Comparative evaluation of enamel wear against monolithic zirconia and layered zirconia after polishing and glazing: An in vitro study

Abstract

Aim:

The aim of this study was to compare the wear behavior of human tooth enamel opposing monolithic zirconia and layered zirconia after glazing and polishing by a two-body wear mechanism using a wear simulator.

Settings and Design:

This In-vitro study was done in Department of Prosthodontics, Sri Sai College of Dental Research, Vikarabad.

Materials and Methods:

Zirconia specimens were divided into four groups (n = 15), Group monolithic glazed zirconia (MG), Group monolithic polished zirconia (MP), Group zirconia layered with E. max ceram and glazed (LG), Group zirconia layered with E. max ceram and polished without glaze (LP). Sixty human premolar teeth were subjected to wear test against the zirconia specimens using a Pin on Disc wear tester under a constant load of 5 kg (49 N) at 30 rpm for 10,000 cycles. The loss of enamel was recorded before and after the wear test and mean loss of height of tooth enamel after 10,000 cycles of wear was measured with a profile projector. The surface characteristics of all the four group zirconia specimens were evaluated qualitatively with scanning electron microscope.

Statistical Analysis Used:

One way ANOVA, Tukey Post hoc.

Results:

One-way analysis of variance test revealed that the mean loss of enamel of four groups was statistically different with P < 0.001. A further Tukey post hoc test revealed that the MP group had lesser mean scores than group LP, MG, and LG.

Conclusion:

It was concluded that MP caused less wear to opposing natural teeth, and polished surfaces of both monolithic and layered zirconia showed less tooth wear compared to glazed surfaces of monolithic and layered zirconia.

INTRODUCTION

Zirconia is used as an alternative to metal ceramics owing to its superior properties and esthetics.[1] In comparison to other dental ceramics, Zirconia has been reported to have superior qualities in terms of biocompatibility, dimensional and chemical stability and fracture resistance. Literature on ceramics has shown that they tend to be more abrasive to the opposing natural teeth when compared to other restorative materials.[2] Zirconia, as claimed by the manufacturers, has high strength and abrasive resistance, and therefore, its effect on the opposing teeth is to be investigated.

Wear of the teeth is influenced by various factors such as thickness of enamel, abrasiveness of the restorative material, and patient's oral habits. Studies have shown that surface finish and hardness of the restorative material are the two prime factors which affect the wear of the opposing teeth.[3] Therefore, there is a necessity to explore the suitable surface finish for zirconia restorations for them to function efficiently without any harm to the opposing natural teeth.[4]

The present study aimed to assess and compare the wear pattern of monolithic zirconia and layered zirconia on opposing natural teeth using a wear simulator following glazing and polishing.

MATERIALS AND METHODS

Fabrication of monolithic zirconia specimens

For the calculation of sample size, G power software was used. Keeping the power of the study as 90% and alpha error 5%, the sample size was calculated to be 15 per group. Institutional approval has not been considered as it is an in vitro study. Milled zirconia specimens prepared using computer-aided design-computer-aided manufacturing were used for the study. Sixty zirconia discs of dimensions 20 mm × 3 mm were milled (SIRONA in Lab MC X5) using an standard triangle language file. After milling, the specimens were subjected to sintering cycle at 1450°C (SIRONA in Fire HTC speed). The specimens acquired were distributed into four groups consisting 15 specimens each (n = 15) based on surface treatments [Figure 1].

Figure 1.

Profilometer height measurement

• Group MG: Monolithic zirconia with glaze (n = 15)

• Group MP: Monolithic polished zirconia without glaze (n = 15)

• Group LG: Zirconia layered with E. Max and glazed (n = 15)

• Group LP: Zirconia layered with E. Max and polished without glaze (n = 15).

All the zirconia specimens were steam cleaned for 10 min, followed by ultrasonic cleaning and air-drying.

