Bonding in dentistry
I. Definitions:
According to LAROUSSE:
To fix, to adhere something to something else by means of a suitable glue or adhesive.
In prosthesis:
Method of fixing prosthetic reconstructions, its principle is the assembly by physicochemical bond of the two interfaces: dental tissue prosthetic part.
II. Different glues:
1. Resins
Resins, which result from the polymerization of methacrylic molecules. They are found in glues and in composite materials. We distinguish:
- The adhesive which is a very fluid resin which infiltrates the roughness of the treated dental surfaces and thus forms a mechanical keying.
- Glues that are loaded with particles and create the mechanical bond between the adhesive layer and the prosthesis.
In adhesive systems, resins are generally combined with “primer” type agents which modify the surface to be bonded.
- Classification of adhesive systems: based on the principles of action of the different adhesive systems, we distinguish 2 main classes of adhesives:
Those products which require etching followed by rinsing, prior to their use.
That of products that are applied directly to dental surfaces without any preliminary treatment. This class includes all self-etching systems.
Depending on the polymerization method, we distinguish:
Self-curing adhesives (base + catalyst mixture)
Light-curing glue (light source)
Mixed polymerization adhesives (or dual photochemical)
2. “4 META” resins:
It is an acrylic resin containing two new compounds:
• 4-META (4-methacryloyloxyethyl trimellitate anhydride)
• tri-n-butyl borane (TBB).
- Features :
- A product of choice for bonding metal alloys
- Adhesion to dental tissues, and in particular dentin, is effective, thanks to the creation of a quality hybrid layer.
3. “Methyl diphosphate” resins: MDP
In 1981, Kuraray developed a phosphate-containing monomer that improved adhesion to dentin.
- “MDP” resin improves adhesion to enamel and dentin
- Provides highly effective bonding to metal alloys.
- The setting of the material is anaerobic and therefore allows a long working time.
- Polymerization is triggered after the application of an isolation gel
- PANAVIA is the trade name of this MDP resin.
4. Resin-modified glass ionomer cements (hybrids), CVIH:
- They have a real chemical adhesion to the mineralized tissues of the tooth, as well as with certain alloys.
- However, the material’s own strength is lower than the adhesive strength achieved.
- CVIs are therefore most often reinforced with resin.
- The adhesion of modified CVIs could be improved by applying an adhesive system before CVIH placement
III. Required qualities:
Ease of handling
Biocompatibility; non-cytotoxic to the pulp-dentin organ
Durability: preservation of the adhesive bond over time
The adhesive character: immediately ensures an adhesive seal
Waterproofing: prevent the penetration of bacteria and fluids that cause sensitivity and cavities.
IV. Membership mechanism:
- Bonding to dental tissues:
The two tissues that make up the tooth, enamel and dentin, are quite different in their chemical composition and physical properties. Enamel is a hard and brittle tissue, while dentin is soft and flexible. This tissue duality complicates the adhesion processes.
Membership:
It is the union of two solids. It is generally obtained by an adhesive agent (normally liquid) which is capable of moistening the two surfaces and which, by solidifying, creates a joint between the two elements.
How ?
The mechanism keeping the adhesive in intimate contact and permanently bonded to the solid surface can be:
- Chemical: the adhesive maintains primary chemical bonds with the solid surface, particularly with the calcium of the hydroxyapatite.
- Mechanical: micromechanical, even nanomechanical due to the penetration of the liquid adhesive into the irregularities of the solid surface.
- Or a combination of both.
- Enamel bonding:
It was Dr. Michael Buonocore who first demonstrated that an acid could alter the surface of dental enamel and allow bonding by a resin.
The greater dissolution of the core of the prisms will in fact create a microrelief on the surface of the enamel.
A resin can then infiltrate these created crevices and ensure adhesion by mechanical keying.
- Clinical application of enamel bonding: bonded bridge
The enamel bonding technique typically involves 03 steps:
- Etching of enamel with orthophosphoric acid gel for 30 s.
- Vigorous rinsing with an air-water spray followed by careful and complete drying with a jet of air: a chalky, matt white appearance of the enamel is observed
- Application of the enamel bonding agent.
