Influence of ultrasonic excitation on microhardness of glass ionomer cement

Abstract

BACKGROUND:

Numerous researchers have attempted to improve the mechanical properties of glass ionomer cement since 1972. In this study, ultrasonic curing treatment was introduced during the mixing of glass ionomer cement (GC Fuji IX) to facilitate intimate mixing, compaction and adaptation of residual glass particle which consequently improves densification of the material.

OBJECTIVE:

To assess the influence of ultrasonic treatment on the microhardness of glass ionomer cement (GC Fuji IX) and compare it with the conventionally cured method.

METHODS:

A total of 40 specimens (2 $\times$ 2 mm) were fabricated and equally divided into two groups: Group I (conventional curing method) and Group II (ultrasonically cured). For Group II, an ultrasonic scaler was used which provides energy to ensure proper mixing of material without leaving any air bubbles or unmixed particles. Vicker’s hardness test was employed to generate the average microhardness values by making three indentations at different points on each specimen. Statistical Package for Social Sciences (SPSS) Version 17 was used, employing independent samples $T$ test to compare the difference in microhardness values between two curing groups.

RESULTS:

The average surface hardness value for conventional cured GIC was 62.21 $\pm$ 13.61 while ultrasonically cured GIC exhibited a higher mean microhardness value of 66.37 $\pm$ 12.83. Additionally, the average microhardness values produced by the two groups showed statistically significant differences ( $p$ value $<$ 0.035).

CONCLUSION:

Ultrasonic excitation treatment leads to intimate mixing and accelerated hardening of glass ionomer cement thereby enhancing its microhardness and reducing early weakness.

Keywords

Glass ionomer cement conventional cure ultrasonic cure micro-hardness

1. Introduction

Glass ionomer cements (GICs) have been used as tooth colored restorative material for replacement of enamel and dentine structure since 1972. GICs comprise of calcium or strontium alumino-silicate glass powder (base) mixed with a soluble polymer (acid) [1]. They have properties comparable to composite restorative material which includes biocompatibility, acceptable handling and esthetics, adhesion to enamel and dentine as well as long-term fluoride release [2, 3, 4, 5]. They also exhibit a low coefficient of thermal expansion identical to the calcified tooth tissues which renders them a suitable choice for dental restorations [6]. However, their applications are limited due to low wear resistance, brittleness and low strength compared to composite material [7].

Mixing alumino-silicate glass particles with the solution of polyalkenoic acid results in the formation of glass ionomer cement. This is an acid base reaction in which acid deteriorates the alumino silicate glass particle, which results in the release of calcium and aluminium cation which then crosslinks the polyalkenoic acid chain [8, 9].

Glass ionomer cements undergo a two-step setting reaction. In the first step, the susceptibility of the material for water uptake is high due to loosely bound water molecules which cause certain integrity problems [10]. The initial phase that results in clinically set material takes place within the first 10 minutes after mixing. The second step involves the release of calcium and aluminosilicate cations in the matrix. It is a slow and long-term reaction of the acid-base complex. It is in this phase that the material is susceptible to dehydration [1]. Hence, its short-term sensitivity to water results in the softening of the surface and poor wear resistance limits the full potential use of GIC in dental applications [11].

‘Set-on-command’ glass ionomer materials were introduced to decrease the reliance of premature water uptake. It is for this purpose that resin modified GICs were developed [12]. These materials are analogous to the conventional GICs but have been incorporated with an organic photo-polymerizable monomer which can be command set by applying an appropriate light source. Resin modified GICs chemically adhere to enamel and dentine just as conventional GICs. However, the resin modified GICs have disadvantages due to the presence of resin, namely monomer toxicity, expansion in aqueous media, and low longstanding mechanical properties in comparison to conventional glass ionomer cements [12].

Initially the ultrasonic excitation was attempted to reduce air bubbles in glass ionomer material. Such pores in the set material are points of crack propagation which decrease the ultimate resistance of restorative material to indentation and wear [13]. Few studies have concluded that ultrasound excitation not only imparts an instant set to the material but also improves the hardness compared to its chemically cured counterparts especially within the first 24 hours of setting [14, 15]. The significant increase in temperature due to ultrasonic waves intensifies the acid-base reaction, thereby rapidly setting the material. Moreover, the rise in temperature results in water evaporation hence an increase in the water/powder ratio. Additionally, the external application of ultrasonic energy does not change the chemical composition of GICs, and therefore overcomes the drawbacks linked with resin modified GICs [16, 17].

