Log in Register

Login to your account

Username *
Password *
Remember Me

Create an account

Fields marked with an asterisk (*) are required.
Name *
Username *
Password *
Verify password *
Email *
Verify email *
Captcha *

Captcha Image Reload image challenge



Authors: Dr. Saravanan Poorni, Dr. Anil Kumar, Dr. Rajamani Indira

ABSTRACT

AIM: The purpose of this study was to evaluate the influence of sodium ascorbate antioxidant gel on the reversal of shear bond strength of bleached enamel surface.

METHODOLOGY: Eighty human permanent maxillary central incisors used were divided into two groups of 40 teeth each. Each group was further subdivided into five subgroups of 8 teeth each. Following bleaching and reversal depending on the group, resin composite was applied to all specimens and shear bond strength testing done and data statistically analysed. RESULTS: There was a marked decrease in the shear bond strength of both normal and fluorosed enamel after bleaching. The shear bond strength of specimens reversed with antioxidant gel was greater than that of bleached specimens but less than that of the control group.

CONCLUSION: Bleaching decreased the bond strength of normal and fluorosed enamel significantly. Bond strengths significantly increased on application of antioxidant gel.

KEYWORDS: Antioxidant gel, Bond strength reversal, Fluorosed enamel, Shear bond strength, Sodium ascorbate.

INTRODUCTION

Cosmetic dentistry, influenced by cultural, racial and ethnic concepts, has become a great concern for people, which includes a perfect smile with white and lined up teeth.1 The advent of bleaching materials represented an important milestone in cosmetic dentistry.2 In certain conditions, bleaching alone may not provide clinically acceptable results, hence an esthetic restoration such as composite or ceramic veneers may be required. Fluorosis is a condition caused mainly due to excessive fluoride in drinking water (greater than 1 to 2 ppm) leading to metabolic alteration in the ameloblasts resulting in a defective matrix and improper calcification of teeth.1

Enamel fluorosis is characterized by surface hyper-mineralization and porosity of the subsurface layer. Fluorosis manifests itself as defects in the subsurface enamel with color ranging from white to brown and occurring as pits and irregular white opaque lines, striations or cloudy areas, which further exacerbate the problem of bonding to the enamel. This also possesses an aesthetic challenge.3

Previous studies have shown that bleaching process adversely affects the bond strength of composite to acid etched enamel when bonding is performed immediately following the bleaching procedure. A waiting period of three weeks is required for resin-enamel bond strengths to return to values obtained for unbleached enamel.2 The bond strength reduction can be reversed by applying a biocompatible and neutral antioxidant such as sodium ascorbate before resin composite application.4 There have been many laboratory studies in the areas of shear bond strength of enamel following vital whitening therapy using hydrogen peroxide and its reversal with sodium ascorbate antioxidant. 4,5,6

However not many studies in the literature have elucidated the reliability of sodium ascorbate antioxidant gel on the reversal of compromised bond strength of bleached fluorosed enamel. The aim of this in-vitro study was to investigate the influence of sodium ascorbate antioxidant gel on the reversal of shear bond strength of bleached enamel surface and to determine the effective duration period (time) for the application of sodium ascorbate gel. The study also compared the bond strength reversal between fluorosed and normal (non-hypoplastic) enamel surfaces.

MATERIALS AND METHODS

A total of eighty [40 normal (non-hypoplastic) and 40 fluorosed] non carious freshly extracted human permanent maxillary central incisors without any visible defects were used in this study. Fluorosed teeth with TFI score 3 were selected, which is based on clinical changes in the teeth. TFI score 3 denotes overall cloudy appearance with brownish discoloration on some parts of the labial surface. After extraction the teeth were cleaned of any residual tissue tags and washed under running tap water. They were stored in distilled water at +4ºC until needed for the study.

Sample Preparation

Teeth were derooted at the level of CEJ using a slow speed diamond disc. Coronal pulp was removed with H files and saline was used for irrigation. The pulp chamber was then filled with flowable composite and light cured. Each crown was individually embedded in an auto polymerising acrylic resin block with their labial surfaces faced up. The specimens were stored in cold water until the resin was completely cured to avoid thermal effects generated by the resin curing process. The labial surfaces of the teeth were polished with fine grit silicon carbide paper on a water irrigated metallurgical polishing wheel. The specimens were stored in artificial saliva except during the bleaching, bonding and testing procedures.

