Clinic meets Research, Part 1 – Adhesion
An article by Giuseppe Marchetti & Giovanna Orsini.
Modern dentistry means adhesive dentistry. We today use adhesive procedures in all our clinical works. So let’s try to understand which procedure we have to use and in which case we have to chose one or another. Here’s some Q&A!
1. Curing time seems to have a great influence on improving bond strength of adhesive systems. But what about the curing “power” (low, soft, high)? Could it be of any influence (good or bad) on the seal’s quality of our adhesive systems, regardless of the brand or system generation or substrate? What would be the best choice to minimize the shrinkage effect (if there’s one)?
Curing is of utmost importance, lamps must have a power output of 1000 mW per cm2 or more, the ideal is between 1,200-1,400, and it is important as well to have a wavelength span capable of polymerizing not only camphoroquinone, but as well leucerin and its possible derivates. This is one of the reasons why modern LED lamps have tri-wave or tetra-wave in light emission. The lack of bonding, composite or cement conversion will surely cause marginal leakage issues and post-operative sensitivity. To compensate polymerization contraction factor, we must use incremental techniques in direct systems, or use an indirect technique, where indicated, to minimize risk for failure.
2. What about the influence of using systematically (or not) sandblasting for direct/indirect adhesive restorations on the effective improvement of the adhesive systems’ seal?
Sandblasting of indirect composite restorations prior to the cementation has been proved to be the best system to increase the bond strength. In direct techniques, the use of 50 micron aluminum oxide or other specific abrasive systems has no sense at all in dental substrate, except if an old restoration or an immediate dentinal sealing is candidate to be adhesively treated. It is important to highlight that aluminum oxide should only be used on old composite or non adhesive materials, avoiding, as much as possible it use on enamel and dentin.
3. Will adhesion be compromised in the long run, when multiple layers of different materials (i.e. multiple interfaces) are used on a same tooth restoration?
There is controversy about this matter; some authors state that there could be problems where there are different materials (such as fiber posts, composite, ceramics), while others consider it a mechanical advantage (as long as they are all adhesive materials). Regardless of the number of materials, the fundamental bond strength occurs at the level of the material that is in contact with the adhesive layer, if bonding was perfectly performed the use of multiple materials should not influence adhesion negatively, and cohesive failure between adhesive materials is very unlikely to happen.
The best possible adhesive interface is tooth-composite one, with enamel and healthy, non-sclerotic, dentin. Every other situation, where there is the presence of sclerotic dentin or an endodontic treated canal, might not be as ideal. Bonding with the correct steps should provide acceptable bonding strength.
4. Can luting translucent veneers with a 3 step etch&rinse technique, and without curing the bonding layer before fitting of the veneer, jeopardize the hybrid layer integrity by contracting the adhesive layer from the substrate?
No, if the bonding steps are followed rigorously, the risk is minimal, especially if the fit of the veneers is good the thickness of the cement will be minimal and the contraction stress will not be influential.
5. Which bonding system is more indicated for restoration of deep cavities with mostly dentin substrate?
2-step etch & dry systems are proven o be optimal materials to manage post-operative sensitivity, so in every situation where a big amount of dentin is exposed, it can be a good alternative to this issue. Probably these systems are less dependent on the operator skills than etch and rinse.
6. Can increasing the primer application time, in etch & rinse protocols, also increase the stability of the hybrid layer and reduce postoperative sensitivity, and at the same time reduce the skill-bound factors?
No, applying the primer according to the manufacturer instructions (between 10-20 seconds) can be enough. Instead, increasing curing time it does augment the stability and durability of the hybrid layer.
7. Does applying chlorhexidine before adhesive procedures improve adhesion? Yes or No? How?
Yes, chlorhexidine can be used before adhesive procedures as a disinfectant for about 40-60 seconds and then rinsed. It can be use after etching to inhibit metalloproteinases and then only dried. It must not be mixed with any vehicle (alcohol, sweeteners, excipients, preservatives) it should be pure, in a 2-3% concentration maximum. However we stress out that there are etching and bonding systems that include disinfectants in their compositions and have optimal results in this sense.
8. How important is it to check the power output of my curing lamps, and how to do it? What are the minimum limits that we must consider not acceptable for polymerization?
