Effects of different bleaching time intervals on fracture toughness of enamel.

AIM: This study was designed to evaluate the influence of different time intervals of a vital home bleaching procedure on the fracture toughness of bovine enamel.
MATERIALS AND METHODS: Labial aspects of 48 bovine incisors were prepared and stored in artificial saliva. The samples were divided into four groups (n = 12) according to the different time intervals of the bleaching procedure: 0, 2, 4, and 6 weeks. The experimental groups were subjected to the bleaching agent; 15% Opalescence PF according to manufacturer's instructions. The fracture toughness (FT) of enamel was assessed using Vickers hardness indentation. Then, FTs were compared statistically by one-way ANOVA (α = 0.05). RESULTS AND DISCUSSION: Different tested time intervals did not significantly influence the fracture toughness of bovine enamel following treatment with 15% carbamide peroxide (P = 0.05)
CONCLUSION: 15% Opalescence PF after 6 weeks did not reduce the fracture toughness of human enamel compared with the baseline, 2 and 4-week values.

INTRODUCTION

The desire to have white teeth and thus a more pleasant smile has become an important esthetic need of the patients. Various in-office vital bleaching techniques are effective for teeth with generalized discoloration.[1] The successful outcome of any of the applied modalities mainly depends on the etiology, diagnosis, and proper selection of bleaching materials and the correct clinical technique.[2]

Among laboratory studies, surface microhardness (SMH) measurement is a simple method to determine the mechanical properties of enamel and dentin surface and it is not only related to the loss or gain of the mineral content of the dental structure,[3] but also to the composition of the applied product and their pH values and the presence of other components in commercial bleaching agents.[4]

In vitro studies have reported some alterations in the mineral content of enamel.[56] A concern about bleaching with an acidic bleaching solution is possible enamel demineralization, which occurs at a pH value lower than 5.5.[7]

Many studies demonstrated that the peroxide bleaching products had no significant effects on SMH.[8] While Basting et al observed a slight reduction in SMH of human enamel following 8 h/day for 2 days of 10% carbamide peroxide treatment[9] and Araujo et al.[10] in a similar study found a significant reduction in enamel SMH.

Recently, it has been recognized that the different tested light sources did not significantly influence the microhardness of human enamel following treatment with 35% hydrogen peroxide.[11]

Fracture toughness, KIC, is defined as the critical stress intensity level at which a given flaw starts extending and provides insight into the potential resistance to crack growth of a material.[12]

There are controversial results about the effect of bleaching agents on enamel and dentin fracture toughness. Seghi and Denry evaluated the effects of a 10% carbamide peroxide gel on the apparent fracture toughness of human enamel.[13] Fracture toughness of enamel was reduced by about 30% after bleaching for a period of 12 h. The percentage changes of fracture toughness in bleached bovine incisor using 10% carbamide peroxide has been estimated –12.0 ± 4.7% after a 10-day whitening time.[14] White et al. evaluated the effect of Crest White strips hydrogen peroxide treatments on the fracture susceptibility of human enamel.[15] The results of their study confirmed that tooth bleaching did not produce changes in fracture susceptibility of human tooth enamel.

One recent study reported that the whitening agent consists of hydrogen peroxide in combination with hydrochloric acid and ethyl ether (McInnes Solution) caused significant decrease in the enamel microhardness when it was compared with the baseline values. The more frequency of application could create lower values of VHN.[16]

The aim of this study was to evaluate the fracture toughness of bleached enamel after different time intervals that has not been assessed. The null hypothesis tested in this study was that the increased bleaching time had no effects on fracture toughness of bovine enamel.

MATERIALS AND METHODS

Forty-eight freshly extracted bovine incisors were stored in 1% thymol solution until used. The teeth were embedded in acrylic resin (Palavit G, Kulzer Wehrheim, Germany). The central aspect of the labial surface was ground flat and polished with water-cooled carborundum discs (4000 grit; water proof silicon carbide paper Strues, Erkrath, Germany) removing approximately 200 μm of the outermost enamel layer.[14] The thickness of the enamel was controlled with a micrometer (Mitutoy, Tokyo, Japan). The bottom of the embedded specimens was aligned parallel to the polished surface with a thermoplastic material applied to the bottom of the sample. Prior to the experiment, all specimens were stored for 10 days at 37 °C in artificial saliva[17] that was renewed every day.

