The main advantage of glass-ionomer cements is their ability to self-adhere to enamel and dentin. The auto-adhesion of glass-ionomer to tooth tissue is thought to be two-fold in nature: first, micro-mechanical interlocking by a shallow hybridization of the hydroxyapatite-coated collagen fibril network (so, their adhesion to the tooth tissue can be considered as a kind of self-etch approach); and second, true primary chemical bonding through formation of ionic bonds between the carboxyl groups of the polyalkenoic acid and calcium of hydroxyapatite that remained around the exposed surface collagen (1). Therefore, an intermediate zone (ion-exchange layer) of calcium and phosphate formed on the interface between the cement and the tooth exists (2,3).

The adhesion to enamel is more straightforward than the adhesion to dentin. The morphology of the dentinal tissue involves presence of dentinal tubules filled with fluid, so water molecules can easily transudate through the interface with the material. This phenomenon of localized water sorption has been extensively observed in self-etch adhesives and resin-modified glass-ionomer cements (4-15). For example, Sano et al. (16) and Li et al. (17) described the appearance of “nanoleakage” at the basis of the hybrid layer when dentin adhesives are applied. They stated that the incomplete penetration of the adhesive system through the collagen network creates a region of demineralized dentin at the bottom of the hybrid layer and the adhesive union therefore becomes vulnerable, since degradation of the collagen fibrils that are not incorporated into the adhesive system appears (16, 18-20). On the other side, the self-etching adhesives demineralize the dentin in much less extent, but the nanoleakage studies confirm that the liquid still moves along the interface of the adhesive and the dentin (16,17).

Likewise, the water movement across bonded resin-modified glass ionomer cement/dentin interfaces was almost completely unknown until the detection of a 5-15 mm thick, amorphous, non-particulate zone formed in this region (21-23). The formation of this, so-called, “absorption layer” has been attributed to water sorption by HEMA, or the diffusion of HEMA from the resin matrices of the resin modified glass-ionomer cements into the water-rich dentin surface. The subsequent polymerization of the HEMA resulted in the form of a soft poly (HEMA) hydrogel layer. The resin-modified glass ionomer cement absorption layer has been thought to act as a stress-breaking layer and may provide a similar function as a dentin adhesive layer in relieving polymerization shrinkage stresses (24,25).

Only a limited number of studies confirm the water movement in traditional glass- ionomers and resin-modified glass-ionomer cements through a formation of structures entrapped into the air voids of the materials’ matrix (13, 26-28).

So far, much attention has been paid to the processes that appear at the interface between the dental tissues and these materials, as well as in the dentin itself. This study was designed to witness the changes that appear into the conventional glass-ionomer cements, while the surfaces of the cement that were in contact with the artificial saliva were sealed with varnish. Therefore, the null hypothesis tested was that there is no difference in the micro-morphological appearance of the glass-ionomer cement bonded to dentin after different storage time intervals.

 

1. Teeth preparation and restoration

Twenty permanent third molar teeth extracted due to orthodontic reasons at the Clinic for Oral Surgery were used in the examination. After the extraction, the teeth were stored in artificial saliva and used within 1 month after the extraction. The preparation involved ultrasonication and cleaning with polishing toothpaste and pumice. The crown was separated from the radices with diamond bur and high speed handpiece with water cooling at the level of the cemento-enamel junction, and afterwards the remnants of the pulp tissue were discarded. Standard Class V cavities were prepared on the vestibular side of each tooth, using diamond bur and high speed dental handpiece. After the preparation, the teeth were conditioned for 10 seconds with GC Cavity Conditioner and rinsed with water for additional 10 seconds. Then, they were restored with Fuji IX (GC Copr., Japan, batch No. 000152) according to the manufacturer’s instructions, and covered with varnish.

The teeth were stored in artificial saliva prepared according to the British Standards Institution, BS 7115, part 2, BSI, London, 1988, and previously used for dental materials testing. Its composition is given in Table 1. The samples were examined after 1 month and 18 months storage period.

  

2. Sample preparation for SEM

After each storage time interval, the teeth samples were cut by half longitudinally; one of the halves was examined under Scanning Electron Microscope (SEM) to obtain secondary and the second one for backscattered electron images.

For the SEM (backscattered electron mode), the cut surfaces were placed on the bottom of plastic moulds (Buehler®, USA, Batch No. 20-8180) with 32 mm internal diameter. The moulds were filled with resin (Epo-Thin, Buehler®, USA, Batch No.20-8140-032) and cured in a vacuum-desiccator for 1 hour. The curing process continued at room temperature for 24 hours. The sample preparation was finished by grinding with different sizes of carborundum grits down to 1µm diamond. The samples were then carbon-coated (Model S105, Edwards Co., UK) and examined with JEOL JSM 5310LV Scanning Electron Microscope at 350x magnification in backscattered electron mode (20 kV accelerating voltage and 15 mm working distance). The analysis involved Energy Dispersive Analysis with X-rays (EDAX) on JEOL JSM 5310LV, Japan on representative points of interest under the same experimental conditions.

