Comparative Evaluation of Sorption, Solubility, Compressive Strength and Diametral Tensile Strength of Zirconomer Improved (ZI) with GIC – An In Vitro Study

Introduction

The lifespan of a dental restoration is greatly impacted by many variables, including type of material used, the age of the patient, and the rate at which caries affect the restored tooth. In the mouth, masticatory forces produce reactions that cause deformation of restorative materials over time, diminishing their durability. The solubility of This work is licensed under a Creative Commons Attribution-NonCommercial 4.0. materials can affect how quickly they break down and whether they are compatible with biological systems. GIC is one of the most regularly used restorative cements. Although it has many advantages, its strength tends to be weaker than other options available. Therefore, scientists are in search for a material with optimal physical properties and a longer shelf life.¹

Glass ionomer cement (GIC) was introduced by A.D Wilson and B.E Kent in 1972 as a suitable material for restoring deciduous teeth. Not only was it aesthetic, but also offered the advantage of fluoride release and chemical adhesion. As such, GIC has been extensively studied as a material for restoration, especially in its primary form. Although it is popular due to its appealing features, GICs are limited in that they are brittle and have poor wear characteristics. In an effort to reduce these drawbacks, different fillers have been incorporated such as silver-cermets, stainless steel powders, carbon and alumino-silicate fibers along with hydroxyapatite into glass-polyalkenoate.²

In dentistry, zirconomer (white amalgam) has been increasingly utilized as a stronger option for restorative material compared to GIC. The name of this material is derived from the Arabic term "Zargon," which translates to "golden in colour". It is a polycrystalline ceramic that lacks a glassy phase. Zirconomer Improved (ZI) also has nano-sized zirconia fillers to increase mechanical strength, aesthetic appeal, and good chemical and dimensional stability. Furthermore, its powder distribution allows for greater packing density with the hydrogel salt matrix. Although clinical evidence is minimal, laboratory tests suggest that the material boasts superior mechanical properties and esthetics.³

Therefore, this in vitro study was conducted to assess and compare the compressive strength (CS), diametral tensile strength (DTS), solubility, and sorption of ZI and GIC Type IX.

Materials and Methods

This study was an experimental in vitro study of 30 days duration.

Sample preparation

The test materials, namely Type IX GC gold label HS posterior extra from GC Corporation in Tokyo, Japan, and ZI from Shofu Inc. in Japan, were separated into two groups - Group A and Group B, respectively. The manipulation was carried out based on the instructions provided by the manufacturer. The interior walls of the teflon mould were coated with petroleum jelly before being set onto a mylar strip and glass slab. Subsequently, the contents were dispensed onto the mixing pad. Blending of the contents was done, and the mould was slightly overfilled to minimize any air bubbles from forming. To create a smooth surface on the sample, another glass slab was placed on top of the mould with another mylar strip in between. The excess material was extruded by pressing the assembly for 20 seconds. After being kept at 37ºC for approximately one hour, the samples were taken out of the mould. Grinding with silicon carbide paper on both ends removed any extra material present.

Compressive strength

To evaluate CS, samples were prepared in cylindrical teflon moulds of dimensions 6.0 mm diameter × 12.0 mm height. For this experiment, we utilized the Instron universal testing machine with a crosshead speed of 1.0 mm/minute. Each sample was placed with the flat ends between the apparatus's platens, so that the load was applied along the specimens' long axis. The maximum load applied to fracture the specimens was recorded, and the CS (MPa) was determined by applying the formula, C=4P/πD2, where P represents the maximum applied load (N) and D represents the sample's measured diameter (mm) (Figure 1a).

Diametral tensile strength

To evaluate DTS, samples of dimensions 6.0 mm diameter × 3.0 mm height were made utilizing teflon moulds. The DTS of the sample was analysed by conducting a diametral compressive test using the Instron universal testing machine. The crosshead speed was set at 1.0 mm/minute. Samples were placed with the flat ends perpendicular to the platens of the apparatus so that the load was applied to the diameter of the specimens. We recorded the maximum load that caused the specimens to fracture and calculated the DTS (measured in MPa) by applying the formula, T = 2P/πDL. In this formula, P represents the maximum load applied (measured in Newtons), D represents the mean diameter of the sample (measured in millimetres), and L represents the length of the sample (measured in millimetres) (Figure 1b).

