Surface degradation of glass ceramics after exposure to acidulated phosphate fluoride

Objective This study evaluated the surface degradation effect of acidulated phosphate fluoride (APF) gel exposure on the glassy matrix ceramics as a function of time. Material and methods Disc-shaped ceramic specimens (N = 120, 10/per ceramic material) were prepared in stainless steel molds (inner diameter: 5 mm, height: 2 mm) using 6 dental ceramics: 3 indicated for ceramic-fused-to-metal (Vita Omega 900, Carmen and Vita Titankeramik), 2 for all-ceramic (Vitadur Alpha and Finesse® Low Fusing) and 1 for both types of restorations (IPS d.SIGN). The specimens were wet ground finished, ultrasonically cleaned and auto-glazed. All specimens were subjected to calculation of percentage of mass loss, surface roughness analysis and topographical description by scanning electron microscopy (SEM) before (0 min) and after exposure to 1.23 % APF gel for 4 min and 60 min representing short- and long-term etching effect, respectively. The data were analyzed using two-way ANOVA with repeated measures and Tukey`s test (α=0.05). Results Significant effect of the type of the ceramics (p=0.0000, p=0.0031) and exposure time (p=0.0000) was observed in both surface roughness and percentage of mass loss values, respectively. The interaction factor between both parameters was also significant for both parameters (p=0.0904, p=0.0258). Both 4 min (0.44±0.1 - 0.81±0.2 µm) and 60 min (0.66±0.1 - 1.04±0.3 µm) APF gel exposure created significantly more surface roughness for all groups when compared to the control groups (0.33±0.2 - 0.68±0.2 µm) (p<0.05). There were no significant differences in percentage of mass loss between the ceramics at 4 min (p>0.05) but at 60 min exposure, IPS d.SIGN showed the highest percentage of mass loss (0.1151±0.11). The mean surface roughness for Vita Titankeramik (0.84±0.2 µm) and Finesse® Low Fusing (0.74.±0.2 µm) was significantly higher than those of the other ceramics (0.59±0.1 µm - 0.49±0.1 µm) and Vita Titankeramik (p<0.05) regardless of the exposure time. A positive correlation was found between surface roughness and percentage of mass loss for all ceramic materials [(r=0.518 (Vitadur Alpha), r=0.405 (Vita Omega 900), r=0.580 (Carmen), r=0.687 (IPS d.SIGN), r=0.442 (Finesse® Low Fusing), r=0.572 (Vita Titankeramik), Pearson`s correlation coefficient)]. The qualitative SEM analysis showed evidence of corrosive attack on all of ceramics at varying degrees. Conclusions The ceramics indicated for either metal-ceramic or all-ceramic restorations were all vulnerable to surface texture changes and mass loss after short-term and long-term APF gel exposure.


INTRODUCTION
Dental ceramics provide similar optical properties with the natural tooth substance, present chemical stability, good physical and mechanical properties, and they have excellent biocompatibility to soft tissues with low plaque adhesion 1,2,18 . The feldspathic ceramics are the conventional ceramic materials for metal-ceramic restorations with the basic composition of a mixture of feldspar and quartz 6 . Such ceramics are high temperature-fused materials based on the basic SiO 2 that acts as the glassy matrix.
Oxides of potassium, sodium, aluminum and boron are so called glass modifiers that are added to the compound in order to decrease the melting temperature by reducing the amount of crosslinking between the oxygen and the glass forming element, silica. However, when they are used in excessive amounts, chemical durability of the ceramic is decreased and it also makes the ceramic more prone to devitrification 1,213,19 .
Controlled use of these oxides is necessary in order to attain the desirable properties such as resistance to pyroplastic deformation, glaze, to maintain hardness, chemical stability and fusing at low temperatures 2,13,17,18 .
Based on the sintering temperature, dental ceramics are traditionally classified as high-, medium-, low-and ultra low-fusing ceramics. In general, the high-fusing feldspathic ceramics are more corrosion resistant than ceramics with lower sintering temperature. However, all low-fusing ceramics per se are more corrosion-prone than high-fusing ceramics 20 . Glass ceramics used in dentistry are polycrystalline ceramics that are produced under controlled crystallization process.
They are characterized by a feldspar glassy matrix in which several crystalline phases such as alumina, tetracyclicfluoromica, leucite, myca crystals with β spodumene crystals are interspersed 19,20 . Dental glasses are amorphous, non-crystalline and ultra-low fusing ceramic materials intended for veneering of metal or ceramic substructures. Recently, research on ceramics has concentrated on developing a fundamental understanding of ceramic damage as influenced by microstructure 8,9 .
The ultra-low fusing ceramics have been developed to be used with titanium and gold alloys 13,17 . Although, high-and medium-fused ceramics exhibit better corrosion resistance than low-and ultra-low fused ceramics, they are reported to create more wear of the antagonist 1,2 .
Some low-fusing ceramics demonstrated less wear of the enamel than conventional feldspathic ceramics 6 . The low-fused ceramics also show higher solubility in water in contrast to mediumfused ceramic 13,17,19 . Variations in the composition and processing techniques could influence their hydrolytic stability and also other environmental conditions may impair their resistance to surface and bulk corrosion 8,9 .  were that all glassy matrix ceramics present similar degradation when exposed to APF gel and the application time increases the degradation.

MATERIAL AND METHODS
Ceramic materials with different compositions and microstructures were selected for the experiments (Figure 1). Ceramic discs (N = 120, 10/per ceramic material) were fabricated according to each manufacturer's recommendations as described in Table 1   on the specimens by the same operator.
All specimens were evaluated before and after to the APF gel exposure using the following methods:

Percentage of Mass Loss
The specimens were weighed in a digital scale with an accuracy of 0.1 mg (Mettler Toledo, Columbus, OH, USA) in order to calculate the mass before and after APF exposure using the following equation where W1 was considered as the specimen weight before APF gel exposure and W2, the weight after APF exposure 14 .

Surface Roughness Analysis
The surface roughness (Ra) of the specimens was measured by one operator randomly using

Topographical Analysis
The surfaces of the ceramic specimens to be evaluated were cleaned ultrasonically in 99.9% ethanol at 35 kHz for 10 min. Then the specimens were mounted on aluminum stubs and coated with Au-Pd, resulting in a thin layer of about 100-300 nm. The topographical analysis of the specimens was made with a a scanning electron microscope (JEOL, JSM-5310 LV, CTA, Tokyo, Japan) at x500 and x5,000 magnifications.

Statistical Analysis
The results were analyzed using two-way analysis of variance (ANOVA) with repeated measures and multiple comparisons were made using Tukey's test at a confidence level of 95%.
The correlation between surface roughness and loss mass percentage was investigated using Pearson's correlation test (p<0.01).

RESULTS
Significant effect of the type of the ceramics (p=0.0000, p=0.0031) and exposure time (p=0.0000) was observed for both surface roughness and percentage of mass loss values, respectively. The interaction factor between both parameters was also significant for both parameters (p=0.0904, p=0.0258) (Tables 2 and   3).    (Table 4).

Percentage of Mass Loss
There were no significant differences in percentage of mass loss between the ceramics at 4 min (p>0.05) but at 60 min exposure, IPS d.SIGN showed the highest percentage of mass loss (0.1151±0.11) ( Table 5) Table 6).

Topographical Analysis
The qualitative description of the SEM analysis In general, corrosive attack of APF gel was more evident when the ceramic materials were exposed to this medium for 60 min.  Most of the dental ceramics developed for metal ceramic restorations contain leucite as the principal crystalline phase 10