An update on antagonistic wear in full zirconia restorations
The new high translucent ZrO₂ generations can hold their own against lithium disilicate ceramics in terms of aesthetics (see DD journal online: The 'X' in the dental industry - DD cubeX²® or 'e.max®'? https://www.dentaldirekt.de/en/dd-journal/dasxinderdentalbranche). In addition, the excellent mechanical properties enable fully anatomical solutions from the individual anterior tooth to the long-span bridge.
Monolithic zirconium oxide has solved the chipping problem of veneering ceramics. Moreover, the reduced space requirement compared to veneered restorations and the associated reduction in preparation depth is a significant factor from a clinical point of view. This is because conventionally cemented, all-ceramic crowns and bridges now can be fabricated in this way, which in the past was only possible with the help of metallic all-cast restorations.
But what about the behaviour of the material to the natural antagonist? Abrasion is an essential aspect when evaluating a monolithic zirconia restoration.
Due to the high hardness of zirconium oxide (>1200 HV) and an increasing tendency towards all-ceramic zirconium oxide restorations, the question of abrasion to the natural antagonist arises. It could be assumed that the harder the tooth replacement material, the greater the antagonist abrasion. However, several in-vitro investigations have shown that polished zirconium oxide ceramic surfaces exhibit less abrasion at the natural antagonist compared to the classical veneering ceramics. (Fig. 1)
Fig. 1: Exemplary result of an abrasion study with different materials. The polished zirconium oxide occlusal surface typically shows the least abrasion, but no aesthetically satisfactory overall result after complete polishing.
Thus, it is not the hardness but the surface quality of the ceramic that is decisive for the abrasion at the antagonist. The rougher the occlusal contact surface of the ceramic restoration, the greater the resulting abrasion on the natural tooth enamel.
Obviously, glazed zirconium oxide also causes greater wear on the antagonist than polished zirconium oxide. It has been proven that conventional glaze is quickly eroded by masticatory loads (Fig. 2) and thus an 'emery paper effect' harmful to the antagonist can occur.
Fig. 2: REM documentation of glaze wear over three years (source: Courtesy of TRAC Research Clinical Studies, CR Foundation, Provo, Utah USA)
Over a period of three years, the CR Foundation (Provo, Utah USA) observed and documented in detail the superficial changes of a glazed crown in situ using a special procedure and REM (reflection electron microscopy). The study was able to show that regular masticatory loading over time leads to very fine fractures in the brittle glaze layer. In the process, fragments of the glaze are increasingly removed; with the result that shardlike particles of the glaze mass remain on the surface of the restoration, which - comparable to the emery paper effect - can continuously cause increasing abrasion on the natural antagonist.
How critical is the study situation to be assessed?
The material-related abrasion is a non-physiological mechanical wear of the teeth. It leads to an increased loss of hard tooth substance and is intensified by faulty contacts and dysfunctions (e.g. bruxism). Serious consequences such as cracks in the enamel, wedge-shaped defects in the neck of the tooth and grinding facets on front and side teeth can occur.
A general prerequisite for the clinical success of fully monolithic restorations is of course a functional design and an occlusal fit. But the finish of the restorations is also of great importance, as the study situation shows.
Veneering ceramics usually cause comparatively high antagonist wear. The crystalline structures of these masses exhibit a high degree of hardness, fragments of the veneer act as abrasive grinding tools over time.
Glazed surfaces, however, usually show less antagonist wear in studies than the 'gold veneering standard' - i.e.: glazes usually produce a less abrasive paper effect than full veneers.
Polished occlusal surfaces, on the other hand, cause significantly less abrasion, sometimes even less than the natural tooth itself. Therefore, polishing is a good clinical recommendation for minimal abrasion. Indeed, due to the 'mother-of-pearl effect', polished restorations are not considered to be highly aesthetic solutions.
In the search for an optimum finish that is not only 'antagonist friendly' but also aesthetic, it is worth taking a closer look at the behaviour and potential of the glaze materials.
Are there differences in glazes that are relevant to wear?
DD contrast® is an innovative color, texture and glaze paste system that gives full monolithic restorations an optimum color dynamics, excellent vivid depth effect and different intensities of fluorescence.
In addition to the aesthetic aspects, the fundamental basis of the pastes was the focus of the development of the DD contrast® system. On the one hand, the composition should ensure optimum handling and, on the other hand, minimise antagonist wear.
Special granulation processes of the glass raw materials made it possible to achieve extremely low application thicknesses (Fig. 3).
Fig. 3: By using the three DD contrast® components, a plastic depth illusion can be achieved with a minimum application of <0.2 mm.
The developed composition with ideally matched amorphous and crystalline fractions and defined particle sizes guarantees an 'antagonist-friendly' wear behaviour.
In order to prove this, the masses of the DD contrast® system were subjected to a wear test at the UKR University Hospital Regensburg (Polyclinic for Dental Prosthetics, Professor Dr. Rosentritt).
Do DD contrast® finishes show similar good wear behaviour to the antagonist as polished zirconium oxide surfaces?
The wear behaviour of different Dental Direct zirconium oxides was investigated by using the 'Pin on Block' (POB) method. Some of the test specimen series were polished, others were finalised with DD contrast®. The finalisation took place as follows: First DD contrast® color, then DD contrast® texture and finally DD contrast® glaze. The antagonist used was a ball made of steatite - a natural ceramic whose mechanical properties are very close to those of human tooth enamel. The various specimens were loaded with impact impulse and lateral movements under an average chewing force of 50 N for 120,000 cycles. In order to reproduce the clinical situation in the best possible way, the wear test was carried out in a water bath.
For evaluation, the mean wear depth of the specimen surface and the percentage wear area of the steatite antagonist in relation to the projected spherical surface were determined.
Fig. 4: The results of the pin on block test show the wear on the steatite ball and thus provide reliable expected values for wear on natural tooth enamel.
Titelbild: © Anton Sawizki, Master dental technician, Essen. Use of DD cubeX²® posterior tooth crown
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