ORIGINAL RESEARCH |
https://doi.org/10.5005/jp-journals-10015-2363 |
Influence of Finishing and Polishing Procedures on the Surface Characteristics and Flexural Strength of Monolithic Zirconia: An In Vitro Study
1Department of Prosthodontics, Meenakshi Ammal Dental College and Hospital, Tiruvallur, Tamil Nadu, India
2Department of Prosthodontics and Implantology, Meenakshi Academy of Higher Education and Research, Faculty of Dentistry Meenakshi Ammal Dental College and Hospital, Chennai, Tamil Nadu, India
3,4Department of Prosthodontics and Crown & Bridge, Meenakshi Ammal Dental College & Hospital, Chennai, Tamil Nadu, India
Corresponding Author: Vyshnavi Devi Janagaraj, Department of Prosthodontics, Meenakshi Ammal Dental College and Hospital, Tiruvallur, Tamil Nadu, India, Phone: +91 8056085589, e-mail: vyshnavijanagaraj@gmail.com
Received: 04 December 2023; Accepted: 05 January 2024; Published on: 20 February 2024
ABSTRACT
Aim: The aim of the present in vitro study was to evaluate the effect of the finishing and polishing system on surface roughness, topography, and flexural strength of monolithic zirconia.
Materials and methods: A total of 20 samples were milled and sintered into bar-shaped specimens (16 × 4 × 1.6 mm) from a presintered yttria-stabilized tetragonal zirconia polycrystal (Y-TZP block). They were divided into two groups containing 10 each. The control group consisted of glazed untreated zirconia. The experimental group consisted of specimens whose surfaces were roughened using a red diamond-coated bur for 20 seconds, and then they were polished using a Diacera intraoral polishing kit. The samples were evaluated quantitatively for surface roughness using a surface profilometer and qualitatively by scanning electron microscope (SEM), and the flexural strength of the two groups was tested using an Instron universal testing machine. The results were statistically analyzed using the Shapiro–Wilcoxon test and Mann–Whitney U test.
Results: Scanning electron microscopic images showed evident differences in the topography of untreated, roughened, polished monolithic zirconia. The polished group had a lower average surface roughness value (Ra = 0.13 μm) when compared to untreated Zirconia. There was no statistically significant difference between the test and control group. The highest flexural strength was seen in the polished test group (max: 320.80 MPa). There was a statistically significant difference (p < 0.05) between the test and control group.
Conclusion: Eve Diacera’s zirconia polishing kit created a smoother and more uniform surface on monolithic zirconia. The flexural strength of roughened and polished monolithic zirconia was relatively higher than that of untreated zirconia. Under a SEM, there was an evident difference in the topography of untreated, roughened, and polished monolithic zirconia.
Clinical significance: Finishing and polishing procedures mainly enhance esthetics, aid in reducing surface irregularities, and maximize biocompatibility by limiting bacterial adherence to the surface, among other advantages. Therefore, finishing and polishing are crucial for the longevity of the restorations.
How to cite this article: Janagaraj VD, Shivasubramanian L, Sekar V, et al. Influence of Finishing and Polishing Procedures on the Surface Characteristics and Flexural Strength of Monolithic Zirconia: An In Vitro Study. World J Dent 2024;15(1):19–24.