Group I
(Non fluorosed Teeth)

Group II
(Fluorosed Teeth)

IA

Teeth were not subjected to bleaching (control)

IIA

Teeth were not subjected to bleaching (control)

IB

Teeth were bleached, rinsed, bonded and subjected to shear bond strength testing

IIB

Teeth were bleached, rinsed, bonded and subjected to shear bond strength testing

IC

Teeth were bleached, rinsed, reversed with antioxidant gel for 2hrs, bonded and then subjected to shear bond strength testing

IIC

Teeth were bleached, rinsed, reversed with antioxidant gel for 2hrs, bonded and then subjected to shear bond strength testing

ID

Teeth were bleached, rinsed, reversed with antioxidant gel for 4hrs, bonded and then subjected to shear bond strength testing

IID

Teeth were bleached, rinsed, reversed with antioxidant gel for 4hrs, bonded and then subjected to shear bond strength testing

IE

Teeth were bleached, rinsed, reversed with antioxidant gel for 6hrs, bonded and then subjected to shear bond strength testing

IIE

Teeth were bleached, rinsed, reversed with antioxidant gel for 6hrs, bonded and then subjected to shear bond strength testing

Experimental Groups

80 teeth were divided into two groups of 40 teeth each. Group I consisted of normal (non-hypoplastic) teeth and Group II consisted of fluorosed teeth. Each group was further subdivided into five subgroups of 8 teeth each. The grouping of specimens are shown in Table 1

Bleaching Procedure

Samples in groups IB, IC, ID,IE, IIB, IIC, IID and IIE teeth were bleached with 35% hydrogen peroxide (Opalescence Extra) as per the manufacturers’ instructions, exposed to a halogen curing light with an intensity of 3000 mW/cm2 for 20 seconds and the bleaching gel was left in contact with the labial surface for 15 mins. The gel was then washed away and a fresh gel was reapplied, light activated, left standing for another 15 minutes and washed away. The teeth were then stored in artificial saliva for 24 hours at room temperature.

Preparation of Artificial Saliva

The samples were stored in artificial saliva as described in British Standard 7115, part 2, BSI London, 1988. The artificial saliva with an electrolyte compostion similar to human saliva was prepared by mixing Sodium chloride – 0.5g, Sodium Bicarbonate – 4.2g, Sodium nitrate – 0.03g and Potassium Chloride 0.2g in 100ml of distilled deionized water.

Preparation of Antioxidant Gel

Sodium ascorbate gel (sodium salt of ascorbic acid) was used as antioxidant preparation. The antioxidant gel (2.5% [wt/wt]) containing sodium ascorbate (10%) was prepared by dispersing the carbopol 976P resin polymer in purified water containing sodium ascorbate under gentle mixing. The mixture was stirred until thickening occurred and then neutralized by drop wise addition of thriethanolamine until a transparent gel appeared. The quantity of thriethanolamine was adjusted to a pH of 7.

Antioxidant Application

After the bleaching procedure, the samples in groups IC, ID, IE, IIC, IID and IIE were subjected to antioxidant gel action. Sodium ascorbate gel was placed on the enamel surfaces of the experimental teeth for 120 mins (2hrs), 240 mins (4hrs) and 360 mins (6hrs) depending upon the group. After the antioxidant treatment, the enamel surface was thoroughly rinsed with distilled water for 30 secs.

Bonding Procedure

A hole of 3mm diameter was cut in the mylar stip to ensure that the adhesive adhered to the surface where the composite cylinder was intended to be made. This mylar strip containing the hole was fixed to the surface of the sample. After etching with phosphoric acid, adhesive (Prime & Bond NT) was applied to the open surface of the sample according to manufactures recommendations. A custom made split Teflon mold, with a circular hole 2mm in diameter and 4mm deep was positioned over the hole in the mylar strip and clamped into place with a special bonding alignment apparatus. A resin composite was placed into the hole in 2 increments of 2mm each. Each layer was light cured for 40 secs with a conventional quartz tungsten LC unit. Additional light curing was done from the sides. Curing the resin in the mold formed cylindrical posts vertical to the dentin surface. Each increment was cured for 40secs each. After removing the bonding alignment apparatus and the split mold along with the mylar strip, the specimens were stored in artificial saliva for 24 hrs.

Shear Bond Strength Testing
 

 

Groups

Group I

Group II

 

p value

Mean

SD

Mean

SD

A

35.42d

0.60

24.28 d

0.93

< 0.001**

B

23.52a

0.62

15.58 a

0.95

< 0.001**

C

31.11b

0.79

20.49 b

0.55

< 0.001**

D

33.35c

0.81

22.40 c

0.72

< 0.001**

E

33.63c

0.72

22.59 c

0.87

< 0.001**

p value

< 0.001**

< 0.001**

 



The shear bond strength was measured with a Universal Testing Machine. A knife edge shearing rod and a crosshead speed of 0.5mm/minute were used. The distance from the probe to the dentin surface was monitored using a spacer of two celluloid matrices. The load at failure was recorded by Labtech Notebook Software (version 6.3, Labtec, Wilmington, MA, USA). The shear bond strengths of the specimens were calculated and expressed in MPa.