Hardness of a material indicates its resistance to wear and creep behavior, which is a measure of flow in a polymer, thereby signifying the extent of cross-linking within a material. Various micro-hardness tests have been performed in the past to analyze the setting behavior of different GICs, namely Brinell, Rockwell, Vickers and Knoop micro hardness testers [18, 19].

The rationale of the current study was to assess the influence of ultrasonic treatment on the microhardness of GC Fuji IX glass ionomer cement and compare it with the conventionally cured method. Clinical implications of study results would benefit clinicians in decision making regarding strength and overall longevity of GIC restorations.

2. Objectives

We aimed to assess the influence of ultrasonic treatment on the microhardness of glass ionomer cement (GC Fuji IX) and compare it with the conventionally cured method.

3. Materials and methods

3.1 Specimen preparation

Specimen were prepared by Fuji IX (GC Corporation, Tokyo, Japan) restorative glass ionomer cement (GIC). A total of 40 specimens were fabricated using a plastic mould measuring 2 mm in diameter and 2 mm in depth to simulate clinical restoration size. The specimens were divided into two groups, each comprising of 20 samples: Group I (conventional curing method) and Group II (ultrasonically cured).

Samples in Group I were mixed in line with the directions supplied by the manufacturer (2:2 powder to liquid ratio). Freshly mixed pastes for each sample were packed into a plastic mould and the mould was slightly overfilled. Both sides of the mould were covered by a polyester strip and the cement was hardened for ten minutes. Grossly extruded excess cement was removed after applying slight pressure.

For setting the experimental Group II samples, immediately after placing the mixed cement into the plastic mould and covering it with a polyester strip on both sides, an ultrasonic scaler tip Woodpecker UDS-K (Guilin Woodpecker, China) was placed on top of the strip for 45 seconds. This provided energy to ensure proper mixing of the material without leaving any air bubbles or unmixed particles. Afterwards, the samples were allowed to set for a further nine minutes. A flat end ultrasonic tip (10 mm length and 2 mm width) was used in a Wookpecker UDS-K ultrasonic scaler (28 kHz $\pm$ 3 kHz frequency).

After the designated setting time, samples of both groups were removed from the mould and specimen finishing was performed. Initially a coarser silicon carbide grit paper #1000 was used, but eventually a finer paper #4000 was used on a rotating polishing device. Prior to microhardness testing, all samples were stored in a distilled water reservoir at room temperature since the method is sensitive to dehydration of the material surface. To minimize variability, all specimen fabrication, finishing and polishing procedures were performed by one operator. The specimens were examined for obvious voids, labeled and randomly distributed into the two groups ( $n=$ 20).

3.2 Microhardness measurements

A standard microhardness testing machine was used for hardness analysis of both the conventionally cured and ultrasonically cured groups employing the Vicker’s hardness method. Three indentations at different points were made on each sample under a load of 300 g for 15 seconds and the microhardness value was obtained as the average of these findings.

Statistical Package for Social Sciences (SPSS) Version 17 was used to enter and analyze the microhardness values. An independent samples $T$ test was used to compare the difference in microhardness values between the two groups. A p value of less than 0.05 was considered statistically significant.

4. Result

The data showed a normal distribution with average surface hardness value of 62.21 $\pm$ 13.61 for Group I (conventionally cured) while Group II (ultrasonically cured) exhibited a mean microhardness value of 66.37 $\pm$ 12.83 (Table 1). The average microhardness values produced by the two groups were statistically compared using the independent samples $T$ test. According to this analysis, the two curing methods exhibited a statistically significant difference ( $p$ value $<$ 0.035) in their surface microhardness (see Table 1).

Both groups had an equal number of glass ionomer cement (GIC) samples prepared by the conventional self-cured and ultrasonic cured method. The above statistics revealed that higher Vicker’s microhardness values were achieved with the ultrasonic curing method.

A $p$ value of $<$ 0.035 reflects statistically significant differences in microhardness between the two groups and demonstrated that the application of ultrasonic energy on GIC during curing displays better surface hardness characteristics in comparison to the conventional self-curing method.

