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Clinical Oral Investigations

, Volume 22, Issue 7, pp 2475–2485 | Cite as

Effect of root canal sealers on the fracture resistance of endodontically treated teeth: a systematic review of in vitro studies

  • Emel Uzunoglu-ÖzyürekEmail author
  • Selen Küçükkaya Eren
  • Sevilay Karahan
Review
  • 343 Downloads

Abstract

Objective

The purpose of this study was to systematically review and critically analyze the published data of in vitro studies testing the effect of root canal sealers on the fracture resistance of endodontically treated teeth.

Methods

A comprehensive literature search was performed by using the Medline, Scopus, Web of Science, Cochrane, and Open Grey databases. A hand search of the reference lists of identified articles was also performed. Two reviewers critically assessed the studies for eligibility against inclusion and exclusion criteria and performed data extraction. Evaluation of the risk of bias of the studies was performed.

Results

A total of 48 studies were assessed for eligibility. Of these, 20 met the inclusion criteria and were included in the systematic review. All studies had a medium or high risk of bias. Although the majority of the studies reported that the use of root canal sealers increased the fracture resistance of endodontically treated teeth, conflicting evidence was found for the reinforcing effect of resin, glass ionomer, and calcium silicate-based sealers while there was moderate evidence for zinc oxide eugenol-based sealers in favor of no reinforcing effect.

Conclusion

On the basis of available evidence, the use of root canal sealer increases the fracture resistance of endodontically treated teeth. However, included studies presented considerable risk of bias. Regarding the comparisons among the sealers, no conclusions could be drawn for the superiority of one sealer type to another.

Clinical relevance

A considerable heterogeneity was found in the methodologies of included studies. Therefore, this review strongly suggests the development of standardized methods to test the reinforcement effect of root canal filling materials in in vitro studies.

Keywords

Fracture strength Root canal sealer Systematic review 

Introduction

Vertical root fracture (VRF) is defined as a longitudinal fracture of the root with fracture lines running parallel to the long axis [1]. VRF is a serious complication that can occur during or after root canal treatment and often leads to tooth extraction [2]. The prevalence of VRFs was reported to be 11–20% in extracted endodontically treated teeth [3]. Excessive loss of tooth structure is associated with the increased risk of VRF of endodontically treated tooth and often results from caries or trauma, access cavity preparation, root canal instrumentation, and obturation procedures [4].

Root canal obturation is classically performed using gutta-percha in combination with a sealer to provide complete filling of the canal in all dimensions [5]. The root canal sealer fills the gaps between gutta-percha cones and the walls of the root canal and the voids between individual gutta-percha cones applied during root canal obturation [5]. It has been suggested that an ideal root canal filling material should bond to the root canal dentin and reinforce the remaining tooth structure against fracture to improve the long-term success of an endodontically treated tooth [4]. In this regard, root canal sealers may play an important role as gutta-percha is incapable of bonding to root canal walls [6].

To date, many studies have evaluated the effect of different root canal filling materials on fracture strength of teeth [2, 4, 7, 8, 9, 10]. However, there is no consensus on whether the use of different types of sealers in conjunction with gutta-percha affects the fracture strength of endodontically treated roots [2, 4, 7, 8, 9, 10]. Therefore, the purpose of this study was to systematically review and critically analyze the published data of in vitro studies testing the effect of root canal sealers on the fracture resistance of endodontically treated teeth.

Methods

Data sources and the literature search strategy

This systematic review was conducted according to the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) guidelines [11]. A comprehensive electronic search was carried out in MEDLINE (via the PubMed and OVID interfaces), Scopus, Web of Science, Cochrane, and Open Grey databases to identify articles published in English language only. The free form of the research question was the following: “Does the use of different root canal sealers in conjunction with gutta-percha increase the fracture resistance of endodontically treated teeth?” The population, intervention, comparison, and outcome (PICO) strategy was used for the structured review question as follows:
  1. 1.

    Population: Extracted fully formed human teeth undergoing root canal treatment

     
  2. 2.

    Intervention: Root canal obturation with different root canal sealers in conjunction with gutta-percha

     
  3. 3.

    Comparison: Root canal preparation but no obturation or obturation only with gutta-percha

     
  4. 4.

    Primary outcome: The effect of using root canal sealers on the fracture resistance of endodontically treated teeth

     
  5. 5.

