Clinic meets Research Vol.1 Adhesion

Shadeguides - Direct anteriors - Direct posteriors
28 Nov 2013

Modern dentistry means to glue. Nowadays we use adhesion procedures in all our clinical works. So let’s understand which procedure we have to use and in which case we have to chose one or another.

A blog by Giuseppe Marchetti and Giovanna Orsini

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2)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 utilization6.

3)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 don’t 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 microleakage 5. 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)

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4)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.

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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 flaws20. 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.

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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.

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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.

Conclusions:

In this 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 appliance 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 intrafi- brillar collagen compartments and immobilize the collagenolytic enzymes: Sauro et al. 2010

TIPS SYSTEM BY SYSTEM:
1) Etch and rinse three steps technique. Respect etching time( 15 seconds for dentine and 30 seconds for enamel). Rinse it for the same time. Do not overdry 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 overdry 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 microleakage 5.

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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

 

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