Comparative Evaluation of Flexural Strength, Surface Hardness and Surface Roughness of Monolithic Zirconia After Surface Alteration with Various Chair Side Grinding and Polishing Methods – An Invitro Study

Background: A standard procedure used by physicians to create the best occlusal contacts is chairside adjustment of a restoration. After making these modifications, the restoration should be reglazed or mechanically polished to regain its smooth surface. There is a debate on how zirconia’s properties are affected by grinding, polishing, and glazing.

Objective: To compare and contrast the effects of various chair-side polishing systems on flexural strength, surface hardness, and surface roughness.

Methods: In this invitro study, 50 bar-shaped specimens of monolithic zirconia were fabricated using CAD-CAM  and divided into 5 groups- Group C: Control group, group GrD+PZ: Grind using diamond bur followed by polishing with diamond interspersed polisher for zirconia, Group GrD+PP: Grind using diamond bur followed by polishing with porcelain polishing kit, Group GrC+PZ: Grind using carbide bur followed by polishing with diamond interspersed polisher for zirconia, Group GrC+PP: Grind using carbide bur followed by polishing with porcelain polishing kit. Surface roughness, surface hardness, and flexural strength were tested and data were analysed statistically.

Results: Grinding with a diamond bur followed by polishing with a diamond interspersed polisher for zirconia provides lower roughness values that are within the threshold surface roughness value (Ra=0.2µm), the highest surface hardness values, and the lowest mean flexural strength compared to the other groups, but mean flexural strength exceeds the average occlusal load (500MPa).

Conclusion: Grinding with a diamond bur followed by polishing with a zirconia specific polishing kit should be more effective for monolithic zirconia.

Keywords: CAD-CAM; Dental polishing; Flexural strength; Surface properties (hardness); Zirconia

Metal-free restorations have replaced metal-based ones as the preferred material for fixed dental prostheses due to the rising desire for aesthetics [1]. Although metal-ceramic restorations remain the gold standard for restoration of the teeth, the popularity of metal-free restorations is a result of patient’s demand for greater aesthetics [1,2]. Zirconia, also known as ceramic steel, is a crystalline form of zirconium dioxide. Its mechanical characteristics are similar to those of metals, and both its color and appearance are akin to teeth [3].

In recent years, there has been an increase in demand for zirconia that is both high in strength and aesthetically acceptable [4]. Zirconia has been used extensively in dentistry for orthodontic material, crowns, bridges, and implant frameworks [5]. Zirconia veneered with porcelain (also known as bi-layered zirconia) and fully contoured zirconia (monolithic zirconia) are the two varieties of zirconia that have been employed as restorative materials [2].

Even though veneered zirconia has demonstrated to provide improved aesthetics, veneering may result in the chipping, breaking, and wearing of opposing teeth [6]. Monolithic zirconia was developed in order to solve these issues [2]. Due to its beneficial properties, including as the need for less tooth preparation, increased strength, less wear on the opposing teeth, and suitability for patients with parafunctional behaviors like clenching and bruxism or in situations with little interocclusal space, monolithic zirconia has become increasingly popular [2].

A standard procedure used by physicians to create the best occlusal contacts is chairside adjustment of a restoration, which can lead to a relatively rough surface of the restoration and potentially result in significant wear of the opposing enamel. The restoration’s surface must be smooth in order to prevent the accumulation of plaque, gingivitis, periodontitis, wear on the antagonist’s teeth and other complications that could result in the restoration’s failure [1].

Ceramic surfaces must be as smooth as possible before glazing or polishing in order to reduce these problems, but the most effective technique for this goal is still up for debate. Although some studies have indicated that glazing can provide the smooth surface that is acceptable, repeated firings may have detrimental effects on ceramic surfaces and may result in phase transformation. Furthermore, reglazing requires several clinic visits. Therefore, polishing is the best option if glazing is not possible since it can be completed in a single visit and adds value to infection management by removing the need for repeated laboratory procedures [2]. Huh et al examined the efficacy of six zirconia polishing systems and demonstrated that every polishing system produced results that were clinically acceptable [7]. The ceramic polishing kits guarantee a flawless surface, a long-lasting result, and cost efficiency [8-10]. Furthermore, polishing is a simple process [9].

There is debate on how zirconia’s properties are affected by grinding, polishing, and glazing [11-13,8,14-18]. Therefore, the purpose of this study was to compare and contrast the effects of various chair-side polishing systems on flexural strength, surface hardness, and surface roughness. The null hypothesis was that no differences exist in surface roughness, surface hardness and flexural strength of monolithic zirconia ceramics after the described procedures.

