An in vitro comparative evaluation of different... : Journal of Conservative Dentistry and Endodontics (2024)

INTRODUCTION

Endodontically treated teeth are more susceptible to fracture than vital teeth because of excessive loss of tooth tissue, dehydration of the dentin, and pressure during obturation procedures. Previous clinical studies have shown that 11-13% of extracted teeth with endodontic treatment are associated with vertical root fractures, rendering it the second most frequent identifiable reason for loss of root-filled teeth.[^1,2] Some authors have indicated that endodontically treated teeth get desiccated and inelastic while others have suggested that root fractures most often occur in teeth after root canal treatment due to loss of tooth structure, use of irrigants and medicaments, and excessive widening of root canals.[³]

Bonded composite restorations can be considered as the first choice for coronal restorations as they play a vital role in increasing the fracture resistance of endodontically treated teeth.[⁴] Furthermore, it has been shown that direct and indirect cusp coverage adhesive restorations can significantly increase the fracture resistance of endodontically treated teeth.[⁵] Bonding endodontic obturation materials to radicular dentin is another approach to increase fracture resistance.[^6,7] Use of sealers and lateral condensation technique significantly strengthen the roots as compared to canals, which are instrumented but not obturated.[⁸]

The adhesive potential of gutta-percha to the radicular dentin has been found to be far from satisfactory.[^6,7] Thus, there is a need for different materials and/or techniques to overcome the shortcomings of current endodontic filling materials to reinforce the roots.

Studies have advocated the use of intracoronal barriers in preventing coronal microleakage.[⁹] Through the use of restorative materials with elastic moduli similar to the dentin, it might be logical to assume that intraorifice barriers can also provide stiffness against forces that generate root fractures.

Very few studies have assessed the reinforcing effect of intraorifice barriers placed over root canal fillings. Hence, the objective of this study was to evaluate the effect of four different intraorifice barrier materials mineral trioxide aggregate (MTA), resin-modified glass ionomer cement (RMGIC), fiber-reinforced composite (FRC), and nanohybrid composite (NC) on the fracture resistance of roots obturated with gutta-percha and AH Plus sealer.

MATERIALS AND METHODS

Specimen preparation

A total of 75 extracted human single rooted mandibular premolars with single canal and less than 10° curvature with approximately same dimension were selected and stored in 1% chloramine-T (Himedia Labs, Mumbai, India) solution for 12 h followed by storage in distilled water until use. All the teeth were examined under a stereomicroscope (Carl Zeiss Stemi 2000 Arlesan Technology Group, Carl Zeiss, Italy) at 10X magnification to ensure the absence of preexisting fractures. In order to standardize the root measurements, the mesiodistal and buccolingual diameters of the coronal plane were measured with a digital caliper (SPAC systems, Pune, Maharashtra, India). Roots presenting with 10% difference from the average mesiodistal and buccolingual diameters of the coronal plane were excluded. Specimens were decoronated with diamond disc (DLC Australia Pty Ltd., Caboolture Qld, Australia) and water as a coolant to a standardized length of 14 mm.

Instrumentation and obturation of root canal system

After determination of the working length, root canals were instrumented with rotary ProTaper universal system (Dentsply Maillefer, Ballaigues, Switzerland) in a sequential manner till F3 using crown down technique (as per manufacturer's instructions). During instrumentation, canals were irrigated with 2 mL of 5.25% sodium hypochlorite after each change of file and final rinse of 5 mL 17% ethylenediaminetetraacetic acid (EDTA). Finally, canals were flushed with 10 mL of distilled water and dried with paper points. Obturation was performed using corresponding gutta-percha (Dentsply Maillefer, Ballaigues, Switzerland) and AH Plus Sealer (De Trey-Dentsply, Konstanz, Germany). Excess gutta-percha protruding out of the root canal was seared off with a hot burnisher. The samples were then stored in an incubator (Weiss Gallenkamp, Königswinter, Germany) at 37°C for 8 h to allow complete set of the sealer.

Placement of intraorifice barrier

Except for control specimens, coronal 3 mm of root canal obturation was meticulously removed with the aid of a customized spoon excavator (API Ashoosons, Mehrauli, New Delhi), heated red hot on a Bunsen burner and later on with alcohol (70%)-moistened microbrushes to remove sealer remnants.

The specimens were randomly divided into five groups with respect to the intraorifice barrier material used over root canal obturation. Each group consisted of 15 specimens.

