Factors affecting the cement–post interface

Abstract

Objectives

To evaluate the effect of different factors on the push-out bond strength of glass fiber posts luted in simulated (standard) root canals using different composite cements.

Methods

Three types of glass-fiber root-canal posts with a different matrix, namely an epoxy resin (RelyX post, 3M ESPE), a proprietary composite resin (FRC-Plus post, Ivoclar-Vivadent), and a methacrylate resin (GC post, GC), and three types of composite cements, namely an etch-and-rinse Bis-GMA-based (Variolink II, Ivoclar-Vivadent), a self-etch 10-MDP-based (Clearfil Esthetic Cement, Kuraray) and a self-adhesive (RelyX Unicem, 3M ESPE) cement, were tested. Posts were either left untreated (control), were treated with silane, or coated with silicated alumina particles (Cojet system, 3M ESPE). Posts were inserted up to 9-mm depth into composite CAD-CAM blocks (Paradigm, 3M ESPE) in order to solely test the strength of the cement–post interface, while excluding interference of the cement–dentin interface. After 1-week storage at 37 °C, three sections (coronal, middle, apical) of 2-mm thickness were subjected to a push-out bond-strength test.

Results

All three variables, namely the type of post, the composite cement and the post-surface pre-treatment, were found to significantly affect the push-out bond strength ( p < 0.001). Regarding the type of post, a significantly lower push-out bond strength was recorded for the FRC-Plus post (Ivoclar-Vivadent); regarding the composite cement, a significantly higher push-out bond strength was recorded for the self-adhesive cement Unicem (3M ESPE); and regarding the post-surface treatment, a significantly higher push-out bond strength was recorded when the post-surface was beforehand subjected to a Cojet (3M ESPE) combined sandblasting/silicatization surface pre-treatment. Many interactions between these three variables were found to be significant as well ( p < 0.001). Finally, the push-out bond strength was found to significantly reduce with depth from coronal to apical.

Significance

Laboratory testing revealed that different variables like the type of post, the composite cement and the post-surface pre-treatment may influence the cement–post interface, making clear guidelines for routine clinical practice hard to define. Further long-term durability testing may help to clarify, and should therefore be encouraged.

Introduction

Since the 1990s fiber-reinforced posts have been employed more frequently to restore endodontically treated teeth that excessively lost tooth structure ; these are intended to solely improve the retention of the build-up material. Because their elastic modulus is claimed to be similar to that of dentin, the risk of vertical root fracture is significantly reduced . Furthermore, quartz or glass-fiber posts (white or translucent) have been developed to better meet with the increasing demand for high-end esthetic restorations .

While root fracture has been reported to be the most severe cause of failure for endodontically treated teeth that were restored with cast metal post-and-cores, less tooth-threatening failures like loss of retention or post fracture are the most frequent failures reported when using glass-fiber posts . Since the bonding effectiveness into the root canal is crucial for the retention of fiber posts , many studies have focused on the dentin–cement interface or on the combined ‘sandwich’ dentin–cement–post assembly . Much less attention has however been paid to the interaction of the composite cements with the fiber post itself . In addition, mostly rather controversial results were reported, while also manufacturers of glass-fiber posts often recommend very different protocols to (pre-)treat the fiber-post surface.

Current fiber posts are composed of unidirectional fibers (carbon, quartz or glass) embedded in a resin matrix . Different matrices, among which epoxy resin, methacrylate resin or a proprietary composite resin, are used by manufacturers. The fibers are responsible for the resistance against flexure, while the resin matrix provides resistance to compression, but also forms the surface the functional monomers contained in the adhesive cements will interact with .

To lute a fiber post into a prepared root canal, a composite cement is commonly used. The elastic modulus of the composite cement is in the same range of that of both the post and dentin . That is why a composite cement is preferentially used to lute fiber posts rather than a conventional glass-ionomer or zinc-phosphate cement. Composite cements were also shown to improve the retention of the post and the overall resistance of the root against fracture . Similar bonding protocols, as when a restorative composite is bonded to coronal dentin, are in general recommended for fiber-post bonding into the root canal. Hence, multi-step composite cements are applied following either an ‘etch-and-rinse’ or ‘self-etch’ approach . In particular, a ‘mild’ self-etch approach using specific functional monomers with high affinity to hydroxyapatite has been claimed to bond most durably to dentin . More recently, one-step self-adhesive composite cements, which do not require any kind of tooth-surface treatment, have been developed. Thanks to their ease-of-use and low technique-sensitivity, they appear especially suitable to bond fiber posts into these ‘difficult’ root canals. They are indeed deep and narrow and thus make any adhesive application protocol difficult to control .

