Journal of International Oral Health

: 2019  |  Volume : 11  |  Issue : 6  |  Page : 376--383

A comparative evaluation of the microhardness of glass ionomer cements modified with chitosan and chlorhexidine: A 1-year in vitro study

Anu Jose1, Abi M Thomas2,  
1 Dental Department, Chittoor Campus, Christian Medical College, Vellore, Tamil Nadu, India
2 Department of Pedodontics and Preventive Dentistry, Christian Dental College, Ludhiana, Punjab, India

Correspondence Address:
Dr. Anu Jose
Dental Department, Christian Medical College Vellore, Chittoor Campus, 190 Ramapuram Village, 189 Kothapalle Post, Gudipala Mandal, Chittoor 517132, Andhra Pradesh.


Aims and Objectives: Enhancement of the therapeutic properties of glass ionomer cements with antimicrobials, without alterations in its existing physical properties is of current research interest. One such innate property is microhardness. The aim of this study was to compare the microhardness of chlorhexidine-modified glass ionomer cement (CHX GIC) and chitosan-modified glass ionomer cement (CHT GIC) with an unmodified glass ionomer cement (FUJI IX GIC) and to study the variations in their microhardness when stored for a year. Materials and Methods: In an experimental analytic study, Fuji IX GIC liquid was modified with chitosan to form 10% vol/vol of CHT GIC. Fuji IX GIC powder was modified with chlorhexidine diacetate powder to form 1% wt/wt CHX GIC. Fuji IX GIC was used as control. Forty cylindrical samples each of the control and experimental cements were prepared, embedded in resin and stored in distilled water. Vickers microhardness testing was performed at stipulated time periods. The mean and standard deviation of each group were calculated. The data were analyzed using analysis of variance and post hoc Tukey test for multiple comparisons. Results: Materials showed significant difference in microhardness (P <0.000) at all time periods chosen for the study. FUJI IX GIC had the highest microhardness values at all time periods. CHT GIC showed the lowest microhardness and CHX GIC showed excellent stability over time. Conclusion: Although the therapeutic effects of modified GICs are proven, these additives can significantly and negatively alter the existing microhardness of the parent material.

How to cite this article:
Jose A, Thomas AM. A comparative evaluation of the microhardness of glass ionomer cements modified with chitosan and chlorhexidine: A 1-year in vitro study.J Int Oral Health 2019;11:376-383

How to cite this URL:
Jose A, Thomas AM. A comparative evaluation of the microhardness of glass ionomer cements modified with chitosan and chlorhexidine: A 1-year in vitro study. J Int Oral Health [serial online] 2019 [cited 2021 Oct 18 ];11:376-383
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Atraumatic restorative treatment (ART) practiced since the mid-1980s uses hand instruments to remove carious tissue. This procedure can address the restorative need in areas with minimal resources. It is mostly pain free and avoids the use of noisy hand pieces and local anesthetics. As a concept, it has been largely adopted even in countries with modern dental services to treat children who are apprehensive or uncooperative.[1]

The first glass ionomer cements (GICs) were not specifically made for ART and also had many shortcomings such as deficient mechanical integrity and an inability to bear fracture loads.[2] Hence in 1995, packable GIC or high-viscosity GICs (HVGICs) with improved strength, lower solubility, and better handling characteristics were developed and is currently the mainstay for ARTs.[3] They have high powder to liquid ratio and set rapidly thus significantly reducing the early moisture sensitivity.

Clinical longevity is a primary concern in selecting any restorative material. It is determined by auditing restorations for clinical failure over a sustained period of time. The survival rate of GIC restorations for multi-surface lesions is still not satisfactory.[4] Although GICs are well known for their anticariogenic property, secondary caries are still prevalent.[5] In addition, cumulative fluoride ion release by HVGICs is less than conventional restorative GICs.[6]

To overcome these problems, an improved filling material would be beneficial for the success of ART. In this quest, researchers have added various antibacterial agents of different concentrations to Fuji IX GIC.[7] These additions, however, should be carried out with caution, to prevent the degradation of its existing physical properties.[8],[9] Experiments incorporating chlorhexidine to GIC in a concentration of 1% wt/wt have shown enhanced antibacterial properties without alteration in its existing physical properties.[8] Petri et al.[10] showed that the addition of chitosan, another antimicrobial, to GIC at a concentration of 10% vol/vol improved its flexural strength and enhanced fluoride release[10] while maintaining similar microleakage as that of unmodified cement.[11] Studies have shown the need to do long-term evaluation of physical properties of new materials, as they can undergo time and material dependent changes.[12]

Microhardness is a physical property valuable in comparing restorative materials. It gives indications of long-term durability and clinical performance parameters such as resistance and wear.[13],[14] For microhardness testing, hardness is measured in a microscopic scale.

