Reinforcing the cervical dentin with bonded materials to improve fracture resistance of endodontically treated roots.

Introduction: Endodontic procedure leads to the loss of tooth structure resulting in fractures. Intraorifice barriers of bonded restorative materials placed in the cervical third of tooth may help in increasing fracture resistance. Materials and
Methods: Human mandibular premolars (n = 75) underwent decoronation to adjust working length at 14 mm and prepared up to F3. They were obturated using gutta-percha and resin sealer AH-Plus and randomly divided into five groups (n = 15), Group 1: Control obturated with gutta-percha only. Groups 2, 3, 4, and 5 had placement of intraorifice barriers after the removal of 3 mm coronal gutta-percha such that Group 2: RMGI, Group 3: Self-adhering flowable composite, Group 4: Bulkfill Flowable Composite, and Group 5: mineral trioxide aggregate (MTA). Mounting of specimens was done in acrylic resin to expose coronal 3 mm and tested using the universal testing machine.
Results: Group 1 (control) showed least fracture strength among all groups. Among those with intraorifice barriers, Group 2 Resin-modified glass ionomer cement showed maximum fracture resistance followed by Group 4 (Bulkfill composite) and Group 5 (self-adhering flowable composite) and least by Group 5 (MTA).
Conclusion: The type of intraorifice barrier had a significant impact on root fracture resistance. Copyright:

INTRODUCTION

Endodontic treatment is the primary approach for the resolution of pulpal and periapical pathology. However, various studies have reported that almost 11%–13% of all teeth that undergo extraction after endodontic treatment show the presence of cracks, craze lines, and vertical root fractures[12] The loss of coronal anatomic structures due to caries and removal of dentin during access cavity preparation, as well as impairment of neurosensory feedback due to pulpal tissue loss, are all the common causes of fracture of endodontically treated teeth.[3] Other reasons include chemical changes in tissue caused by endodontic agents such as sodium hypochlorite, ethylenediaminetetraacetic acid (EDTA), and intracanal medicaments such as calcium hydroxide.[456]

It is well-known that teeth with inadequate postendodontic restoration are more prone to fracture and coronal leakage, causing diffusion of oral fluids, bacteria, bacterial products, and possibly root canal treatment failure.[7] Thus, one of the objectives of postendodontic restoration is providing an impermeable hermetic seal and increase root fracture resistance. Roghanizad and Jones, inorder to reduce leakage, first proposed replacement of 3-mm of gutta-percha with restorative material at the root canal orifice.[8] Many studies have since confirmed the effectiveness of intraorifice barriers in preventing coronal microleakage and improved fracture resistance.[91011] Moreover, studies have advocated the use of restorative materials for endodontically treated teeth which have a similar or higher elastic modulus than the tooth can be proposed for providing stiffness against forces that generate root fracture.[1213] Thus, intraorifice barriers with restorative materials that could bond to radicular dentin could be used for additionally reinforcing the pericervical dentin as well as preventing coronal microleakage.

The common restorative materials which have been evaluated as intraorifice barriers for resisting microleakage in roots obturated with gutta-percha are the glass ionomer cement, composites and calcium silicate-based cements.[1415161718] However, the improved and recent advances of these materials which offer more ease of placement and superior properties than the conventional have not been compared in previous studies to compare their efficacy as root reinforcing material.

Hence, the objective of the present study was to investigate and compare the fracture resistance between endodontically treated roots obturated with gutta-percha and those along with placement of four different restorative materials as intraorifice barriers. The null hypothesis was that there is no difference in fracture resistance of roots obturated with gutta-percha and placement of glass ionomers cement (RMGIC), self-adhering flowable composite, bulkfill flowable composite, and white mineral trioxide aggregate (MTA) as intraorifice barriers.

