The Role of Resin Cement on Bond Strength of Glass-fiber Posts Luted Into Root Canals: A Systematic Review and Meta-analysis of In Vitro Studies
Because there are several ways to cement glass-fiber posts (GFPs) into root canals, there is no consensus on the best strategy to achieve high bond strengths. A systematic review was conducted to determine if there is difference in bond strength to dentin between regular and self-adhesive resin cements and to verify the influence of several variables on the retention of GFPs. This report followed the Preferred Reporting Items for Systematic Reviews and Meta-Analyses statement. In vitro studies that investigated the bond strength of GFPs luted with self-adhesive and regular resin cements were selected. Searches were carried out in the PubMed and Scopus databases. No publication year or language limit was used, and the last search was done in October 2012. A global comparison was performed between self-adhesive and regular resin cements. Two subgroup analyses were performed: 1) Self-adhesive × Regular resin cement + Etch-and-rinse adhesive and 2) Self-adhesive × Regular resin cement + Self-etch adhesive. The analyses were carried out using fixed-effect and random-effects models. The results showed heterogeneity in all comparisons, and higher bond strength to dentin was identified for self-adhesive cements. Although the articles included in this meta-analysis showed high heterogeneity and high risk of bias, the in vitro literature seems to suggest that use of self-adhesive resin cement could improve the retention of GFPs into root canals.SUMMARY
INTRODUCTION
The use of glass-fiber posts (GFPs) has increased in recent years compared with other types of posts.1 In addition to their esthetics,2 GFPs have similar elastic modulus to that of dentin, providing a more homogeneous dissipation of loading stresses to the tooth/cement/post structure compared with more rigid posts.3 However, the main reason for failure of GFPs is still debonding,4 which occurs mainly because of the difficulties in achieving proper adhesion to intraradicular dentin. Cementing GFPs into root canals is a clinical challenge because of the complex cementation techniques and high level of technique sensitivity.1
Resin-based cements are commonly used for luting GFPs into intraradicular dentin. A combination of the etch-and-rinse adhesive system and regular resin cement is the approach most often used in dental practice.1,5,6 In the past decade, self-adhesive resin cements were introduced to provide easier clinical application compared with regular resin cements.7 Despite some clinical studies testing different types of posts reported in the literature,5,6,8,9 most information about the retention of GFPs cemented with resin cements is available from in vitro studies, which have tested several cementation strategies and performed different bond strength tests.10,11
Irrespective of recent advances in materials and techniques to make cementation procedures easier, it is important to understand all factors involved in cementing posts, not only the type of resin cement used but also the different approaches attempted to improve bond strength. It is still difficult for clinicians to choose the best and most efficient strategy for luting GFPs. Clinical studies provide little evidence on the performance of GFPs on which to base clinical decisions, leading clinicians to rely on their clinical experience or on data from in vitro studies for choosing a cementation strategy. Therefore, pooled in vitro data could provide more solid conclusions on which strategy to use.
The aim of this study was to systematically review the literature for in vitro studies comparing the bond strength of GFPs cemented with regular and self-adhesive resin cements and to conduct a descriptive analysis to verify the influence of cementation strategies among studies on the retention of GFPs to intraradicular dentin. The hypothesis tested was that no significant difference in bond strength would be detected between GFPs cemented into root canals with regular resin cements or self-adhesive resin cements.
MATERIAL AND METHODS
Search Strategy
This systematic review was performed according to the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) statement.12 Two electronic databases (Medline and Scopus) were searched to identify articles that met the following inclusion criteria: in vitro studies that evaluated and compared the retention (bond strength values in MPa) of GFPs cemented into root canals of human or bovine teeth using both regular resin cement and self-adhesive resin cement.
The following strategy was used for the searches: (glass fiber post) AND (resin cement) AND (bond strength); (glass fiber post) AND (push out); (self* resin cement) AND (glass fiber post) AND (bond strength); (glass-fiber OR glass fiber), and (post) AND (bond* OR adhes*). The same strategy was then performed changing the term post for dowel.
Screening and Selection
No publication year or language limit was used, and the last search was done in October 2012. Reference lists of included studies were hand searched for additional articles. Excluded from the investigation were studies including in vivo or in situ analyses, studies testing posts other than GFPs (ie, carbon-fiber or metal posts), studies with cementation of posts performed in substrates other than teeth (artificial devices), and studies that did not compare bond strength between the two types of resin cements.