Mode of Bond Failure

Specimens were thoroughly washed with distilled water, dried and secured onto metal stubs with colloidal carbon adhesives. They were then sputter coated with gold and examined at X1000 – X2000 magnification under Scanning Electron Microscope at 15Kv. Photomicrographs were taken using Kodak Verichrome Pan 120mm black and white negative films VP120.

RESULTS

The shear bond strength in Mpa (mean + sd) for the groups are shown in Table 2. One Way ANOVA test followed by Tukey HSD test showed a statistically significant difference amongst various subgroups concerning the discrepancy in the shear bond strength in each group while Student’s t-test showed a significant difference in the shear bond strength between the groups (Table 2).

A comparison of the shear bond strength of normal and fluorosed enamel is shown in figure 1. Shear bond strength of normal enamel is greater than that of fluorosed enamel. There was a marked decrease in the shear bond strength of both normal and fluorosed enamel after bleaching. The shear bond strength of specimens reversed with antioxidant gel was greater than the shear bond strength of bleached specimens but less than that of the control group. Shear bond strength of specimens reversed with sodium ascorbate antioxidant gel for 4 hours was greater than that of 2 hours. There was no difference between the bond strength of specimens reversed with sodium ascorbate for 4 hours and 6 hours.

SEM analysis predominantly showed mixed failures in all the groups. Cohesive failure in resin composite and enamel was minimal. The mode of failure was cohesive in the resin composite in group IA while all other demonstrated a mixed failure. In teeth with mixed failure exposed dentinal tubules could be seen suggesting detachment of enamel at the dentino-enamel junction. Fractured resin tags were attached to the dentinal tubules. In cases with cohesive failure in resin, SEM examination revealed typical resin particles all over the surface.

DISCUSSION

Esthetics has strongly influenced patients’ perception and paved way to the development and expansion of cosmetic dentistry. In addition to solving the problem of chromatic inconsistencies, it should also impart a healthy and harmonious smile. Dental bleaching represents a conservative treatment option or it may be an integral part of restorative procedures in conditions that require elimination of color change abnormalities.7 Bleaching agents release free radicals as nascent oxygen and hydroxyl or peri-hydroxyl ions that react with electron rich regions of pigments inside dental structure, breaking down large pigmented molecules into smaller less pigmented ones. On the other hand, this property is deleterious to the bonding of resinous materials.5
Recommendations of waiting periods for the application of composite materials onto the bleached enamel surface ranges from one day to six weeks. However in order to remove the dissolved peroxide remnants on bleached enamel, it has been demonstrated that the application of a catalase or 10% sodium ascorbate on bleached enamel and dentin immediately after the bleaching treatment makes the above mentioned waiting periods unnecessary. 5,8,9 Ascorbic acid and its sodium salt are potent antioxidants capable of quenching reactive free radicals in biological systems. Since Vitamin C and its salts are non-toxic and widely used in the food industry as antioxidants, it is unlikely that their use on enamel and dentin will create any adverse biological effect or clinical hazards.4 In this study sodium ascorbate was used to prevent the acidic effect of ascorbic acid as indicated by Lai et al.8 Several studies have reported that antioxidant gel causes a reversal of compromised bond strengths in non-fluorosed bleached enamel. 4,5,6 Till date, no data is available concerning the effect of antioxidant gel on shear bond strength of bleached fluorosed enamel. Therefore this study was designed to investigate not only the effects of bleaching agent and antioxidant on shear bond strength of normal enamel but also compared it with that of fluorosed enamel. The results of the present study demonstrated a significant difference in the shear bond strength of normal and fluorosed enamel. This indicates that fluorosis significantly reduced the shear bond strength of resin composites to enamel. This is consistent with Adanir et al who reported that severity of fluorosis affected the shear bond strength to fluorosed enamel.10 According to this study, bleaching treatment decreased the bond strength of resin composite to normal and fluorosed enamel. Various theories have been proposed to explain this decrease in bond strength. Changes in enamel structure are one of the explanations. It may be probably due to loss of mineral content and increased porosity of bleached surface that manifests as an over etched surface with a loss of the prismatic enamel. It has been noted that resin tags are reduced in number, less definite and shorter in bleached enamel.2 In addition to loss of calcium, decrease in microhardness and alterations in the organic substance might be important factors to cause a decrease in enamel bond strengths.11 Sundfeld et al observed that immediate application of adhesive system after bleaching resulted in inferior resin tags that were smaller, thinner, less frequent, lacking in uniformity and poorly defined.12 His study supported the possibility of an interaction between the resin adhesives and the peroxide located on, or close to, the bleached enamel surface. This is in agreement with finding of Titley and others, who proposed that this interaction could prevent polymerization of adhesives due to presence of oxygen as well as increased porosity of the resin material as a result of release of oxygen. This oxygen is known to inhibit vinyl polymerization in resins used for restorative dentistry.13The lowest mean value of shear bond strength in the present study was obtained for the fluorosed specimens that were subjected to bonding without antioxidant reversal. This may be due to morphological alterations induced by bleaching in the most superficial enamel crystallites. Results of this study revealed that antioxidant application caused an increase in the shear bond strength of normal and fluorosed enamel. However the post reversal bond strengths were not equal to those of non-bleached group. This was consistent with the study results of Kaya et al 5 and Kimayi et al 6. Increase in bond strength of groups that were reversed with sodium ascorbate for 4 hrs was significantly higher when compared to the groups that were reversed for 2 hrs. This may be due to the fact that as the application time for antioxidant increases, the bond strength for composite resin increases. however there was no significant increase in bond strength between the 4 hrs and 6 hrs application. Sodium ascorbate removes the oxidative effect of bleaching agent by restoring the altered redox potential thus allowing free radical polymerization of adhesive to proceed without premature termination and hence reverse the compromised bonding. 5,6,8 The specimens were evaluated for the bond strength failures under scanning electron microscope. Failure is said to be adhesive when more than 75% of the bonding area appeared to be clean enamel, cohesive in resin when more than 75% of the bonding area was covered with remnants of resin composites and mixed when 25%-75% of the failure was both adhesive and cohesive. Specimens when viewed under SEM showed predominantly mixed failures while the unbleached non-fluorosed group showed cohesive failure in resin. However, when analyzing the fracture site, it is evident that the group that was not treated with the hydrogen peroxide gel produced cohesive failure in resin more than the other groups. All the other groups showed a similar surface with a mixture of both adhesive and cohesive failures and hence stated as mixed failures. Differences among various groups could be discerned by the variation in extent of resin infiltration and resin tag integrity along fractured hybrid layers. It must be emphasized that this study was performed in-vitro. Therefore, shear bond strengths obtained in this study may not correspond well with clinical success. Even though the in-vitro results are promising in terms of using sodium ascorbate to reverse oxygen inhibition in normal and fluorosed enamel; however further in vivo studies are still needed to substantiate the results obtained in this study.

