: 2019  |  Volume : 31  |  Issue : 2  |  Page : 150--157

Fracture resistance of teeth undergoing postendodontic bleaching: Comparison of four treatment modalities – An in vitro study

Abe Antony, Rajesh Pillai, NO Varghese, U-Nu Sujathan, A Afzal, Sheila George 
 Department of Conservative Dentistry and Endodontics, PMS College of Dental Science and Research, Thiruvananthapuram, Kerala, India

Correspondence Address:
Dr. Abe Antony
PMS College of Dental Science and Research, Golden Hills, Vattapara, Venkode, Thiruvananthapuram - 695 028, Kerala


Aims: The aims of the study were: (1) to evaluate the fracture resistance of endodontically treated teeth restored with microhybrid composite following combination bleaching and (2) to assess the antioxidizing effect of 10% sodium ascorbate hydrogel. Settings and Design: This was a randomized control trial in a tertiary care setting. Subjects and Methods: Endodontic treatment was performed on 40 freshly extracted human mandibular premolars. Following removal of gutta-percha 2 mm apical to cementoenamel junction and application of resin-modified glass ionomer as cervical barrier, all teeth were embedded in acrylic resin using cylindrical molds. Specimens were divided into five groups: Group 1 (n = 8): Specimens subjected to inside and outside bleaching using 10% hydrogen peroxide followed by composite restoration. Group 2 (n = 8): Specimens subjected to inside-outside bleaching with 10% hydrogen peroxide and sodium perborate followed by composite restoration; Group 3 (n = 8): Specimens subjected to conditioning with 10% sodium ascorbate after inside-outside bleaching using 10% hydrogen peroxide followed by composite restoration; Group 4 (n = 8): Specimens subjected to conditioning with 10% sodium ascorbate after inside-outside bleaching with 10% hydrogen peroxide and sodium perborate followed by composite restoration; and Group 5 (negative control) (n = 8): After endodontic treatment specimens were restored with composite. Finally, all the specimens were subjected to fracture resistance test using Universal Testing Machine. Statistical Analysis Used: ANOVA test followed by post hoc comparison of groups taken together by Tukey's analysis was performed. Results: Statistically significant difference in fracture resistance was present between Group 5 and Group 1 and also between Group 5 and Group 2. Unpaired t-test showed statistically significant difference between Group 1 and Group 3 and also between Group 2 and Group 4. Conclusion: The use of 10% sodium ascorbate antioxidant gel was effective in compensating for the decreased fracture resistance following combination bleaching.

How to cite this article:
Antony A, Pillai R, Varghese N O, Sujathan UN, Afzal A, George S. Fracture resistance of teeth undergoing postendodontic bleaching: Comparison of four treatment modalities – An in vitro study.Endodontology 2019;31:150-157

How to cite this URL:
Antony A, Pillai R, Varghese N O, Sujathan UN, Afzal A, George S. Fracture resistance of teeth undergoing postendodontic bleaching: Comparison of four treatment modalities – An in vitro study. Endodontology [serial online] 2019 [cited 2020 Feb 28 ];31:150-157
Available from: http://www.endodontologyonweb.org/text.asp?2019/31/2/150/275460

Full Text


Tooth discoloration varies in etiology, appearance, localization, severity, and adherence to tooth structure. It may be classified as intrinsic, extrinsic, or a combination of both.[1] Scaling and polishing of the teeth remove many extrinsic stains. For more stubborn extrinsic discoloration and intrinsic stain, various bleaching techniques may be attempted.[2] Tooth bleaching can be performed extracoronally in-home or in-office vital tooth bleaching as well as intracoronally in root-filled termed as nonvital tooth bleaching.

