Current Insights on Fiber Posts: A Narrative Review of Laboratory and Clinical Studies

Purpose: The aim of this study was to review the literature related to the clinical performance and laboratory findings regarding fiber posts, as well as the cementation technique employed with their use. Materials and Methods: A literature search was performed using an electronic database, PubMed/Medline, between 2010 and 2023. The terms used were “intra coronal post, fiber post, post cementation, and post length”. Titles and abstracts were initially screened, and a full-text assessment was conducted for those that fulfilled the inclusion criteria. The reference list of the collected papers was also screened for further relevant citations. Results: In this work, 135 potentially eligible studies were analyzed. Titles and abstracts of 90 studies followed the inclusion criteria and were selected for a full-text assessment, resulting in 50 studies selected. Moreover, additional studies from relevant citations were included, totaling 57 studies. Conclusion: According to the laboratory and clinical studies revised, the survival rate between fiber and prefabricated and cast metal posts was similar, and failures were mainly related to the loss of retention. The intra-canal post length of less than two-thirds of the root length presented successful results when ferrule was present. Furthermore, the ferrule increased the longevity of teeth restored with fiber posts. Additionally, the use of a surface treatment protocol for fiber posts and the adhesive cementation technique both contributed to the clinical success and longevity of the intra-canal post.


Introduction
Endodontically treated teeth commonly present a high risk of biomechanical failure due to the loss of tooth substance and frequently require a prosthetic restoration [1].The decision for the placement of a post mostly depends on the quantity and the quality of the remaining tooth structure.Several studies related the presence of a ferrule (±2 mm) [2][3][4][5] and the number of remaining walls [5][6][7][8][9][10][11] to the ability of the post and core complex to resist intraoral forces regardless of the type of post and final restoration.Previous in vitro studies [6,7] observed that the fracture resistance of endodontically treated teeth enhanced significantly with the increase in the number of coronal walls.In addition, biomechanical aspects and tooth location have been evaluated in several studies, and it has been shown that anterior teeth are at a higher risk of failing due to the lateral forces they are subjected to during function, whereas posterior teeth are subject primarily to vertical forces [4,[12][13][14].
The increasing demand for aesthetics, especially in the anterior region, led to the search for restorative alternatives for metallic post systems, such as fiber posts.The prefabricated fiber post's microstructure comprises a resin matrix, which is usually made of epoxy resin or its derivatives [15], allowing the post to present an elastic modulus similar to the dentine (fiber post ∼ = 30 GPa; dentine ∼ = 18 GPa) compared to the metallic post (108.6 GPa) [16].As a result, the absorption and distribution of stress are more uniform throughout the radicular reminiscent, reducing the risk of nonrestorable fracture [16].
Most fiber post systems allow some light transmission through the root canal; however, the light transmission and bonding strength decrease from the cervical to the apical Dent.J. 2023, 11, 236 2 of 15 third [17].Moreover, intraradicular dentine adhesion is still a clinical challenge due to its limited access and visibility associated with a reduced number of dentinal tubules at the apical third due to the presence of irregular secondary dentine among other structures that could be associated with the risk of the system debonding after long periods in the oral environment [18].
The selection of posts deserves special attention since their physical and mechanical properties can influence the pattern of stress distribution throughout the tooth.A variety of prefabricated post systems have been introduced, such as cast metallic posts, prefabricated metallic, and more recently, translucent fiber posts [15].Likewise, a large number of luting agents presenting different compositions and technique protocols are available for cementation [19].Therefore, the purpose of this review was to summarize the most updated literature from laboratory and clinical studies regarding intra-canal posts in an attempt to provide an updated evidence-based guideline to help clinicians in the selection of the most appropriate system and cementation protocols in dental practice.

Eligibility Criteria
The criteria for selecting articles in the present review followed these specific aspects: articles had to be written in English, undergo peer review, and be dated between 2010 and 2023 (Table 1).

