Surface roughness of glazed feldspar, alumina, and zirconia-based ceramics Rugosidade das superfícies vitrifi

The aim of this study was to compare the mean surface roughness (Ra) of feldspar-, alumina-, and zirconia-based ceramics, testing the hypothesis that the feldspathic ceramics have lower average surface roughness (Ra) than the ceramics used for infrastructure. Eight disk specimens (5mm in diameter; 2mm in thickness) of each ceramic material were fabricated according to the manufacturer’s specifi cations: V7-feldspathic veneer ceramic (Vita VM7); VA-feldspathic veneer ceramic (Vitadur-α); IA-slip casted, glass-infi ltrated alumina-based ceramic (Vita In-Ceram Alumina); IZS-slip casted, glass-infi ltrated zirconia-reinforced alumina-based ceramic (Vita In-Ceram Zirconia); IZB-dry-pressed block of glass-infi ltrated zirconia-reinforced alumina-based ceramic (Vita In-Ceram Zirconia 2000 for Cerec InLab). All materials were glazed as recommended by the manufacturer. Four Ra readings (Mitutoyo SJ 400) per specimen were performed, averaging the value per specimen (n=8). Ra values were statistically analyzed using one-way ANOVA and Tukey test (α=0.05). V7 showed the lowest mean Ra value (0.43±0.07 μm) compared to the other ceramics. There were no statistical differences between the mean Ra values of VA (0.94±0.2 μm), IA (0.7±0.13 μm), IZS (0.98±0.3 μm) and IZB (0.75±0.4 μm). The testing hypothesis was partially accepted. V7 showed a smoother ceramic surface than the VA. There were no statistically differences between the mean Ra value of the high crystalline content ceramics (IA, IZS, IZB).


INTRODUCTION
With the increase in the crystalline content of dental ceramics and increase in their mechanical properties, it has become possible to use them more safely in oral rehabilitation 18,28 .Currently, alumina/ zirconia-reinforced ceramics can be indicated for fabrication of framework in fi xed prosthodontics and implant abutments, as alternative or substitute to the metallic framework 2 .
However, the high crystalline content ceramics can present a rougher surface, since they are mostly composed of zirconium polygonal crystals with irregular shape 21,29 ; besides, their surface roughness can be increased after grinding 1,5 . On the othr hand, feldspathic veneering ceramics are constituted of approximately 50wt% of a glass phase (silica oxide) and 40wt%-50wt% of crystalline phase (leucite), which are customarily able to generate a smoother surface than high-ceramics for frameworks 7 .
The high crystalline content ceramic framework of metal-free bonded prostheses and implant abutments is often exposed to the oral environment.In these cases, the framework ceramic surface should be as smooth as possible, with the aim of minimizing the bacterial colonization and dental biofi lm formation 26 .
Grinding and polishing procedures to adjust ceramic restorations may also produce a rougher surface 1,5 , which may cause an increased rate of biofi lm accumulation, producing gingival infl ammation and adverse soft tissue reaction 16,17,26 .In addition, the occlusal adjustments may cause excessive wear of the opposing teeth 15,22 , and also impair the strength of the ceramic restorations 4,11 .
The correlation between quantity of dental biofi lm and surface roughness was verifi ed on the surface of different dental materials, such as ceramics 16 , titanium 27 and acrylic resins 30 .Several studies have demonstrated that the bacterial adhesion begins around irregularities and expands to the whole surface 19.20.23 .Moreover, the dental biofi lm is formed more quickly on rough areas 12,24,25 .There is a general understanding that bacteria are the primary cause of gingivitis, periodontitis and caries.Although these diseases are explained by specifi c plaque theories, the removal of all bacterial deposits remains essential to prevent these diseases 3 .Bacterial recolonization on a cleaned surface occurs rapidly 6 , being facilitated on rough surfaces and the biofi lm maturation occurs more rapidly with presence of more pathogenic bacteria 8,[23][24][25]27 .
Some studies have found a correlation between roughness and mechanical resistance of dental ceramics 4,11 .Namely, increasing the surface roughness of ceramics makes the material weaker.Fischer et al. 11 (2003) found that stress distribution and the respective characteristic strength values based on a Weibull stress distribution can be directly correlated to the roughness parameters Ra, Rz, and Rmax for all studied ceramics.
Considering the importance of the surface features of dental ceramics on the mechanical properties and the biofi lm formation, the purpose of this study was to compare the mean surface roughness (Ra) of 2 feldspar veneer ceramics and 3 high crystalline content ceramic used as framework material, testing the hypothesis that the feldspathic ceramics have lower mean Ra value than the ceramics used for framework.

