Evaluation of push-out bond strength of two fiber-reinforced composite posts systems using two luting cements in vitro.

INTRODUCTION: The concept of using a "post" for the restoration of teeth has been practiced to restore the endodontically treated tooth. Metallic posts have been commonly used, but their delirious effects have led to the development of fiber-reinforced materials that have overcome the limitations of metallic posts. The use of glass and quartz fibers was proposed as an alternative to the dark color of carbon fiber posts as far as esthetics was concerned. "Debonding" is the most common failure in fiber-reinforced composite type of posts. This study was aimed to compare the push-out bond strength of a self-adhesive dual-cured luting agent (RelyX U100) with a total etch resin luting agent (Variolink II) used to cement two different FRC posts.
MATERIALS AND METHODS: Eighty human maxillary anterior single-rooted teeth were decoronated, endodontically treated, post space prepared and divided into four groups (n = 20); Group I: D.T. light post (RTD) and Variolink II (Ivoclare vivadent), Group II: D.T. light post (RTD) and RelyX U100 (3M ESPE), Group III: Glassix post (Nordin) and Variolink II (Ivoclare vivadent) and Group IV: Glassix post (Nordin) and RelyX U100 (3M ESPE). Each root was sectioned to get slices of 2 ± 0.05-mm thickness. Push-out tests were performed using a triaxial loading frame. To express bond strength in megapascals (Mpa), load value recorded in Newton (N) was divided by the area of the bonded interface. After testing the push-out strengths, the samples were analyzed under a stereomicroscope.
RESULTS: The mean values of the push-out bond strength show that Group I and Group III had significantly higher values than Group II and Group IV. The most common mode of failure observed was adhesive between dentin and luting material and between post and luting material.
CONCLUSIONS: The mean push-out bond strengths were higher for Groups I and III where Variolink II resin cement was used for luting the fiber post, which is based on the total etch adhesive approach. In most of the samples, failure was observed between cement-dentine interface, followed by post-cement interface, which shows difficulty in bonding between post-cement-dentine interface.

INTRODUCTION

For many years, posts have been used for the restoration of endodontically treated teeth.[1] Different materials have been used for the fabrication of posts. The progress made in the technology of fiber-reinforced materials has overcome some of the limitations of metallic posts; their advantage is more flexibility and same modulus of elasticity as dentin.[2]

When bonded in place with resin cement, it was thought that the forces would be distributed more evenly in the root, resulting in fewer root fractures.[3]

“Debonding” is the most common failure in fiber-reinforced composite (FRC) type of posts. The bonding capacity of FRC post systems is influenced by various factors.[4] Controlling these factors is essential in achieving a good bond between post–cement–dentine interface either by variation in the technique or by use of different materials for bonding of the post in the root canal.[5]

Adhesive resin cements have been used for the luting of these types of posts inside the root canal. A multiple-step technique is rather time consuming and technique-sensitive; self-adhesive resin cements that do not require any pre-treatment in such a context have gained popularity.[6] Adhesion to the tooth tissue is due to a chemical reaction between phosphate methacrylates and hydroxy-apatite.[78]

The objective of this study was to compare the push-out bond strength between two fiber posts and two luting cements and also to evaluate the mode of failure between the fiber post, luting agent and root dentin.

MATERIALS AND METHODS

Eighty human maxillary anterior single-rooted teeth (previously extracted for periodontal reasons) with fully developed apices, straight roots without cracks, caries and fractures were selected.

Teeth were cleansed using ultrasonics and disinfected by immersion in 2.5% sodium hypochlorite solution for 2 h and stored in normal saline.

All the teeth were decoronated 1.5-2.0 mm coronal to the cemento–enamel junction with a straight fissure bur. The pulp tissue was extirpated and canal patency was assessed with a size 10 K-file. Working length was established 1 mm short of the apical foramen and canals were enlarged up to size 40 K-file following the step-back technique with an intermediate irrigation using 1 mL of 2.5% sodium hypochlorite. After final irrigation with normal saline, the canals were completely dried and obturated using AH plus sealer and gutta-percha by cold lateral compaction. The samples were stored in dampened gauze with normal saline for 1 week at 37°C.

