Evaluation of Surface Roughness of Multilayered Monolithic Zirconia Following Chairside Adjustment with 3 Different Polishing Systems Sterilized for Multiple Cycles: An In Vitro Study

Introduction

Zirconia, in any of its several forms, is becoming progressively preferred as a substitute for cast metal in the fabrication of single crowns, short-span fixed partial dentures, and implant-supported fixed prostheses [1,2]. The preference is attributed to its enhanced aesthetic qualities, comparable strength, and the convenience of clinical or laboratory production, facilitated by computer assisted design and manufacturing (CAD/CAM) technology. Monoclinic (m-ZrO2), tetragonal (t-ZrO2), and cubic (c-ZrO2) phases are the 3 primary forms of this polymorphic substance. From room temperature up to 2370°C, these different forms tend to remain stable without undergoing any physical or chemical change [3]. Yttrium oxide (Y2Ot) and magnesium/aluminum oxides (MgO, Al2Ot) are examples of stabilizing oxides that may be used to stabilize zirconia phases at certain temperatures [1,3]. Cubic/tetragonal phases are able to maintain their stability even after sintering at normal temperature because of this [1,3]. Materials researchers have produced 4 generations of Y-TZP [4,5]. The first and subsequent generations of partly stabilized zirconia include 3Y-TZP, which contains 3% yttrium oxide and 0.25% and 0.5% aluminum oxide. Fully stabilized zirconia is the third generation, with 5Y-TZP containing 5% yttrium oxide and 0.5% aluminum oxide. The fourth generation, 4Y-TZP, contains 4% yttrium oxide and 0.5% aluminum oxide [6,7]. Monolithic zirconia (MLZ) restorations from a single CAD/CAM block do overcome drawbacks like ceramic chipping but lack translucency [8,9]. Third-generation zirconia, with a 50% cubic phase instead of 98% tetragonal, shows enhanced light transmission and decreased refractive effects due to its opaque nature, thereby increasing translucency [10,11]. The increased translucency increases fracture risk; however, fourth-generation zirconia addresses this. Fourth-generation zirconia emerged from the development of multilayer (polychromatic/pre-shaded) monolithic restorations that blended 3Y-TZP and 5Y-TZP strengths [11]. These multilayer restorations use a high-strength 3Y-TZP base at the cervical (dentin/body) region, and a transparent 5Y-TZP layer at the incisal region to overcome reduced flexural strength. The clinical benefits of multilayered MLZ (MMLZ) restorations include conservative tooth preparation, increased flexural strength for short-extent fixed partial dentures [12], negligible opposing attrition, improved translucency, reduced fabrication time, and no ceramic chipping [6,13,14].

MMLZ’s longevity depends on its surface roughness, like any other dental ceramic. Surface adjustment, performed clinically to adjust for occlusion and proximal contacts, imparts varied roughness. Sabrah et al found that glazed zirconia surfaces had the smoothest texture, with an average roughness (Ra) of 0.42 μm and a root mean square roughness (Rq) of 0.63 μm [5]. Surfaces finished with diamond burs had a slightly higher roughness value of 0.89 μm compared with glazed zirconia surface (Ra: 0.84 μm). The process of polishing in another study yielded intermediate average roughness values of Ra: 0.49 μm [15]. Janyavula et al noted that glazed zirconia (Ra: 0.76±0.12 μm) and zirconia polished prior to glazing (Ra: 0.69±0.1 μm) demonstrated smoother surface characteristics compared with natural enamel (Ra: 2.6±1.1 μm) and veneered zirconia (Ra: 1.6±0.16 μm). Nonetheless, polished zirconia surfaces exhibited the lowest roughness, with an Ra value of 0.17±0.07 μm, making them the smoothest among all tested surfaces [16]. Preis et al [4], while examining different adjustment techniques on MLZ surface characteristics, concluded that grinding increased while polishing reduced surface roughness. Furthermore, the wear simulation exhibited a negligible impact on surface roughness and did not induce any phase transformation. Amer et al observed that average roughness values of monolithic zirconia did not change after undergoing 3-body wear against human enamel, which is comparable to conventional feldspathic porcelain [10]. Moreover, it is widely recognized that surface roughness influences plaque accumulation, with a recommended Ra of less than 0.5 μm, since values above this threshold can be easily sensed by the tongue [6,10,14,17]. Surface roughness is typically assessed using profilometers and a variety of microscopes, such as optical, atomic force, and scanning electron microscopes. The parameter most frequently employed to evaluate surface texture is Ra. Conversely, Rq provides a quantification of surface height variation and can be beneficial in clinical settings for the assessment of film thickness, which is achieved through refining or glazing [7,12,18–21].