    Secondary outcome: Comparison of the reinforcing effect of root canal sealers

     

The articles published through to April 2017 were screened. The search terms were root canal sealer, endodontic sealer, root canal sealant, endodontic sealant, fracture resistance, fracture strength, reinforce, and root fracture. These keywords were combined as (root canal sealer* OR root canal sealers* OR endodontic sealer* OR endodontic sealers* OR root canal sealant* OR root canal sealants* OR endodontic sealant* OR endodontic sealants*) AND (fracture resistance* OR fracture strength* OR reinforce* OR root fracture*). These keywords and terms were selected from articles published in following endodontic journals: Journal of Endodontics and International Endodontic Journal and Australian Endodontic Journal. The search terms were modified for each database. Reference lists of all included articles were hand-searched for additional articles not identified through the above methods.

Screening and selection of the studies

Initially, two reviewers independently scanned the titles identified in electronic searches and decided which articles were relevant to the topic. The abstracts of all relevant articles were then carefully appraised to identify eligible studies. The full text of the article was read in any case that the information obtained through title and abstract screening was insufficient. Studies were selected for inclusion if they fulfilled all of the following criteria:
  1. 1.

    In vitro studies performed on fully formed human teeth and used gutta-percha as the main root canal obturating material.

     
  2. 2.

    Studies testing at least two different types of sealers.

     
  3. 3.

    Studies comparing the fracture resistance of endodontically treated teeth.

     
  4. 4.

    Studies using teeth prepared but not obturated or obturated only with gutta-percha as controls.

     

Inclusion was based on consensus between two reviewers. Studies failing to meet any of these criteria were excluded.

Assessment of risk of bias

The risk of bias assessment was based on and adapted from previous studies [12, 13]. Evaluation was based on the description of the following parameters for the quality assessment of study: randomization of teeth, presence of control, standardization of teeth dimensions, reporting of age, description of sample size calculation, the use of materials according to the manufacturers’ instructions, samples prepared by the single operator, and blinding of the observer during fracture test. If the authors reported the parameter, the article had a Y (yes) for that specific parameter; if it was not possible to find the information, the article received an N (no). The articles that reported 1–3 items were classified as high risk of bias, 4–6 as medium risk, and 7–8 as low risk. Assessments were made independently by the two reviewers, with any disagreements resolved by consensus.

Every effort was made to retrieve any missing data from the included studies. To request missing data, emails were sent to the corresponding authors of the included studies.

Data extraction

Full texts were obtained for all eligible studies, and two reviewers extracted the data simultaneously by using a standardized outline. The variables extracted included authors, year of publication, journal name, type of teeth, decoronization procedure, presence of control groups, preparation technique, final apical diameter, final irrigation regimen, obturation method, tested root canal sealers, simulation of periodontal ligament, diameter, position, and speed of plunger tip during fracture test, expression of results (N, kg, or lb) and comparative outcomes among groups in each study. The data were extracted from the included studies to evaluate the effect of sealer type on the fracture resistance of endodontically treated teeth.

Each of the included studies was analyzed in terms of similarities so that a meta-analysis could be performed. However, due to the heterogeneity of the studies, it was not feasible to perform a meta-analysis. Instead, a descriptive analysis of the results of the individual studies was undertaken. The results of the studies regarding the effect of sealers on the fracture resistance of teeth were summarized as reinforcing or no effect. On the basis of their prime constituent or structure, root canal sealers were grouped as resin-, zinc oxide–eugenol (ZOE)-, glass ionomer (GI)-, and calcium silicate (CS)-based sealers.

A synthesis of evidence was performed as follows [12, 13]:
  1. 1.

    Strong evidence: Provided by 2 or more studies with high quality and/or generally consistent findings in all studies (≥ 75% of the studies reported consistent findings)

     
  2. 2.

    Moderate evidence: Provided by 1 study with high quality and/or 2 or more studies with low quality and generally consistent findings in all studies (≥ 75% of the studies reported consistent findings)

     
  3. 3.

    Limited evidence: Provided by only 1 study with low quality

     
  4. 4.

    Conflicting evidence: Inconsistent findings in multiple studies (< 75% of the studies reported consistent findings)

     
  5. 5.

