Effect of fiber reinforcement on the fracture resistance of bulk-fill composite restorations in endodontically treated premolars: an in vitro study

Abstract

Background

This in vitro study aimed to evaluate the fracture resistance of endodontically treated premolars restored with different bulk-fill composites with and without polyethylene fiber reinforcement.

Methods

A total of 144 extracted premolars were prepared with standardized mesococclusodistal cavities and allocated to 12 groups (n = 12). Root canal treatment was performed in all but the intact control group. Restorations were carried out via a nanohybrid composite (Filtek Z250) or one of four bulk-fill composites (SDR, SonicFill 2, Tetric EvoCeram Bulk Fill, Filtek Bulk Fill Posterior), each with or without polyethylene fiber reinforcement. After 24 h of water storage at 37 °C, the samples were loaded axially with a steel sphere until fracture. Fracture resistance values (mean ± standard deviation, N) were recorded, and the data were analyzed via one-way ANOVA and Tukey’s test (α = 0.05).

Results

Intact teeth presented the highest fracture resistance (1278.82 ± 124.19 N), whereas unrestored teeth presented the lowest fracture resistance (255.88 ± 59.17 N). Among the restored groups, Filtek Bulk Fill Posterior (996.77 ± 55.90 N) provided the greatest resistance, approaching intact values. Fiber reinforcement did not consistently increase fracture resistance and significantly reduced it in the Filtek Bulk Fill Posterior group (p < 0.05).

Conclusions

Within the limitations of this in vitro study, high-viscosity bulk-fill composites, particularly Filtek Bulk Fill Posterior, effectively restored the fracture resistance of premolars to levels approaching intact teeth. The routine use of polyethylene fiber reinforcement does not provide consistent benefits and may compromise performance.

Background

The structural integrity of endodontically treated teeth (ETT) is critical for long-term clinical success, particularly in maxillary premolars, which are prone to fracture owing to their narrow crown morphology and the weakening effect of mesio-occluso-distal (MOD) cavity preparations [1–3].

Adhesive composite restorations are commonly used to rehabilitate ETT, yet the conventional incremental technique is time-consuming and technique-sensitive, predisposing restorations to interfacial defects and incomplete polymerization [4, 5]. Bulk-fill composites, which allow placement in increments of up to 4–5 mm, were introduced to overcome these limitations. They offer improved depth of cure, reduced polymerization shrinkage stress, and enhanced clinical efficiency, with favorable clinical and laboratory outcomes reported [4, 6, 7].

Fiber reinforcement, particularly with polyethylene and glass fibers, has also been proposed to improve the biomechanical behavior of structurally compromised teeth [8, 9]. However, evidence remains inconsistent: while some studies report improved fracture resistance, others indicate minimal or even adverse effects depending on fiber type, bonding capacity, and placement technique [10–12].

Given these discrepancies, the combined use of bulk-fill composites and fiber reinforcement remains controversial, especially in MOD-prepared premolars that are subjected to high occlusal stresses. Further investigation is needed to clarify whether polyethylene fiber reinforcement provides additional mechanical benefits when it is used with bulk-fill composites.

The null hypothesis of this study was that neither the type of composite resin (conventional nanohybrid or bulk-fill) nor the use of polyethylene fiber reinforcement would significantly influence the fracture resistance of endodontically treated maxillary premolars.

Methods

Specimen selection and preparation

A total of 144 extracted, intact, noncarious, single-rooted human maxillary premolars of similar dimensions were selected. All teeth were extracted for orthodontic or periodontal reasons, unrelated to this research. After extraction, the teeth were stored in 0.1% thymol solution at room temperature until use. All the samples were examined under 10× magnification with a stereomicroscope (Stemi 2000-C, Carl Zeiss MicroImaging, Göttingen, Germany) to exclude cracks or structural defects. Teeth with similar crown dimensions (± 10% variation in buccolingual and mesiodistal widths) were selected to ensure standardization. Measurements were performed using a digital caliper. All root canal treatments were performed by a single experienced endodontist (IO), and all cavity preparations and restorative procedures were carried out by a single operator (FAB) to standardize the protocol and eliminate inter-operator variability. Standardized MOD cavities were prepared with an isthmus width of 3 mm, depth of 5 mm, and remaining wall thickness of 2.5 mm using a high-speed flat-end fissure diamond bur (#856, Komet, Germany). Endodontic access was performed with a round diamond bur (#801, Komet, Germany). All procedures were completed under copious water cooling. The remaining 12 teeth served as the intact control.