Source of support: Nil
Conflict of interest: None
Keywords: Flexural strength, Monolithic, Polishing Kit, Scanning electron microscopy, Zirconia
INTRODUCTION
The development of newer materials is on the rise for their esthetic properties, biocompatibility, color stability, and low thermal conductivity.1 Polycrystalline-based zirconia-based restorations are becoming increasingly popular due to their excellent properties like higher strength, lower wear, and lower cost compared to other silicate bonded restorations.2
During the clinical try-in of any ceramic restoration, it is necessary to check and adjust any occlusal interference, which can lead to the removal of the surface glaze and exposure of the underlying unglazed rough ceramic surface.3 Unglazed ceramics may increase plaque retention, increase wear on the opposing teeth, and reduce the strength of the ceramic material.3 These restorations are returned to the laboratory for reglazing.3 Adjusted prostheses must be polished to reduce surface irregularities. Polishing is the process of producing a smooth, shiny surface that has similar reflection and refraction characteristics as natural teeth through the use of abrasives.4
Instruments and techniques advocated for contouring and polishing must create a smooth surface to maximize the flexural strength of the restoration, minimize the risk of chipping or fracture, minimize abrasive wear of opposing teeth and restorations, and maximize biocompatibility by limiting adherence of bacteria to the surface of the restorations.5
Many intraoral polishing protocols are available to eliminate or decrease the fissures arising during the adjustment and save working time after small ceramic adjustments.6 Several studies have shown that an adequately polished ceramic surface can be achieved clinically in the dental laboratory without the need for glazing.7
Different surface roughness formed through different finishing procedures can cause various stress concentrations and, consequently, a reduction in strength.8 Surface roughness (Ra) refers to the finer irregularities on the surface texture that usually result from the action of the production process or material condition and is measured in micrometers (μm).9
Surface profilometry is commonly used for quantitatively assessing the surface roughness of materials. Since only some areas are probed by the profilometer, the roughness values do not represent the actual topography of entire surfaces. Scanning electron microscopy qualitatively assesses the surface roughness by providing high-resolution images.10
The strength of materials is often described in terms of flexural strength and fracture toughness. The influence of polished surface and absence of microcracks is proportional to the quality of ceramic surface and its influence on flexural strength. The use of the indentation fracture technique in studying the behavior and properties of brittle materials is specifically appropriate because only small dimensional specimens are required, and the crack growth parameter is similar to those cracks expected under clinical conditions.11
Although finishing and polishing procedures are important for clinical success, the effects of surface treatment protocols in combination with popular polishing systems have not been fully investigated,12 thus the aim of this in vitro study is to evaluate the effect of finishing and polishing procedures on the surface roughness, topography, and flexural strength of monolithic zirconia. The null hypothesis was there was no difference in surface roughness, topography, and flexural strength of roughened and polished monolithic zirconia.
MATERIALS AND METHODS
The study was approved by the Institutional Review Board (MAHER University, Chennai, Tamil Nadu). IRB number: (MADC/IRB-XXXVI/2018/413). The study was conducted from 12/2018 to 06/2019.
Specimen Preparation
A presintered yttria-stabilized tetragonal zirconia polycrystal (Y-TZP block) (K2 Zircon blank, Yetti, Germany) was milled and sintered into 20 bars measuring of length 16 mm width, 4 mm, and a thickness of 1.6 mm10 using computer-aided design/computer-aided manufacturing (CAD/CAM) technology by a dental ceramist. It was then glazed to mimic the zirconia crowns. The 20 zirconia bars were divided into two groups of 10 bars in each group.
Sample Size Calculation
A pilot study was performed with five samples in each group; based on the results of the pilot study, the minimum appropriate sample size was computed after assigning the power of the study as 80% and α-error as 5% to be around 10 per group [study parameter delta = 22.1620, m 1 = 259.500, m 2 = 281.662, standard deviation (SD) 1 = 13.763, SD 2 = 18.205].
The 20 milled samples were divided into two groups, the control and test groups, each group consisting of 10 samples. The control group consisted of 10 samples of glazed bars of Y-TZP. The test group consisted of 10 samples of surface-treated bars of Y-TZP, which were subjected to surface roughening and polishing. The surface roughing was done throughout the length of the specimen, using a red band diamond-coated bur (53–63 μm) (TF 12F ISO 173/015 MANI, Japan) for 20 seconds with a high-speed handpiece (NSK, Tochigi-Ken, Japan) under water cooling in a forward-backward motion to stimulate that of the clinical setting, a new bur was used for every five specimens. After roughening procedures, all 10 samples were rinsed with air-water sprayed for 15 seconds and dried. After roughening, the 10 test samples were subjected to polishing using an Eve Diacera zirconia polishing kit (Eve Diacera, Germany), which comprises a two-step polishing system that was diamond-impregnated. The two-step polishing system consisted of a prepolishing bur and a Final polishing bur. Each bur was used for 1 minute of polishing at 20,000 rpm using a slow-speed handpiece (NSK, Tochigi-Ken, Japan). Again, the samples were rinsed with air-water spray for 15 seconds and dried. All the procedures were done by the same operator, and the specimens were mounted in addition to silicone putty (Aquasil Soft Putty, Dentsply, United States of America) to standardize the evaluations and treatments.
