Surface Roughness of Glass Ionomer Cements after Application of Different Polishing Techniques.

Glass Ionomer Cements (GIC) have been widely used in clinical practice since they have a wide range of positive characteristics: chemical bonding to the tooth surface, fluoride release, a heat-expansion coefficient similar to the tooth, do not require an absolutely dry working area, less volumetric contraction, good color stability. Physical properties can be improved by using external energy such as ultrasound and radiant heat (thermo-curing), which also accelerates chemical curing.
Objectives: The aim of this study was to determine the most effective polishing technique and to compare the surface roughness of two Glass Ionomer Cements after treatment with heat (thermo-curing), and without heat treatment during the setting process. Materials and methods: Two polishing systems (Tungsten carbide burs and Sof-Lex discs) were used on two types of GIC (Equia Fil and Ketac Molar Universal). Bluephase 16i LED (Vivadent, Schaan Liechtenstein) light was used for the specimens treated with heat (thermo-curing). Samples without heat treatment are left for 10 minutes to chemically cure. Surface profilometar was used for measuring the mean surface roughness value (Ra).
Results: Group with Mylar strip (control group) of each material showed the lowest (Ra) value. The Equia Fil material samples treated with heat (thermo-curing) achieved lower surface roughness values (Ra), and showed lower surface roughness values (Ra) after polishing with a Sof-Lex discs (p<0.05). The results for Ketac Molar Universal samples showed no statistically significant difference (p>0.05) between polishing with Sof-Lex discs and Tungsten carbide burs.
Conclusion: Based on the obtained results, it can be concluded that the smoothest surface roughness is achieved by the Mylar strip. Some types of Glass Ionomer Cements can obtain better surface polishing with heat treatment (thermo-curing).

Introduction

Glass Ionomer Cements (GICs) were introduced by Wilson and Kent in 1970 (). They have been widely used in clinical practice because they possess a wide range of positive characteristics: chemical bonding to the tooth surface, fluoride release, the heat-expansion coefficient similar to the tooth, do not require an absolutely dry working area, less volumetric contraction, good color stability (, ).
Finishing is defined as shaping of teeth morphology with instruments to achieve ideal anatomy. Polishing eliminates the scratches resulting from finishing instruments and reduces surface roughness (). Residual surface roughness may result in gingival inflammation, bacterial colonization, increased surface staining and dental plaque accumulation. Finishing and polishing are mandatory steps in restorative dentistry that enhance both longevity of restorations and esthetics (-). Surface characteristics and wear resistance of restorations are important criteria to predict the clinical deterioration of restorative materials. Poorly polished and rough surfaces contribute to a faster accumulation of dental plaque and bacteria, which increases the gingival inflammation and caries risk. Conversely, a highly polished surface minimizes it and contributes to better esthetics and color stability (-).

Several articles have reported that the lowest roughness of GIC surfaces was found after treatment with the Mylar strip. However, the correct morphology of the restoration is rarely achieved by using only the Mylar strip (-). Bollen et al. reported that the critical surface roughness (Ra) for bacterial colonization is 0.2 μm. Bacterial accumulation, plaque maturation and acidity significantly increase when the surface roughness exceeds 0.2 μm, which acts on material surfaces, thus increasing caries risk (). For that reason, finishing and polishing are necessary steps in restorative dentistry that enhance longevity of restorations and esthetics. Additionally, surface roughness can directly affect marginal integrity and the wear behavior of restoration (, ).

Surface roughness is micromorphology created by various physical processes used for surface modification. Average roughness (Ra) is the most commonly used parameter to describe surface roughness that is measured with a profilometer. Profilometers provide two-dimensional information, but a scanning electron microscope (SEM) is needed for a complete picture of a detailed analysis. An arithmetic average roughness that can assist clinicians in their treatment decisions can be calculated for each material after polishing treatment (-).

The acid base neutralization reaction of GIC can be accelerated by the use of external energy such as ultrasound (, ) and heat (, ). This is particularly useful in overcoming the moisture sensitivity which adversely affects the properties of GIC material (-). Although ultrasound accelerated the setting, its use was clinically difficult. On the other hand, heat can be applied through portable LED lamps. Commercial Glass Ionomer Cements are now available on the market with manufacturer's instructions for applying thermo-curing technique using radiant heat from portable LED lamps (). There was a concern that such heat exposure could have a potentially harmful effect on the dental pulp. Based on the obtained results of Van Duinen et al, it could be concluded that the use of external heat during the setting of GIC material does not lead to harmful overheating of the pulp tissue; hence it does not cause any pathological conditions. On the contrary, for improving the adhesion of GIC material and mechanical properties, the application of external heat (thermo-curing) as a “Command set „method can be part of regular clinical practice ().