Root canal treatment

The root canals were prepared with ProTaper Universal rotary instruments (Dentsply Maillefer, Ballaigues, Switzerland) up to size F3. Irrigation was performed with 2 mL of 5% sodium hypochlorite (NaOCl) after each file. The final irrigation consisted of 5 mL of 5% NaOCl, 5 mL of 15% EDTA (Imicryl, Konya, Turkey), and 5 mL of 5% NaOCl. Canals were dried with paper points and obturated with gutta-percha and an epoxy resin–based sealer (2Seal, VDW, Munich, Germany). The orifices were sealed with a 1 mm-thick layer of glass ionomer (Ionofil U, Voco, Cuxhaven, Germany). The samples were stored at 37 °C and 100% humidity for seven days to allow complete sealer setting before restoration.

Group allocation and restorative protocols

The specimens were randomly assigned to 12 groups (n = 12) via a computer-generated randomization sequence. The group allocation data are summarized in Table 1.

Table 1.

Experimental groups

Group	Restoration protocol	Endodontic treatment	Fiber reinforcement
INT	Intact (negative control)	No	No
UR	Endodontically treated, unrestored	Yes	No
Z250	Filtek Z250	Yes	No
Z250-F	Filtek Z250 + fiber	Yes	Yes
SDR-Z250	SDR base + Filtek Z250	Yes	No
SDR-F-Z250	SDR base + fiber + Filtek Z250	Yes	Yes
SF2	SonicFill 2	Yes	No
SF2-F	SonicFill 2 + fiber	Yes	Yes
TEC	Tetric EvoCeram Bulk Fill	Yes	No
TEC-F	Tetric EvoCeram Bulk Fill + fiber	Yes	Yes
FBP	Filtek Bulk Fill Posterior	Yes	No
FBP-F	Filtek Bulk Fill Posterior + fiber	Yes	Yes

The fiber reinforcement used was a polyethylene fiber ribbon (Construct Fiber Ribbon, Kerr, Orange, USA). SDR: Smart Dentin Replacement (Dentsply, USA)

In the fiber-reinforced groups, a polyethylene fiber ribbon (Construct Fiber Ribbon, Kerr, Orange, USA; 9 mm length, 1 mm width) was adapted buccolingually on the pulpal floor with a transparent instrument before adhesive application (Fig. 1).

Fig. 1.

Standardized MOD cavity preparation (3 mm isthmus width, 5 mm depth, 2.5 mm wall thickness) and restoration scheme. A layer of glass ionomer cement and polyethylene fiber reinforcement are illustrated

All teeth were etched with 35% phosphoric acid (enamel for 30 s, dentin for 15 s) and bonded with Adper Single Bond 2 (3 M ESPE, St. Paul, MN, USA). Restorations followed the manufacturer’s instructions:

• Filtek Z250 (nanohybrid composite): 2-mm increments, each light-cured for 20 s.

• SDR (flowable bulk-fill): single increment up to 4 mm, light-cured for 20 s; occlusal surface capped with nanohybrid composite. The combination of SDR as a flowable bulk-fill base with a nanohybrid occlusal layer (Filtek Z250) reflects a commonly used clinical protocol that aims to optimize cavity adaptation, reduce polymerization contraction stress, and enhance occlusal wear resistance.

• SonicFill 2, Tetric EvoCeram Bulk Fill, Filtek Bulk Fill Posterior: single increment up to 5 mm, light-cured for 20 s.

Polymerization was performed with an LED curing unit (Woodpecker LED-D, Guilin, China; 1000 mW/cm², 430–480 nm). The output was verified with a radiometer (Demetron LED Radiometer, Kerr, USA) before use. The details of all the restorative materials, manufacturers, and lot numbers are summarized in Table 2.

Table 2.