All 20 samples were subjected to all three tests to assess the surface roughness qualitatively, quantitatively, and flexural strength.
Surface Roughness Assessment
The surface roughness of the samples was assessed using a profilometer (Mitutoyo, SJ 310 Kawasaki, Japan). The measurements per specimen were made over a transverse length of 4 mm, with a stylus speed of 0.5 mm/second. The average surface roughness (Ra in μm) was measured.
Flexural Strength Assessment
A universal testing machine (ElectroPulsTM E3000, Instron Ez Pulse Dynamic Digital Machine, United States of America) was used for flexural strength testing. Initially, the actual dimensions of each ceramic specimen were measured with a digital caliper (Mitutoyo, Kawasaki, Japan) for later calculation of flexural strength (MPa). The specimens were placed centrally in a self-aligning fixture. The edges of the specimen had approximately 3 mm distance from the fixture, leaving a 10 mm test span (center-to-center between bearers). The load was applied perpendicular to the long axis of the sample with the processing speed of 1 mm/minute. The flexural strength (MPa) of the samples was calculated using the following equation:
M = 3WL / 2bd2
Where W is the applied load (N), L is the test span (mm), b is the width of the sample (mm), and d is the thickness of the sample (mm).
Qualitative Surface Topography Assessment
A 15 nm thick gold coating was applied on the surface of all the specimens using a sputter coater (K450X; Emitech, Cambridge, United Kingdom). Then all the 20 samples, that is, the 10 samples from the control group, were individually viewed under scanning electronic microscopy in 5000× magnification, and the 10 samples from the test group were viewed individually twice under scanning electronic microscopy in 5000× magnification, that is, initially, after surface roughening of the samples and finally, after polishing of the samples (Fig. 1).
Figs 1A to C: Scanning electron microscope (SEM) image of; (A) Glazed zirconia (5000×); (B) SEM image of surface roughed zirconia (5000×); (C) SEM image of polished zirconia (5000×)
Statistical Analysis
The distribution of the data was analyzed by using the Shapiro–Wilcoxon test. Surface roughness and flexural strength between the groups were analyzed using the Mann–Whitney U test. The significance level for all statistical analyses was set at (p-value of <0.05, the power of the study as 80%). All statistical analyses were performed with the STATA 16.1 software (STATA Corporation, college station, TX, United States of America).
RESULTS
Surface Roughness
The mean and SD values of surface roughness (Ra values in μm) are presented in Table 1 and Fig. 2. The statistical analysis table for the average surface roughness (Ra in μm) stated that, for the control group, the mean ± SD was 0.124 ± 0.0069921 μm with a minimum value of 0.12 μm and maximum value of 0.14 μm and for the test group, the mean ± SD was 0.125 ± 0.0052705 μm with a minimum value of 0.12 μm and maximum value of 0.13 μm. The statistical analysis of the intergroup significance was 0.6499 (p < 0.05). There was no statistically significant difference in this regard among tested groups (p < 0.05) (Table 1).
| Variable | Groups | Mean ± SD | Min | Max | Intergroup significance (p < 0.05) |
|---|---|---|---|---|---|
| Surface roughness value (Ra in μm) |
Untreated zirconia: group I | 0.124 ± 0.0069921 | 0.12 | 0.14 | 0.6499 |
| Polished zirconia: group II | 0.125 ± 0.0052705 | 0.12 | 0.13 | ||
| Flexural strength value (MPa) |
Untreated zirconia: group I | 259.045 ± 21.27422 | 236.81 | 298.01 | 0.0355* |
| Polished zirconia: group II | 285.217 ± 27.15453 | 245.08 | 320.80 |
*p-value is less than 0.05
Fig. 2: Figure representation of the Ra values
Flexural Strength
The mean and SD values of flexural strength are presented in Table 1 and Figures 2 and 3.
Fig. 3: Figure representation of the maximum load to fracture
The statistical analysis table for the flexural strength (MPa) stated that, for the control group, the mean ± SD was 259.045 ± 21.27422 MPa, and for the test group, the mean ± SD was 285.217 ± 27.15453 MPa. The statistical analysis of the intergroup significance was 0.0355 (p < 0.05).
