Influence of cement type and thickness on polyfiber post adhesion.

INTRODUCTION: To evaluate the effect of two different post space diameters and related resin cement film thicknesses on the bond strength of a polyfiber post.
MATERIALS AND METHODS: A total of 48 premolars were randomly divided into two according to the post space diameter: 1.1 mm and 1.5 mm. Then each group was divided into three sub-groups according to luting cement used: RelyX U100, Panavia F2.0/ED primer, Clearfil SA cement. Spirapost was then luted into the canal using luting cements. Two slices were obtained from each root specimen. Push-out tests were performed. Data was analyzed with Kruskal-Wallis and Connover post-hoc and Mann-Whitney U-test (P < 0.05).
RESULTS: Push-out bond strength was found to vary significantly according to type of adhesive system and post space diameter size (P < 0.05). The self-adhesive resin cement RelyX U100 had significantly higher bond strengths compared with the other adhesive system (P < 0.05). The self-etch adhesive system (Panavia F2.0) showed significantly lower bond strengths compared with the other systems (P < 0.05). There was a significant interaction between the luting systems and post space diameter (P < 0.05).
CONCLUSION: The increases in post space diameter significantly reduced the bond strength of Spirapost to root dentine for both groups.

INTRODUCTION

Posts are commonly used to provide retention for the core and increase the resistance form of the endodontically treated teeth.[1] Since, the 1990s fiber-reinforced posts have been employed more frequently to restore endodontically treated teeth that excessively lost tooth structure. Fiber reinforced posts (18-22 GPa) have a modulus of elasticity similar to the dentine (18 GPa);[2] they produce a stress field similar to that of natural teeth, thereby reducing the risk of root fractures[34] In addition, fiber posts showed increased incidence of repairable fracture when compared to cast dowel posts.[5]

Various luting cements combined with adhesives have been recommended for bonding fiber posts into root canals. It has been reported that the cementation of the fiber post to the root dentine with adhesive techniques increases post retention and reduces microleakage at the dentine–cement interface.[67] Self-etching adhesives are generally applied on dry dentine and do not require rinsing with phosphoric acid, thereby solving the problem of dentine wetness control.[8] The trend toward the simplification of clinical luting procedures has led to the introduction of self-adhesive resin cements. These newer systems, that produce bonding in a simple application, eliminate the need for enamel or dentine pretreatment, simplify the cementation technique and diminish the chances of failures of the clinical procedures.[9] The bonding mechanism of self-adhesive cements is based on micromechanical retention and chemical adhesion.[1011]

Post type, diameter, and cement thickness all have been shown to affect the retention of post systems.[1213] One of clinically, relevant problems dentists face when restoring endodontically treated teeth is the mismatch between the diameter of the post space and that of the post.[14] Although the use of size-matched drills supplied by post manufacturers permits good fitting of posts to the canal walls, root canals can have different shapes;[15] thus, the resin cement thickness around the post can also vary.[16] Several previous studies demonstrate that debonding occurs when an increased cement film thickness is present.[1718] Conversely, Perez et al.[19] have shown that bond strength was not impaired by increased cement thickness. Therefore, the optimal thickness of resin cements surrounding the fiber post is still unclear.

There are commercially available fiber posts that can be prefabricated or custom fabricated. Prefabricated carbon and glass fiber-reinforced composite posts are designed by the manufacturer to fit specific dimensions and there remains a need for straight-line access for the post to be placed into these specific dimensions. A novel post has been developed that is constructed of surgical stainless-steel wires that are twisted around a biocompatible polyfiber strand. According to the manufacturer, this flexible post self-adapts to the configuration of the root canal after minimum post space preparation thus maintains the natural shape and strength of the root and eliminates the possibility of canal perforation. However, there is limited data regarding the properties of this new type of post. There is no study evaluating the effect of post space diameter on the bond strength of Spirapost in conjunction with different self-adhesive luting systems.

The purpose of this in vitro study was to compare the effects of different resin cements and post space diameters on the bond strength of a new polyfiber post to root canal dentine. The null hypothesis was that the push out strength of the Spirapost does not vary significantly on the basis of the post space diameters and cement type.

