Master Dental Technician
When a dental restoration is integrated into the oral cavity, the quality of the superficial polish is one of many decisive factors for the quality of the work.
It’s not only the visual appearance of the restoration which defines good craftsmanship, other important characteristics, such as plaque affinity or abrasion properties, are also crucial for a long-term success of the work and the preservation of the remaining dentition. Especially in this day and age, where new materials are constantly being processed, in some cases through new brand-new technologies, coordinated processing steps to include the final polish have become necessary. Until a few years ago, the range of materials included PMMA acrylics, precious/non-precious metal and model casting alloys, veneer composites and ceramics. Today, apart from the “traditional” materials, the dental technician is additionally confronted with nano-composites, polycrystalline high-performance framework materials, hybrid ceramics, etc. It therefore goes without saying, that new materials, in addition to new processing techniques, also require new products for the polishing procedure.
Hence this article describes the tests carried out with the polishing paste Renfert Polish all-in-one. The tested materials include veneering ceramics, LiSi2 ceramics from different processing methods (CAD and press technology), PEEK, hybrid acrylics and veneering composites. The selection is exemplary for each respective product group.
By definition, to polish is to smooth a surface without significant material removal. This process causes a change in the chemical behavior and the physical or mechanical properties of the material due to a reduction of the effective surface. This results in a decrease in susceptibility to damage from cracking and biological reactions and can therefore also be regarded as a kind of “refinement”.¹
In particular with materials which are significantly harder than natural enamel (e.g. ZrO2), a good high-gloss polish is essential, because a glaze which has been superficially applied and then fired can only stand up to the abrasion forces for a short period of time.²
The glass-like structure of a glazed surface is subject to wear by the antagonist and the associated roughening of the surface leads to a sandpaper-like effect. A polished surface is therefore necessary to avoid the risk of antagonist abrasion/attrition.³
During the development of the Renfert Polish all-in-one polishing paste, it was tested on 10 different materials (table 1), to assess how surface roughness can be reduced by pre-treatment and subsequent high-gloss polishing. The pre-treatment consisted of mechanical plan grinding (Fig.1, Buehler MetaServ 250, ITW Test & Measurement GmbH, Esslingen A.N., Germany) and subsequent sandblasting with aluminum oxide blasting material (Al2O3, 25µm, 1.5 bar) and glass beads (glass 50µm, 1.5 bar). During this procedure, care was taken that the surface was blasted evenly and there was no direct heat generation caused by blasting at an individual point for a longer period of time. The subsequent surface treatment (pre-polish with Renfert silicone polisher and high-gloss polish with the Renfert Polish all-in-one) was determined by means of an optical measurement of the surface topography according to DIN EN ISO 25178 (FRT GmbH, Bergisch Gladbach, Germany).
Fig.1: Grinding and polishing device MetaServ 250
Table 1: Framework and veneering materials, test descriptions
When evaluating the test results (table 2), a so-called Sk value was used, which describes the distance between the highest and lowest level of the surface core structure (Fig.2). This value is also known as core roughness depth and allows the assessment of a measuring area and not just an individual point.
Fig.2: Sk value (statement from DIN EN ISO 25178)
Table 2: Evaluation of the test results
If the average values of the pre-polish are compared with those of the high-gloss polish, then the polishing properties can assessed by the reduction of surface roughness. The appearance of a surface before and after the treatment can be seen very well in the tests Z I (Fig. 3) and Z II (Fig. 4) with CAM-manufactured zirconium oxide (ZrO2). The color shades show that the surface has been clearly smoothed over, from specific areas with original values of >1.2µm to almost consistent values of <0.2µm after pre-polishing.
Fig. 3: Confocal microscopic image, ZrO2 Test Z I
Fig. 4: Confocal microscopic image, ZrO2 Test Z II
When the results of the zirconium oxide are compared with polishing results from selected competitors (production of samples under cp.), the good polishing properties of the Renfert Polish all-in-one become clearly visible.
Fig. 5: Competitor 1: Confocal microscopic image, ZrO2 Test Z II
Fig. 6: Competitor 2: Confocal microscopic image, ZrO2 Test Z II
Competitor 1 shows a lot of light areas in this display that have values of up to 0.6µm. Competitor 2 clearly shows better results after the high-gloss polish than Competitor 1 (significantly more dark areas), but does not match the test results from the Renfert Polish all-in-one paste.
As table 3 shows, the Renfert Polish-all-in-one polishing paste proves it can be used for a wide range of applications with good to very good results for the tested materials.
In particular, the very hard materials such as zirconium oxide and lithium disilicate show a high reduction of surface roughness in this standardized test procedure. The Renfert Polish all-in-one also produces good high-gloss polishing results on the new, tested materials such as PEEK or the hybrid ceramics, even though slightly more time is required.
¹ Kappert, Heinrich F., Eichner, Karl; Zahnärztliche Werkstoffkunde und ihre Verarbeitung, Band 1 Grundlagen und Verarbeitung, S. 373 Schleif- und Poliermittel; Hüthig Verlag Heidelberg
² Kern, Manfred; ZrO2 -Monolithen – Lösung mit Zukunft?; AG-Keramik, Ettlingen; ZWL 04/2014
³ Prospiech, P.; Monolithische Rekonstruktionen aus Zirkoniumdioxid (ZrO2), Deutsche Zahnärztliche Zeitschrift 2013; 68 (10)