About the power output, there is a thorough answer in question one. Instead, to control the lamp efficacy we need a radiometer, it can be part of the lamp or it can be separated. The ideal thing to do is to replace the built-in bulb or the LED inside the lamp when it does not reach the desired numbers.
9. IDS “Immediate Dentinal Sealing”: when to perform it? How to inhibit the adhesive? Does it makes sense to perform it over conventional prosthetic prepared teeth?
The immediate dentinal tubule sealing is done always, most of all in indirect partial bonded restorations in posteriors. There is not one single reason not to do a build-up in posteriors, because it reduces post-op sensitivity, reduces unnecessary space forcing the restoration to have an ideal thickness and thus letting us using conventional composites during cementation instead of dual-cement. Every time that we can isolate we can perform it. The adhesive or the flowable composites that are used for the build-up is deactivated with glycerin after polymerization or by mild polishing. In conventional prosthetic preparations, the IDS is as well indicated, there’s no reason no to do it.
It is mandatory to highlight that the clinician must be aware that the use of the rubber dam does not always guarantee a perfect bonding and viceversa and must be thoroughly evaluated.
10. Why is enamel absence in bonding an issue? is it because bonding to dentin is unstable? And how can indirect help in that?
The lack of enamel is a problem because the adhesion to enamel is micro-mechanical, while to dentin it’s chemical and it is more unstable and conditioned by several factors, among which the quality of the dentin, its degree of wetness, etc… Indirect techniques are ideal in those situations where, for whatever reason, the implementation of a direct restoration can be particularly difficult to perform, or where the need to counteract the shrinkage is imperative.
11. How do you manage deep proximal boxes in class II cavities, and apical class V when there is no enamel but just cementum? What is the bonding procedure in such case? And what the precautions to take?
In those cavities, where at the cervical level, there is a little of enamel or no enamel at all, indirect restorations can be done. That allows us to manage the polymerization shrinkage in a critical area, where to obtain and to maintain a perfect seal can be a problem. The bonding procedures are the same as in every situation, paying attention to the steps of the etch and rinse or etch and dry techniques.
The stability of a dental adhesive depends on the materials, dental substrates, patient-related factors and clinical steps. The limited durability of resin-dentin bonds severely compromises the lifetime of aesthetic restorations. Bond degradation occurs via hydrolysis of sub-optimally polymerized hydrophilic resin components and degradation of water-rich, resin-sparse collagen matrices by matrix metalloproteinases (MMPs) and cysteine cathepsins (Limitations in Bonding to Dentin and Experimental Strategies to Prevent Bond Degradation – Y. Liu, L. Tjäderhane, L. Breschi, A. Mazzoni, N. Li, J. Mao,D.H. Pashley, and F.R. Tay – J Dent Res 90(8):953-968, 2011). Other factors that can influence the result are the skills of the operator, isolation, etching time and adhesive application, type of light source and type of finishing can influence the final result. Different adhesive systems need different protocols. Dental adhesives can be classified according to: a) their action – either removing the smear layer (etch-and-rinse technique) or maintaining it as the substrate for bonding (etch-and-dry or self-etch technique) and b) the number of clinical steps.
Now we can summarise adhesive systems thusly:
1) Etch and rinse, 3-step technique
In the etch and rinse three steps technique the protocol leads to etch for 15/30 seconds the enamel, 15 seconds the dentin, and an extensive rinse for the same time. Etch and rinse adhesive systems are the oldest of the multi-generation evolution of resin bonding systems. In the 3-step version, they involve acid-etching, priming and application of a separate adhesive. Each step can accomplish multiple goals. Acid-etching, using 32–37% phosphoric acid (pH 0.1–0.4) not only simultaneously etches enamel and dentin, but the low pH kills many residual bacteria. Results: Some etchants include anti-microbial compounds such as benzalkonium chloride that also inhibits matrix metalloproteinases (MMPs) in dentin. Etch-and-rinse adhesives produce higher resin–dentin bonds that are more durable than most 1 and 2-step adhesives. Incorporation of protease inhibitors in etchants and/or cross-linking agents in primers may increase the durability of resin–dentin bonds.( State of the art etch-and-rinse adhesives David H. Pashley, Franklin R. Tay, Lorenzo Breschi, Leo Tjäderhane, Ricardo M. Carvalho, Marcela Carrilho, Arzu Tezvergil-Mutluay – Dental materials 27 (2011) 1–16)
2) Etch and rinse 2-step technique
The two steps etch-and-rinse-technique despite the one-bottle concept is extremely user friendly, acetone-based adhesives may lose their efficacy with constant usage (6).
