Comparative evaluation of microhardness of dentin treated with 4% titanium tetrafluoride and 1.23% acidic phosphate fluoride gel before and after exposure to acidic pH: An ex vivo study.

AIM: The aim of this study was to comparatively evaluate the effect of 4% titanium tetrafluoride (TiF4) and 1.23% acidic phosphate fluoride (APF) gel on the microhardness of human coronal dentin.
MATERIALS AND METHODS: Thirty noncarious extracted premolars were collected and sectioned buccolingually with the help of diamond disk. Exposing the sectioned surface, teeth were embedded in self-cure acrylic. Exposed coronal dentin was polished with abrasive papers starting with 220-5000 grit. Microhardness was evaluated by Vickers microhardness evaluator, at four different stages as follows - stage 1: Baseline values, Stage 2: Exposure of specimens to acidic environment at a pH 1 for 5 min, Stage 3: Application of 1.23% APF gel and 4% TiF4 (after dividing the specimens into two groups, i.e., Group A and B, respectively), and Stage 4: Followed by exposure of fluoridated specimens to acidic protocol as mentioned above.
RESULTS: Paired t-test was used to compare the readings between Groups A and B. Group B has shown greater resistance to decrease in microhardness of coronal dentin (P < 0.05) on exposure to acidic protocol.
CONCLUSION: Due to acidic pH (1.5) of 4% TiF4, amount of increase in microhardness of dentin is <1.23% APF gel. 4% TiF4 was more effective in resisting demineralization than 1.23% APF gel.

INTRODUCTION

In recent years, much focus has been placed on extrinsic causes of dental erosion, such as acidic dietary beverages and citric and phosphoric acids which are often added to soft drinks. Only few studies have focused on the erosive effect of intrinsic acids such as gastric juices even though they have a higher erosive potential than soft drinks.[1]

Prolonged exposure to gastric content can cause dental erosion and in severe cases result in pathological tooth wear with loss of both function and esthetics. It has been shown that people suffering from eating disorders or gastric reflux are at increased risk of developing dental erosion.[1] Hence, the use of procedures such as fluoride rinses and varnishes has been recommended; however, these are presentably ineffective because at low pH fluorapatite gets dissolved.[2]

It was thought for many years that fluoride acted basically by its incorporation in the hydroxyapatite lattice and the reduced solubility of the so-formed fluoridated hydroxyapatite. On the contrary, it was shown that shark enamel, which consists of solid fluorapatite, also developed carious lesion in an in situ experiment in high caries challenge regimen.[3] Later, studies revealed that it was not the intrinsic fluoride content of tissues but rather the loosely bound fluoride present at the tooth–oral fluid interface, which was responsible for effective inhibition of lesion formation.[4] Erosion progression can be diminished by application of fluoride preparations,[5] such as acidic sodium, stannous, or amine fluoride preparations. The principle of its protective effect is the precipitation of CaF2-like product on the tooth surface. One major disadvantage of this precipitate is that they readily get dissolved in acidic solutions. Unlike above-mentioned agents, titanium tetrafluoride (TiF4) has shown to offer greater protection against caries progression and erosion. The unique interaction of TiF4 with tooth structure leads to the formation of a resistant tenacious coating (referred as a glaze-like layer) on the tooth surface.[6] The advantage has been credited to the titanium group present in the compound, which synergizes the effect of fluoride.[7]

Reed and Bibby's[8] preliminary test in children showed that the topical application of TiF4 (1%) was more effective in reducing enamel solubility and in preventing caries than the application of acidic phosphate fluoride (APF). These results along with stability, lack of irritating properties, and nontoxicity of titanium suggested its use in human beings.

Büyükyilmaz and Sen[9] observed that the glaze formed after application of 4% TiF4 on the occlusal surface of deciduous enamel retained in the area of pits and fissures even after a period of 12 months despite the masticatory and abrasive forces in the oral cavity.

In advanced stages of gastroesophageal reflux disorders, dentin gets exposed due to erosion. Hence, in our present study, the effect of fluoridating agents on dentin has been evaluated. The study described here sought to comparatively investigate the effect of 1.23% APF and 4% TiF4 on human coronal dentin using Vickers microhardness test.

