Evaluation of the effect of micro-hydroxyapatite incorporation on the diametral tensile strength of glass ionomer cements.

AIM: The aim of this study was to evaluate the effect of micro-hydroxyapatite (micro-HAP) incorporation on the diametral tensile strengths (DTSs) of a conventional glass ionomer cement (GIC) and a resin-modified glass ionomer cement (RMGIC).
MATERIALS AND METHODS: Forty disc-shaped specimens (diameter: 6.5 mm, height: 2 mm) were prepared into four groups (n = 10) as follows: group 1, conventional GIC; Group 2, GIC + micro-HAP (15 wt %); Group 3, RMGIC; and Group 4, RMGIC + micro-HAP (15 wt %). All the specimens were stored in distilled water for 24 h at room temperature. The DTSs of the specimens were measured using a universal testing machine. Data analysis was performed using one-way ANOVA and Tukey's test (P < 0.05).
RESULTS: No significant difference was found in the DTS of conventional GIC with and without micro-HAP incorporation (P > 0.05). Moreover, the DTS of RMGIC incorporated with micro-HAP was significantly lower than that of RMGIC without micro-HAP incorporation (P < 0.05).
CONCLUSIONS: Micro-HAP incorporation did not affect the DTS of conventional GIC. The DTS of RMGIC was negatively influenced by the micro-HAP incorporation. Conventional GIC (with and without hydroxyapatite) exhibited a lower DTS than RMGIC (with or without hydroxyapatite).

INTRODUCTION

Glass ionomer cements (GICs) have many applications in dentistry because of their unique properties.[12] GICs have an anticariogenic property, the ability to release fluoride, chemical adhesion to the tooth structure, minimal microleakage at tooth–restoration interface, low shrinkage coefficient, low cellular toxicity, and similar coefficient of thermal expansion and modulus of elasticity to the tooth structure.[345] However, one of the most significant shortcomings of GICs is their poor physical properties such as low flexural strength and low fracture resistance which prevent their use in high stress-bearing areas.[6]

To overcome some of the shortcomings of GICs, resin-modified glass ionomer cements (RMGICs) have been introduced which have a higher initial strength than the original formulations.[7] Moreover, different filler particles such as silver, stainless steel powder, aluminosilicate fibers, and hydroxyapatite have been previously incorporated into GICs to improve their mechanical properties.[89]

In recent years, hydroxyapatite powders have been produced at different sizes and added to GICs to improve their physical and mechanical properties. Hydroxyapatite (Ca10[PO4]6[OH]2) is considered as the principal mineral component of enamel and comprises over 60% of the weight of dentin.[10] Considering the high biocompatibility of hydroxyapatite, some researchers have made attempts to evaluate the effects of incorporating hydroxyapatite powder into restorative dental materials such as GICs.[811121314] The interaction between GICs and hydroxyapatite takes place through carboxylate groups in the polyacid structure. It has been reported that hydroxyapatite incorporation into GICs may improve the mechanical properties of GICs. An increase in the bond strength of GIC to the tooth structure can also occur due to similarity of hydroxyapatite to the structure of enamel and dentin.[1213]

Mechanical strength is one of the most important factors in analyzing the clinical success of restorative materials. The diametral tensile strength (DTS) test is an easy technique to determine the tensile strength of brittle materials such as GICs.[15] However, to our knowledge, there is no published study investigating the effect of micro-HAP incorporation into GIC and RMGIC on the DTS of the mentioned cements. Therefore, the present study was undertaken to compare the DTS of a conventional GIC and a RMGIC with and without hydroxyapatite incorporation.

MATERIALS AND METHODS

In this experimental study, 40 disc-shaped specimens (6.5 mm in diameter and 2.5 mm in height) were prepared.

The powder and liquid of a conventional GIC (GC Fuji II, GC Corporation, Tokyo, Japan) with the powder-to-liquid ratio of 1.8:1.0 were mixed for 20 s on a cold slab with a plastic spatula according to the manufacturer's instructions to prepare the specimens of Group 1 (n = 10). Then, the cylindrical brass molds [Figure 1] were overfilled with the experimental mixture and a Transparent Mylar Matrix (M-TP, Pulpdent Corporation, USA) was placed on the upper surface of the mold until the complete setting of the GIC (10 min).

