Coronal restorations of pulpless teeth after endodontic therapy are still a challenging issue. ¹ The strength of endodontically treated teeth (ETT) could be enormously reduced due to the weak tooth structure resulting from caries, trauma, or previous restorations and loss of pulp chamber roof. The design of restoration and type of restorative materials are very determining in this situation; they not only restore and seal the weakened tooth but also reinforce it. ² Today, resin composites have become the preferred choice of many dentists and patients for the coronal restorations because of their sufficient retention and esthetic and mechanical properties with maximum conservation of tooth structure. ¹ The latter is a result of their bonding ability to dental substrates that leads to cusp splinting/reinforcing effect. ³ It has been debated that intracoronal adhesive composite resin restorations provide internal reinforcement of mesio-occluso-distal (MOD) restorations in ETT without occlusal coverage. ⁴ These restorations may be an alternative restorative treatment if a normal occlusion, without parafunctional habit, is present. ⁵

Polymerization shrinkage of composite resin, as an inherent property, is considered a major problem so that if the resultant force exceeds the interfacial bond strength, adhesive failure negates the adhesive reinforcement of the weakened cusps. ⁶ With high bond strength, tooth structure may be involved in stress fracture. ⁷ Consequently, incremental technique was suggested to overcome these problems and ensure complete curing to achieve optimum performance of the restoration. ⁸ However, the effectiveness of this sensitive and time-consuming placement on reduction of shrinkage stress was not confirmed by some authors. ⁹ Recently, the “bulk-fill”

materials were introduced into the market. Development in photoinitiator dynamics and increased translucency could allow a deeper light penetration and curing. Therefore, bulk-fill composite resins could be applied in a thickness of 4–5 mm and cured at once with low shrinkage stress. Similar to the conventional composite (CC) resins, this new category is provided in low-viscosity (flowable) and high-viscosity (paste) types. ¹⁰

Bulk-fill materials simplify the restorative procedure and save clinical time, especially in wide and deep cavities. Recently, better performance of these composite resins, compared to conventional ones, in extensive MOD cavities has been reported in terms of shrinkage stress and fracture resistance (FR). ¹¹ However, this result was not supported in two other situations on FR of ETT, with similar FR for two types of composite resins. ¹² , ¹³

The development of different types of fibers, such as polyethylene or glass fiber in fiber-reinforced composite, has increased the resin composite applications. Fibers have the ability to reduce polymerization shrinkage, tolerate tensile stress, and stop crack propagation in resin composite materials. ¹⁴ Ribbond (Ribbond, Seattle, WA, USA) is a polyethylene fiber with woven network that allows an infusion of the resin into the fibers. The fiber network, higher modulus of elasticity, and lower flexural modulus of the polyethylene fibers are believed to have a modifying impact on the interfacial stresses developed along the adhesive interface and allow efficient force transmission. ¹³ , ¹⁵ Therefore, they could act as stress relievers in restored teeth and may prevent unfavorable subgingival fracture of composite restorations and increase the reparability of fractured teeth. ¹⁶ Despite some reports on the beneficial effect of fiber reinforcement in composite resin restorations on FR of ETT, ¹³ , ¹⁷ no significant strengthening effect has also been reported. ¹² To the best of our knowledge, there was no study evaluating the effectiveness of using fibers beneath bulk-fill resin composites. Therefore, the present study aimed to analyze fiber reinforcement on the fracture strength (FR) and failure mode of flowable and paste bulk (PB)-fill resin composites in endodontically treated premolars.

The materials used in this in vitro experimental study are listed in Table 1. Eighty freshly extracted human maxillary premolars with approximately the same sizes (measured mesiodistally and buccolingually by a digital caliper) and complete root formation were selected. The teeth were checked to be free of caries, any restorations, fractures, or cracks through transillumination technique. After removing any soft-tissue deposits by hand scaler and being stored in chloramine-T solution at 4°C for 24 h, the teeth were stored in distilled water up to 1 month and randomly assigned to eight groups, ten teeth in each.{Table 1}

Group 1: Intact teeth with no cavity preparation or root canal treatment were used as control samples

Remaining in distilled water at 37°C for 24 h, all restorations were finished and polished with Sof-Lex disc (3M ESPE, St. Paul, MN, USA), and rubber points. The teeth were subjected to thermocycling (5000 cycles at 5°C and 55°C, with 30-s dwell time and 5-s transfer time). All the specimens were embedded vertically in self-curing acrylic resin (Acropars, Tehran, Iran) in a Teflon mold up to 1 mm apical to the CEJ while the tooth's long axis was perpendicular to the base of the mold.

