Dental Adhesion

Dental Adhesion

Dr. Manaf Basil

Adhesion or Bonding

Adhesion or bonding: The forces or energies between atoms or molecules at an interface that hold two phases together.
Adhesive: A material that can join substances together, resist separation and transmit loads across the bond.

Dental Adhesion

• The principles of adhesive in dentistry date back to 1955 when Buonocore applied acid to tooth to render the tooth more receptive to adhesion.

• In dentistry, bonding of resins to tooth structure is a result of four possible mechanisms, as follows:
Mechanical penetration of resin and formation of resin tags within the tooth surface.
• 2.Adsorption chemical bonding to the inorganic component (hydroxyapatite) or organic components of tooth structure.
• 3.Diffusion precipitation of substances on the tooth surfaces to which resin monomers can bond mechanically or chemically. 4.A combination of the previous three mechanisms.

Enamel

Enamel, the hardest tissue in the human body consists of 88% mineralized inorganic substance, hydroxyapatite arranged in a dense crystalline structure and a small amount of protein (organic) and water .






Etching It is the process of increasing the surface reactivity by demineralizing the superficial calcium layer and thus creating the enamel tags. These tags are responsible for micromechanical bonding between tooth and restorative resin.

• Enamel adhesion:

Acid etching transforms the smooth enamel into an irregular surface and increases its surface free energy. When a fluid resin-based material is applied to the irregular etched surface, the resin penetrates into the surface.





• Various concentrations of phosphoric acid have been used to etch enamel. Liquid and gel etchants are available, most in concentrations of 37% phosphoric acid.
• An etching time of 20 seconds originally was recommended for permanent enamel and 15 seconds for dentin.
• After etching the enamel appear as chalky (frosty).


Deciduous teeth require longer time for etching than permanent teeth because of presence of aprismatic enamel in deciduous teeth.

• Advantages of enamel adhesion:

• 1. Such bond strengths provide adequate retention (or a broad variety of procedures and prevent micro leakage around enamel margins of restorations.
• 2. Cusp reinforcement after tooth preparation.
• 3. Adhesive restorations substantially reinforce remaining enamel and dentin, making them less susceptible to fracture.

Factors affecting effects of acid etching on enamel

1.Form of acid used that is solution or gel
2.Concentration of acid used and time of etching 3.Type of acid used
4 .Chemical nature of enamel
5 .Whether enamel is fluoridated or
demineralized
6.Type of dentition, i.e. primary or permanent

• Dentin adhesion:

• The classic concepts of operative dentistry were challenged in the 1980s and 1990s by the introduction of new adhesive techniques, first for enamel and then for dentin. Nevertheless, adhesion to dentin remains difficult.

• Bonding to dentin presents a much greater challenge.

• Several factors account for this difference between enamel and dentin bonding.
• 1.In enamel, it is 88% inorganic hydroxyapatite by volume, in dentin it is 50%. Dentin contains more water than does enamel.
• 2.Hydroxyapatite crystals have regular pattern in enamel whereas in dentin, hydroxyapatite crystals are randomly arranged in an organic matrix.
• 3.The presence of the smear layer makes wetting of the dentin by the adhesive more difficult.






4. Dentin contains dentinal tubules which contain vital processes of the pulp, odontoblasts. This makes the dentin a sensitive structure.
5. Fluid present in dentinal tubules constantly flows outwards which reduces the adhesion of the composite resin to dentin bond.
6. Different quality of dentin including the number, diameter and size of dentinal tubules in deep and superficial dentin.

7. Amount of collagen: As dentin ages, there is increase in mineralization and decrease in the number of dentinal tubules which overall affects the adhesion quality of dentin.

• Whenever tooth structure is prepared with a bur or other instrument, residual organic and inorganic components form a "smear layer" of debris on the surface. The removal of the smear layer and smear plugs with acidic solutions results in an increase of the fluid flow onto the exposed dentin surface.

• Development of Bonding:

• 1. First generation 2. Second Generation:
• The first- and second-generation bonding agents used during the late 1960s and 1970s did not recommend etching the dentin and relied on adhesion to the attached smear layer. The weak bond (2 MPa to 6 MPa) to the smear layer still allowed dentin leakage with clinical margin stain.