MATERIALS AND METHODS

The study was approved by the Noninvasive Clinical Research Ethics Committee of Hacettepe University (Approval number: B.30.2.HAC.0.20.05.04/122), Ankara, Turkey. Forty-eight freshly extracted human mandibular premolars with single straight root canals and fully formed apices were used in this study. Radiographic images were obtained from the mesiodistal and buccolingual views to exclude teeth with calcifications, anatomic abnormalities, or previous endodontic treatment. To standardize procedures and materials, teeth with similar mesiodistal and buccolingual dimensions were selected. The selected specimens were stored in 0.9% saline at 4°C until utilization. The crowns were separated 2 mm coronal to the cementoenamel junction using a 0.3-mm-thick, diamond-coated slow-speed band saw (Isomet 1000; Buehler, Lake Bluff, IL) under copious water coolant. To standardize the root canal lengths, all roots were cut to 14 mm in length. Apical patency was verified using an ISO size-10 K-file (Dentsply Maillefer, Ballaigues, Switzerland). The working length of each tooth was determined visually by subtracting 1 mm from the length of an ISO size-10 K-file placed at the apical foramen. The root canals were mechanically enlarged according the manufacturer's instructions using endodontic files (ProTaper up to #F3); between each file 2 mL of 5.25% NaOCl was used as an intra-canal irrigant during instrumentation. After preparation, the canals were irrigated with 5 mL NaOCl followed by 5 mL 17% ethylenediaminetetraacetic acid (EDTA) to remove the smear layer. Finally, the specimens were irrigated with distilled water to avoid the prolonged effect of the EDTA and NaOCl solutions, dried with paper points and obturated with lateral condensation of gutta-percha and endodontic sealer (AH26). After the completion of endodontic treatment, cervical root canal openings were filled with a provisional restorative material (Cavit-G; 3M ESPE AG, Seefeld, Germany). Then, teeth were stored in 100% humidity for 7 days. The roots were randomly divided into two groups of 24 specimens in each group according to the post drill used: Group 1: Gates glidden #4, (ISO sized 1.1 mm in diameter, recommended by the company) and Group 2: Gates glidden #6 (ISO sized 1.5 mm in diameter). Gutta-percha was removed with heated endodontic pluggers (Sybron Dental Specialties, Romulus, MI), maintaining at least 4 mm of filling material in the apical third. Post space preparation was performed at a depth of 10 mm from the sectioned surface with the use of Gates Gliddens. A new drill was used for every six specimens. Following the post space preparations, the spaces were rinsed for 1 min with 5% NaOCl. Final irrigation was accomplished with distilled water using an endodontic syringe and then the post spaces were dried with paper points. Post holes were checked radiographically for any residual gutta-percha. These specimens were divided into three subgroups according to the luting cements (n = 8 per group) [Table 1].

Table 1

Chemical compositions of the evaluated materials

A polyfiber post (888102 Spirapost PFS Tapered) then was coated with the luting cements showed in Table 1 according to the manufacturer's instructions. All posts were then seated to full depth in the prepared spaces using finger pressure, ensuring that all posts were extended the same length from the orifice of the canal. Excess luting cement was immediately removed with a small brush. The luting cement was light cured with a light emitting diode light-curing unit (Elipar S10, 3M ESPE, St. Paul, MN) for 20 s in each of four directions (buccally, lingually and proximally). After the cementation procedures, the coronal part of the exposed dentine was completely covered with a temporary filling (Cavit; Espe, Seefeld, Germany) and the teeth were stored in distilled water for 7 days at 37°C. All specimens were prepared by the same operator. To evaluate the adhesion of the posts, all specimens of both groups were serially sectioned into 1.0-mm slices (±0.1 mm) using a water-cooled, 0.3-mm-thick, diamond-coated slow-speed band saw from the root canals. Each specimen was marked on its coronal side with an indelible marker, and specimen thickness was measured with a digital caliper with accuracy of 0.001 mm. Root sections demonstrating oval root canal form were discarded and replaced with another specimen prepared in accordance with the experimental protocol of that group. The specimens were then subjected to a push out test in a universal testing machine (Lloyd LR30K; Lloyd Instruments Ltd, Fareham, UK) managed by software (Nexygen-Ondio Version 4.0; Lloyd Instruments Ltd). The jig's 0.76 mm wide stainless steel pin was aligned with the center of the post, and the crosshead was lowered at 1.0 mm/min until the post was dislodged from the roots section. The maximum force required to dislodge each post was recorded in Newtons (N). The push-out strength was calculated by dividing the load at debonding (N) by the interface between post and cement surface (mm2).[20] Push out bond strength values were converted to MegaPascals (MPa).