The aim of this study was to analyze the effect of polishing instruments on surface roughness of GIC and to determine the differences in the quality of polishing of GIC treated with heat (thermo-curing) and without the application of heat treatment. The null hypothesis was that GIC treated with heat (thermo-curing) obtain the lower surface roughness value.

Material and methods

Two types of GIC were used in this study, Equia Fil (GC Corp, Tokyo, Japan) and Ketac Molar Universal (3M ESPE, Seefeld, Germany). The specimens of each material were divided into three groups: treated with heat (thermo-curing) without heat and the control group (Table 1).

Table 1

GIC samples divided into six groups (n=10).

GROUP 1	Ketac Molar Universal - without heat
GROUP 2	Ketac Molar Universal - with heat (thermo-curing)
GROUP 3	Equia Fil - without heat
GROUP 4	Equia Fil - with heat (thermo-curing)
GROUP 5	Ketac Molar Universal - control group (Mylar strip)
GROUP 6	Equia Fil - control group (Mylar strip)

All 60 specimens were prepared using standardized rubber molds with dimensions 2x2x10mm and divided into six groups (n=10). Material for each sample was capsulated and mechanically mixed according to the manufacturer’s instructions and inserted into the molds using a hand applicator for the capsules. The material was covered with a transparent Mylar strip on the top of the filled mold. A glass plate was placed against the top surface of the transparent Mylar strip and pressed with light pressure to expel the excess material from the mold. Bluephase 16i LED light (Vivadent, Schaan Liechtenstein) was used at an intensity of 1600 mW/cm2 for the GIC specimens treated with heat (thermo-curing). The samples without heat treatment were left for 10 minutes to chemically cure. During heating, the tip of the polymerization device was in contact with the Mylar strip to standardize the distance between the heat source and the sample on the thickness of the Mylar strip. The samples treated by thermo-curing were treated for 60 seconds in three places for 20seconds each (in the canter and at the ends of molds) so that all parts of the cement were equally affected. Subsequently, the prepared samples were stored in a petroleum jelly as a storage media for a week.

After seven days, the samples were cleaned with alcohol. Apart from control group, half of the samples of each experimental group, were polished with Sof-Lex discs system with water cooling (3M ESPE, St. Paul, USA), and the other half with Tungsten carbide drills (Komet Dental, Brasseler GmbH & Co KG) (Table 2).

Table 2

Polishing instruments.

Sof-Lex discs	3M ESPE, St. Paul, USA
Tungsten-carbide drills	Komet Dental, Brasseler GmbH & Co KG

Each sample was polished according to the manufacturer's instructions with respect to the speed of rotation of the spindle and the application time of the polishing agent. In order for the samples to have a smooth surface, the polishing was performed evenly and flatly from the left to the right direction. All samples were made by the same operator to eliminate individual operator differences and made equal pressure on the GIC samples. Four grades of Sof-Lex discs (rough, medium, fine, and ultra-fine) and three gradations of Tungsten carbide drills (blue, yellow and white) were used. After the completed polishing process, the samples were stored for 24 hours in distilled water. The control groups were not polished. Half of each control group was treated by thermo-curing and the other half without thermo-curing.

Prior to surface roughness measurement (Ra), all samples were dried. The surface roughness (Ra) was measured by a profilometer (KairDa, China) (Figure 1). Each sample was measured in five different places and the average value of the obtained results was taken. The obtained results were subjected to statistical analysis. The statistical analysis methods used were the analysis of variance (ANOVA) and the Tukey HSD test at a level of 95% significance (p <0.05).

Figure 1

Surface roughness measurement with profilometer.

Results

The specimens of GIC Ketac Molar Universal obtained smoothest surface roughness in the control group with the Mylar strip (experimental group 5). There were no statistically significant differences (p <0.05) between the samples treated with a heat (thermo-curing) and samples without heating treatment within the control groups. The results obtained for the experimental groups 1 and 2 after polishing the samples with Sof-lex discs system and Tungsten carbide drills did not show a statistically significant difference (p> 0.05) in the mutual comparison of these two polishing agents. Thus, the results indicate that there were no significant differences in the choice between these two polishing agents for this type of GIC. A comparison of samples polished with Sof-lex discs with thermo-curing and without thermo-curing showed the lower roughness values than the samples treated without thermo-curing. The results obtained by testing samples of polished Tungsten carbide drills treated with thermo-curing and non-heating samples showed no statistically significant differences (p> 0.05) (Table 3).

