Comparative evaluation of the effect of adhesive restorative composite resins on the reinforcement of peri-cervical dentin: An in vitro study.

Background: Peri-cervical dentin (PCD) and its reinforcement play a crucial role in the fracture resistance of root canal-treated teeth. Aims: The aim of this study was to compare the fracture resistance of dual-cure, nano-hybrid, and short-fiber reinforced composite resins restored PCD with conventional hybrid composite (CHC) resin restored endodontically treated mandibular premolars. Settings and Design: Academic, in vitro study. Materials and
Methods: The prepared mandibular premolar samples were randomly divided into five groups of 10 each. In Group 1, teeth were left intact. The remaining 40 teeth were endodontically treated and obturated as follows: In Group 2, teeth were obturated with gutta-percha till cementoenamel junction and restored with CHC. Teeth in Groups 3, 4, and 5 were obturated to a depth of 5 mm from the cervical line and restored with dual-cure, nano-hybrid, and short-fiber reinforced composite resins, respectively. Fracture resistance was tested using a universal testing machine. Statistical Analysis Used: One-way analysis of variance and post hoc Tukey's test.
Results: Short-fiber reinforced composite resin showed a significantly higher mean fracture resistance value compared with other experimental groups. The mean fracture resistance values were obtained as Group 1 > Group 5 > Group 4 > Group 3 > Group 2. Conclusions: Reinforcement with short-fiber reinforced composite showed significantly higher fracture resistance compared with nano-hybrid and dual-cure composite resins. Copyright:

INTRODUCTION

Restoring the tooth with a definitive, permanent, postendodontic restoration plays a crucial role in the success of root canal therapy.[1] The well-executed restoration with a good coronal seal prevents microleakage, maintains occlusal stability, and protects the tooth from cuspal flexure and future fracture. Root canal-treated teeth were claimed to be weaker and more likely to fracture than are the vital teeth.[2] The causes most often cited were loss of dentin, removal of important anatomic structures such as cusps, marginal ridges, and arched roof of the pulp chamber, all of which provide the necessary support for the tooth.[3]

Dentin loss, either iatrogenic or noniatrogenic, makes the tooth more susceptible to mechanical failure and generates higher stress concentrations.[4] Peri-cervical dentin (PCD), extending 4 mm above and 4 mm apical to the crestal level of bone, is of interest in this regard.[5] PCD transfers the occlusal stress along the root long axis. Research suggests that long-term retention of tooth and fracture resistance are in direct relation to the remaining amount of PCD.[6] It has been shown that adhesive restorative materials as intracanal and intracoronal restorations increased the fracture resistance of endodontically treated teeth.[7] Composite resin restorations due to their adhesive property require minimal cavity preparation, thereby resulting in the conservation of tooth structure and providing intracoronal reinforcement.[17]

Nanocomposites serve both functional needs and superior esthetics through the application of nanotechnology.[8] Nanotechnology is the process of creating materials with dimensions ranging from 0.1 to 100 nanometers using a variety of physical and chemical methods. The Nano-hybrid composite (NHC) used in the current study is a methacrylate-based NHC resin. Restorative resins differ in their application consistency from flowable to packable and in their mode of curing from autopolymerizing to dual curing. Dual-cure composites (DCCs) were developed to overcome the limitations of reduced interlayer strength, increased interfacial porosity, and depth of cure by incorporating a redox initiator system along with photoinitiators.[9] The DCC resin used in the current study contains low consistency fluoride fillers, which allows for easy application into the root canal space. Short-fiber reinforced composite resin, i.e., everX Posterior, has gained attention as a restorative material and is commonly suggested for use in high stress-bearing areas.[10] Although studies have shown usage of this material as Onlays, core buildup materials, there is limited literature till now on its effect in reinforcing PCD.[210]

Hence, this study aimed to evaluate and compare the fracture resistance of restored PCD in root-filled premolar teeth using various adhesive restorative composite resins.

