochester, NY.

Oral Health welcomes this original article.

REFERENCES

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