In restorative dentistry, a primary objective is to replicate the biomechanical performance of the natural dentition. This is particularly challenging when treating teeth with extensive coronal structure loss, where large cavity preparations or endodontic treatment further compromise structural integrity (1, 2). Composite resin is the material of choice for direct esthetic reconstructions due to its superior aesthetics and adhesive properties. However, when used in extensive restorations, its inherently brittle polymer matrix can lead to inadequate fracture resistance and catastrophic failure under functional occlusal loads (2).

To address these mechanical limitations, various fiber reinforcement strategies have been developed. Among them, polyethylene fibers have gained attention due to their unique properties, including high tensile strength, a low modulus of elasticity similar to dentin, and superior fracture toughness (3, 4). When incorporated into a resin composite matrix, these fibers are proposed to act as a stress-bridging network, improving impact resistance, distributing occlusal forces more uniformly, and reducing crack propagation (4–6). This mechanism holds promise for enhancing the longevity of large direct restorations and restorations involving fiber posts in endodontically treated teeth.

Despite the theoretical advantages, the in vitro evidence regarding the efficacy of polyethylene fiber reinforcement remains inconclusive. Several studies report a significant increase in fracture resistance for teeth restored with polyethylene fiber-reinforced composites or posts (7, 8), while others found no statistically significant difference compared to non-reinforced controls (9, 10). This discrepancy may be attributed to variations in study design, including fiber type, placement technique, cavity configuration, and testing protocols.

Given these inconsistent findings, a systematic evaluation and quantitative synthesis of the existing in vitro data are necessary to clarify the actual effect of polyethylene fiber reinforcement on fracture resistance. Therefore, this systematic review and meta-analysis aimed to assess whether the use of polyethylene fiber-reinforced composite resin or fiber post restorations improves the fracture resistance of permanent teeth compared to non-fiber-reinforced restorations. The study tested the null hypothesis that no significant difference exists between the fracture resistance values of teeth restored with and without polyethylene fiber reinforcement.

The present study was conducted following the PRISMA (Preferred Reporting Items for Systematic reviews and Meta-Analyses) guidelines (11). The protocol was developed a priori and can be accessed at: https://doi.org/10.17605/OSF.IO/5K8XB. The following PICOS framework was applied: Population, permanent teeth requiring restorative treatment; Intervention, restoration with polyethylene fiber-reinforced composite resin or polyethylene fiber-reinforced fiber posts; Control, restoration with non-fiber-reinforced composite resin, or conventional fiber posts; Outcome, fracture resistance (in Newtons); Study design, in vitro studies. Eligible controls thus included any standard restorative alternative against which the polyethylene fiber-reinforced intervention was compared within a study. The research question was: Does the use of polyethylene fiber-reinforced composite restorations increase the fracture resistance of permanent teeth?

A total of 4,289 papers were recognized in all databases searched. A flowchart that forms the report selection procedure agreeing to the PRISMA Statement is displayed in Figure 1. The literature review rescued 3,276 articles for the initial inspection after removing the duplicates. Afterward, 3,210 studies were excluded after reviewing the titles and abstracts, leaving 66 articles to be assessed by full-text interpretation. After the full-text assessment, 34 studies were excluded due to the following reasons: fifteen lacked a control group (13–27), four evaluated other physical properties different to fracture resistance (24, 26, 28, 29), four were clinical trials (30–33), three evaluated fracture resistance in permanent teeth with immature apices (34–36), three applied facture strength test to other points but not the teeth (16, 20, 37), two used another fiber type (38, 39), one did not use the teeth as specimens (40), one did not evaluate fracture resistance in permanent teeth (41), and one because access to the full document could not be obtained (42). The specific reason for each excluded study, mapped to the violated PICOS criterion, is provided in Supplementary Table S1.

A total of thirty-two manuscripts were included in the qualitative analysis, and from these, two were excluded from the meta-analysis because they did not have complete statistical data (43, 44). Finally, thirty manuscripts were included in the meta-analysis (3, 5, 22, 42, 45–70).

This review identified that the overwhelming majority of the included studies investigated the Ribbond™ brand (Ribbond Inc., Seattle, WA, USA) of polyethylene fibers. Thus, the findings of this synthesis pertain primarily to this specific commercial product applied alongside resin composite in Class I and II cavities of permanent teeth, with or without prior endodontic treatment. It also included fiber post restorations reinforced with Ribbond in teeth that underwent root canal treatment. The various composite resins and fiber posts were evaluated using fracture resistance tests, and the storage conditions of the samples included distilled water, saliva, chloramine solution, or thermocycling regimes for varying durations (Table 2). A detailed summary of the cavity configurations and fiber placement strategies employed in the included studies is also provided. It is noteworthy that all included studies assessed fracture resistance using a monotonic static loading protocol; no studies utilizing cyclic fatigue tests met the inclusion criteria for this review.