Materials used in the study

Material name	Type/Category	Manufacturer (Country)	Lot number
Filtek Z250	Nanohybrid composite	3 M ESPE, St. Paul, MN, USA	N560885
SDR	Flowable bulk-fill	Dentsply DeTrey GmbH, Konstanz, Germany	09301
SonicFill 2	Sonic-activated bulk-fill	Kerr, Orange, CA, USA	5,767,355
Tetric EvoCeram Bulk Fill	Bulk-fill composite	Ivoclar Vivadent, Schaan, Liechtenstein	U17294
Filtek Bulk Fill Posterior	High-viscosity bulk-fill	3 M ESPE, St. Paul, MN, USA	N682702
Construct Fiber Ribbon	Polyethylene fiber	Kerr, Orange, CA, USA	5,073,955
Ionofil U	Glass ionomer cement	Voco, Cuxhaven, Germany	1,522,052
Adper Single Bond 2	Adhesive	3 M ESPE, St. Paul, MN, USA	N563309
2Seal	Epoxy resin–based sealer	VDW, Munich, Germany	1,603,000,534

Storage and fracture resistance testing

After restoration, the samples were stored in distilled water at 37 °C for 24 h. No thermocycling or mechanical loading was applied to evaluate the initial fracture resistance without the influence of aging effects. Each tooth was embedded vertically in self-curing acrylic resin up to 2 mm below the cementoenamel junction using cylindrical molds to ensure standardized positioning. Fracture resistance was measured in a universal testing machine (Shimadzu, Kyoto, Japan) by applying an axial compressive load with a 5-mm stainless-steel sphere at a crosshead speed of 1 mm/min until failure. The maximum load at fracture (N) was recorded.

Statistical analysis

Normality was tested with the Shapiro–Wilk test, and homogeneity was tested with Levene’s test. The data were analyzed via one-way analysis of variance (ANOVA) followed by Tukey’s post hoc test. The level of significance was set at α = 0.05. The effect size (η²) was calculated. An a priori power analysis (G*Power 3.1.9.7) determined that a minimum of 12 specimens per group (total N = 144) would provide > 0.80 statistical power to detect medium effect sizes (f = 0.25) at α = 0.05.

Results

The intact teeth (INT) presented the highest fracture resistance (1278.82 ± 124.19 N; mean ± standard deviation (SD)), whereas the endodontically treated but unrestored teeth (UR) presented the lowest fracture resistance (255.88 ± 59.17 N; p < 0.001). Compared with the plants in UR group, those in the restored groups presented significantly greater resistance.

Among the restored groups, Filtek Bulk Fill Posterior (FBP) demonstrated the highest resistance (996.77 ± 55.90 N), which was significantly lower than that of intact teeth but closest to their performance. The lowest resistance among the restored groups was observed in Filtek Z250 (Z250) (3: 635.18 ± 83.12 N), whereas SDR base + Filtek Z250 occlusal (SDR-Z250), SonicFill 2 (SF2), and Tetric EvoCeram Bulk Fill (TEC) presented intermediate values.

Polyethylene fiber reinforcement had an inconsistent effect. Its addition significantly reduced fracture resistance in the FBP group (Group 12: 759.83 ± 68.43 N vs. Group 11: 996.77 ± 55.90 N; p < 0.05). For the other composites (Z250 group compared with Z250-F group, SDR group compared with SDR-F group, and TEC group compared with TEC-F group), no significant differences were observed.

One-way ANOVA confirmed significant differences among the groups (p < 0.001), with a large effect size (η² = 0.88), indicating that 88% of the variance in fracture resistance was explained by the type of restoration. Multiple comparisons (Tukey’s post hoc test, α = 0.05) are presented in Table 3. The distribution of fracture resistance across groups is illustrated in Fig. 2.

Table 3.

Fracture resistance values of the experimental groups (mean (SD)N)

Group	Fracture resistance (N)
INT (Intact)	1278.82 (124.19) a
UR (Unrestored)	255.88 (59.17) b
Z250 (Filtek Z250)	635.18 (83.12) c
Z250-F (Filtek Z250 + Fiber)	722.47 (102.38) cd
SDR-Z250 (SDR base + Filtek Z250)	693.49 (129.17) cd
SDR-F-Z250 (SDR base +Fiber + Filtek Z250)	677.07 (66.84) cd
SF2 (SonicFill 2)	776.22 (90.11) d
SF2-F (SonicFill 2 + Fiber)	660.95 (87.07) cd
TEC (Tetric EvoCeram Bulk Fill)	755.82 (83.02) d
TEC-F (Tetric EvoCeram Bulk Fill + Fiber)	691.16 (60.65) cd
FBP (Filtek Bulk Fill Posterior)	996.77 (55.90) e
FBP-F (Filtek Bulk Fill Posterior + Fiber)	759.83 (68.43) d

The values are the means (SDs) N. Groups with different supercript letters indicate statistical differences (Tukey’s test, p < 0.05). Groups sharing the same superscript show similar statistical behavior; INT showed the highest values, followed by FBP, an intermediate cluster consisting of most bulk-fill groups, Z250 as a lower-resistance group, and UNR with the lowest values

Fig. 2.