The specimens were then randomly assigned to four groups (n = 12) according to the time intervals of the bleaching procedure using 15% carbamide peroxide; Opalescence PF (Ultradent products Inc, South Jordan, UT USA) as described below:

Control

2 weeks

4 weeks

6 weeks

A home bleaching technique was conducted 8 h daily in a humid atmosphere at 37 °C. The bleaching agent was applied on the dried enamel surfaces. After bleaching and before retransferring to saliva, the bleaching substance was carefully removed with a soft toothbrush under tap water for 3 min.[10] After polishing the specimens with a rubber cup, the fracture toughness was assessed. It should be noted that hardness testing can only be performed on a flat highly polished surface.[18]

The Vickers hardness indentation was applied using a 9.8 N load hardness testing device (Typ 3212001, Zwick, Ulm, Germany). In the Vickers hardness test, a 136° diamond pyramid-shaped indenter is forced into the material with a definite load application.[19] The Vickers hardness indentation and fracture toughness were performed in one area in the central aspect of the exposed enamel surface.

For enamel fracture toughness evaluation, the diagonal length of the Vickers hardness indentation was first measured and the hardness calculated according to the following formula by Lawn and Marshal.[20]

Fracture toughness = 0.016 × (E/H) 1/2 × P/C3/2

where H is the Vickers hardness (GPa); C is the maximum lateral length of the fracture line (m); P is the applied load (MN), and E is the Young modulus (GPa).

The fracture toughness of groups was analyzed by one-way ANOVA (α = 0.05). Since there was no difference among experimental groups, the post hoc test has not been followed. All statistical analyses were performed with SPSS, version 16.0.

RESULTS

The fracture toughness values are presented in Table 1. The one-way ANOVA showed no significant differences among groups (P = 0.05).

Table 1

The mean, standard deviations, and the result of the one-way ANOVA in each group.

DISCUSSION

The hypothesis of this study was accepted because there was no significant difference among groups. In this study, a model comprising periods of treatment using 15% carbamide peroxide; Opalescence PF and remineralization in artificial saliva were utilized, attempting to simulate physiological conditions during bleaching procedures. During the experiment, the enamel specimens were kept in artificial saliva, which has been proven to act as an effective agent in rehardening the softened either human,[16] or bovine enamel.[21] One recent review by Attin et al. showed that a significant higher number of bleaching treatments resulted in enamel microhardness reduction when artificial instead of human saliva was used for storage of the enamel samples in the intervals between the bleaching applications.[14] However, the use of human saliva was less associated with microhardness reduction as compared to artificial saliva, although artificial saliva would be able to reharden the surface-softened enamel.[16] It should be noted that fluoridated carbamide peroxide bleaching gels were able to reduce microhardness loss and accelerated microhardness recovery in the post-treatment phase to a better extent than unfluoridated gels.[2223] In this study, bovine specimens were used as a substitute for human teeth because the chemical composition and structure of bovine teeth are similar to human enamel.[24] A 37 °C humid atmosphere can simulate the intraoral clinical situation during home bleaching. Attin et al. claimed that simulating the intraoral conditions as closely as possible in laboratory studies could decrease the risk of enamel microhardness due to bleaching treatments.[14] This study evaluated the enamel fracture toughness with the indentation technique described by two previous studies.[13] Additionally, this technique is well established for fracture toughness measurement of brittle dental restorative materials such as ceramics.[25] Normal values for fracture toughness in enamel and dentin are, respectively, 1.3 and 3.1 MN m-3/2 .[9] In this study, the enamel fracture toughness ranged from 1.1 to 1.4, which is similar to the enamel fracture toughness values reported by Powers and Sakaguchi.[12] This study showed no significant differences in experimental groups of bleached enamel compared with the control group regarding fracture toughness. The results of studies conducted by Seghi and Denry[13] and Attin et al.[14] are in contrast with the findings of this study, as they found a reduction in enamel fracture susceptibility in vitro. These may be explained by the differences in the methods and materials used. In this study, the placement of indentation resulted in the crack formation required for determining fracture toughness with the indentation technique, while Attin and others showed severe softening of enamel instead of crack formation in 10% Opalescence bleached enamel.[17] If specimens were exposed to human whole saliva between bleaching treatment and after the final treatment as opposed to phosphate buffer saliva, they would be better simulated in vivo conditions.[3] Microstructural alterations in bleached enamel may be reversed due to repair by salivary compounds.[16] In addition, 15% Opalescence contains fluoride, which may have contributed to better remineralization even after 6 weeks of bleaching treatment. In summary, in vitro 6-week vital bleaching using 15% carbamide peroxide caused no significant decrease in fracture toughness of human enamel when it was compared with the baseline value, two- and four-week bleaching procedure. In future, further research is needed to evaluate the effect of different brands of bleaching agents in combination with different time intervals on the fracture toughness of enamel.

CONCLUSION

With the limitation of this study, it is concluded that 15% Opalescence PF in different time intervals showed no reduction in fracture toughness of bovine enamel.