Regarding surface pre-treatment of fiber posts, both chemical and micro-mechanical treatment protocols have been proposed to enhance the bond strength at the post–cement interface . Chemical post-surface pre-treatments that are today employed clinically, involve coating of the post with a silane primer, and/or with an adhesive resin, this potentially combined with beforehand acid-etching of the post surface. In particular, silanization of the post has quite often been investigated, but unfortunately has also often revealed contradictory results . The most common silane-coupling agent used in dentistry is a pre-hydrolyzed monofunctional γ-methacryloxypropyl-trimethoxysilane (γ-MPS) that is diluted in an ethanol-water solution to a pH between 4 and 5 . Its working mechanism is based on improved wetting along with chemical bridge formation between the glass phase of the post and the resin matrix of the adhesive resin or composite cement.

Most common micro-mechanical post-surface pre-treatment is sandblasting, which is intended to remove the top layer of resin, making the glass fibers reachable for chemical interaction. Naturally, sandblasting also roughens the surface, thereby significantly increasing the surface area and energy . The Cojet system (3M ESPE, Seefeld, Germany) uses silicate-coated alumina particles, by which the surface area/roughness is not only increased, but also a silicate layer is welded onto the post surface following a process being referred to as ‘tribo-chemical coating’. The formed surface can then be silane-treated, by which micro-mechanical and chemical bonding mechanisms are combined . Again controversial data have been reported when the Cojet system (3M ESPE) was applied to enhance the bond strength to fiber posts ; even serious doubt was raised as this surface-treatment has been considered by several authors as being very aggressive .

The aim of this study was to evaluate the effect of different factors on the push-out bond strength of glass-fiber posts luted in simulated (standard) root canals using different composite cements. The hypotheses tested were that (1) the type of fiber post, (2) the type of composite cement, (3) the type of post-surface pre-treatment, and (4) the depth within the root canal do not affect the push-out bond strength.

Materials and methods

Post cementation

Three types of glass-fiber root-canal posts with a different matrix, namely an epoxy resin (RelyX post, 3M ESPE), a proprietary composite resin (FRC-Plus post, Ivoclar-Vivadent, Schaan, Liechtenstein), and a methacrylate resin (GC post, GC, Tokyo, Japan), were tested. Only double-tapered posts with a cylindrical coronal part and a tapered apical part were used. All posts had a similar size with regard to the apical and coronal diameter, and a similar taper of the apical part. The posts were luted using three types of composite cements, namely an etch-and-rinse Bis-GMA-based (Variolink II, Ivoclar-Vivadent), a self-etch 10-MDP-based (Clearfil Esthetic Cement, Kuraray) and a self-adhesive (RelyX Unicem, 3M ESPE) cement. The main composition of the posts and composite cements is listed in Table 1 . Finally, all possible combinations of post and cement were tested when the post-surface was either not pre-treated, or when it was silanized, or sandblasted with Cojet (3M ESPE) ( Table 1 ).

