Effect of different surface treatments on prefabricated fiber posts: a literature review.
Dr. Najib Abou Hamra
Dr. Danielle Hakim
Dr. Maha Ghotmi
This literature review summarizes the recent research on fiber posts regarding surface treatment to improve their adhesive properties. Actual methods to treat the surface relied on chemical and/or micro-mechanical techniques. The majority of data was based on in vitro studies that investigated different chairside post surface treatments for enhancing and/or modifying the surface available for bonding. The tests used were microtensile and push out bond strength in combination with SEM analysis. Few studies included aging procedures and 3 point bending test to evaluate mechanical properties of the treated posts. According to the reviewed in vitro studies, surface conditioning improves fiber posts bonding properties and leads to a good bond strength between the post and the restorative materials. Results are promising especially regarding some new acidic treatments. Long term clinical studies are still needed before recommending their use.
Fiber posts are currently widely used in restoration of endodontically treated teeth.Many studies have investigated factors that may affect the retention of a post, such as design, length, diameter and surface treatments.
The similar elastic modulus of fiber posts, resin cement, resin composites and dentin is considered to be advantageous for improving their performance. A proper bonding at the dentin/ cement, post/ cement interfaces is needed for dissipation of stresses generated by occlusal loads
In vitro and in vivo studies indicates that failure of fiber post and core restorations often occurs because of debonding between the fiber post- resin and/or resin-root canal dentin interfaces as a result of inadequate bond strength. Perdigao et al. estimated that 60% of fiber posts failures occurred between post and resin cement.
Retention of the composite core to prefabricated posts is affected by various factors, including surface treatment of the post, design of the post head and the composite core material.
Surface treatments are commonly used for enhancing the adhesive properties of the post, by allowing chemical and micromechanical retention between the fiber post and the composite core in contact.27
These procedures fall into 3 categories
1- Treatments to optimize chemical bond between the post and the cement or the composite (coating with priming solutions)
2- Treatments that roughen the surface (sandblasting and etching)
3- Combine micromechanical and chemical components either by using the two above or a unique system (Co-Jet).
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 and 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. This resin matrix also forms the surface to which the functional monomers contained in the adhesive cements will interact with.
Most of the matrix components of fiber reinforced composite (FRC) posts are epoxy resin which has a high degree of conversion and has also highly cross linked structures. In fact, no functional groups of the fiber post would react with the methacrylate group, which is the major component of dental composite resin. This results in the absence of chemical bonding between the 2 substrates. On the other hand, methacrylate resin matrix of some FRC posts exhibit a good affinity in terms of bonding between the post matrix and the methacrylate based adhesive and resin cements, this affinity will result in good chemical bonding between the fiber post and cement or core build up.
This discrepancy in adhesive behavior opened the door for a large amount of research to study the optimal pretreatment of each kind of fiber post.
This review will summarize the major surface treatments mentioned, their benefits in term of improving bonding to resin composites, their limitations, and their potential clinical indications.
1- Chemical Bonding to Fiber Posts
Silane coupling agents are hybrid organic-inorganic compounds that can mediate adhesion between inorganic and organic matrices through an intrinsic dual reactivity.
Treating the post surface with a silane-coupling agent may be advisable for enhancing adhesion. However opinion differs about the efficiency of post silanization.
Perdigao et al. reported that silane did not improve the bond strength of several posts cemented with the respective cements at any level of the root. This outcome corresponds to other findings.The ineffectiveness of silanization is more likely attributed to a rather weaker or even absent chemical union between methacrylate based resin composites and the highly cross- linked epoxy resin, main constituent of fiber posts matrix.It is also probable that incomplete evaporation of the solvent of the silane may compromise the coupling.
On the contrary other studies reported improved bond strength between silanized fiber posts and flowable composites used for core materials, Aksornmuang et al. confirmed the benefit of silane application for enhancing microtensile bond strength of a dual cure resin core material to fiber posts. Soares et al. assessed microtensile bond strength of glass fiber posts treated with silane and airborne particle abrasion and concluded that treatment with silane only was sufficient as surface treatment for adhesive bonding. These results rely on silanes capability to increase surface wettability, creating a chemical bridge with OH-covered substrates, such as glass (glass fibers on the post surface). However the interfacial strength is still relatively low when compared to the values achieved with dental substrates.
Furthermore, silane and consecutive application of bonding have been investigated: Ferrari et al.10 reported no significant improvement in bond strength by separately applying silane and different adhesives on methacrylate based quartz fiber posts. The formation of a thick multi phase coupling layer with possible errors during each separate phase of application possibly explain the outcomes. On the contrary satisfactory results have been reported on epoxy resin based fiber posts2 and that by using the combination of a silane/ primer solution and a bonding agent. These combined silane / bonding agents allow the formation of siloxane bonds and the polymerization of functional groups in the resin in the same time. These separate silanes seem to perform more efficiently than completely prehydrolysed solutions.In addition the use of a separate hydrophobic resin coating after silane/primer solution created better seal of post surface than the one step self etch adhesive.