Sorption and solubility

To evaluate sorption and solubility, the prepared specimens were let to sit untouched for 30 minutes before subjecting them to a thermocycling test. This test involved 500 cycles in water spanning from 5°C to 55°C. A precision balance was utilised to weigh a total of 10 samples, five for each material. The initial weight was recorded as W1 (in g) once the samples were weighed. Following this, the samples were submerged in artificial saliva with a pH of 7 and kept at a temperature of 37°C for one entire day (24 hours). Following this, the samples were taken out, rinsed and dried with water and absorbent paper respectively, air dried for 15 seconds, and were weighed one minute after being removed from the medium. The weight recorded was mentioned as W2 (in μg). Next, the samples were dried out in an oven for 24 hours at a temperature of 37°C. They were weighed again, and this weight was noted down as W3 (μg). By taking the average of two measurements taken at right angles to one another, the diameter and thickness of each sample was measured utilizing a digital vernier calliper having a precision of ±0.01 mm.

To determine the volume (V) of each sample in cubic millimeters, the mean thickness and diameter were utilised according to the following formula:

V = π × r² × h.

The variable "r" represents the average sample radius, measured in millimetres. The variable "h" represents the average sample thickness, measured in millimetres.

To calculate the solubility, we measured the baseline and final drying mass of each specimen and subtracted them. This gave us the material loss (W1 - W3). To determine water sorption, the initial weight (W1) of the sample was subtracted from its wet weight (W2). To calculate the water sorption (Wsp) and solubility (Wsol) values for each sample, the following equations were used:

Wsp = (W2 - W1) / V

Wsol = (W1 - W3) / V

V represents the volume of the sample in mm3 and the values are measured in μg/mm³ .

The obtained data from the available samples were subjected to statistical analysis using SPSS 21.0 software. One way analysis of variance (ANOVA) was used to compare the two groups at the same time to determine if a relationship exists between them.

Results

An in vitro research was conducted to assess and compare the CS, DTS, solubility and sorption of ZI and GIC type IX.

The mean CS and DTS were found to be significantly greater for ZI (CS=190.29 MPa; DTS= 42.65 MPa) compared to that of type IX GIC (CS=82.60 MPa; DTS= 11.98 MPa) (P <0.05). (Table 1)

The solubility and sorption levels were notably higher for GIC type IX (sorption =93.28 g; solubility =32.37g) compared to ZI (sorption =47.47g; solubility =12.13g) (P <0.05) (Table 1).

Discussion

Dental caries is the most common disease afflicting the human race, dating back to ancient times. Despite various preventive measures, dental caries remains a major challenge for clinicians. When dental caries occurs, it is necessary to restore the carious lesion.⁴

The most commonly used material for restoring deciduous teeth is GIC. Wilson and Kent introduced GIC as one of the first aesthetic restorative materials in the dental arena in 1972^.1,2

Over the past 15 years, manufacturers have made significant efforts to develop GIC systems that address the three main issues associated with these materials: handling difficulties, low resistance to surface wear, and susceptibility to fractures. The products they have developed have undergone significant improvements, resulting in the elimination or reduction of major drawbacks to acceptable levels.³

Through a rigorous manufacturing technique, Zirconomer® (White Amalgam) or Zirconia reinforced GIC was developed to exhibit the strength, comparable to amalgam. By uniformly integrating Zirconia particles into the glass component, the material is strengthened, achieves prolonged durability and is able to withstand high occlusal loads. The glass component of this highstrength GIC is exposed to finely controlled micronization to attain ideal particle dimensions and properties. The polyalkenoic acid and glass components have undergone special processing.^1,2

According to various studies, when Zirconomer material is reinforced with nano-zirconia fillers, it is responsible for imparting enhanced mechanical properties, particularly making it suitable for posterior load bearing areas.⁵  This material with nano-zirconia filler is ZI.