Many indirect restorations need chairside changes once a crown or fixed partial denture is placed. Research shows that diamond burs reduce zirconia’s flexural strength [7,15,22]. Repolishing zirconia may increase flexural strength by 75%. Zirconia polishing reduces opposing tooth enamel wear [23]. Polished zirconia may achieve high luster without glaze. Since glazes degrade over time, polished zirconia resists long-term wear better. Clinically, the lack of surface roughness is highly relevant for treatment outcomes. Roughness affects the surface properties of the ceramic, especially its gloss and translucency, and allows extrinsic staining on the restoration [24]. Light is reflected less from a rough surface and is also distorted while being reflected. It is also possible that surface degeneration leads to the accumulation of plaque, subsequent infections of surrounding periodontium, and the destruction of enamel [25]. Zirconia that has been polished correctly has a high success rate over the long term, particularly in regions that experience high occlusal pressures or in situations where longevity is more important than aesthetics [26,27].

Polishing and finishing tools need to be sterilized to prevent infection, especially in implant-supported prostheses [28]. Standard steam sterilization, which takes 15 minutes at 121°C under 1 bar pressure in a pre-vacuum environment, and flash sterilization, which takes 3 minutes at 134°C under 2 bars pressure, are 2 widely used sterilization methods [19,29]. Studies show that sterilization reduces the effectiveness of finishing and polishing equipment, particularly equipment used on composite resins, altering surface roughness [27]. Lacerda et al found that 5 steam sterilization cycles reduced diamond bur polishing efficacy [30]. After usage, polishing tips’ elastomeric matrix softens, reducing tear resistance and tensile strength. Thermal degradation mechanisms include oxidation and hydrolysis fracture and leach matrix components [30]. Shah et al found that numerous steam sterilization treatments increase rubber elastomer surface hardness [31]. Polishing tips that use silicone rubber as a binder lose efficiency, especially surface roughness, after numerous usage and sterilization cycles [29]. There are several commercially available polishing systems, with each one claiming its superiority in MLZ chairside finishing and polishing, but their efficacy and use are unknown. Kozmacs et al [18] compared polishing methods and dentists’ opinions on their use in dental clinics. After finalizing dental materials, the authors summarized each polishing kit’s performance and reported equivalent findings. The authors provided a summary of the effectiveness of each polishing kit following the final finishing of dental materials, reporting comparable results.

With the above mentioned contextual background, the present study aimed to evaluate the effects of sterilization cycles on the effectiveness of 3 commercial polishing kits (2 of which were manufacturer recommended: EVE Diacera and SS White; and 1 which was generic: Ultradent Jiffy) on the surface roughness of MMLZ crown restorations. The surface roughness values were measured after the first, fifth, and tenth autoclave cycles. The objectives of the study were to ascertain whether generic polishing systems are equally efficient as manufacturer-recommended ones, whether different manufacturer-recommended polishing systems have equal efficiency after recurrent sterilization, and whether the changes are general or specific to a particular polishing system. We hypothesized that generic polishing systems will not be as efficient as manufacturer-recommended ones, and that all polishing systems will eventually show surface roughness after increased sterilization cycles. Alternatively, the null hypothesis states that all polishing, whether generic or manufacturer recommended, will be unaffected by sterilization cycles.

Material and Methods

ETHICAL CONSIDERATIONS:

The protocol for this experimental study was submitted as a student research proposal to the ethical committee of the concerned college/university during the 2022–2023 academic year (vide registration number CODJU-23291).

STUDY DESIGN:

An in vitro investigation was conducted using a comparative analysis between 3 experimental groups and 1 control group. MMLZ disc-shaped samples and polishing kits were considered as independent variables. The dependent variable was the outcome, the Ra, which was objectively evaluated with profilometer and scanning electron microscopy (SEM). All operators performing the various measurements were blinded to the sample groups to avoid bias in the results.

CONTEXTUAL DEFINITION:

Finishing was defined as the act of applying a final coat or surface as part of the refining process before starting to polish, while polishing was defined as using friction to smooth a surface until it looked glossy [29].