Group 1 — MTA (ProRoot, Dentsply, Tulsa Dental) (n = 15)

MTA was mixed according to the manufacturer's recommendation and placed incrementally with MTA gun (Dentsply Maillefer, Ballaigues, Switzerland) into prepared coronal space and condensed with the help of plunger of MTA gun. A moist cotton pellet was placed on the top and left undisturbed to set completely for 24 h.

Group 2: Resin-modified glass ionomer cement (Vitremer, 3M ESPE, USA) (n = 15)

After mixing according to the manufacturer's instructions, the material was placed into the cavity using plastic carrier instrument (SKU: PFI1 Hu Friedy, India) and condensed using plastic instrument (SKU: PLGOR1 Hu Fiedy) and light-cured for 40 s.

Group 3: Fiber-reinforced composite (Turku, Southwest Finland, Finland) (n = 15)

After dentin conditioning with 37% phosphoric acid for 15s, the preparation was rinsed with distilled water for 10s, and excess moisture was removed with moist cotton. A thin layer of bonding agent (Adper Single Bond 2, 3M ESPE, USA) was applied with applicator tips and light-cured for 10 s. Fibers (Ribbond, Seattle, Washington, USA) were packed in the composite already placed incrementally in the prepared space. Light-curing was performed for 40 s for each increment.

Group 4: Nanohybrid composite (Filtek Z250 XT, 3M ESPE, USA) (n = 15)

After dentin conditioning with 37% phosphoric acid for 15 s, the preparation was rinsed with distilled water for 10 s and excess moisture was removed. A thin layer of bonding agent (Adper single bond 2, 3M ESPE, USA) was applied with applicator tips and light-cured for 10 s. The prepared intraorifice space was then restored with NC (Filtek Z250 XT, 3M ESPE, USA) using three increments of 1 mm each and light-cured.

Group 5: Control

The coronal 3 mm of gutta-percha was not removed.

After placement of the intraorifice barrier materials, all the specimens were stored at 37°C and 100% humidity for 1 week in an incubator to allow the materials to set completely.

Mounting and testing of specimens

The apical root end of each tooth was aligned vertically along their long axis in self-curing acrylic (Quick - Ashvin, Delhi, India) filled in 10 × 10 × 20 mm dimension (length × breadth × height) stainless steel blocks, leaving 3 mm of each root exposed. Periodontal ligament (PDL) simulation was performed using light body elastomeric impression materials (Express, 3M ESPA, USA). The specimens were mounted on a universal testing machine (Micronix, South Korea) and a compressive force was applied at a crosshead speed of 1 mm/min until fracture occurred. The force necessary to fracture each specimen as displayed on the monitor was recorded in newton (N).

Statistical analysis

Software Statistical Package for the Social Sciences (SPSS Inc, Chicago, USA) 16.0 was used for statistical analysis. The required force to fracture the specimens was recorded and subjected to statistical analysis using mean and standard deviation. Analysis of variance (ANOVA) and unpaired t-test were applied to find the significant difference between the different groups at 5% level of significance.

RESULTS

The mean force required for vertical fracture to occur in all five groups can be arranged in the following manner: RMGIC > FRC > composite > MTA > control [Table 1].

The RMGIC group showed maximum resistance to root fracture, whereas the control group showed minimum resistance. Statistically significant difference was observed between fracture scores of Groups 1, 2, 3, 4, and 5. A statistically significant difference was observed between fracture scores of MTA and FRC, MTA and RMGIC, MTA and composite, MTA and control, FRC and composite, FRC and control, RMGIC and composite, RMGIC and control, and composite and control [Table 2]. A statistically significant difference was observed between the fracture scores of FRC and RMGIC.

RMGIC and FRC showed considerable resistance to vertical root fracture than MTA and composite. However, RMGIC showed better resistance to root fracture than FRC.

DISCUSSION

Much of the fracture susceptibility of endodontically treated teeth is intrinsic to the root canal morphology, dentin thickness, canal shape, and size and curvature of the external root;[¹⁰] thus, special attention should be given for securing sufficient remaining dentin. However, enlargement of the coronal third of the root canal space is considered important to support root canal length measurement, debris removal, effective irrigation, and canal obturation. However, extensive use of rotary instruments during preparation of the root canal space by cutting the dentin to gain straight lines access weakens the root structure. Desiccation and dehydration of the dentin are also a few of the causes that may predispose to the weakening of tooth. Rundquist et al. (2006) stated that with increasing taper, root stresses decreased during root filling but tended to increase for masticatory loading, resulting fracture originating in the cervical portion.[¹¹]

