Analoog of digitaal

Primero Lab System

Ik was vier weken geleden uitgenodigd spreker op een tandheelkundig congres in Garmisch-Partenkirchen. De lezingen vonden plaats op de Zugspitze, met 3000 meter het hoogste punt in Duitsland. Iedere ochtend gingen de sprekers en deelnemers met de kabelbaan naar bergspits, want daar kun je niet met een auto komen. Mijn lezing ging over de vraag of we het potentieel van de digitalisering van de tandtechniek wel volledig benut hebben. De conclusie was dat het potentieel van de digitalisering nog lang niet is benut en dat dit alsnog in de toekomst moet gebeuren.   

Dertig jaar geleden was het maken van een kroon van A tot Z analoog handwerk. Tandtechnici verdienden goud geld aan kronen die toen 350-500 euro per stuk kosten. Ik weet nog goed dat in Duitsland veel tandtechnici in een Porsche 911 reden. Het met de hand aanbrengen van porselein was een soort kunst en tandtechnici die dat goed konden, stonden in hoog aanzien.

Ik heb zelf, dertig jaar geleden, aan de wieg gestaan van de digitalisering van de tandheelkunde. Al het analoge handwerk moest volgens mij voortaan digitaal, namelijk beter voorspelbaar en alles onder controle. Nu het allemaal digitaal is, kan iedereen die het geld ervoor heeft een digitale scanner, software en een freesbankje kopen en kronen uit een stuk massief materiaal frezen. Hierdoor is de concurrentie verhevigd en blijft er voor de tandtechnici minder marge over. De kwaliteit van deze digitale kronen is niet slecht, maar de esthetiek van analoge kronen wordt bij lange na niet bereikt. Gerenommeerde tandtechnici grijpen daarom vaak terug naar een analoge manier van werken.

Wat er fout gegaan is, is dat er bij de overgang naar digitaal, genoegen werd genomen met een wat mindere kwaliteit. Mijn mening was dat digitalisering ook kon, zonder concessies te doen aan de kwaliteit. Dus geen kronen uit een stuk massief materiaal, maar net als bij een analoge kroon: een sterke onderbouw en daaroverheen een laag porselein. De tandtechnicus moet met de juiste software en materialen een kroon kunnen ontwerpen die er precies zo uit ziet als een echte tand. Op deze manier wordt de tandtechnicus uitgedaagd om ook op een digitale manier zo dicht mogelijk de natuurlijke esthetiek van de bestaande tanden van de patiënt te benaderen.

Met deze gepatenteerde technologie werden bij Cyrtina in Zwaag in zeven jaar tijd langs digitale weg 50.000 prachtige kronen voor Nederlandse tandartsen geproduceerd, maar vaak werd de voorkeur gegeven aan de goedkopere digitale kronen uit een massief materiaal. De enigen die het potentieel van de Cyrtina manier van gelaagde kronenproductie zagen zitten, waren de Chinese eigenaren van het allergrootste tandtechnische laboratorium van China in Shanghai. Zij kochten in 2019 de licenties van onze software en materialen en produceren deze kronen nu zelf.  

De les die ik hieruit trek is, dat de beste digitale technologie weinig kans maakt als het digitaal ook sneller en goedkoper kan, ook als het kwaliteitsniveau lager ligt.

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

Primero Lab System

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

Primero Lab System

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

Primero porcelain

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?   

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