SAMPLE SIZE:

The sample size was calculated using the G-power sample size calculator (version 3.1.9.7). Based on the results of previous studies, an effect size (Δ2=2) and a Type I error rate (α=0.05) were considered [28,32]. The calculation yielded a total sample size of 100 specimens, with 10 samples in the control group and 30 samples in each experimental group, and 10 samples in each sub-group. This was sufficient to achieve a statistical power of 80%. To account for any defects in the samples due to manufacturing, 3 additional samples were included in all groups. Manufacturing of zirconia samples is bound to result in some defective samples, which would not be used for final testing. To compensate for any defective specimen, these 3 samples were kept as reserves.

SPECIMEN PREPARATION:

Zolid Gen-X is a highly translucent MLZ manufactured through CAD/CAM milling, and it is classified as Type II, Class 5 in accordance with Deutsches Institut für Normung (DIN), European Norm (EN), and International Organization for Standardization (ISO) 6872 standards [33]. It comes in 8 Vita shades (A to D), along with an additional bleach shade. The material incorporates a natural color gradient, with both shade and translucency varying from the cervical to the incisal regions. For this study, shade A2 was chosen, which combines elements of Vita shades A1 and A2. The material is supplied in disc-shaped blanks of different heights, ranging from 12 mm to 25 mm (Table 1) [33]. The manufacturer suggests measuring the specimens’ dimensions and choosing the proper sintering temperature based on these data prior to processing. This is to calibrate the machine to a particular ceramic type, since the machine is used for multiple ceramic types. The specimens were designed utilizing the recommended software, and the blocks were arranged for milling in a 5-axis machine employing Dental Cam software (version 8.0). Each specimen was processed in accordance with the manufacturer’s specifications, which included an 800 W power output, a rotational speed of 160 000 rpm in unidirectional motion, dry milling, and a repetition accuracy of 3 μm. Various finishing and polishing tools were utilized (Table 1). Figure 1 illustrates the study design, including the dependent and independent variables, as well as the study groups involved in the current research. A total of 100 disc-shaped specimens (10 mm in diameter, 3 mm in thickness) were milled from multilayered zirconia blocks. The discs were finished, polished, and glazed on one surface of each disc. All glazed samples (n=10), which served as controls, were tested for surface roughness using a profilometer. The roughness of the control group specimens was measured without any treatment. After that, the remaining specimens were split into 3 equal groups of 30 specimens each, according to the polishing techniques that were used.

INTERVENTION AND GROUPING:

The manufacturer recommends conducting the grinding process with diamond grinding points of 40 μm grain size, followed by finishing/polishing using diamond-impregnated polishing burs. To standardize the grinding procedure, a high-speed handpiece was fixed to a pressure capsule gauge to ensure consistent pressure across all specimens. A rotary bur (Diamond, GmbH, Tokyo) was operated at 160 000 rotations per minute (RPM) in a unidirectional motion, with the handpiece attached to a surveyor. Two external reference points were marked on each specimen, and the operator guided the specimen horizontally between them across a flat platform. To maintain consistency, a single operator used a rotary diamond instrument for approximately 20 seconds in one direction, utilizing a high-speed handpiece (956 LX; W&H GmbH) with water spray. After grinding, the abraded zirconia surface was rinsed with triple commercial distilled water and air-dried before polishing. After that, the specimens were randomly allocated into 3 groups of 10, each of which used a different polishing kit. Allotment of each specimen to a particular group was random. Each specimen group (n=30) was further subdivided into 3 subgroups of 10 specimens each (n=10). The group distribution for the zirconia discs was as follows: Group C: Control group (glazed samples); Group 1: polishing with the EVE Diacera polishing kit and flash autoclaving of burs for 1 (sub-group 1E), 5 (sub-group 5E), and 10 (sub-group 10E) cycles; Group 2: polishing with SS White polishing kit and flash autoclaving of burs for 1 (sub-group 1W), 5 (sub-group 5W), and 10 (sub-group 10W) cycles; Group 3: Ultradent Jiffy universal polishing kit and flash autoclaving of burs for 1 (sub-group 1U), 5 (sub-group 5U), and 10 (sub-group 10U) cycles. For polishing with the EVE Diacera kit, a speed of 4000-8000 RPM was applied for 30 seconds with the medium polishing bur, while the SS White and Ultradent Jiffy kits were used at a speed of 7000–10 000 RPM for 30 seconds with coarse and fine polishing burs. Finishing and polishing were performed according to the clinical and manufacturers guidelines. The autoclaving of all kits was performed as recommended by the manufacturers, which included use of the flash (immediate) sterilization cycle, under the settings of a temperature of 134 degrees Celsius, for 3 to 5 minutes under pressure (2 bars). The total operating time of each cycle was 15 minutes out of which 5 minutes included the drying time.