Many studies have reported the adverse effects of bleaching agents when applied to dental structures. These include external cervical resorption,[3],[4] cervical caries, increase dentin permeability,[5] reduction in microhardness of dentin and enamel,[6],[7] reduction in bond strength,[8],[9],[10] and increased microleakage in composite resin restorations performed after dental bleaching [11],[12],[13] These effects could be related to the presence of residual hydrogen peroxide in the interprismatic spaces as well as in the dentinal matrix and tubules.[9],[14] Bleaching agents also can cause chemical alterations in the hard dental tissues, changing the ratio of organic to inorganic composition, and increasing the tooth's solubility, which subsequently reduces the bond strength of resin-based composite restorations.[15]

Some techniques have been suggested to remove the remnant oxygen-free radicals in enamel and dentin after bleaching and also to reverse the compromised infiltration and polymerization of resin at the tooth interface. Cvitko et al.[16] proposed to remove the superficial layer of enamel; Barghi and Godwin [17] treated the bleached enamel with alcohol before restoration, while Kalili et al.[18] and Sung et al.[19] suggested the use of adhesives containing organic solvents. However, the general approach is to postpone any bonding procedure for a while after bleaching since the reduction of bond strength has been shown to be temporary. The waiting period for bonding procedures after bleaching has been reported to vary from 24 h to 4 weeks.[8],[9],[20],[21]

Dental bleaching results in deterioration of mechanical properties of enamel and dentin such as microhadrness and modulus of elasticity.. It is also associated with a decreased bond strength of adhesive restorations to bleached enamel. This can be reversed with application of 10% sodium ascorbate solution. Khoroushi et al.[22] reversed the decreased fracture resistance of endodontically treated teeth that were subjected to combination bleaching using 10% sodium ascorbate hydrogel. A study conducted by Lai et al.[23] has shown that hydrogen peroxide or sodium hypochlorite-induced reduction in bond strength of resin to enamel is reversed with the use of sodium ascorbate as an antioxidant. If the antioxidant treatment of bleached tooth before bonding reverses the reduced bond strength, it may be an alternative to the delayed bonding procedure after bleaching.

The aims of the current study are:

To evaluate the change in fracture resistance of endodontically treated teeth following combination bleaching with 10% hydrogen peroxideTo evaluate the change in fracture resistance of endodontically treated teeth following combination bleaching with 10% hydrogen peroxide and sodium perborateTo assess whether the application of 10% sodium ascorbate hydrogel can improve the fracture resistance of teeth following combination bleaching after endodontic treatment.

 Subjects and Methods

Following approval from institutional ethics committee, 40 human mandibular premolars with completely formed apices and Type I Vertucci's canal morphology, scheduled for therapeutic extractions, were collected. Teeth with similar coronal and radicular dimensions were included in the study. Teeth with radiographic evidence of calcifications, internal or external resorption were discarded. The teeth were examined under stereomicroscope (SteREO Discovery.V20, Zeiss) at a × 20 to exclude teeth with fracture lines/cracks or fissures. Teeth were stored in 0.2% thymol solution (Sigma–Aldrich) at room temperature until further use. Before initiating endodontic treatment, the teeth were mounted on plaster of paris bases up to level of cementoenamel junction (CEJ).

Following access cavity preparation, the working length was calculated by subtracting 1 mm from root length using No. 20 K-file (MANI). Instrumentation was done with Protaper Universal rotary files up to size F3 (Tip Size 30; Taper 9%). Irrigation was carried out with 2 ml of 5.2% NaOCl, 17% EDTA, and final rinse with 0.9% isotonic saline for all specimens. The root canals were be obturated using F3 gutta-percha points (Dentsply) using lateral condensation technique with accessory gutta-percha points and AH plus resin sealer (Dentsply). Radiographs were taken to assess the quality of obturation. Teeth were stored for 48 h at room temperature for the sealer to set.

Heated plugger (Hu–Friedy) was be used to remove gutta-percha from the pulp chamber up to the level of 2 mm apical to CEJ. All teeth were embedded in autopolymerizing acrylic resin up to the CEJ, using cylindrical molds and root canal orifices were be sealed with gutta-percha stop. Resin-modified glass ionomer (Vitremer Core Build Up/Restorative A3 Shade) was placed as cervical barrier up to CEJ and light cured for 40 s (Epilar S10 LED).

At this point specimens are divided into five groups (n = 8) as follows:

Group 1: Specimens are subjected to inside-outside bleaching using 10% hydrogen peroxide followed by composite restoration.Group 2: Specimens are subjected to inside-outside bleaching with 10% hydrogen peroxide and sodium perborate followed by composite restorationGroup 3: Bleaching protocol similar to Group 1 followed by conditioning with 10% sodium ascorbate for 10 min followed by composite restorationGroup 4: Bleaching protocol similar to Group 2 followed by conditioning with 10% sodium ascorbate for 10 min followed by composite restorationGroup 5 (−ve control): Teeth subjected to endodontic treatment followed by composite restoration.