CONCLUSION

Within the limitations of this present study, it can be concluded that
  1. Shear bond strength of normal (non-hypoplastic) enamel is greater when compared to that of fluorosed enamel.
  2. Bleaching causes a decrease in the shear bond strength of both normal (non-hypoplastic) and fluorosed enamel.
  3. Application of 10% sodium ascorbate gel improves the shear bond strength of bleached normal (non-hypoplastic) and fluorosed enamel causing bond reversal.
  4. Four hours was found to be optimum time duration for application of antioxidant gel.

REFERENCES
  1. Joiner A. The bleaching of teeth: A review of literature. J Dent 2006; 34: 412-416.
  2. Nour El-din AK, Miller BH, Griggs JA, Wakefield C. Immediate bonding to bleached enamel. Oper Dent 2006; 31(1): 106-114.
  3. Noble J, Karaiskos NE, Wiltshire WA. In vivo bonding of orthodontic brackets to Fluorosed enamel using an adhesion promoter. The Angle Orthodontist 2008; 78(2): 357-360.
  4. Kaya AD, Turkun M. Reversal of dentin bonding to bleached teeth. Oper Dent 2003; 28(6): 825-829.
  5. Kaya AD, Turkun M, Arici. Reversal of compromised bonding in bleached enamel using antioxidant gel. Oper Dent 2008; 33(4): 441-447.
  6. Kimyai S, Valizadeh H. The effect of hydrogel and solution of sodium ascorbate on bond strength in bleached enamel. Oper Dent 2006; 31(4): 496-499.
  7. Miranda CB, Pagani C, Benetti AR, Matuda FS.Evaluation of the bleached human enamel by scanning electron microscopy. J Applied Oral Science 2005; 13(2): 204-211.
  8. Lai SCN, Mak YF, Cheung GSP, Osario R, Tolendano M, Carvalho RM, Tay FR, Pashley DH. Reversal of compromised bonding to oxidized etched dentin. J Dent Res 2001; 80(10): 1919-1924.
  9. Lai SCN, Tay FR, Cheung GSP, Mak YF, Carvalho RM, WEI SHY, Toledano M, Oaorio R, Pashley DH. Reversal of compromised bonding in bleached enamel. J Dent Res 2002; 81(7): 477-481.
  10. Adanir N, Turkkahraman H, Gungor AY. Effects of fluorosis and bleaching on shear bond strengths of orthodontic brackets. Euro J Dent 2007; 1: 230-235.

Add comment


Security code
Refresh