Figure 2 shows the meta-analysis of the fracture resistance values of the polyethylene fibers reinforced fiber post. According to the analysis, fracture resistance was enhanced when the polyethylene fibers system was used (p < 0.01). A high heterogenicity (93%) was observed. The pooled Standardized Mean Difference (SMD) was 0.76 (95% CI: [1.17, 0.36), representing a moderate-to-large effect size according to Cohen's criteria (SMD > 0.8).

Figure 3 shows the meta-analysis of the fracture resistance values of the polyethylene fibers reinforced composite resin restorations. According to the analysis, fracture resistance was enhanced when the polyethylene fibers system was used (p < 0.01). A high heterogenicity (89%) was observed. The pooled SMD was −0.82 (95% CI: [−1.16, −0.49]), representing a large effect size.

Figures 4, 5 show the meta-analysis of the number of unfavorable or catastrophic failures identified after the fracture resistance test. For both situations (with or without fiber posts), the use polyethylene fibers reduced the number of catastrophic failures in the restored teeth (p < 0.001). It is important to note that the specific definitions for “catastrophic” vs. “favorable” varied across studies, and no standardized re-classification was possible.

The high statistical heterogeneity observed in the meta-analyses reflects considerable clinical and methodological diversity across the included in vitro studies. Qualitative examination suggests potential sources include: the type of cavity preparation, the amount of remaining tooth structure, variations in the placement and orientation of polyethylene fibers, the use of different commercial brands of composite resins and adhesive systems and differing aging protocols.

The potential for publication bias in the meta-analyses was assessed through visual inspection of funnel plots. For the analysis of fracture resistance in teeth restored with polyethylene fiber-reinforced fiber posts (Figure 2), the funnel plot (Supplementary Supplementary Figure S2) showed a relatively symmetric dispersion of study estimates around the pooled effect size (SMD), with points scattered within the expected inverted funnel shape. This symmetry suggests a low likelihood of significant publication bias for this outcome.

In contrast, the funnel plot for the analysis of fracture resistance in teeth restored with polyethylene fiber-reinforced composite restorations (Figure 3, Supplementary Figure S3) revealed a more asymmetrical distribution. A noticeable gap was observed in the lower-left quadrant, indicating a potential absence of smaller studies reporting negative or null effects (i.e., no benefit from fiber reinforcement). While the heterogeneity (I² = 89%) precludes definitive conclusions, this asymmetry hints at the possibility of publication bias, where smaller studies with non-significant results might be missing from the literature.

Regarding the analysis of catastrophic failures, both funnel plots exhibited high symmetry. The plot for the comparison involving fiber posts (Figure 4, Supplementary Figure S4) and the plot for direct composite restorations (Figure 5, Supplementary Figure S5) displayed a balanced distribution of studies on both sides of the pooled risk difference (RD) line. This consistent symmetry across both analyses strengthens the robustness of the finding that polyethylene fiber reinforcement significantly reduces the incidence of unfavorable fractures, as it is less likely to be driven by unpublished data.

The risk of bias in the articles studied in this review is recapped in Table 3. Most of the articles included failed to meet the parameters of blinded operator and sample size calculation.

This systematic review and meta-analysis were directed to assess the fracture resistance of permanent teeth restored with polyethylene fiber-reinforced composite resin or fiber post restorations. The overall findings revealed that the polyethylene fibers in conjunction with composite resin or fiber post favored the fracture resistance of permanent teeth. Based on the findings of this study, the null hypothesis was rejected, indicating that the fracture resistance of permanent teeth with polyethylene fiber-reinforced composite restorations (p < 0.01) and polyethylene fiber-reinforced fiber posts (p < 0.01) significantly differs from that of non-fiber-reinforced composite and fiber post restorations. The large positive effect sizes (SMD > 0.8) indicate that the improvement in fracture resistance is not only statistically significant but also clinically substantial.

Ribbond fibers are composed of longitudinally oriented and crystallized polyethylene fibers. These fibers are used in combination with composite resin to reinforce teeth that have lost significant coronal structure. When applied adhesively, Ribbond fibers help enhance the mechanical properties of the tooth, increasing its strength and resistance to fracture by distributing forces uniformly and preventing crack propagation (48, 51, 71, 72). Also, the polyethylene fibers have been used to reinforce the fiber post in endodontically treated teeth to increase the durability and distribute forces along the teeth (51, 72). There are several studies evaluating the fracture resistance of teeth with polyethylene fiber-reinforced restorations (5, 45, 72). The mechanism of polyethylene fibers is to transfer stress from the polymer matrix of the resin composite into the fibers, which causes less stress to be transmitted to the remaining tooth structure (71).