MATERIALS AND METHODS

Thirty noncarious extracted premolars were collected and stored in distilled water after cleaning them out of their debris. Teeth were sectioned buccolingually with the help of diamond disk (Confident Dental Equipments Ltd., India) in a micromotor straight handpiece (NSK, Nakanishi Inc, Tochigi-ken, Japan), at a speed of 20,000 rpm, using water spray as a coolant. Among the two halves of the buccolingually sectioned tooth, the one with less surface aberrations on dentin was selected to prepare the sample. Exposing the sectioned surface, teeth were embedded in self-cure acrylic, surrounded by a plastic pipe (sectioned to required thickness before the procedure) to give the specimen correct shape. Exposed coronal dentin was polished with abrasive papers starting with 220–5000 grit (coarse grit to fine grit). Then, specimens were stored in a closed container in a dry environment to preserve the surface fineness. As a part of the evaluation of microhardness of coronal dentin, four indentations were given 100 μm apart on each specimen by Vickers microhardness indenter and average of four indentations was calculated to minimize the error. The load of 500 g was applied for 10 s. These readings stand as baseline values of microhardness of dentin. All the specimens were treated with hydrochloric acid at a pH of 1 for 5 min, followed by the evaluation of microhardness of treated coronal dentin. All the thirty samples were divided into two groups:

Group A - Fifteen samples in the group on coronal dentin were applied carefully with 1.23% APF gel (Dentsply, York, PA, USA) as an approximately 2 mm thick layer in one application, with the help of the cotton pellet by tweezers and were left undisturbed.[10] After 4 min, it was gently removed with the help of smooth absorbent paper, followed by gentle rinsing with water and air dried

Group B - Fifteen samples in the group on coronal dentin were applied carefully with freshly prepared 4% TiF4 solution (Sigma-Aldrich Co., St. Louis, MO, USA). It was prepared by dissolving 3.4 g of TiF4 powder in 100 ml deionized distilled water. The pH of fresh solution was 1.5. A solution of 4% TiF4 was applied in drops for 4 min, the drop was left undisturbed till surface appeared dry, and the additional drops were applied in the same manner.[11] After 4 min, the specimens were gently rinsed with water spray and air dried.

Change in microhardness of the dentin in both Groups A and B after application of 1.23% APF gel and 4% TiF4, respectively, was evaluated. It was followed by exposure of both Group A and B specimens to the acidic protocol as mentioned above, and microhardness readings were recorded to evaluate the ability of 1.23% APF gel and 4% TiF4 to resist demineralization in an acidic environment.

RESULTS

Table 1 represents the result of paired t-test between Groups A and B after application of APF gel and 4% TiF4 to demineralized dentin.

Table 1

Represents result of paired t-test between group A and group B after application of APF gel and 4% TiF4 to demineralized dentin

Table 2 represents the result of paired t-test between Groups A and B after exposure of 1.23% APF gel- and 4% TiF4-treated dentin to an acidic environment.

Table 2

Represents result of Paired T-Test between Group A and Group B after exposure of APF gel and 4% TiF4 treated dentin to Low pH

Statistical analysis was performed using (GraphPad Prism, version 5, La Jolla, CA, USA). Paired t-test was used to compare the Vickers microhardness values between Groups A and B. Group A has shown greater increase in microhardness of coronal dentin (P < 0.001), followed by application of APF gel than Group B. Greater resistance to decrease in microhardness of coronal dentin (P < 0.05) followed by exposure to acidic protocol was shown by Group B.

DISCUSSION

According to Buyukyilmaz, 4% TiF4-treated enamel surface is more resistant to crack formation and the glaze appears to be more tenacious and thicker,[6] than with 1% TiF4. Hence, in the present study, 4% TiF4 was used.

Few studies that evaluated the impact of TiF4 solution on dentin in vitro[1213] have confirmed it was effective in reducing erosive dentin loss.

Jones et al.[14] have demonstrated that concentrated fluoride gels were able to provide some degree of protection to enamel against endogenous erosion. Mok et al.[15] have demonstrated that these fluoride gels have shown the same level of protection against exogenous erosion. In both the studies, APF gel has shown the highest level of protection.