SURFACE ROUGHNESS:

The evaluation of roughness was conducted utilizing a profilometer (Talysurf Intra, Leicester, England), with a particular focus on measuring the Ra value. The purpose of this evaluation was to assess the influence of autoclave cycles on the effectiveness of different polishing systems utilized on zirconia discs. A surface profilometer is an appropriate tool for the qualitative and quantitative evaluation of surface roughness. A contour optical microscope (GT-K 3D, Bruker) was used for the imaging and characterization operations; this microscope used non-contact surface metrology using interferometry. Vertical Scan Interferometry employed a 5× Michelson lens, which exhibited a 1×1 mm view. A Gaussian regression filter was implemented, with the scan speed set to 1× and a threshold determined at 4. Aside from physically positioning the samples, the instrument setup, data processing, and graphical output were all carried out using Vision 64 (Bruker) software. The broadband light source enabled the assessment of Ra, conducting 3 scans for each sample, with the Ra values averaged across these intervals. For each specimen, triple measures were recorded on the treated surface, and the surface roughness was calculated as the arithmetic mean of the values obtained from 3 separate locations on each specimen.

SCANNING ELECTRON MICROSCOPE:

A specimen was randomly selected from each sub-group of the different polishing kits (control, first, fifth, and tenth cycle) for analysis utilizing SEM (JSM, 6360 LV, Japan). Initially, the specimens were soaked with distilled water, then dried and subsequently mounted onto cylindrical holders measuring 13 mm in diameter and 10 mm in height. After the sputter coating procedure employing a gold-palladium alloy with the SPI-Module sputter system (USA), micrographs were taken at an enlargement of 1000× in secondary electron mode to analyze surface roughness.

STATISTICAL ANALYSIS:

Data obtained through profilometer images for each sample was entered in Microsoft excel spreadsheets with each sheet named after each group. All data was first analyzed for correctness, then refined, followed by coding. Statistical analyses were performed utilizing a software program (IBM SPSS v21.0 for Windows; IBM Corp). After testing the data for normality (Shapiro-Wilk test), the recommended test for inferential analysis was a one-way ANOVA rank test (Kruskal-Wallis) which uses median, interquartile range (IQR), and mean rank scores for each group. Differences between the 3 groups were expressed as the H statistic, which determines differences between 2 or more groups of an independent variable on a continuous dependent variable. To know the differences between subgroups of each group and other subgroups, a post-hoc multiple comparison test (Dunn test) was applied after Bonferroni’s correction for probability value ‘P’. the corrected alpha was calculated using a formula-corrected α=α/m, where α was the P value (0.05) and m was the number of groups. For comparison between groups, the probability ‘P’ value was considered significant at ≤0.05 (P≤0.05) while for post-hoc tests, the ‘P’ value was significant at ≤0.0011.

Results

NORMALITY TESTS:

Unlike other parametric tests, the Kruskal-Wallis test used in statistical analysis does not require the assumption of normality or homogeneity of variance. In our situation, however, the normality test was conducted with the Shapiro-Wilk test to determine which statistical test would be more appropriate. The P value for the residuals from the Shapiro-Wilk test was 0.00016, indicating that all groups had a normal distribution.

AVERAGE SURFACE ROUGHNESS:

