Mechanical Properties of High Viscosity Glass Ionomer and Glass Hybrid Restorative Materials

Abstract

Objectives

to determine the mechanical properties of hybrid and high-viscosity glass ionomer cements. Compressive strength and hardness of three glass ionomer cements (GIC) were measured: Ketac ™ Universal Aplicap ™, EQUIA Fil® and EQUIA FORTE Fil®, and the SEM sample analysis were performed.

Materials and Methods

The samples for measuring the compressive strength were prepared using silicone molds with standard dimensions of 6 mm x 4 mm and stored in deionized water for five days, while the samples for hardness measurement were prepared using Teflon molds with a cylindrical opening in the middle, dimensions 2 mm in height and 5 mm in width. For each material, one sample was made (n = 1) and stored in deionized water at 37ºC for 25 days. A representative sample of each material was analyzed using SEM. For the comparison of obtained values, the ANOVA test was used, while Tukey test was used for the multiple comparison.

Results

There were no significant differences between the compressive strength of the three tested materials (p <0.05). The hardness values were: 157 HV0,2 for Ketac ™ Universal Aplicap ™, 47 HV0,2 for EQUIA Fil® and 39 HV0,2 for EQUIA FORTE Fil®, respectively, and were significantly different, implying that Ketac ™ Universal Aplicap ™ has much higher hardness values than the other materials tested. SEM sample analysis revealed similar fracture modes of the tested materials.

Conclusion

It was concluded that there were no statistically significant differences in compressive strength and fracture modes between the tested materials, while Ketac ™ Universal Aplicap ™ hardness results were significantly higher than the ones measured for EQUIA Fil® and EQUIA FORTE Fil®.

Introduction

The ideal in developing new restorative materials in dental medicine is achieving appropriate biological, mechanical and esthetic properties. Glass ionomer cements (GIC) were introduced into dental practice during the 1970s of the last century (1). The GICs are made of powder and liquid; the powder consists of calcium-aluminum fluorosilicate glass and the liquid is 35-65% polyacrylic acid (2). The improvements of physical-mechanical properties over the past decades have resulted in their application in various areas: restorative dentistry, pediatric dentistry, endodontic surgery, postendodontic tooth restoration, atraumatic restorative treatment (ART) and dental prosthetics (3 - 6).

The GICs are bioactive materials. They have the ability to release fluoride (7), which is believed to enhance the remineralization of hard tooth tissue and prevent secondary caries. The degree of fluoride release is furthermore influenced by the composition and storing methods of the material (8). For instance, incorporation of Nano-sized particles can further enhance the fluoride release (9).

Because of their chemical composition, they connect directly with the enamel and the dentin by achieving a chemical bond with hard tooth tissue. There are many different types of GICs; the basic ones are conventional, light-curing and hybrid glass ionomer cements. Conventional GICs are divided into high-viscosity and low-viscosity GICs according to the proportion of powder and liquid part and the amount of Ca ⁺ and Al^{3 +} ions. Hybrid materials based on GIC technology have been modified with glass particles of different sizes. This feature significantly affects physical and mechanical properties of the material (10). Furthermore, it is considered that by applying a Nano-protective coating (micro-laminated technique), the properties of the material improve (11). Hybrid (EQUIA FORTE Fil®) and high-viscosity (EQUIA Fil®) materials are clinically applied for permanent restorations by micro-laminated technique. Ketac ™ Universal Aplicap ™ is a high-viscosity GIC not combined with coating agent. Mechanical properties of GIC materials are usually estimated by compressive strength and hardness measurements, and are associated with the microstructure of the material (12). Various techniques may be used to analyze the material structure. The scanning electron microscopy (SEM) proved to be the most acceptable procedure for inspecting the surface of materials, particle size and porosity (13); these elements give significant insight into the properties of GICs and are, therefore, suitable for this study (14).

The aim of this research was to determine the mechanical properties of two types of materials: hybrid and high-viscosity glass ionomeric cements. The zero hypothesis was that there would be no differences in compressive strength, hardness and fracture modes between the tested materials

Materials and methods

Examined materials and preparation of specimens

The materials used in this study are listed in Table 1. The materials belong to the group of encapsulated GIC systems; hence each capsule was prepared in the mixer 3M ™ ESPE ™ CapMix^TM (3M ESPE, Seefeld, Germany) for eight seconds, according to the manufacturer's instructions.

Table 1. The results of compressive strength and hardness of tested materials.

