Conventional glass-ionomer cements: a guide for practitioners

Antibacterial properties

The initial low pH of GICs may confer antibacterial activity, particularly when placed over caries-affected dentine. Additionally, laboratory testing indicated that freshly mixed or set GIC and mature GIC inhibit the growth of Streptococcus mutans and affect the acidogenicity of the overlying plaque biofilm.^26,27 Ions released from GICs, including fluoride, aluminium and strontium, have exhibited antimicrobial effects.^{28,29,30,31,32} Some studies have suggested that the antibacterial properties of GICs could be related to fluoride release,³³ the acidity,^33,34 or even zinc.³⁵ Given there are conflicting reports on this matter, the exact mechanism by which fresh and set GIC exhibit antibacterial properties is still not fully understood.³⁴

Bio-interactive properties

The terms ‘bio-active’ and ‘bio-interactive’ describe two different properties for a given dental material. Bio-active dental materials can induce apatite-containing material formation (eg hydroxyapatite) in simulated body fluid, or induce a pulpal response to simulate reparative dentine formation. Bio-interactive dental materials contain and release ions similar to those found within the tooth structure (eg calcium) that can interact with adjacent tooth structure to drive remineralization.^36,37,38 GICs therefore belong in the bio-interactive category because of their ability to release calcium and fluoride into the surrounding tooth structure and environment.

Polyalkenoic acids are both ionic and polymeric in nature. Clinically, this is important because GICs are both hydrophilic and acidic, and can interact chemically with dentine and enamel, resulting in chemical adhesion and ion exchange (calcium and fluoride) between the GIC and adjacent tooth structure.^17,39 The calcium and fluoride ions found within the GIC can aid in tooth remineralization and provide cariostatic properties that are not observed in conventional resin composites.³⁹

Physical properties

As GICs have been refined, they have been successfully used across a wide range of clinical scenarios, such as the definitive restoration of primary teeth, and stabilization in adults with high caries susceptibility.⁴⁰ However, in comparison to resin composites, the reduced mechanical properties of GICs have traditionally limited their comprehensive clinical application as definitive long-term restorations, especially as posterior, load-bearing restorations.^9,41 Compressive and flexural strength are most commonly used to describe GIC mechanical properties because they have suitable in vitro analogues that allow the replication of typical masticatory loading seen clinically.⁴¹ Wear resistance is another requirement in load-bearing scenarios. Conventional GICs have been demonstrated to exhibit lower wear resistance compared with dental amalgam and resin composites; however, their physical and mechanical properties improve as maturation proceeds.⁴²

Aesthetic properties

Aesthetics is a key property that determines the overall clinical success or failure of a tooth-restoration complex. In vitro laboratory studies have found that the colour stability of GICs differs for several reasons, including the additives in the formulation, contamination from extrinsic sources and the storage solution.^43,44

Clinical trials have also found good long-term colour stability for GICs. In a 2-year study, EQUIA (GC Corp, Tokyo, Japan) was found to rarely show distinct colour mismatch (less than 1%) in class I and II restorations in permanent teeth.⁴⁵ This was later confirmed by a series of studies in which EQUIA (GC) exhibited no significant colour match or margin discolouration issues at any recall up to 5 years, with no differences found compared to the hybrid resin composite Gradia Direct Posterior (Dentsply, PA, USA).^46,47

Clinical use

Delivery systems

GICs are presented with different delivery systems according to their clinical use and formulation, and are available in a powder/liquid combination, either hand-mixed or encapsulated and auto-dispensed. Most manufacturers provide the same GICs with different delivery systems. For example, Fuji IX GP (GC) and Chemfil Rock (Dentsply) are available both as encapsulated and manually mixed powder/liquids. GICs with the same brand name and overall formulation do have subtle differences in the filler:liquid ratio according to their clinical delivery system.^5,9

Hand-mixed GICs allow the clinician to control the quantity of final GIC required for their restoration. It is easier to restore a large cavity using a large quantity of hand-mixed GIC compared with using multiple capsules, some of which may not be used in their entirety. However, previous research has indicated that there are large inconsistencies in the mixing ratios of powder/liquid and mixing techniques that can influence the mechanical properties and setting time/handling properties of the GIC.^{48,49,50,51,52} Pre-dosed encapsulated GICs (powder/liquid) offer the advantage of improved consistency and repeatability of mixing and dispensing.