    No evidence: No studies could be found

     

Results

The electronic systematic searches yielded a total of 681 results from the databases. Of these, 305 were in Pubmed, 175 were in Web of Science, 140 were identified in Scopus, 44 were in Ovid, 17 were in Cochrane library, and 0 was in Open Grey databases. After removal of duplicates and data screening based on title and abstract, a total of 48 citations were selected for full-text reading. Twenty-eight studies were excluded and the reasons for exclusion were the following: studies that only compared the fracture resistance of teeth obturated with gutta-percha to synthetic polymer-based filling materials [14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28] or coated core materials [29], solely use of cements as root canal filling materials [30], studies that performed the fracture analysis using dentin slices [31], studies that only compared different brands of one sealer type [8, 32, 33], studies that have no control group [34, 35], or used only intact teeth as controls [9, 36, 37]. One study was also excluded because of inadequate presentation of the statistical data of the experimental groups [38] and another was excluded because the full text was not available [39]. Following the full-text reading, 20 studies were found eligible for this systematic review [2, 4, 6, 7, 10, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, 52, 53, 54] (Fig. 1). No additional study was identified at the cross-reference analysis.
Fig. 1

A flowchart of the systematic review process

Twelve studies compared controls (roots prepared but not obturated or obturated only with gutta-percha) with intact specimens and all of them reported significantly lower fracture strength values in control group. In all studies, decoronation of teeth was performed. However, no standardization was found among the studies regarding the root canal preparation, irrigation, and obturation procedures. In addition, differences were also found in the parameters of fracture test such as diameter and position of plunger tip and speed of tip movement. The characteristics of all included studies are presented in Tables 1, 2, and 3.
Table 1

Descriptive data of included studies: part 1

Authors

Selected teeth

Decoronation

Control group/s

Preparation technique

Final apical diameter

Apicella et al [7]

Maxillary and mandibular single-rooted teeth

Y

PNO

Balanced force with hand instruments

#50

Chadha et al. [40]

Mandibular premolars

Y

PNO

Rotary instrumentation with ProTaper files

F3

Cobankara et al. [41]

Maxillary canines

Y

PNO, NP

Step back technique with hand instruments

#55

Ersoy et al. [42]

Mandibular premolars

Y

PNO, NP

Rotary instrumentation with ProTaper files

F4

Hegde and Arora [43]

Mandibular premolars

Y

PNO, NP

Rotary instrumentation with ProTaper files

F3

Jhamb et al. [44]

Maxillary central incisors

Y

PNO, NP

Standardized technique with hand instruments

NS

Johnson et al. [4]

Mandibular incisors

Y

PNO

Step back technique with hand instruments

#45

Karapinar Kazandağ et al [45]

Single-rooted teeth

Y

PNO, NP

Rotary instrumentation with Endosequence 0.06 taper files

#35

Lertchirakarn et al. [46]

Mandibular incisors

Y

NP, OGP

Step back technique with hand instruments

#30 or #35

Mandava et al [47]

Mandibular premolars

Y

PNO, NP

Rotary instrumentation with ProTaper files

F3

Nagpal et al [48]

Single-rooted premolars

Y

NP, OGP

Standardized technique with hand instruments

#40

Ribeiro et al. [49]

Mandibular incisors

Y

PNO

Rotary instrumentation with ProFile 0.06 taper files

#40

Sagsen et al. [50]

Maxillary central incisors

Y

PNO

Standardized technique with hand instruments

#40

Sagsen et al. [6]

Maxillary central incisors

Y

PNO, NP

Rotary instrumentation with ProTaper files

F3

Sungur et al. [51]

Mandibular premolars

Y

PNO, NP

Rotary instrumentation with ProTaper files

F4

Topçuoğlu et al. [52]

Mandibular premolars

Y

PNO, NP

Rotary instrumentation with ProTaper files

F3

Trope et al. [2]

Maxillary and mandibular canines

Y

PNO, NP

Step back technique with hand instruments

#55

Ulusoy et al. [10]

Mandibular premolars

Y

PNO

Rotary instrumentation with ProTaper files

F3

Vishwanathan et al. [53]

Maxillary incisors

Y

PNO

Rotary instrumentation with ProTaper files

F4

Zamin et al. [54]

Mandibular incisors

Y

PNO

Rotary instrumentation with K3 files

#40

Y yes, PNO prepared and non-obturated, NP non-prepared, NS non-standardized, OGP obturated with only gutta-percha

Table 2

Descriptive data of included studies: part 2

Authors

Irrigation solutions

Obturation technique/s

Tested sealers (type and brand of sealers)

Simulation of PDL

Tip diameter of plunger

Tip position in relation to the long axis of tooth

Apicella et al. [7]

5% NaOCl, 17% REDTA

CLC, SC

*Ketac Endo (GI, 3M ESPE, Seefeld/Oberbay, Germany)

*Roth’s 801 (ZOE, Roth International, Chicago, IL, USA)

N

6.35 mm

P

Chadha et al. [40]

5.25% NaOCl, 17% EDTA

CLC

*AH Plus (Epoxy resin, Dentsply-DeTrey, Zurich, Switzerland),

*ZnOE cement (ZOE, NM)

