Adhesion
The various adhesive restorative materials along with their properties, modes of adhesion and much more are covered in this section.
1. Why Orthophosphoric acid was attempted for acid etching in dentistry?
Wide spread industrial use of 85% Orthophosphoric acid for adhesion of paints and resins to metallic surfaces, stimulated Buanocore to attempt Orthophosphoric acid for acid etch technique in dentistry.
Ref: Theodore.M.Roberson,Harald.O.Heymann,Edward.J.SwiftJR.Sturdevant's Art and Science of Operative Dentistry,Mosby Publications.2002;38
2. Why the acid component of dentin bonding system has weaker acids than enamel etchant?
The primary function of the dentin bonding agents(DBA) is to form "hybrid layer" with the help of its acidic and resin component. The weaker acids present in DBA removes the smear layer but leaves the smear plugs intact as they greatly reduce dentin permeability and sensitivity. These mild etchants will not damage the exposed collagen fibers as much of the bond strength develops from resin encapsulating these fibers. The demineralized intertubular dentin and exposed collagen fibres impregnated by resin is known as "hybrid layer". Hence, DBA contains weaker acids to achieve these objectives.
Ref: Theodore.M.Roberson,Harald.O.Heymann,Edward.J.SwiftJR.Sturdevant's Art and Science of Operative Dentistry.Mosby publications;2002;28-30
3. Why most of the acid etchants contain 30-40% of ortho phosphoric acid?
Application of 50% ortho phosphoric acid for 60 seconds to enamel surface results in formation of a mono calcium phosphate monohydrate precipitate that can be rinsed off. However concentration below 27% may create a dicalcium phosphate monohydrate that cannot be rinsed off. Both these precipitates interfere with adhesion and hence the concentration of 30-40% is used widely.
Ref: Theodore.M.Roberson,Harald.O.Heymann,Edward.J.SwiftJR.Sturdevant's Art and Science of Operative Dentistry.Mosby publications;2002;238-239
4. Why dentin is a challenging substrate for resin bonding?
Dentin contains more amount of water and organic material (Type I collagen) than enamel. Also, it contains dentinal tubules that is filled with dentinal fluid which communicates with the pulp. These make the dentin a highly hydrophilic substrate and possess formidable challenge for hydrophobic resin to bond with dentin.
Ref: Theodore.M.Roberson,Harald.O.Heymann,Edward.J.SwiftJR.Sturdevant's Art and Science of Operative Dentistry.Mosby publications;2002;240-242
5. Why immediate bond strength of composite resin restoration is important for success of the restoration?
Composites shrink as they polymerize, creating stress upto 7Mpa within the restoration. To counteract these stresses, a minimum of 17Mpa of bond strength is required to prevent debonding of composite restoration. Hence immediate bond strength is of prime importance for a successful restoration.
Ref: Theodore.M.Roberson,Harald.O.Heymann,Edward.J.SwiftJR.Sturdevant's Art and Science of Operative Dentistry.Mosby publications;2002;243
6. Why "wet bonding" concept is accepted over "dry bonding" for bonding composites to dentin surface?
Drying of etched enamel and dentin surface before application of bonding agents to acheive dry dentin and enamel surface, was followed as standard protocol. Drying the dentin surface leads to collapse of collagen fibres which prevent the resin monomers from penetrating the dentin. This causes poor bond strength of the restoration. On the other hand, "wet bonding" concept which dictates the dentin surface to be moist before application of dentin bonding agent, preserves the spatial orientation of the collagen fibres, thus providing wide gaps between collagen fibres for increased retention and bond strength of the dentin bonding agent.
Ref: Theodore.M.Roberson,Harald.O.Heymann,Edward.J.SwiftJR.Sturdevant's Art and Science of Operative Dentistry.Mosby publications;2002;251
7. Why "re-wetting" of dentin surface is acceptable only for acetone based dentin bonding agents?
Acetone based dentin bonding agents are water-free agents and etched dentin must be re-wetted before applying the adhesive. Re-wetting done with water or aqueous re-wetting agents will cause the collapsed collagen network to raise and restore its spatial configuration. This will lead to bond strength value similar to wet bonding concept. Hence the operator should be aware whether acetone-based or water-based dentin bonding agent is being used.
Ref: Theodore.M.Roberson,Harald.O.Heymann,Edward.J.SwiftJR.Sturdevant's Art and Science of Operative Dentistry.Mosby publications;2002;251-254
8. What is adhesion/ bonding?
It is a process of solid and/ or liquid interaction of one material with another at a single interface.Dental Adhesion is also called as Dental Bonding.