Two independent reviewers first screened the titles identified in the searches. If the title indicated possible inclusion, the abstract was then evaluated. After the abstracts were carefully appraised, articles considered eligible for the review were identified; if there was any doubt, the full text of the article was read. In case of disagreement, a third reviewer decided if the article should be included or not (Figure 1).
Citation: Operative Dentistry 39, 1; 10.2341/13-070-LIT
Data Collection
Two reviewers extracted all data simultaneously using a standardized outline. To more easily identify variables found in the articles, the authors categorized similar information into two or three groups (eg, cement application mode). In case of measurement of bond strength values for different root thirds (push-out test, for instance), the arithmetic average of the values of the thirds was used. In studies where bond strength test was performed, including other types of cement or post, only the data of interest were extracted.
Assessment of Risk of Bias
Risk of bias was evaluated according to the articles' description of the following parameters for study quality assessment: randomization of teeth, use of teeth free of caries or restoration, use of materials according to the manufacturer's instructions, use of teeth with similar dimensions, endodontic treatment performed by the same operator, description of sample-size calculation, and blinding of the operator of the testing machine. If the authors reported the parameter, the article had a “Y” (yes) on that specific parameter; if it was not possible to find the information, the article received an “N” (no). Articles that reported one to three items were classified as having high risk of bias, four or five items as medium risk of bias, and six or seven items as low risk of bias.
Statistical Analysis
Initially, each possible comparison of the bond strength of regular resin cement and self-adhesive resin cement in each study was carried out; for example, a study using two regular resin cements and four self-adhesive cements resulted in eight possible comparisons. Pooled-effect estimates were obtained by comparing the means of each resin cement and were expressed as the weighted mean difference between groups. A P value ≤.05 was considered statistically significant.
Statistical heterogeneity of the treatment effect among studies was assessed using the Cochran's Q test, in which a threshold P value of .1 was considered statistically significant, and the inconsistency I2 test, in which values greater than 50% were considered indicative of high heterogeneity.13
The first global analysis was carried out using a fixed-effect model, and two subgroup analyses were carried out to explore heterogeneity between studies: 1) regular resin cement (etch-and-rinse adhesive) vs self-adhesive resin cement and 2) regular resin cement (self-etch adhesive) vs self-adhesive resin cement. The same analyses were carried out using random-effects models. All analyses were conducted using Review Manager Software version 5.1 (The Nordic Cochrane Centre, The Cochrane Collaboration, Copenhagen, Denmark). The influence of cementation strategies among studies on the bond strength of luted GFPs was analyzed using descriptive statistics.
RESULTS
Risk of Bias
Of the 22 studies included, 3 studies presented medium risk of bias and 9 studies showed high risk of bias. The results are described in Table 1, according to the parameters considered in the analysis.
Meta-analysis
Meta-analysis was performed with 23 data sets, although 22 studies were included,11,14-34 because one study22 presented two distinct data sets (one from microtensile test and one from push-out test). Characteristics of the 22 studies (23 data sets) are summarized in Table 2. In the global analysis, 148 comparisons were included.
In the first analysis using a fixed-effect model (Figure 2), the self-adhesive resin cements had higher in vitro bond strengths (1.25 MPa; p≤0.01). The values of the Cochran's Q and I2 tests were p≤0.01 and 98%. In the subgroup analysis of self-adhesive resin cement vs regular resin cement with etch-and-rinse adhesive, the self-adhesive resin cements had higher bond strengths (0.9 MPa; p≤0.01). The values of the Cochran‘s Q and I2 tests were p≤0.01 and 98%. In the subgroup analysis of self-adhesive resin cement vs regular resin cement, the self-adhesive resin cements again had higher bond strengths (1.88 MPa; p≤0.01). The values of the Cochran's Q and I2 tests were p≤0.01 and 96%.