GIC conditioners

GIC tooth conditioners (also known as surface or cavity conditioners) are not the same as acid-etchants used prior to resin composite placement. They differ according to the acid type, strength, and effect on the smear layer.^5,9 A smear layer is always created after tooth preparation and contains a mixture of bacteria, necrotic organic tooth tissue, minerals, oils from the dental handpiece and other debris. Over time, this smear layer is susceptible to dissolution under restorations, which encourages microleakage, microbial ingress and possibly pulp inflammation.⁵³

GIC surface coatings

During the setting reaction, GICs are susceptible to excess water loss or gain, which can significantly affect the chemical and mechanical properties of the set material.^5,57,58 The concept of surface protection for conventional GICs was first investigated in 1993, using the then-available dental adhesives to investigate their adhesion in vitro.⁵⁹ As a result, manufacturers may recommend the use of a GIC surface coat after placement to help protect the GIC. These can be of three types:

Emollients can be petroleum- or lipid-based products.⁵⁷ Solvent-based varnishes are simple solutions of different polymers in solvents that evaporate, leaving behind a layer of polymer on the GIC surface.⁹ Light-cured resin coatings generally consist of a mixture of methacrylate monomers, photo-initiators, with/without filler particles.⁹

Comparisons between GIC surface coats have been undertaken primarily in laboratory-based studies studying water loss or gain, or the penetration of dyes into the surface of GIC samples coated with different GIC surface coats. Surface emollients have been reported to have limited success in protecting GICs because they can be easily wiped or washed off. They do, however, offer some protection where no GIC coat is available.^57,60 No differences between the protective effects of solvent-based varnishes and light-cured resin-based coats have been reported. All coatings that were tested performed equally well in minimizing dye penetration and preventing water loss, and both types were significantly better than no coating at all.^57,61 A previous study indicated that varnishes might peel from the GIC surface and the use of a light-cured resin-based coat may be preferred.⁶² In 1990, the American Dental Association (ADA) stated the importance of coating conventional GICs with either a varnish or a light-cured resin-based coat to limit water movement during the maturation stage.⁶³

Improved clinical survival rates have been demonstrated for GICs protected with light-cured coats compared with no surface coat.⁶⁴ However, the mechanism by which this occurs is not fully understood.^65,66

Clinical placement of GICs: technical considerations

The decision to use GIC must be considered before any cavity preparation is undertaken. The decision is made together with the patient with the understanding of why the material is being used and what will most likely be required in the future, ie GIC removal and replacement, or cut-back and overlaying with a more durable material such as resin composite.

Cavity preparation and caries management must be carried out using minimally invasive techniques to ensure full use of the benefits of the GIC material, improved clinical longevity and tooth-restoration complex survival. Because the material is moisture tolerant to a degree, the use of rubber dam isolation is not mandatory, but is recommended to improve placement owing to the rubber dam's soft tissue control and cavity field isolation.

The use of a proprietary GIC cavity conditioner and GIC coat is dependent on the GIC being used and its initial pH. Because this information is often not readily available, it is recommended that a conditioning step is included in most cases, using a proprietary mild concentration polyacrylic acid (10–25%) for 10–15 seconds on enamel and dentine. This should be thoroughly washed off and the tissue gently air dried to ensure obvious water droplets are removed from the tooth surface prior to GIC placement.

Another important consideration is when to finish the GIC surface after placement. Clearly, gross material excess whether occlusal, approximal, or otherwise, must be removed with a sharp instrument to ensure conformity to the existing occlusion and aid patient oral hygiene. The finishing of GICs must only be carried 24 hours (minimum) after placement to avoid dehydration and loss of water from the maturing GIC.⁶⁸

Figure 3 provides an overview of the ideal GIC placement for a small class II approximal restoration, and the relevant clinical steps clinicians should consider. Each clinical scenario must be considered on an individual basis and manufacturers' guidelines followed.