Table 1
Composition and application procedure of the materials investigated.
Material (batch no.) Composition Adhesive strategy/curing mode Application procedure
Post system
RelyX post n. 2 (055920705) (3M ESPE, Seefeld, Germany) Glass fibers, epoxy resin, zirconia filler
FRC-Plus post n. 1 (K13203) (Ivoclar-Vivadent, Schaan, Liechtenstein) Glass fibers, dimethacrylates, ytterbium fluoride
GC Post n. 3 (0612071) (GC, Tokyo, Japan) Glass fibers, methacrylates
Composite cement
CLF
Clearfil ED Primer II (Kuraray, Tokyo, Japan) Primer A : HEMA, 10-MDP, water, accelerator.
Primer B : methacrylate monomers, water, initiator, accelerator
Self-etch Mix one drop each of Primers EDII-A and EDII-B. Apply the mixture to the root canal, leave it in place for 30 s. Remove excess primer with paper points. Dry with gentle air flow.
Clearfil Esthetic Cement (41111)
(Kuraray)
Paste A : Bis-GMA, TEGDMA, other methacrylate monomers, silanated glass filler, colloidal silica Dual-curing Mix paste A and paste B for 20 s; Apply the mixed paste and seat the post; Remove the excess and light-cure for 60 s.
Paste B : Bis-GMA, TEGDMA, other methacrylate monomers, silanated glass filler, silanated silica, colloidal silica, benzoylperoxide, di-camphorquinone, pigments
VAR
Excite DSC (Ivoclar-Vivadent) Excite DSC Adhesive : dimethacrylates, alcohol, phosphonic acid acrylate, HEMA, SiO 2 , initiator, stabilizer Etch-and-rinse Apply Total Etch (37% phosphoric acid) for 15 s. Rinse with water and dry with paper points. Apply Excite DSC and gently agitate for 10 s.
Variolink II (J22596) (Ivoclar-Vivadent) Paste A : Bis-GMA, urethane dimethacrylate, TEGDMA, inorganic filler, ytterbium trifluoride, initiator, stabilizer Dual-curing Mix base paste and catalyst in a 1:1 ratio; Apply the mixed paste and seat the post; Remove the excess and light-cure for 60 s.
Paste B : Bis-GMA, urethane dimethacrylate, TEGDMA, inorganic filler, ytterbium trifluoride, benzoylperoxide, initiator, stabilizer
UNI
RelyX Unicem (257929) (3M ESPE) Powder : glass powder, silica, calcium hydroxide, substitute pyrimidine, peroxy compound, pigment, initiator Self-adhesive/Dual-curing Activate the capsule and mix the cement into a mixer for 15 s (Rotomix, 3M ESPE); Apply the mixed paste and seat the post; Remove the excess and light-cure for 60 s.
Liquid : methacrylated phosphoric ester, dimethacrylate, stabilizer, initiator
Silane primer
Clearfil Ceramic Primer (003CA)
(Kuraray)
Ethanol (>80%), 3-trimethoxysilylpropyl methacrylate (<5%), 10-MDP Dispense the necessary amount into a well of the mixing dish immediately before application; Apply to the adherent surface with a disposable brush tip; Dry the adherent surface sufficiently by blowing mild oil-free air.
Monobond S (K10155) (Ivoclar-Vivadent) Silane methacrylate, phosphoric acid methacrylate, sulfide methacrylate, ethanol (50–52%) Apply with a brush and allow the material to react for 60 s; Disperse with a strong stream of air.
ESPE-SIL (290117)
(3M ESPE)
Ethanol (>90%), 3-trimethoxysilylpropyl methacrylate Dose on a clean grease-free dish; Soak the brush into the dish and apply to the adherent surface with a disposable brush just once; Allow the volatile silane solution to dry for 5 min.

Two-hundred and sixteen specimens were randomly distributed among 27 groups of 8 specimens each, this following the different combinations of post, cement, and post-surface pre-treatment ( Table 1 , Fig. 1 ). Instead of extracted teeth that biologically vary substantially, composite CAD-CAM blocks (Paradigm MZ-100, size ‘Small’, 3M ESPE) were used as artificial (and standard) root material, in which (nearly identical) post spaces were prepared. In this way, we could test solely the cement–post interface in a maximally standardized way, while excluding the cement–dentin interface. The post space was prepared in each block using a low-speed bur, as provided by the respective manufacturer of the post, up to a standard depth of 9 mm. After preparation, the post space was flushed with deionized water, and dried with successively ethanol and paper points. Posts were tried in, cleaned with ethanol and as mentioned above, either left as such (no post-surface pre-treatment), or silane-treated using the respective silane primer ( Table 1 ), or Cojet-blasted with silicate-coated alumina particles with a diameter of 30 μm at a pressure of 2.3 bar (2.3 × 10 5 Pa) and from a distance of 10 mm. The latter tribo-chemical coating was completed by application of a layer of ESPESil (3M ESPE). Finally, all posts were luted with one of the three different composite cements, this strictly following the respective manufacturer’s instructions ( Table 1 ). The adhesive supplied with Clearfil Esthetic Cement (Kuraray) was applied using extra-small micro-brushes (Microbrush, Grafton, WI, USA), and that supplied with Variolink II (Ivoclar-Vivadent) using specific endo-micro-brushes (Ivoclar-Vivadent). Both cements were injected into the post space using a disposable AccuDose Low Viscosity in an C-R syringe (Centrix, Shelton, Ct, USA). The self-adhesive composite RelyX Unicem (3M ESPE) was applied using its own capsule, to which a specific root-canal elongation tip was attached. Once the post was luted, the cement was polymerized from the top of the post with an Optilux 500 light-curing device (Demetron/Kerr, Danbury, CT, USA) with a light output not less than 550 mW/cm 2 for 60 s. All specimens were prepared using 4.5× magnification loupes (Carl Zeiss, Jena, Germany).

Fig. 1
Study set-up illustrating specimen preparation and the actual push-out bond-strength testing. Posts were cemented into artificial root canals that were before prepared with calibrated burs into composite CAD-CAM blocks. The specimens were then sectioned perpendicularly to the long axis of the root using a high-speed diamond blade (Accutom-50, Struers, Denmark). Three sections of 2 mm thickness and two sections of 1 mm thickness in between were cut in order to obtain 3 sections per tooth: a coronal, middle and apical section. All specimens were then subjected to a push-out bond-strength test using an InstronMicroTester. Because of the taper of the posts, stainless steel pins of different diameters were used to apply the load on the post. Care was taken to assure a perfectly positioned pin in the center of the post surface.

