Reinforcing bleached roots: A comparative study of intraorifice barriers

Abstract

Aim:

The aim of this study was to assess the influence of three types of intraorifice barriers (IOBs) on the fracture toughness of root canal-treated teeth subjected to intracoronal bleaching.

Materials and Methods:

Endodontic therapy was performed on 60 extracted maxillary central incisors, which were subsequently randomized into four groups (n = 15): Group I – glass-ionomer cement, Group II – TheraCal, Group III – Ionoseal, and Group IV – control (no barrier). Following placement of the designated IOB materials, intracoronal bleaching was performed using 35% hydrogen peroxide. The fracture toughness of all specimens was evaluated using a universal testing machine.

Statistical analysis:

A statistical evaluation of the data was carried out with one-way analysis of variance and subsequent post hoc analysis.

Results:

All the IOB groups showed higher fracture toughness than the control group. Among the tested materials, TheraCal exhibited the highest reinforcement, followed by Ionoseal and GIC, with statistically significant differences (P < 0.05).

Conclusion:

Endodontically treated teeth subjected to intracoronal bleaching demonstrated greater fracture resistance when IOBs were applied. TheraCal proved to be the most effective material, highlighting its clinical relevance in preserving tooth integrity during bleaching procedures.

INTRODUCTION

Tooth discoloration is a common esthetic concern and may arise due to a wide range of etiological factors. It can be broadly categorized as extrinsic or intrinsic based on its location and origin.[1]

One of the most effective treatment modalities for intrinsic discoloration in endodontically treated teeth is nonvital tooth bleaching, which involves the application of oxidizing agents like hydrogen peroxide (H₂O₂) within the pulp chamber to lighten discolored teeth.[2]

While effective, intracoronal bleaching may compromise tooth structure, causing decreased microhardness, increased permeability, and risk of cervical resorption, primarily due to residual oxidizing agents.[3] To counter these effects and reinforce the tooth, intraorifice barriers (IOBs) are placed in the coronal third of the canal, improving fracture resistance and sealing ability.

Conventional glass-ionomer cement (GIC), composed of fluoroaluminosilicate, provides chemical adhesion to tooth structure, fluoride release, and mechanical properties comparable to dentin. However, its slow setting and limited wear resistance restrict its use to nonload-bearing areas.[4]

Recent developments have focused on creating GICs with shorter working time and improved properties compared to earlier generations. Ionoseal represents one of the latest resin-modified GIC (RMGIC).

Ionoseal is a light-cured RMGIC containing resin monomers such as 2-hydroxyethyl methacrylate and bisphenol A-glycidyl methacrylate (Bis-GMA) with photoinitiators, offering improved biocompatibility, adhesion to tooth structure, and enhanced mechanical properties.[5]

TheraCal LC is a light-curing, resin-modified, tricalcium silicate-based cement, used as a protective base/liner or in direct and indirect pulp capping agent, offering rapid polymerization, resistance to dissolution, superior mechanical strength, and calcium ion release that promotes odontoblast differentiation and new dentin formation.[6,7]

Several materials have been utilized as IOBs, though limited research exists regarding their impact on enhancing strength and fracture resistance. This study was conducted to assess the fracture resistance of newer materials used as IOBs in endodontically treated teeth. The primary objectives were to compare the performance of various materials and to identify the one that provides the greatest resistance to fracture in such teeth.

MATERIALS AND METHODS

Sample preparation

For this study, 60 freshly extracted human maxillary central incisors were selected, and ultrasonic scalers were used to clean their surfaces. After debridement, the specimens were stored in 0.9% of saline.

Maxillary central incisors were selected due to their relatively uniform root canal anatomy and thin pericervical dentin, which makes them more susceptible to fracture following endodontic treatment and intracoronal bleaching. Using a single-tooth type minimized anatomical variability and allowed a more accurate assessment of fracture resistance.

Endodontic access cavities were prepared in a standardized manner using a round diamond bur, followed by an Endo Z bur with continuous water irrigation. To ensure consistency and reduce operator-related bias, all procedures were carried out by a single operator. The canal working length was measured using a K-file. The canals were then instrumented and shaped with ProTaper Gold rotary files up to size F3, with 5.25% NaOCl used for irrigation between instruments. Then, the canals were irrigated with 5 mL of 17% ethylenediaminetetraacetic acid for 1 min, followed by 5 mL of normal saline and dried. Obturation was performed with an F3 master cone and ZOE sealer using the cold lateral compaction technique, after which the access cavities were sealed with Dentemp. Samples were incubated for 24 h. In the experimental groups, gutta-percha was removed with a hot plugger to a depth of 3 mm below the cementoenamel junction (CEJ), while the control group remained unaltered. Depth was verified using a Williams periodontal probe.

Grouping of samples

The teeth were randomly distributed into four groups depending on the IOB material applied over the obturated canals.

• Group 1 – Conventional GIC (GC Gold Label 2) (GC corporation, Tokyo, Japan) (n = 15)

• Group 2 – TheraCal LC (BISCO, Inc., Schaumburg, IL, USA) (n = 15)

• Group 3 – Ionoseal (VOCO America Inc., Briarcliff Manor, USA) (n = 15)

• Group 4 – Control group (n = 15).