MATERIAL AND METHODS
Eight disc specimens (5mm in diameter and 2mm in thickness) of each ceramic were fabricated according to the manufacturer's specifi cations and described as follows: • V7: The powder and liquid of the feldspathic ceramic (Vita VM7 VITA Zahnfabrik, Bad Sackingen, Germany) were mixed and applied into a metallic matrix.The specimens were sintered in the Vacumat furnace (VITA Zahnfabrik), polished using a silicon-carbide abrasive paper #1000 (3M, St Paul, USA) and sonically cleaned (Vitasonic, VITA Zahnfabrik) in distilled water for fi ve minutes.The specimens were glazed using the Vita Akzent 25 (VITA Zahnfabrik).• VA: The feldspathic ceramic (Vitadur-α, VITA Zahnfabrik) specimens were fabricated using same methodology as for V7 specimens, except for the sintering program and glazing (Akzent 24, VITA Zahnfabrik).• IA: The slip material for the glass-infi ltrated alumina-based ceramic (Vita In-Ceram Alumina, VITA Zahnfabrik) was prepared, applied into the metallic matrix and allowed to dry.The specimens were sintered (Inceramat furnace, VITA Zahnfabrik) and the glass was applied and infi ltration was carried out in the same furnace.All specimens were air-abraded with 110 µm alumina particles to remove the excess of glass.
Polishing and cleaning were performed as for V7 specimens.Specimens were glazed using Vita Akzent 25 (VITA Zahnfabrik).rounded to a disk shape of 5mm-diameter using a diamond bur under a light microscope (Carl Zeiss 350, Germany) (x4.5).The remaining fabrication procedures were as for IA specimens.
The average roughness parameter (Ra) was calculated using a roughness analyzer (Mitutoyo SJ-400, Tokyo, Japan).On one face of each specimen, four readings (2 in the x direction and 2 in the y direction, 1 mm apart from each other and cut-off value = 3 mm) were averaged and used to calculate (n=8) the mean value and standard deviation of Ra (in µm).
The Ra values (in µm) were statistically analyzed using one-way analysis of variance (Anova) and Tukey post-hoc test, at 5% signifi cance level.Statistical analysis was performed using Statistics 8.0 for Windows (Analytical Software Inc, Tallahassee, USA).