Inclusion Criteria Exclusion Criteria
English language Before 2010 Studies focused on fiber posts intra-canal length; cementation protocol; survival studies Finite element analyses paper without in vitro validation; meta-analyses; systematic review; literature review

Prospective and retrospective clinical trials and in vitro studies
In vitro studies conducted on bovine teeth; primary teeth or immature teeth In vitro study which used a simulated root resembling a composite block

Data Sources
An electronic literature search was conducted using the PubMed search engine combining the keywords "fiber post", "intra coronal post", "post cementation", and "post length".The studies encompassed prospective and retrospective clinical trials as well as in vitro studies.Furthermore, as part of the review process, the search was extended by reviewing the reference sections of the retrieved articles to gather additional relevant information.

Search Strategy
After eliminating duplicate papers, 135 abstracts of potential studies underwent screening, resulting in the selection of 90 articles that focused on fiber posts intra-canal length, cementation protocol, and survival.Exclusion criteria entailed eliminating articles relying solely on finite element analysis without any in vitro validation, in vitro studies conducted on bovine teeth, primary teeth, or immature teeth, or those using a simulated root resembling a composite block.Regarding clinical trial studies, in cases where multiple publications from the same study reported different follow-up periods, the article with the longest evaluation time was selected.Following a thorough examination of the full articles, 51 studies were selected.In addition, the references of the selected papers were screened for further relevant citations, resulting in the inclusion of 6 additional studies, totaling 57 studies (Figure 1). the longest evaluation time was selected.Following a thorough examination of the full articles, 51 studies were selected.In addition, the references of the selected papers were screened for further relevant citations, resulting in the inclusion of 6 additional studies, totaling 57 studies (Figure 1).

Data Included
The information extracted from each study, such as authors and year of publication, type of posts, luting agent, and tooth location, is presented in Table 2. Studies that did not include one of these pieces of information were excluded.Data regarding the mechanical properties of the materials used, the influence of remaining dental structure on the outcome, and the survival rate including reasons for failures from clinical and in vitro studies were evaluated and discussed in the present investigation.

Data Included
The information extracted from each study, such as authors and year of publication, type of posts, luting agent, and tooth location, is presented in Table 2. Studies that did not include one of these pieces of information were excluded.Data regarding the mechanical properties of the materials used, the influence of remaining dental structure on the outcome, and the survival rate including reasons for failures from clinical and in vitro studies were evaluated and discussed in the present investigation.

Features of Fiber Posts
The use of prefabricated posts with core buildups has shown several advantages, such as the use of a direct and less time-consuming technique [20].Specifically, nonmetallic prefabricated post systems have shown high fracture resistance due to their ability to bond to the tooth structure and capacity to better absorb and distribute stress compared to the metallic posts [21].This fact has been claimed to provide a short-term strengthening effect called an endodontic "Monoblock" [22].
Several in vitro studies have compared the use of posts with different elastic modulus, such as zirconia [23], carbon fiber [24,25], stainless steel [25], prefabricated titanium [26], glass fiber, and cast posts [13,16,[23][24][25][26][27][28], and verified that posts with high elastic modulus transmit stress to the root tip resulting in root fracture, while posts with a dentin-like modulus are able to distribute occlusal stresses more evenly in the root canal, reducing the risk of fractures.
On the other hand, the most frequent failures of teeth restored with fiber posts have been related to post/core debonding [4,[26][27][28][29], which, most often, have resulted in favorable fractures likely to be restored.In regard to the mechanical properties (Table 1), different results have been published.While several laboratory studies have verified higher fracture resistance [20,23,30] and bond strength to the dental structure [31] of glass fiber posts compared to different types of posts, other studies have observed similar bond strength and fracture resistance when comparing prefabricated metal and cast posts with fiber posts [13,25,31,[38][39][40].Furthermore, some studies reported lower bond strength and fracture resistance values for glass fiber posts compared to metal posts [21,24,[26][27][28][32][33][34].It is essential to note that the maximum bite force for posterior teeth has been reported to range from 420 ± 112 N to 632 ± 174 N [48].Interestingly, this range coincides with the fracture resistance of most available fiber post systems.

