INTRODUCTION

As a restorative material, conventional glass-ionomer cements (C-GIC) have certain desirable properties. C-GIC includes chemical bonding to enamel and dentin substrates, the release of anticariogenic fluoride into adjacent hard tissues and a low coefficient of thermal expansion similar to that of dentin.1–2 However, C-GICs are susceptible to fracture and exhibit low wear resistance. Therefore, these deficiencies have limited their use to areas subject to low masticatory stresses.2 Because of low fracture toughness, mechanical strength and the brittleness of C-GICs, efforts were made to improve their mechanical properties by adding powder.3–5 Highly-viscous C-GICs (HV-GICs) were developed to overcome early moisture sensitivity and the low mechanical properties associated with conventional materials. They were then designed as an alternative to amalgam for posterior preventive restoration.1–2 Highly-viscous or high powder-liquid ratio C-GICs, such as Fuji IX GP, Ketac Molar and GlasIonomer FX-II, provide a condensable feel and are specifically used for the atraumatic restorative treatment (ART) technique introduced by the World Health Organization (WHO) for use in developing countries.6–7 Indications for these cements in general practice are to small Class I cavities, deciduous teeth and long-term temporaries.28–12

The polishing period is another factor that may influence sealability around a cervical or cavity restoration. Polishing after storage in water for one day resulted in improved gap formation for cervical restorations or dentin cavities of a resin-modified glass-ionomer cement (RM-GIC) or a C-CGI.13–17 Due to the structure of RM-GIC or C-GIC and its hydrophilic nature, water sorption and subsequent swelling may lead to partial compensation of the shrinkage. The preservation of sealing around a restoration would benefit most if water sorption and setting shrinkage could proceed simultaneously. However, water sorption occurs only at a later stage compared with setting shrinkage.18 Currently, no information is available regarding interfacial-gap formation behavior around Class I restorations of a C-GIC.

In the oral environment, C-GICs must also withstand masticatory and parafunctional stresses. These stresses vary markedly in clinical situations. Thus, thresholds in the mechanical properties needed for success may vary considerably from case to case, with stronger C-GICs being required where greater stresses are anticipated. Flexural tests are appropriate for assessing the mechanical properties of restorative and luting cements.91316 In previous studies, C-GICs were proposed to improve their marginal seal by enhancing their flexural strength and bonding ability after 24-hour water-storage.13 Appropriate elastic moduli and proportional limit values are also desirable.16

Therefore, this study evaluated both gap-formation integrity around butt-joints in model restorations, analogous to Class I restoration of HV-GICs and determined the early development of their flexural and adhesive properties, compared with those of a C-GIC. An important clinical variable was to be assessed in this connection; namely, the effect of an immediate versus a 24-hour-delayed finishing procedure on these properties. Hence, a major hypothesis to be tested was that premature finishing would significantly reduce interfacial integrity relative to delayed finishing. Several additional properties, including shear-bond-strength, were also to be measured to further elucidate the effects of water-uptake over 24 hours upon intrinsic and interfacial material behavior and to discriminate between the different types of material.

METHODS AND MATERIALS

The basic properties of three HV-GICs and one C-GIC, as a control, are summarized in Table 1. Human pre-molars, extracted for orthodontic reasons, were used in this experiment. After extraction, each tooth was immediately stored in distilled water at 4°C for one to two months before use.

Table 1 Restorative Materials and Conditioner Agents Investigated

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Four C-GICs, which were placed according to the manufacturers' instructions, were investigated (Table 1). Dentin Conditioner was applied for 20 seconds and rinsed with water. Ketac Conditioner was applied for 10 seconds and rinsed with water. The cavity was filled with mixed GIC using a syringe tip (Centrix C-R Syringe System, Centrix, CT, USA) covered with a plastic strip and stored in an incubator at 37°C and 100% relative humidity for four minutes after mixing as setting procedure. The restored teeth were then coated with a varnish (Fuji Varnish, GC, Tokyo, Japan).

Gap-Formation Around Class I Restorations

Preparing and Polishing Procedures

A Class I cavity, having a length of 3.5 mm, a width of approximately 2 mm and a depth of 1.5 mm, was prepared on the human premolar surface using a tungsten carbide bur (200,000-rpm) and a fissure bur (8,000-rpm) under wet conditions (Figure 1). The cavosurface walls were finished to a butt joint. This design differed from a Class I clinical cavity in that the cavity corners were geometric-box angles used to prepare a constant-volume model. One cavity was prepared in each of 80 teeth (4 materials × 2 polishing or inspecting times 10× repeats = 80). The surfaces of the designated restorations were polished immediately after setting, using abrasive points (Silicone Mide, Shofu, Kyoto, Japan), while rinsing with distilled water to avoid desiccation and breakdown. The other designated specimens were stored after setting in distilled water at 37°C for 24 hours. The restoration surfaces were then polished, as described above.