Post space preparation was done by drills provided by the respective manufacturer of the post after removal of gutta-percha from the middle thirds of the canal, keeping 4-5 mm of the apical gutta-percha intact. Canals were irrigated with saline and radiographs were taken to check for any residual gutta-percha.

The roots were randomly assigned into four groups (n = 20):

Group I: D.T. light post (RTD) and Variolink II (Ivoclare vivadent)

Group II: D.T. light post (RTD) and RelyX U100 (3M ESPE)

Group III: Glassix post (Nordin) and Variolink II (Ivoclare vivadent)

Group IV: Glassix post (Nordin) and RelyX U100 (3M ESPE).

In groups where Variolink II was used as the luting agent, the walls of the post space were etched with 37% phosphoric acid for 15 s and rinsed and dried. Scotch Bond (3M ESPE) was applied to the post space walls for 10 s with microbrush and excess was removed with paper points. Roots were polymerized using a halogen light unit for 20 s with the tip directed toward the post space opening. Equal amounts of base and catalyst were mixed and applied on the surface of the posts and the orifice of the root canals. The posts were then inserted into canals to a full depth using gentle finger pressure and excess was immediately removed with an explorer. The samples were subjected to light curing for 60 s, with the tip of the light unit in direct contact with the coronal end of the posts.

In groups where RelyX U100 was used as a luting agent, the dual-curing, self-adhesive resin luting agent was prepared according to the manufacturer's instructions [Table 3]. Cementation and polymerization was performed as mentioned previously. All samples were placed in distilled water at room temperature.

Table 3

Materials information

Push-out tests

The coronal portion of each root was sectioned perpendicular to the long axis of the tooth with a diamond disk to get 2 ± 0.05-mm-thick slices. The thickness of each slice and the radius of the canals were measured using a Vernier caliper. Push-out tests were performed with a cross-head speed of 1 mm/min using a triaxial loading frame with the push-out jig being attached to a testing machine. Care was taken to center the push-out pin (diameter 1.0 mm) on the post surface without causing stress on the post space walls. Load was then applied to the apical side of the root slice to avoid resistance to movement of the post due to post space taper. The peak force that caused extrusion of the post segment from the slice was taken as the point of bond failure and the value was recorded in Newton (N). To express bond strength in megapascals (Mpa), the load value recorded in Newton was divided by the area of the bonded interface.

This was calculated as follows:

A = 2πrh

Where, “π” is constant (3.14), “r” is the post radius and “h” is the thickness of the slice.

RESULTS

The mean push-out bond strengths [Table 1] were higher for Group I and Group III where Variolink II resin cement was used for luting the fiber post. In most of the samples, failure was observed between the cement–dentine interface, followed by the post–cement interface, which shows difficulty in bonding between post–cement–dentine interface.

Table 1

Comparison of push-out bond strengths between the groups

Statistical analysis was carried out using SPSS software version 16.0, which showed significant differences among the groups (P < 0.05) according to analysis of variance (ANOVA one-way). Scheffe multiple comparison showed statistically significant differences (P < 0.05) between Groups I and II, Groups I and IV, Groups II and III and Groups III and IV. No significant difference was found between Groups I and III and Groups II and IV.

After testing the push-out strengths, the samples were analyzed using a stereomicroscope at the original magnification (40X) to determine the type of failure as follows: [Table 2]

Table 2

Mode of failure between the different groups

Type A: Adhesive failure between post and luting material

Type B: Adhesive failure between dentin and luting material

Type C: Cohesive failure of post system

Type D: Cohesive failure of luting material

Type E: Mixed type; combination of any two of the above-mentioned types.

76.25% of the samples showed adhesive failures while 18.75% were mixed failure and only 5% samples showed cohesive failures. In adhesive failure, 47.5% were seen between dentin and luting material, while 28.75% adhesive failures were between post and luting material. Cohesive failure was seen only for the post system.