N

5 mm

P

Çobankara et al. [41]

5.25% NaOCl, 17% EDTA (±)

CLC

*AH 26 (Epoxy resin, Dentsply-DeTrey, Zurich, Switzerland),

*Ketac Endo (GI, 3M ESPE, Seefeld/Oberbay, Germany)

N

1 mm

15 degree

Ersoy et al. [42]

%5 NaOCl, 15% EDTA

CLC, SC, WVC

*AH Plus (Epoxy resin, Dentsply de Trey GmbH Konstanz, Germany),

*MTA Fillapex (CSC, Angelus, Londrina, Brazil)

Y

NM

P

Hegde and Arora [43]

5.25% NaOCl, 17% EDTA

SC

*AH Plus (Epoxy resin, Dentsply de Trey GmbH Konstanz, Germany),

*EndoSequence BC (CSC, Brasseler USA, Savannah, GA, USA)

Y

3 mm

P

Jhamb et al. [44]

%5 NaOCl, 17% EDTA (±), 17%EGTA (±)

CLC

*Acreoseal (Calcium hydroxide containing epoxy resin, Septodont, Saint-Maur des Fossés France),

*Ketac Endo (GI, 3M ESPE, Seefeld/Oberbay, Germany)

N

3 mm

P

Johnson et al. [4]

5.25% NaOCl, 17% EDTA or 25% polyacrylic acid

CLC, SC

*Roth’s 801 (ZOE, Roth International, Chicago, IL, USA),

*Ketac Endo (GI, ESPE-Premier, Norristown, PA, USA)

N

3 mm

P

Karapinar Kazandağ et al. [45]

5.25% NaOCl, 17% EDTA

CLC

*Activ GP (GI, Brasseler USA, Savannah, GA, USA),

*AH Plus (Epoxy resin, Dentsply de Trey,Gmbh Konstanz, Germany)

N

NM

P

Lertchirakarn et al. [46]

5.25% NaOCl, 17% EDTA

CLC

*AH Plus (Epoxy resin, Dentsply de Trey GmbH Konstanz, Germany),

*Ketac Endo (GI, 3M ESPE GmbH, Seefeld, Germany),

*Tubliseal (ZOE, Kerr, Romulus, MI, USA)

Y

0.45 mm

P

Mandava et al. [47]

3% NaOCl, 17% EDTA, 10% ascorbic acid

SC

*AH Plus (Epoxy resin, Dentsply de Trey GmbH Konstanz, Germany),

*MetaSeal (Methacrylate resin, Parkell, Inc., Edgewood, NY, USA),

*MTA Fillapex (CS, Angelus, Londrina, Brazil)

Y

2.2 mm

P

Nagpal et al. [48]

3% NaOCl, 17% EDTA

CLC

*AH Plus (Epoxy resin, Dentsply de Trey GmbH Konstanz, Germany),

*Tubliseal EWT (ZOE, Sybron Endo, Orange, CA)

N

4 mm

P

Ribeiro et al. [49]

1% NaOCl, 17% EDTAC

CLC

*AH Plus (Epoxy resin, Dentsply de Trey GmbH Konstanz, Germany),

*EndoFill (ZOE, Dermo Lab. Ltda, Petropolis, Brazil),

*Epiphany (Methacrylate resin, Pentron Clinical Technologies, Wallingford, CT, USA),

*Sealer 26 (Epoxy resin, Dentsply Herpo, Petropolis, Brazil)

N

NM

45 degree

Sagsen et al. [50]

2.5% NaOCl, 17% EDTA

CLC

*AH 26 (Epoxy resin, Dentsply de Trey GmbH Konstanz, Germany),

*MCS canal sealer (ZOE, Lone Star Technologies, Westport, CT, USA)

N

4 mm

P

Sagsen et al. [6]

%5 NaOCl; 17% EDTA

CLC

*AH Plus (Epoxy resin, Dentsply de Trey GmbH Konstanz, Germany),

*iRoot SP (CS, Innovative Bioceramix, Vancouver, Canada),

*MTA Fillapex (CS, Angelus, Londrina, Brazil)

N

6 mm

45 degree

Sungur et al. [51]

5.25% NaOCl, 17% EDTA, 2% CHX (±)

SC

*AH 26 (Epoxy resin, Dentsply de Trey GmbH Konstanz, Germany),

*iRoot SP (CS, Innovative Bioceramix, Vancouver, Canada),

*MTA Fillapex (CS, Angelus, Londrina, Brazil)

N

NM

P

Topçuoğlu et al. [52]