Table 3

Average surface roughness (Ra) for Ketac Molar Universal

Material	Polishing instrument	N	Ra	Std.Dev	min	max
Ketac Molar U without heating	Tungs-carb	10	0,727800	0,138397	0,563000	0,992000
Ketac Molar U. without heating	Sof-Lex	10	0,629609	0,162293	0,372000	0,851000
Ketac Molar U. without heating	Mylar	10	0,277200	0,021091	0,244000	0,308000
Ketac Molar U. with heating	Tungs-carb	10	0,779300	0,086876	0,643000	0,988000
Ketac Molar U. with heating	Sof-Lex	10	0,795280	0,190414	0,606000	1,129000
Ketac Molar U. with heating	Mylar	10	0,342100	0,054803	0,287000	0,399000

The specimens of GIC Equia Fil obtained the smoothest surface roughness in the control group with the Mylar strip (experimental group 6). Within the control groups, there were no statistically significant differences (p <0.05) between the samples treated with thermo-curing and the samples without thermo-curing. In the experimental groups 3 and 4, the samples polished with Sof-lex discs system yielded better results compared to samples polished with the Tungsten carbide drills. In the comparison of the samples polished with Tungsten carbide drills, the smoothest surface roughness (Ra = 0.88) was achieved with thermo-curing treatment samples compared to polished Tungsten carbide drills without thermo-curing treatment (Ra = 1.07). The samples polished with Sof-lex discs system treated with thermo-curing achieved a lower average roughness Ra (Ra = 0.69) compared to non-heated samples (Ra = 0.76), although the observed difference was not statistically significant (p > 0) (Table 4, 5 and 6).

Table 4

Average surface roughness (Ra) for Equia Fil

Material	Polishing instrument	N	Ra	Std.Dev	min	max
Equia Fil without heating	Tungs-carb	10	1,070320	0,175747	0,717000	1,332000
Equia Fil without heating	Sof-Lex	10	0,767240	0,241597	0,469000	1,206000
Equia Fil without heating	Mylar	10	0,233300	0,010499	0,215000	0,246000
Equia Fil with heating	Tungs-carb	10	0,886480	0,045677	0,812000	0,967000
Equia Fil with heating	Sof-Lex	10	0,694400	0,125892	0,441000	0,849000
Equia Fil with heating	Mylar	10	0,344000	0,023132	0,312000	0,378000

Table 5

ANOVA of the surface roughness (Ra) of GIC materials with and without heating treatment after polishing with Tungsten carbide drills and Sof-Lex discs.

	1	2	3	4	5	6	7	8	9	10	11	12
Ketac Molar Universal TC {1}		0,452503	0,000018	0,990591	0,895154	0,000018	0,000018	0,998473	0,000018	0,006571	0,999820	0,000018
Ketac Molar Universal Soflex {2}	0,452503		0,000018	0,037246	0,004719	0,000028	0,000018	0,049597	0,000018	0,000018	0,952295	0,000031
Ketac Molar Universal Mylar {3}	0,000018	0,000018		0,000018	0,000018	0,997805	0,000018	0,000018	0,999948	0,000018	0,000018	0,997158
Ketac Molar grijanje TC {4}	0,990591	0,037246	0,000018		1,000000	0,000018	0,000018	1,000000	0,000018	0,370136	0,793916	0,000018
Ketac Molar grijanje Soflex {5}	0,895154	0,004719	0,000018	1,000000		0,000018	0,000018	0,999943	0,000018	0,539377	0,470126	0,000018
Ketac Molar grijanje Mylar {6}	0,000018	0,000028	0,997805	0,000018	0,000018		0,000018	0,000018	0,881824	0,000018	0,000018	1,000000
Equia fil TC {7}	0,000018	0,000018	0,000018	0,000018	0,000018	0,000018		0,000018	0,000018	0,000508	0,000018	0,000018
Equia fil Soflex {8}	0,998473	0,049597	0,000018	1,000000	0,999943	0,000018	0,000018		0,000018	0,142633	0,883241	0,000018
Equia fil Mylar {9}	0,000018	0,000018	0,999948	0,000018	0,000018	0,881824	0,000018	0,000018		0,000018	0,000018	0,868933
Equia fil grijanje TC {10}	0,006571	0,000018	0,000018	0,370136	0,539377	0,000018	0,000508	0,142633	0,000018		0,000683	0,000018
Equia fil grijanje Soflex {11}	0,999820	0,952295	0,000018	0,793916	0,470126	0,000018	0,000018	0,883241	0,000018	0,000683		0,000018
Equia fil grijanje Mylar {12}	0,000018	0,000031	0,997158	0,000018	0,000018	1,000000	0,000018	0,000018	0,868933	0,000018	0,000018