Citation: Operative Dentistry 39, 1; 10.2341/13-070-LIT
The second global analysis using random-effects model showed no statistically significant difference between groups (p=0.31). The values of the Cochran's Q and I2 tests were p≤0.01 and 98%. The subgroup analysis of self-adhesive resin cement vs regular resin cement with etch-and-rinse adhesive showed no statistically significant difference between groups (p=0.41). The values of the Cochran's Q and I2 tests were p≤ 0.01 and 98%. The subgroup analysis of self-adhesive resin cement vs regular resin cement with self-etch adhesive showed no statistically significant difference between groups (p=0.63). The values of the Cochran's Q and I2 tests were p≤0.01 and 96%.
Descriptive Analysis
From the studies included in the review, a total of 47 experimental groups testing regular resin cements were detected, including 13 studies using Panavia F 2.0 (Kuraray, Osaka, Japan) and eight studies using Variolink II (Ivoclar Vivadent, Schann, Liechtenstein). A total of 39 experimental groups testing self-adhesive resin cements were detected, including 27 studies with RelyX Unicem (3M ESPE, St Paul, MN, USA).
Several attempts to verify how pretreating the post influenced bond strength results were used: cleaning with ethanol,18,19,26,33 silane application,15-19,21,23,25-28,30-33,34,35 use of acids,11 or even no pretreatment of the post.14 Although no statistical analysis was performed, the retention of GPFs that had been pretreated with silane seemed to be higher compared with posts that were not pretreated or that were pretreated with other products.
Application of the resin cement was performed using three different approaches: inserting the cement into the root canal, inserting the cement around the post, or inserting the cement into the root canal and around the post. The studies that used mixed techniques showed lower bond strength values.17,26,30 In two studies that did not perform endodontic treatment before luting,23,25 the bond strength results were similar or higher than those for studies in which endodontic treatment was performed.
Each study used its own protocol for aging and storing the samples, including storage for 1 week in water,18,22,26,33 storage in a light-proof container with 100% humidity at 37°C for 9 months,23 or storage in water at 37°C + 3000 thermal cycles (between 5°C and 55°C) for 60 seconds in each water bath with a 6-second dwell time.29 The overall results did not seem to be influenced by the aging protocol.
The type of post cementation failure was not taken into consideration in our analyses, that is, adhesive between the post and cement versus adhesive between the dentin and cement or cohesive within the different types of cement because of the wide variation between the classifications of failure modes in the different studies included. In the same way, we performed no statistical analysis regarding how different types of post pretreatment affected their retention because there were many confounding factors that could influence the results based on the heterogeneity among the studies.
DISCUSSION
This systematic review and meta-analysis is the first to verify the pooled effect of data from in vitro studies that tested the retention of GFPs using resin cements. Several cementation strategies and different bond strength tests were used; more consistent results could be obtained if data were analyzed together, giving support for the clinician on evidence-based decision-making.
The global result (regular vs self-adhesive resin cement) using a fixed-effect model favored the use of self-adhesive resin cement. This result could be explained by the different characteristics of the resin cements. The most commonly used self-adhesive resin cement was RelyX Unicem (3M ESPE),7 which has adhesive properties based on acid monomers that demineralize and infiltrate the tooth substrate, creating micromechanical retention and chemical bonding to hydroxyapatite.7 The water resulting from the acid-base interactions may improve the tooth-cement interaction and the cement moisture tolerance.36,37 The consequent use of water available in the cement matrix and ionization of residual acidic methacrylates culminates in transformation to a hydrophobic material with neutral pH values.7 In addition, the higher bond strength of self-adhesive cements may be a result of the lower polymerization stress compared with regular resin cements.38 The high C-factor and the conical shape of the root canal are critical for the development of polymerization stress; thus, cements with higher stress values may present poorer bonding to the canal walls. Although the hypothesis was rejected, it is important to highlight that the study was conducted using in vitro studies (see the Limitations of the Study section).
Our analysis also demonstrated high heterogeneity (98%); thus, the subgroup analysis was carried out to verify the influence of the adhesive used (etch-and-rinse or self-etch) with regular resin cements in the heterogeneity. The two subgroup analyses favored the use of self-adhesive resin cement. Regular resin cements require multiple bonding steps compared with self-adhesive materials. Etch-and-rinse adhesives require an accurate technique mainly concerning the control of dentin moisture and proper infiltration of the adhesive solution into the root canal, a procedure that might be considered critical and might affect post retention. The etch-and-rinse approach has been also reported to leave a non-encapsulated collagen zone beneath the hybrid layer, which could interfere with the longevity of the bonds.
