An In Vitro Assessment Of Anticariogenic Property Of Three Fluoride Containing Restorative Materials Using Artificial Caries Model

INTRODUCTION

One of the main aims of clinical dentistry is to provide restorations which seal the margins of cavity preparation against ingress of various kinds of irritants from oral environment. Most restorative materials fail to adapt intimately with the tooth structure to provide a leak proof seal. Even if restorative material adapts well at the time of insertion, gaps form around it during the life of restoration leading to microleakage. This can cause secondary caries, post-operative sensitivity, pulpal injury, discoloration of tooth structure and break down of the restorative material.

Reducing or preferably elimination of microleakage is therefore an important step in reducing the incidence of secondary caries. Elimination of microleakage in a clinical situation is practically impossible, so our focus of attention should be directed towards development of restorative materials with anti-cariogenic property.

It is well established that fluoride is effective in preventing development of dental caries¹ .Glass ionomer cement was introduced in 1972 which possess anti-cariogenic property. The anti-cariogenic property of Glass ionomer cement is attributed to its long term fluoride release. However owing to the limitations of Conventional Glass ionomer, while still preserving its clinical advantages, researchers have developed Resin modified Glass ionomer and Poly acid modified resin composite materials.

Secondary caries is recognized as one of the three major causes for replacement of composite restorations². So, several attempts have been made to incorporate fluoride in composite resin to impart anti-cariogenic property. Several in vitro studies have shown that fluoride releasing composites inhibits enamel demineralization³, but few studies have shown inconsistent results⁴.

There are several in vitro studies available in the literature, which assess the anti-cariogenic property of restorative materials. These studies measure either, the fluoride release by the restorative material or fluoride uptake by the tooth structure, but do not consider the aspect of micro-leakage. So, in-vitro artificial caries model would be regarded as the most useful test to study the anti-cariogenic property of restorative materials in situ which can assess the combined effects of both microleakage and fluoride release and uptake.

The aim of this in vitro study is to compare the ability of fluoride containing composite resin, resin modified glass ionomer, ployacid modified resin composite to inhibit caries at restorative margins using acidified gel technique to create caries like lesions around restorations.

METHODOLOGY

Twenty five freshly extracted human maxillary premolars, free of caries, restorations and other defects were selected for this study.

Preparation of the samples:

All the teeth were cleaned with aqueous slurry of pumice using a hand piece and rubber cup. For each tooth, two Class V cavities were prepared, one each on the buccal and lingual surface near the cementoenamel junction. Both the cavities in each tooth were restored with the same restorative material. The occlusal cavo-surface margin was placed on enamel and gingival cavo-surface margin was placed on the root surface. The approximate dimensions of the prepared cavity were: 3mm in mesiodistal direction, 1.5mm in occluso-gingival direction and 1.5mm in depth. Teeth were then randomly divided into five groups comprising of five teeth each. Teeth were then restored with five different restorative materials as mentioned below according to manufacturer's instructions

Group I: Conventional Glass ionomer (Fuji II)- (G C Corporation ,Japan)

Group II: Composite resin without fluoride content (Z-100)- (3M ESPE Dental Products)

Group III: Composite resin containing fluoride (Tetric Ceram) - (Ivoclar Vivadent)

Group IV: Resin modified Glass ionomer (Vitremer)- (3M ESPE Dental Products)

Group V: Polyacid modified resin composite (Dyract) - (Dentsply India Pvt Ltd)

Group I acted as a positive control and Group II acted as a negative control

All restored teeth were stored at 37^oC in 100% relative humidity for 24 hrs to ensure maximum polymerization of the restorative materials. Restorations were finished and polished with Shofu finishing and polishing kit (Shofu Inc. Japan). Teeth were then subjected to thermocycling for 800 cycles between 5^o and 55^o C with a dwell time of 30 seconds.

Subsequently all restored teeth were painted with an acid resistant nail varnish to within 1mm of the cavo-surface margin surrounding the restorations. Teeth were then immersed in jars containing acidified gel for a period of 5 weeks to induce caries like lesions around restorations. The gel contained 10% methyl cellulose, 0.1M lactic acid and pH was adjusted to 4.5 by addition of potassium hydroxide⁵ .