5.25% NaOCl, 17% EDTA

SC

*AH Plus Jet (Epoxy resin, Dentsply de Trey GmbH Konstanz, Germany),

*EndoSequence BC (CS, Brasseler USA, Savannah, GA, USA)

*Tech Biosealer Endo (CS, Isasan, Como, Italy)

Y

3 mm

P

Trope et al. [2]

0.5% NaOCl

SC, CLC

*Ketac Endo (GI, ESPE, Seefeld/Oberbay, Germany)

*Roth’s 801 (ZOE, Roth International, Chicago, IL, USA)

N

1 mm

15 degree

Ulusoy et al. [10]

5.25% NaOCl, 17% EDTA

CLC

*AH 26 (Epoxy resin, Dentsply de Trey GmbH Konstanz, Germany),

*Ketac Endo Aplicap (GI, 3M ESPE, Maplewood, MN, USA)

N

5 mm

P

Vishwanathan et al. [53]

3%NaOCl, 17%EDTA, 0.2%CHX

CLC

*AH 26 (Epoxy resin, Dentsply de Trey GmbH Konstanz, Germany),

*ZnOE sealer (ZOE, NM)

N

6 mm

P

Zamin et al. [54]

1% NaOCl, 17% EDTA

LC

*AH Plus (Epoxy resin, Dentsply-DeTrey, Zurich, Switzerland),

*EndoFill (ZOE, Dentsply-Maillefer, Petropolis, Brazil)

N

NM

45 degree

NaOCl sodium hypochlorite, EDTA ethylene diamine tetraacetic acid, CHX chlorhexidine digluconate, CLC cold lateral compaction, SC single cone, WVC warm vertical compaction, PDL periodontal ligament, Y yes, N no, NM not mentioned, mm millimeter, CS calcium silicate, ZOE zinc-oxide eugenol, GI glass ionomer, P parallel

Table 3

Descriptive data of included studies: part 3

Authors

Units of force

Speed of tip movement (mm/min)

Reinforcing sealers

No effect

Apicella et al. [7]

N

1

 

Ketac-Endo, Roth’s 801

Chadha et al. [40]

N

1

AH Plus

ZnOE cement

Çobankara et al. [41]

KG

NM

Ketac-Endo, AH 26

 

Ersoy et al. [42]

N

1

 

AH Plus, MTA Fillapex

Hegde and Arora [43]

N

1

Endosequence BC, AH plus

 

Jhamb et al. [44]

KG

0.5

Ketac-Endo, Acroseal

 

Johnson et al. [4]

KG

0.5

 

Ketac-Endo, Roth’s 801

Karapinar Kazandağ et al. [45]

N

1

AH Plus, Activ GP

 

Lertchirakarn et al. [46]

N

0.5

Ketac-Endo

AH Plus, Tubliseal

Mandava et al. [47]

MPa

0.5

AH Plus, MetaSeal

MTA Fillapex

Nagpal et al. [48]

N

1

 

AH Plus, Tubliseal EWT

Ribeiro et al. [49]

N

1

 

AH Plus, EndoFill, Epiphany, Sealer 26

Sagsen et al. [50]

N

1

AH 26, MCS Canal Sealer

 

Sagsen et al. [6]

N

1

AH Plus, iRoot SP, MTA Fillapex

 

Sungur et al. [51]

N

1

AH 26, iRoot SP

MTA Fillapex

Topçuoğlu et al. [52]

N

1

Endosequence BC, AH plus

Tech Biosealer

Trope et al. [2]

LB

NM

Ketac-Endo

Roth’s 801

Ulusoy et al. [10]

N

1

AH 26

Ketac-Endo

Vishwanathan et al. [53]

N

1

AH 26

ZnOE sealer

Zamin et al. [54]

N

1

 

AH Plus, EndoFill

NM not mentioned, N Newton, KG kilogram, LB libra, MPa mega Pascal

All included studies were assessed for the risk of bias. Of the 20 studies included, 11 studies (55%) showed medium risk of bias, whereas 9 studies (45%) presented high risk of bias. The results are presented in Table 4, according to the parameters considered in the analysis.
Table 4

Bias risks of individual studies

Authors

Sample size calculation

Control group

Teeth dimensions

Age of patients

Teeth randomization

Manufacturer’s instructions

Single operator

Blinding of the observer

Risk of bias

Apicella et al. [7]

N

Y

N

N

Y

Y

N

N

High

Chadha et al. [40]

N

Y

Y

Y

Y

Y

N

N

Medium

Çobankara et al. [41]

N

Y

Y

N

Y

Y

N

N

Medium

Ersoy et al. [42]