2- Micromechanical Bonding to fiber posts
Airborne Particle Abrasion
Airborne particle abrasion with Al2O3 is the most studied. It increases the surface area and enhances mechanical interlocking between the cement and the roughened surface of a post. Several protocols were described. They all seem to be effective in term of enhancing the bond strength and that by partial removal of the resin matrix, which increases the number of exposed glass fibers and consequently the surface area available for reaction. Of importance is the fact that even for a mild abrasive protocol (50[eth]-m aluminum oxide, 2 bars, 10 seconds, 10 mm distance) the SEM analysis of post surface showed a disruption of the interface between the matrix and the fibers with fracture of fibers, even though the authors stated that no influence was reported on the mechanical properties of the posts.27 D'Arcangelo et al. have also reported that flexural strength and flexural modulus of fiber posts were not influenced by pretreatment with silane, airborne particle abrasion and hydrofluoric acid. Lately, more promising results were achieved by Kern and Asmussen with the use of a specific protocol: 50[eth]-m aluminum oxide, 2.5 bars, 5 seconds, 30 mm distance. This regimen did not produce any alteration in the shape of the post and resulted in increased bond strength with resin cement.
Some studies preconize airborne particle abrasion as treatment of choice without additional treatment, and some others (Magni et al.24) show better bond strength with silane application.
To optimize the bonding of resin cements to FRC posts, etching with phosphoric acid and hydrofluoric acid has been proposed. Different concentrations and etching times were used in studies to investigate the effect of surface treatment of posts by conditioning with these 2 acids, since clear information regarding the most appropriate etching conditions is still lacking. When H3PO4 was used at 35% for 3 min, it showed better bond strength to epoxy based fiber posts without modifications in the post surface in SEM examination.Albashaireh et al. stated that treating post surface with 36% phosphoric acid for 15 sec before cementation produced no significant improvement in post retention. His results were in agreement with other findings26, 28 ; this may be due to the removal of small amount of the uppermost layer of epoxy resin, thereby leading to weak micro-mechanical retention. Whereas conditioning the post surface with hydrofluoric acid seems to be very aggressive and attacks both fibers and epoxy resin.12 These SEM observations were confirmed by Vano and others: despite the improvement in post-to-composite bond strength, a remarkable surface alteration, ranging from micro cracks to longitudinal fractures of fiber layer, was detected. As a consequence, it is not possible to suggest general guidelines for using hydrofluoric acid in the surface etching of aesthetic fiber posts.
Treatment with Hydrogen Peroxide (H2O2)
Sandblasting and hydrofluoric acid can sometimes damage the glass fibers and affect the integrity of the posts. Therefore substances that selectively dissolve the epoxy matrix without interfering with the fibers have been studied: Potassium permanganate, methylene chloride, and hydrogen peroxide (H2O2) may effectively remove the epoxy resin and expose the fibers which are then available to be silanated.
Hydrogen peroxide is frequently used in immunological electron microscopy to partially dissolve the resin surface of epoxy resin-embedded tissue section, and expose tissue epitopes for immunolabeling enhancement.28 The etching effect of hydrogen peroxide depends on its capacity to partially dissolve the resin matrix, breaking epoxy resin bonds through a mechanism of substrate oxidation. A similar hydrogen peroxide etching procedure is employed to improve the micromechanical retention between the epoxy resin matrix of fiber posts and methacrylate-based resin composites.30 H2O2 at concentrations of 10% and 24% for 10-20 min effectively removes the surface layer of the epoxy resin, but since these applications periods were clinically impractical, further studies were made to investigate more applicable protocols.
Menezes et al. reported that both 24% and 50% hydrogen peroxide exposures increased the bond strength of resin to the post irrespective of the application time. Both concentrations were able to partially dissolve the epoxy resin and expose the glass fibers after a 1 minute exposure. Despite the slight etching obtained by 24% H2O2 after 1 minute exposure, it was sufficient to produce bond strength similar to that obtained with concentrations or longer application times.
However the results of Elsaka9 were in conflict with the previous study as higher concentrations of H2O2 30% enhanced the bond strength of the different post/ core systems.
The differences in the concentrations of H2O2 used, the testing methods, and the test tested materials could be the causes of such a discrepancy in the results. It is important to note that all treatments with H2O2 exposed the fibers without damaging them. Dissolution of the epoxy resin probably relies on an electrophilic attack of the H2O2 to the cured secondary amine. Thus, the spaces created between the fibers provide conditions for the micromechanical interlocking of the resin adhesive with the post.
Etching with 24% H2O2 for 1 minute represents an easy and clinically feasible method to enhance interfacial strength between fiber posts and resin core build up.
Alkaline Potassium Permanganate
It is a chemical treatment usually applied in an industrial process for conditioning epoxy resin surfaces for metal plating of printed circuit boards. This etching procedure consists in 3 complicated steps (swelling, etching and neutralizing). Commonly defined as desmearing, it is a process to remove the smeared epoxy resin byproducts, providing superior topography for increased adhesion. Monticelli et al.21 found that the application of 10% potassium permanganate for 10 minutes had a significant influence on microtensile interfacial bond strength values and that these results were superior to those found with 10% H2O2 for 20 minutes. But given the complicated technique of application, use of H2O2 or CH2CL2 was thought to be less aggressive and time consuming. Etching with potassium permanganate will remove the superficial layer of epoxy resin, exposing more fibers to react with silane. An improved interfacial strength will than occur because of increased chemical union between silanized glass fibers and methacrylate based core material or luting cements.28 Besides exposing the quartz fibers, it may also activate the latter by improving their hydrophilicity.
