Restorative strategies for weakened roots : Systematic review and Meta-analysis of in vitro studies

This study evaluated fracture resistance values and failure mode of weakened roots restored with cast post and core, fiber post and anatomical posts. In vitro studies, published in English, Portuguese and Spanish, that evaluated weakened roots restored with at least two of mentioned strategies were searched for in three databases MEDLINE, PubMed and Scopus until June 2017; there was no restriction regarding the publication year. Fracture resistance values were obtained, and the following comparisons: cast post and core vs. fiber post and vs. anatomical post and fiber post vs. anatomical post were performed. Meta-analyses were conducted using the random effects model to calculate the combined effect of the difference between the averages. Six articles were included in the study. There was no significant difference between the fracture resistance values, but more catastrophic failures were observed on cast post and core, and the included studies presented a low risk of bias for the most domains. It was concluded that weakened roots restored with the searched strategies seem to have similar fracture strength, but fiber and anatomic post decreases the possibility of catastrophic failure. ReSumo


INTRoDuCTIoN
T he loss of tooth structure caused by caries, trauma or endodontic retreatments leads to the gradual weakening of the roots.When teeth reach this condition, it is primordial the use of intracanal posts to retain the coronary restoration [1,2].
Among the restorative strategies, cast post and core (CPC) is widely used [3,4], although its technique requires more clinical steps and increases the risk of catastrophic fractures of the remaining tooth [4][5][6][7].The advent of the fiber posts (FP) reduced clinical steps and reduces the chances of catastrophic failures of the root, once its mechanical properties are similar to dentine [2,[6][7][8][9][10].However, the use of fiber posts to restore weakened roots creates a thick cement layer between root dentine and the post, which reduces the bond strength of the set [11,12].Anatomic post (AP) technique was developed to solve this problem with the application of composite resin around the fiber post and consequently precise adaptation between the post and the canal walls [13,14].
An important characteristic of a restorative strategy is to present high values of fracture resistance and allow possible repairs in case of structure fracture.Some studies had evaluated which restorative strategy would be more appropriated for the restoration of weakened teeth considering these characteristics [7,10,13].Nevertheless, there is no consensus in the literature about which technique present better behavior to restore weakened roots, also there are no clinical trials regarding this subject.
Thus, this systematic review aimed to compare the load to fracture values of weakened roots restored with cast post and core, glass fiber posts and anatomic posts, as well as the fracture patterns generated among strategies.
This study focused on answering the following patient, problem, or population; intervention; comparison, control, or comparator; outcome (PICO) question: Witch Intra-radicular (cast post and core, fiber post and anatomical posts) retainer presents the best biomechanical behavior on weakened roots submitted to the load-to-fracture in vitro test?
The null hypothesis tested was that there would be no difference between the restorative strategies regarding the load to fracture values.

mATeRIAl AND meThoDS
The basic structure of this review was conducted based on the Cochrane Handbook for Systematic Reviews of Interventions [15] and followed the four steps from the flow chart of "Preferred Reporting Items for Systematic Reviews and Meta-analysis (PRISMA) Statement" [16].

Search Strategy and Eligibility Criteria
Searches were performed within electronic databases (MEDLINE/Pubmed and Scopus), and the last search was carried out in 2017 June.The search strategy used is described in Table I.The strategy was created based on Medline/Pubmed database and adapted to Scopus.Literature search results were de-duplicated using EndNote X7 software (Thomson Reuters, New York, NY, USA).After an initial search in electronic databases, a hand search was made on the references of the selected articles, to verify the presence of any other possible source of information.

Terms Description
Retainers "Post and Core Technique"[Mesh] OR "Post and Core Technique" OR "Post-Core Technic" OR "Post-Core Technics" OR "Technic, Post-Core" OR "Technics, Post-Core" OR "Post and Core Technic" OR "Post Technique" OR "Post Techniques" OR "Technique, Post" OR "Techniques, Post" OR "Post Technic" OR "Post Technics" OR "Technic, Post" OR "Technics, Post" OR "Dental Dowel" OR "Dowels, Dental" OR "Dental Dowels" OR "Dowel, Dental" OR "Lightpost [Supplementary Concept]" OR "cast post and core" OR "Fib* Post" OR "cast metal post" OR "Anatomic Post" OR "Relined Fiber Post"