MATERIALS AND METHODS

The present study was conducted in the Department of Conservative Dentistry and Endodontics of Maharashtra Institute of Dental Sciences and Research, Latur with the donors’ informed consent and ethics committee approval. A total of 75 single-rooted human mandibular premolar teeth extracted for orthodontic reasons with single canal with and <10° curvature with approximately same dimensions were selected and stored in normal saline until the use. Teeth with more than one canal, existing caries, cracks, open apices, and curvature of roots more than 10° and with mesiodistal and buccolingual diameter of coronal plane exhibiting more than 10% difference from average were excluded. Specimens were decoronated to standardize the length of 14 mm. The canals were instrumented with rotary ProTaper Universal system (Dentsply) till F3 using the crown-down technique. During instrumentation, canals were irrigated with 2 mL of 5% NaOCl and distilled water after each file change and final rinse of 5 ml of 17% EDTA. Obturation was performed using the single cone technique with corresponding gutta-percha points and AH plus sealer. The teeth were randomly assigned to five groups (n = 15) for the placement of intraorifice barriers such as Group I: control group, Group II: RMGIC (Vitremer), Group III: self-adhering flowable composite (Dyad Flow), Group IV: flowable Bulkfill Flowable Composite (Smart Dentin Replacement, Dentsply [SDR]), Group V: MTA Angelus. Except for control group, all specimens were prepared by removing coronal 3 mm of gutta-percha with spoon excavator heated on Bunsen burner followed by 70% alcohol moistened microbrushes used to remove sealer remnants. The intraorifice materials will be placed over the obturated roots in the coronal 3 mm space as follows:

Group II: RMGIC (Vitremer, 3M ESPE, USA), Shade A3. The primer was applied with a brush for 30 s to dentin and air dried. The primer was then light cured for 20 s. After mixing according to the manufacturer's instructions, the material was placed into the cavity, condensed and light-cured for 40 s.

Group III: self-adhering flowable composite (Dyad Flow, Kerr), Shade A3.

Dyad Flow was syringed into the cavity to obtain a thin layer (<0.5 mm) and was applied to the entire cavity wall by rubbing on all surfaces for 20 s. It was then light cured for 20 s. After lining the cavity wall, the cavity was completely filled with more dyad flow and then light cured for 20 s.

Group IV: Bulkfill Flowable Composite (SDR, Dentsply).

The cavities were bonded using Optibond All-in-One (Kerr) using the self-etch technique as specified by the manufacturer and light cured for 10 s. SDR resin was placed into the entire depth of cavity using the gun and ampule injection system and light cured for 20 s.

Group V: Tricalcium silicate-based cement: MTA Angelus (Angelus, Brazil).

MTA was dispensed on a glass slab in P/L ratio of 3:1 and mixed and placed incrementally with MTA gun into prepared coronal space and condensed. A moist cotton pellet was placed on the top and left undisturbed to set completely for 24 h.

After the placement of intraorifice barrier materials, all specimens were stored at 37°C and 100% humidity for 1 week in an incubator. The apical root end of each tooth was mounted vertically along the long axis in self-curing acrylic resin such that 3 mm of each root was exposed [Figure 1a]. In order to acquire periodontal ligament simulation as in a tooth socket, light body elastomeric impression material was used between the acrylic mold surface and root [Figure 1a and b].[16] The specimens were mounted on a universal testing machine and compressive force applied along the long axis of roots with the velocity of 1 mm/min until fracture occurs [Figure 1c and d]. The force upon the sample breaking was recorded in Newton and converted into MPa.

Figure 1

(a) Finished sample showing 3 mm exposed root embedded in acrylic resin. (b) Finished sample as viewed from the occlusal aspect showing periodontal ligament simulation. (c) Sample testing on universal testing machine. (d) Fractured Sample after testing

Statistical analysis

Data were collected by using a structured proforma and expressed in terms of mean and standard deviation and analyzed using the SPSS 24.0 version IBM USA (software SPSS 24, IBM, Chicago, IL, USA). The mean and SD between all groups were compared using the one-way ANOVA, and the Post hoc Tukey's HSD test was used to determine whether the mean difference between two groups was significant or not. P < 0.05 was considered statistically significant, whereas a P < 0.001 was considered as highly significant.

RESULTS

When mean fracture resistance [Table 1] values of Group I (Control) (606.33 ± 35.88 N), Group II (RMGIC) (976.33 ± 35.02 N), Group III (Flowable composite) (833.87 ± 33.51 N), Group IV (bulkfill flowable composite) (906.40 ± 38.77 N) and Group V (MTA) (738.20 ± 38.05 N) were compared to each other by the ANOVA test, it showed statistically significant difference (P = 0.048). The highest mean fracture resistance among all groups was shown by Group II for RMGIC (976.33 ± 35.02 N) and least was shown by Group V for MTA (738.20 ± 38.05 N). The order of fracture resistance of all groups in the decreasing order is such that RMGIC > SAFC > BFC > MTA > Control. All groups showed statistically significant difference from each other after analysis using Post hoc Tukey's test [Table 2] except Group I (606.33 ± 35.88 N) and Group V (738.20 ± 38.05 N) where the difference was not significant (P > 0.05).