Layering of zirconia specimens

Zirconia specimens belonging to group zirconia layered with E. max ceram and glazed (LG) and group zirconia layered with E. max ceram and polished without glaze (LP) were layered. Prior to layering the specimens were sandblasted with aluminum oxide particles (50 μm) to increase surface roughness and enhance bond strength. After sandblasting, they were steam cleaned and air dried. Later Z-liner was applied to the discs and subjected to firing at 960°C. Porcelain powder dentin (IPS E. max Ceram) was mixed with adequate amount of modelling liquid and applied with the use of incremental brushing technique. Porcelain was added in excess on the zirconia discs to compensate for shrinkage. A thickness of 1 mm of ceramic layer over zirconia discs was attained and subjected to firing. To ensure that the porcelain layer on all the samples was flat and symmetrical, the samples were checked using an Iwanson's gauge.

Glazing of monolithic zirconia and layered zirconia disc specimens

Group monolithic glazed zirconia (MG) and group LG were subjected to glazing. IPS E. max Ceram Glaze paste and glaze liquid were mixed and applied in an even layer on the entire surface of the specimens followed by firing in a calibrated porcelain furnace (Ivoclar Vivadent Programat P310). Contents of IPS E. max Ceram shade and Glaze pastes include Silicone dioxide, oxides (Al₂O₃, ZnO₂, Na₂O, K₂O, ZrO, CaO, P₂O₅), glycerine, butandiol, and poly (vinyl pyrrolidone).

Polishing of monolithic zirconia and layered zirconia disc specimens

Polishing of group monolithic polished zirconia (MP) was done with Zi-finish range (Bredent UK). First, medium grit and fine grit Edenta Exa-Cerapol polishing wheels were used for 20 s at a speed of 3000 rpm. Once adequate smoothness was achieved, Zi-finish range prepolishers were used followed by polishers to gain high luster.

Polishing of group LP was done with polishing wheels and diamond paste. Medium grit followed by fine grit Edenta Exa-Cerapol polishing wheels was used for 20 s at a speed of 3000 rpm. After finishing, diamond paste (Renfert polish) was applied with a felt wheel to gain high luster. Finishing and polishing procedure was done in a unidirectional manner and excess contact time and force was avoided during the polishing procedure to avoid heat generation.

Fabrication of teeth specimens

Sixty freshly extracted maxillary first and second premolars that are nondecayed and in good shape were disinfected and mounted with auto-polymerizing acrylic resin (length 15 mm and width 10 mm × 10 mm). The teeth specimens were randomly divided into four groups of 15 each to be tested against zirconia groups.

• Group I: To be tested against group MG

• Group II: To be tested against group MP

• Group III: To be tested against group LG

• Group IV: To be tested against group LP.

All the teeth specimens were viewed under profile projector (Metzer M profile projector) to assess the height before testing. The specimen's silhouette is enlarged and displayed on the projection screen via the projector. Because the image is magnified, the X-Y axis of the grid can be aligned with a straight edge of the part to be viewed or measured on this screen, making linear measurements easy to calculate. The teeth specimens were put on the profile projector's worktable in the proper order, and the X, Y, and Z axes were adjusted as needed, and the profile of each tooth specimen was drawn. From the height of the cusp tip to the base of the tooth, a vertical line was dropped. This height was used to determine the baseline height of that specific tooth.

Wear test

Wear test was carried out using pin on disc wear and friction test rig. The tooth specimens were inserted into the pin holder and the zirconia specimens were attached to the lower custom-made metal disc of diameter 165 mm and 5 mm thickness. To hold the test specimens, a provision was given in the center of the disc of dimensions 20 mm in diameter and 2-mm depth such that the specimens were securely seated in the rotating disc. Both the specimens moved in a rotational movement with a load of 5 kg (49N) at 30 cycles per minute for 10,000 cycles in the presence of distilled water.

The loss of height of all the tooth specimens after testing was determined using the profile projector (Metzer M profile projector) in reference to the baseline data [Figure 1]. To evaluate the wear patterns, test materials were assessed by Scanning electron microscope which produces signals on the interaction of the electrons and show the image with desired magnification (ZEISS ultra 55). A magnification of 500 × at 5.00 kV was chosen and surface roughness of each test specimen was recorded prior and after 10,000 wear cycles to analyze the influence of the material and the surface finish on the amount of enamel wear [Figures 2–5].