Mean fracture resistance values (mean (SD) N) of the experimental groups. Letters above bars indicate statistical groupings; groups not sharing the same letter differ significantly (Tukey’s post hoc test, α = 0.05)

Discussion

The present study confirmed that, compared with a conventional nanohybrid composite, bulk-fill composites demonstrated superior mechanical performance. Given the significant differences observed among the restorative materials and between fiber-reinforced and non-reinforced groups, the null hypothesis stating that neither composite type nor fiber reinforcement would affect fracture resistance was rejected. This finding is consistent with previous in vitro evidence attributing the improved behavior of bulk-fill systems to optimized filler loading, reduced polymerization shrinkage, and more favorable stress distribution [13]. Among the tested materials, FBP group exhibited the highest fracture resistance, performing closer to intact teeth than the other restorative protocols did. These results support earlier reports that high-viscosity bulk-fill composites are particularly advantageous for posterior teeth under heavy occlusal loading [14, 15]. Previous studies have demonstrated that bulk-fill composites differ considerably in their mechanical behavior, with high-viscosity formulations showing superior stress distribution under occlusal loading [13–15]. Our results align with these findings, as Filtek Bulk Fill Posterior outperformed the other materials, consistent with reports by Hegde and Sali [14] and Al-Nahedh and Alawami [15]. Conversely, the inconsistent effect of polyethylene fibers agrees with the heterogeneous outcomes reported in systematic reviews, which emphasize technique sensitivity and placement-dependent performance [16–18]. Some studies reported improved fracture resistance with glass or polyethylene fibers [8, 9], whereas others demonstrated neutral or even adverse effects depending on fiber orientation, polymerization, or bonding limitations [11, 12]. Such discrepancies may explain why fiber reinforcement significantly reduced the performance of Filtek Bulk Fill Posterior in the present study, suggesting that fiber placement may interfere with light transmission or stress distribution, as also discussed by Garoushi et al. [18].

This study also provided one of the few direct comparisons of multiple bulk-fill composites with and without polyethylene fiber reinforcement in standardized MOD-prepared maxillary premolars. While bulk-fill materials consistently improved fracture resistance, polyethylene fibers produced inconsistent outcomes, sometimes significantly reducing resistance. This observation underscores the technique sensitivity of fiber reinforcement and highlights the need for further research to establish protocols that optimize its clinical effectiveness [16, 17]. The unexpected reduction in resistance contrasts with the theoretical benefits of fiber reinforcement, which are thought to derive from enhanced crack deflection and toughness [8]. The inconsistent performance of polyethylene fibers observed in the present study aligns with previous findings reporting technique-dependent or unfavorable effects depending on fiber location, hydrophobicity, impaired interfacial bonding, and reduced light transmission beneath the fiber layer [11, 12, 18, 19]. Possible explanations include the hydrophobic nature of polyethylene fibers and impaired polymerization beneath the fiber layer due to reduced light transmission [11, 12, 18].

Not all the bulk-fill composites performed equally. Compared with high-viscosity systems, the flowable SDR composite exhibited inferior resistance, which can be attributed to its lower filler loading and reduced modulus of elasticity [19]. These findings emphasize that bulk-fill composites do not form a homogeneous category and that high-viscosity formulations are better suited for posterior restorations subjected to high occlusal demands. Moreover, the efficiency of fiber reinforcement depends strongly on its position and orientation, with recent studies showing more favorable results when fibers are placed closer to the occlusal surface or combined with cusp reduction [20, 21]. Additional factors, such as adequate polymerization and surface pretreatment of fibers, also appear critical [22].