Weakening
"Thin-walled roots" OR "Weakened roots" OR "Tooth, Nonvital"[Mesh] OR "Tooth, Nonvital" OR "Nonvital Tooth" OR "Tooth, Devitalized" OR "Devitalized Tooth" OR "Pulpless Tooth" OR "Teeth, Pulpless" OR "Pulpless Teeth" OR "Teeth, Devitalized" OR "Devitalized Teeth" OR "Teeth, Nonvital" OR "Nonvital Teeth" OR "Endodontically-Treated Teeth" OR "Teeth, Endodontically Treated" OR "Tooth, Endodontically-Treated" OR "Endodontically-Treated Tooth" OR "Tooth, Endodontically Treated" OR "Tooth Root"[Mesh] OR "Tooth Root" OR "Root, Tooth" OR "Roots, Tooth" OR "Tooth Roots" OR "Tooth Fractures"[Mesh] OR "Tooth Fractures" OR "Fracture, Tooth" OR "Fractures, Tooth" OR "Tooth Fracture" OR "flared teeth" OR " compromised roots" OR "flared root" Load to fracture "failure" OR "fracture strength" OR "load to fracture" OR "fracture load" OR "Fracture resistance" The inclusion criteria were: (1) articles published in English (2) in vitro studies that compared fracture resistance of weakened roots with at least two of the following restorative strategies: cast post and core, glass fiber post and anatomic post; (3) use of single-root bovine or human teeth; (4) roots with a maximum of 1 mm of dentin remaining thickness; (5) the post space preparation should be 2/3 of the remaining radicular length; (6) specimens should be restored with single crown (metallic or metal-ceramic); ( 7) load to fracture test should be conducted with an angulation of 45° with the long axis of the teeth and ( 8) highest speed of mm/min.

Study Selection
The literature search was made by two independent reviewers.Initially, the reviewers evaluated the article titles, and if the titles had inclusion potential their abstracts were assessed.After a detailed evaluation of the abstracts, the articles considered eligible were identified and a complete assessment was performed by both reviewers, to define its inclusion or not.In case of discordance between the reviewers about the article inclusion, a third reviewer was consulted.

Data collection
The following parameters were extracted simultaneously by both reviewers, using a standard outline: Author(s) and year of publication, journal, alloy used to produce the cast post and core, coronary diameter of the post, type of fiber, post design, load to fracture values and standard deviation, type of fracture, specimen length, depth of weakening, remaining thickness, type of cement and material of cast metal post and core, aging of the specimens and presence of ferrule.Also, the number and the mode of the failures for each strategy were collected and classified in two groups: Favorable (any fracture that was possible to repair, as core fracture, root fracture above the periodontal ligament, adhesive failure of the post, adhesive failure of the crown); Unfavorable (root fractures bellow the periodontal ligament).

Risk of bias
The risk of bias of included studies was assessed based on previous studies [4,17,18].The risk of bias assessed the selected articles according to the report of the following items: the presence of sample size calculation, teeth randomization, previously training operators, followed manufacturer's recommendation and blinding of test operator.If the author had reported the parameter, this domain was considered as "Low risk", if it was not reported or if it was unclear for the reviewers on the article, the domain was considered as "Unclear", and if the authors reported that the parameter was not followed or controlled the domain was classified as "High risk".One researcher made the analysis and reviewed by another using the Review Manager 5.3 Software (Copenhagen: The Nordic Cochrane Centre, The Cochrane Collaboration, 2014).

Statistical Analysis
The studies characteristics were descriptively summarized.The outcome considered was the load to fracture in Newton.The meta-analyses were conducted using random effect models to calculate the pooled mean differences between glass fiber post vs. cast post and core, glass fiber post vs. anatomic post and cast post and core vs. anatomic post since these models provide.All summary estimates were reported with point estimates and corresponding 95% confidence intervals (CIs).Heterogeneity was assessed using the Cochrane Q statistics and the I2 test (>75% indicating high heterogeneity).The analyses were conducted using Review Manager 5.3 Software.Considering the analysis between glass fiber post and cast metal post and core, two separate analyses were performed, once Balkaya et al. 2013 [19] presented two different groups of glass fiber post with different diameters (ϕ=1.3mm e ϕ1.7mm) and one group restored with CPC.

Studies Characteristics
The characteristics of included studies are presented in Table II.

Type of post
Studies evaluating CPC used Ni-Cr alloy for post and core production.One study reported [19] that the CPC had a 4 mm diameter and a parallel format, the others did not report format and dimension.All studies evaluating fiber posts used glass fiber as post material, and on the majority of the studies the fiber post had a conical format, and the diameters ranged between 1.1 mm and 1.8mm.

Cementation material used
All studies used resin cement for post cementation.Four [7,10,21,22] studies used dual-cured resin cement.One [19] study used dual-polymerized self-adhesive cement and the other study [20] used self-curing resin cement.

Specimens aging
Only one study did not perform the specimens aging [19], four studies made the aging of all specimens [10,[20][21][22], and one study compared the effect with and without aging [7].

Ferrule effect
Only three studies presented ferrule on the specimens [7,10,19] Relining material Among the studies with anatomic post groups, four of them used micro-hybrid composite resin to reline the post [10,[20][21][22], while one article used flow composite resin [7].

Failure mode
Specimens restored with glass fiber post and anatomic post presented a higher percentage of favorable failures, while the specimens restored with cast post and core presented a higher percentage of unfavorable failures (Table III).