N

Y

Y

N

Y

N

N

N

High

Hegde and Arora [43]

N

Y

Y

Y

Y

N

N

N

Medium

Jhamb et al. [44]

N

Y

N

Y

Y

N

N

N

High

Johnson et al. [44]

N

Y

Y

N

Y

Y

N

N

Medium

Kazandağ et al. [45]

N

Y

Y

N

N

N

Y

N

High

Lertchirakarn et al. [46]

N

Y

N

N

Y

Y

N

N

High

Mandava et al. [47]

N

Y

Y

N

Y

Y

N

N

Medium

Nagpal et al. [48]

N

Y

N

N

Y

N

N

N

High

Ribeiro et al. [49]

N

Y

Y

N

Y

N

N

N

High

Sagsen et al. [50]

N

Y

Y

N

N

Y

N

N

High

Sagsen et al. [6]

N

Y

Y

N

Y

Y

N

N

Medium

Sungur et al. [51]

N

Y

Y

N

Y

N

Y

N

Medium

Topçuoğlu et al. [52]

N

Y

Y

Y

Y

Y

N

N

Medium

Trope et al. [2]

N

Y

Y

N

Y

Y

N

N

Medium

Ulusoy et al. [10]

N

Y

Y

Y

N

Y

Y

N

Medium

Vishwanathan et al. [53]

N

Y

N

N

N

Y

N

N

High

Zamin et al. [53]

N

Y

Y

N

Y

Y

N

N

Medium

Y yes, N no

Reinforcing effect of sealers

In the included studies, different categories of root canal sealers [resin-, zinc oxide–eugenol (ZOE)-, glass ionomer (GI)-, and calcium silicate (CS)-based] were tested. According to 6 studies, none of the tested root canal sealers presented any reinforcing effect when compared with the control [4, 7, 42, 48, 49, 54], while 14 studies reported an increase in fracture resistance values with the use of some of the tested sealers [2, 6, 10, 40, 41, 43, 44, 45, 46, 47, 50, 51, 52, 53].

Resin-based sealers

Seventeen studies evaluated the fracture resistance of endodontically treated teeth with epoxy resin-based sealers [6, 10, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, 52, 53, 54], while 2 of them also evaluated the fracture resistance of endodontically treated teeth with methacrylate resin-based sealers [47, 49]. One study reported higher fracture resistance results with methacrylate resin-based sealer [47] and the other one reported no significant increase in fracture resistance values compared with the control [49], providing conflicting evidence.

Twelve studies reported reinforcing effect with the use of epoxy resin-based sealers when compared with the control [6, 10, 40, 41, 43, 44, 45, 47, 50, 51, 52, 53]. According to 5 studies, the epoxy resin-based sealers did not increase the fracture resistance of endodontically treated teeth [42, 46, 48, 49, 54]. Therefore, there was conflicting evidence in the results (< 75% of the studies reported consistent findings).

Seven studies compared the reinforcing effect of epoxy resin- and ZOE-based sealers [40, 46, 48, 49, 50, 53, 54]. Of these, 5 studies reported no significant difference between the sealers [46, 48, 49, 50, 54] and 2 studies reported increased fracture resistance results with epoxy resin-based sealer [40, 53]. The evidence was regarded as conflicting evidence.

Six studies compared the effect of epoxy resin- and CS-based sealers on fracture resistance of teeth [6, 42, 43, 47, 51, 52]. Two studies reported similar effect for the sealers [6, 42], 1 study associated epoxy resin-based sealer with higher resistance results [47] and 1 study reported higher resistance results with the use of CS-based sealer [43]. According to 2 studies, 1 brand of CS-based sealers presented similar effect with epoxy resin-based sealer while the other tested brand exhibited lower fracture strength results [51, 52]. There was conflicting evidence in the results regarding the comparison between resin- and CS-based sealers.

Five studies compared the effect of epoxy resin- and GI-based sealers on resistance to root fracture [10, 41, 44, 45, 46]. Of these, 2 studies reported higher fracture resistance results with epoxy resin-based sealers [10, 45], 2 studies reported higher fracture resistance results with GI-based sealers [44, 46] and 1 study reported no significant difference between the sealers [41], indicating conflicting evidence.

ZOE-based sealers

Ten studies tested the reinforcing effect of ZOE-based sealers [2, 4, 7, 40, 46, 48, 49, 50, 53, 54] and only 1 study reported a significant increase in the fracture resistance of teeth when compared with the control [50]. Therefore, the review found moderate evidence in favor of no reinforcing effect of ZOE-based sealers (≥ 75% of the studies reported consistent findings).