Figure 2.

SEM images monolithic glazed zirconia. SEM: Scanning electron microscopy

Figure 5.

SEM images of layered polished zirconia. SEM: Scanning electron microscopy

Figure 3.

SEM images of polished monolithic zirconia. SEM: Scanning electron microscopy

Figure 4.

SEM images of layered glazed zirconia. SEM: Scanning electron microscopy

RESULTS

The loss of enamel height against zirconia groups was tabulated and subjected to statistical analysis. One-way analysis of variance test with Tukey's post hoc was performed to analyze mean loss of enamel height among four zirconia groups (MG, MP, LG, and LP) [Table 1 and Graph 1] and two within group categories (monolithic and layered) [Tables 2 and 3, Graphs 2 and 3]. The confidence intervals were set to 95% as P < 0.05 was considered statistically significant. The results revealed that Group II (teeth against MP zirconia) showed statistically significant lesser mean scores when compared to Group IV (teeth against LP zirconia), whose mean was comparable with Group I (teeth against MG zirconia) followed by Group III (teeth against LG zirconia), which showed greater mean score. Mean surface roughness values of zirconia groups before and after wear test were tabulated and analyzed [Tables 4, 5 and Graphs 4, 5].

Table 1.

Comparison of mean loss of height of enamel in all the study groups

Group	Mean±SD	One-way ANOVA	Tukey’s post hoc
GROUP I (MG)	0.76±0.17	<0.001	LG>MG=LP>MP
GROUP II (MP)	0.19±0.08
GROUP III (LG)	1.01±0.11
GROUP IV (LP)	0.65±0.10

It was observed that four groups had statistically significant mean difference with P<0.001. SD: Standard deviation, MG: Monolithic glazed, MP: Monolithic polished, LG: Layered glazed zirconia, LP: Layered polished zirconia

Graph 1.

Graphical bar diagram representation showing mean loss of height after 10,000 wear cycles

Table 2.

Comparison of mean loss of height of enamel in Group I (teeth specimens against monolithic glazed zirconia) and Group II (teeth specimens against monolithic polished zirconia)

Monolith zirconia	Mean±SD		P
	Group MG	Group MP
Enamel wear (loss of height)	0.76±0.17	0.19±0.08	<0.001

The test showed that there was a significant mean difference between the MG and MP specimens with P<0.001. MG: Monolithic glazed, MP: Monolithic polished, SD: Standard deviation

Table 3.

Comparison of mean loss of height of enamel in Group III (teeth specimens against zirconia layered with E.max and glazed) and Group IV (teeth specimens against zirconia layered with E.max and polished without glaze)

Layered zirconia	Mean±SD		P
	Group LG	Group LP
Enamel wear (loss of height)	1.01±0.11	0.65±0.10	<0.001

There was a significant mean difference between the layered glazed zirconia and layered and polished zirconia with P<0.001. LG: Layered glazed zirconia, LP: Layered polished zirconia, SD: Standard deviation

Graph 2.

Graphical bar diagram representation showing mean loss of height after 10,000 wear cycles between monolithic glazed group (Group MG) and monolithic polished group (Group MP)

Graph 3.

Graphical bar diagram representation showing mean loss of height after 10,000 wear cycles between layered glazed group (Group LG) and layered polished group (Group LP) 0.3

Table 4.

Comparison of surface roughness in all the study groups before wear test

Group	Mean±SD	P One way ANOVA	Tukey’s post hoc
Group MG	0.30±0.04	<0.001	LG>LP>MG>MP
Group MP	0.19±0.02
Group LG	0.59±0.06
Group LP	0.39±0.05

A one-way ANOVA was performed to compare surface roughness in 4 different types of material. There was a significant difference between the mean surface roughness of the 4 groups. On further post hoc analysis, it was revealed that the LG group had significantly greater roughness than that of LP, MG, and MP, respectively. LP was significantly greater than MG and MP, respectively, while MG was significantly greater than that of MP. SD: Standard deviation, MG: Monolithic glazed, MP: Monolithic polished, LG: Layered glazed zirconia, LP: Layered polished zirconia

Table 5.