Data were analyzed with Kruskal-Wallis post-hoc Connover and Mann–Whitney U-tests using the SPSS 11.5 for Windows (SPSS, Chicago, IL) statistical software at P < 0.05. After push-out testing, the specimens were analyzed under stereomicroscope (Olympus SZ61; Olympus Optical Co., Tokyo, Japan) at ×40 magnification and the failure mode was classified according to the following criteria: adhesive failure between dentin and the luting agent [Figure 1a], adhesive failure between the luting agent and the post [Figure 1b], mixed failure [Figure 1c], cohesive failure within the luting agent, cohesive failure within the post [Figure 1d].[21]

Figure 1

Stereomicroscope images of failed specimens. (a) Adhesive failure between luting agent and the post, (b) adhesive failure between dentin and the luting agent, (c) cohesive failure within the post, (d) mixed failure (×40)

RESULTS

Mean and standard deviation push-out bond strength values for each luting system and post space diameters are displayed in Table 2. Push-out bond strength was found to vary significantly according to type of adhesive system (P < 0.05). The self-adhesive resin cement RelyX U100 had significantly higher bond strengths compared with the other adhesive system (P < 0.05). The self-etch adhesive system (Panavia F2.0) showed significantly lower bond strengths compared with the other systems (P < 0.05). There was a significant interaction between the luting systems and post space diameter (P < 0.05). However, bond strength values of groups decreased with the increasing post space diameter, significant differences were observed only within the RelyX U100 and Clearfil SA groups. Although the self-etch system (Panavia F2.0) showed a slight decrease in bond strength with increasing cement thickness, differences among its sub-groups were insignificant (P > 0.05). Table 3 exhibits the analysis of failure modes in each group. Cohesive failure within the post was the most frequent type of failure mode in each group. Thicknesses of the cements used had no effect on failure modes of the tested posts for each adhesive system.

Table 2

Mean push-out bond strength values (MPa) and SD of the tested materials

Table 3

Failure modes of experimental groups

DISCUSSION

The purpose of this study was to evaluate the effects of luting cements and different post space diameters on the bond strength of a new polyfiber post to root canal dentine by using push-out test method.

Several methods have been performed to measure the bond strength to root dentine, such as microtensile, shear test, pull-out and push-out tests.[22] The push-out test provides a better estimation of the bond strength than the does the conventional shear test, because with the push-out test, the fracture occurs parallel to the dentine-adhesive interface, which makes it a true shear test.[23] In addition, the push-out test has been considered more dependable than the microtensile test for bonded posts.[24] The thin-slice push-out test has been reported to be a practical tool for evaluating the bond strength of fiber posts.[22] Hence, the thin-slice push-out test was used in the present investigation.

There is no study evaluating the effect of post space diameter on the bond strength of Spirapost in conjunction with different self-adhesive luting systems. The aim of our study was to evaluate the accuracy of fit between the post and post space diameter on the bond strength of a polyfiber post. Spiraposts are interproximal brushes used to fill the root canal space in combination with resin composite. Unlike a classic post, the Spirapost does not have two distinct interfaces (post cement and cement-dentin). In the present study, post spaces were prepared by using two different sized post drill and so different thicknesses of cements was created. To obtain standard cement thickness throughout the slices, apical portions of the posts were not included due to their tapered design.

The results of this study demonstrated that different post space diameters affect the push-out bond strength, necessitating acceptance of the first null hypothesis. The results of this study demonstrated that the push-out bond strength of Spiraposts decreased as the cement thickness increased regardless of cement type. When the cement was thick, significantly lower bond strengths were observed for the self-etch and self-adhesive systems. These results could be explained by reduced shrinkage of the thinner cement results in less stress at the dentine-post interfaces.[25] In each luting cement groups bond strength results were found to be statistically higher for post space in 1.1 mm diameter than the in 1.5 mm diameter. It may be caused by the fact that more luting cement penetrates polyfiber strands and may result lower retention in 1.5 mm diameter post space. When the cement layer is thicker, more air is incorporated by surface area. Air bubbles may weaken the composite substantially, which may explain in part the debonding between the resin cement and the root dentin. Post space might be related with the formation of bubbles or voids, representing the areas of weakness within the material, which is less likely to be seen in a thin and uniform layer of cement.