Figure 1

The split mold and the specimen

In Group 2, the specimens contained 85% conventional GIC with 15 wt% of micro-hydroxyapatite (micro-HAP). The conventional GIC (85 wt %) and micro-HAP (Sigma-Aldrich, St. Louis, USA) (15 wt %) were weighed carefully using a weighing machine accurate to 0.0001 g (A and D, GR + 360, Tokyo, Japan). Then, micro-HAP was added to the GIC and mixed in the amalgamator (Ultramat, SDI, Bayswater, Victoria, Australia) using clean amalgam capsules for 20 s to have a uniform mixture. Then, the mixing and preparation of the specimens were done as described for Group 1.

In Group 3, the discs were prepared from RMGIC powder (GC, Tokyo, Japan) which was mixed with liquid according to the manufacturer's instructions with the powder-to-liquid ratio of 3.2:1 by weight. Then, the molds were overfilled as mentioned above. The upper surface of each sample was then light cured for 40 s using a halogen light-curing unit (BlueLEX, Monitex, Taiwan) with a light intensity of 470 mW/cm2 according to the manufacturer's directions. The curing tip was placed at a 1-mm distance from each side of the sample.

Group 4 consisted of RMGIC with 15 wt% of micro-HAP. The samples were prepared and evaluated as described for Group 3. After complete setting, all specimens were removed from the mold and the samples were coated with varnish to be protected against moisture.

Then, all the specimens were stored in distilled water at room temperature for 24 h. The upper and lower surfaces of the samples were polished with 600-grit sandpaper. Before performing the test, the diameters and heights of all the samples were measured with calipers. Then, the samples were subjected to diametral forces in a universal testing machine (Zwick-ZOB, Germany) at a crosshead speed of 0.5 mm/min. The data were analyzed with one-way ANOVA and Tukey's test (P < 0.05).

RESULTS

The mean DTSs of the experimental groups are presented in Table 1.

Table 1

The mean values of the diametral tensile strength (MPa) among different groups

Group	Mean	SD
Conventional GIC	8.69	0.89
Conventional GIC incorporated with micro-HAP	8.79	1.4
RMGIC	27.08	2.26
RMGIC incorporated with micro-HAP	19.72	2.15

GIC: Glass ionomer cement, micro-HAP: Micro-hydroxyapatite, RMGIC: Resin-modified glass ionomer cement, SD: Standard deviation

One-way ANOVA showed significant differences in the DTSs of the four groups (P < 00001).

Based on the results of two-by-two comparisons using Tukey's test, HAP incorporation did not affect the DTS of the conventional GIC (P = 0.99). However, the DTS of RMGIC was higher than that of RMGIC incorporated with HAP (P < 0.0001).

The DTS of the conventional GIC was significantly lower than those of RMGIC and RMGIC with hydroxyapatite (P < 0.001). Furthermore, the DTS of the conventional GIC incorporated with HAP was lower than those of RMGIC and RMGIC incorporated with HAP (P < 0.001).

DISCUSSION

The results of the present study showed that incorporation of 15 wt% of micro-HAP into conventional GIC did not affect the DTS of the conventional glass ionomer. However, the DTS of RMGIC was significantly decreased after micro-HAP incorporation.

Some previous studies have reported that the incorporation of hydroxyapatite into GICs could improve the mechanical properties and bond strength to tooth structure.[51617] Moreover, the fluoride ion release was improved after HAP incorporation into GIC.[18] Considering the positive effects of HAP incorporation into GICs, the effect of HAP incorporation on the DTSs of GIC and RMGIC was evaluated in the present study. It has been previously demonstrated that the incorporation of 15 wt% of micro-HAP into RMGIC improves microhardness, while the addition of 15 wt% of HAP has a negative effect on the microhardness of RMGIC.[13] Therefore, 15 wt% of micro-HAP was incorporated into a conventional GIC and a RMGIC in the present study. The bioactive hydroxyapatite (HAP)/zirconia-filled GIC demonstrated increased DTS and compressive strength compared to the unmodified GIC in a previous study.[16] In addition, the incorporation of nano-hydroxyapatite and fluorapatite into the structure of conventional GIC resulted in a higher compressive strength, DTS, and flexural strength compared to conventional GIC.[5] In addition, conventional GIC containing 4 wt% HAP exhibited higher DTSs and compressive strengths compared to conventional GIC without HAP particles.[19] The DTS of the GIC containing 20 wt% HAP was significantly higher than that of GIC containing 10, 15, 25, and 30 wt% synthetic hydroxyapatite in a previous study. This result was attributed to the chemical reactions between polycarboxylic acid and hydroxyapatite and the formation of strong chemical bonds following HAP incorporation into GIC.[20] In contrast to the mentioned studies, 15 wt% micro-HAP incorporation did not affect the DTS of GIC and decreased the DTS of RMGIC in the present study. This difference might be attributed to the different types of HAP which were used. In the current study, micro-HAP (Aldrich, Sigma, USA) dissolved in distilled water was used without being heated.

