TBU # 018: Dental Adhesion and Bonding Systems

Adhesion has been a game-changer in dentistry. Restorative procedures are way more advanced and adhesion reliable rather than retention reliable, the important part is to understand adhesion principles and apply them correctly and efficiently in our practice. Adhesion is simply defined by the state in which two surfaces are held together by interfacial forces which may consist of valence forces or interlocking forces or both (two surfaces that are held together) it has many forms that are applied in dentistry:

1- Mechanical\ macro mechanical adhesion (amalgam, metal indirect restorations..etc)
2- Chemical Adhesion\adsorption (Glass ionomer): this type of adhesion is considered a true adhesion.
3- Micro mechanical (composite)

Let's discuss adhesion in general before we jump into the bonding systems, In nature, any object tends to be in a reduced energy state, if the surface energy is high then the object isn't stable and to reduce or stabilise its energy, it adheres to another object. Tooth structures (enamel, dentin, cementum) are considered the adherend, they bond to the bonding agent (adhesive) and form a micro mechanical adhesion, then by applying composite on top of the bonding agent we will get a chemical adhesion between the composite and the bonding agent. Many factors are needed to be known to understand adhesion: surface energy, surface tension and wettability, failures of the adhesion should also be in mind.

The substrate (ex: enamel, dentin) has surface energy and the adhesive (liquid state) (ex: bonding agent) has something called surface tension. If the surface tension of the adhesive is high, it will not spread, so to have a good adhesive we need one with low surface tension, so it will have a better spread-ability (wettability). In other words, I need a substrate with high surface energy (to attract and have the need to stabilise itself) and an adhesive with low surface tension so it can spread easily on the substrate, this is what wettability means. Another thing that we need to discuss is the critical surface tension and this is about the substrate, this property is an indication of its relative water-hating or water-loving character. This also means that high critical surface tension in the substrate leads to high surface energy.

Failures in the adhesion could happen, it might happen in the substrate which is enamel or dentin (structural failure), it might happen between the adhesive and the substrate (adhesive failure), or it might happen inside the adhesive itself ( cohesive failure).

Moving on to the interesting dental part and leaving chemistry behind, to better know how to bond to dental structures we should understand the components of each part of the tooth and the bonding systems. Enamel is easier to bond to because of its structure, it contains almost 95% of inorganic material (calcium, phosphate and hydroxyapatite). It is also more homogeneous (contains less water) than dentin. Based on this we can state that bonding to enamel is stronger because it has high surface energy (caused by the inorganic components) and that adhering to a homogenous substrate is easier. It's important to focus on and optimise dentin bonding, dentin has 70% inorganic components, 12% organic and 18% water, which makes it harder to bond to.

As we stated before, if we have high surface energy then we have better adhesion, so how do we increase the surface energy of our substrates? In enamel, we can achieve that by phosphoric acid etching which produces irregularities and therefore increases the surface area which will increase the surface energy, it also helps in cleaning the area which is also needed because the pellicle reduces the surface energy of the enamel. Hydroxyapatite has a better surface energy and it accepts adhesion. When we look into dentin, we have both hydroxyapatite and collagen. Collagen has a low surface energy and so does the composite. Here we are having an issue. Bonding to dentin is not as easy as bonding to enamel!. If we etched the dentin the inorganic part will be removed leaving us with weak collagen that is poor with adhesion!. Dentin bonding will be mentioned below along side the bonding systems.

Bonding systems

Bonding agents are defined as unfilled composite, they have almost no filler. They are mostly hydrophobic, unfortunately, it's not completely hydrophobic, but the main ingredients which are resin and monomers are hydrophobic, that's why they add HEMA which is a monomer that is hydrophilic and hydrophobic at the same time and can combine the hydrophilic components of the bonding agent with the others. It may also contain an MDP monomer that provides chemical adhesion instead of micro mechanical adhesion.

4th generation (Three-Step Etch-and-Rinse Adhesives “Total-Etch”)

This bonding system is considered the gold standard adhesive material. This technique is sensitive and requires more clinical time. It is composed of three components: 1- Phosphoric acid etching: Removes the inorganic part of the dentin

(hydroxyapatite crystals), clean the smear layer, open dentinal tubules and exposes the collagen.

2- Primer: Collagen is not dimensionally stable, it acts like a sponge and it is fragile without the hydroxyapatite crystals, that's why we need the primer to prepare the collagen to receive the bonding agent, the primer usually contains hydrophilic monomers (ethanol \acetone). The role of primer is to increase the critical surface tension of dentin.

3- Bonding agent: Unfilled resin monomer.

5th generation (Two-Step Etch-and-Rinse Adhesives “Total-Etch”)

This system is produced to simplify the bonding procedure, Primer and the bonding agent are mixed in a single bottle making it a two-step approach: Phosphoric acid etching followed by the bonding agent. The chance of leaving a dry dentin surface is higher in this system when a rewetting agent such as chlorhexidine is not used.