This study was carried out to evaluate and compare the microhardness variations of HVGIC and GIC modified with chitosan and chlorhexidine when stored over a period of 1 year.

 Materials and Methods

This is an experimental, analytic, in vitro study performed for a duration of 1 year. Products used for the study, their manufacturers, location, and batch numbers are listed in [Table 1]. This study was performed using conventional, self-curing, high-viscous GIC FUJI IX as the control.{Table 1}

For the preparation of experimental chitosan-modified glass ionomer cement (CHT GIC), chitosan was first dissolved in 1% acetic acid to get 0.1mg/mL chitosan solution. One milliliter of 0.1mg/mL of chitosan solution was added to 9mL of glass ionomer liquid to get 10% vol/vol CHT GIC.[10]

For formulating experimental chlorhexidine-modified glass ionomer cement (CHX GIC), 0.15g of the chlorhexidine diacetate powder was weighed electronically and added into 15g of Fuji IX powder and mixed thoroughly to obtain the desired formulation of 1% wt/wt of experimental cement.[15]

Cylindrical Teflon molds of 5-mm internal diameter and 4-mm thickness were coated with polytetrafluoroethylene dry film lubricant to facilitate easy retrieval of the hardened specimen.[16] The specimens were prepared by mixing powder and liquid in the ratio[3] of 3.6:1 as specified by the manufacturer for 25 seconds. The homogenous mixture thus formed was inserted in excess into the Teflon molds within 2 minutes from start of mixing. The samples were covered with glass slabs and sealed. After surface protection with varnish, self-cure GIC samples were left at room temperature for 15 minutes. After the initial setting, the specimens were retrieved from the molds and transferred into appropriate labeled containers with distilled water [Figure 1]. Specimens with surface defects or pores were discarded. These containers were left at room temperature and were not subjected to any dynamic changes. A total of 120 samples were studied.{Figure 1}

Group I: 40 samples of FUJI IX GIC

Group II: 40 samples of CHT GIC

Group III: 40 samples of CHX GIC

The effect size was calculated to be 1.782; taking alpha error 0.05 and power required 80%, the number of samples required from each group at the specified time period was calculated as 10.

These samples were then embedded in epoxy resin (Lapox Epoxy Resin, Lapox India, Atul, Gujarat, India) [Figure 1]. The surfaces were ground flat manually with water-cooled silicon carbide particles of varying abrasiveness (grit sizes 600, 800, and 1200, respectively). The samples were further polished with felt paper wet with diamond suspensions (particle grit size of 3, 1, 0.5, and 0.3 µm, respectively using a micro grinder and polisher (EcoMet III, Buehler, Lake bluff, Illinois, USA).[17] Then the samples were cleaned ultrasonically for 10 minutes to remove any surface debris.

At specified time intervals (24 hours, 15 days, 6 months, and 1 year) 10 samples from each group were tested. The Vickers microhardness (VHN) of the sample was measured with a Wolpert microhardness tester (Wilson Instrument, Shanghai, China), which uses a square-based pyramid diamond indenter. Applied load was carefully selected to achieve a perfect square-based pyramid indentation for accurate hardness values. Thus, a loading of 10g for 10 seconds was selected appropriate for the test material. The specimen was supported so that the indenter axis was perpendicular to the test surface and then the indentation was made. Each sample was subjected to five indentations [Figure 2] placed 300 µm apart so that stress formation from any two indentations does not overlap.[17]{Figure 2}

The Vickers hardness number was then calculated using the standard formula, VHN = 1854 P/d2 where VHN is the Vickers Hardness Number, P is the applied load, and d is the length of mean diagonal of the indentation (micrometers). Any pin-cushioned indentations or barrel-shaped indentations were not measured as it could give erroneous hardness values.[18]

Statistical analysis

VHN measurements obtained from the respective samples were tabulated. The mean and standard deviation of each group were calculated. One-way analysis of variance (ANOVA) was performed to calculate whether there is an overall difference between the groups under study, followed by a post hoc Tukey test for multiple comparisons. Statistical analysis was performed using SPSS version 17 (IBM, Armonk, NY).


The mean microhardness values of each material at different time periods are represented in [Table 2]. ANOVA analysis performed on means of different groups showed significant difference in the microhardness between groups and within groups (P = 0.000) [Table 3]. Multiple comparison of means of different groups using Tukey post hoc analysis revealed that each group showed significant difference in microhardness (P ≤ 0.05 between all groups) except when comparing the microhardness of CHT GIC and CHX GIC at 15 days and 6 months, where the difference was not significant (P = 0.730, P = 0.144, respectively) [Table 4]. [Graph 1] shows the variations in the microhardness of the materials after 1 year of storage.{Table 2}, {Table 3}, {Table 4}, {Graph 1}