Risk of bias
From the five evaluated domains on the articles, the majority was defined as "Low risk", followed by the "unclear" definition with 2 criteria.None of the domains evaluated was classified as "High risk" on this study (Figure 3).

DISCuSSIoN
The preservation of remaining coronal walls is a key factor for the success of the restoration of endodontically treated teeth [23].Thus, the absence of coronal walls, called weakened teeth, is the worst-case scenario for restoration.Several studies were performed trying to find the best restorative option for this scenario.However, there is no consensus on the literature.This review was carried out in order to obtain results to assist in the selection of the technique that presents the higher resistance values and the more favorable failures modes.This review is the first to suggest that the fracture resistance of weakened roots restored with CPC, FP or AP are similar, accepting the null hypothesis.The main reason for this result is because the use of crowns as final restoration has an important role in the absorption of the forces during the tests.Without the use of crowns, the Braz Dent Sci 2019 Jan/Mar;22 (1) core becomes more suitable to failure, and the load is transmitted directly to post/core complex [24].
Another important fact should be highlighted.The anatomic posts presented values of fracture resistance comparable to the other posts demonstrating that the fit of the post with the root walls and the lower cement thickness obtained have an important role on the biomechanical behavior of the set [11,12].
Studies showed that in specimens restored with FP the composite resin core is the first structure to fail [25].This structure presents lower resistance than the root, so, sound roots restored with FP presents lower fracture resistance than sound root restored with CPC.In weakened specimens the mechanical behavior is different, as the root is fragilized, teeth restored with CPC tend to fracture with lower forces, presenting similar values than teeth restored with glass fiber posts.
Analyzing the failure modes, it is possible to observe a higher number of unfavorable fractures for the groups restored with cast post and core.This is related with metal mechanical properties, particularly, its elastic modulus (200GPa), which is much higher than the dentin (18GPa).This high elastic modulus is responsible for less bending of the post under mechanical load, resulting in high resistance of the post and therefore a higher load until fracture.Metal concentrates the tension, mainly on the apical third of the root, and this force concentration is transmitted to the canal walls which can result in catastrophic failures of the remaining structure [26,27] and this phenomenon can explain the unfavorable fracture mode when CPC is used.
On the other hand, the use of fiber posts and anatomic post presented higher favorable failures in comparison with the use of CPC (Table III).The highest percentage of favorable failures using fiber post is in accordance with the literature, and because fiber posts present an elastic modulus (30 to 50 GPa) similar to dentine [28,29] and thus a more uniform stress distribution.Also, there is a tendency that in this scenario the post or composite resin core fail before the root, avoiding unfavorable fractures [23].The best favorable fractures percentage presented by the anatomic post in relation to the fiber post may be related to the hypothesis that this type of posts tends to bend less than the isolated post, due to the better fit with the root walls, causing less strain on the radicular portion [25].
The small number of studies that met the inclusion criteria in this review and the use of two databases represents a limitation of this study because a greater number of studies could allow some analysis to identify the variables with direct relation on the results.Furthermore, the statistical analyses showed high heterogeneity among the studies.Thus the random effects models were used since they are the most accepted mainly due to the high heterogeneity of the included studies.Factors and variables contributing to this high heterogeneity are hard to identify because a large number of variables involved, this heterogeneity and great number of variables may have affected the results obtained.Maybe with a greater number of studies and with more careful data reporting, this high heterogeneity would be attenuated.Regarding the risk of bias, despite no assessed domain has been classified as "High risk", a great number of the judgments was "Unclear", showing problems on the methodological aspects or poor reporting, also the publication bias should be considered as a limitation factor for this study.Lastly, although the review demonstrates similar behavior among restorative strategies, this result should be interpreted with caution because longterm longitudinal studies in the topic are scarce especially considering the use of anatomic posts.
Notwithstanding the limitations of this systematic review, the results showed a similar behavior of the different strategies used to restore weakened roots, it also showed that the fragilized teeth issue is poorly explored on the literature and more studies, mainly clinical trials, are needed to better explain the mechanical behavior of this kind of restorations.Also the authors need a better reporting protocol, in order to reduce heterogeneity among studies.So, it can be concluded that the load to fracture values in weakened roots was not affected by the restorative strategy (CPC, FP, AP), and that weakened roots restored with glass fiber posts or anatomic posts showed higher probability to present favorable fracture, when compared to those restored with cast post and core.

BrazFigure 1 -
Figure 1 -Flow chart of the article selection.

Figure 2 -
Figure 2 -Forest Plots comparing load to fracture values between: (A) fiber post system (FP with 1.7mm) and CPC; (B) fiber post system (FP with 1.3mm) and CPC; (C) anatomic posts and FP; (D) anatomic post and CPC.

Figure 3 -
Figure 3 -Risk of bias for each study.

Table I -
Description of the search terms used on the survey

Table II -
Characteristics of each included study

Table III -
Failure mode and distribution for each retainer.