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Table of Contents
Year : 2021  |  Volume : 11  |  Issue : 3  |  Page : 114-118

Evaluation of bonding strength of conventional glass ionomer cement modified with micro- and nano-hydroxyapatite: An in vitro study

Department of Conservative Dentistry and Endodontics, CSMSS Dental College and Hospital, Aurangabad, Maharashtra, India

Date of Submission23-Sep-2020
Date of Decision20-Aug-2021
Date of Acceptance12-Oct-2021
Date of Web Publication22-Dec-2021

Correspondence Address:
Dr. Karishma Kishor Patel
At Post Pandhari, Khanampur, Tq-Anjangaon Surji, Amravati - 444 808, Maharashtra
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Source of Support: None, Conflict of Interest: None

DOI: 10.4103/jid.jid_76_20

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Context: With the dawn of esthetic dentistry, esthetic restorative materials are the preferred choice for various applications. Because of their limited strength and wear resistance, glass ionomer cement (GIC) is indicated generally for the restoration of low-stress areas where caries activity potential is of significant concern. Therefore, hydroxyapatite (HA) materials were added to improve the consistency, compressive, and bonding strength of GIC. In dentistry, the adhering ability of GIC to HA resulted in variety of clinical applications. Aims: The aim of this study is to evaluate and compare the effects of incorporation of micro-HA and nano-HA on the bonding strength of conventional GIC to tooth structure. Settings and Design: There was an experimental randomized analytical in vitro study. Materials and Methods: Twenty-one extracted human permanent molars were divided into three groups (n = 7). Each specimen was buried in a cylinder-shaped epoxy resin, and coronal portion was sectioned to expose the dentin surface available for bonding GIC. The smear layer on the exposed dentin was removed by etching with 35% phosphoric acid. Mixed cement was placed on tooth in cylindrical form, and specimens were immersed in artificial saliva for 4 weeks. After 4 weeks, shear bond strength was measured using a universal testing machine with 1 mm/min crosshead speed. Statistical Analysis Used: Obtained shear bond strength values were statistically analyzed using one-way ANOVA and Tukey's Post hoc test. Results: Results showed that there are significant differences between GIC reinforced with HA than conventional GIC. The bonding strength is higher in nano-HA GIC compared to micro-HA and conventional GICs. Conclusions: The addition of 15% HA to conventional GIC increased its bond strength to dentin. Nano-HA added GIC showed the highest bonding strength to tooth structure.

Keywords: Bonding strength, glass ionomer cement, hydroxyapatite, universal testing machine

How to cite this article:
Daokar SG, Patel KK, Pawar KS, Wahane KD, Kulkarni SS, Mantri AR. Evaluation of bonding strength of conventional glass ionomer cement modified with micro- and nano-hydroxyapatite: An in vitro study. J Interdiscip Dentistry 2021;11:114-8

How to cite this URL:
Daokar SG, Patel KK, Pawar KS, Wahane KD, Kulkarni SS, Mantri AR. Evaluation of bonding strength of conventional glass ionomer cement modified with micro- and nano-hydroxyapatite: An in vitro study. J Interdiscip Dentistry [serial online] 2021 [cited 2023 Mar 30];11:114-8. Available from: https://www.jidonline.com/text.asp?2021/11/3/114/333345

   Clinical Relevance to Interdisciplinary Dentistry Top

  • Restoration of class III and class IV caries
  • Restoration of traumatic anterior teeth
  • Tooth-colored restoration
  • Cementation of orthodontic brackets.

   Introduction Top

Glass polyalkenoate cements are materials made of calcium or strontium alumina fluorosilicate glass powder (base) combined with a water-soluble polymer (acid).[1] Kent called such materials “glass ionomer” cements and that name has become part of the dental vernacular.[2] It offers several advantages, including fluoride release, similarity of the coefficient of thermal expansion, and elastic modulus to that of tooth structure, ability of chemical bonding to calcified enamel and dentin, biocompatibility, and easiness of handling.[3]

However, glass ionomer cement (GIC) has several disadvantages such as brittleness, inferior mechanical strength, and moisture sensitivity.[3] To improve physical properties of GIC incorporation of various fillers has been attempted, including silver cements, stainless steel powder, carbon and aluminosilicate fibers, and hydroxyapatite (HA).[3] Considering the high biocompatibility of HA, some researchers have made attempts to evaluate the effects of incorporating HA powder into restorative dental materials such as GICs.[4]