MATERIALS AND METHODS

The institutional review board approved (VDC/IEC/2017/37) the study protocol. Fifty intact, non-carious, unrestored human mandibular premolars with single canals that were extracted for orthodontic or periodontal reasons were selected for the study. A sample size of 10 teeth per group was used with 80% power and 5% significance. Anatomic crowns of identical dimensions (i.e., 7 ± 1 mm mesiodistal diameter and 8 ± 1 mm buccolingual diameter) were measured using a caliper to standardize. Teeth with previous root canal therapy, developmental disturbances, root caries, and canal curvature of >15° were excluded from the study. All teeth were examined under a dental operating microscope (Carl Zeiss surgical GmbH, Germany) at ×10 magnification to confirm the absence of any preexisting fractures. Teeth were cleaned, all the soft-tissue deposits were removed with an ultrasonic scaler and stored in distilled water at room temperature. The maximum storage period for the extracted teeth was 4 months.

Crowns of all the samples were resected to obtain a dimension of 4 mm from 1 mm below the highest point of the proximal cervical line. The enamel was then removed with a diamond abrasive bur from all the surfaces.[6] Access cavities were made in 40 teeth using endo access bur (Dentsply, Maillefer, Switzerland). With a 15 K-file, the working length was established by inserting it into the canal till the apical foramen, then reducing 1 mm from the measured length. Working length was confirmed radiographically. Root canals were instrumented with rotary ProTaper files (Dentsply, Maillefer, U.S.A.) till F3 (#30) size file. During the procedure, glide path verification was done with a size 15 K-file. Intracanal irrigation was done with 2 ml of 5.25% sodium hypochlorite (NaOCl) (Prime Dental Products Pvt. Ltd., Thane) after every instrumentation. The root canals received final irrigation of 1 ml of 17% ethylenediaminetetraacetic acid (EDTA) (Prime Dental Products Pvt. Ltd., Thane) followed by 3 ml of 5.25% NaOCl, after which the canals were flushed with 10 ml distilled water to avoid the prolonged effect of EDTA (Prime Dental Products Pvt. Ltd., Thane) and canals were dried with paper points.

Table 1 provides a description of the materials used in this study. The samples were randomly allocated into five groups (n = 10/each group), and the following procedures were implemented:

Table 1

Description of materials used in the study

Material	Manufacturer	Composition
		Resin matrix	Fillers
Short-fiber reinforced composite	EverX Posterior: GC, Europe	Bis-GMA, TEGDMA, PMMA	Short E-glass fibers, Barium borosilicate glass
Dual-cure composite	MultiCore Flow: Ivoclar Vivadent, Schaan, Liechtenstein	UDMA, Bis-GMA, TEGDMA	Barium glass, BaAlFSiO4, YF3, SiO2, mixed oxide
Conventional hybrid composite	Te-Econom Plus: Ivoclar, Vivadent	UDMA, TEGDMA	Barium glass, YF3, SiO2, mixed oxide
Nano-hybrid composite	Tetric N-Ceram: Ivoclar Vivadent, Schaan, Liechtenstein	UDMA, Bis-GMA, ethoxylated Bis-EMA, TEGDMA	Barium glass, YF3, SiO2, mixed oxide

Bis-GMA: Bisphenol A-glycidyl methacrylate, TEGDMA: Triethylene glycol dimethacrylate, PMMA: Polymethyl methacrylate, UDMA: Urethane dimethacrylate, BaAlFSiO4: Barium aluminum fluorosilicate glass, SiO2: Silicon dioxide, Bis-EMA: Bisphenol A ethoxylated dimethacrylate, YF3: Ytterbium trifluoride

Group 1 – Negative control (NC) group – Teeth were left intact and were not subjected to root canal treatment.

Group 2 – Conventional hybrid composite (CHC) group: Teeth were obturated with gutta-percha cones using resin-based sealer (AH Plus sealer, Dentsply) till the level of cementoenamel junction and restored with CHC (Te-Econom Plus, Ivoclar Vivadent).