Four studies compared the effect of ZOE- and GI-based sealers on resistance to root fracture [2, 4, 7, 46]. Of these, 2 studies reported higher fracture resistance results with GI-based sealer [2, 46] and 2 studies reported no significant difference between the sealers [4, 7], indicating conflicting evidence.

There was no study comparing the effect of ZOE- and CS-based sealers on fracture resistance of endodontically treated teeth. Therefore, no evidence was found to arrive at any conclusion regarding the comparison of these sealers.

GI-based sealers

Eight studies evaluated the fracture resistance of teeth obturated using GI-based sealers [2, 4, 7, 10, 41, 44, 45, 46]. According to 5 studies, GI-based sealers positively affected the fracture strength of endodontically treated teeth when compared with the control [2, 41, 44, 45, 46] while 3 studies reported no significant reinforcing effect for this sealer type [4, 7, 10]. Therefore, there was conflicting evidence in the results (< 75% of the studies reported consistent findings).

There was no study comparing the effect of GI- and CS-based sealers on resistance to root fracture. Therefore, no evidence was found to arrive at any conclusion regarding the comparison of these sealers.

CS-based sealers

Four different brands of CS-based sealers were tested in 6 studies [6, 42, 43, 47, 51, 52]. According to 2 studies, CS-based sealers had no significant reinforcing effect when compared with the control [42, 47], while 2 studies reported a significant increase in the fracture resistance of teeth with the use of CS-based sealers [6, 43]. In 2 studies, one brand of CS-based sealers was found to increase the fracture strength of endodontically treated teeth, while the other tested brands were not found to effect the fracture strength [51, 52]. This review regards the available data as conflicting evidence (< 75% of the studies reported consistent findings).

Discussion

To the best of the authors’ knowledge, the present systematic review is the first to evaluate the effect of root canal sealers on the fracture resistance of endodontically treated teeth. It should be noted that studies that did not use samples prepared but not obturated or obturated only with gutta-percha as controls were excluded in the present review, because such studies were not able to reveal the reinforcing effect of gutta-percha/sealer combination. Moreover, studies only comparing the reinforcing effect of gutta-percha with other core materials including carrier based core materials or synthetic polymer based root canal fillings were also excluded. The root canal filling with gutta-percha used in conjunction with root canal sealers is probably the best combination available to date and accepted as the gold standard of root canal fillings [55]. Therefore, this study was conducted to fill the gap in literature regarding the reinforcement effect of root fillings with gutta-percha used in conjunction with different root canal sealers. However, the effect of root canal fillings with different core materials on the fracture resistance of endodontically treated teeth can be evaluated in future studies.

Based on the findings, endodontically treated teeth presented lower fracture resistance results than intact teeth regardless of the obturation material used. This finding indicates that root canal treatment procedures weaken the tooth structure and increase its susceptibility to fracture and is consistent with previous studies [56, 57]. According to the majority of the included studies in the present review, the use of root canal sealers in conjunction with gutta-percha increased the fracture resistance of endodontically treated teeth. However, this effect seems to be dependent on the sealer type used. This review demonstrated that resin-based sealers were the most commonly tested root canal sealers in the evaluation of the fracture resistance of endodontically treated teeth. Other types of sealers were also tested including ZOE-, GI-, and CS-based sealers. Conflicting evidence was found for the reinforcing effect of resin-, GI-, and CS-based sealers while there was moderate evidence for ZOE-based sealers in favor of no reinforcing effect. Regarding the comparisons among the sealers, no conclusions could be drawn for the superiority of one sealer type to another based on the available evidence.

The different results among the studies could be related to the presence of several variables in the design of the studies. The dimensions and age of teeth varied among the studies. In a recent study, changes were reported in the microstructure of root dentin with age that caused reduction in strength and resistance to fatigue [58]. Furthermore, it was concluded that the greatest degradation occurred near the apex and this could contribute to the incidence of vertical root fracture [58]. In the present review, there were also differences in the final apical diameter of teeth and the root canal preparation technique used among the studies. According to a previous study, greater apical enlargement or increased canal taper did not increase the fracture susceptibility of roots [59]. However, other studies reported that the different canal tapered preparation techniques with different rotary files reduced resistance to fracture [60, 61]. Final irrigation procedures may also have an effect on the fracture susceptibility of endodontically treated teeth [62, 63]. In the majority of the included studies, final irrigation was performed with NaOCl and EDTA solutions. However, the concentration, amount and application time of these solutions varied among the studies. These solutions had a negative effect on microhardness values of root canal dentin, which might result in vertical root fracture [64, 65]. Different obturation techniques were used including SC, CLC, and WVC techniques. Pişkin et al. [66] reported that even spreader size used during lateral compaction of gutta-percha could affect the fracture resistance of roots. The method of sealer placement and the amount of sealer placed in the root canal were not clear in the studies. Previous studies reported that bond strengths of thick layers of resin-based sealers to root dentin tended to be higher than with thin layers [67, 68]. In this manner, the thickness of the sealer may also affect the fracture resistance of teeth. However, more research is necessary on this issue.