DISCUSSION

FRC posts, as an alternative to cast post & core and prefabricated metal posts, were introduced in the early 1990s to restore endodontically treated teeth with an excessive loss of tooth structure. Several in vitro studies have shown that FRC posts distributed occlusal stresses more evenly in the root dentin, thereby resulting in fewer and more favorable root fractures, which were often reparable.[910]

Bonding of FRC post systems may be influenced by various factors like the degree of hydration of root canal dentin, surface conditioning agent and luting cement used, cavity configuration factor, use of eugenol-containing sealers and the anatomic differences in density and orientation of the dentinal tubules at different levels of the root canal area.[45]

Adhesive resin cement was used to cement the posts as they have the ability to bond to both the radicular dentin and the post, allowing the use of conservative post insertion techniques as well as reducing potential stress.[1112]

The behavior of luting cements is influenced by factors such as the adhesive approach and the type of initiation.[13] Both the resin cements used were dual cure, in which initiation is dependent on light source that is used in conjunction of light-transmitting fiber posts, which did not affect the bond strength as when metal posts were used.[14] A significant difference was seen between the two resin cements used, which may be related to the fact that thick smear layers and other debris retained on the root canal walls[15] might prevent optimal adhesive infiltration and that self-etching primers might not be able to etch through thick smear layers.[16]

The groups in which Variolink II was used as a luting agent showed the highest mean push-out bond strength compared with that of the groups luted with RelyX U100, which were similar to study conducted by Kececi et al.[5] The reason for the lower push-out bond strength in RelyX U100 cement might be the fact that methacrylated phosphoric esters responsible for substrate conditioning are not as effective as phosphoric acid used with Variolink II to dissolve the thick smear layer created on the root canal walls during the post space preparation.[17]

The push-out test is a more reliable method for determining bond strengths between fiber posts and post space dentine because of the high number of premature failures occurring during specimen preparation and large data distribution spread associated with microtensile testing.[17] Push-out tests result in a shear stress at the interface between dentine and cement as well as between post and cement, and are comparable with the stress under clinical conditions.[18]

Different authors have recommended different thickness of dentin disk for the push-out test.[192021222324] The use of thicker discs seems to increase the area of friction and may lead to an overestimation of the bond strength.[25] In the present study, the disk thickness was 2 ± 0.5 mm, which was acceptable for testing. Furthermore, if the force is not completely centralized, friction with part of the dentin wall can be increased[26]; for this reason, the pin diameter of the push-out jig was 1 mm, which was smaller than the diameter of the post (1.5 mm).

The analysis of failure modes in this study revealed that most of the failures occurred at the interface between dentin and luting agent, which was similar to previous studies.[24] Adhesion between resin cements and root dentin is difficult because of polymerization stress occurring at the cement–dentin interface,[27] which may be affected by root canal geometry, responsible for its high configuration factor (C-Factor), i.e., a high force of resin cement contraction opposite to the cavity walls competing with the bond strength of the adhesive area.[28] Also, the use of sodium hypochlorite as an irrigant may interfere with the bond strength to dentin.[29]

The second most observed failure modes were adhesive at the cement and post interfaces. This may have resulted from the absence of a chemical union between epoxy resin-based post and methacrylate-based resins, and led to the proposed use of silane to improve the bonding mechanism.[30] In this study, there was no pretreatment on the post surface, and this may have resulted in this type of failure mode.[31] Cohesive failure was observed only of the post system in the group having Glassix post luted with RelyX U100 resin cement.

Both the FRC posts used in the study were translucent fiber posts that eliminated lack of light transmission, which affects the degree of polymerization of dual cure resin cements.[32]

CONCLUSIONS

Within the limitations of this study, it can be concluded that:

The mean push-out bond strengths were higher for Groups I and III where Variolink II resin cement was used for luting the fiber post, which is based on the total etch adhesive approach. Therefore, pretreatment for removal of smear layer is recommended before the use of any kind of resin cement inside the root canal for luting of post

In most of the samples, failure was observed between the cement–dentine interface, followed by the post–cement interface, which shows difficulty in bonding between post–cement–dentine interface.