Does the Moment of Fiber Post Cutting Influence on the Retention to Root Dentin?

Abstracts

Despite several advantages associated with pre-fabricated glass-fiber posts, the coronal portion of these posts must be cut to allow their use in various clinical situations. However, cutting the cemented post can generate stress on the bonding interface and affect the bond strength. The aim of this study was to investigate the effect the of fiber post cutting on the bond strength of root canals. Sixty bovine incisor roots were included in resin cylinders with simulated periodontal ligaments. Glass-fiber posts were luted using regular resin cement RelyX ARC (3M ESPE) or self-adhesive cement RelyX Unicem (3M ESPE). The posts were cut prior to cementation, immediately after luting or after building up the core (n=10). After storage for 24 h, the samples were cut and subject to push-out testing using a mechanical testing machine (EMIC DL 2000). Data were analyzed using two-way ANOVA (resin cement x moment of post cutting) and Tukey's post hoc test (α=0.05). The moment of fiber post cutting did not affect the bond strength when Unicem was used. However, the bond strength was reduced when ARC was used and when the post was cut immediately. In conclusion, the moment of fiber post cutting may affect the retention of root canal posts when a regular resin cement is used.

dental bonding; resin cements; post and core technique.

Apesar das diversas vantagens associadas aos pinos de fibra de vidro pré-fabricados, o corte da sua porção coronária deve ser realizado para permitir seu uso em algumas situações clínicas. Entretanto, o corte do pino cimentado pode gerar tensões na interface adesiva e afetar a resistência de união. O objetivo deste estudo foi avaliar o efeito do momento do corte do pino na resistência de união ao canal radicular. Sessenta raízes de incisivos bovinos foram incluídas em cilindros de resina com simulação do ligamento periodontal. Pinos de fibra de vidro foram cimentados utilizando um cimento resinoso convencional RelyX ARC (3M ESPE) ou autoadesivo RelyX Unicem (3M ESPE). Os pinos foram cortados previamente à cimentação, imediatamente após a cimentação, ou após a reconstrução do núcleo de preenchimento (n=10). Após o armazenamento por 24 h, as amostras foram seccionadas e submetidas ao ensaio de cisalhamento por extrusão utilizando máquina de ensaio mecânico (EMIC DL 2000). Os dados foram analisados por ANOVA dois fatores (cimento resinoso x momento do corte do pino) e teste de Tukey (=0,05). O momento do corte do pino de fibra de vidro não afetou a resistência de união quando o RelyX Unicem foi utilizado. No entanto, o corte do pino imediatamente após a cimentação reduziu os valores de resistência de união quando o RelyX ARC foi utilizado. Em conclusão, o momento do corte do pino pode afetar a resistência de união ao canal radicular quando o cimento resinoso convencional for utilizado.

Introduction

Fiber posts are commonly used to restore endodontically treated teeth when the coronal remains are unable to retain the restoration ⁽¹⁾. These post systems present biomechanical properties similar to dentin ⁽²⁾, leading to a more homogeneous stress distribution than metal posts ⁽³⁾ and thus decreasing the risk of root fracture ⁽⁴⁾. The use of these glass fiber posts may also enhance aesthetic restorations and reduce the time required to finalize the restoration due mainly to the elimination of laboratory procedures ⁽⁵⁾. These effects are attributed to the fact that pre-fabricated posts are used; thus, the restorative procedures, including direct restorations, can be performed immediately after cementation of the post ⁽⁶⁾.

The pre-fabricated posts are available in standardized lengths and require partial sectioning to the coronal portion to adjust the clinical condition. Post sectioning can be performed prior to post insertion into the root canal, immediately after post cementation or after core/restoration build-up. Sectioning of cemented posts may generate stress via bur vibrations on the bonding interface between the luting agent and dentin. This stress can reduce the bond strength or promote areas of debonding, compromising the longevity of the post-retained restoration. These deleterious effects mainly occur when post sectioning is performed immediately after post cementation. Since cement setting is mainly activated by chemical initiation during the post cementation ⁽⁷⁾, the resin cements usually exhibit a low degree of conversion immediately after the cementation and thus reduce the bond strength to dentin ⁽⁸⁾. In contrast, when post sectioning is performed after the core build-up, the resin cement exhibits an increased degree of conversion. Furthermore, the core that is bonded to the remaining coronal dentin aids in dissipation of the stress generated by bur vibrations.