Table 1
Mean microhardness values of glass ionomer cement in both groups

Group	$n$	Mean $\pm$ SD values
Group I	20	62.21 $\pm$ 13.61
Group II	20	66.37 $\pm$ 12.83
$p$ value ${}^{*}$		$<$ 0.035

${}^{**}$ Test of significance: Independent samples $T$ test. (Significance value: $p<$ 0.05.)

5. Discussion

Surface hardness of a material determines the wear stability of the material. Various studies demonstrated that superior hardness values in materials signify its strength and durability [20, 21].

During the formation of powder/liquid mixture of glass ionomer cements, there are chances of incorporation of air bubbles or pores. The pores that remain in the material negatively affect the properties of the restoration. A few studies have reported that the presence of these pores render the material weak, since they act as the points of crack propagation [11, 22].

Few studies have shown that excitation by ultrasonic facilitate proper mixing of glass powder and polyacid. The energy wave produced by the ultrasonic tip may also accelerate the dissolution of the polyalkenoic acid chains and glass, thereby enhancing the overall cross-linking process [14, 16]. In addition, the mechanical mixing caused by the ultrasonic tip results in better compaction and adaptation of the residual glass particle which consequently improved densification of the solid.

In the present study, two methods were employed for the setting of glass ionomer cement (GIC): the conventional self-cure and ultrasonic command-set method, and their effect on the microhardness of Fuji IX glass ionomer cement was evaluated. The results of the current study utilizing Vickers’s microhardness tester displayed higher surface hardness values for the ultrasonically cured group in comparison to the conventionally cured group. This is in line with a study by Baloch et al. which also reported similar microhardness differences [23]. Similar significant differences in microhardness values were reported by Towler et al., who also used Fuji IX glass ionomer cement [14]. These study findings prove the notion stated earlier that ultrasonically cured samples show higher microhardness values than conventionally cured samples.

According to a few studies, the ultrasonic excitation method used for the setting of the material exhibits an increase in the surface hardness values, improves enamel adhesion, and provides resistance to compression [13, 24]. It is believed that the reaction rate of the material increases because additional kinetic energy is produced by the ultrasonic excitation method. However, it should be noted that a large temperature difference can also cause irreversible damage to the pulp.

This research proved that the additional energy provided by the ultrasonic instrument imparted a command set on the conventional GIC material. The aforementioned physical and chemical disadvantages due to the light cure GIC can also be eliminated using conventional GICs. These study findings are also supported by Towler et al. [17].

The rapid setting of the material will reduce the clinical chair time and since scaling is regularly employed at clinics, it will not cause a financial burden to the clinician.

6. Conclusion

The ultrasonic excitation method leads to intimate mixing and accelerated hardening of glass ionomer cement. Such a clinical technique can be advantageous because, when pressed against the setting glass ionomer cement material, it will improve the surface mechanical strength of the restoration in comparison to conventionally cured GIC restoration and hence can also be useful in posterior load bearing areas.

7. Study limitation

Since this was an in vitro study, it might not depict the exact intra-oral performance of ultrasonically cured glass ionomer restoration. Therefore, further in vivo studies should be undertaken to mimic natural oral conditions. Due to limited funding, the wide range of glass ionomer cement materials could not be analyzed and compared for mean microhardness values.

Footnotes

Acknowledgments

The authors greatly appreciate NED University of Engineering and Technology for their cooperation in testing specimens for microhardness values. The authors declare that this was a self-funded study and no support of any sort was received from any funding agencies, institutes, companies or individuals.

Conflict of interest

The authors declare that they do not have any competing interests.

References

Sidhu

Nicholson

2016. A review of glass-ionomer cements for clinical dentistry. Journal of Functional Biomaterials, 7, 16.

Francisconi

L.F.

Scaffa

P.M.C.

Barros

V.R.D.S.P.

Coutinho

Francisconi

P.A.S.

2009. Glass ionomer cements and their role in the restoration of non-carious cervical lesions. Journal of Applied Oral Science, 17, 364–369.

Lohbauer

2009. Dental glass ionomer cements as permanent filling materials? – properties, limitations and future trends. Materials, 3, 76–96.

Sidhu

2011. Glass-ionomer cement restorative materials: a sticky subject? Australian Dental Journal, 56, 23–30.

Malik

Ahmed

M.A.

Choudhry

Mughal

Amin

Lone

M.A.

2018. Fluoride release from glass ionomer cement containing fluoroapatite and hydroxyapatite. Journal of Ayub Medical College Abbottabad, 30, 198–202.