	EQUIA Fil®		EQUIA FORTE Fil®		Ketac™ Universal Aplicap™,		ANOVA
	average	st.d.	average	st.d.	average	st.d.	p
Maximum force (N)	1259.7	(635.9)	1277.3	(695.2)	1046.4	(519.2)	0.66
Compressive strength (N/mm2)	97.6	(49.1)	99.6	(53.6)	80.0	(38.7)	0.60
Hardness (HV0,2)	47		39		157

EQUIA Fil® and EQUIA FORTE Fil® belong to the group of micro-laminated GICs, while Ketac ™ Universal Aplicap ™ is regarded as a conventional high-viscosity material. Micro-laminated GICs are materials of newer generation with improved properties and, according to manufacturers, the first GICs that can be used to create durable fillings in posterior region (15). No protective coating was used during sample testing and the specimens were not lit by a polymerization lamp. Sample dimensions and method of preparation for testing both properties were made according to other relevant studies (16, 17). One measurement for each sample was performed by an independent researcher at the Department of Materials at the Faculty of Mechanical Engineering and Naval Architecture of the University of Zagreb.

Compressive strength test specimens were prepared using a silicone mold with standard dimensions of 6 mm x 4 mm. The mold was placed on a glass tile and filled with material immediately after capsule mixing and covered with the acetate foil. Lightweight pressure was applied to the surface of the foil to prevent bubble formation and to achieve smooth surface (18). After three minutes, foil, tile and excess material were removed, and the specimen was then polished with standard metallographic grinding paper (P1000, Pace Technologies, and Tucson, USA) and stored in deionized water for five days at 37 ° C water temperature.

For the determination of hardness of the materials, specimens were prepared using Teflon molds with a cylindrical opening in the middle, dimensions 2 mm in height and 5 mm in width. For each material, one sample was made (n = 1). The molds were placed on glass tile covered with acetate foil and filled with the tested GIC. The mold was then covered with acetate foil on the other side and put under light pressure by using a glass tile. After curing for 3 minutes, glass tiles, acetate foil and excess material were removed and the specimens were stored in deionized water for 25 days at 37 ° C water temperature.

SEM analysis was used for one specimen of each test material (n = 1). The specimens were prepared in Teflon molds with a cylindrical opening in the middle, dimension 2 mm in height and 5 mm in width. After curing for three minutes, the specimens were ground and polished. They were ground with standard metallographic grinding paper, Pace Technologies, Tucson, USA, from rough to finer - P320, P500, P1000, P2400, and P4000 - with water cooling. The speed was 300 revolutions per minute and the pressure force was equal to the manual force. The specimens were polished with diamond pasta 3 um and 1 um in two steps at a speed of 150 rpm and a force of 30 N pressure. Prior to analysis, all specimens were cooled and lubricated.

Determination of compressive strength, hardness and SEM analysis

The compressive strength was measured in a VEB device (WPM Werkstoffprüfsysteme Leipzig GmbH, Markkleeberg, Germany; EU 40 mod., Serial number 83/35) at a temperature of 24.5 ° C with a load speed of 20 N / sec. 10 specimens of each material were prepared (n = 10). The average dimensions of the specimens used to determine the compressive strength are approximately 6 mm in height and 4 mm in width (ISO Standard ISO9917-1). Prior to placing the specimens in the testing machine, their dimensions were measured by a micrometer with a precision of 0.01 mm. The compressive strength (RH) was calculated according to the formula

where FH is the maximum force due to the sample fracture, S0 the cross-sectional area of the sample, and d the width of the sample.

Hardness was measured using PMT-3 device (OKB Spectr, Sankt-Peterburg, Russia). A Vickers method with a load of 0.2 x 9.81 N was used, which corresponds to the HV0,2 method.

The Vega electron microscope (Tescan, Brno, Czech Republic) was used to determine the microstructure of each tested specimen.

Statistical Analysis

ANOVA test, usually used to analyze the differences among group means in a sample, was used to compare the value of the different types of filling, while the Tukey's test was used for multiple comparisons. The analysis was made using the SAS statistical package, a software application for statistical analysis, on the Windows platform. All the tests were performed with level of significance α = 0.05.

Results

The results of the compressive strength and hardness measurements are shown in Table 1. Maximum Force (FH) was used to calculate the compressive strength. The difference in compressive strength between the tested materials was not statistically significant. On the other hand, Ketac™ Universal Aplicap ™ hardness values were significantly higher than the ones measured for EQUIA Fil ® and EQUIA FORTE Fil ® GIC.

The SEM analysis of the structure showed similar cohesive cracks in all tested specimens. (Figures 1a, 1b and 1c), showing the need for further material improvements.

Figure 1a.

SEM image of Ketac ™ Universal Aplicap ™ GIC sample surface (200x)

Figure 1b.