Additionally, it is reasonable to assume that the interaction between luting agents and dentin may also exacerbate the possible deleterious effects of stress caused by post sectioning. During the past several years, resin cements have been used as an adhesive system for luting fiber posts to root canals. More recently, self-adhesive resin cements (SARCs) were introduced to facilitate luting procedures ⁽⁹⁾. The bonding mechanism of SARCs is attributed to both a chemical reaction between phosphate methacrylates and hydroxyapatite as well as the infiltration of these materials into the tooth tissues ⁽¹⁰⁾. Various studies have demonstrated that SARCs exhibit a more stable bonding interface compared with regular resin cements ⁽¹¹⁾.

Thus, the aim of this study was to evaluate the effect of the moment of post cutting on its retention to root canal when cemented with regular or self-adhesive resin cement. The hypotheses tested were: (I) the moment when post cutting occurs does not affect the bond strength of the fiber post to the root canal; and (II) the type of resin cement (regular or self-adhesive) does not influence on the retention of posts cut at different moments.

Material and Methods

Study Design

This in vitro investigation was conducted using a 2×3 factorial study design to evaluate the factors "moment of fiber post cutting" in three levels (before the luting procedure, immediately after post luting or after resin core build-up) and "adhesive luting agents" in two levels. The evaluated luting agents included the regular resin cement RelyX ARC (3M ESPE, St. Paul, MN, USA) and the self-adhesive resin cement RelyX Unicem (3M ESPE). The bond strengths of the fiber posts to the root canals of bovine incisors were evaluated by push-out testing (n=10). The experimental design is illustrated in Figure 1.

Figure 1.
Experimental design.

Sample Preparation

Sixty bovine incisor roots with similar dimensions and shapes were chosen for this study. Coronal portions of the teeth were sectioned 15.0 mm above the root apex, using a diamond double-faced disk (KG Sorensen, Barueri, SP, Brazil) mounted in a slow-speed hand-piece that was cooled with an air/water spray. The root canal was instrumented in its full extension using #2 and #3 Gates Glidden drills (Dentsply Maillefer, Ballaigues, Switzerland). The #4 drill was used exclusively in the cervical third of the root canal. Following, the canals were rinsed with 1% sodium hypochlorite solution (Asfer; Industrial Química, São Paulo, SP, Brazil). Instrumented root canals were obturated with gutta-percha cones and Sealer-26 resin sealer (Dentsply Ind. e Com. LTDA, Petrópolis, RJ, Brazil) using the lateral condensation technique.

Post-hole spaces 10 mm deep were prepared using #4 largo drills followed by burs provided by the post manufacturer (White Post DC3; FGM, Joinvile, SC, Brazil). Then, each root was embedded in a polystyrene resin cylinder (Cristal; Piracicaba, SP, Brazil) 2 mm below the cervical limit and the periodontal ligament was simulated. For this purpose, the external surfaces of the root remainders were dipped in melted wax, resulting in a 0.2- to 0.3-mm thick wax layer. Then, the wax-covered roots were included in the acrylic resin cylinders. Following resin polymerization, the roots were removed from the cylinder and wax was removed from the root surface, creating a space in the resin cylinder. The polyether impression material (Impregum Soft; 3M ESPE) was mixed and placed in the space created in the resin cylinder. The tooth was re-inserted into the cylinder, and the excess material was removed with a scalpel blade ⁽¹²⁾.