Group 1 (glass-ionomer cement)

Powder and liquid were mixed in the standard ratio of 3:1 on the paper pad with the plastic spatula applied in the orifice and condensed with the small round condenser. A uniform barrier thickness of 3 mm was maintained.

Group 2 (TheraCal LC) and Group 3 (Ionoseal)

The material was dispensed from a syringe and applied to the canal orifice to a uniform thickness of 3 mm following the manufacturer’s guidelines (materials were placed incrementally and light-cured for 20 s per increment).

Group 4 (control)

No gutta-percha was removed, and no intraorifice barrier was placed.

The teeth were kept at 37°C and 100% humidity for 24 h post-IOB placement to permit complete setting of the materials.

In each group, a cotton pellet soaked in 35% H₂O₂ was inserted into the access cavity, and the teeth were subjected to light-activated heat for 2 min. This procedure was repeated twice, with a fresh cotton pellet impregnated with 35% H₂O₂ placed each time. Decoronation was done with diamond discs till the CEJ. Decoronated root surfaces were coated with a thick layer of wax extending 3 mm below the CEJ. The teeth were placed in self-cured acrylic resin blocks, leaving 3 mm of the coronal portion exposed, and fracture strength was evaluated using a universal testing machine [Figure 1].

Figure 1.

Figure1(a-f) placement of intraorifice barrier and testing done under UTM. (a) 3mm removal of gutta-percha, (b and c) GIC placed as a barrier, (d) TheraCal placed as a barrier, (e) Ionoseal placed as a barrier, (f) Fracture resistance testing under Universal Testing Machine

Testing method

Compressive forces were applied to the root canal orifice of each tooth along its long axis using a round-headed rod at a crosshead speed of 1 mm/min, and the fracture resistance was recorded in Newtons.

RESULTS

Intergroup analysis of mean fracture resistance among the groups using the analysis of variance test. The TheraCal group exhibited the highest mean fracture resistance (314.13 N), followed by the Ionoseal group (286.23 N) and the GIC group (213.24 N). The control group showed the lowest mean fracture resistance (123.13 N), and these differences were statistically significant (P = 0.000) [Table 1].

Table 1.

Intergroup comparison of fracture resistance among the study group

Study groups	n	Mean	SD	SE	P
GIC	15	213.24	77.99	20.14	0.000*
TheraCal	15	314.14	117.94	30.45
Ionoseal	15	286.23	108.16	27.93
Control	15	123.13	56.17	14.50

*Significance. ANOVA, P≤0.05 considered statistically significant. SD: Standard deviation, SE: Standard error, GIC: Glass-ionomer cement

Tukey’s post hoc test demonstrated a significant difference between the GIC and TheraCal groups (P = 0.023), whereas no significant differences were found between GIC and Ionoseal (P = 0.153) or between GIC and the control group (P = 0.050).

The TheraCal and Ionoseal groups did not differ significantly (P = 0.845), whereas both the TheraCal versus control and Ionoseal versus control comparisons were statistically significant (P = 0.000) [Table 2].

Table 2.

Multiple pairwise comparison among the study groups

Reference group	Comparison group	Mean difference	P
GIC	TheraCal	−100.89333	0.023*
	Ionoseal	−72.99133	0.153
	Control	90.10733	0.050
TheraCal	Ionoseal	27.90200	0.845
	Control	191.00067	0.000*
Ionoseal	Control	163.09867	0.000*

*Significance. Tukey’s post hoc test, P≤0.05 considered statistically significant. GIC: Glass-ionomer cement

DISCUSSION

Endodontically treated teeth exhibit a higher susceptibility to biomechanical failure than vital teeth due to significant alterations in their physiologic characteristics and physical properties, particularly the reduction of immature collagen levels and a decline in the modulus of elasticity caused by dehydration.[8]

According to Dietschi et al., the reduction in tooth strength caused by the loss of coronal tissue due to carious lesions or restorative procedures directly correlates to the amount of remaining tooth structure, highlighting the critical importance of preserving dental integrity to ensure optimal resistance against occlusal forces.[9]

In the current study, the specimens underwent endodontic treatment prior to bleaching and fracture testing. The biochemical and biomechanical modifications in dentin resulting from endodontic treatment, alongside the loss of tooth structure during access opening, significantly compromise the fracture resistance of dental specimens, highlighting the critical need for integrated approaches in endodontic procedures to preserve tooth integrity.