Naasan

Watson

1998. Conventional glass ionomers as posterior restorations. A status report for the american journal of dentistry. American Journal of Dentistry, 11, 36–45.

Cho

Cheng

A.C.

1999. A review of glass ionomer restorations in the primary dentition. Journal-Canadian Dental Association, 65, 491–495.

Asif

Ahmed

M.A.

Malik

Choudhry

Mughal

Naz

2019. Evaluation of loosely bound water loss from different compositions of glass ionomer cement. Journal of Ayub Medical College Abbottabad, 30.

Khoroushi

Keshani

2013. A review of glass-ionomers: from conventional glass-ionomer to bioactive glass-ionomer. Dental Research Journal, 10, 411.

10.

Nicholson

J.W.

2018. Maturation processes in glass-ionomer dental cements. Acta Biomaterialia Odontologica Scandinavica, 4, 63–71.

11.

Xie

Brantley

Culbertson

Wang

2000. Mechanical properties and microstructures of glass-ionomer cements. Dental Materials, 16, 129–138.

12.

Berzins

D.W.

Abey

Costache

Wilkie

C.A.

Roberts

2010. Resin-modified glass-ionomer setting reaction competition. Journal of Dental Research, 89, 82–86.

13.

Navarro

2007. Influence of ultrasonic setting on microhardness of glass-ionomer cements.

14.

Towler

Bushby

Billington

Hill

2001. A preliminary comparison of the mechanical properties of chemically cured and ultrasonically cured glass ionomer cements, using nano-indentation techniques. Biomaterials, 22, 1401–1406.

15.

Algera

Kleverlaan

De Gee

Prahl-Andersen

Feilzer

2005. The influence of accelerating the setting rate by ultrasound or heat on the bond strength of glass ionomers used as orthodontic bracket cements. The European Journal of Orthodontics, 27, 472–476.

16.

Tanner

D.A.

Rushe

Towler

2006. Ultrasonically set glass polyalkenoate cements for orthodontic applications. Journal of Materials Science: Materials in Medicine, 17, 313–318.

17.

Towler

Crowley

Hill

2003. Investigation into the ultrasonic setting of glass ionomer cements Part I Postulated modalities. Journal of Materials Science Letters, 7, 539–541.

18.

Buschow

K.J.

Cahn

R.W.

Flemings

M.C.

Ilschner

Kramer

E.J.

Mahajan

2001. Encyclopedia of materials. Science and Technology, 1, 11.

19.

Arcoria

Butler

Wagner

, and Vitasek

1992. Bending strength of Fuji® and Ketac® glass ionomers after sonication. Journal of Oral Rehabilitation, 19, 607–613.

20.

Fleming

G.J.

Kenny

S.M.

Barralet

J.E.

2006. The optimisation of the initial viscosity of an encapsulated glass-ionomer restorative following different mechanical mixing regimes. Journal of Dentistry, 34, 155–163.

21.

Bala

Arisu

H.D.

Yikilgan

Arslan

Gullu

2012. Evaluation of surface roughness and hardness of different glass ionomer cements. European Journal of Dentistry, 6, 79.

22.

Nomoto

Komoriyama

Mccabe

J.F.

Hirano

2004. Effect of mixing method on the porosity of encapsulated glass ionomer cement. Dental Materials, 20, 972–978.

23.

Baloch

Mirza

A.J.

Baloch

2010. An in-vitro study to compare the microhardness of glass ionomer cement set conventionally versus set under ultrasonic waves. International Journal of Health Sciences, 4, 149.

24.

Twomey

Towler

Crowley

Doyle

hampshire

2004. Investigation into the ultrasonic setting of glass ionomer cements Part II Setting times and compressive strengths. Journal of Materials Science, 39, 4631–4632.

Influence of ultrasonic excitation on microhardness of glass ionomer cement

Abstract

BACKGROUND:

OBJECTIVE:

METHODS:

RESULTS:

CONCLUSION:

Keywords

1. Introduction

2. Objectives

3. Materials and methods

3.1 Specimen preparation

3.2 Microhardness measurements

4. Result

Table 1 Mean microhardness values of glass ionomer cement in both groups

6. Conclusion

7. Study limitation

Footnotes

Acknowledgments

Conflict of interest

References

Table 1
Mean microhardness values of glass ionomer cement in both groups