- Bonding to dentin:
Etching rinsing: at the level of the dentin surfaces, the acid attack:
-Removes most of the dentinal sludge,
-Opens tubular orifices,
-Superficially demineralizes the peri and inter tubular areas (a few μm)
This superficial area of dentin is made up of a network of interwoven collagen fibrils dispersed in the rinsing water.
The presence of water hinders the infiltration of the surface treated by the hydrophobic adhesive resin.
The evaporation of the rinsing water by drying causes at this stage a fusion of the protein fibrils
The collapsed surface becomes compact and not conducive to resin penetration.
What to do to eliminate water and allow the resin to penetrate is to use an agent called a primer.
- The primary:
The primer is a liquid which allows:
- Either to keep the collagen network sufficiently porous
- Or to allow it to re-expand if it has collapsed during drying.
The application of a primer appears to be essential to allow permeability of demineralized dentin after evaporation of the water it contains.
Once the water is removed, the surface has a hydrophobic character conducive to the penetration of the resin.
- The adhesive resin:
- The third step of the adhesive treatment is simply the application of the adhesive resin which must penetrate the tubules and infiltrate the channels of the inter and peri tubular protein network.
- After polymerization, an adherent and waterproof interphase is formed between the restoration and the intact dentin.
- This interphase is made up of a hybrid inter and peri tubular layer
- The hybrid layer:
The hybrid layer is an interweaving of two types of polymers: the collagen fibers of the dentin matrix, a polymer of natural origin, on the one hand, and the macromolecules of the adhesive, a synthetic polymer on the other hand.
- Clinical application of dentin bonding:
- Direct Class V restorations where there is minimal enamel
- Indirect restorations: inlay/onlay, ceramic crown
- Bonding to reconstruction materials:
■ Bonding of ceramics
■ Resin bonding
■ Metal bonding
- Bonding the ceramic:
Bonding is an effective way to overcome the fragility of ceramics; we distinguish:
- Etchable ceramics: feldspathic or pressed Empress type
These are ceramics that contain silicates in varying quantities. This vitreous phase can be etched with a strong acid, hydrofluoric acid, which creates a relief suitable for bonding. This acid is applied with great caution due to its toxicity and volatility (wearing gloves, mask and goggles). The acid must be rinsed thoroughly.
In a second step, a silane is deposited on the surface of the ceramic. This coupling agent allows, on the one hand, to create a chemical bond with the glass phase, and on the other hand to bind to the bonding resin. The silane also allows to increase the wettability on the surface of the ceramic.
Since this type of ceramic is sufficiently translucent, a photopolymerizing adhesive system can be used.
- Non-etching reinforced ceramics:
- Alumina or zirconia reinforced ceramics (Inceram®, Procera®)
- Treatment with hydrofluoric acid is ineffective and does not create a relief on the surface of such ceramics suitable for bonding.
- The treatment is done by:
- Sandblasting the intrados of the prosthesis with alumina
- An artificial silica deposit the adhesion values obtained immediately are very high.
- The use of 4-META or MDP type glue allows significant adhesion values to be obtained.
- Since these ceramics are more or less opaque, it is important to use an adhesive system that is partly or completely chemopolymerizing, and not just photopolymerizing.
- Resin bonding:
- Treatment of the intrados of the prosthetic part to improve bonding by systematically carrying out sandblasting with alumina.
- We etch with hydrofluoric acid and then apply a silane.
- If the prosthetic part is not too thick (< 2-3 mm), a light-curing adhesive can be used. Beyond this thickness, a chemo-curing adhesive must be used.
- Metal bonding:
Bonding to metal alloys was first achieved by mechanical retention:
• first in the form of macroretention (perforated fins of Rochette bridges);
• then microretention by sandblasting with alumina.
Thanks to 4-META (Superbond®) (Panavia®) type adhesives, the adhesion values obtained on alloys are of the same order of magnitude as the adhesion to dental tissues.
Bonding with a noble alloy is less effective than with a non-precious alloy.
In order to increase this adhesion, surface treatments have been proposed, which consist of depositing silica on the surface of the metal.
This silica is then coupled to the bonding resin by the application of a silane.
If the adhesion values obtained are then very high, mechanical and thermal fatigue seem to degrade the adhesive layer quite quickly.
In the medium term, the collage loses its effectiveness.
The recommended protocol is therefore to sandblast the prosthetic part with 50 μm alumina, then to use a 4-META or Panavia® type resin .
Bonding in dentistry
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