Therefore, the results signify that the highest flexural strength was seen in the test group, which consisted of roughened and polished zirconia (max: 320.80 MPa). There was a statistically significant difference in this regard among tested groups (p < 0.05) (Table 1).
Scanning Electron Microscopic Test
The scanning electron microscopic images of the samples, which were surface roughening with a diamond bur, created a surface with significant irregularities on the surface, when compared to untreated glazed zirconia samples, which had a smooth surface. Eventually, when the surface roughened samples were subjected to finishing and polishing procedures using the Eve Diacera Zirconia polishing kit, the scanning electron microscope (SEM) image showed a smoother surface devoid of irregularities. These images show significant evidence of the change in surface structure after the finishing and polishing procedure.
DISCUSSION
Polycrystalline-based zirconia often requires intraoral adjustments to enable adequate occlusion, which results in rough surfaces.2 Surface roughness should be minimized to achieve superior aesthetics, better mechanical properties, reduced plaque accumulation, and to resist wear from the opposing teeth.10 Intraoral polishing and finishing procedures can reduce surface roughness and have a significant effect on stress concentration and fracture strength of ceramics.13
The purpose of the current study was to assess the effects of monolithic zirconia on the surface roughness, topography, and flexural strength after finishing and polishing procedures in comparison with glazed zirconia.
The results of the current study showed that the surface roughness and flexural strength of monolithic zirconia were influenced by finishing and polishing procedures done by commercially available zirconia finishing and polishing kits (EVE Diacera, Germany), and the maximum load to fracture was comparatively more in the test group which was subjected to finishing and polishing procedures when compared to that of the control group which consisted of untreated glazed zirconia samples. The surface roughness of monolithic Zirconia was well modified by commercially available polishing systems after surface roughening. The results showed that surface roughness values between the untreated, roughened, and polished zirconia samples were similar to each other in surface profilometer analysis and scanning electron microscopic images.
Chairside adjustments of the restorations, which subsequently cause roughness of the clinical crowns, can be removed either by glazing procedures or finishing and polishing after clinical adjustments.14 Sabrah et al. evaluated the surface roughness and studied the wear behavior of glazed, grounded, and polished monolithic zirconia. The study stated that the glazed group had the smoothest surface, and the wear behavior of the glazed monolithic Zirconia was not preferable to the unglazed group.
In spite of various studies comparing the effects of polishing procedures on ceramics, no significant conclusion has been drawn because of the interdependence of various factors, which always have to be considered.2,10,12,15,16 There are various factors that can influence surface roughness, which are types of ceramic, polishing system, speed, duration, amount of applied pressure, and the presence or absence of water during finishing.16 There are no proper specifications about handpiece speed, characteristics of abrasives, polishing load, or application time.17 A lot of parameters make it difficult to compare studies on the effects of polishing procedures on monolithic zirconia.
The polishing of polycrystalline zirconia is not a conventional polishing process but rather a mechanical micro abrasion of the material peaks, where the remaining microscratches are not filled by plasticized material, as seen during the polishing of resin or metal.6 On the other hand, surface polishing of silicate ceramic is always a combination of filling the micro scratches with a liquefied glass matrix, removing surface peaks, and polishing procedure with diamond particles.18 It has been studied that higher surface modification ability is achieved with larger-sized diamond grains.18 But, the surface roughness is increased, so the rotary instrument should be changed sequentially from a large to smaller grain size of the diamond abrasives of the polishing instrument. A standard polishing kit usually comprises two steps; where initially it is polishing points in the prepolished step, followed by super-polished steps. The recommendation for the surface finish of zirconia is mirror polishing with appropriate polishing materials and instruments containing fine diamond particles.19 The finishing and polishing kit used in the current study is a Diacera two-step polishing kit, which also comprises an initial prepolishing diamond-impregnated polishing disk followed by a super polishing diamond-impregnated polishing disk.
The surface roughness of the materials is to be evaluated using surface profilometric analysis. Using Ra and Rz as parameters. Ra is the arithmetical mean roughness value—the arithmetical mean of the absolute values of the profile deviations from the mean line of the roughness profile.