Length of Fiber Posts
Conventionally, it has been established that the intra-canal post length should respect two-thirds of the total length of the remnant dental structure, or at least the height of the crown, or no less than half of the alveolar bone height surrounding the root to provide sufficient retention and avoid catastrophic fracture of the root walls [1].Also, the post size that best fits the dowel space will be chosen, respecting the amount of dental structure remaining [6,7,13].Besides these aspects, several other factors should be analyzed when placing an intra-canal post, including root length, crown height, level of bone support, and ferrule [15].
Due to the features of fiber posts, such as better stress distribution and a more favorable fracture mode, several in vitro studies [11,15,25,28,34,35] have suggested the use of fiber posts with short intra-canal length (one-third of the root length ≥ 6 mm).A study from Thakur A and Ramarao S [20] verified that fiber posts cemented just above half of the root length (or about 8 mm long) presented mechanical behavior statistically similar to longer posts cemented at two-thirds of the root length.
Although decreasing the post length presents the advantages of preserving dentin structure, reducing the risk of root perforation [25,35], and allowing re-intervention on the canal when needed, short posts may reduce the intra-canal retention [1].On the other hand, an increased intra-canal post length may not provide increased retention of fiber posts cemented with resin luting agents since several investigations [38,[40][41][42][43] have reported a decreased quality of bonding at the apical area due to both anatomic features and technical difficulties.Additionally, some studies [2,15,34] found no significant difference among the intra-canal post lengths analyzed.They reported that unless the intra-canal post length is less than one-third of the root length, the result may not significantly influence the fracture resistance of the treated teeth, considering that the correct cementation protocol was followed [2].

Cementation
The use of resin luting agents has been preferred for post cementation, due to satisfactory retention and resistance against post fracture [35].Some factors related to the anatomical and histological characteristics of the root canal are likely to influence the adhesion of luting cement, resulting in a variation of dentin bonding among different areas of the same tooth [42].Regarding the types of adhesive systems available, although the etchand-rinse mode has been traditionally used with resin cement [10,14,49,50], the advances in the adhesive systems resulted in simplified adhesive protocols through the use of self-etch adhesives and self-adhesive resin cement, allowing to shorten chair time and simplify the clinical procedures.Additionally, the use of self-adhesive resin cement avoids the difficult task of applying and rinsing the phosphoric acid in the apical area of the prepared canal, resulting in a more predictable and less technique-sensitive procedure [37,51].
Although a variety of materials and protocols for post cementation have been extensively analyzed, in vitro investigations have provided contradictory findings.Several studies have shown significantly higher bond strength values (Table 1) for self-adhesive luting cement [37][38][39][41][42][43][44][45]51,52], while others have shown comparable behaviors among the adhesive systems available [22,46].In fact, it has been reported that the composition of the self-adhesive resin cement favors a positive performance, due to a greater moisture tolerance than self-etching cement for instance [38,42].Contrary to these findings, other studies have reported a less favorable bond strength and fracture resistance of self-adhesive cement compared to etch-and-rinse adhesives and self-etching adhesive cement [36].The large variation in results may be related to several factors including the variation in teeth's anatomy, different preparation and pretreatment utilized.These factors may play an important role during the cementation protocol [46].
Furthermore, it has been mentioned that the longevity of the cementation procedure can be influenced by the type of bonding generated between the luting material and the tooth structure [9,41,51].Additionally, their unique compositions can affect water sorption and subsequent hygroscopic expansion of the cement [19,41].

Surface Treatment of Fiber Posts
Regarding the surface treatment of fiber posts, different protocols have been tested.Among them, etching the post with phosphoric acid (36% phosphoric acid for 15 s) and/or coating the post with a silane primer [37] and/or with an adhesive bonding agent [47] have been the most common approaches.The silanization of fiber posts has been considered advantageous to improve their retention into the root canals depending on the post type [37,46].Previous studies that have evaluated the use of silane on titanium [4] and fiber posts observed no debonding after an average follow-up of 8.8 years [50] and after a follow-up of 11 years [4].In addition, a micro-mechanical post-surface pretreatment consisting of airborne-particle abrasion has been reported to significantly improve retention due to the roughness created, which contributed to an increased surface area and surface energy [47,53].Furthermore, the idea of customizing glass fiber posts has also been mentioned in the literature.It is claimed that a thicker layer of adhesive cement may lead to unfavorable stress distribution at the post-cement-dentine interface and reduce post retention [35].

