2. Basic concepts of adhesion
3. Requirements for good adhesion
4. Applications of Adhesive restorative techniques
5. Enamel adhesion
6. Mechanism of etching and etch patterns
7. Dentin adhesion
8. Challenges in dentin bonding
9. Generations of dentin bonding agents
• Adhesion is one of the ideal requirements of a restorative material.
• The obvious advantage of adhesion would be the absence of
marginal gap between the restoration and tooth structure
• This minimizes microleakage and the possibility of secondary caries.
• The possibility of adhesion to tooth structure has opened up many
new avenues that have radically altered all aspects of restorative
The state in which two surfaces are held together by
interfacial forces which may consist of valance forces or
interlocking forces or both.
(The American Society for Testing and Materials Sp. D 907)
• An adhesive is a material, frequently a viscous fluid, that joins two
substrates together and solidifies and is able to transfer load from
one surface to the other. (Sturdevant)
ADHESIVE : this refers to the bonding agent that when
applied to the surfaces or substances, can join them
together, resist separation and transmit loads across the
ADHEREND : this refers to the surface or substrate that
is adhered to.
MECHANISMS OF ADHESIONS :
1. Mechanical adhesion : interlocking of the adhesive with
irregularities in the surface of the substrate, or adherent.
2. Adsorption adhesion : chemical bonding between the adhesive and
3. Diffusion adhesion : interlocking between mobile molecules, such
as the adhesion of two polymers through diffusion of polymer
chain ends across an interface,
In dentistry bonding of resins to the tooth structure is a result of four
possible mechanisms, as follows :
4. Electrostatic adhesion : an electrical double layer at the interface of
a metal with a polymer that is part of the total bonding mechanism
1. 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 (mainly type I collagen) of
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
• The surface of the substrate should be clean
• The adhesive should wet the substrate well, have a low
contact angle, and spread onto the surface.
• There should be intimate adaptation between the adhesive
and the adherent.
• The bond strength between the adhesive and the adherent
should be strong enough to resist debonding.
• The adhesive should be well cured.
Adhesive restorative techniques are currently used to
accomplish the following :
1. Restore Class I,II,III,IV,V and VI carious or traumatic defects.
2. Change in shape and the colour of anterior teeth.
3. Improve retention for metallic crowns or for porcelain-fused-to-
4. Bond all ceramic restorations
5. Bond indirect resin-based restoration
6. Seal pits and fissures
7. Bond orthodontic brackets
8. Bond periodontal splints and conservative tooth replacement
9. Repair existing restorations (composite, amalgam, ceramic, or
10.Provide foundations for crowns
11.Desensitize exposed root surface
12.Seal beneath or bond amalgam restorations to tooth structure
13.Impregnate dentin that has been exposed to the oral fluids,
making it less susceptible to caries.
14.Bond fractured fragments of anterior teeth
15.Bond prefabricated and cast posts
16.Reinforce fragile roots internally
17.Seal apical restorations placed during endodontic surgery.
• Buonocore in 1955 introduced the acid-etch technique for the
adhesion of resin to the tooth.
• He envisioned the use of acids to etch enamel for sealing pits and
• 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, aided by capillary action.
• Monomers undergo polymerization and the material becomes
interlocked with the enamel surface.
• The formation of resin microtags within the enamel surface is the
fundamental mechanism of resin-enamel adhesion
• Originally Buonocore suggested the use of 85% phosphoric acid for
2 minutes on the enamel surface to etch it.
• Silverstone et al found that the application of 30-40% phosphoric
acid resulted in retentive enamel surfaces.
• Presently a 37% concentration of phosphoric acid is preferred.
• An etching time of 60 seconds originally was recommended for
permanent enamel using 30-40% phosphoric acid.
• But studies using scanning electron microscopy showed that a 15
second etch resulted in a similar surface roughness as provided by
a 60 second etch
• Acid may be available as a liquid or gel form.
• Gel etchants are preferred due to better control in placement
over the enamel surface.
• The acid may be applied by means of syringe or brush. Syringe
placement is easy and precise.
MECHANISM OF ETCHING AND ETCH PATTERNS
• Acid etching converts smooth enamel into a very irregular surface
with high surface energy.
• It removes 10µm of surface enamel and creates a microporous layer
which is 5 to 50µm deep.
• Optimum concentrations of acid produces monocalcium phosphate
monohydrate precipitate which can be easily rinsed off.
Microscopically, three types o etch patterns have been described :
Type I Dissolution of the prism cores leaving the prism
Type II Dissolution of the prism peripheries leaving the prism
Type III No prism structures are evident.
• After etching, the enamel surface should be thoroughly rinsed
with a continuous stream of water spray for 5-10 seconds so
that the acid is completely washed off.
• This should be followed by proper drying which will produce a
frosty, white appearance.
ENAMEL BONDING AGENTS
• Originally enamel bonding agent consisted of Bis-GMA or UDMA
resins with a diluent like TEGDMA to lower their viscosity.