SEM image of Equia FORTE Fil^® GIC sample surface (200x)

Figure 1c.

SEM image of Equia Fil^{® GIC} sample surface (200x)

Discussion

It is believed that the information about compressive strength and hardness of the material can give insight into their mechanical integrity. Moreover, the compressive strength results ​​are often used to estimate the durability of the material when exposed to masticatory forces (19). Although it might be very interesting from clinical perspective to compare the values exhibited by the materials in our research with other restorative materials, it is very hard to do so because of different experimental settings. The obtained results showed that there was no statistically significant difference in the compressive strength of the tested materials, thus the first null hypothesis was confirmed.

Specimens of standard dimensions of 6 mm x 4 mm according to ISO 9917-1 were used in this study. There are other standardized systems such as ANSI / ADA Specification No. 66, which prepare specimens of larger dimensions. Mallman et al. (16) have demonstrated that specimens with larger dimensions exhibit better mechanical properties. However, larger size specimens have the potential for creating irregular structures (16). For this reason, in this study, smaller specimens were used to ensure a good material structure. The mixing behavior affects the structure of the material. The examined materials come into the market in capsulated form. Such packaging, apart from facilitating clinical use, reduces the possibility of mistakes when mixing the material. In this study the compressive strength of capsulated GIC was tested, which according to the manufacturer can be used for definite restorations. The storage time of the specimens was five days, due to the fact that immediately after mixing GICs have weak mechanical properties until they mature (20). In this study, lower compressive strength values ​​were obtained compared to the manufacturer's claims (21, 22). A possible explanation for the obtained values ​​was that the specimens were stored in deionized water and that ions were released into the fluid possibly leading to lower values ​​for all the tested materials (23). The influence of early water uptake on GIC materials tends to be reduced by applying protective coatings. Despite the manufacturer's recommendation for Equia Fill® and Equia FORTE Fil®, the materials were tested without application of protective coating, which could have also lead to the results which are inferior to those obtained by the manufacturer (24). The results of this study have shown that Ketac ™ Universal Aplicap ™ has higher hardness values ​​compared to EQUIA Fil® and EQUIA Forte Fil®, therefore the second null hypothesis that there is no difference in hardness between hybrid and high-viscosity GICs has been rejected.

Brinell, Rockwell, Shore, Vickers and Knoop test methods (25) can be used to measure the hardness of dental materials. Although the Knoop’s method is most commonly used method, due to availability of equipment, the Vickers test method was used in this study, which was also used in other relevant studies (26, 27). It is interesting that both bulk fill systems, EQUIA Fil® and EQUIA FORTE Fil®, had lower hardness values. Therefore, it can be concluded that the application of protective coating on these materials is mandatory in order to achieve the hardness comparable to other GIC materials (28). The manufacturer recommends a Nano punctured light-curing resin-based coating. It is stated that the coating, due to its low viscosity, fills the micro-gaps in the GICs structure. In this way, the improved surface better tolerates the impact of the masticatory forces and protects against erosion (29). Furthermore, the manufacturer claims that this protective coating reduces the wear of GIC when used in restorations in posterior region (29).

The influence of all the forces acting on the specimens reflects on their microstructure and fracture modes. SEM analysis showed cohesive cracks in all examined specimens which is in accordance with data from the literature (30). This may be due to the dehydration of the specimens which is necessary in preparations for SEM analysis (9). Thus, the third null hypothesis that there was no difference in the fracture modes of hybrid and high-viscosity materials was accepted. Despite the continuous development of the materials and the improvement of their properties, imperfections such as cohesive fractures are yet to be eliminated. This fact is confirmed by this research, using the newest materials on the market. That is a clinically important feature because it shows that the material and tooth bond is stable, and in a minimally invasive concept of restoration it is more important that, if a fracture occurs, the restorative material is affected, not the tooth.

As this is an in vitro study, the oral conditions could not be completely simulated. However, all the samples were exposed to the same experimental conditions. Within the limitations of this study, a good insight into material endurance was given, but the absolute values of the mechanical properties recorded must be interpreted with respect to the experimental conditions.

Conclusion

There was no significant difference in compressive strength between the tested materials. The hardness values were significantly higher for Ketac™ Universal Aplicap ™ compared with EQUIA Fil^® and EQUIA FORTE Fil^® materials, which probably came as a result of not using the recommended protective coating when preparing the EQUIA Fil^® and EQUIA FORTE Fil^® samples. SEM analysis showed similar microstructure and fracture modes of the three tested materials, justifying their use in in a minimally invasive concept of restoration.