Table 1

Comparison of mean fracture resistance, standard deviation and P value strength between Group I, Group II, Group III, Group IV, and Group V

	n	Mean±SD	F	P	Inference
Group I (control)	15	606.33±35.88	139.49	0.048 (<0.05)	Significant
Group II (RMGIC)	15	976.33±35.02
Group III (flowable)	15	833.87±33.51
Group IV (bulkfill flowable)	15	906.40±38.77
Group V (MTA)	15	738.20±38.05
Total	75	812.23±135.28

MTA: Mineral trioxide aggregate, RMGIC: Resin-modified glass ionomer cement, SD: Standard deviation

Table 2

Post hoc Tukey’s Honest significant difference test to see the mean difference between individual group is significant or not

	Group II (RMGIC)	Group III (flowable)	Group IV (bulkfill flowable)	Group V (MTA)
Group I (control)	−370.0*	−227.53*	−300.06*	−131.86
Group II (RMGIC)		142.46*	69.93*	238.13*
Group III (flowable)			−72.53*	95.66*
Group IV (bulkfill flowable)				168.2*

*Indicates statistically significant difference. MTA: Mineral trioxide aggregate, RMGIC: Resin-modified glass ionomer cement

DISCUSSION

The newer concept of biomimetic restoration of teeth needs to be executed while choosing restorative materials to ensure long-term prognosis of endodontically treated teeth. The occlusal forces acting on teeth are principally compressive in nature. During function and parafunction, compressive and tensile forces are produced intraorally which make it imperative for the restorative material to demonstrate resistance to these forces in the form of compressive strength.[19] Furthermore, when occlusal forces act, it causes a lateral or axial bending of tooth in the form of tooth flexure.[20] According to tooth flexure theory, occlusal forces are transmitted through the axial surface to cervical areas. The property of flexural strength of a given material demonstrates the resistance to fracture and helps to indicate the flaws within the material that may give rise to failure upon loading.[2122] Thus, the core material demands to possess the adequate values of compressive and flexural strength inherently, in order that it resists the occlusal forces and also prevents dislodgment of the core while in function. Concurrently, another important aspect for consideration in this regard is the modulus of elasticity of a material which gives an idea of the rigidity of a material. Cervical restorations flex along with the tooth when in function, and hence, they should possess a low modulus of elasticity.

Coronal restorations are under constant occlusal loading. Hence, the type of material used for coronal sealing in the pericervical dentin zone will affect both the fracture resistance of roots and sealing ability from coronal leakage. Thus, we can logically assume that choice of restorative material can influence the stress distribution patterns and affect fracture resistance of endodontically treated teeth. Bonded restorations have been shown to provide coronal reinforcement to the tooth in a study.[15] In such scenario, it is desirable that the materials possess a modulus of elasticity similar to that of dentin (14–16 gigapascals) so that it will minimize the stress concentrations at the dentin-material interface during flexure.[121323]

In the present study, the results of ANOVA test showed statistically significant difference (P < 0.05) when mean fracture resistance of Group I, Group II, Group III, Group IV, and Group V were compared. The results of our study are in accordance with similar studies conducted by Aboobaker et al.,[15] Gupta et al.,[16] and Yasa et al.[17] and support the finding that teeth with intraorifice barriers show higher fracture resistance as compared to roots obturated with gutta-percha and no placement of intraorifice barrier.

In the present study, the highest values of fracture resistance were showed by Group II (RMGIC) in comparison with other groups. The manufacturer states flexural strength of RMGIC as approximately 60 MPa. According to Goldberg, RMGIC has high flexural strength and elastic moduli values that is similar to dentin which is 10–14 GPa.[24] Hence, it has the potential to withstand significant amounts of stress before it transmits the forces to the root.[25] It forms a chemical bond with the root dentin and confers more strength at dentin-cement interface.[26] Dyad Flow showed statistically significant difference in fracture resistance as intraorifice barrier when compared with Control Group and Group II (RMGIC). When a resin of low viscosity is used, it possess low elastic modulus and acts like a stress absorbing layer.[2728] This decreases the incidence of crack formation in dentin as it itself absorbs the energy and offers protection to the root dentin[29] which is radicular dentin. In a study conducted by Aboobaker et al.(2015),[15] flowable composite has shown no significant difference in fracture resistance when compared with RMGIC as intraorifice barrier. This difference could be due to no preinsertion preparation done for RMGIC in their study, whereas in the present study, we have preconditioned dentin with primer before insertion of Vitremer. This pretreatment of dentin may have resulted in the better adaptation of RMGIC to the internal cavity wall, greater elasticity, and chemical and micromechanical bonding to tooth. Furthermore, Vitremer has additional self-curing property which may result in better polymerization of the resin component as compared to Fuji GC LC cement and both flowable composites which rely only on light curing for polymerization. Furthermore, the flexural strength of Vitremer is less (61.7 MPa) as compared to Dyad Flow (125.4 MPa) which is considerably high. This means that RMGIC is less stiff and more elastic as compared to flowable composite. Hence, RMGIC can flex better under occlusal loads and transfer stresses to the dentin in a better way. Hence, it may have given better and significant results in the present study.