Many studies had only used Ra as the roughness parameter.2,12,20 Similar to experiments conducted by Steiner et al.,21 studied 20 specimens which were first roughened with a red band rotary diamond finishing instrument (46 μm grit) to simulate occlusal adjustment. Some defects may manifest due to adjustments, which can propagate to a critical defect upon clinical loading and may cause some form of failure. To avoid fracture of the restorations, they must be reglazed or polished after adjustment.22 Same as other previous studies, even in this study, there were no statistically significant differences in Ra and Rz values of the glazed control group and test group, which was surface roughened and polished using a commercial polishing kit.22
Bollen et al.23 also stated that bacterial retention could not be expected below the threshold surface roughness value (Ra = 0.2 μm). The findings of this are similar to threshold surface roughness values (Ra = 0.12 μm) of a dental prosthesis for the prevention of plaque accumulation, which means that the surface roughness values for the control group and test group were clinically acceptable. Therefore, the results of the current study showed that the zirconia specimens that were subjected to surface roughening and then polished with commercially available intraoral zirconia polishing kit, Eve Diacera zirconia polishing kit produced a similar surface to that of glazed zirconia bars, which showed the importance of usage of commercially available zirconia polishing kit after surface modification.17 Huh et al. used EVE Diacera zirconia and Meisinger polishing systems, which were used to polish the ground surface, and the study showed that surface roughness values were similar to this study.
There was no statistically significant difference in surface roughness between the control group and the test group. Surface roughening of the glazed zirconia surface with a diamond bur created a significant change on the surface. The scanning electron microscopic image also confirmed it, showing that there were evident changes on the surface with a lot of surface irregularities in all the specimens that were subjected to surface modification.
Eventually, after the finishing and polishing procedures using the Eve Diacera Zirconia polishing kit, which had a significant role in reducing the surface roughness in the test samples in this study, and this was very evident in the SEM analysis, which showed a clear surface in the smoothness of zirconia after polishing with Eve Diacera zirconia polishing kit (Diacera, Germany).
Evaluation of flexural strength shower variable results in relation to the control and the test groups. After grinding, polished specimens showed better resistance to fracture when compared to the load of it in the control group when load was applied. A few researchers believe that there is an inverse relationship between flexural strength and surface roughness of ceramics since most of the stresses are concentrated at the tensile zone of zirconia.24,25 The results of the study show that resistance to fracture when the load was better in the polished groups when compared to that of glazed zirconia control samples. The results also showed a statistical significance between the two groups (p = 0.03).
Using a polishing kit with finer diamond particles for polishing decreases the stress, which indicates the superiority of the zirconia polishing kit for polishing, as it yields better flexural strength than the control specimen.
The SEM image of the fracture junction was also evaluated in both the samples, which showed smoother and uniform surfaces in control samples, whereas in treated samples, there were a lot of surface irregularities.
The effect of polishing and finishing instruments depends on various factors like the structure, size, and shape of abrasive particles, mechanical properties of the substrate, and physical properties of the binder.
The limitation of the study was that the samples were not subjected to any thermocycling or aging processes, and the sample size used in this study was comparatively smaller than that of other previous studies. Only one polishing kit was assessed in this study. Therefore, a comparative analysis with various polishing kits can be further investigated for a proper conclusion. Evaluating the effect of polishing systems on fracture toughness is a very useful topic for future studies.
CONCLUSION
The scanning electron microscopic images showed that there was an evident difference in topography of untreated glazed, surface roughened, and polished monolithic zirconia. The surface roughened, and polished zirconia had a statistically significant higher flexural strength when compared to untreated glazed zirconia. Glazed and polished samples had similar surface roughness values, which showed that polishing of zirconia removed surface irregularities. Therefore, the Diacera zirconia polishing system created a smoother and more uniform surface in monolithic zirconia in terms of quality and quantity. The flexural strength of roughened and polished monolithic zirconia was relatively higher than that of untreated zirconia.
ORCID
Vyshnavi Devi Janagaraj https://orcid.org/0000-0003-0089-4465
Vigneswaran Sekar https://orcid.org/0000-0002-1471-5002
Annapoorni Hariharan https://orcid.org/0000-0003-1175-5882
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