3) Etch and dry 2-step technique
The self-etch technique can include either two or one-step systems, if the etching/primer agent is separated from the adhesive or combined with it to have a single application. With the multi-bottle system we dont have to rinse and we can apply it fast. It is important to respect the application time of the acidic primer and the bonding before polymerization; it may not work on sclerotic dentin and it may request the etching procedures of the enamel to avoid enamel micro-leakage. In contrast to the control three-step etch & rinse adhesive, the bond strength to dentin of both one-step and two-step self-etch adhesives decreased after six-month water storage. These failures just under the hybrid layer may be attributed to insufficient encapsulation of surface smear. (Filler Debonding & Subhybridlayer Failures in Self-etch Adhesives – K.L. Van Landuyt*, J. De Munck, A. Mine, M.V. Cardoso, M. Peumans, and B. Van Meerbeek – J Dent Res 89(10):1045-1050, 2010)
4) Etch and dry 1-step technique
The all-in-one application may be a very quick alternative, but it has a very low bond strength value, needs multiple coatings to bond effectively to dentin and has not proven their efficacy over time 5. One-step adhesives have been especially documented with problems, such as insufficient polymerization, water-uptake and subsequent plasticization, water- and enzyme-induced nanoleakage, and/or the presence of voids due to phase-separation or osmosis. It was hypothesized that these shortcomings may weaken the adhesive layer and, as such, may jeopardize long-term bonding(Filler Debonding & Subhybridlayer Failures in Self-etch Adhesives – K.L. Van Landuyt*, J. De Munck, A. Mine, M.V. Cardoso, M. Peumans, and B. Van Meerbeek – J Dent Res 89(10):1045-1050, 2010) So causes of failure for the etch-and-rinse techniques may be: over-etching, no proper sequence, multi-layering. In the self-etch technique (especially in all-in-one application) failures can be due to: stability over time, phase separation, low degree of conversion, continuous etching.
TARGETS AND CLINICAL CONSIDERATIONS
The target of a bonded restoration is to reach a close adaptation of the restorative material with the dental substrate (enamel, dentin) and the different restorative materials (composite resins and ceramics). While adhesion to enamel is stable over time, adhesion to dentin is more unstable, because of its heterogenous characteristics (10). The wetness of dentin surfaces, the presence of pulpal pressure, and the thickness of dentin are extremely important variables during bonding procedures, especially when testing bond strength of adhesive materials in vitro with the intention of simulating in vivo conditions. While enamel is predominantly mineral, dentin contains a significant amount of water and organic material, mainly type I collagen (11, 12). Several other substrate-related variables may affect the clinical outcome of bonded restorations.
Bonding to caries-affected dentin is stopped by its lower hardness, presence of mineral deposits in the tubules and disorganized collagen (13). Hybrid layers in caries-affected dentin are usually thicker but more porous than those in sound dentin (14). Non-carious cervical areas contain hypermineralized dentin and denatured collagen, which is not the ideal combination for a bonding substrate. An increase in number of tubules with depth and, consequently, increase in dentin wetness, make bonding to deeper dentin more difficult than to superficial dentin. Dentin permeability increases with cavity depth and it can be also increased by acids that remove the smear layer. Etch-and-rinse adhesives result in higher micro-permeability compared to self-etch ones (16). The application of acidic agents open the pathway for the diffusion of monomers into the collagen network, it also facilitates the outward seepage of tubular fluid from the pulp to the dentin surface, deteriorating the bonding for some of the current adhesives. Further factors that might influence with the bonding efficacy are: the orientation of the dentinal tubules and other regional variables; the instruments used to create the smear layer. For instance, dentin surfaces ground with diamond burs tended to present compact smear layers of which denseness may compromise bonding action, especially of self-etch systems (17). Special attention should be directed to water-based agents, mainly the all-in-one agents. A multiple layer application under a continuous brushing technique has also been claimed to increase the bond strength of these materials (14,23).