3. Third Generation:

• In the third-generation systems, new material introduced. Kuraray (Osaka, Japan) introduced Clearfil New Bond in 1984. This phosphate-based material which includes a long hydrophobic (promoting adhesion to resin) and a short hydrophilic component (promoting adhesion to dentin) called primer.


Primers are used to increase the diffusion of resin into moist and demineralized dentin and thus optimal micromechanical bonding. For optimal penetration of primer into demineralized dentin, it should be applied in multiple coats. Also it is preferred to keep the dentin surface moist, otherwise collagen fibers get collapsed in dry condition resisting the entry for primer and adhesive resin.







These systems, employed the concept of conditioning and priming before application of bonding agent. In other words, in third generation systems, alteration or removal of the smear layer is done before bonding.

• 4. Fourth generation:

• Removal of the smear layer via acid etching led to significant improvements in the in vitro bond strengths of resins to dentin.
• Because the clinical technique involves simultaneous application of an acid to enamel and dentin, this method is commonly known as the total-etch technique. Also called the etch and rinse tech­nique, it was the most common strategy for dentin bonding during the 1990s and remains popular today.

Total etch technique involves the complete removal of the smear layer by simultaneous acid etching of enamel and dentin.

• The mechanism of bonding of resin-based dentin bonding agents is via a hybrid layer, this is a micromechanical interlocking of resin around dentinal collagen fibrils that have been exposed by demineralization ,the interlocking occurs by the diffusion of the resins inside the enamel or dentin surfaces.

5. Fifth Generation:

• In the fifth-generation bonding sys­tems, introduced during the mid 1990s, which combined primer and adhesive in one bottle while maintaining high bond strengths.
• The fifth generation consists of two different types of adhesive materials: the so-called “one-bottle systems” and the self-etching primer bonding systems.

• A. One-Bottle Systems:These are called the "one-bottle" systems because they combine the primer and bonding agent into a single solution. A separate etching step still is required.
• B. Self etching Primer Systems:
• These acidic primers include a phosphonated resin molecule that performs two functions simultaneous) etching and priming of dentin and enamel. In contrast to conventional etchants, SEPs are not rinsed off. the combination of etching and priming steps reduce the working time, eliminate the washing out of the acidic gel and also eliminate the risk of collagen collapse.

• One disadvantage of SEPs available to date is that they do not etch enamel as well as phosphoric acid, particularly if the enamel has not been instrumented.

• 6. Sixth generation (Self Etching Adhesives) (All in One):

• They have been made available since 2000. These single bottle adhesives combine etching, priming and bonding is mixed together into a single solution and as a single step.

7. Seventh Generation

They achieve the same objective as the 6th generation except that they simplified the multiple sixth generation materials into a single component, single bottle system, thus avoiding any mistake in mixing. In other words seventh generation bonding consists of only one component.

• Adhesive Amalgam Restorations:

• . Delayed interfacial marginal leakage occurs at the amalgam-preparation interface. Corrosion products from amalgam seal the interface after a few months; however, this process may take more than 6 months for copper rich amalgam alloys.
• To overcome the marginal microleakage, dentin adhesive systems have been used under amalgam restorations.

• The attachment mechanism between the adhesive and the amalgam is not fully understood, but it may be micromechanical engagement of uncured adhesive material with the setting amalgam mix during condensation of the amalgam.

• Advantages of Amalgam Adhesive systems:

• 1. Reduce or prevents marginal leakage.
• 2. Improves marginal integrity of the restoration when compared to the use of a copal varnish.
• 3. Additionally dentin adhesives reinforce the amalgam restoration margins.

• 4. The additional adhesive liner may provide an increased retention to adhesive amalgam restorations, therefore allowing for preparations with lower demand for additional retention features such as dovetails, slots, holes, or even pins.
• 5. The residual tooth structure becomes more resistant to fracture.

• This bonding mechanism actually may depend on the type of amalgam used; for example, spherical amalgam alloys typically have higher bond strengths than dispersed phase or admixed amalgam alloys.

• Shear bond strengths between amalgam and dentin were in the range of 3 to 5 MPa in the late 1980s and early 1990 However; recent studies have demonstrated that some current adhesive systems provide bond strengths in the range of 10 to 14 MPa. As a safety precaution, primary mechanical retention features are still recommended when an adhesive system is used with amalgam.

Thank You