Survival from Longitudinal Studies
Since oral conditions are dynamic, long-term clinical evaluation studies provide the most reliable evidence.Some studies have reported encouraging results on the clinical longevity of fiber posts [3,4,49] with a survival time of over 10 years.Several investigations that evaluated prefabricated metal and fiber posts [54][55][56] observed similar survival rates for both types of retainers.
A long-term randomized clinical trial [4] evaluated glass fiber and titanium posts (Fiberpoints Root Pins Titanium) luted with a self-adhesive cement (RelyX Unicem; 3M ESPE).A 2 mm ferrule and post length of 9 mm were employed.The root canal and tooth surface were cleaned with an air abrasion system, and the post space was rinsed with 2 mL of 99.6% ethanol solution and dried with paper points.The titanium posts were treated with a tribochemical silica coating (2.8 bar, 13 s, Rocatec Soft, 3M ESPE), and a thin layer of silane (ESPE-SIL; 3M ESPE) was applied and air-dried in both types of posts.The authors reported that the survival rate decreased rapidly after the 5-year evaluation as survival rates decreased from 86.4% to 58.7% and from 92.5% to 74.2% for fiber glass and titanium posts, respectively, at the 5-and 8-year follow-ups.In addition, the most common failure mode was horizontal root fracture, followed by endodontic failure with apical periodontitis.
A study by Naumann et al. (2012) [49] evaluated two types of fiber posts, consisting of a parallel-sided post with serrated surface configuration (FibreKor, Jeneric Pentron) and a tapered post shape (Luscent Anchors, Dentatus), both in the sizes of 1.0, 1.25, and 1.5 mm.The posts were luted with an etch-and-rinse adhesive system (EBS-Multi, 3M ESPE, Germany) with a 10-year follow-up.Posts were cleaned with alcohol, airdried, and coated with a thin layer of bonding agent.In spite of the fact that the authors observed a 2-fold increased failure rate for anterior teeth compared to premolars and molars, there was no significant difference among the fiber posts used.Other studies have also verified a higher number of failures for anterior teeth [3,49,54].Contrary to these findings, some investigations reported more failures in the posterior region [14].This finding might be attributed to the fact that only 72 teeth were analyzed and the short 3-year evaluation period since posterior teeth are predisposed mostly to vertical forces while anterior teeth are predisposed to lateral forces.Therefore, anterior teeth are at a greater risk of failing [4,[12][13][14].
A randomized clinical study [10] investigated the use of a prefabricated glass post (DT Light Post) and a customized glass FRC post (everStick Post).Both systems were luted to the root canal using resin cement (Calibra and BisCore) to restore premolars with single crowns.The authors reported an overall survival rate of 94.1% after 6 years of evaluation with higher effectiveness for the prefabricated fiber posts compared to the custom-made posts.A significantly increased success rate was observed on teeth with an increased number of remaining walls.
Another randomized clinical trial [14] compared the survival of glass fiber (White Post DC, FGM) and cast metal (CoCr) posts used to restore teeth with no remaining coronal walls.Posts were inserted in two-thirds of the root canal and restored with single crowns.The filling was removed from the root canal with #5 Gates Glidden burs (Dentsply Maillefer).Fiber posts were cleaned with ethanol, pretreated with silane (ProSil, FGM), and luted with two types of resin cement (RelyX ARC, 3M ESPE) and self-adhesive resin cement (RelyX U100, 3M ESPE).After up to 3 years of follow-up, no significant difference was observed between the types of cement and types of post, and survival rates of 97.1% (n = 35) for fiber posts and 91.9% (n = 37) for cast metal posts were observed.A higher survival rate was verified for anterior teeth (97.5%) compared to posterior teeth (90%).
In addition, a randomized clinical trial [54] compared the clinical outcomes of a prefabricated glass fiber post (Parapost FibreLux, Coltène-Whaledent, Cuyahoga Falls, OH, USA), a custom-made glass fiber post (everStick, StickTech-GC America, Alsip, IL, USA), and a gold cast post and core (gold-alloy-based wrought-Parapost, Coltene-Whaledent, and Medior 3 Cendres + Métaux) luted with self-etching adhesive cement (Panavia F 2.0/ED Primer II, Kuraray).After 5 years of follow-up, the authors reported an overall survival rate of 91.4% for fiber posts, 92.1% for custom-made glass fiber posts, and 91.2% for gold cast post and core.The most common type of failure observed was dislodgement of posts, consisting of 30.9% for anterior teeth and 18.02% for posterior teeth, and mostly with custom-made glass fiber posts (everStick, StickTech).Furthermore, a previous investigation [57] evaluated the effectiveness of a quartz translucent fiber post (DT Light SL9; VDW GhB, Munich, Germany) assessed at a 2-year evaluation.Posts were silanized and cemented with self-etching cement (Calibra, Dentsply, Kostanz, Germany).The authors reported an excellent clinical performance of the fiber posts with no periapical lesions.
A longitudinal retrospective study [50] analyzed endodontically treated teeth with and without posts after an average period of 8.8 years (Easy Post and Easy Post Lux, Dentsply).The initial post-space preparation was performed with a Largo Peeso Bur and precision drills.The post was placed in teeth possessing only one wall and/or less than one-third of the remaining height of the clinical crown.The post length used was 8 mm, and they were cleaned with alcohol and coated with silane (Monobond S/Monobond Plus; Ivoclar Vivadent, Schaan, Lichtenstein).Posts were luted with resin cement using an etch-and-rinse technique.The survival rate of teeth restored with fiber post (94.3%) was significantly higher than the teeth without a post (76.3%), irrespective of the restoration type, and no post debonding was observed.The authors concluded that teeth with only one remaining wall restored with fiber posts yielded significantly less tooth loss and reported that the use of crowns did not improve the prognosis when compared with noncrowded teeth.