Comparison of surface roughness in all the study groups after wear test

Group	Mean±SD	P One-way ANOVA	Tukey’s post hoc
Group MG	0.50±0.05	<0.001	LG>(LP=MG)>MP
Group MP	0.26±0.06
Group LG	0.77±0.08
Group LP	0.54±0.06

A one-way ANOVA was performed to compare surface roughness in 4 different types of material. There was a significant difference between the mean surface roughness of the 4 groups. On further post hoc analysis, it was revealed that LG group had significantly greater roughness than that of LP, MG, and MP, respectively. LP was equal to MG and group MP showed the lowest mean score. SD: Standard deviation, MG: Monolithic glazed, MP: Monolithic polished, LG: Layered glazed zirconia, LP: Layered polished zirconia

Graph 4.

Graphical bar diagram representation showing surface roughness before wear test

Graph 5.

Graphical bar diagram representation showing surface roughness after wear test

DISCUSSION

Tooth enamel is a very hard, highly mineralized tissue that acts as a barrier to protect the tooth against mechanical and chemical insults but it can also be susceptible to wear. Enamel wear opposing restorative materials is a major concern as it is affected by various internal and external factors.[5] In the present study, enamel wear opposing the monolithic zirconia and layered zirconia was evaluated as each material differs in its properties and hence a different wear behavior occurs. Another important factor which affects the opposing enamel wear is the surface finish of the restorative material. The study was performed to check the difference in the enamel wear after glazing and polishing of monolithic zirconia and layered zirconia to ascertain which surface finish produces the least effect on the opposing enamel.

Glazing and Polishing have various advantages and disadvantages over each other. Glazing is highly esthetic and lustrous. Natural teeth shades can be matched with glazing which cannot be achieved by polishing. However, it is difficult to do, consumes much time, and takes better practice. Polishing on the other hand is easy to do, gives efficient results and less technique sensitive. However, the aesthetic value of polishing is lesser than glazing but the restoration can always be polished even after cementation which is not possible with glazing. Another disadvantage with glazing is that it tends to wear off with time in the functional regions revealing the underlying rougher ceramic surface which may harm the opposing dentition.

In the present study, Zi-finish range has been used to polish zirconia samples which makes surface polishing on zirconia much simpler. This includes prepolishers for smoothing and Polishers for high luster polishing. They are available in three shapes – lens, wheel, and pointed cone which make polishing more accessible in difficult regions like pits and fissures. Advantages of the new Zi-finish products include fast surface polishing of zirconia; two stage system of prepolish and polish, simplifies the work and reduces working time; can also be used with ceramic and nonprecious metals.

Study conducted by Mohammadi-Bassir et al. and Amaya-Pajares et al. showed that the porcelain polishing system produced higher surface roughness values in the range of 2.12–3.10 μ.[6,7] Research done by Mohammadi-Bassir et al. and Park et al. reported lower values, ranging between 0.08 and 0.9 μ after polishing with different zirconia polishing systems.[6,8] However, Vieira et al. Stated that the mechanical finishing and polishing methods were not able to provide a surface as smooth as the glazed surface.[9] A surface roughness test was performed in the present study before the test which showed surface roughness value of Ra-0.19 μ for the monolithic polished specimens, Ra-0.39 μ for the layered polished specimens, Ra-0.3 μ for MG, and Ra-0.59 μ for layered glazed zirconia. These findings were in conformity with the above studies.

Results showed that the wear was greater in Group MG compared to Group MP. Among the layered zirconia groups, Group LG showed greater wear compared to Group LP. Amidst all the groups tested Group LG showed the highest enamel wear whereas Group MP showed the lowest enamel wear suggesting that surface finish and the type of material has a role in the wear mechanism.

On qualitative analysis with scanning electron microscopy (SEM) in accordance to Kadokawa et al. and Ortega et al. Both MG specimens and layered glazed zirconia had more asperities with loss of glaze layer which might be a cause for increased wear among glazed groups.[10,11] The SEM image of MP showed a comparatively intact surface with mild roughening of the surface which might be the reason for relatively lower enamel wear compared to other groups. The SEM image of layered polished zirconia showed loss of surface finish but it appeared to be smoother compared to the monolithic glazed and layered glazed zirconia.