Wefel and Wei have stated in their study that 4-min application time of 1.23% APF gel is better over 1-min application time.[16] While Ten cate JM et al[17] have found no difference in application time of 1.23% APF gel. However, in the case of 4% TiF4, various time periods of application have been used. Reed and Bibby,[8] Von Rijkom et al.,[18] and Exterkate and Ten Cate[19] have considered 1-, 4-, 5-min application time, respectively, in their studies to evaluate the effect of TiF4. Hence, to standardize the application time of both fluoridating agents in the present study, 4-min application time was considered.

In the case of endogenous erosion, the teeth exposed to acids from the gastrointestinal tract are most likely intermittent and for short periods but, on the other hand, may last for weeks or months. Thus, the present direct acid exposure model is presumably more severe than exposure in the oral cavity.[6]

The relationship between microhardness and mineral structure of teeth has been documented as directly proportional. Angker et al.[20] stated that mechanical properties of dentin are dependent on its mineral content.

Microindentation hardness testing can be done using Vickers as well as Knoop indenters. Lips and Sack described the first Vickers tester using low loads in 1936.[21] The hardness number is based on the surface area of the indent itself divided by the applied force, giving hardness units in kgf/mm2. Microindentation tests typically have forces of <2 N and produce indentations of about 50 μm. In addition, microhardness values vary with load and work-hardening effects of materials.[22] The depth of the indentation is about 1/7th the diagonal length.[23]

The indentation has two measurements of the length of the diagonal (d1 and d2) which are measured in an optical microscope and reading of microhardness appears on the screen. Vickers hardness is a measure of the resistance of material to deformation. Vickers indenter is recommended for resilient materials (like dentin).[21] Hence, Vickers microhardness test was performed in the study.

Results obtained were statistically evaluated by paired t-test; all at P < 0.05 to find statistical significance of microhardness values. A t-test is any statistical hypothesis test in which the test statistic follows a Student's t-distribution if the null hypothesis is supported. It can be used to determine if two sets of data are significantly different from each other and are most commonly applied when the test statistic would follow a normal distribution if the value of a scaling term in the test statistic was known.

Comparison of results obtained between Groups A and B after application of 1.23% APF gel and 4% TiF4 solution to demineralized dentin has shown statistically significant (P < 0.001) difference in the increase of microhardness. The amount of increase in microhardness of Group B less than Group A was probably due to low pH of 4% TiF4 solution (pH - 1.5). Slight demineralization of dentin surface following topical 4% TiF4 application was previously reported.[24] However, the mineral loss was considered partial and limited to outermost 8–10 μm.[6] On the other side, APF gel has a pH range of 3–3.5, which is comparatively less acidic to 4% TiF4, thereby causes less surface mineral loss.

Comparison of results obtained between Groups A and B after exposure of 1.23% APF gel- and 4% TiF4-treated dentin to the acidic environment has shown statistically significant difference in the decrease in microhardness. The results confirmed that 4% TiF4- and 1.23% APF gel-treated dentin was resistant to demineralization in an acidic environment. However, 4% TiF4 has shown better results than 1.23% APF gel. This might be due to glaze-like layer formed on the surface of dentin, which is a product of interaction between 4% TiF4 and proteins on the surface of dentin.[25] Another explanation for this acid-resistant glaze-like layer includes Ti ion reacts with oxygen groups on the tooth surface, i.e., phosphate-bound oxygen.[26] This mechanism would provide covalently bound titanium on the enamel surface properties of the glaze. This is probably the best explanatory model for glaze formation available at present. The protective action of 1.23% APF gel is the precipitation of CaF2-like product on the eroded tooth surface and its major disadvantage is that they possibly readily dissolve in acidic solution, so the effectiveness of 1.23% APF gel in preventing dentinal erosion is limited.

CONCLUSION

Within confines of the study, it can be stated that both 1.23% APF gel and 4% TiF4 solution are effective in resisting the demineralization of dentin in acidic environment

However, 4% TiF4 has shown better efficacy than 1.23% APF gel.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.