The use of various tooth bleaching materials can significantly compromise the fracture resistance of teeth by inducing detrimental changes in dental structure, including increased porosity, demineralization, diminished adhesion of restorative materials to dentin, higher dentin permeability, decreased microhardness, and lower diametral tensile strength.[10]

Based on the findings of Kawamoto and Tsujimoto, the degradation of H₂O₂ releases hydroxyl radicals that play a critical role in tooth whitening by targeting the organic components of intertubular and peritubular dentin; this process not only enhances the permeability of the dentin but also compromises its hardness and elasticity modulus.[11]

Rundquist and Versluis stated that the use of greater taper instrumentation in root canal procedures often leads to increased widening of the coronal third,[12] necessitating a strategic focus on reinforcing this vulnerable region using restorative materials that closely mimic the modulus of elasticity and compressive strength of dentin, as well as effectively bonding to dentinal walls; this approach may enhance coronal sealing, augment structural integrity, and reduce the risk of root fractures under functional stress.

The use of nonvital bleaching in dentistry, while effective in improving esthetic outcomes, carries significant risks such as the infiltration of H₂O₂ into dentinal tubules, which compromises the integrity and mechanical properties of dental hard tissues, increases the likelihood of dental fractures and relapses, and poses a serious threat of external cervical root resorption. Therefore, the implementation of a protective barrier material of 3 mm over the root filling is essential to mitigate these adverse effects and preserve dental health.

Placement of a 3-mm intraorifice barrier plays a crucial role in reinforcing the cervical third of endodontically treated teeth, which is the region most susceptible to fracture. An adequately thick barrier acts as a stress-absorbing and stress-distributing layer, reducing the concentration of occlusal forces at the canal orifice. In addition, it prevents penetration of bleaching agents into the root canal system, thereby limiting chemical weakening of dentin and preserving fracture resistance.

To date, although the ideal material has not yet been understood, various materials have been proposed as an intraorifice canal barrier.

The present study assessed the fracture toughness of root canal-treated teeth following intracoronal bleaching, emphasizing the importance of a dependable IOB to prevent cervical root resorption. Hence, this study evaluated the root fracture resistance following the placement of different IOBs: conventional GIC, Ionoseal, and TheraCal.

GIC was preferred in this study because it possesses the majority of the optimal properties first suggested for IOBs. It was the most commonly preferred IOB material because it is a self-adhesive substance that has a good chemical interaction with the root dentine, is biocompatible, has low pH, and releases fluoride ions, and has antibacterial activity near teeth and their capacity to enhance enamel and dentin remineralization.[13]

Ionoseal RMGIC has enhanced physical properties, self-adhesion capabilities, biocompatibility, and fluoride release. According to Croll, Ionoseal demonstrates high compressive strength and a transverse strength, which indicates that it not only withstands significant forces but also maintains structural integrity under various conditions, making it suitable for clinical use as a sealing and lining agent.[14]

TheraCal is a light-cured, resin-modified calcium silicate cement composed of calcium oxide, type III Portland cement-based calcium silicate particles, strontium glass, fumed silica, barium sulfate, barium zirconate, and a resin matrix of Bis-GMA and polydimethacrylate. The setting reaction is fundamentally dependent on the polymerization of its resin component, which distinctly differentiates it from hygroscopic dental cements such as mineral trioxide aggregate and those based on bioceramics, calcium silicate, or calcium sulfate, highlighting the need to reevaluate material classifications in dental applications based on their chemical and physical setting mechanisms.[15] Its capacity to form apatite greatly enhances dentine repair and mineralization by fostering the development of hydroxyapatite-like crystals. These crystals not only reinforce the chemical bond to the dentine but also establish a crucial biological seal.[16]

The results demonstrated that TheraCal exhibited the highest mean fracture resistance, followed by Ionoseal and GIC, with the control group showing the lowest mean fracture strength.

TheraCal exhibited the highest fracture toughness, which may be attributed to its ability to penetrate and hydrate dentin, enhancing micromechanical retention and promoting uniform stress distribution, thus lowering the risk of fracture. In addition, the release of calcium ions from TheraCal during intracoronal bleaching, along with their diffusion into dentin, further reinforces the tooth structure.

Ionoseal has lower fracture strength compared to TheraCal because it offers adhesion and fluoride release but remains brittle due to its glass-ionomer base. Its low resin content improves properties slightly but is insufficient to overcome the limited toughness and fracture resistance. However, the findings of Güray Efes et al. contradict this study, which demonstrated that Ionoseal exhibited high fracture strength.[17]

GIC showed the lowest fracture strength among the groups, likely due to porosity from glass–polyacrylic acid interaction, which creates stress concentration points. Its lower compressive strength compared to resin-based materials further increases susceptibility to occlusal stress and fracture. The observations of Poornima et al. and Malhotra et al. are consistent with the present study, indicating that GIC exhibits lower fracture and compressive strength.[18,19]

The control group, in which obturation was performed without an IOB, showed the lowest fracture strength. This finding is consistent with previous studies reporting reduced fracture resistance in endodontically treated teeth when an IOB is not used.

CONCLUSION

Within the study’s limitations, IOBs were found to significantly improve the fracture resistance of endodontically treated teeth. Among the materials tested, TheraCal demonstrated the highest resistance, followed by Ionoseal, while GIC showed the lowest. These findings suggest TheraCal as a promising option for reinforcing tooth structure, though further studies are needed to validate long-term performance under varying conditions.

Conflicts of interest

There are no conflicts of interest.

Funding Statement

Nil.