Compressive and Flexural Strengths of High-strength Glass Ionomer Cements: A Systematic Review

INTRODUCTION

Glass ionomer cements (GICs) were invented in 1969 and their use was reported by Wilson and Kent in the early 1970s. They are composed of an aluminosilicate glass and a polyalkenoic acid.

Glass ionomer cements possess unique properties such as biocompatibility, elasticity similar to dentin, anticariogenic action due to fluoride release, and direct bonding to the tooth structure. Conversely, GICs have also presented clinical limitations such as low wear resistance, low fracture toughness, low mechanical properties, prolonged setting rate, and high early moisture sensitivity.^1,2 These limitations have restricted the use of conventional GICs in high stress-bearing areas (e.g., posterior teeth).

High-strength/highly viscous GICs have since been developed to address the relatively poor physical and mechanical properties of conventional GICs and to be used as restorative materials in higher stress-bearing posterior teeth. However, these materials may still experience restoration failures, e.g., in multiple-surface atraumatic restorative technique (ART) restorations, restorations in high stress-bearing sites,^3,4 indicating the need for further improvement in their physical and mechanical properties.

Efforts have been made toward improving high-strength GICs’ physical and mechanical properties without affecting their biological properties, by the addition of a variety of filler materials including montmorillonite clay,⁵ zirconia,⁶ glass fibers,⁷ hydroxyapatite, bioactive glass particles as prereacted glass ionomer particles, and changing the type of alkenoic acid.^8,9

With the wide variety of dental products constantly being launched, selecting an ideal restorative material has become a difficult task for the clinician. Prior to clinical trials, laboratory tests are a fundamental necessity in guiding researchers and clinicians to analyze and understand the factors affecting the variability in test results of a material, thus facilitating evidence-based practice.

However, many of the studies that have evaluated the properties of high-strength GICs have shown variability in protocols, in terms of temperature of specimen storage, duration of storage, dimensions of the specimens, and the load applied during tests. This lack of uniform methods and reporting hampers meaningful comparisons of these studies.^9,10

Thus, in view of the new materials being developed, it is necessary to verify the standardization of laboratory tests performed with these materials so that future manufacturers, clinicians, and researchers can rely on in vitro studies to deduce inferences.

Therefore, the objectives of this review are the following:

• To systematically analyze and compare the test methodologies employed for testing the compressive strength (CS) and flexural strength (FS) for new/experimental high-strength GICs for compliance with the recommendations of testing established by the ISO.
• To compare the above-mentioned properties of different GICs that have been tested according to the International Standards Organization (ISO) recommendations.
• To assess the quality of evidence of these studies.

MATERIALS AND METHODS

RESULTS

Initially, 123 studies were found, from which 115 were selected after removing 8 duplicates. These 115 studies were then screened by reading the titles and abstracts, and 49 were selected. The full-text of these 49 studies was accessed, and 23 studies were excluded based on the exclusion criteria; 26 studies were included. These 26 studies were checked for compliance with ISO recommendations, and 2 studies were found to comply with the ISO recommendations for testing. Flowchart 1 depicts this process.

The number of studies not complying with the ISO recommendations is shown in Table 1. It was observed that only two studies evaluating CS complied with ISO recommendations, while none studying FS were compliant with the ISO recommendations. The characteristics of these studies are displayed in Table 2.

With respect to the quality assessment, both studies were considered of fair quality. The studies did not provide information pertaining to how the samples were allocated to the different groups or blinding of evaluators to the type of material or procedure.

DISCUSSION

A previous systematic review on mechanical and optical properties of conventional GICs found that only one study complied with the ISO recommendations CS testing.¹⁶ Similar results of this review indicate that there is a need for researchers to reconsider the test methodologies that they follow for uniform reporting and better interpretation of results.

From the results of this review, depending on the property being tested, only two or no studies followed the recommendations laid down by the ISO. Many of the studies reported having followed the specifications but with modifications in dimensions, storage media/conditions, storage time, etc., of the specimens. Studies have demonstrated that CS can be influenced by specimen dimensions as well as storage media and testing conditions.^17–19 While the ISO provides guidance for the evaluation of dental materials with the intention of ensuring reproducibility of test results for selected testing methodologies between different test centers under standard conditions, such modifications make it difficult to draw comparisons.

Only two studies in this review adhered to the ISO recommendations for CS testing of GICs.