GICs have been found to interact with HA through the carboxylate groups in the polyacid.[5] HA is one of the most biocompatible and bioactive materials, and its nanosized particles are similar to the apatite crystal of tooth enamel in morphology, crystal structure, and crystallinity.[6] Apart from enhancing the mechanical properties, it provides a positive atmosphere for the remineralization of enamel.[7]

Compared to micro-HA, decreased particle size of nano-HA, similar to the size of minerals in teeth, leads to increased surface area and higher solubility.[8],[9] Microparticles of HA have been shown to be easily mixed with GIC powder, including resin, while porous spherical HA particles have been shown to increase mechanical properties and the release of fluoride ions most effectively.[10]

Therefore, nano-HA with high solubility can fill the micropores of enamel defects more effectively by releasing inorganic ions, such as calcium and phosphate, and enhance bonding strength between restorative material and teeth.[11] Its bonding mechanism should be attributed to micromechanical interlocking provided by the surface roughness, most likely combined with chemical interaction through its acrylic/itaconic acid copolymers.[12]

Thus, the aim of this study was to compare bonding strength among three groups: control group (conventional GIC), Exp 1 group (nano-HA containing GIC), and Exp 2 group (micro-HA containing GIC).

   Materials and Methods Top

Sample preparation

GIC (GC Gold Label) was used as the control group and base material for experimental groups. For micro-HA [Figure 1], calcium phosphate tribasic (CLARION Pharma, New Delhi) was used. Its molecular formula was Ca5(OH) (PO4) 3 with a mean diameter of 5–10 μm.
Figure 1: Microhydroxyapatite crystals

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For nano-HA [Figure 2], medical-grade extra pure (99.5%) nano-HA powder (NANO Research Lab, Jharkhand) was used; its molecular formula was Ca5(OH) (PO4) 3, with a mean diameter of 100–150 nm.
Figure 2: Nano-hydroxyapatite crystals

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For experimental groups, the specimens contained 85% conventional GIC with 15 wt% of micro-HA (micro-HAP). The conventional GIC (85 wt%) and micro-HAP (15 wt%) were weighed carefully using a weighing machine accurate to 0.0001 g. Then, micro-HAP was added to the GIC and mixed in the amalgamator for 20 s to have a uniform mixture.

Bonding strength

Twenty-one extracted human permanent molars free of caries, defects, or restorations were collected and stored in normal saline. The crowns were cut using a diamond disc accompanied with a water coolant and divided into three groups (n = 7).

  • G1 – Conventional GIC (control group)
  • G2 – GIC + Micro-HA
  • G3 – GIC + nano-HA.

The exclusion criteria were,

  • Teeth with preexisting fractures and cracks
  • Teeth with root caries
  • Teeth with intracanal calcification
  • Teeth with external and internal resorption
  • Previously endodontically treated teeth
  • Carious teeth
  • Teeth with an open apex
  • Teeth with previous restorations.

Each specimen was buried in a cylinder-shaped epoxy resin mold (25 mm diameter × 25 mm height), and its coronal portion was sectioned perpendicular to the long axis of the tooth to expose the dentin surface available for bonding GIC [Figure 3] and [Figure 4]. The smear layer on the exposed dentin was removed by etching with 35% phosphoric acid. Mixed cement was placed on it in cylindrical form with 6 mm of diameter and 5 mm of height using ring-shaped mold made up of Tofflemire band.
Figure 3: Diagrammatical representation of epoxy resin mold

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Figure 4: Mixed cement placed in cylindrical form

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When setting time had elapsed, specimens were immersed in artificial saliva (Milestone Healthcare) (pH 7.4) for 4 weeks and were changed every week. After 4 weeks, shear bond strength was measured using a universal testing machine (Central Institute of Petrochemicals Engineering and Technology, Aurangabad) with 1 mm/min crosshead speed. A chisel-shaped rod was aligned in the crosshead so that the force delivered to the specimen was immediately adjacent and parallel to the dentin surface. Each specimen was continuously loaded until fracture occurred. The shear bond strength values were calculated by dividing the force at which bond failure occurred by the bonding area.

Obtained data were recorded and subjected to statistical analysis for results.