Final Remarks/Considerations
Limitations of this review can be attributed to the limited number of longer-term clinical trials published in this area and studies lacking precise clinical guidelines for different clinical situations.Therefore, long-term evidence from clinical studies with betterdefined guidelines is needed.Furthermore, this review paper aimed to search for the most updated papers (from 2010 and onward) available on the PubMed search engine, which may have constrained the search.However, the primary objective of this review was to provide current insights related to fiber posts and cementation protocol, with a focus on reviewing the most up-to-date literature over the last 13 years.
Based on the papers evaluated in the present review, a similar survival rate was observed for fiber and prefabricated metal posts, as well as cast metal, which ranged from 3 to 10 years.The lower modulus of elasticity of fiber posts was associated with favorable outcomes.The primary causes of failures observed in fiber posts were related to post/core debonding.Moreover, employing a surface treatment protocol, like silane, was demonstrated to enhance adhesion for most types of fiber posts.Both conventional and self-adhesive cement contributed equally to the clinical success and longevity of intra-canal posts.
Furthermore, the presence of a ferrule was deemed crucial for the longevity of the post/restoration complex.Under these conditions, intra-canal fiber post lengths of less than two-thirds of the root length can be effectively utilized.Future research should delve into areas that have not been addressed in this review, such as the optimal canal irrigation and cleansing methods prior to adhesive cementation, protocols for gutta-percha removal, and techniques for extracting fiber posts.

Figure 1 .
Figure 1.Flow diagram (PRISMA format) of the screening and selection process.

Figure 1 .
Figure 1.Flow diagram (PRISMA format) of the screening and selection process.

Table 1 .
Inclusion and exclusion criteria.

Table 2 .
Results from in vitro studies.