Ref: Theodore.M.Roberson,Harald.O.Heymann,Edward.J.SwiftJR.Sturdevant's Art and Science of Operative Dentistry.Mosby publications;2002;177
9. What are the types of bonding?
Bonding may be physical, chemical or mechanical. Physical involves vanderwalls or electrostatic interactions. Chemical involves formation of atoms across the interface and mechanical involves undercuts and other irregularities which interlocks the restorative material.
Ref: Theodore.M.Roberson,Harald.O.Heymann,Edward.J.SwiftJR.Sturdevant's Art and Science of Operative Dentistry.Mosby publications;2002;179
10. What is a hybrid layer?
Hybrid layer is the layer that is formed in the interface of bonding agents and the dentinal surface of composite resin restorations.The dentin bonding systems have acids that remove the smear layer and partially demineralise the intertubular dentin. The smear plugs are not removed by the etchants and the exposed collagen fibres are also not damaged excessively. When hydrophilic adhesive resins are applied, it penetrates the moist dentin surfaces forming resin tags and also adapts to the demineralised intertubular dentin and collagen fibres. This resultant resin interdiffusion zone is called the Hybrid layer.
Ref: Theodore.M.Roberson,Harald.O.Heymann,Edward.J.SwiftJR.Sturdevant's Art and Science of Operative Dentistry.Mosby publications;2002;29
11. What are amalgam bonding systems?
They are used to seal the underlying tooth structure and bond amalgam to enamel and dentin. It should be both hydrophilic and hydrophobic to wet both the enamel& dentin and the amalgam respectively. 4-Methyloxy ethyl trimellitic anhydride (4 –META) based systems are frequently used.
Ref: Theodore.M.Roberson,Harald.O.Heymann,Edward.J.SwiftJR.Sturdevant's Art and Science of Operative Dentistry.Mosby publications;2002;185
12. What are the general considerations for etching the tooth surface?
30 - 40% Orthophosphoric acid gel is applied on the tooth for 15 – 20 seconds and rinsed with air / water spray . No acid should remain on the tooth after rinsing. Excess water is evacuated or removed with small sponges or applicator tips so that surface remains moist and the collagen fibres are not destroyed . An air spray can also be used, but the dentinal surface should be moist for bonding.
Ref: Kenneth W.Aschheim,Barry G.Dale,Esthetic Dentistry,Mosby publications,II edn,45
13. How is roughness created on a surface for a better mechanical bonding?
Surface roughness is commonly produced by grinding or etching the surface.
Ref: Theodore.M.Roberson,Harald.O.Heymann,Edward.J.SwiftJR.Sturdevant's Art and Science of Operative Dentistry.Mosby publications;2002;179
14. What is the difference between grinding and etching of a surface for a better mechanical bonding?
Grinding produces gross mechanical roughness but leaves a smear layer of hydroxyapatite crystals and denatured collagen about 1-3um thick. Acid etching dissolves the smear layer produced and produces microscopic relief with undercuts on the surface for mechanical bonding.
Ref: Theodore.M.Roberson,Harald.O.Heymann,Edward.J.SwiftJR.Sturdevant's Art and Science of Operative Dentistry.Mosby publications;2002;179
15. What is micromechanical bonding or micro retention?
If the mechanical roughness produces a microscopically interlocked adhesive and adherend with dimensions of less than approximately 10 um, then it is called as micromechanical retention.
Ref: Theodore.M.Roberson,Harald.O.Heymann,Edward.J.SwiftJR.Sturdevant's Art and Science of Operative Dentistry.Mosby publications;2002;179
16. What is wetting?
Wetting is the measure of the energy of interaction of the material. It produces a chemical bond. In a complete state of wetness, the contact angle between the solid and the liquid approaches zero degrees.
Ref: Theodore.M.Roberson,Harald.O.Heymann,Edward.J.SwiftJR.Sturdevant's Art and Science of Operative Dentistry.Mosby publications;2002;179
17. How is gold for direct filling restorations classified?
Gold for direct filling restorations may be classified on the basis of
• geometric form in which it is supplied
• the surface condition of the piece
• the microstructure of the piece.
Ref: Theodore.M.Roberson,Harald.O.Heymann,Edward.J.SwiftJR.Sturdevant's Art and Science of Operative Dentistry.Mosby publications;2002;211
18. What is non cohesive gold?
The surface of the gold piece may be intentionally protected by ammonia gas adsorption to prevent adsorption of unwanted hard to remove gases. At the time of use, the layer of ammonia can easily be removed by proper heating of the piece.
