Operative Dentistry - Bonding System - Part 1 ( Enamel bonding and Enamel etching )

Bonding System

•Adhesion: Attraction between two unlike molecules.

•Cohesion: Attraction between two similar molecules, the material/ film used to cause adhesion is adhesive, the material to which it is applied is adherent.

*Types of bonding

-Mechanical bonding: Strong attachment to one another accomplished by mechanical retention rather than molecular attraction. Most dental adhesion is based on this type of bonding .

-Physical bonding:  Here weak Van Der Waals forces are developed.

-Chemical bonding: Chemisorption, a chemical bond is formed. An example is oxide layer over metallic surfaces.


•Wetting : To force two solids to adhere, a thin layer of liquid is required between them, this liquid should flow easily over the solid, this called as wetting.
•Wetting influences the solid by cleanliness of surface and surface energy.
• Waxes have low surface energy and prevent wetting.
•The extent to which the adhesive wets the adherend is determined by the angle between adhesive and adherend.
•If adhesive spreads completely over the adherend then the Ø=0 and as increases the wetting is decreased.
Contact Angle


 


Adhesion to tooth structure:

•Composition of tooth is not homogeneous and the organic and inorganic content of both enamel and dentin varies, so adhesive must bond both organic and inorganic component. 
• After tooth preparation, a tenacious smear layer is formed of microscopic debris which reduces wetting, so the greatest problem with adhesion to tooth is smear layer and contamination by saliva.

•Dental adhesives should be able to displace water, react with it, or wet the surface more effectively and bonding must be long standing in aqueous environment.

Adhesive Joint

Clinical applications of adhesion are :

- Pit and fissure sealant
- Bonded amalgam 
- Composite restorations 
- Orthodontic brackets
- All ceramic restorations luting
- Repair of ceramic restorations


Enamel etching and bonding:

•Enamel bonding achieved through acid etching of highly mineralized tissues.
•Etching of enamel results in etch pores or pits through which resin (unfilled and lightly filled) can flow and form resin tags.
•The goals of enamel etching are to clean enamel, remove enamel smear layer, increase microscopic roughness, increase surface free energy (from 28 dynes/cm to 72).
•The pattern of etching either dissolution of prism cores and peripheries left intact (Honeycomb appearance), or dissolution of prism peripheries and core left intact (cobblestone appearance)
 




Resin tags

•Resin tags are resinous extensions into microporosities of enamel. These tags become interlocked (on polymerization) with surface irregularities created by etching – micromechanical bonding.
•Two types:
1.Macrotags are formed circularly between enamel prisms at periphery of enamel rod/ 3-5 microns
2.Microtags are formed at cores of enamel prism  where multitude of individual crypts formed due to removal of hydroxyapatite crystals. This is more important for bond strength. Because of larger number and greater surface are 0.2 microns. These form basis of micromechanical bonding.



Factors affecting etching procedures

1.Concentration of acid

- Usually 30-50%  of phosphoric acid is used. Commonly 37% is preferable.

- Above 50% result in formation of monocalcium phosphate monohydrate which can inhibit further dissolution but can be rinsed off. Concentration below 27% result in dicalcium phosphate monohydrate which cannot be easily rinsed off.

2- Etching time

-15 secs of etching conserves enamel.

- Primary and fluorosis teeth require greater time.

3- Rinsing time :

Rinsing time should be done for about 20 secs to remove dissolved calcium phosphate.

4- Types of etching:

•Gels are preferable than solution, entrapment of air should be avoided,
•If contamination of saliva occurs, enamel should be rinsed and re-etched for 10 secs.




Alternative to phosphoric acid :

•EDTA
•Pyruvic acid
•Sulphuric acid
•Laser
•Air abrasion

Enamel Bonding Agents

•Early systems consisted of polymethyl- methacrylate, but these were discontinued because of curing or thermal shrinkage. Todays agents composition is same as that of composite resin with BIS-GMA, UDMA, TEGDMA this can be either unfilled or slightly filled.
•This resin penetrate into microporosities created by etching forming resin tags, also potential chemical interaction with etched enamel, carbon-carbon double bond formation with matrix phase of composite resin.
•These separate agents (for enamel were used before introduction of total etch concept) has been replaced by dentin bonding agents that has the ability to wet etched enamel surface to simplify process.

Smear Layer

•A smear layer is an adherent layer of debris (1-2µm) on tooth surfaces when they are cut with rotary or hand instrument.
•In dentin surface, the orifices of the dentinal tubules are obstructed by dentin tags called smear plugs, which may extend into the tubule to a depth of 1-10µm, these smear plugs are contiguous with the smear layer.


•The thickness and morphology of smear layer varies with the method used for producing, smear layer with location within dentin in relation to the pulp.
•It is concluded that its thickness increases with increasing roughness of bur, also smear layer composition reflect the composition of underlying dentin which is generally consist of shattered and crushed hydroxyapatite, as well as fragment of denaturated collagen.
•Clinically, smear layer is small with varying directions, irregular and weak structure making bonding agent covering this layer weak to underlying dentin. A smear layer may also be contaminated by bacteria and saliva.
•To overcome low bond strengths due to limited strength of smear layer; either :
1. Removal of smear layer prior to bonding or
2. The use of bonding agents that can penetrate beyond the smear layer while incorporating it, both techniques have been proven successful.
•Removal of smear layer, however, increases the permeability of dentinal tubules permitting fluid flow from outside pulp chamber and vice versa according to hydrodynamic theory, this dentinal fluid movement associated with postoperative sensitivity more than bonding system that do not require smear layer removal.
•After removal of smear layer dentin permeability increases (more than 90%) and subsequent wetting of dentin that may affect bond strength as dentinal fluid may dilute primers and bonding agents.
•Open dentinal tubules may also permit access of bacteria and its toxins towards the pulp, a continuous bacterial irritation due to microgaps and microleakage is likely to cause damage to the pulp and post-operative pain.



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