Microscopic observation:

After 5 weeks teeth were removed from the acidified gel and were then sectioned longitudinally through the center of the restoration using a diamond disc .The sections were ground to a thickness of approximately 80-100µ in a hard tissue microtome. After 24 hours of imbibition in water, the sections were mounted on glass slides and observed under polarizing microscope for caries like lesions both at enamel and dentin margins. Measurements of depth and length of lesions were made using a calibrated eyepiece reticule.

Evaluation criteria [Fig 1]:

The caries like lesions which were formed at the enamel restoration interface consisted of two parts; they are outer surface lesion and cavity wall lesion. Additionally there was formation of dentin lesion near the dentin restoration interface. Measurements of these lesions were made as mentioned below:

1. Outer lesion depth was measured as largest distance between the enamel surface and inner most border of the lesion

2. Wall lesion depth was measured as largest distance between the restoration tooth interface and inner most border of the lesion

3. Wall lesion length was measured from the enamel surface to the innermost extended portion of wall lesion along the tooth restoration interface

4. Dentin lesion depth was measured as largest distance between the outer dentin surface and the innermost border of the lesion

RESULTS

The sectioned teeth were viewed under polarizing microscope and measurements were made using calibrated eye piece reticule. The values obtained were then subjected to statistical analysis namely

1. Analysis of Variance - ANOVA

2. Student Neuman Keul test

Outer lesion depth:

The mean outer lesion depth ranged from 138µ for teeth restored with conventional Glass ionomer (Fuji II) to 275 µ for teeth restored with Composite resin without fluoride (Z-100)

The mean outer lesion depth of teeth restored with Conventional Glass ionomer (Fuji II) was significantly lesser than those for teeth restored with Composite resin containing fluoride (Tetric Ceram), Composite resin without fluoride (Z100) and Polyacid modified resin composite ( Dyract)

There was no statistically significant difference in mean outer lesion depth in teeth restored with Conventional Glass ionomer (Fuji II) and Resin modified Glass ionomer (Vitremer).

Wall lesion:

There was no wall lesion formation in teeth restored with Conventional Glass ionomer (Fuji II) and Resin modified Glass ionomer (Vitremer).

The wall lesion depth ranged from 58 µ for teeth restored with Polyacid modified resin composite (Dyract) to 93 µ for teeth restored with Composite resin without fluoride (Z100).

The wall lesion length ranged from 219 µ for teeth restored with Polyacid modified resin composite (Dyract) to 263 µ for teeth restored with Composite resin without fluoride (Z100).

There was no statistically significant difference in wall lesion depth and wall lesion length in teeth restored with Composite resin without fluoride (Z100), Composite resin containing fluoride (Tetric Ceram) and Polyacid modified resin composite (Dyract) .

Dentin lesion depth:

The mean dentin lesion depth ranged from 225 µ for teeth restored with Conventional Glass ionomer (Fuji II) to 388 µ for teeth restored with Composite resin without fluoride (Z100).

The mean dentin lesion depth in teeth restored with Conventional Glass ionomer (Fuji II) was significantly higher than those for teeth restored with Composite resin without fluoride (Z 100).

There was no statistically significant difference in mean dentin lesion depth when other groups were compared.

DISCUSSION

The aim of the present study was to compare the ability of fluoride in Composite resin, Resin modified glass ionomer and Poly acid modified resin composite to inhibit caries at restorative margins using acidified gel technique to create caries like lesions around restorations.

Anatomical location of secondary caries lesions in permanent teeth clearly indicate that cervical and proximal areas are more vulnerable than occlusal or incisal areas⁶ . Hence in the present study, Class V cavities were prepared near the cementenamel junction with occlusal cavo-surface margin on enamel and gingival cavo-surface margin on root surface.

Artificial secondary caries like lesions have been produced in vitro, using either bacterial culture or chemical systems. Various chemical systems used for the formation of artificial caries like lesions are Acidified gelatin gel, Acidified methyl cellulose gel, Acetic acid-sodium acetate buffer and diphosphate buffer system. In this study acidified methyl cellulose gel was used because, it is efficient in creating caries like lesions at rates comparable to those occurring in vivo and utilizes a gel medium with organic and inorganic elements as a substitute for plaque occurring in vivo⁵.