Although bonded obturation materials might increase the fracture resistance of root-filled teeth, the current endodontic obturation systems are not suitable to obtain this goal. In the present study, the core material (gutta-percha) combined with the tested endodontic sealer (AH Plus) was not able to increase the root fracture resistance significantly in all the groups including the control group. Zandbiglari et al. (2006) also observed that roots get significantly weakened with the use of greater taper instruments and obturation with AH Plus sealer was not able to increase the fracture resistance.[¹²]

Based on this premise, the present study evaluated the reinforcing ability of commonly used postendodontic materials MTA, RMGIC, FRC, and NC. The presence of intraorifice barriers strengthen the fracture resistance of endodontically treated teeth as compared to endodontically treated teeth without intraorifice barriers. The fracture strength values of the test groups revealed that fracture resistance of the roots was significantly affected by the type of intraorifice barrier used. To reinforce endodontically treated tooth, stress concentrations at the dentin material interface should preferably be minimized by using materials with a modulus of elasticity similar to that of the dentin, which is about 14-16 GPa.[⁶] Both RMGIC and FRC significantly increased the fracture resistance of the root specimens. The results indicated that RMGIC (852.7 N) reinforced the tooth most followed by FRC (819 N), NC (679 N), and MTA (573 N). Our results are in conjunction with the previous study conducted by Nagas et al. (2010) although NC was not included in his study.[¹⁰] NC have higher filler content which further strengthens and improves elastic modulus with less shrinkage, so it was included in the study. Aboobaker et al. (2015) also have reported RMGIC and flowable resin to be an effective intraorifice barrier with significantly high resistance.[¹³]

MTA provides a superior seal against microleakage when used as an intracanal medicament; so MTA was evaluated in the present study for its fracture resistance. Similar to the study conducted by Nagas et al., results of our study also showed the lowest values for fracture resistance among all the tested groups.[¹⁴] Low fracture resistance may be attributed to its lack of bonding to the dentin, high stiffness in compression, and little strength in tension.

Composites bond to the tooth structure micromechanically and thus, provide good marginal seal, reinforcement of remaining tooth structure,[¹⁵] and conservation of tooth structure. Composite resins reportedly absorb and distribute forces in a uniform manner, thereby increasing resistance to fracture and providing an improved prognosis. Incorporation of fibers increases the elastic modulus of nonreinforced resin from 6-9 GPa to 9-15 GPa, which is close to the dentin,[¹⁶] thus resulting in higher values of fracture resistance for FRC than conventional resin composite. Moreover, fibers used in FRC have a unique, patented interpenetrating polymer network structure, which can be reactivated even after the final polymerization, resulting in superior anchorage.

RMGIC shows superior performance as an acceptable coronal seal due to water sorption by the material, resulting in setting expansion.[¹⁷] RMGIC has high flexural strength and modulus of elasticity (10-14 GPa) close to the dentin.[¹⁸] Thus, the material can withstand a large amount of stress before transmitting the load to the root.[¹⁹] Moreover, it chemically bonds with the dentinal surface, rendering more strength at the dentin cement interface.[²⁰] All these properties might have resulted in RMGIC being the most fracture-resistant material tested in the present study.

Both FRC and RMGIC have high flexural strength and high modulus of elasticity;[²⁰] with values that are similar to dentin, both materials can withstand large amounts of stress before transmitting the load to the root.[²¹] Moreover, both materials exhibit good adhesive properties to the dentin,[²²] which might have significantly contributed to the greater fracture resistance values obtained in the present study.

Root reinforcement with the tested intraorifice barriers did not totally reduce the susceptibility of roots to fracture. However, within the limitations of this study, it might be concluded that the reinforcement of obturated roots with FRC or RMGIC as intraorifice barriers can be regarded as a viable choice to reduce the occurrence of postendodontic root fractures. Further laboratory research with different materials coupled with clinical trials is necessary to validate the results of this in vitro study.

However, more studies with simultaneous testing of both microleakage and fracture resistance are needed including more materials and parameters.

CONCLUSION

Endodontically treated roots with an intraorifice barrier are more resistant to fracture compared with those without ones. Fracture resistance of roots was significantly affected by the type of intraorifice barrier. RMGIC and FRC followed by NC significantly increase the fracture resistance of endodontically treated teeth. RMGIC yielded the highest fracture resistance followed by FRC, NC, and MTA. MTA is not suitable for root reinforcement.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.

REFERENCES