In general, the studies included in this systematic review and meta-analysis indicate that fracture resistance improves with the use of polyethylene fibers. However, this improvement is influenced by other factors, such as the amount of remaining crown structure, the type of cavity preparation, and the arrangement or orientation of the fibers within the restoration. These variables play a significant role in determining the overall effectiveness of fiber-reinforced restorations in enhancing the tooth's mechanical stability (5).

The theoretical rationale for using polyethylene fibers is their potential to reinforce composite resin by improving stress distribution and bridging microcracks, thereby enhancing fracture resistance (47). However, the empirical evidence synthesized in this review reveals that this theoretical benefit is highly contingent on technical execution. While many studies report significant improvements (5, 48, 57), others—particularly those where fiber placement, orientation, or impregnation may have been suboptimal—found no significant difference or even a decrease in performance (60, 62). This dichotomy provides a compelling explanation for the high statistical heterogeneity observed (I² > 89%). It suggests that the fibers' high tensile strength and low modulus are advantageous only when they are correctly integrated to absorb and distribute occlusal forces. Improper placement, such as positioning fibers in compression zones rather than tension zones, or poor adhesion creating voids, could inadvertently create stress concentrators, explaining the diminished performance in some studies.

Unlike conventional composite materials, which may exhibit brittle failure under high loads, polyethylene fibers introduce a toughening mechanism by bridging microcracks and dissipating stress along their length (49). Their high tensile strength and low-density modulus allow them to absorb and distribute occlusal forces more effectively, reducing the risk of catastrophic fractures (43). Additionally, their ability to conform to cavity walls and integrate within the composite matrix contributes to a more uniform load distribution, further enhancing the mechanical stability of the restoration (56).

Some studies have reported that the way in which polyethylene fibers are placed is related to fracture resistance. The most common way of placing them in mesio-occluso-distal (MOD) cavities is on the buccal, lingual/palatal wall and on the floor of the cavity (47, 54); however, two studies positioned the fibers on the occlusal surface and found that extending the fiber ends through the occlusal third of the buccal or lingual/palatal walls significantly enhanced fracture resistance. This strategic placement allowed the fibers to bind the cusps together, improving the structural stability of the restoration and enabling better distribution of forces during occlusion, ultimately leading to higher resistance against fractures (49). Regarding type of cavity, a study showed that Class I cavity preparation restored with fiber-reinforced resin composite was stronger than Class II cavities, due to the marginal ridges (49); Supporting this, fracture resistance is lower in MOD cavities restored with polyethylene fiber-reinforced composite resin compared to occluso-distal (OD) cavities, due to the loss of both marginal ridges (43, 68).

In this sense, it is important to highlight the fact that the position of polyethylene fibers is crucial for optimizing their reinforcing effect in composite restorations. For fibers to effectively absorb and distribute occlusal forces, they should ideally be placed on the opposite side of the applied stress, allowing them to stretch and resist tension (66). If fibers are positioned in areas where compressive forces prevail, their reinforcing function may be compromised. This emphasizes the need for careful consideration of fiber orientation in clinical applications to ensure the maximum fracture resistance of the restoration.

Other studies have reported that in teeth with root canal treatment, fracture resistance increases when they are reconstructed with polyethylene fiber reinforcements in conjunction with composite resin or fiber post, even when an extracoronal restoration such as an overlay or endocrown is placed after this (3, 56).

Research on tooth restoration using various fiber reinforcement materials, including long polyethylene fibers (Ribbond), short glass-fibers (EverX Posterior), and combinations of fibers, has yielded intriguing results. One study found that fracture resistance with polyethylene fibers was lower than with EverX Posterior (53, 56, 58). Conversely, another study reported that Ribbond outperformed EverX Posterior (73). Additionally, a third study indicated that when EverX Posterior was combined with Ribbond in MOD cavities, it resulted in an increase in fracture resistance values. These findings highlight the variability in outcomes depending on the type of fibers and how they are used in restorative procedures (57).

On the other hand, few studies have concluded that there is no significant difference in the fracture resistance of teeth restored with composite resins reinforced with polyethylene fibers compared to those restored only with composite resins (56, 58), however they also do not report that the fracture resistance is lower or affected when polyethylene fibers have been used, so more studies that include all the variables are necessary to have a clearer result regarding fracture resistance.

Beyond improving fracture resistance, the present meta-analysis also revealed that polyethylene fibers significantly reduced the number of unfavorable or catastrophic failures, both in teeth restored with and without fiber posts (p < 0.001). This finding suggests that the inclusion of polyethylene fibers not only enhances the strength of restorations but also influences the failure pattern, favoring more favorable, repairable fractures. Similar outcomes have been reported by previous in vitro studies, where the incorporation of Ribbond fibers into resin composite restorations promoted a shift from catastrophic root fractures toward restorable failures confined to the coronal structure (68–70). This effect has been attributed to the fibers' ability to absorb and redistribute stresses throughout the resin matrix, preventing stress concentration at critical points of the tooth-restoration interface (71, 72).