FR test was performed by a universal testing machine (Instron model 4302, Darmstadt, Germany). The compressive load was delivered using a 6-mm stainless steel sphere at crosshead speed of 1 mm/min, perpendicular to the long axis of the tooth. The sphere touched the buccal and lingual cusps of the teeth until a fracture occurred. The forces needed for fracture were recorded in Newton (N). To determine the failure modes, the fractured specimens were observed under a stereomicroscope at magnification of ×40. Failure modes were classified as:

Restorable failures when the fracture line was above the CEJ or 1 mm or less apical to the CEJ

After verifying normal distribution with the normality test (Kolmogorov–Smirnov test), data were analyzed using one-way analysis of variance and Tukey's honest significance difference multiple comparison tests at the significance level of P < 0.05.

The mean and standard deviation of FR in each of the eight groups are presented in Table 2and Figure 1. Power analysis showed that the sample size of this study resulted in acceptable power values (80% ≤ power). Group 1 (intact teeth, 1204 ± 252) demonstrated the highest strength and a significant difference with all groups (P < 0.001). Group 2 (unrestored teeth: 352 ± 143) had the lowest FR showing a significant difference (P < 0.05) with the other groups except G7. In our assessment, there was no statistically significant difference among the FR of G3, G4, and G5 (P > 0.05), indicating similar FR for three types of composite resin without fiber.

Fiber insertion resulted in a significantly higher FR only for CC resin (G6 = 858 ± 215 vs. G3 = 579 ± 114, P = 0.01). G6 had the highest strength with a significant difference with other experimental groups (P < 0. 03) except for G5 (624.9 ± 182) and G8 (802 ± 201). There was no noteworthy difference between G5 and G8. Fiber in combination with PB-fill composite resin revealed no reinforcing effect and showed the lowest FR (G7 = 529 ± 124) among the experimental groups that were significantly lower than that of G6 and G8 (P = 0.003 and P = 0.002, respectively.).{Table 2} Figure 1

Mean fracture resistance and standard deviation among the groups. G1 (Control, Intact teeth), G2 (Control, unrestored teeth), G3 (conventional composite), G4 (paste bulk-fill), G5 (flowable bulk-fill), G6 (Fiber+ conventional composite) G7 (Fiber + paste bulk-fill), G8 (Fiber + flowable bulk-fill).

The present study evaluated the FR of ETT with MOD cavities and intracoronally (no cuspal coverage) restored with different types of composite resins reinforced by fiber. Although cusp coverage is recommended as the definite restoration in ETT, we used intracoronal restoration to mimic a clinical scenario in which ETT cannot be restored permanently like in case of endodontic or periodontal problems. In the maintenance phase, preservation of restored ETT with no catastrophic fracture is an important issue. It is evidenced that cusp fracture is one of the main reasons for loss of ETT. In this study, we used TB as the bulk-filled material, because of its sculpt-able consistency, and that it can be applied in one increment; also, it was considered suitable for posterior final restoration as described by the manufacturer. ¹⁹ Furthermore, x-tra base (low-viscosity bulk fill) was used due to its higher filler content (approximately 75 wt%) and flexural strength similar to other employed composite resins. ²⁰

The results of our study revealed that also high-viscosity TB composite resin had a comparable FR compared to conventional counterpart (TN composite). These findings are in agreement with the results of Atalay et al. and Kemaloglu et al. and Yasa et al. studies. ¹² , ¹³ , ²¹

These authors reported that CC resin and bulk-fill composite resin showed no difference in the strength of restored teeth. Composition, filler content of resin composites, and their elastic modulus are the important factors attributed to polymerization shrinkage stress and subsequent clinical fractures. ²² Benetti et al.'s study showed that Tetric-EvoCeram (TEC) bulk-fill composite resin had the same polymerization contraction stress and gap formation similar to TEC CC resin. ⁶ Do et al. also showed that TEC bulk fill had the lowest cuspal flexure than other tested bulk-fill composite resins. ²³ According to El-Damanhoury and Platt, TEC bulk fill exhibited lower stress than the control material with good mechanical properties, enabling it as a final restoration. ²⁴ Rosatto et al. revealed that the use of bulk-filling technique resulted in significantly lower cuspal strains and shrinkage stresses with higher FR. ¹¹ TB used in this study demonstrated a higher depth of cure due to the improvements in their initiator (Ivocerin) and increased translucency. ²⁵ , ²⁶ It has filler content similar to TN (around 77%–80%). This may contribute to the similar FR obtained for two types of composite resins. Akbarian et al. also indicated that silorane-based composite with less volumetric shrinkage compared with dimethacrylate-based composite, both had a similar FR. ²⁷ It seems that polymerization shrinkage stress does not directly affect FR. Such conflicting results might be due to the variety of types and dimensions of the cavities and the direction of the applied load on the examined teeth. Our study showed that the FR of ETT restored with CC resin with Ribbond fiber was significantly higher than that without fiber. Ribbond is a leno-woven ultra-high-molecular-weight polyethylene ribbon. It has a high tensile strength, modulus of elasticity and fracture toughness, and biocompatible and excellent optical properties. ²⁸ This is consistent with Ayad et al.'s results, indicating that polyethylene fiber has a modifying effect on the developing interfacial stresses and has a strengthening effect on the remaining tooth structure. ²⁹