Table 2 shows the one-way ANOVA results for zirconia’s average surface roughness after 3 finishing/polishing systems (EVE Diacera, Ultradent Jiffy, and SS White) and 3 autoclaving cycles (first, fifth, and tenth). Gp C (control glazed) had the highest surface roughness [Mdn (IQR), 0.0325(0.00)], indicating that glazed zirconia exhibited higher surface roughness compared with the polished (experimental) groups. In comparison with the control glazed group, the EVE Diacera group showed changes in surface roughness after the tenth autoclaving cycle [0.0245(0.009)]. Ultradent Jiffy and SS White exhibited similar changes in surface roughness after the tenth autoclaving cycle, with Ultradent Jiffy at [0.023(0.003)] and SS White at [0.025(0.002)]. The result of one-way ANOVA showed statistical significance among the subgroups included in the studies. The mean rank sum score showed that the maximum score was seen in Group C [MRS (90.95)], followed by Group 1U [MRS 55.6] and Group 10W [MRS 55.3]. Table 3 shows the post-hoc results for multiple group comparisons of surface roughness across different finishing/polishing systems and autoclaving cycles using the Dunn test after Bonferroni correction. The results are presented in terms of mean rank differences (MRD) and their associated P-values, comparing each group against the control and among subgroups. Significant differences in surface roughness were found between the control group (group C) and several experimental groups: for Group 1, statistical significance was reported in 1E (P=0.0002) and 10E (P=0.0010); for Group 2: 5U (P=0.0003) and 10U (P=0.0001); for Group 3: 1W (P<0.0001) and 5W (P<0.0001). These significant P-values (≤0.0011) indicate that the control group had statistically significant surface roughness compared with the experimental groups. There were no significant differences between 1E, 5E, and 10E (P>0.0011), indicating that autoclaving did not significantly affect the surface roughness within the EVE Diacera system. Similar non-significant results were seen within the Ultradent Jiffy (1U, 5U, 10U) and SS White (1W, 5W, 10W) groups, indicating consistent roughness across autoclaving cycles for these systems. The majority of pairwise comparisons between different systems (eg, 1E vs 1U, 5E vs 5W, and so forth) were not statistically significant (P>0.0011). This suggests that the systems had similar surface roughness outcomes after autoclaving, despite minor differences in rank. The post-hoc analysis revealed that significant differences in surface roughness exist between the control and several experimental groups after autoclaving, particularly in the 1E, 10E, 5U, 10U, 1W, and 5W groups. However, within each polishing system (EVE Diacera, Ultradent Jiffy, SS White), autoclaving cycles did not lead to significant differences in roughness. Most cross-system comparisons were also non-significant, suggesting overall consistency in the performance of the polishing systems after autoclaving. These results imply that autoclaving cycles may influence surface roughness for some systems but not consistently across all groups.

SEM AND PROFILOMETRIC ANALYSIS:

Figure 2 illustrates a comparative analysis of profilometric and SEM results between the control and experimental groups. The data presented illustrate the average surface roughness of MMLZ that had been polished using various polishing systems following the completion of the tenth autoclaving cycle. The results indicate comparable surface roughness within the group following autoclaving, with slight variations observed in the EVE Diacera group at the tenth cycle. The SEM images of the glazed control group exhibited a surface structure characterized by slight irregularity and non-homogeneity, featuring a wavy surface (Figure 2A). The EVE Diacera group exhibited characteristics that were comparable to the control group, which consisted of glazed zirconia, as illustrated in Figure 2C. The Ultradent Jiffy group (Figure 2E) and the SS White groups (Figure 2G) exhibited a more homogeneous and smoother surface.

Discussion

STRENGTHS AND LIMITATIONS:

The surface roughness of MMLZ after flash autoclaving cycles was compared using 3 distinct polishing techniques in this first-of-its-kind investigation. However, there are a number of caveats to this in vitro research. Before the burs were autoclaved, their surface roughness was measured using a special kind of zirconia disc. Additional parameters, such as the flexural strength of the zirconia discs after several autoclaving and polishing treatments, were not evaluated by the authors. Furthermore, flash steam sterilization was the only technique used for sterilization in this investigation; no other procedures that may have affected the findings were considered. Third, the MMLZ discs’ surface roughness was tested; however, findings can vary depending on the thickness of the crowns used to measure the roughness. The results may have been affected by the study’s omission of polishing systems wear. Despite having large numbers of specimen for the entire study, the number of samples in each group was small due to a greater number of groups, which adds to another limitation of this study. It would be beneficial to carry out more research comparing different sterilization methods with an emphasis on clinical circumstances.

Conclusions

The findings of this study indicated that repeated autoclaving of up to 10 cycles of a flash sterilization cycle did not decrease the efficiency of the 3 polishing systems used, in terms of producing clinically acceptable surface finish of MMLZ samples. Since manufacturers recommend a maximum of 10 such cycles for polishing systems, future studies may be conducted to determine at which cycle the efficacy of these polishing systems diminishes. The present study concludes that all 3 polishing systems were equally effective to produce clinically acceptable surface finish of MMLZ samples, with none being significantly superior to another.