Therefore, the present findings should not be interpreted as an unconditional recommendation for polyethylene fiber use. Instead, they highlight a technique-dependent intervention. The consistent positive directional effect across the meta-analysis indicates a robust underlying potential for reinforcement. Yet, the inclusion of studies with null or negative results within that same analysis serves as a critical caveat: realizing this potential in clinical practice requires meticulous attention to protocol. Future research should move beyond asking if fibers work, to defining how they must be used—delineating the specific placement techniques, material combinations, and clinical indications that reliably unlock their reinforcing benefit while avoiding the pitfalls that lead to ineffective outcomes.

The reduction in catastrophic failures is particularly relevant in endodontically treated teeth, where the remaining dentin is structurally weakened and more susceptible to unfavorable fracture patterns. Polyethylene fibers act as a mechanical interlocking network within the composite, bridging cracks and delaying their propagation under functional loads (69, 73). Consequently, their incorporation may extend the longevity of restorations and facilitate future repair rather than replacement in case of failure. This characteristic represents a significant clinical advantage, particularly in minimally invasive restorative dentistry, where preserving as much sound tooth structure as possible remains a primary goal (72).

The finding that polyethylene fibers reduce the risk of unfavorable failures must be interpreted with consideration of an important methodological limitation. The criteria for classifying a failure as “catastrophic” or “favorable” were not standardized across the included studies. While some studies used strictly anatomical criteria (e.g., root fracture = catastrophic), others incorporated reparability. Therefore, the pooled risk difference from the meta-analysis does not represent a single, universally defined outcome but an aggregate of each study's own classification. Despite this limitation, the consistent direction of effect across all studies is notable and aligns with the proposed mechanical mechanism of fibers: by bridging cracks and distributing stress, they appear to redirect failure towards patterns that are more likely to be confined to the coronal restoration, as perceived by different research groups. Future in vitro studies would greatly benefit from adopting a common, clinically relevant failure classification system to allow for more robust quantitative synthesis.

Despite the consistent findings of this meta-analysis, several limitations should be acknowledged. First, only in vitro studies were included, which do not fully reproduce the complex clinical conditions of the oral environment, such as humidity, masticatory dynamics, and thermal fluctuations. Second, most of the included studies presented a medium to high risk of bias due to the absence of blinding and sample size calculation. Also, the lack of standardized testing protocols and long-term fatigue loading models limits the extrapolation of these results to clinical performance. Furthermore, the evidence synthesized is almost exclusively based on one commercial brand of polyethylene fibers (Ribbond™). While this reflects the focus of the existing in vitro literature, it limits the generalizability of our conclusions to other polyethylene fiber systems, which may differ in fiber architecture, surface treatment, and mechanical properties.

Worth is mentioning that our analyses revealed high statistical heterogeneity, which is a major limitation. This heterogeneity is an inherent feature of the in vitro literature in this field, stemming from substantial variations in study methodologies. This limitation underscores that our findings represent an average effect across a wide range of experimental conditions and highlights the need for more standardized reporting in future in vitro research. The high statistical heterogeneity precludes a precise, single estimate of the effect size of polyethylene fiber reinforcement. However, this variability itself is clinically informative. It reflects the reality that the restorative outcome is influenced by multiple factors at the chairside. The positive and significant pooled effect across such a spectrum of in vitro conditions—different cavity types, tooth states, and material combinations—is a robust indicator of a fundamental reinforcing capacity. For the clinician, this suggests that incorporating polyethylene fibers is a sound strategy to generally increase fracture resistance. Crucially, the heterogeneity signals that the degree of improvement may vary. For instance, a severely compromised endodontically treated molar with an MOD cavity may experience a more pronounced benefit from strategic fiber reinforcement than a vital premolar with a conservative restoration. Thus, the review's findings validate the reinforcement concept while simultaneously emphasizing that its execution—guided by principles of biomechanics and adhesive dentistry—is key to harnessing its full potential.

In conclusion, this study demonstrates that polyethylene fiber reinforcement can significantly enhance the fracture resistance of permanent teeth, but this effect is modulated by technical factors. Clinicians should be aware that its successful application depends on adherence to techniques that ensure optimal fiber integration and orientation. Moreover, in the included in vitro models, the use of polyethylene fibers was associated with a significant reduction in failures classified as catastrophic by the investigators. This suggests a favorable shift in failure pattern under laboratory conditions, which, if replicated in vivo, could potentially influence the clinical reparability of restored teeth. The clinical implications of this failure mode shift remain to be investigated.