Step by step clinical adhesion 

To obtain high-quality adhesion, one must strictly follow a series of steps, each with a specific function. Thus, this section is intended to provide and support as concise as possible, the rationale for the clinical application of adhesive systems. To not limit the relevance of the information presented to one or another material, a discussion will be held on the conceptual level - how to do, why to do. 

Isolation of the operatory field : Adhesive procedures should not be performed without adequate control of the contamination of the operatory field by saliva, blood and moisture. Thus, the first step in establishing successful adhesive bonding is to establish adequate isolation - either relative or absolute.

Acid etching: The use of phosphoric acid in a concentration ranging between 30 and 40 % prepares the surface of enamel and dentin for the adhesive system to be applied. For enamel to be properly conditioned, it is ideal that the acid remains for 15 to 30 seconds. In dentin, however, the optimal time of action for the acid is only 15 seconds. Thus, when the cavity involves both enamel and dentin, conditioning should always be started in enamel, passing only to dentin at the moment it is already adequately coated by the acid. Following the conditioning time, the cavity should be rinsed for 15 to 30 seconds with the aid of an air/water spray. This step is critical and should be conducted very carefully to ensure the removal of any acid, and debris generated by conditioning. Then, excesses of moisture must be removed carefully, so that the components of the adhesive system are not diluted - remember that during the establishment of adhesive bonding, water 💦 is a contaminant. 

In cavities or preparations restricted to enamel, common in situations like diastema closure and laminate veneers, excessive moisture can be easily removed by air jets - an approach that often results in an opaque white surface on areas where there was contact between enamel and the acid, but the absence of such feature does not mean that the etching was not effective. In dentin, due to its structure, the use of air jet is not preferred, since it promotes changes that compromise the effectiveness of adhesion. Thus, in cavities involving enamel and dentin, excess moisture ideally should be removed using air jets on enamel and by combining air jets and cotton pellets on dentin - this approach maintains dentin moist .
Interestingly, while most adhesive systems involve the simultaneous application of acid to enamel and dentin - in a technique known as total etching - the roles and effects of the conditioning are completely different from one tissue to another. In enamel, a high mineralized tissue, the conditioning has as its main role increasing wettability and surface free energy- natural effects of the removal of the acquired pellicle. Furthermore, demineralization of the enamel surface results in the creation of micro-retention and thereby an increased contact area- favorable conditions for subsequent mechanical imbrication of the adhesive agent. In dentin, on the other hand, the application of acid has the primary goal of removing the smear layer - a superficial layer formed by debris generated during cavity preparation. The removal of the smear layer is accompanied by the dissolution of the mineral surface of dentin and collagen fibers, which result in increased tubule openings, allowing for the output of dentinal fluid. With this, the post-conditioning dentin surface becomes extremely moist and with considerable organic content - opposite to the characteristics of enamel after etching - which causes dentin to have low surface energy representing a major challenge to the establishment of successful adhesive bonding. To overcome the difficulties imposed by the organic structure of dentin, we should first understand why air jets should not be used. 💁🏻‍♂️

Rather, cotton balls or absorbent spongation of the adhesive.  

Application of primer : Thanks to its organic nature and moist and low surface energy, etched dentin is not a good substrate for adhesion. Therefore, before applying the adhesive agent, it is necessary to apply a primer on dentin surface. The primer consists of bifunctional monomers - with a hydrophilic part ( affinity to water ) and a hydrophobic part ( without affinity to water but a high chemical affinity for the adhesive monomers ) and acts as a link between the moist surface of the etched dentin and the adhesive agent. By penetrating the demineralized surface and filling the spaces once occupied by hydroxyapatite crystals, the components of the primer stabilize the collagen network and promote the evaporation of excess water 💦 

The result is the increase of surface free energy of dentin, making it capable to interact with the bonding agent . To ensure good results the primer should be applied to the full extent of dentin with the aid of a disposable applicator and after approximately 30 seconds - optimal time required for infiltration of the monomers - the solvents are evaporated with gentle air jets. It should be clear that application of the primer to the enamel surface is totally unnecessary since it doesn’t present any collagen fibers, it can be dried with air jets and presents high surface energy. When the cavity involves enamel and dentin, however, the primer can also be safely applied on etched enamel without any loss of adhesion. This is extremely important since it is very difficult to ensure complete coating of the dentin by the primer without the occurrence of contact of the disposable applicator with enamel.