Preparation of 10% hydrogen peroxide gel

10% hydrogen peroxide gel was prepared by mixing 33.3 ml of 30% hydrogen peroxide (Nice Chemicals, India) with 2% carbopol 974 P (Yarrow Chem Products, India) to a final volume of 100 ml. pH of the gel was adjusted to 7.2 ± 0.2 units using triethanolamine (Emplura, India) and verified using pH meter (OAKION pH 700, Coleparmer, USA). To further the process of gelation, the prepared gel was processed at 1250 rpm for 1 h using a magnetic stirrer (ANM, India).

Bleaching procedure in Group 1 and 3

Group 1 and 3 were subjected to a single session of inside-outside bleaching using 10% hydrogen peroxide. 10% hydrogen peroxide gel was applied on the labial surface as well as into the pulp chambers. Entire buccal surface of specimens is coated with 1 mm to 2 mm thick layer to ensure uniform effect. After 9 h, gel was removed, and treated teeth were thoroughly rinsed with air water spray.

Bleaching procedure in Group 2 and 4

Specimens of Group 2 and 4 are subjected to single session of inside-outside bleach using 10% hydrogen peroxide and sodium perborate. About 1–2 mm thick layer of 10% hydrogen peroxide gel was applied on the entire buccal surface of specimens. Sodium perborate (Microfine Chemicals, India) was mixed with hydrogen peroxide solution IP 6% w/v (JP PHARMA) in a beaker and placed into the pulp chambers of the specimens. After 9 h, the bleaching agents were removed, and treated teeth were thoroughly rinsed with air-water spray.

Preparation of 10% sodium ascorbate antioxidant gel

Sodium ascorbate (sodium salt of ascorbic acid) (Yarrow Chem Products, India) was used for the antioxidant preparation. The antioxidant gel (2.5% [wt/wt]) containing sodium ascorbate (10%) was prepared by dispersing the Carbopol 976P resin polymer (Yarrow Chem Products, India) in purified water containing sodium ascorbate under gentle mixing. pH of the gel was adjusted to 7.2 ± 0.2 units using triethanolamine (Emplura, India) and verified using pH meter (OAKION pH 700, Coleparmer, USA).

To further the process of gelation, the prepared gel was processed at 1250 rpm for 1 h using a magnetic stirrer (ANM, India). Post bleaching specimens of Group 3 and 4 were subjected to conditioning with 10% sodium ascorbate hydrogel for 10 min each. Postconditioning specimens are thoroughly rinsed with distilled water. Between the sessions, teeth would be stored in normal saline (0.9% isotonic saline, Nirlife, Nirma Limited). Control group remains in normal saline.

Fracture testing of specimens

Following restoration of access cavities with Z100 Universal Restorative Microhybrid composite all the specimens were subjected to fracture resistance test using the universal testing machine (INSTRON). As the maximum width a specimen that could be accommodated in the Instron machine is 1.2 mm, the test specimens which had a width of >1.5 cm had to be trimmed to a width of 1 cm.

Test was carried out using a 5 mm diameter round bar was positioned parallel to long axis of the teeth and centered over the teeth until the bar just contacted the slopes of the buccal and lingual cusps of the tooth near the composite tooth interface.

Force necessary to fracture teeth was measured in Newtons (N) as calibrated by Instron machine. Crosshead speed of 1 mm/min was applied till the teeth fractured. Moment of fracture was determined by sudden decrease in force measurements in testing machine gauge.

Statistical analysis

One-way analysis of variance (ANOVA) was done among Group 1, Group 2, and Group 5 together, followed by post hoc comparison of two groups taken together (multiple comparison) by Tukey's analysis in finding out the most efficient and least efficient modality. Unpaired t-test was used to compare Group 1 with Group 3 and Group 2 with Group 4.