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Inspection Procedures

Each tooth was sectioned in a buccolingual direction through the center of the restoration using a low-speed diamond saw (Isomet, Buehler Ltd, Lake Bluff, IL, USA). The presence or absence of marginal gaps was measured at 14 points (each 0.5-mm apart) along the cavity restoration interface (n=10; total points measured = 140) using a traveling microscope (1000×, Measurescope, MM-11, Nikon, Tokyo, Japan) (Figure 1). The number of gaps in each position was totaled and expressed as a sum for each sample.14–1517

RESULTS

Table 2 summarizes the interfacial gap formation observed in Class I specimens with three HV-GICs and a normal C-GIC (as a control), when the specimen was polished immediately after light-activation and after delayed polishing. For all materials, the sum of the gaps was significantly less with delayed polishing compared to immediate polishing.

Table 2 Effect of Polishing Time on Gap Formation Around Class I Restorations

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Table 3 summarizes the marginal gap width among the four GICs and Teflon molds under two conditions. The two columns represent the linear (diametral) setting shrinkage-strain immediately after setting and after one-day storage. The data for polishing after one-day storage was significantly better compared with immediate polishing. With both the immediate and after one-day stages, the values of Fuji IX GP and GlasIonomer FX-II were significantly less than for Fuji II.

Table 3 Effect of Polishing Time on Marginal Gap-width in a Teflon Mold (micrometer)

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Table 4 Shear Bond Strength to Enamel Surface (MPa) Immediately After Setting and One-day Storage

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Table 5 summarizes the shear bond strength to the dentin surface and mode of fracture, respectively. Immediately after setting, the value of the shear bond strength of Fuji IX GP and Ketac-Molar were significantly less than Fuji II. After one-day storage, the value of shear bond strength was not significantly different among the three C-GICs, except for GlasIonomer FX-II. The data for polishing after one-day storage were significantly better compared with that for polishing immediately. For all groups, no significant differences in fracture mode were observed between immediate and 24 hour polishing.

Table 5 Shear Bond Strength to Dentin Surface (MPa) Immediately After Setting and After One-day Storage

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All the materials showed significantly higher flexural strength after one day than immediately after setting (Table 6).

Table 6 Flexural Strength (MPa) Immediately After Setting and After One-day Storage

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Table 7 summarizes the flexural modulus under two conditions. The tendency of the results was similar to flexural strength—increasing after storage. All the materials showed significantly higher value than when the specimens were measured immediately after setting.

Table 7 Flexural Modulus (GPa) Immediately After Setting and After One-day Storage

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DISCUSSION

This study used a model cavity for the geometry of typical Class I cavities. This approximates only the Class I morphology and is not the typical morphology for a C-GIC; it has the advantage of a constant volume and reproducible geometry that is beneficial for an in vitro scientific study.

This study demonstrated that polishing four GICs should not be performed immediately after the filling and setting procedure; instead, polishing should be delayed to a later time to prevent interfacial gap-formation between the material and the Class I cavity. In contrast to the marginal gap of approximately 80–100 gaps when the specimen was polished immediately after setting, the gap was near zero when the specimen was polished after storage in water for one day. GICs shrink during the setting reaction, interfacial gaps form an adhesion between the tooth-cavity and glass-ionomers do not resist shrinkage-stress.1320–21 However, after one-day water storage, shrinkage-stresses of the materials are effectively compensated for or even converted into expansion-stress due to water uptake and swelling.1–2418 Water absorption of C-GICs reportedly affects cavity adaptation and reduces microleakage.313 Although hygroscopic expansion may not be sufficient to compensate for setting shrinkage, it plays an important role in reducing shrinkage caused by the cement setting reaction and thus improves interfacial gap-formation in the restoration.14–15

With respect to polishing condition, the marginal gap measured using the Teflon mold showed a similar pattern to that obtained using the Class I restoration, as mentioned above. The marginal gap observed even after the specimen was stored in water for one day indicated that hygroscopic expansion did not fully compensate for setting-shrinkage.

The bond-strength, flexural-strength and flexural-modulus of one-day storage were significantly higher than what was measured immediately for all C-GICs, and inter-relationships have been reported previously.31322 As expected, cements show higher bond and mechanical strengths when fully set than during the setting reaction. Also, the pH, an index of the extent of the hardening reaction for GICs, gradually increases for 24 hours.1–223 Therefore, it can be presumed that completion of the setting reaction of a GIC requires at least 24 hours.

After one-day storage, an HV-GIC (Fuji IX GP) performed significantly better than its corresponding conventional C-GIC (Fuji II). Increasing the powder-liquid ratio is the primary reason for improving these results, as the two C-GICs are otherwise very similar. This improvement is achieved by a reduction in the size of the glass particle. However, GlasIonomer FX-II and Ketac Molar Aplicap did not clearly show this pattern. This may be explained by differences in density, distribution or content of the powder, and the polyacrylic or maleic acid concentration or molecular weight of polyacrylic or maleic acid of the liquid. A number of variations led to an HV-GIC with improved physical properties.24

CONCLUSIONS

For C-GICs, investigating interfacial gap-formation after 24 hour storage had value. The greater interfacial integrity of GICs resulted from harmony between good bond-strength, low-setting shrinkage or possibly some hygroscopic expansion. With HV-GICs, it is thus generally advisable to adjust the occlusion immediately after initial setting and perform a final contouring and finishing using the delayed polishing procedure. It is thought that an HV-GIC is the most useful and significant restorative material for some pediatric or geriatric patients.

A more extensive approach to the evaluation of sealing efficacy with C-GICs would require longer-term durability testing or load cycling.