6th generation (Two-Step Self-Etch Systems: Type 1 and 2 )

This bonding mechanism is simplified, it's considered less technique sensitive and is based on the stimulants etching and priming of enamel and dentin, this system can help in the reduction of the possible over-wetting or over-drying because the rinsing and drying steps are eliminated. The primer here contains a self etch acid, the PH of this primer is 2 which allows the etching of dentin, etching in this generation is considered mild, which means the risk of over-etching dentin is of the table, however, we need multiple applications to ensure that the dentin is properly etched. For enamel, this type of etching is

not sufficient that's why selective etching of enamel with phosphoric acid is needed. The smear layer isn't completely removed thus a reduction in post-operative sensitivity. The steps here are:

1- PA selective etching for enamel. 2- Multiple applications of the primer. 3- Bond.

Type 1: Primer and bond are applied alone (Two steps).

Type 2: Two bottles of acidic primer and adhesive that needs to be mixed before application; a drop of each liquid (primer+bond). This type, however, requires more air thinning.

7th generation (Self-Etch System)

Here we have one bottle (single solution, all in one) that contains the etch, primer, and the bond. It is very simplified and requires fewer steps. Multiple applications may be needed.

Universal Bonding Systems

It can be used as any other previous system depending on what you believe in. It provides the flexibility for the dentist to use it as needed. It can be used with self etch, etch and rinse or selective etch systems.

Adhesion to dentin: hybrid layer

The hybrid layer is a 3-dimensional polymer/collagen network that provides both a continuous and stable link between the adhesive and dentin substrate. The hybrid Layer is the result of the diffusion and impregnation of monomers into the subsurface of pretreated dentin substrates. It consists of collagen and the bonding agent. When the bonding agent infiltrates the collagen and forms a micromechanical adhesion. The bonding agent here is replacing the removed hydroxyapatite crystals, giving collagen stability and strength.

Micromechanical adhesion to dentin is made by two concepts: The hybrid layer and the resin tags (adhesive inside the dentinal tubules).

The hybrid layer is the weakest part of the restoration, it's not about the thickness, it's about the quality of the hybrid layer. If we over etch dentin we will increase the thickness of the hybrid layer. The bonding agent should infiltrate all the collagen network and substitute all the gaps of the etched hydroxyapatite crystals, so the quality of the hybrid layer is that the bonding agent should encapsulate all the collagen network.

There is also a problem with this layer, if we don't air thin the bonding agent properly water sorption will occur, because the bonding agent usually contains acetone or ethanol and they both love water, water attracts water and therefore an expansion of the area causing the deterioration\degradation of the hybrid layer and the loss of its mechanical properties. Matrix metalloproteinases (MMPs) is an enzyme that breaks down protein. As we know collagen is a type of protein. Now water sorption results from improper air thinning. The MMPs come from the dentin itself, it used to be attached to the hydroxyapatite crystals before etching, after the removal of the crystals we have freed the MMPs and it will start to attack the collagen over time causing the hybrid layer to deteriorate. Phosphoric acid etching activates MMP because of the low PH environment and the removal of the hydroxyapatite. Chlorhexidine (CHX) is found to inactivate the MMPs and is considered the gold standard MMP inhibitor. CHX is also used as a cavity disinfectant and a re-wetting agent(reexpand the collagen). CHX has a property called adsorption, it binds to dentin and is then releases itself slowly over time giving antibacterial and MMP inhibitory effects.

When we etch dentin the dentin permeability will increase causing postoperative sensitivity, the MMP monomers will get activated, and etching will result in a collagen network that is weak, demineralised, vulnerable, degradable and fragile. If you over etch the dentin the demineralisation area will be very big, and if the amount of exposed collagen network isn't completely encapsulated by the bonding agent, you will be left with weak collagen at the bottom that will be attacked by the MMPs (the depth of the collagen band is deep and the monomer cant go inside all of it). Another issue is overdrying after you etch dentin, you rinse and dry it, if you over-dry the dentin the collagen network will collapse and there won't be enough places for the resin monomer to infiltrate resulting in unprotected collagen. Over-drying and over-etching will result in post-operative sensitivity, microleakage, secondary caries, and loss of bonded restorations. Cross-linking agents such as glutaraldehyde are used to strengthen and support the collagen network.

Wet bonding technique is the used method to bond to dentin, the dentin should be glistening, which means that the surface should be moist but there is no visible droplet of water. The use of this technique was made possible by the incorporation of organic solvents (acetone or ethanol) in the primer or adhesive, the solvent displaces water thus allowing resin infiltration. This will stop the collapse of the collagen. If over-drying of the dentin occurs you can use chlorhexidine, or the primer itself to reexpand the collagen. That can be applied in the 4th generation where the primer alone is provided, but in the 5th generation, it is not quite possible so I need to have chlorhexidine in the clinic. You can differentiate between the enamel and dentin in colour, when you over dry dentin it becomes white and it should be yellow. Selective etching is also used to avoid over- etching of the dentin.

Adhesion to enamel : Microtags

The main concept of enamel adhesion is the formation of resin micro tags within the surface. The acid etching results in the micromechanical function in the form of micro tags and macro tags. The ends of enamel rods are more effectively etched producing deeper micro-undercuts than when only the sides of the rods are etched. Factors that affect enamel adhesion:
1- The nature of enamel (fluoridated or demineralised enamel).
2- Type of dentition.
3- The type and concentration of acid used.
4- Duration of acid application.