In a previous study Mastoras et al.[26] reported that Spirapost had higher mean bond strength values compared to quartz-fiber post. The mean bond strength values of Spirapost were lower in the present study compared to this previous study. The possible reasons for this difference may be due to the differences between post space diameter and luting cement used. In this study, it was performed with the use of Gates Glidden #3 for Spirapost; while in the present study it was performed with the use of Gates Glidden #4 according to manufacturer's instructions.

The effect of cement thickness on the pull-out strength of a fiber post using a self-etch system for cementation of the fiber post was investigated by D’Arcangelo et al.[27] They reported that when the cement thickness was too great, the bond strength was significantly decreased. In addition, Grandini et al.[28] evaluated the resin cement thickness after luting anatomic posts and standardized fiber posts into root canal preparations. The formation of bubbles or voids, representing areas of weakness within the material, is less likely in a thin and uniform layer of cement. Moreover, they stated that the polymerization stress, developing within a relatively thin film of cement, would be minimal.[28] Similar studies have reported that the resin cement thickness in fiber reinforced posts significantly influences bonding because an excessively thick layer of resin cement around a fiber post is correlated with a higher frequency of post debonding.[1829] To solve this problem, several studies have recommended using the thinnest cement layer to increase the bond strength. Conversely, Perez et al.[19] have reported that an increase in cement thickness did not significantly affect the bond strength of fiber posts to the root dentine. Moreover, Perdigão et al.[20] implied that the thickness of the resin cement has no effect on push-out bond strengths of fiber reinforced composite posts.

Self-etch and self-adhesive resin cements were developed in order to reduce the number of application steps and technique sensitivity compared to conventional resin cements while obtaining comparable results.[18] Previous studies evaluated bond strength of etch-and-rinse, self-etch and self-adhesive systems and concluded that the self-etch approach may offer less favorable adhesion to root dentine in comparison with the other systems.[1830] Similarly, in the present study push-out bond strength of the self-adhesive systems (RelyX U100, Clearfill SA cement) was better than that of the self-etch system (Panavia F2.0). This result is consistent with the study of Zorba et al.[31] In addition, Toman et al.[32] also showed that displacement resistance values for the self-adhesive system were significantly higher than for the self-etch system.

Self-adhesive resin cements contain multifunctional phosphoric acid methacrylates that demineralize the tooth structure and react with the hydroxyapatite of the hard tissue.[33] In addition, self-adhesive systems exhibit moisture tolerance, which may explain the superior bonding performance of the self-adhesive system to the self-etch system in the present study because of difficulties in moisture control of post space after rinsing the root canal.[34] The acidic resin monomers in the self-etch ED primer from Panavia F 2.0 (10-methacryloyloxydecyl dihydrogen phosphate) and in the self-adhesive cement Clearfil SA cement (10-methacryloyloxydecyl dihydrogen phosphate), both with a pH of ≈ 2.0, seemed to be less effective.[35] Conversely, several previous studies reported similar bond strength for the self-adhesive system compared with the self-etch system.[1136]

Analyses of the failure modes in the present study revealed cohesive failures within Spirapost, which is in accordance with a previous study. Mastoras et al.[26] explained that the reason for cohesive failure may be the mechanical interlocking between resin cement and polyfiber strands of Spirapost at different planes. In addition, failure modes did not vary according to the cement thickness used. It is essential to consider that these examinations were performed using an optical microscope, which does not offer the highest magnifications or accuracy for this type of analysis.

Based on these findings and within the limitations of this study, it was demonstrated that the post space diameter and luting agent significantly influenced the push out strengths of Spirapost. The increases in cement thickness significantly reduce the bond strength of Spirapost to root dentine for both the self-etch and self-adhesive resin systems.