Treatment with Methylene Chloride Methylene chloride (CH2CL2) had been proposed for use to improve the adhesion between acrylic resin denture base materials and acrylic resin repair materials by changing the chemical features and surface morphology of denture base resins increasing their repair strength. When first studied, CH2CL2 was applied to epoxy resin based fiber post for 5 seconds in order to improve the adhesion between fiber post and composite resin; however the results showed that this treatment was not effective. Recently, surface treatment of methacrylate resin--based glass fiber posts with CH2CL2 for 5 or 10 min enhanced the adhesion between fiber posts and resin core materials. It appears to be a simple, effective and inexpensive method that might improve the clinical performance of fiber posts.
3- Micromechanical and Chemical Bonding to fiber posts
Sandblasting and Silane
When sandblasting a fiber post, more fibers are exposed and thus able of chemical bonding with the alcoxy groups of the silane molecules, thus the advantage of using silane after sandblasting. However data is controversial on this issue, Choi et al.6 and others who compared sandblasting with sandblasting and silane found no improvement in bond strength with silane added. These results are most likely because of partial removal of the resin matrix as a result of air particle abrasion, the bond strength achieved between resin based cements and glass fibers is most likely mechanical
This treatment uses the 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 making it chemically more reactive to the resin via coupling agent. The most used is a chairside system called Co-Jet (Co-Jet, 3M ESPE, St Paul, MN, USA). This active sandblasting improved bond strength of FRC posts especially with methacrylate resin,32 whereas less effectively improved bond strength is observed with epoxy resin based posts. This may be explained by an inferior compatibility between cements and this kind of posts.
However, a prolonged sandblasting time can reduce the post diameter and cause misfit resulting in lower retentive bond strength. The sensitivity of both matrix and fibers to these treatments impose the risk of damage of post structure. More recent studies confirmed that Cojet (3M ESPE) sandblasting under well controlled conditions may improve bond strength without visible changes and damages to the post surface.
When comparing the retention of endodontic dowels following treatment with airborne particle abrasion and Cojet, Kelsy et al. it was concluded that no difference was found in the retention of the fiber post to the resin cement.
Acidic treatments and Silane
Most of the studies reviewed tested the treatments with different etching procedures with silane and they reported that surface pretreatment increased the responsiveness to silanization, achieving better results for the tested chemicals. Pretreatments with hydrofluoric acid, phosphoric acid and hydrogen peroxide and silane were compared to silane application and were found to improve bond strength, but there is no clear data comparing these pretreatments with and without silane in order to preconize it as additional step before application of resin cements. Further studies are needed to clarify this subject.
Plasma is a partially or wholly ionized gas with a roughly equal number of positively and negatively charged particles. Plasma consists of energetic species that bombard the surfaces they contact transferring their energy. The transferred energy has various effects resulting in acid-base interactions and covalent linking, an improvement in bond strength is then achieved. Ar, N2 and H2+N2 plasma treatments seem to improve the shear bond strength of both types of fiber posts. Hence, further studies are needed on the parameters of plasma treatment, hydrothermal stability of the interface, and the aging effects of plasma-treated posts.
It has been used to improve adhesion property of coatings as well as wettability and printability of polymers, due to its capability to change the morphology and chemical properties of the polymer surface. UV radiations are generated in ultraviolet sterilizing ovens. When used from a distance of 1 cm for 3 min, they seem to activate the epoxy resin matrix of the surface of fiber post inducing chemical binding with resin cement without destroying surface structure of fiber posts. UV irradiation demonstrated stronger effect than silane and is much less destructive to the surface of the post than the other chemical and mechanical treatments.
This technique is a less expensive, more applicable way to improve interfacial adhesion without affecting the integrity of FRC posts. Further studies have to focus on the optimal intensity for clinical application, and the aging effect on bond strength.
Surface post treatments represent an important factor for improving bonding of resin cements on core material of FRC posts especially when dealing with epoxy resin-based fiber posts. Chairside post pretreatments are still considered a technique-sensitive step. However treatments with methylene chloride, hydrogen peroxide seem promising because of their simple, not expensive application resulting in the improvement of bond strength to fiber posts without damaging the post structure. Further studies are still necessary:
- to clarify the potential effect of dissolving the epoxy matrix on the mechanical properties of the post in term of affecting the fatigue resistance and the flexural strength,
- to evaluate whether the positive performance of the treated posts is similar in vivo as it is in vitro
- to evaluate the durability of fiber post/core bond strength subjected to the proposed chemical treatments to give reliable recommendations for dental practitioners.
The possibility of an industrial conditioning of fiber posts may be a good alternative in order to simplify clinical procedures.
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