On the other hand, the effect of the incorporation of HAP nanoparticles on the flexural strength of RMGIC at 0, 1, 2, 5, 7, and 10 wt% was previously evaluated and demonstrated that the groups with up to 5 wt% HAP exhibited an increase in the flexural strength. However, the flexural strengths of the groups with more than 5 wt% HAP were significantly lower than that of the control group.[21] This decrease was attributed to the formation of the agglomerates of HAP nanoparticles in the matrix and the resultant increased porosity which might serve as a weak point, resulting in a decrease in the flexural strength of the mixture. Moreover, it has been demonstrated that nanoparticles at high wt% function as nonreactive fillers and can interfere with acid–base reactions.[22] Furthermore, excess water can penetrate the space between the agglomerates of HAP nanoparticles in the matrix and finally result in decreased strength.[22] These explanations may justify the findings of the present study which showed no improvement in the DTS of the GIC incorporated with 15 wt% micro-HAP incorporation.

In the present study, 15 wt% micro-HAP resulted in a decrease in the DTS of RMGIC. 15 wt% micro-HAP incorporation into RMGIC might serve as a weak point and compromise the mechanical properties of the cement. In contrast to the present study, some previous studies found that the DTS of the GIC incorporated with HAP increased.[51619] This difference may be due to the lower wt% of the HAP incorporated with the GIC in the mentioned studies compared to the current study.

In this context, it has been reported that the incorporation of HAP up to 5 wt% does not affect the compressive strength of GIC. However, higher concentrations of HAP resulted in decreased compressive strength.[23] Generally, the scanning electron microscopy and Fourier transform infrared spectroscopy have shown that the presence of HAP in the structure of GIC decreases the number of acid–base reactions during the cement's setting reaction. Therefore, more calcium ions are released from the cement surface at the beginning of the immersion, and the dissolution of calcium ions inhibits the inhibitory effect of polycarboxylic acid to form apatite. As a result, nonhomogeneous apatite crystallization occurs in silicone and carboxyl cement groups.[4] On the other hand, due to their small size, the use of hydroxyapatite nanoparticles in the GIC powder up to 5 wt% results in the widespread diffusion of the HAP particles which occupy the empty spaces between the GIC particles, serve as a reinforcing agent in the chemical structure of GIC, and finally improve the mechanical properties.[19]

The mechanical properties of GICs incorporated with granular (rod-shaped) nanoparticles of HAP have been evaluated in a previous study which concluded that HAP could play an important role in improving the mechanical and bioactive properties of GICs.[4] In contrast, no positive effect on the DTS of the GIC incorporated with the spherical HAP was observed in the present study. This difference may be due to the fact that granular hydroxyapatite nanoparticles have higher solubility than the spherical nanoparticles used in the present study. This higher solubility can affect the mechanical behavior of GIC.[4]

Based on the results of the present study, the DTS of RMGIC, with and without hydroxyapatite, was higher than that of conventional GIC. Furthermore, the incorporation of micro-HAP into RMGIC resulted in a decrease in its DTS. It seems that HAP incorporation can interfere with the complete reaction of the components of RMGIC which can negatively affect the DTS.[13] However, the probable effects of different concentrations and different particle sizes of the HAP on mechanical and bond strength properties of GICs should be investigated in future studies.

CONCLUSIONS

The incorporation of hydroxyapatite into conventional GIC is recommended because it does not decrease the DTS of conventional GIC and may exhibit positive biological effects. However, the DTS of RMGIC decreased following micro-HAP incorporation.

Financial support and sponsorship

Shiraz University of Medical Sciences, Shiraz, Iran, supported the study.

Conflicts of interest

There are no conflicts of interest.