The result of the present study shows a statistically significant difference (P < 0.05) between Group II (RMGIC) and Group IV (BFC). In the present study, RMGIC fared better than SDR. This is due to the difference in flexural strength of RMGIC (61.7 MPa) and SDR (approx 120 MPa). SDR is stiffer as compared to RMGIC. However, Yasa et al.[17] found no significant difference between fracture resistance of values between glass ionomer cement, nanohybrid composite, and Bulkfill Flowable Composite. This may be due to difference in materials used in their study (Filtek Bulk Fill flowable; 3M Espe) as compared to our study (SDR, Dentsply). He also used conventional glass ionomer cement (Equia; GC Corp., Tokyo, Japan) as opposed to RMGIC (Vitremer, 3M Espe) used in our study. This may have given the difference in results. Similarly, a study by Chauhan et al. found the fracture resistance of RMGIC inferior to that of nanohybrid composite the reason for which they stated a low rigidity of RMGIC when compared to stiffer nanohybrid composite.[30] The different results in our study are thus attributed to the difference in type of composite used in their study as compared to the two flowable composite varieties used in our study.

In the present study, Group V (MTA) showed highly significant difference (P < 0.001) when compared with Group II (RMGIC). These results are in conjunction with results of Gupta et al.[16] and Nagas et al.[14] who have obtained least fracture resistance of MTA when compared Vitremer. A study by Yasa et al.[17] gave statistically significant difference between fracture resistance of glass ionomer cement used as intraorifice barrier in comparison to MTA which showed least fracture resistance. In a recent study, Oskoee et al.[18] have not found any significant difference between MTA and RMGIC as intraorifice barriers. However, their study involved the use of treating teeth with different bleaching agents which also affected the fracture resistance of teeth. Hence, the difference in findings as compared to our study.

In the present study, statistically significant difference (P < 0.05) is seen between Group IV (BFC) and Group III (SAFC). SDR performed better than Dyad Flow. This may be attributed to the filler content of two materials. SDR has filler content of 68% by weight and 45% by volume and average particle size is 4.2μ. The average particle size of Dyad Flow is 1 micron and the percentage filler loading of flowable composites is also less due to greater concentration of diluents monomers. An important factor that can have an impact on the fracture strength value is the transfer of stress from the polymer matrix to the embedded filler particles. Thus, SDR performed better due to greater size and concentration of filler particles which could better absorb the occlusal load. Apart from the properties of the filler system used, the structure of monomers constituting the resin matrix also affects the mechanical properties. According to, Burgess and Cakir,[31] the matrix of SDR is chemically designed in such a way that it slows down the rate of polymerization, and reduces the stress of polymerization shrinkage without any effect on the levels of polymerziation shrinkage. Thus, it has very low overall shrinkage (3.5%) compared to other conventional flowable composites.[32] It thus decreases the stresses on cavity walls encountered during polymerization shrinkage.[33] Less polymerization shrinkage reduces the stresses that develop at dentin-resin interface due to shrinkage of resin component. Hence, it may have given better results as compared to Dyad Flow. Group V (MTA) presents statistically significant difference (P < 0.05) with Group III (SAFC) in the present study. There has been no study in the literature to investigate this finding, which could be due to a lack of bonding to the dentin, high stiffness in compression, and little strength in tension.[16]

Within the limitations of this study, we can recommend that the use of intraorifice barriers shall enhance the outcome of endodontic treatment by providing a reinforcing effect at the cervical third of canals. Further research along with clinical trials is needed to determine the best outcome of this technique with different materials and with or without full coverage crowns after endodontic treatment. In the era of minimally invasive endodontics, the findings of this study will help clinicians to obtain a viable and easy chairside technique to strengthen the obturated teeth using a wide range of routinely used restorative materials of their choice.

CONCLUSION

The fracture resistance exhibited by roots with RMGIC, Self Adhering Flowable Composite and Bulkfill Flowable Composite placed as intraorifice barrier was significantly higher than control group. The highest fracture resistance was exhibited by roots with RMGIC and lowest was exhibited by MTA placed as intraorifice barrier among all experimental groups.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.