Fracture resistance in Newton along with mean and standard deviation of five groups are given in [Table 1]a. ANOVA revealed significant difference in fracture resistance between Group 1 and 5 and also between Group 2 and 5 (P < 0.05). No significant differences were observed between Group 2 and 3 (P > 0.05) [Table 1]b and [Table 1]c and [Graph 1]. Unpaired t-test revealed statistically significant difference between Group 1 and 3 and also between Group 2 and 4 [Table 2]a and [Table 2]b and [Graph 2]a and [Graph 2]b.{Table 1}[INLINE:1]{Table 2}[INLINE:2]


Endodontically treated teeth have been reported to present a higher risk of biomechanical failure than vital teeth, suggesting the need for additional restorative considerations.[24] The dentin of endodontically treated teeth undergoes changes in both its physiologic characteristics, such as a decrease in the immature collagen levels [25] and its physical properties, whereby dehydration causes a decrease in the modulus of elasticity.[26] These changes accompanying root canal therapy influence the approach and selection of restorative procedures. In the current study, prior to fracture testing and combination bleaching, the study specimens were subjected to endodontic treatment. Biochemical and biomechanical changes in dentin following endodontic treatment and tooth structure lost during access opening must definitely have influenced the fracture resistance of the specimens.

Restoration of root canal-treated teeth with a permanent, definitive, postendodontic restoration is a final step for successful root canal treatment, as these teeth are considered more susceptible to fracture.[27] Therefore, intracoronal strengthening of teeth is important to protect them against fracture, particularly in posterior teeth where stresses generated by occlusal forces can lead to fracture of unprotected cusps. An optimal final restoration for endodontically treated teeth maintains esthetics, function, preserves the remaining tooth structure, and prevents microleakage.[28]

With the recent advancements in adhesive technology and stronger adhesive materials, it is now possible to create conservative, highly esthetic restorations [29] that are bonded directly to the tooth structure and strengthen it.[30] Adhesive restorative materials promote enough retention and create an adhesive bridge between the facial and lingual cusps of a significantly weakened tooth.[31] These materials may have the potential to decrease deflection and fracture of cusps under occlusal load.[32] The fracture resistance of endodontically treated premolars is increased when they receive composite resin restorations. In the present specimens, all the groups were restored with Z100 Universal Restorative microhybrid composite.

However, studies have shown compromised bonding of composite restorations immediately carried out following bleaching.[33],[34],[35] This is attributed to the presence of residual oxygen and interference with resin penetration and resin polymerization.[36] Bleaching is also shown to cause physical alteration in enamel and reduced composite bond strength.[37]

Hydrogen peroxide is a strong oxidizing agent that can act on dentin modifying its mechanical and chemical properties.[38],[39],[40] Hydrogen peroxide is able to generate hydroxyl radical (OH _) in the presence of ferrous salts,[40] which has been described as being responsible for dental bleaching.[39] Due to its high oxidation potential, OH _ radicals act in intertubular and peritubular dentin breaking the polypeptide chains [39] and degrading components of connective tissue, particularly collagen and hyaluronic acid,[40] thereby attacking the organic component of dentin. These ultrastructural alterations increase dentin permeability,[41] reduce hardness and elasticity.[6],[42],[43] Chng et al.[6] reported a significant decrease in the ultimate tensile and shear strengths of dentin after intracoronal bleaching with 30% hydrogen peroxide for 24 h. Ghavamnasiri et al.[43] also showed that the flexural strength of bovine dentin decreased after the application of 20% carbamide peroxide for 2 weeks. Dental modifications caused by bleaching appeared to be time-related, showing a greater decrease in mechanical properties after 2 months.[44]

In the current study, the specimens of Group 1 and Group 2 showed decreased fracture resistance when compared to the negative control group (Group 5). This can be partly attributed to the alteration of mechanical and chemical properties of enamel and dentin by 10% hydrogen peroxide and sodium perborate used to bleach the teeth. Remnants of peroxide or peroxide-related substances resulted in compromised bond strength to both enamel and dentin. This reduced their fracture resistance when they were subjected to fracture testing using Instron machine.

Recently, some authors have described an inside/outside technique for bleaching teeth with intrinsic discolorations.[45],[46] This technique was first described by Settembrini et al.[41] and was later modified Liebenberg.[47] This technique allows the endodontically treated tooth to be bleached both from within the sealed pulp chamber (inside) and from the facial enamel (outside) simultaneously, and the process can carried out as in-office and/or at-home treatment. The in-office procedure can also be accelerated using a light source.[11] The access cavity remains open during the entire treatment process. One advantage of this technique is that a low concentration of bleaching gel is sufficient to obtain the desired effect.