Khan et al. demonstrated that both polyethylene ribbon and glass fiber significantly enhanced the FR of MOD composite resin restorations with no difference between the two groups. It was thought that the fiber network would absorb and change the stress at the restorative/adhesive interface and reduce the risk of fracture. ³⁰ Furthermore, fibers would replace a part of the composite resin increment and decrease in the overall volumetric polymerization shrinkage of the composite resin. Hshad et al. also reported the considerably beneficial effect of polyethylene ribbon fiber on the fracture strength of ETT restored with CC resin. ³¹ In contrast to our results, Göktürk et al. found no significant difference between the fracture strength of the restored premolars using CC resin with and without fiber insertion. ³² Such contradictory results could be attributed to the variation in the composite resin brands, types of the teeth (premolars vs. molars), cavity size, loading device, and the experimental situation.

In contrast to the effect of fiber reinforcement with CC resin in our study, this effect was not observed using PB-fill composite resin and fiber + PB-fill composite resin showed the lowest mean FR among the experimental groups. It was not statistically different from CC resin and bulk composite resin groups. This phenomenon might be explained due to the presence of a weak interaction and gap formation between the PB-fill composite resin and the fiber. The possibility of this effect might be less in case of using flowable composite resin; hence, establishment of a unique united structure and chemical bond between flowable composite resin, fiber, and restorative resin is essential in obtaining a positive effect of fiber in the FR, ¹³ which is easily achieved in CC resin placed in increments of maximum 2 mm while this integrity may be not achieved with 4 mm bulk layer. The possible presence of void between the bulk layer of the composite resin and the fiber might create some flexure of the restoration and subsequent flaw formation and the reduced FR of the teeth. When bulk-fill flowable composite resin was used with fiber, a significantly higher strength was obtained although this difference was not statistically significant. Considering similar fracture strength for bulk-fill flowable and CC resins with fiber reinforcing, the former approach could be suggested to simplify the composite resin placement in ETT with extensive MOD cavities. This provides improved adaptation and shortened restorative time. In the literature, only in a recent study by Takce et al., the fiber was used beneath a bulk-fill flowable composite resin (smart dentin replacement). These authors concluded that this approach behaves similar to CC resin with fiber in terms of FR in ETT. However, bulk-fill flowable without fiber and PB fill with or without fiber were not compared in their study. In the present study, the majority of the restorable failure pattern was observed in groups with fiber reinforcement which was similar to the intact tooth. ³³ Recently, Eliguzeloglu Dalkılıç et al. evaluated the effect of two fiber insertion techniques on the fracture strength of restored teeth using FB-fill composite resin. ³⁴ They found no increase in the fracture strength values; however, fiber increased the favorable fracture modes consistently with our findings. ³⁴ These results are attributed to the stress distributive effect of polyethylene fiber, as shown in Yanyu et al. and Hshad et al.'s studies. ³¹ , ³⁵ However, the specimens restored with conventional nanohybrid and bulk-fill composite resins were more prone to unrestorable fractures.

This in vitro study was conducted under a static load with no simulation of in vivo situation. Considering the intraoral conditions, further in vivo studies should be performed to evaluate the effect of different fibers with different bulk-fill composite resins in restoring the ETT.

Based on the limitations of this study, the following results were reached:

The conventional nanohybrid composite resin and flowable and PB-fill composite resins without fiber had comparable FRs

Acknowledgement

The authors thank the vice-chancellery for research, Shiraz University of Medical Sciences, Shiraz, Iran, for supporting this research (Grant #12597).This article is based on the thesis by N.Ghaderi. The authors all thank Dr. Vosough of the Dental Research Development Center of the school of Dentistry for the statistical analysis and Dr. Shokrpour for his editorial assistance.

Financial support and sponsorship

This work was supported by Shiraz University of Medical Sciences, Shiraz, Iran (grant no. 12597).

Conflicts of interest

The authors of this manuscript declare that they have no conflicts of interest, real or perceived, financial or non-financial in this article.