Application of the adhesive: The adhesive is nothing more than low-viscosity resin used to wet the substrate, acting as an intermediary agent between the tooth 🦷 structure and the restorative materials. In enamel, the interaction of the adhesive with the tissue involves filling of irregularities and micro porosities created by etching. While being polymerized within those surface grooves, the adhesive is retained micro-mechanically on the surface. In conditioned primed dentin, the adhesive fills the interspaces of the exposed collagen fiber network, penetrating some dentinal tubules and then is light-cured. The result is a polymer inter-diffusion region between the adhesive and dentin components. This region, known as the hybrid layer, extends from the area of dentin unaffected by etching until the surface of the exposed collagen fibers. In the dentinal tubules, the adhesive can penetrate considerably deep forming extensions or resin tags. 

Clinically, the application of the adhesive is performed with a disposable applicator, used carefully to ensure coating of the entire tooth surface - this means that the adhesive should always be applied beyond the limits of the cavity. Mild air jets may be used to smooth the thickness of the adhesive layer since pooling areas of the bonding agent are not desirable . The air stream does not promote controlled removal of any excess adhesive, it only spreads the material found on the surface. Thus gross pooling of adhesive , common in regions such as the interior angles of the preparation should be removed with dry disposable brushes before applying the air stream.

Latest adhesive systems : what are the differences ? In order to simplify the operatory procedures and reduce clinical time , manufacturers seek to develop alternative materials and techniques, usually through a combination of steps ( e.g. acidic primer is able to combine the functions of the primer and conditioning in a single bottle ). Evidently, there is not any problem in seeking to simplify operatory process, provided that the speed and ease don’t compromise the effectiveness of the adhesive bonding.

1️⃣ Multicomponent “ total-etching “ adhesive system :
They are the most traditional systems commonly described as three-step adhesive systems, in which each component - acid , primer , adhesive - is available in a separate vial .
These materials has a long history of successful results in clinical and laboratory evaluations and still represent the gold standard in dental adhesion.
⛔️ A caution to achieve good results with these materials is properly applying the primer - which is facilitated by using more than one layer - in order to ensure complete infiltration of the demineralized dentin . Between each layer , it is interesting to use gentle air jets in order to promote volatilization of solvents and allow enhanced infiltration of the monomers.
⛔️ By using three-step systems in situations where there is no exposed dentin, one can eliminate the application of the primer. In this case, after etching with phosphoric acid the surface must be thoroughly dried with air jets and then the adhesive is applied taking care to prevent any pooling of the adhesive not to produce a thick layer. 

2️⃣ Monocomponent “ total-etching “ adhesive system :
These materials represent an attempt of simplification in relation to the traditional systems. Like the three-step system, they depend on the conditioning of dental substrates with 30-40 % phosphoric acid. The major difference is that the components of primer and adhesive are available from the manufacturer in a single bottle. Therefore, these materials are commonly described as two-step adhesive systems - etching + application of primer/adhesive.
Although this nomenclature would suggest that the use of these systems is faster and simpler than the three-step systems, it is not necessarily true since most manufacturers recommend the application of multiple layers of primer/adhesive prior to light curing. Applying several layers contributes greatly to the proper infiltration of the adhesive at full depth of etched dentin. In this concept, the first layers would function similar to the primer and the subsequent layers would have a similar action to the traditional adhesive systems. 

3️⃣ Two-step self-etching :
They are systems consisting of an acidic primer and bonding agent . In these materials, there is a separate etching step, the primer is responsible for the modification of dental substrates in order to make them able to interact with the adhesive agent. For this, it is important that the primer presents low pH, sufficient to demineralize the hydroxyapatite crystals of enamel and dentin.

⛔️ Importantly, although acidic primers play an equivalent role to phosphoric acid used in the etching step of the total-etching technique, they should not be rinsed after application.
This results in a fundamental difference between traditional systems and self-etching systems; in the later, the smear layer is not removed but modified and incorporated into the hybrid layer.
After applying the primer, a thin uniform layer of the adhesive agent is used, with the aid of disposable brushes. Despite being conceptually different from conventional systems - after all, the smear layer is changed, rather than being removed - the two-step self-etching systems exhibit excellent performance in tests of adhesion to dentin. 
⛔️ Conversely, on enamel, where very high inorganic content represents a real challenge for the acidic primer, there is evidence that two-step self-etching systems have lower performance compared to conventional systems that rely on a preliminary stage of phosphoric acid etching. 

4️⃣ One-step self-etching adhesive systems : 
They represent another attempt at simplification by the manufacturers. In these materials, all components - etchant, primer, adhesive - are applied simultaneously on the dental substrates. They may be available in a single or two bottles. In the first, all components of the adhesive bonding are already mixed and ready for use. In the two bottle systems, on the other hand, mixing one drop of each bottle immediately prior to their use is sufficient. Then the mixture is applied as a single product, performing all hybridization stages in a single clinical step. As in two-step etching systems, the smear layer is not removed but incorporated into the hybrid layer.