No prior studies have been conducted to compare the CS, DTS, solubility and sorption of the new ZI with GIC type IX restorative material. I

n this study, CS and DTS were used to measure the mechanical strength of the restorative materials. In order to estimate the molecular arrangement of a substance, two forces were exerted on a sample in opposite directions to gauge its resistance to compression. Testing the CS is used widely by clinicians and researchers to anticipate how a dental restorative material will perform in the oral environment. Many clinical failures occur due to low tensile stress, which is why the DTS is an important requirement.^2,5

In this study, on comparing the CS and DTS, it was seen that ZI had greater values (CS=190.29 MPa; DTS= 42.65 MPa) than GIC type IX (CS=82.60 MPa; DTS= 11.98 MPa). Studies done by Dheeraj et al., (2019) Bhatia et al., (2017) Chalissery et al., (2016) support the results obtained. The rise in CS and DTS may be because of the even distribution of microsized zirconia particles in the glass component. This strengthens the material and makes it highly durable and able to tolerate occlusal load for a long time.⁶ Zirconia’s unique mechanical properties are also due to its transformational toughening property which is its ability to prevent crack growth.⁴ Zirconomer enhances the strength of structures and is suitable for load-bearing regions, such as posterior restorations.² Researchers Gu et al., have found that replacing the amalgam alloy in miracle mix with yttria stabilized ZrO2 particles results in improved mechanical properties of ZrO2 infused miracle mix. The DTS of the new mixture also increases with longer soaking time and surpasses that of the amalgamated miracle mix. Gu et al., conducted a research on the impact of hydroxyapatite (HA)/ZrO2 particle inclusion. They discovered that there was a consistent dispersion of these particles within the GIC matrix and that the mechanical properties were superior to those of hydroxyapatite GICs. They also noticed a decrease in mechanical properties as the HA/ZrO2 content of the GIC increased above 12 vol%.⁷

The ability of restorative materials to dissolve in a liquid affects how quickly they break down and how well they work with the body. This is the first study to measure the solubility and sorption of ZI. In this study, GIC type IX had greater sorption (93.28 g) and solubility (32.37g) values compared to ZI (sorption =47.47g; solubility =12.13g).

Meşe et al. found that the solubility and sorption values can be influenced by various factors, including the type and amount of filler, concentration of filler, particle size, filler particles’ makeup, use of coupling agents, and the type of solvent.⁸ Benhameurlain and his colleagues found that traditional glass ionomer cement had the greatest amount of sorption and material loss compared to alternative tooth-colored restorative materials. According to Sidhu and colleagues, the modified version of GIC shares a similar water balance with standard glass ionomer cements, making it equally sensitive to moisture.⁹

The result from this study shows that ZI has high durability and better CS and DTS than GIC type IX. Various research have shown that using nano-zirconia fillers in reinforcing materials can improve their mechanical properties, making them ideal for areas that need to bear heavy loads, such as in the posterior region.

It should be noted that the sample size utilized in this study is relatively small. In order to ensure more precise outcomes, it is imperative to conduct further research with a larger sample size. Parameters like microleakage and fluoride release of ZI should be studied and compared to strengthen the knowledge. Further research is necessary to investigate the effects of moisture on GIC and their modifications, particularly in Pediatric dentistry where achieving proper isolation can be challenging. Further research is necessary to evaluate the impact of saliva on the extended clinical longevity of conventional and modified GICs.

Conclusion

This study revealed that

1. ZI has the highest CS and DTS compared to other groups. The lowest value was observed in glass ionomer cement type IX-Extra and the differences between the groups were statistically significant with a P value less than 0.05.
2. ZI demonstrated lower water absorption and solubility compared to glass ionomer cement type IX-Extra. The differences between the two were statistically significant, with a P value less than 0.05.

Conflict of Interest

No conflict of interest was raised by any of the authors during the study.