   Results Top

There were statistically significant differences between the control and experimental groups (P > 0.05).

Lowest bonding strength was observed in the control group (9.00 MPa), followed by micro-HA added GIC group (11.05 MPa), and nano-HA added GIC group, which presented greatest bonding strength (13.88 MPa) [Graph 1] and [Table 1].
Table 1: Descriptive statistics for mean stress (mpa) of the bonding agents

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Statistical analysis

One-way ANOVA test showed a significant difference in bonding strength when compared among control group (conventional GIC) and experimental groups (micro-and nano-HA GIC) (P < 0.0005) [Table 2]. The mean maximum stress has significant difference at 95% level of confidence.
Table 2: ANOVA test for difference in mean maximum stress

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The Tukey's post hoc test was used to determine intergroup differences (P < 0.05). Results are presented with median values and ranges. It revealed significant difference in the bonding strength between the control group (conventional GIC) and nano-HA added GIC group [Graph 2].

   Discussion Top

In the field of dentistry, studies to obtain better biocompatible and conservative materials had been conducted, and GIC was invented out of such efforts.[3] GIC chemically bonds to both enamel and dentin, and it has similar coefficient value of thermal expansion with that of tooth structure.[3]

There have been many efforts to enhance its mechanical properties as well as its anticariogenic effect.[3] It is then hypothesized that the addition of HA, regardless of its morphology, to GIC may improve the mechanical properties of the cement without compromising its inherent favorable properties.[13]

In this study, we evaluated the effect of incorporation of micro-HA or nano-HA into conventional GIC on bonding strength of tooth. HA has shown promising advantages in restorative dentistry, including its biocompatibility, hardness similar to that of natural tooth, and intrinsic radiopaque response.[14]

Various reinforcement materials such as HA had shown high success when added to either composite or adhesive bone cements.[14] Studies have reported that, when HA is added to cement, it helps in improving mechanical properties such as surface hardness, toughness, flexural strength, and modulus.[14]

Yoshida et al. analyzed the chemical interaction of a synthesized polyalkenoic acid with enamel and synthetic HA and pointed out that carboxylic groups of the polyalkenoic acid replaces phosphate ion of the substrate and make ionic bonds with calcium ions of Ha.[15]

This suggests that the HA is participating in chemical changes that are taking place during the initial setting of the cement.[13] The HA is soluble in acidic solution, and its solubility rate is rapidly increased with a pH below 2.05 upon contact with polyacrylic acid (pH 1.23), calcium ions may be liberated from the surface of the HA.[13]

It is possible that calcium from the added HA is available earlier than other metal ions from the glass surface, that is, calcium, aluminum, and strontium to react with polyacrylic acid.[13]

Therefore, this extra calcium which is available for the cement formation, polysalt bridge formation, and cross-linking, all of which reinforce the glass ionomer matrix may increase the bonding inside the matrix but may be affecting the bonding ability of the GIC to the HA surface.[1]

Bonding strength was lowest in the control group followed by micro-HA group. It was found highest in the nano-HA added GIC group, which showed statistically significant difference (P < 0.05).

Bonding strength was greater in the nano-HA added GIC group compared to micro-HA added GIC group, which can be attributed to smaller particle size of nano-HA; smaller particles have increased surface area, which contributes to enhanced solubility and infiltration capacity to enamel surface, thereby reinforcing the bonding strength [Table 3].[3]
Table 3: Significance level of intergroup comparison

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Jong-Jin Lee et al. reported that, in nano-HA added GIC group, deposition of increased amount of bone-like apatite particles was observed compared to micro-HA-added GIC group.[3] In addition, dentinal tubules were narrowed by their deposition.[3]

Increased deposition of bone-like apatite particles in nano-HA-added GIC group can be explained by two means.[3] First, high solubility of nano-HA may enhance active ionic exchange with SBF, leading to increased formation of bone-like apatite particles.[3] Second, nano-HA has similar composition to teeth, and therefore it has higher affinity to tooth structure.[3]

However, the role of nanosize particles on the micro-size particles of GIC and the ratio and proportions of nano-HA to the GIC needs further elucidation.[1]

   Conclusions Top

Based on the observation of the present study, it can be concluded that, by the addition of 15% nano-HA and micro-HA to conventional GIC, it maintains long-term bond strength to dentin.[13] The difference in bonding strength between micro-HA and conventional GIC was insignificant [Table 2]. In comparison to conventional and micro-HA added GIC, nano-HA-added GIC demonstrated the highest bonding strength to tooth structure [Graph 2].

