Is zirconia the best available technology (BAT) for dental restorations?

Patients with missing tooth structure should receive restorations that at least mimic or exceed the functional and esthetic requirements of the remaining teeth. This can be accomplished by using a strong substructure and a translucent layer of porcelain, therewith mimicking the structure of a natural tooth. The technology that is regarded as best available technology (BAT) is computer aided design (CAD) and computer aided milling (CAM) of a reduced substructure of zirconia and subsequently milling the outer contour in hardened paste glass ceramic, known as prosthetic mimetic restorations (PRIMERO, J Dents Dent Med. 2019 Sep;2(2):4-11). However, at present most restorations produced by CAD/CAM technology are monolithic products of either color and structure graded zirconia or lithiumdisilicate glass ceramic. That the best available technology is not used, has economical reasons, because direct milling of the final restoration from one block of material is easier than having to mill a second porcelain layer. The three materials involved in present restoration differ considerably in their Vickers surface hardness: zirconia in all its modifications 1200, lithium disilicate glass ceramic 700 and PRIMERO 470, while human enamel has a hardness between 390 and 450. Because zirconia has four times the hardness of metal, initial concern about antagonist abrasion could with numerous studies be negated. Most previous work conducted on the wear behavior of dental enamel has focused mainly on wear rates and surface damage. There is, however, scarce information regarding the subsurface damage (SSD) arising from sliding contact fatigue. Enamel probes were submitted to cyclic contact fatigue test in off-axis mouth-motion cycling machine with 200 N load and 2 Hz frequency in distilled water at 37°C for up to 106 cycles a zirconia, lithiumdisilicate and PRIMERO indenter. Subsurface damage of enamel is often covered with a smearing layer generated by wear mechanisms and rather difficult to directly observe and detect by optical microscopy. Subsurface damage was therefore assessed using sagittal and transverse sections of the samples and lateral, median and cone cracks observed and quantified. Massive crack formation in enamel probes opposing monolithic zirconia was observed. The zirconia caused twice as much cracks in the enamel probe as lithiumdisilicate and three times more cracks than the PRIMERO indenter. Both wear and subsurface fracture in enamel are determining factors for the long-term decay of teeth. We must fear for the subsurface integrity of zirconia opposing enamel on the long term and the use of porcelain glass ceramics should remain the “best available technology”.

The esthetic “e”Crown

The esthetic e-Crown

Jef van der Zel*, Orestes Foundation, The Netherlands

*Corresponding Author: Jef van der Zel, former professor of endowed Chair in Computerized Dentistry, University of Amsterdam, The Netherlands.

A CAD/CAM crown, or better an e-Crown, is nowadays increasingly synonymous with monolithic zirconia crown produced from a color and structure graded zirconia block, and coated with a thin layer of glaze stains, which also provides some kind of fluorescence. The “graded” zirconia crown has a darker cervical area, consisting of tetragonal zirconia, a main tooth color in the buccal area and a translucent incisal edge consisting of cubic zirconia. The only thing a dental technician has to do is use the proper height of the zirconia block so that the crown fits in all the color zones. Although on the outside the color gradient is mimicking the color gradient of natural teeth, they are still far away from the optical, physical, biomimetic and esthetic properties of natural teeth. Esthetic prosthetic restorations, with natural reflection, color from within and color gradients influenced by the internal dentinal core anatomy can best be accomplished by veneered zirconia, rather than with crowns of monolithic zirconia. In the production of dental restorations specifically made for one patient, dental technicians with their problem-solving skills, dexterity and cognitive skills are until recently the only way to provide the required esthetics, individuality and artistry with porcelain. Fear for chipping of conventional zirconia porcelains on the longer term and price pressure on manual application of porcelain, are possible drivers for the monolithic zirconia restorations.

However the application of porcelain is no issue with prosthetic mimetic restorations (PRIMERO) were an e-Crown follows a model of the natural tooth in two layers: a histo-anatomic dentin layer mimicking the dentin shape of the dentition of the patient and an enamel layer. These restorations that mimic the structure of natural teeth by cognitive design of the dentin core presents a new production paradigm to fabricate natural restorations of veneered zirconia using a high strength porcelain with CAD/CAM. These e-Crowns are produced with a core of tooth-colored tetragonal zirconia, on which a high strength translucent porcelain layer has been applied and subsequently milled to size. In the subtle cooperation between the dentin-colored zirconia and the veneering porcelain, the zirconia shines through the translucent porcelain layer, all the more as the porcelain layer is thinner. This creates the natural color dynamics with color “from the inside” as found in natural elements, instead of color “on the outside”, with monolithic zirconia. As a result, the natural tooth, in terms of esthetics and hardness, is approached closer than crowns made from solid monolithic zirconia. This implies that the histo-anatomic dentin core is the key to the esthetic e-Crown.

Jef van der Zel

In contact with Zirconia

Since the introduction of zirconia in 1998 as a substructure for veneered crown- and bridgework, we learned a lot about its clinical behavior, mostly the hard way. Soon, many problems were reported, concerning the chipping or delamination of porcelain on zirconia. Failure rates after 5 years were reported of a much as 20% of the restorations. This was unprecedented, because in metal-ceramics chipping was negligible and delamination might reach 5% maximum after 5 years. The reason lies in the high strength of zirconia, even at the firing temperature of the porcelain. During cooling the porcelain builds up pressure immediately from the beginning, in contrast with materials such as metal, alumina or press ceramics. The first reaction to the shared concerns of dentists and technicians, was to do away with the porcelain layer and go monolithic with zirconia. This, without taking in consideration that the problem might be lack of resistance or the low strength of the porcelains used. In the meantime, it has become evident that when a crack-stopping mechanism such as leucite crystals is present in the porcelain matrix, no chipping whatsoever is reported, for the porcelains with this build-in mechanism.

Since then monolithic zirconia, either individually colored by applying coloring liquids in the green stage or the use of color-graded zirconia blanks, has become extremely popular both under dentists and technicians. Because zirconia is the hardest material used up to now in dentistry (it is almost twice as hard as cobalt-chromium), initial concerns came up about the possibility of unacceptable abrasion of antagonists, when in direct contact with zirconia. Laboratory abrasion tests on polished zirconia against natural teeth proved its positive tribological properties. However, more and more clinicians reported recently heavy abrasion of antagonists, because zirconia does not abrade itself, as the rest of the dentition, therefore after a few years exerting more load on the antagonists, then was anticipated in the laboratory abrasion tests. Although researchers that performed the abrasion tests did not find clinically unacceptable abrasion of the opposing natural tissue, all reported fine cracks in the samples. Although there is little abrasion, the repeated contact with hard, resistant zirconia gives a “hammer” effect, of which effect we do not know its long-term effect.

It this a reason to avoid contact with zirconia and go back to zirconia veneered with chip-resistant porcelains with their “cushioning” and natural abrasion?