The rationale of using self-etch adhesives and self-adhesive cements is based on the same principle of dental demineralization and simultaneous infiltration by methacrylate monomers. The bonding mechanism of these adhesive techniques has been linked to an additional chemical bond to tooth structures; the self-etch and self-adhesive strategies, however, have the same possible problem of poorer surface conditioning. Interestingly, the two subgroup analyses favored the use of self-adhesive resin cement; a possible explanation for these results is twofold. On the one hand, application of self-etch solutions into root canals is more complex than self-adhesive cements, particularly regarding proper solvent evaporation, excess adhesive removal, and photopolymerization in the apical areas. On the other hand, some studies use strong self-etch adhesives, which might lead to deposition of calcium phosphates on dentin that are not rinsed and are very unstable in an aqueous environment, thus interfering with the interfacial integrity and bonding ability.7,39 Compared with the use of regular resin cements associated with conventional or self-etch adhesives, the self-adherence potential and dual-cure mechanism of self-adhesive resin cements seems to improve the bonding of GFPs into the confines of the root canal.
Nevertheless, the subgroup analyses showed high heterogeneity because there are great differences among studies. The articles included in this review demonstrated differences, particularly in such aspects as aging or storage of samples, cement application mode, and approaches used to pretreat the posts. The variability related to multiple steps in the bonding process could increase the retention of GFPs to intraradicular dentin in some cases; in other cases, however, the multiple steps might just make the procedures harder and more time consuming. In addition, the included studies generally had a small number of samples and consequently high standard deviation, favoring heterogeneity. This finding made it hard to identify the reasons and variables that influenced the high heterogeneity.
The global result (regular vs self-adhesive resin cement) using a random-effects model showed no difference between resin cements, although the data remained with high heterogeneity (98%). The subgroup analyses were carried out to verify the influence of the adhesive used (etch-and-rinse or self-etch) with regular resin cement in the heterogeneity. The results demonstrated no difference between groups and high heterogeneity, confirming the differences between methodologies used in the studies included in the review. This finding made it hard to identify the reasons and variables that influenced the high heterogeneity. Furthermore, the parameters the authors developed to assess risk of bias showed that the studies included had high or medium risk of bias, thus demonstrating that the variables that could influence the results of the studies were not controlled by researchers favoring the high heterogeneity of the findings of this study.
Yet, post debonding has been described as the most common mode of failure in vitro,4 and this type of failure can be more related to inappropriate bonding techniques than to problems inherent to the materials themselves. The bonding techniques using either regular or self-adhesive resin cements can still be regarded as good options for the luting of GFPs into root canals. The use of self-adhesive resin cements, however, appears to be a suitable and perhaps less technique-sensitive option than luting strategies that involve pretreating the canals with adhesive solutions.
To date the literature has no clinical studies comparing different cementation strategies for GFPs, and clinical studies with self-adhesive resin cements are still scarce. The few clinical studies available40,41 using regular or self-adhesive resin cements to lute GFPs show high survival rates in the short term. The differences between resin cements shown by in vitro studies could be clinically irrelevant, but longer clinical follow-ups are not available.
Limitations of the Study
The results of the present review should be interpreted with caution because laboratory studies have intrinsic limitations when trying to simulate in vivo conditions. In addition, there was a predominance of one particular self-adhesive resin cement (RelyX Unicem) in the studies included, and this should be taken into account when comparing regular resin cements with other self-adhesive cements. Well-designed randomized controlled trials with long follow-up periods are needed to provide the ultimate answer as to whether self-adhesive resin cement will result in improved clinical success rates compared with regular resin cements.
CONCLUSION
Although the articles included in this meta-analysis showed high heterogeneity and high risk of bias, the in vitro literature seems to suggest that the use of self-adhesive resin cement could improve the retention of GFPs into root canals.
Selection procedures according to the PRISMA statement.
Results for analysis using fixed-effect model. The analysis 1.1.1 represents the subgroup analysis between self-adhesive resin cement vs regular resin cement with etch-and-rinse adhesive, while the analysis 1.1.2 represents the subgroup analysis between self-adhesive resin cement vs regular resin cement with self-etch adhesive. The total analysis stands for the global results.
Contributor Notes