Both the studies were judged to be of fair quality according to the CRIS guidelines for in vitro studies as they did not provide information about randomization/sample allocation and investigator blinding. Randomization in in vitro experiments reduces the chances of bias and favors the difference in outcome between groups to be by chance, just as in randomized clinical trials. Reporting sample size calculation, meaningful difference, and randomization would make results of in vitro studies more robust and significant.13

The study by Yesilyurt et al.¹⁴ evaluated the antibacterial activity and physical properties (CS and bond strength to dentin) of GICs containing an antibiotic mixture of ciprofloxacin, metronidazole, and minocycline in concentrations of 1.5, 3.0, and 4.5% w/w for use with ART. The results of this study showed that the experimental GICs were effective in inhibiting bacteria associated with caries. They also displayed reduced bond strength to dentin. At the end of 24 hours, all the experimental groups as well as the control groups showed CS lower than that specified as the minimum required CS by the ISO, i.e., 100 MPa. The ISO specifies that at least four out of five test results should be above the minimum specified strength. If three or more of five results are below the specified minimum strength, the material fails the test, in which case, another batch of five specimens must be tested. Subsequently, at least 8 out of a total of 10 results must be above the minimum specified strength. It has been suggested that at least 20 similar specimens are required to determine statistically significant differences when assessing the CS of brittle dental materials.^20,21 However, this study considered a sample size of 5 per group for CS testing. According to the authors, a possible explanation for these results is that increasing concentration of antibiotic powders may contribute to incomplete reactions between the glass particles and liquid, thereby increasing the number of unreacted particles in the structure. The powdered antibiotic particles used in this study were hygroscopic. Absorption of water by these additives could be a contributory factor to the decrease in the CSs of the GIC. The low CS of these GICs may make them unsuitable for use as materials in posterior teeth for ART.

The second study, by El-Wassefy et al.,¹⁵ studied the effect of incorporating silver nanoparticles in concentrations of 0, 1, 3, and 5 wt% to GIC on the Staphylococcus aureus biofilm, in terms of bacterial growth and effect on hardness and CS. The CS reported by the study was more than the minimum specified strength by the ISO. However, there was a decrease in the CS of the experimental GICs, which was considered to be statistically insignificant. A possible explanation for this, as given by the authors, is that the nanosize of the silver nanoparticles and physical addition instead of bonding the particles to the matrix did not alter the mechanical properties significantly. The nanosize of the particles allows the dispersion in-between and around polymer chains. It is also assumed that silver does not interfere with the matrix setting reactions. The authors concluded that the incorporation of silver nanoparticles in GIC powder could limit the formation of biofilm with a nonsignificant effect on the mechanical properties. However, the influence of silver nanoparticles on color was noted, which would restrict its use to areas where esthetics is not of concern.

Despite CS being considered as one of the parameters for evaluating strength by the ISO, its validity has been questioned.^22,23 A major criticism of the CS testing methodology is that CS testing provides the stress at failure and neglects the mechanism of failure. Uniaxially compressed cylinders fail due to “some unresolved combination of tension and shear” stresses. The CS data do not help to determine “service performance.”²²

Alternatives to CS for GICs are the TFS and the biaxial flexure strength (BFS) test.²¹ The ISO 9917-2 provides specifications for the TFS testing of GICs, and none of the studies in this review complied with ISO specifications for TFS testing.

However, researchers have reported BFS testing to be more advantageous over uniaxial tests such as TFS because the specimens are considered to be easier to prepare and closely match the volume of a clinical restoration. The BFS testing also creates a stress field away from the supports and away from the specimen periphery, reducing the sensitivity to specimen edge defects.10

In the absence of strict guidelines for testing other strength parameters that may be potentially advantageous over the existing ones in use, different authors have used different experimental techniques,¹⁰ which again makes it difficult to draw comparisons.

To summarize, despite the large number of studies that have been conducted on high-strength posterior GICs, lack of adherence to guidelines makes it difficult to compare results of different studies. Lack of guidelines for potentially better test methodologies and guidelines for test methods that may not be as apt make it difficult to scrutinize and compare the validity of the research being conducted.

CONCLUSION

The evidence from this review on the CS and FS of high-strength posterior GIC indicates that:

• Only two published studies that tested the CS of high-strength posterior GIC followed the ISO standards. Both studies were considered of fair quality.
• Both studies showed a decrease in the CS of the experimental groups compared to the control groups.
• None of the studies that tested FS adhered to the ISO recommendations.

There is a need for researchers to adhere to the guidelines laid down by the ISO to ensure validity and comparability of the data.

There is a need for ISO to reconsider the parameters that are currently in use in favor of potentially more advantageous parameters.

Adhering to a checklist or guidelines similar to CRIS guidelines for in vitro studies would make reporting of in vitro studies more significant and reliable.

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