This procedure is also indicated when where an endodontically treated tooth is present within the arch, and the arch as a whole is to be bleached.[48] In this case, the whitening gel is applied by means of a bleaching tray to bleach both the buccal surface and pulp chamber through the access opening.[49] There are certain risks with this technique, in that an unsealed access opening enables bacteria and stains to penetrate into dentin. Even with a good root filling, the passage of bacteria through the tooth can be observed. Therefore, a restorative material such as glass-ionomer cement or resin composite should be used to seal the root filling at the orifice. In the present study, all specimens except negative control group were subjected to combination bleaching.

In the current study, the effect of applying sodium ascorbate subsequent to the bleaching procedure as an antioxidant on fracture resistance was investigated. Khoroushi et al.[22] used 10% sodium ascorbate gel in the access cavities and buccal surfaces of specimens for a period of 24 h after bleaching and found that it compensated the decreased fracture resistance found in other groups.

Lai et al.[50] recommended using sodium ascorbate gel for a duration that corresponds to one third of the bleaching time. Since the duration of bleaching was 9 h which corresponded to an overnight combination of home bleaching, the specimens should have been subjected to 3 h application of antioxidant gel. Other researchers [51],[52] have also recommended that a 10 min application time of antioxidant is sufficient. Freire et al.[53] in his research has shown that even an application time of 5 min can neutralize the bleaching effects in terms of chemical process involved. Since a shorter application time was clinically desirable, specimens in the present study were subjected to a 10 min application of antioxidant gel.

Regarding the concentration of sodium ascorbate used, Türkün et al.[52] proved that 10% sodium ascorbate was more effective in reversing the decreased shear bond strength to composites when compared to 2.5 and 5% of same reagent. While a study conducted by Kimyai and Valizadeh [54] that there was no difference in efficacy of 10% sodium ascorbate solution and 20% sodium ascorbate hydrogel. Regarding the reaction kinetics of sodium ascorbate with hydrogen peroxide, Freire et al.[53] proved that there was a direct relationship between the initial amount of sodium ascorbate submitted to the action of the bleaching gel and the mass of ascorbate that reacted with the hydrogen peroxide.

In previous investigations, sodium ascorbate has been used in its solution form.[23],[55],[56] However, the gel form of sodium ascorbate is easy to apply (because of its higher viscosity and better control) and its application is less expensive for patients compared to the application of sodium ascorbate solution by the dental practitioner due to the shorter chair time needed.[54]

In the present study, application of sodium ascorbate increased the fracture resistance of in specimens of Group 3 and Group 4 when compared to that of Group 1 and Group 2 where restoration of teeth immediately followed bleaching procedures without application of antioxidant gel. Results of the present study concur with results obtained by Khoroushi et al.,[22] Türkün et al.,[52] and Kimyai and Valizadeh [54] have also attempted to reverse the compromised bond strength to composites after bleaching with 10% carbamide peroxide gel.

The increase in fracture resistance, which is directly proportional to the bond strength of the post endodontic composite restoration to enamel and dentin, could be due to the potent antioxidant capacity of sodium ascorbate by quenching free radicals in biological systems.[27] Sodium ascorbate, being a reducing agent, is capable of donating two high-energy electrons to scavenge the free radicals by the mechanism called passive detoxification. The neutralizing effect of antioxidant leads to better resin tooth adhesion due to the release of free oxygen radicals. According to Soeno et al.,[57] ascorbic acid acts a polymerization promoter and coinititator and has high positive effects on resin bond to tooth. Rather than delaying bonding procedures to reduce the effects of bleaching application of antioxidant gel can also decrease the treatment period.


Within the limitations of the current study, it can be concluded that:

The combination bleaching procedures significantly decreases the fracture resistance of endodontically treated teethThe use of 10% sodium ascorbate antioxidant gel is effective in compensating for the decreased fracture resistance of endodontically treated and bleached teeth.

Financial support and sponsorship


Conflicts of interest

There are no conflicts of interest.


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