Between monolithic glazed and layered glazed group the surface of layered glazed zirconia appeared rough. In regards to the above findings, due to the damage of the glaze layer after repeated cycles, the surface tends to become rough thereby causing greater wear.

Test results showed that Layered and Glazed group showed significantly greater enamel wear (1.01 mm ± 0.11) compared to Monolithic Glazed group (0.76 mm ± 0.17), followed by Layered Polished group (0.65 mm ± 0.10) and Monolithic Polished group (0.19 mm ± 0.08) showed the least enamel wear among the groups tested indicating that the mechanical polishing of zirconia is the best method to reduce the antagonist wear. The results obtained were in conformity with the study done by Rosentritt et al., Preis et al., Wang et al., Mitov et al., and Park et al.[12,13,14,15,16] To substantiate these findings, surface roughness test was performed for the material specimens following the completion of the wear cycles. Surface roughness values obtained were: Monolithic glazed group (Ra-0.5 μ), monolithic polished group (Ra-0.26 μ), layered glazed group (Ra-0.77 μ), and layered polished group (Ra-0.54 μ). The rate of enamel wear may be correlated with the increase of the surface roughness among the test specimens during the wear simulation.

Previously conducted research showed the wear of enamel against zirconia through various study designs. In vitro studies conducted by Mitov et al., Stawarczyk et al., Elmaria et al., Preis et al., Wang et al., Janyavula et al., Mulay et al., and Mundhe et al. have shown that polished zirconia produces less wear on enamel antagonists than glazed zirconia.[3,13,14,15,16,17,18,19,20] However, using a modified Leinfelder wear testing equipment, Shar et al. discovered that polished zirconia causes more enamel loss than glazed zirconia, and suggested that glazed zirconia should be preferable when the restoration opponent is natural tooth.[21] In contrast, Lawson et al., Janyavula et al., and Mitov et al. in their research determined that the polished surfaces of monolithic zirconia were smoother than glazed surfaces and showed a lower surface roughness than glazed and ground zirconia.[3,15,22]

The study's limitations are that, while enamel is a perfect antagonist, differences in natural substrate and storage media make it less practical and exact than synthetic materials, as stated by S D Heintze et al.[23] Furthermore, the outcomes obtained with nonstandardized enamel specimens were significantly variable. As proposed by Preis et al. and Attin et al., this variance can be due to opposing inhomogeneity, tooth tissue with varied shape or thickness of enamel layers.[13,24] Steps involved in the fabrication of the specimens like sandblasting, layering, application of glaze and polishing were done manually. These factors alone or in combination may contribute to the inconsistencies.

Altogether, the type of restorative material and the surface condition have an influence on the wear potential of the restorative materials. When choosing a restoration, the wear behavior of the material against enamel should be considered as it is an irreversible damage. In order to preserve the enamel, it is essential that proper measures be taken. Chairside adjustments of the zirconia restorations leave a rough surface which in turn can be associated with increased enamel loss. This might be attributed to partial disruption of the glaze layer, incorporation of surface irregularities, etc. Therefore, it is essential that chairside polishing of the restoration is done, irrespective of the surface finish method, before the cementation of the restoration. In this way, zirconia can be effectively used against natural teeth. Several chairside polishing kits are available in the market, but the effectiveness and choice of the polishing agent are of question and stands as a further scope of the study.

CONCLUSION

Following observations might be concluded from the study taking in consideration its limits:

• On preevaluation: Surface roughness of specimens was found to be least in Group MP (Ra = 0.19 μ) followed by Group MG (0.3 μ), Group LP (0.39 μ), and Group LG (0.59 μ)

• Group MP caused the least wear of opposing enamel followed by Group LP, Group MG, and Group LG caused the highest wear

• Both Polished groups (MP and LP) caused lower wear compared to glazed groups (MG and LG)

• Both Monolithic groups (MP and MG) caused lower wear compared to layered zirconia groups (LP and LG).

Further studies may be required to conclude the relation between the surface roughness and the wear pattern.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.