Ref: Theodore.M.Roberson,Harald.O.Heymann,Edward.J.SwiftJR.Sturdevant's Art and Science of Operative Dentistry.Mosby publications;2002;211
19. What are the possible mechanisms through which resins bond to the tooth structure?
Resins can bond to the tooth structure through
• Mechanical penetration of the resin and formation of resin tags within the tooth surface
• Diffusion precipitation of substances on the tooth surfaces to which resin monomers can bond mechanically or chemically
• Adsorption – chemical bonding to the inorganic or organic component of the tooth structure
• A combination of the previous three mechanisms.
Ref: Theodore.M.Roberson,Harald.O.Heymann,Edward.J.SwiftJR.Sturdevant's Art and Science of Operative Dentistry.Mosby publications;2002;237
20. What are the basic requirements of an adhesive to achieve a good adhesion?
For a good adhesion to occur, a close contact must exist between the adhesive and the enamel or dentin. The surface tension of the adhesive must be lower than the surface energy of the enamel and dentin.
Ref: Theodore.M.Roberson,Harald.O.Heymann,Edward.J.SwiftJR.Sturdevant's Art and Science of Operative Dentistry.Mosby publications;2002;237
21. What are the locations in which failures of adhesive joints can occur?
Failure of the adhesive joints can occur in three locations.
• Cohesive failure in the substrate
• Cohesive failure within the adhesive
• Adhesive failure at the interface of substrate and adhesive.
Ref: Theodore.M.Roberson,Harald.O.Heymann,Edward.J.SwiftJR.Sturdevant's Art and Science of Operative Dentistry.Mosby publications;2002;237
22. What are the conditions that show an increase in the mineral content of the dentin?
The mineral content of the dentin increases in conditions like ageing of the dentin, dentin in viscinity of carious lesion and dentin exposed to oral cavity in non carious cervical lesions.
Ref: Theodore.M.Roberson,Harald.O.Heymann,Edward.J.SwiftJR.Sturdevant's Art and Science of Operative Dentistry.Mosby publications;2002;257
23. What is the advantage of wet bonding?
The adhesive systems used in wet bonding are incorporated with inorganic solvents like acetone or ethanol. This solvent displaces the water from both the dentin surface and the moist collagen network and promotes infiltration of resin monomers throughout the dense collagen web.
Ref: Theodore.M.Roberson,Harald.O.Heymann,Edward.J.SwiftJR.Sturdevant's Art and Science of Operative Dentistry.Mosby publications;2002;251
24. How does ‘wet bonding’ improve the bond strength?
The wet bonding technique enhances the bond strength because water preserves the porosity of collagen network available for monomer interdiffusion. If the surface of the dentin is dried with air, the collagen undergoes immediate collapse and prevents the resin monomer from penetrating.
Ref: Theodore.M.Roberson,Harald.O.Heymann,Edward.J.SwiftJR.Sturdevant's Art and Science of Operative Dentistry.Mosby publications;2002;251
25. ‘Pooled Moisture’ should not remain on the tooth – Why?
Pooled moisture should not remain on the tooth because excess water can dilute the primer and make it less effective. Hence a glistening hydrated surface is preferred.
Ref: Theodore.M.Roberson,Harald.O.Heymann,Edward.J.SwiftJR.Sturdevant's Art and Science of Operative Dentistry.Mosby publications;2002;251
26. What are the possible factors for adverse pulpal reactions following restorative procedures?
Adverse pulpal reactions can be due to a combination of factors like:
• Bacterial invasion of the pulp either from the tooth preparation or the carious lesion:
• Faulty restoration causing bacterial penetration into the pulp
• Excessive pressure during cementation
• Traumatic injuries
• Iatrogenic tooth preparation – excessive heat , pressure etc
• Polymerization contraction of composites/adhesives producing stress
Ref: Theodore.M.Roberson,Harald.O.Heymann,Edward.J.SwiftJR.Sturdevant's Art and Science of Operative Dentistry.Mosby publications;2002;256
27. Why is it important to keep the preparation margins with the enamel peripheries?
The peripheries of the tooth preparation are preferred to be placed on the enamel since the polymerization shrinkage stresses at the interface are counteracted by strong enamel adhesion. So marginal gaps are less likely to form and the restoration is sealed against bacteria.
Ref: Theodore.M.Roberson,Harald.O.Heymann,Edward.J.SwiftJR.Sturdevant's Art and Science of Operative Dentistry.Mosby publications;2002;256