The Study On Retentive Mechanism Of Fiber Posts And Factors Related To The Retention Of Fiber Posts In Root Canal

A post-core restoration is usually recommended for endodontically treated teeth in cases when little coronal tooth tissue remains due to caries, traumas or removal of previous restorations. Post and core built-up is necessary to provide support to the full crown and thus enable the final restoration to sustain various oral functional load. Recently, fiber posts have been more popularly used than conventional cast post-core and pre-fabricated metallic post in the restoration of endodontically treated teeth. Due to the similar elastic modulus with dentin, fiber posts could allow more favorable stress distribution and reduce the risk of root fractures. Fiber posts in combination with resin cements could reinforce the root weakened by the root canal preparation. Lots of glass- or quartz- fiber posts have natural tooth color, supporting all-ceramic and esthetic restoration for anterior teeth. Owe to the good biocompatibility and resistance to corrosion of fiber posts, the problems of discoloration, corrosion and secondary infection induced by metallic posts can therefore be delimited when using fiber posts. However, many laboratorial and clinical studies have revealed that the most frequent failure mode of fiber post restoration is adhesive failure. Therefore, it is clinical important to develop procedures that can effectively enhance the adhesion between intraradicular dentin and resinous luting materials and/or between luting materials and fiber posts.As to the adhesion between intraradicular dentin and luting resin materials, it is relatively weak considering three parts of factors. The first is that the post space after sequential endodontic treatment and post-space preparation is covered by a thick smear layer, which may hamper the infiltration of adhesive resin. Secondly, root canal is a narrow and confined space, so that resin polymerization may produce contraction stress and influence the bonding to dentin according to the C-factor theory. The last factor is that the dentinal structure is different within different root regions and the deep region is hard to access by both instrument and luting materials. The bond strength in deep root region is therefore lower than that in upper region. Concerning the adhesion between fiber posts and various composite resins, it is still inferior to the adhesion of dental substrate because of the chemical incompatibility or an absence of chemical bond between the epoxy resin based fiber posts and the Bis-GMA based composite resin.This study is designed to improve the retention of fiber posts when taking into account these factors. The purpose of this study was to evaluate the effect of bonding substrate, luting material, and luting procedure etc. on the retention of fiber posts, to investigate the retentive mechanism of fiber posts, and to attempt to improve the retentive strength of fiber posts using different surface treatments and luting procedures as well.The experimental study could be divided into two parts. Part 1 is composed of three sections and is about the evaluation of retentive strength of fiber posts in root canal. Part 2 is composed of two experiments and is about the adhesion between fiber posts and different composite resin, including resin cements and resin core.In Section 1 of Part 1, the hybrid layer and resin tags formed by different adhesives, as well as the morphological aspects of post-space dentin either with or without root canal obturation, were observed using SEM. Also, the effect of root canal obturation, root region and luting procedure on the push-out strength of fiber posts in roots were evaluated in this section. The following results were achieved:â‘ r oot canal obturation, root region and luting procedure all have an effect on the push-out strength of fiber posts;â‘¡root canal obturation could produce a smear layer containing large amount of dentinal debris, gutta-percha/sealer remnants, which decreased the push-out strength of fiber posts by interfering with the infiltration of adhesives;â‘¢the use of adhesive could only improve the retention of fiber posts in coronal root region, while the use of mild self-etching adhesive showed no effect on the improvement of post retention in the apical root region;â‘£the push-out strength of fiber posts using a total-etching adhesive was affected by the root region, while the push-out strength of fiber posts using a self-etching adhesive was not affected by the root region.There are two experiments in Secion 2 of Part 1. In experiment 1 the effect of two chemical irrigations -3%H2O2 and 5.25%NaOCl+17%EDTA on the retention of fiber posts using four adhesives (1 total-etching and 3 self-etching) was evaluated. In experiment 2, the efficacy of three post-space treatments, 35% phosphoric acid etching, irrigation using 5.25%NaOCl+17%EDTA and ultrasonic agitation associated with 5.25%NaOCl+17%EDTA, in the removal of smear layer on post spaces were evaluated with SEM. The effect of these post-space treatments on push-out strength of fiber posts were also investigated in this experiment. The following results were achieved:â‘ The effects of 3%H2O2 and 5.25%NaOCl+17%EDTA irrigations on the retention of fiber posts were both dependent on the type and chemical composition of adhesives;â‘¡All the treatments tested in Experiment 2 were effective in the removal of the smear layer on the post-spaces;â‘¢Treatment with 35% phosphoric acid or ultrasonic agitation associated with EDTA/NaOCl irrigation on the post space improves the retention of fiber posts in the apical region;â‘£A solo irrigation with an EDTA/NaOCl solution had no effect on the retention of fiber-posts.There are two experiments in Secion 3 of Part 1. In experiment 1 the push-out strength of fiber posts luted using four luting materials were compared. In experiment 2 the effects of curing modes of dual-cured luting systems and root regions on the push-out strength of fiber posts were evaluated. The following results were achieved:â‘ Luting materials, luting procedures and root regions all showed a significant effect on the retention of fiber posts;â‘¡the retention of fiber posts were affected by the curing modes of dual-cured luting materials, which depended on the type of the materials;â‘¢I t is not necessary to separately light cure the dual-cured dentin adhesives before post cementation when translucent fiber posts were used;â‘£T he photoirradiation of dual-cured resin cement after post cementation may give an increase to both bonding and mechanical strength of fiber posts.There are two experiments in Part 2. In experiment 1 the effect of coupling agents on the adhesion of pre-silanized fiber posts to two composite resins (One flowable resin cement and one hybrid core resin) were evaluated. In experiment 2 the durability of bonding interfaces between adhesive-mediated fiber posts and two types of composite resins were investigated using a simplified model of accelerated aging. The following results were achieved:â‘ The use of self-cured coupling agent could increase the bond strength of pre-silanized fiber posts to hybrid core resin, while it showed no effect on adhesion of fiber posts to flowable resin cement;â‘¡The use of light-cured coupling agent had no effect on the adhesion of pre-silanized fiber posts to composite resin regardless of the type of composite resin;â‘¢The bonding interface between light-cured adhesive mediated fiber posts and composite resin presented degradation after storage in water for one month;â‘£No degradation were detected at the bonding interfaces between self-cured adhesive mediated or no treated fiber posts and composite resin after storage in water for one month.