Although all tested restorative strategies provided fracture resistance values above average physiological masticatory forces [23, 24], high-viscosity bulk-fill composites clearly demonstrated superiority. From a clinical perspective, the present results suggest that high-viscosity bulk-fill composites should be preferred when restoring endodontically treated maxillary premolars exposed to high occlusal loads. Polyethylene fiber reinforcement, although theoretically beneficial, does not consistently increase fracture resistance and may even reduce fracture resistance in certain configurations. Therefore, its application should be restricted to carefully selected clinical scenarios and applied under well-controlled protocols [16, 18]. As all tested restorations exceeded functional thresholds, failures in practice are more likely to result from fatigue or parafunctional stresses, underscoring the importance of patient-specific occlusal evaluation in treatment planning [23–25]. These recommendations are consistent with clinical evidence showing satisfactory long-term performance of bulk-fill composites in posterior teeth [25, 26] and with randomized clinical trial results confirming their reliability in Class II restorations [27].

This in vitro study presents several methodological limitations that should be acknowledged when interpreting the findings. First, only static load-to-fracture testing was performed, without the use of thermocycling, mechanical fatigue loading, or long-term aging procedures. The absence of such protocols limits the extrapolation of the results to the dynamic and thermomechanical conditions of the oral environment. In addition, axial compressive loading was used, whereas oblique forces may more accurately represent functional masticatory loads.

The study also did not evaluate fracture modes; therefore, distinctions between restorable (supracervical) and non-restorable (subcervical) failures could not be identified. This restricts the clinical interpretability of the fracture resistance values, particularly for fiber-reinforced groups, where differences in crack propagation patterns may be expected.

Periodontal ligament simulation was not included, which may have altered stress distribution during loading. Only one type of polyethylene fiber and a single placement orientation were tested, and no assessment was made regarding the polymerization efficiency beneath the fiber layer—an aspect that may influence its mechanical contribution due to potential light attenuation.

Furthermore, the restorative and fiber-reinforcement techniques were limited to specific materials and protocols, and the findings can not necessarily be generalized to other bulk-fill systems, adhesive strategies, or fiber designs. Although teeth were standardized dimensionally, natural anatomical variability may still have influenced fracture behavior.

Considering these limitations, future research should incorporate cyclic fatigue loading, thermal aging, periodontal ligament simulation, evaluation of multiple fiber orientations and materials, and detailed fracture mode classification to enhance clinical relevance.

Conclusion

Within the limitations of this in vitro study, high-viscosity bulk-fill composites demonstrated superior fracture resistance compared with conventional nanohybrid composites. FBP was performed most closely to intact teeth, supporting its reliability for restoring endodontically treated premolars. Polyethylene fiber reinforcement did not provide consistent benefits and, in some cases, reduced fracture resistance, emphasizing its technique-sensitive nature. Therefore, high-viscosity bulk-fill composites can be recommended as the preferred restorative option in clinical practice, whereas fiber reinforcement should be reserved for carefully selected and well-controlled applications.

Abbreviations

ANOVA

Analysis of variance

EDTA

Ethylenediaminetetraacetic acid

ETT

Endodontically treated teeth

F

Fiber

FBP

Filtek BulkFill Posterior Composite Resin

INT

Intact

LED

Light-emitting diode

MOD

Mesio-occluso-distal

NaOCl

Sodium hypochlorite

SD

Standard deviation

SDR

Smart Dentin Replacement Composite Resin

SF2

Sonic Fill2 Composite Resin

TEC

Tetric EvoCeram Composite Resin

UR

Unrestorated

Z250

Filtek Z250 Composite Resin

Authors’ contributions

IO and FAB conceived and designed the study. IO performed the specimen selection and endodontic treatment and contributed to the methodology and investigation. FAB performed restorative procedures, supervised the laboratory workflow, and provided resources. IO drafted the manuscript; FAB critically revised the manuscript for important intellectual content. Both authors read and approved the final manuscript.

Clinical trial number

Not applicable.

Funding

No funding was received. The study was self-funded by the authors.

Data availability

The datasets generated and/or analyzed during the current study are available from the corresponding author upon reasonable request.

Declarations

Ethics approval and consent to participate

This in vitro study was conducted in accordance with the ethical principles of the Declaration of Helsinki and approved by the Scientific Research Ethics Committee of Biruni University (Approval No: 2024-BIAEK/12–16). As the study involved extracted human teeth collected under institutional and national regulations, informed consent to participate was waived by the Committee, as no living human subjects or identifiable data were included.

Consent for publication

Not applicable.

Competing interests

The authors declare no competing interests.