The SEM (secondary electron mode) analysis of the other half of the samples involved desiccation, gold-sputtering (Edwards 150B) and examination under high-resolution Scanning Electron Microscope in secondary electron mode Model Cambridge Stereoscan 360 Scanning Electron Microscope, Cambridge Instruments, Co., UK.

 

 

 

As a result of the desiccation (dehydration) during the preparation of the teeth samples for the SEM, cracks appeared within the polyalkenoate matrix. Since the vacuum for the secondary electron images is higher, the cracks that can be seen at these images were more frequent and larger in comparison to the ones seen in the backscattered mode.

After one month of storage in artificial saliva, numerous air voids were found in the Fuji IX samples. The samples appeared empty, i.e. the ones visualized under backscattered electron mode were filled only with the epoxy-resin used for embedding of the samples, Figure 1.

After 18 months, the situation was completely different. Namely, there was presence of numerous spherical bodies within the air-voids inside the polyalkenoate matrix, Figure 2. Two types of spherical bodies were found: a. hollow and “egg-shell-like” , Figure 3, Figure 4 and b. solid ones, Figure 5. The spherical bodies can be easily distinguished from the angular fluoroaluminosilicate glass particles.

The graph obtained by EDAX of these structures showed that they are made mainly from strontium, but quite high quantities of aluminium and silicon in their composition were also found.

 

Common microscopic observation is the appearance of porosities into glass-ionomer cement matrices that resemble to bubbles and are formed by inclusion of air during the mixing of the material. Once the material sets, these voids become entrapped into the matrix. The presence of these structures is one of the reasons for the low compressive strength found in glass-ionomer cements, which is considered as one of their main disadvantages.

Recently, however, it was noticed that these air voids tend to fill up in function of time (26,28). Our electron microscope images prove that after one month of storage in aqueous medium these voids were empty. Later on, after 18 months, two types of spherical bodies formed: the first ones have only an outer layer which looks like a crust, and the others are solid. Therefore, we have to reject the null hypothesis, which implied similarity between the samples in the different time intervals.

These results are only partially in accordance with the results of Tay et al (27), because they found only “egg-shell-like”, hollow structures in conventional glass-ionomer cements next to dentin surfaces. This may be attributed to the shorter storage time they applied. The presence of solid spherical structures within the air-voids in the material’s matrix was noted only in resin-modified glass-ionomers, and only if the fractured surfaces were on 3mm from the interface (26). We suspect that if the storage time interval is longer, more structures will be formed. Also, the smaller voids can be occluded faster than the bigger ones.

The probable mechanism is initiation of a delayed acid-base setting reaction during the maturation phase of glass-ionomer cements into the air voids of the original matrix. Since the materials were protected with varnish, the water diffusion could only result from movement of the dentinal fluid across the bonded interface from the underlying dentin.

The EDAX analysis of the formed structures found very high quantity of strontium, which is not strange, having in mind the composition of Fuji IX, but our observations imply that there is a tendency for concentration of this element in these formations. This may be detrimental for the material’s strength, because only lower concentrations of strontium may lead to its increase (29).

Along with the strontium, our results found high quantity of silica and aluminium. Previous studies also witnessed the silica-rich peripheral phase of these formations. They found that the composition of these bodies is closer to the structure of the original polyalkenoate matrix than to the siliceous phase which envelopes the fluoroaluminosilicate glass particles (27). The presence of this secondary phase resembles to the silica phase that was observed following the aging of glass-ionomer cements (30,31).

The appearance of these structures is highly important, because this may be an instrument for increasing of the compressive strength found in old glass-ionomer cement restorations, probably as a result of repairing the cracks that spread throughout the matrix and postponing the appearance of new ones by the occlusion of the voids.

The shortcoming of this phenomenon is that the glass-ionomer cements may draw water from the dentin quickly, so it may result in the appearance of the post-operative sensitivity (27,32). This feature may be an explanation for the manufacturers’ recommendations that glass ionomer-based materials should be used on slightly moist dentin.

 

1. Glass-ionomer cements bonded to dentin will persistently take-up water from the underlying moist dentin and will propagate an additional reaction of neutralization.

2. The presence of the spherical bodies indicates that movement of water into the air voids across the interface of conventional glass-ionomer cements exists.

3. These structures are mainly composed of strontium, with some silica and aluminium.

4. The partial or complete filling up of the air voids by the spherical bodies can be regarded as a mechanism which leads to improvement of the compressive strength.

 

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