The lesions produced by artificial caries technique consist of two parts, an outer surface lesion and cavity wall lesion. The outer surface lesion occurs because of cariogenic attack on the tooth surface and typically has the characteristic features of primary caries. The wall lesion forms at the tooth restoration interface. It is thought that wall lesions are formed by diffusion of hydrogen ions from an artificial caries medium into the microspace between the cavity wall and the restorative material. The wall lesion once formed progresses perpendicular to the cavity wall. Therefore caries challenge may be quantified by measuring the depth of cavity wall lesion perpendicular to the interface between the restorative material and the cavity wall⁷.

In the present study Conventional Glass Ionomer (Fuji II) was selected as a standard (Positive control) to compare with the anti-cariogenic property of other restorative materials. This is because of the proven anti-cariogenic effect of Glass ⁸ Ionomer.

On comparative evaluation , in the present study it was revealed that Conventional Glass Ionomer (Fuji II) and Resin modified glass ionomer (Vitremer) had complete inhibitory effect on the development of wall lesion and decrease in the depth of outer lesion and wall lesion when compared with Composite resin containing fluoride(Tetric Ceram) and Polyacid modified resin composite (Dyract). This may be due to high initial fluoride release, fluoride uptake by enamel and dentin and lesser marginal leakage around the restorations⁹.

The higher magnitude of outer lesion depth of Resin modified glass ionomer (Vitremer) compared to Conventional Glass Ionomer (Fuji II) may be due to decreased fluoride release of Resin modified glass ionomer (Vitremer)¹⁰. It can be speculated that hydrophilic molecules like HEMA in Resin modified glass ionomer could favor the absorption of enough water to allow for fluoride ion diffusion that would otherwise be prevented, because the ions are firmly trapped in the polyalkenoic matrix. The resin network reduces the diffusion of water into the cement and thus the release of unbound fluoride from the material matrix.

Polyacid modified resin composite (Dyract) and Composite resin containing fluoride (Tetric Ceram) were not fully effective in preventing the development of experimental wall lesion. Even though there was no statistically significant difference between the two, Polyacid modified resin composite (Dyract) seemed to perform better when the mean values of the lesions were considered. The decreased inhibitory effect of Polyacid modified resin composite (Dyract) on the development of experimental wall lesion and the outer lesion may be because of lesser fluoride release than Conventional Glass Ionomer. The material takes up water with time and carboxylic groups of acidic monomers can undergo acid base reaction with metal ions of glass fillers. This in turn leads to formation of carboxylate salts and release of fluoride. It seems that thus the reaction is slow and results in a low fluoride release^10,11.

Composite resin containing fluoride (Tetric ceram) did not have a favorable inhibitory effect on the development of wall lesion and outer lesion. This may be due to the bonding agent blocking the transfer of fluoride ions from the restorative material to the tooth structure. Also the amount of fluoride released from the composite resin was not sufficient to inhibit secondary caries^4,10.

Composite resin without fluoride content (Z100) which acted as a negative control showed the highest magnitude of experimental wall lesions. The reason behind this observation may be because of absence of fluoride release and gap formation around the restoration¹².

When dentin lesions were compared, there was no statistically significant difference between the groups except composite resin without fluoride content (Z-100). It has been shown that anti-cariogenic effect of glass ionomer was not as great for radicular as for coronal surface and when photo-activated materials are cured, there will be shrinkage of material towards enamel which has a higher bond strength than dentin creating gaps along tooth restoration interface. This may be the reason for higher magnitude of lesions in dentin¹³.

The ability of these fluoride releasing restorative materials to resist caries at the tooth restoration interface would appear to be of great importance in prevention of secondary caries. Therefore, fluoride release and incorporation data alone cannot determine the anti-cariogenicity of a restorative material. The ability of a restorative material to resist microleakage has to be considered, which potentially imparts acid resistance to tooth structure. Also this in vitro data suggests the need for well controlled clinical trial to further evaluate the clinical effectiveness of these restorative materials.

CONCLUSION

It can be concluded that

1. The anticariogenic property of Conventional Glass Ionomer (Fuji II) was superior to all the other restorative materials investigated in this study

2. The ranked efficacy of anticariogenic property of the restorative materials investigated in this study is Conventional Glass Ionomer (Fuji II) ˃Resin modified Glass Ionomer Vitremer) >Polyacid modified resin composite (Dyract) > Composite resin containing Fluoride ( Tetric Ceram ) > Composite resin without fluoride content ( Z 100).