The remaining 30 teeth were obturated with gutta-percha cones using resin-based sealer (AH Plus sealer, Dentsply) till a depth of 5 mm from the cervical line and were restored as follows:

Group 3 – Teeth were restored with DCC resin

Group 4 – Restored with NHC resin (NHC)

Group 5 – Teeth were restored using short-fiber reinforced composite resin (SFC).

The roots of all teeth after restoration were mounted in self-cure acrylic resin up to the level of 1 mm apical to the cementoenamel junction. Simulation of periodontal ligament was done with polysiloxane impression material (Reprosil, Dentsply, Switzerland). The specimens were then positioned in a holding slot on the lower arm of the universal testing machine (Instron testing machine, Model No. 8801, U.K.). A metal indenter was fixed to the upper arm of a universal testing machine that was set to deliver increasing loads until fracture (25% drop in applied force). At a crosshead speed of 1 mm/min, the load was applied along the long axis of the tooth. The force required for the fracture of each tooth was recorded in newtons (N).

RESULTS

The collected data were subjected to statistical analysis using IBM SPSS Statistics for Windows, Version 21.0. Armonk, NY, USA: IBM Corp. Data were explored for normality using the Kolmogorov–Smirnov Z test. One-way analysis of variance was used to assess the mean fracture resistance values, followed by post hoc analysis using Tukey's test. P < 0.05 was considered statistically significant for all the comparisons. The mean fracture resistance values are presented in Table 2. Group 1 (NC group) showed a mean fracture resistance of 1483.11N, which was significantly higher than the other groups (P <.05). Group 2 (CHC group) showed the lowest fracture resistance (553.838N). Among the experimental groups, Group 5 (SFC group) showed a mean fracture resistance value of 1238.07 N, which is significantly higher than Group 4 (NHC group; 846.06 N), followed by Group 3 (DCC group; 696.85 N). All the pair-wise comparisons were statistically significant with P < 0.05 [Table 3].

Table 2

Comparison of mean fracture resistance between the study groups

Group	n	Mean±SD	F	P
Group 1 (PC group)	10	1483.11±148.78	193.90	0.000*
Group 2 (CHC group)	10	553.83±78.76
Group 3 (DCC group)	10	696.85±51.43
Group 4 (NHC group)	10	846.06±58.84
Group 5 (SFC group)	10	1238.07±64.09

SD: Standard deviation, CHC: Conventional hybrid composite, DCC: Dual-cure composite resin, NHC: Nano-hybrid composite resin, SFC: Short-fiber reinforced composite resin, PC: Positive Control

Table 3

Intergroup comparison of fracture resistance using Tukey’s test

Intergroup comparison	Mean difference	SE	Significance	95% CI
				Lower bound	Upper bound
PC group
CHC group	929.28100*	39.27208	0.000	817.6914	1040.8706
DCC group	786.26800*	39.27208	0.000	674.6784	897.8576
NHC group	637.05500*	39.27208	0.000	525.4654	748.6446
SFC group	245.04900*	39.27208	0.000	133.4594	356.6386
CHC group
DCC group	−143.01300*	39.27208	0.006	−254.6026	−31.4234
NHC group	−292.22600*	39.27208	0.000	−403.8156	−180.6364
SFC group	−684.23200*	39.27208	0.000	−795.8216	−572.6424
DCC group
NHC group	−149.21300*	39.27208	0.004	−260.8026	−37.6234
SFC group	−541.21900*	39.27208	0.000	−652.8086	−429.6294
NHC group
SFC group	−392.00600*	39.27208	0.000	−503.5956	−280.4164

SE: Standard error, CI: Confidence interval, CHC: Conventional hybrid composite, DCC: Dual-cure composite resin, NHC: Nano-hybrid composite resin, SFC: Short-fiber reinforced composite resin, PC: Positive Control