In this invitro study, 50 bar shaped zirconia specimens of dimension 20x4x2mm (figure 1) were fabricated from presintered zirconia blanks (Nexx Zr T, Sagemax, USA) were designed with the help of CAD software (Exclusive dental CAD, Exocad, Germany) based on digitized data and processed by computer assisted milling machine (Roland DWX-52D, Roland, Japan) to specified dimension and then fully sintered using sintering machine (Ceramill therm, Amanngirbacch, Austria) Then the actual dimension of the specimen was measured using a digital calliper (ULJ judgement, Digi-Kanom, India) (Figure 2). Then they were divided into 5 groups (n=10): of control (C), Grind using diamond bur (Green band diamond bur, iris, India) followed by polishing with diamond interspersed polisher for zirconia (Toboom Polishing Kit For Zirconia HP0109D, Toboom, China) (GrD+PZ), Grind using diamond bur followed by polishing with porcelain polishing kit (Porcelain adjustment kit, Shofu, India) (GrD+PP), Grind using carbide bur (Meisinger HP tungsten carbide burs, Meisinger, Germany) followed by polishing with diamond interspersed polisher for zirconia (GrC+PZ), Grind using carbide bur followed by polishing with porcelain polishing kit (GrC+PP).

Figure1: Bar shaped monolithic zirconia specimens.

Figure2: Verifying specimen dimensions using a digital caliper

Figure3: Customized apparatus for grinding

Figure4: Outlined area at the center of each specimen for grinding

For standardization of specimens in the grinding group, a custom-made grinding apparatus (Figure 3) was designed to mount the specimens. It had two perpendicular planes; a linear guide was attached to the horizontal plane for constant linear forward and backward movement of specimens while the straight micromotor handpiece (SDE-H37L1, Marathon, Korea) was clamped to the vertical plane, resulting in the contact of specimens with the rotating bur. An area of 5mm diameter was outlined at the centre of each specimen and marked (Figure4). A new bur was used for every 5 specimens. Grinding was performed for 30 seconds in a continuous forward–backward sweeping motion in the designated area.[2] Then the polishing was performed in sweeping motion in the same direction as the grinding procedure. Finally, the specimens were ultrasonically cleaned for 10 minutes at room temperature and air-dried.

To test flexural strength of specimens, a universal testing machine was used. The specimens were placed in a self-aligning fixture. The load was applied perpendicular to the longitudinal axis of the specimen. Finally, the flexural strength of specimens was calculated in Megapascals (MPa) using the following equation recommended by ISO6874.2

M=3WL/2bd2

Where W is the applied load(N), L is the length of the specimen (mm), b width of the specimen (mm) and d is the thickness of the specimen (mm).

Surface roughness of specimens was determined with a profilometer. The outlined area of the specimen to be tested were placed parallel to the detector and stylus is in proper contact with the surface to be measured. The Ra value describes the average roughness value for a surface that was traced by the profilometer. The surface roughness value was represented in µm.

Surface hardness of the specimens was determined using a Vickers microhardness tester. The outlined area of the Zirconia specimen was fixed on the microhardness stage in perpendicular alignment to the indentation head. The diamond pyramid head of Vickers microhardness testing machine was applied to the intaglio surface of zirconia specimen at a force of 0.5 kg for 30 seconds. The length of the two indentation lines was measured at 40× through the built-in scale microscope as shown in. Three indentations were applied for each specimen at three different locations (left, right, and central regions). Values were averaged and reported as a single value.

The data were collected, coded and fed in SPSS version 25 for statistical analysis. The descriptive statistics included mean and standard deviation. The analytical statistics included Kruskal Wallis test of the comparison. The level of significance was set at 0.05 at 95% confidence interval.

Graph 1 presents the mean of surface roughness. The highest mean surface roughness was recorded in group GrD+PP and lowest was found in group GrD+PZ. Pairwise comparison for the surface roughness showed that there was a significant difference between the surface roughness of Group GrD+PZ and group GrD+PP with P=.000 and Group C and group GrD+PP with P=.001. There was no significance between other group (P<0.05; Table 1).

Graph 1: Mean surface roughness

Table 1: Results of multiple comparisons using Kruskal Wallis test between five groups   of surface roughness values.

Graph 2 presents the mean of surface hardness. The highest mean surface roughness was recorded in group GrD+PZ and lowest was found in group GrC+PZ. Pairwise comparison for surface hardness showed that there was a significant difference between the surface hardness of group GrC+PZ and group GrC+PP with P=.000, group GrC+PZ and group GrD+PZ with P=.000, group GrD+PP and group GrC+PP with P=.010, group GrD+PP and group GrD+PZ with P=.001, group C and group GrD+PZ with P=.006. There was no significance between other group (P<0.05; Table 2).

Graph 2: Mean surface hardness

Table 2: Results of multiple comparisons using Kruskal Wallis test between five groups   of surface hardness values

Graph 3 presents the mean of flexural strength. The highest mean flexural strength was recorded in group C and lowest was found in group GrD+PZ. Pairwise comparison for the flexural strength showed that there was a significant difference between the flexural strength of group GrD+PZ and group C with P=.001. There was no significance between other group (P<0.05; Table 3).