Microhardness values of FUJI IX GIC were significantly higher than CHT GIC and CHX GIC at all time periods but showed some deterioration as the storage period increased. CHX GIC showed the lowest microhardness value initially but showed a significant increase in microhardness by 15 days (P = 0.000) [Table 5] and the values remained stable over time. CHT GIC showed significantly higher value than CHX GIC at 24 hours (P = 0.000) [Table 4]. CHT GIC and CHX GIC showed comparable measurements at 15 days and 6 months, but at the end of the study, microhardness value drastically decreased for CHT GIC and the difference was statistically significant when compared with control (P = 0.000) as well as with CHX GIC (P = 0.000) [Table 4].{Table 5}


Modified GICs due to the diversity of composition are expected to undergo significant variations in their complex setting reactions and cement properties.[19] Hence the primary objective of the study was to evaluate the effect of adding chlorhexidine and chitosan on the microhardness of FUJI IX GIC. In this study, we have chosen 1% chlorhexidine diacetate and 10% vol/vol of chitosan. These concentrations were studied previously and are proven to be the most beneficial.[10],[20]

Physical properties of GIC change with time[17] due to a continued long-term acid–base interaction. In other words, initial physical properties may not truly represent long-term prognosis. The secondary objective of this study, therefore, was to analyze whether the material property changed over time. To avoid desiccation of the sample and for practical purposes, distilled water was used as it was the most commonly employed storage media.[21]

Microhardness testing provides a quick and practical assessment of material and is performed in a microscopic scale. The most commonly used unit is VHN as it is more accurate and convenient over Knoop hardness measurement. Vickers indentations also allow us to group indentations closer to each other, allowing for a tighter grouping of measurements. Microhardness measurements are employed using test loads in the range of 1gF–1000gF based on the material under study. The load determined for this study is 10gF for 10 seconds.[18]

The control material FUJI IX showed the maximum microhardness at all periods. This result corroborates the findings of previous studies.[9] Microhardness value at 24 hours was noted as VHN = 67.358. Material achieved maximum microhardness by 15 days (VHN = 76.416) [Graph 1]. Similar result was reported by Ellakuria et al.[17] where another HVGIC (Ketac Molar) stored in distilled water also achieved maximum microhardness in 15 days and then remained constant. This could be due to the time taken by GIC to achieve maturation. Further on there was a significant decrease in microhardness value (VHN = 71.942) while comparing 6 months to 1 year samples (P = 0.012), but the 1-year value was not significantly different from the 15-day measurement (P = 0.182) [Table 5]. So, we can conclude that although FUJI IX underwent significant variations in microhardness over time, its microhardness remained stable in the 1-year period. This could be because of the decrease in the amount of surface ions leaching out from the GIC surface, due to the equilibrium achieved between the surface and the medium. Studies have also confirmed that as part of maturation process of GIC, much of the strength is achieved after 24 hours and it continues for few months.[22]

Experimental CHT GIC also achieved maximum microhardness mean value at 15 days (VHN = 64.046). But this value was not significantly different from the initial value measured at 24 hours (VHN = 62.622) [Graph 1]. Microhardness of CHT GIC was significantly less than the control GIC at all time periods. VHN remained stable from 24 hours to 6 months but showed a drastic decrease from 6 months to 1 year, which was statistically significant (P =0.000) [Table 5]. Due to scarcity of research on the microhardness of this experimental cement, the exact reason for the deterioration cannot be explained and our results could not be compared. But we assume that increased and sustained fluoride release could be the causative factor. Petri et al.[10] in 2007 reported that chitosan catalyzed the rate of fluoride release from FUJI IX when it was modified with 10% wt/wt of chitosan. They observed that a huge amount of fluoride was released by CHT GIC after 1 month, which was in the range of 3740 µg/cm2 whereas in a similar situation, commercial resin modified glass ionomer cement released only 500 µg/cm2.[10] It is a well-established fact that materials that release fluoride deteriorate over time if these ions are not replenished periodically.[23] Also the ionic strength of the medium in which the samples are placed also plays a role in fluoride release. It was noted that higher amounts of fluoride were released into deionized water than into neutral artificial saliva by compomers.[24] A study carried out on dimethacrylate-based dental resin sealant modified with chitosan whiskers also showed reduced hardness.[25] Hence, although chitosan has inherent property to reduce interfacial tension between the components of GIC and thus is expected to contribute to the enhancement of mechanical property of the restoration, this study shows poor microhardness over time. If this is due to the increased fluoride release, further studies aiming at recharging the restoration and providing a storage media that will have better ionic strength, and that will simulate the oral environment, should be undertaken for a fairer judgment of this experimental cement. Newer GICs have strontium instead of calcium, and studies have shown that release of strontium ions along with fluoride has improved anticariogenic effect.[26] Future studies on the amount of fluoride and strontium released by this experimental cement, their remineralization potential, cross-linking density within the matrix, etc., will be beneficial for the better understanding of this cement.