DISCUSSION

The selection of ideal restorative modality to compensate coronal tooth structure loss is considered a key factor for the success of postendodontic restorations.[11] Dentin is the collagen-rich organic matrix reinforced with calcium-phosphate mineral particles. The essential components of dentin are well optimized for various mechanical demands of oral cavity, and the loss of dentin was found to eventually compromise mechanical integrity of residual tooth structure.[4] After a careful understanding of forces responsible for stress development in endodontically treated teeth, the cervical portion of the tooth is the most susceptible area for concentration of stress.[12] It was reported that the mechanical interlocking between enamel and dentin in the cervical portion is weaker than in other areas of the dentinoenamel junction.[13]

Dentin in the cervical portion of the tooth was termed as PCD. It acts as the neck portion of the tooth.[5] Restoring PCD with composite resin resulted in a shift of microstrain away from apical root dentin, causing least apical microstrain with reduced flexing of cervical root dentin and aids in redistribution of the functional loads away from apical region.[14] Propensity to vertical root fracture might be expected to be decreased due to reduction in the apical microstrain. Considering the above findings, the present study focused on evaluating the reinforcement of the critical zone, i.e., the PCD.

In this in vitro study, during the cleaning and shaping of root canal, 17% EDTA (Prime Dental Products Pvt. Ltd., Thane) followed by 5.25% NaOCl (Prime Dental Products Pvt. Ltd., Thane) was used as a final rinse. Weiger et al. recommended using EDTA followed by NaOCl to optimize the adhesion of sealers to the root canal walls.[15] Epoxy resin-based sealer (AH Plus sealer) was used as it showed higher bond strength to the root canal dentin due to its inherent property of volumetric expansion, which contributes to better bond strength.[16]

In the present study, Group 1 (NC group) showed significantly higher fracture resistance than all the other groups (P < 0.05). This could be due to the presence of tooth-reinforcing structures like roof of pulp chamber and marginal ridges as there was no obvious loss of tooth structure in intact teeth.[17] The roof of the pulp chamber in intact tooth has the configuration of an arch, which is highly resistant to pressure and stress.[18] Group 2 (CHC group) showed significantly lower resistance to fracture (553.83 N) than all the other groups (P < 0.05). This could be because the elastic modulus of filling materials such as gutta-percha presents little or no capacity of reinforcing roots after treatment.[19] Dentin has an elastic modulus of approximately 16GPa whereas, modulus of elasticity of gutta-percha is only 77 MPa which is much lesser than the modulus of elasticity of dentin and therefore cannot strengthen roots.[20]

All the experimental groups of the current study showed significantly higher fracture resistance when compared with the CHC group. The result obtained in this study is consistent with the findings of a similar study wherein the PCD restored with bonded composite resin reduced flexing of cervical root dentin resulting in redistribution of functional loads away from the apical region.[14] Arora et al., in their study on PCD reinforcement ability of postobturation materials, have shown that adhesive postobturation materials significantly reinforced PCD.[6]

In the current study, SFC group showed significantly higher fracture resistance among the experimental groups. The reason could be because of the resin matrix of short-fiber reinforced composite that contains cross-linked bisphenol A-glycidyl methacrylate, triethylene glycol dimethacrylate, and linear polymethyl methacrylate forming a polymer matrix called semi-interpenetrating polymer network.[21] It provides good bonding and enhances the toughness of composite.[22] Furthermore, it was shown that everX Posterior had improved bond strength to the underlying dentin surface due to the presence of a micromechanical interlocking between the protruding short fibers of everX Posterior and dentin.[23] Short-fiber reinforced composite has an elastic modulus of 12.3 GPa, which is greater than the elastic modulus of all other adhesive composite resins used in the current study.[24] Studies have shown that fiber fillers in short-fiber reinforced composite resin could stop propagation of crack and provides multidirectional isotropic reinforcement effect with greatest reinforcement efficiency and thereby resulting in increased fracture resistance of composite resin.[1225] All these factors could have influenced the results obtained in this study. Within the limitations of this in vitro study, it can be concluded that restoring PCD with adhesive composite resins showed a significant increase in fracture resistance of endodontically treated teeth when compared with the CHC group. Among the experimental groups, short-fiber reinforced composite resin showed significantly higher reinforcement capability compared to that of nano-hybrid and DCC resins.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.