Financial support and sponsorship


Conflicts of interest

There are no conflicts of interest.

   References Top

Choudhary K, Nandlal B. Comparative evaluation of shear bond strength of nano-hydroxyapatite incorporated glass ionomer cement and conventional glass ionomer cement on dense synthetic hydroxyapatite disk: An in vitro study. Indian J Dent Res 2015;26:170-5.  Back to cited text no. 1
[PUBMED]  [Full text]  
Croll TP, Nicholson JW. Glass ionomer cements in pediatric dentistry: Review of the literature. Pediatr Dent 2002;24:423-9.  Back to cited text no. 2
Lee JJ, Lee YK, Choi BJ, Lee JH, Choi HJ, Son HK, et al. Physical properties of resin-reinforced glass ionomer cement modified with micro and nanoHA. J Nanosci Nanotech 2010;10:5270-6.  Back to cited text no. 3
Sharafeddin F, Karimi S, Jowkar Z. Evaluation of the effect of micro-HA incorporation on the diametral tensile strength of glass ionomer cements. J Conserv Dent 2019;22:266-9.  Back to cited text no. 4
[PUBMED]  [Full text]  
Gu YW, Yap AU, Cheang P, Khor KA. Effects of incorporation of HA/ZrO(2) into glass ionomer cement (GIC). Biomaterials 2005;26:713-20.  Back to cited text no. 5
Enan ET, Hammad SM. Microleakage under orthodontic bands cemented with nano-hydroxyapatite-modified glass ionomer. Angle Orthod 2013;83:981-6.  Back to cited text no. 6
Moheet IA, Luddin N, Ab Rahman I, Kannan TP, Abd Ghani NR. Evaluation of mechanical properties and bond strength of nano-HA-silica added glass ionomer cement. Ceramics Int 2018;44:9899-906.  Back to cited text no. 7
Arcís RW, López-Macipe A, Toledano M, Osorio E, Rodríguez-Clemente R, Murtra J, et al. Mechanical properties of visible light-cured resins reinforced with hydroxyapatite for dental restoration. Dent Mater 2002;18:49-57.  Back to cited text no. 8
Domingo C, Arcís RW, López-Macipe A, Osorio R, Rodríguez-Clemente R, Murtra J, et al. Dental composites reinforced with HA: Mechanical behavior and absorption/elution characteristics. J Biomed Mater Res 2001;56:297-305.  Back to cited text no. 9
Bilić-Prcić M, Rajić VB, Ivanišević A, Pilipović A, Gurgan S, Miletić I. Mechanical properties of glass ionomer cements after incorporation of marine derived porous cuttlefish bone HA. Materials 2020;13:3542.  Back to cited text no. 10
Huang M, Feng J, Wang J, Zhang X, Li Y, Yan Y. Synthesis and characterization of nano-HA/PA66 composites. J Mater Sci Mater Med 2003;14:655-60.  Back to cited text no. 11
Lyapina MG, Tzekova M, Dencheva M, Krasteva A, Yaneva-Deliverska M, Kisselova A. Nano-glass-ionomer cements in modern restorative dentistry. J IMAB-Ann Proc Sci Pap 2016;22:1160-5.  Back to cited text no. 12
Lucas ME, Arita K, Nishino M. Toughness, bonding and fluoride-release properties of HA-added glass ionomer cement. Biomaterials 2003;24:3787-94.  Back to cited text no. 13
Bali P, Prabhakar AR, Basappa N. An in vitro comparative evaluation of compressive strength and antibacterial activity of conventional GIC and HA reinforced GIC in different storage media. JCDR 2015;9:ZC51.  Back to cited text no. 14
Yoshida Y, Van Meerbeek B, Nakayama Y, Snauwaert J, Hellemans L, Lambrechts P, et al. Evidence of chemical bonding at biomaterial-hard tissue interfaces. J Dent Res 2000;79:709-14.  Back to cited text no. 15


  [Figure 1], [Figure 2], [Figure 3], [Figure 4]

  [Table 1], [Table 2], [Table 3]


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