Post Cutting and Cementation

Twenty posts (21 mm long) were cut 5 mm below the upper side of the post prior to the luting procedure using a #2200 diamond bur (KG Sorensen) operated by a high-speed hand-piece using copious water spray, whereas the other 40 posts had the original dimensions maintained. Prior to cementation, the fiber posts were etched with 24% hydrogen peroxide for 1 min, rinsed with distilled water and air-dried ⁽¹³⁾. A layer of silane coupling agent Prosil (FGM) was applied on the etched surface of post and gently air-dried after 60 s. The Scotchbond Multi-Purpose Plus adhesive system (3M ESPE) was applied over the post surface and light-cured for 20 s. The fiber post was cemented into the root canal using the regular resin cement RelyX ARC (ARC; 3M ESPE) associated with the Scotchbond Multipurpose Plus adhesive system or a self-adhesive cement RelyX Unicem (UNI; 3M ESPE) associated with one of the protocols explained below.

RelyX ARC: The intraradicular dentin was etched with 37% phosphoric acid for 15 s and rinsed. The water excess was removed with absorbent paper points. The Scotchbond Multipurpose Plus (3M ESPE) activator was applied into the root canal with a microbrush of compatible size and air-dried for 5 s. Afterwards, the primer followed by the catalyst were applied and air-dried. The resin cement was placed over the post, and the post/cement was inserted into the root canal and light-cured for 40 s after removal of the excess. After 5 min, the resin cement was light-cured for 80 s.

RekyX Unicem: The root canal walls were rinsed with water using a syringe and then gently dried with absorbent paper points. The self-adhesive cement RelyX Unicem (3M ESPE) was mixed and placed over the posts, which were inserted into the root canal using light pressure. The luting material excess was removed and light activation was performed for 40 s. After 5 min, the resin cement was light-cured for 80 s.

All light activations were performed using a LED light-curing unit (Radii Cal, SDI, Bayswater, Victoria, Australia) with 800 mW/cm² irradiance. Immediately after cementation, 20 posts were cut 6 mm above the root using a previously described bur. For the other 20 samples, a 6-mm-high core was made using Filtek Z-350 composite (3M ESPE) before the cutting procedure. To standardize the core, a matrix was designed using BIO-CAD software (Computer Assisted Desing; Rhino 3D, Rhinoceros, USA). A prototypal resin with the dimensions of the designed cores (6 mm high) was created and used to obtain acetate matrixes. The fiber posts were sectioned to the same level of the resin core.

Push-Out Testing

After storage for 24 h, each root was removed from the resin cylinder and sectioned in 1-mm-thick 6 slabs perpendicular to its long axis using a slow-speed diamond saw (Isomet 1000; Buehler, Lake Bluff, IL, USA). The slabs were analyzed using optical microscopy at 40x magnification (Mitutoyo, Kanagawa, Japan) to measure both major and minor post diameters in millimeters and to identify potential defects caused by cutting. Samples with defects were excluded. Each slab was positioned in a push-out jig and subjected to a compressive load (0.5 mm/min) in a mechanical testing machine (EMIC DL 2000; EMIC, São José dos Pinhais, PR, Brazil) until the post was dislodged. The bond strength value for each slab was calculated in MPa, and the average values of all slabs for each root were used in the statistical analysis. Data were statistically analyzed using two-way ANOVA (resin cement x moment of post cutting) and Tukey's post hoc test (α=0.05).

Results

Values were considered for analysis regardless of root thirds because no significant differences were observed between the root regions for any experimental condition (Table 1). Significant effects were detected for the "resin cement" factor (p<0.001) and the interaction between the factors (p=0.021) but not for the factor "moment of post cutting" (p=0.258). The results are displayed in Figure 2. For RelyX Unicem, no difference was observed between the moments of post cutting. In contrast, post cutting immediately after cementation resulted in the lowest bond strength values when RelyX ARC was used, whereas no differences were observed for other post cutting moments. Bond strength differences between the resin cements were only observed for post cutting immediately after fiber post cementation, whereas RelyX Unicem exhibited the highest values.