The simulation of periodontal ligament for the fracture resistance analysis is controversial. Soares et al. [69] reported that root embedment method and periodontal ligament simulation have a significant effect on the fracture resistance of teeth. On the other hand, Marchionatti et al. [70] reported that simulation of periodontal ligament did not affect the fracture resistance results. In the present review, periodontal ligament was simulated in only 5 studies [42, 43, 46, 47, 52]. The diameter, position, and speed of the plunger tip also varied among the studies. The influence of such parameters during the analysis on fracture strength results should be assessed in future studies.

The present review revealed that none of the studies performed blinding, and sample size calculation. In the majority of the studies, the procedures were not performed by a single operator. Overall, these increased the risk of bias. According to the quality assessment in the present review, all studies had a medium or high risk of bias. Therefore, the results of the studies should be interpreted with caution. It is also important to mention that the results of the present review were derived from in vitro studies and thus direct extrapolation of the findings to clinical situation is difficult. Although randomized controlled clinical trials provide the most reliable and robust results, well-designed in vitro studies with high methodological quality could provide beneficial solutions for clinical problems [12, 13]. Systematic reviews of in vitro studies could also provide valuable information and guide future research by assessing the need for further investigations and addressing the limitations of the previous studies.

Meta-analysis is a powerful tool to cumulate and summarize the knowledge in a research field and to identify the overall measure of a treatment’s effect by combining several conclusions [71]. In the present study, each of the included study was analyzed in terms of similarities in order to perform meta-analysis. However, due to the heterogeneity of the studies, it was not feasible to perform a meta-analysis. It is important to mention that even small violations of certain rules during meta-analysis can lead to misleading conclusions [71]. The differences among root canal treatment procedures (type and concentration of irrigation solutions, root canal preparation methods, obturation techniques, simulation of PDL, etc.) in the included studies could possibly prevent the meta-analysis from giving accurate and sound results.

For future in vitro fracture resistance studies, it can be recommended to consider and indicate age range and to use similar teeth in dimensions. It is also advised to perform power analysis for sample size determination before starting the experiments. To avoid inter-operator variability, each procedure including root canal preparation, irrigation, and obturation should be performed by one operator. To avoid performance bias, this operator should be blind to the groups. Furthermore, the observer should also be blind during the fracture testing to avoid detection bias. If the reinforcing effect of root canal sealer is tested, the root canal preparation, irrigation, and obturation procedures similar to previous studies in literature can be performed to provide more comparable findings. The diameter, direction, and speed of plunger tip and the simulation of PDL may affect the outcome. Therefore, these test parameters should also be taken into account when planning these studies. Such standardizations would promote quality and transparency in reported results of in vitro fracture resistance studies of endodontically treated teeth.

Conclusion

Within the limitations of this review, the evidence indicates that the use of root canal sealers in conjunction with gutta-percha increased the fracture resistance of endodontically treated teeth. There was moderate evidence for ZOE-based sealers in favor of no reinforcing effect. Although the evidence was conflicting regarding the reinforcing effects of resin-, GI-, and CS-based sealers, the type of sealer could affect the fracture resistance of endodontically treated teeth. The findings were borne out in the medium to high risk of bias (low quality) studies. Therefore, studies with low risk of bias or high methodological quality are needed to evaluate the fracture resistance of root canal sealers against vertical root fractures. This review strongly suggests the development of standardized methods to test reinforcement effect of root canal filling materials in in vitro studies.

Notes

Compliance with ethical standards

Conflict of interest

The authors declare that they have no conflict of interest.

Ethical approval

For this type of study, ethical approval is not required.

Informed consent

For this type of study, formal consent is not required.

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Copyright information

© Springer-Verlag GmbH Germany, part of Springer Nature 2018

Authors and Affiliations

  1. 1.Department of Endodontics, Faculty of DentistryHacettepe UniversityAnkaraTurkey
  2. 2.Department of Biostatistics, Faculty of MedicineHacettepe UniversityAnkaraTurkey

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