Experimental CHX GIC showed the lowest microhardness to begin with [Graph 1]. At 24 hours, the microhardness value (VHN = 51.180) of this cement was close to the value reported in the study conducted by Türkün et al.[8] in 2008 (VHN =50.46). Within the next 15 days, CHX GIC showed significant increase in microhardness (VHN = 62.124; P = 0.000) [Tables 2] and [5]. This was also in line with the aforementioned study where the value was VHN = 62.75.[8] The low initial microhardness value could be attributed to the increased time taken for initial maturation due to the addition of CHX salts. According to Takahashi et al.,[20] “the cationic properties of CHX may have interfered with the setting mechanisms such as proton-attack and leaching of ions from glasses.” Researchers have opined that elution of CHX from the cement can also alter the surface property. But it was confirmed in the study conducted by Takahashi et al.[20] that due to the increased viscosity and hardness of FUJI IX, CHX elution from the cement was minimal, but this did not affect the antibacterial property of the cement.[20] This material showed a steady increase in microhardness values till 1 year (VHN = 63.80) [Graph 1], though it was not statistically significant from the 15-day value (P = 0.706) [Table 5]. Also, interestingly, the microhardness values of CHX GIC remained stable from 6 months to 1 year of study period. It is proven that more the aluminum (Al) cross-linking in both the polyacrylates and silicate networks, better the stability and strength of GIC.[19] So it is assumed that although this cement showed slow initial gelation, the progressive cross-linking that occurred in the secondary setting reaction stage of GIC made it stable over time. This unique property of CHX GIC has long-term implications as a probable restorative material.

Scanning electron microscope (SEM) studies of different conventional and resin-modified GICs revealed that irrespective of the chemical composition, they showed voids, cracks, and microporosities on the surface.[17] Also, the fact that final set cement being a complex composition of glass particles enclosed by a sheath of siliceous gel bonded by matrix of hydrated fluoridated calcium and aluminium polyacrylates[19] explains the highly deviated values from the same sample (high standard deviations). Hence, to avoid this limitation, multiple readings were taken from the same sample and the values were averaged. All the cements were mixed by single operator to ensure uniformity of the mix. However, its accuracy may be questionable and whether it can affect the microhardness is debatable. Similar microhardness values for FUJI IX GIC and CHX GIC in previous studies validate our methodology and results.[8],[9],[21] As there are no studies evaluating the microhardness of CHT GIC, the results could not be compared.

A recent study by Moheet et al.[27] comparing the mechanical properties of conventional FUJI IX GIC with 10% nanohydroxyapatite silica–added GIC also reported improved microhardness values VHN = 64.77 (6.18) at a 24-hour period, which is comparable with the values of CHT GIC VHN = 64.04 (12.8) and CHX GIC VHN = 62.12 (8.7) on the 15th day of our study. After conducting an SEM study of the material, they attribute this higher value to the unique structure of the modified cement with elongated particle shape of hydroxyapatite surrounded by round silica particles. Also, particle size of nanoscale allows it to fill up the pores and hence better packing density.[27] Long-term durability of this material is yet to be studied.

A comprehensive review of literature conducted by Sajjad et al.[28] also has pointed out that mechanical properties of GIC can be improved only by increasing the number of polysalt bridges and cross-linking within the glass matrix. He has also suggested that incorporation of nanoceramics will provide better surface area and particle size distribution and enhance bonding at the cement–tooth interface.[28] SEM studies of our proposed experimental cements will give a clearer understanding on their structural aspects, and addition of these modifiers in the nanometer scale may give us improved results.

From the inferences of this study and critically analyzing its limitations, further research in in vivo conditions may throw light on real-life scenarios. Re-evaluation of the microhardness properties of these cements after recharging with topical fluorides and analyzing the amount of fluoride, strontium, and antibacterial substances released by these cements will give us more definite information about the properties of these experimental cements. More research on the stability of these formulations, their shelf life, working times, bioactivity, and the exact composition of these cements at an ultrastructural level is also needed in the future.


On the basis of our results, it can be concluded that modification of GICs with 10% vol/vol chitosan and 1% wt/wt chlorhexidine caused significant differences in microhardness, and all materials in this study suffered variations in microhardness over time when stored in an aqueous media. Hence, the long-term durability of these modified cements is questionable. However, in a society where the incidence of dental caries is high among children, a restorative material with high fluoride release and additional antibacterial properties would be highly desirable. In this regard, CHT GIC and CHX GIC may be promising alternatives in restoring primary teeth of patients with high risk of caries, where its immediate therapeutic benefits far outweigh its long-term physical properties.


We would like to thank the Head and staff of ISRO Satellite Centre, Bengaluru for the help rendered in the experimental work.

Financial support and sponsorship


Conflicts of interest

There are no conflicts of interest.


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