DISCUSSION
In the present study, the feldspar ceramic VM7 presented surface roughness signifi cantly lower when compared to the other feldspar ceramic evaluated (Vitadur-α).The differences with relationship to the surface roughness observed among the ceramic can be, probably, attributed to the micro structural characteristic of the materials as size and it forms of the crystals.The manufacturers of the ceramic VM7 comments that its microstructure presents more homogeneous distribution of the vitreous phases, consequently smoother surfaces are obtained, presenting high resistance to the biofi lm formation when compared to the conventional ceramic.
The framework ceramics displayed the higher roughness values, when compared to the feldspar SURFACE ROUGHNESS OF GLAZED FELDSPAR, ALUMINA, AND ZIRCONIA-BASED CERAMICS Cienc Odontol Bras 2006 out./dez.; 9 (4): 12-17 ceramic VM7.The framework ceramics investigated have a high crystalline content with irregular shape crystals 9 .
Considering this study results, care should be taken when the ceramic framework is exposed to the oral environment, due to the reported association of increased surface roughness and formation/accumulation of dental biofi lm on dental materials 16,27,30 .Changes in this variable might, therefore, facilitate the prevention of recurrent caries and periodontal disease.
The ceramic framework materials investigated in this study (glass-infi ltrated alumina and zirconium) can be indicated for small fi xed prostheses and bonded prostheses, and in both clinical situations the framework may be kept without a ceramic veneer.This is especially true for inadequate tooth preparation and bonded prostheses, in which the palatal extension of the prosthesis is usually uncovered.
Scanning electron microscopy studies revealed that the initial adhesion of microorganisms begins in irregularities and is subsequently extended to the entire surface 19,20,23 .These irregularities increase the area available for adhesion and especially protect the bacteria from the shear forces in the oral cavity, such as salivary fl ow, chewing, swallowing and oral hygiene procedures 24 .
As bacterial adhesion theoretically evolves from an initial reversible stage to a stronger adhesion stage, considered irreversible, authors suggested that this change primarily occurs in the irregularities, in which the microorganisms are protected from the shear forces 12,24,25 .Consequently, the biofi lm may present faster maturation at these areas.
Rimondini et al. 27 (1997) evaluated the bacterial colonization on titanium samples with different surface roughness values.In smooth samples, there was smaller accumulation of bacteria, and only cocci were observed.In specimens with intermediate roughness (presence of grooves), short and long rods were found.In rough samples, with the presence of grooves and porous, there were long bacteria rods aggregated or in layers.Thus, as cocci are considered pioneer species and rods are regarded as subsequent species in colonization, the presence of long rods was considered by the authors as an advanced stage of maturation of the biofi lm on rough surfaces.Similar fi ndings were reported for fl uoride-ethylene-propylene, cellulose acetate, titanium, enamel and cementum specimens 8,23- 25 .Thus, the surface roughness of materials increases both the bacterial adhesion and faster maturation of the biofi lm formed, which presents clinical implica-tions, since this biofi lm may present more pathogenic microorganisms.
Further in vivo studies on bacterial adhesion and dental biofi lm formation on the ceramics evaluated in the current study must be conducted.
Other argument widely discussed in the scientifi c literature is related to the effect of surface roughness on the mechanical strength of dental ceramics.Some authors have shown the direct effect of roughness on the resistance properties of ceramics 4,11 .Fischer et al. 11 (2003) found that stress distribution and the respective characteristic strength values based on a Weibull stress distribution can be directly correlated to the roughness parameters Ra, Rz, and Rmax for all studied ceramics.As also mentioned by Fischer et al. 11 (2003), ceramics normally have a low mechanical reliability, since no exact failure limit can be defi ned.If the peak-to-valley height of the surface roughness is in the range of the critical defect size value, then roughness can affect the fl exural strength.The fundamental fracture mechanics theory (Griffi th criterion) shows that an increase in defect size produces a decrease in strength.Since ceramic materials fail because of the 'weakest-link principle', the maximum (critical) microscopic defect will cause failure at the critical stress 10,11 .It is important to consider that restorations are adjusted in the mouth using diamond burs, and thus should be polished to produce a smoother or less rough surface 1,5 .
Final polishing reduces the roughness by using extremely fi ne abrasive materials.Polishing of the external restoration surface is very relevant, once rough surfaces have great potential to bacterial adhesion  , and can be more capable of wearing the opposing teeth 15 . Morever, effective polishing prevents discoloration of rough areas and leads to a more natural appearance of ceramic restorations.Rough or irregular ceramic surfaces, i.e. produced after intraoral adjustment of the restorations, may concentrate stresses and initiate crack propagation, resulting in early restoration failure 4,11 .Therefore, the results partially accepted this study hypothesis, since only VM7 ceramic showed the lowest mean Ra value, probably due to its fi ner microstructure.

CONCLUSIONS
The feldspar ceramic VM7 presented the lowest surface roughness values when compared to the feldspar ceramic Vitadur-α, and the to framework ceramics evaluated.