Adhesive systems and restorative substrates
We know that adhesion to composite resins is stable over time but adhesion on other restorative materials is more unstable, for their different characteristics. Bonding to traditional silica-based ceramics, generally employing both mechanical and adhesive retentions, has been well researched, and bond strengths are predictable. A strong resin bond relies on chemical adhesion between the cement and ceramic (by way of silane chemistry), and on micromechanical interlocking created by surface roughening. Current roughening techniques are: (1) grinding, (2) abrasion with diamond (or other) rotary instruments, (3) air abrasion with alumina (or other) particles, (4) acid etching (5% hydrofluoric), and (5) a combination of any of these techniques. Unfortunately, the composition and physical properties of certain materials like Zirconia differ from conventional silica-based materials like porcelain and requires very aggressive mechanical abrasion methods to increase surface roughness, possibly creating strength reducing surface flaws (20). Therefore, in order to achieve acceptable cementation in a wide range of clinical applications, alternate attachment methods, ideally utilizing chemical adhesion in addition to mechanical retention, are required.
Different restorative procedures may need different types of bonding systems. Literature reports generally differentiated adhesive approaches for direct versus indirect restorations Luting indirect restorations: if margins are in enamel, the etch-and rinse adhesives and the etch-and-dry systems with pre-etching treatment performed well. When margin are in dentin, the total-etch technique with a 3-steps etch and-rinse is still considered the gold standard (22). The final film thickness of a resin adhesive and a resin cement could be affected by previous polymerization of the adhesive systems on dentin surfaces. Instructions regarding polymerization of the adhesive layer must be followed when adhesive systems are used in combination with dual polymerized resin based cements (23).
Clinical interpretations and applications
In case of inlays, onlays and overlays, the following uses can be suggested:
1) a 2-step self etch adhesive, to make the build-up and to treat the dentin, on the day of inlay preparation;
2) a total-etch adhesive, on the day of the indirect restorations cementation.
In case of luting ceramic veneers with margins on enamel it can be suggested the total-etch technique. Translucency and opacity of the veneers have to be also taken into account. For instance, when the veneer is opaque, it is advisable to treat the prepared tooth by applying and light-curing a simplified adhesive. On the other hand, when the veneer is translucent the adhesive can be also left uncured and a 3-step system can be used. Low-viscosity and extremely thin bonding agents are indicated for young dentin, V and IV class direct restorations, and porcelain veneers and inlays (because they do not compromise the fit). High-viscosity (high filler content) bonding systems can be effective for replacing old restorations with sclerotic dentin or in case of slow caries-affected regions.
TIPS SYSTEM BY SYSTEM
1) Etch and rinse three steps technique.
Respect etching time (15 seconds for dentin and 30 seconds for enamel). Rinse it for the same time. Do not over-dry dentine. Apply primer for 10 seconds and dry it. Apply adhesive for 20 seconds and gently dry it.
2) Etch and rinse two steps technique
Respect etching time (15 seconds for dentine and 30 seconds for enamel). Rinse it for the same time. Do not over-dry dentine. Apply primer&bonding multilayering. Dry to evaporate solvents.
3) Etch and dry two step technique
Respect the application time of the acidic primer and the bonding before polymerization. Requires the etching procedures of the enamel to avoid enamel micro-leakage 5.
4) Etch and dry one step technique
Has a very low bond strength value, needs multiple coatings to bond effectively to dentin and has not proven their efficacy over time
In the aesthetic era, managing different adhesive systems could be difficult, but it is the way to allow good results which can be stable in time. So, for a correct application of these systems, the tips are:
1) Extended polymerization time. Cadenaro et al. 2005-2008
2) Improved impregnation i.e. prolonged impregnation time multilayering. Pashley et al. 2004.
3) Use of an hydrofobic coating to reduce water sorption and stabilizes hybrid layer. De Munck et al.2005, Van Landuyt et al. 2007
4) Improve solvent evaporation by air-blowing. De Munck et al. 2005. Van Landuyt et al. 2007
5) Use of inhibitors of metallo-proteinases (MMPs), which are the enzymes responsible of the bonding degradation. Breschi et al. 2008, Hebling et al. 2005, Carrilho et al. 2007
6)Ethanol wet-bonding with hydrophobic resins to completely replace water from the extrafibrillar and intrafibrillar collagen compartments and immobilize the collagenolytic enzymes. Sauro et al. 2010.