Graph 3: Mean flexural strength

Table3: Results of multiple comparisons using Kruskal Wallis test between five groups   of flexural strength values

A standard procedure used by physicians to create the best occlusal contacts is chairside adjustment of a restoration, which can lead to a relatively rough surface of the restoration and potentially result in significant wear of the opposing enamel. After making these modifications, the restoration should be reglazed or mechanically polished to regain its smooth surface. The restoration’s surface must be smooth in order to prevent the accumulation of plaque, gingivitis, periodontitis, wear on the antagonist’s teeth and other complications that could result in the restoration’s failure. Reglazing, though, isn’t always practical or feasible. In order to restore the surface smoothness and characteristics, polishing is recommended [1].

Sabrah et al [19]. investigated the surface roughness and wear behavior of monolithic zirconia that has been glazed, ground, and polished. They claimed that, while the glazed group had the smoothest surface, the wear behavior of the glazed monolithic zirconia was inferior to the unglazed group. Heintze et al [20]. discovered that glazed surfaces outperformed polished surfaces in terms of antagonist wear. The unifying conclusion of these investigations is that glaze treatment provided the best surface smoothness, however, the lifespan of glaze is unknown when restorations are in use. As a result, adequate polishing can help to prevent or reduce opposing abrasion. In the current investigation, we assessed the polishing effect following the grinding technique rather than glazing. According to research done by Bollen et al [21,22], surfaces with a roughness of more than 0.2 microns increase the risk of caries, bacterial adherence, and plaque maturation. Because a rough zirconia surface causes more wear on the opposing tooth and compromises the clinical efficacy of the restoration, a polished zirconia surface is desirable [1,8,14]. The results of the present study showed that occlusal modifications had a considerable impact on surface roughness levels. Polishing decreased surface roughness levels whereas grinding increased them. The surface smoothness of zirconia and porcelain polishing methods was significantly higher (P <0.05).

Grinding causes a significant change in surface roughness which will be decreased after the polishing procedure and the smoothest surface was found for group GrD+PZ (0.2 µm). These findings are similar to threshold surface roughness values (Ra = 0.2 μm) of the dental prosthesis for prevention of plaque accumulation which means that the surface roughness values for group GrD+PZ were clinically acceptable. However, group GrD+PP (1.43) exhibit surface roughness value than this threshold. This could be due to the higher cutting efficiency of diamond burs compared to carbide burs.1 Park et al.23 reported that zirconia polishing systems on monolithic zirconia showed lower surface roughness values than porcelain polishing systems, which is similar to the findings of the present study. The hardness of the material determines how the polishing systems are made. The efficiency of porcelain polishing systems is debatable when used with zirconia restorations since they include ceramic particles that are less hard than zirconia. Therefore, to control the flaws caused by the occlusal correction, the surface of monolithic zirconia restorations should be polished with a zirconia polishing system.

Hmaidouch et al [8] found that coarse grinding increased surface roughness significantly. After polishing the same specimen, a smooth surface was obtained that was comparable to untreated glazed zirconia surfaces. This was made possible by the removal of weakly adherent surface grains and the elimination of grinding trace lines. They concluded that polished surfaces outperformed glazed surfaces in terms of wear on the opposing enamel. Glazing, according to Azeez S M et al [24], does not reduce surface roughness as successfully as polishing; this may be due to the coating layer being insufficiently thick to properly complete the ceramic surface micro-cracks and grooves.

Surface hardness is a critical physical feature influencing the clinical success of zirconia restorations. It contributes to the material’s resistance to external forces [25]. Hashim A R et al [26] found that surface roughness and VHN had an inverse connection, i.e., a decrease in surface roughness leads to an increase in micro-hardness. Traini et al [27] reported a greater hardness value for machined surfaces than for finely polished surfaces, but a lower hardness value for coarse polished surfaces. Based on the findings of this study, GrD+PZ lower roughness values and the highest VHN values compared to the other groups. However, few research revealed the comparative evaluation of chairside grinding and polishing on the VHN of monolithic zirconia, therefore no further comparison with the results published in previous studies can be established.

Ceramics are brittle and have a low tensile strength [28]. Flexural strength is a reliable method for researching brittle materials since the peak tensile stress occurs within the centre loading area and edge failures are excluded [8]. Previous research on the impact of grinding on zirconia yielded conflicting results [2,6,23,29,30]. According to certain research, grinding increased flexural strength by producing residual surface compressive stress [2,6,24]. Others have argued that surface imperfections caused by grinding can have a negative impact on mechanical characteristics if they extend beyond the depth of the surface compressive layer [31]. Including disparities may be attributable to differing study methodology, as numerous factors, such as applied stress, grinding speed, and grit size, can influence the effects of grinding on zirconia [2,32]. The current study’s findings revealed that polishing greatly reduced flexural strength. Similarly, Traini et al [33] revealed that polishing reduced flexural strength by forming microcracks. Grinding, according to Iseri et al [31], reduced flexural strength, particularly in groups processed with a micromotor. It was established that reduced temperature rise during handpiece grinding is responsible for higher maintenance of the monoclinic phase, resulting in a smaller drop in flexural strength.

Within the limitations of the present invitro study, it may be concluded that grinding with a diamond bur followed by polishing with a zirconia specific polishing kit should be more effective for monolithic zirconia.