Preparation of specimens for push-out bond-strength testing

One week after luting, specimens were cut perpendicularly to the long axis of the post at three levels, starting at 0.2 mm below the surface of the composite CAD-CAM block and subsequently at a depth of 2.5 mm to obtain a ‘coronal’ disk, next at 3.3-mm and 5.5-mm depth to obtain a ‘middle’ disk, and finally at 6.3-mm and 8.8-mm depth to obtain an ‘apical’ disk; all disks have a 2-mm thickness (in total, 648 sections). The actual thickness of each section was measured with a digital caliper (Mitutoyo, Aurora, Illinois, USA) at an accuracy of 0.01 mm. The diameter of the cross-sectioned post at the top and bottom side of each section was measured using a custom-adapted measuring stereomicroscope (Wild M5A, Heerbrugg, Switzerland). In addition, all sections were likewise checked for potential artifacts caused by the cutting process. No artifacts were observed.

Each specimen was next subjected to a push-out bond-strength test using a universal material tester (5848 MicroTester, Instron, Norwood, MA, USA). The test was performed at a cross-head speed of 0.5 mm/min with the load applied in the apical-coronal direction until the post was dislodged. The maximum load at failure was recorded in Newton (N) and converted in MPa by dividing the applied load by A, the bonded area. Because of the tapered post shape, the bonded area was calculated using the formula:

<SPAN role=presentation tabIndex=0 id=MathJax-Element-1-Frame class=MathJax style="POSITION: relative" data-mathml='A=πr+Rh2+R−r2′>A=π(r+R)h2+(Rr)2A=πr+Rh2+R−r2
A = π r + R h 2 + R − r 2

with r and R being respectively the smallest and largest diameter of the cross-sectioned tapered post, and h being the thickness of the section.

Failure analysis by light-microscopy (LM) and scanning electron microscopy (SEM)

After testing, the mode of failure was determined for all sections using the stereomicroscope (Wild M5A, Heerbrugg) at a magnification of 50×. In addition, the failure mode of selected sections of each group was also analyzed using field-emission-gun SEM (Feg-SEM; Philips XL-30, Eindhoven, Netherlands) at higher magnification. Therefore, for each experimental group, two sections belonging to each of the three canal levels were fixed in 2.5% gluteraldehyde in 0.1 M sodium cacodylate buffer for at least 36 h. Then, they were rinsed with 0.2 M sodium cacodylate buffer and distilled water, after which they were air-dried in a desiccator. They were mounted on aluminum SEM stubs with carbon cement, gold-coated and eventually examined by Feg-SEM (Philips XL-30).

SEM characterization of post-surface pre-treatment

For each post system, three posts (9 in total) were either kept untreated (no post-surface pre-treatment, solely cleaned with ethanol), or were silane-treated using the respective silane primer, or Cojet-blasted following the procedure described above in detail ( Table 1 ). They were mounted on an aluminum SEM-stub, gold-coated and imaged using Feg-SEM (Philips XL-30) to morphologically characterize the micro-structure of the post surface.

Statistical analysis

Statistical analysis was performed using the software package Statistica (StatSoft 7.1, Tulsa, OK, USA). Analysis of variance (repeated-ANOVA), with the different levels as within-subject experimental groups, and Tukey-HSD for post-hoc comparison were used in order to find statistically significant differences between the push-out bond strength data ( p < 0.05).

Materials and methods

Post cementation

Three types of glass-fiber root-canal posts with a different matrix, namely an epoxy resin (RelyX post, 3M ESPE), a proprietary composite resin (FRC-Plus post, Ivoclar-Vivadent, Schaan, Liechtenstein), and a methacrylate resin (GC post, GC, Tokyo, Japan), were tested. Only double-tapered posts with a cylindrical coronal part and a tapered apical part were used. All posts had a similar size with regard to the apical and coronal diameter, and a similar taper of the apical part. The posts were luted using three types of composite cements, namely an etch-and-rinse Bis-GMA-based (Variolink II, Ivoclar-Vivadent), a self-etch 10-MDP-based (Clearfil Esthetic Cement, Kuraray) and a self-adhesive (RelyX Unicem, 3M ESPE) cement. The main composition of the posts and composite cements is listed in Table 1 . Finally, all possible combinations of post and cement were tested when the post-surface was either not pre-treated, or when it was silanized, or sandblasted with Cojet (3M ESPE) ( Table 1 ).

Nov 28, 2017 | Posted by in Dental Materials | Comments Off on Factors affecting the cement–post interface
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