Table 1.
Means (95% CI) of push-out bond strength in each root third

Figure 2.
Push-out bond strength results. Distinct letters (lowercase letters for RelyX ARC; capital letters for RelyX Unicem) indicate statistically significant difference among the moments of fiber post cutting (p<0.05). The p-values for comparison between cement at each moment of fiber post cutting are indicated.

Discussion

Despite the advantages of using pre-fabricated posts to extensively restore damaged teeth, partial cut of the coronal portion of the post is necessary to allow the build-up of the core restoration moment of post cutting on the bond strength of the fiber post to the root canal. In the present study, fiber post cutting immediately after post cementation resulted in reduced bond strength values when compared with other post cutting moments only when the post was luted using the regular resin cement RelyX ARC. Thus, the study hypotheses (I - effect of moment of post cutting, and II - effect of resin cement type) were rejected.

Fiber posts are commonly luted to root canals using dual-cured resin cements that are either associated or not with adhesive systems. The use of dual-cured luting agents is preferable in this procedure to achieve proper cement polymerization along the entire root canal space and to obtain an initial stabilization of the fiber post ⁽¹⁴⁾. Once the chemically activated reaction is slowed, light activation of the cement promotes the polymerization of the cement in more cervical regions, thereby establishing post retention in the first minutes after post cementation ^{( 10 , 15 , 16 )}. However, even using translucent fiber posts, a significant reduction in light intensity occurs in the deepest areas of the root canal, and only the slower chemically activated polymerization reaction occurs ^{( 17 , 18 )}. Thus, reduced mechanical strength of the cement and decreased bonding to the dentin root canal is evident, particularly in the early stages after cementation procedures ⁽¹⁹⁾.

The presence of areas of resin cement in deeper regions of root canals in the initial minutes following the fiber post cementation can explain the reduced bond strength observed for post cutting immediately after cementation with RelyX ARC. The use of diamond burs to cut the post vibrates the fiber post and may induce stress in the bonding interface, thereby reducing the bond strength. In this scenario, it is reasonable to hypothesize that the cervical region, which exhibits increased polymerization, acts as a fulcrum; thus, the stress is transmitted to the poorly polymerized deeper areas of the root canal. Since a large number of failures in the adhesive procedure is observed in the apical region of the root canal when adhesive systems are used, debonding areas may appear and propagate the defect through the bonding interface ^{( 20 , 21 )}. Therefore, bond strength is reduced throughout the entire bonding interface without differences between the root thirds. In contrast, when the post was cut after core building up, the bond strength values of posts luted with RelyX ARC were similar to those observed when the fiber post was cut prior to cementation. The time that elapsed between the light activation and the core build-up permits increased cement polymerization ⁽¹⁰⁾, thereby increasing the bonding stability. In addition, the core reconstruction in the dental remnant can potentially increase the post stabilization.

An important finding of this study was that the moment of post cutting did not affect the bond strength when the self-adhesive resin cement RelyX Unicem was used, compared with RelyX ARC. In contrast to regular resin cements, the main bonding mechanism of self-adhesive cements involves chemical chelation between functional acid methacrylates and calcium from tooth tissues ⁽¹⁰⁾. It could be hypothesized that chemical bonding produced by these materials results in a more stable bonding interface. A previous study demonstrated that self-adhesive resin cements support mechanical stress better than conventional resin cements ⁽⁹⁾.

Within the limitations of this in vitro study, the results indicate that the moment of fiber post cutting can reduce the retention of glass fiber posts to root canal, whereas this reduction appears to be material-dependent. However, it is important to emphasize that the immediate bond strengths of only two resin cements were evaluated. Further studies with additional resin cements and aged samples are required to confirm these findings.

Acknowledgements

This study received financial support from FAPEMIG (grant 2014-SAU027).

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Publication Dates

• Publication in this collection
  Apr 2015

History

• Received
  27 Sept 2014
• Accepted
  15 Dec 2014