Table 6

Tukey HSD post hoc test of the surface roughness (Ra) of GIC materials with and without heating treatment after polishing with Tungsten carbide drills and Sof-Lex discs.

	SS Effect	df Effect	MS Effect	SS Error	df Error	MS Error	F	p
Ra	11,38404	11	1,034913	4,581942	216	0,021213	48,78743	0,00

Graphic presentation of average surface roughness (Ra) after treatment of GIC materials with Sof-Lex discs system; Tungsten carbide drills and with Mylar strips (Figure 2).

Figure 2

Graphic presentation of average surface roughness (Ra) after treatment of GIC materials with Sof-Lex discs, Tungsten carbide drills and with the Mylar strip. Legend: TC – Tungsten carbide drills; Soflex – Soflex discs; Mylar – the Mylar strip

Discussion

Glass-ionomer cements (GICs) have been widely used in dental medicine, especially in pediatric dentistry due to a large number of good features. However, lower mechanical properties and reduced wear resistance put a GIC restoration into less durable restorations (). Surface roughness of GIC materials has several clinical implications and changes in surface often defined as a measure of wear of materials. An increased roughness can be a predisposing factor for bacterial colonization that increases the risk of oral disease. Also, increased roughness of the surface can cause deterioration of the material (, ).

Bollen et al. reported that critical surface roughness (Ra) for bacterial colonization is 0.2 μm. Bacterial accumulation, plaque maturation and acidity significantly increase when the surface roughness exceeds 0.2 μm, which acts on material surfaces, thus increasing caries risk (). However, GICs partially compensate for their anti-caries release of fluoride that is incorporated into the hydroxyapatite lattice and slow down the processes of demineralization and contribute to the remineralization process (, , ). From a clinical point of view, the increased surface roughness of the restored tooth surface causes accumulation of plaque, secondary caries, gingivitis and loss of periodontal attachment, and ultimately restoration loses its shine and color (). One of the flaws of today's GIC is worse polishing properties than composites, but newer GICs are increasingly approaching the quality of polishing composite materials.

Several authors in their studies showed the results similar to this study that the lowest surface roughness of GICs materials was found in the surface in contact with the Mylar strip. However, a correct anatomy and morphology of filling is rarely achieved only by using the Mylar strip (, ). Sof-Lex disks system and Tungsten carbide drills were used in this study. Ketac Molar Universal did not show statistically significant differences (p> 0.05) between these two polishing agents, but Equia Fil material showed better results in samples treated with Sof-Lex disks. The results suggest that polishing performance may also depend on the type of restorative material and the particle size, as confirmed by Bala et al. ().

Poor mechanical properties and reduced wear resistance to GICs materials represent a sort of clinical problem. However, thermo-curing of GICs materials improves mechanical properties and adhesion quality. Alegra et al., in his research, obtained a much better adhesion of GIC materials on the enamel after treatment of cement by heating with external energy (induction and thermal trough LED lamp) (). Kleverlaan et al. in their research have demonstrated a significant improvement in mechanical properties after thermo-curing of GIC materials. The strength of the GIC materials grew proportionally to the received energy; hence the samples with the highest heating temperature had the highest strength (). In addition, Goršeta et al. have proved that thermo-curing of GIC materials reduces micro-discharge and increases edge alignment (). The results of this research show that some GICs can be treated by using external heat (thermo-curing) to obtain a better surface polishing quality. Such results were obtained with samples of Equia Fil GIC material.

Conclusions

Based on the obtained results it can be concluded that the smoothest surface roughness is achieved by the Mylar strip.

The null hypothesis of this study was not fully validated because part of the samples obtained less surface roughness after they had been treated by thermo-curing according to the assumption of the null hypothesis, and the part of them achieved greater surface roughness compared to the samples without heating. Some types of GICs can obtain better surface polishing with heat treatment of the material (thermo-curing).