• These agents flow easily into the microporosities of the enamel
surface and when polymerized by light activation form ‘resin tags’
which lock them into the enamel surface.
• Bond strength of composite resins to etched enamel is in the range
from 15 to 25 MPa.
• The classic concepts of operative dentistry were challenged in the
1980-90s by the introduction of new adhesive techniques to dentin
as well as to the enamel.
• Dentin adhesion primarily relies on the penetration of adhesive
monomers into the filigree of collagen fibers left exposed by acid
CHALLENGES IN DENTIN BONDING
• Bonding to enamel is a relatively simple process. Adhesion to
dentin presents a much greater challenge
• Structurally dentin consist of a substantial proportion of water and
organic material as compared to the 90% mineral composition of
• Type I collagen is the principal organic content of dentin.
• Dentin is an intrinsically hydrated tissue, penetrated by a maze of 1-
2.5µm-diameter- fluid filled dentinal tubules.
• Dentinal tubule connects pulp with the DEJ. The constant pressure
from the pulp causes the fluid to move towards the DEJ.
• Dentin is a dynamic tissue that shows changes due to ageing,
caries, or restorative procedures.
• Diameter of dentinal tubules reduces as we move from the
periphery towards the pulpal side. Inversely the relative area
occupied by the tubules increase.
• Cut dentinal surface form an unique structure called as the ‘smear
layer’. It is composed of debris of hydroxyappatite crystals and
• The smear layer fills the orifices of dentin tubules, forming ‘smear
plugs’ and decreases dentin permeability by 85%.
• Submicron porosity of the smear layer still allows for diffusion of
• The removal of smear layer and smear plugs with acidic solutions
will result in an increase of fluid flow onto the exposed dentin.
• This fluid can interfere with adhesion because of the hydrophobic
nature of resins even if the resin tags are created.
STRESSES AT RESIN-DENTIN INTERFACE
• Composites shrink as they polymerize, creating a stress within the
• This is depended on the configuration of the cavity preparation – the
• Stresses within the composite are relieved by flow from the
• Stress relief within a three- dimensional bonded restoration is
• During 1950s, it was reported that a resin containing
glycerophosphoric acid dimethacrylate (GPDM) could bond to a
hydrochloric acid-etched dentin surface.
• A few years before that report, another researcher had used the
same monomer chemically activated with sulfinic acid.
• The bond strength of these primitive adhesion techniques were
severely reduced by immersion in water
• NPG-GMA(N-phenyl glycine glycidyl methacrylate), a surface active
comonomer is considered as the first generation dentin bonding
• Both Invitro and invivo clinical study results were discouraging.
• Theoretically, the comonomer could chelate with calcium on the
tooth surface to generate water-resistant chemical bonds of resin to
• Based on C-13 nuclear magnetic resonance analysis, It seems that
no ionic bonding develops between NPG-GMA and hydroxyapatite.
• Example : Cervident
• They are phosphate ester material (phenyl-P and hydroxyethyl
methacrylate in ethanol)
• Mechanism of action is based on polar interaction between
phosphate group and calcium in the smear layer.
• The smear layer was the weakest link in the system because of its
loose attachment with dentin.
• Bond strength was also poor (<10MPa). In addition to this, the resins
were relatively devoid of hydrophilic groups.
• They show less wettability and penetration into the dentin crossing
the smear layer.
• Example :Clearfil Bond, Scotchbond, Bondlite
• The third generation dentinal adhesives attempted to deal smear
layer in two ways.
• Either by modification of the smear layer to improve its properties
or by the removal of the smear layer by keeping the smear plugs
• Fusayama et al in 1979 introduced the concept of phosphoric acid
etching of dentin prior to the use of phosphate-ester type bonding
• Materials like phenyl-P or PENTA was used to achieve smear layer
modification by penetration of acidic monomers.
• Treatment of smear layer using acid primers were also attempted.
2.5% maleic acid, 55% HEMA, and traces of methacrylic acid were
used for this.
• The removal of smear layer using chelating agents such as EDTA
was also tried (GLUMA system).
• Example : Clearfil New Bond, Scotchbond 2, GLUMA system
• They are; three step, total etch adhesive systems.
• Although smear layer acts as a ‘diffusion barrier’ that reduces the
permeability of dentin, it also can be considered as an obstacle to
• Based on this this consideration a fourth generation dentin
adhesives was introduced for use on acid etched dentin.
• This method is commonly known as the total-etch technique or the
etch and rinse technique.
• The acid will result in complete or partial removal of smear layer
with demineralization of underlying dentin. They also exposes the
• The fourth generation dentin bonding system consist of three
1. Phosphoric acid etching gel that is rinsed off
2. A primer containing reactive hydrophilic monomers in
ethanol, acetone, or water
3. An unfilled or filled resin bonding agent
• Acid treatment not only alters the mineral content of the dentin
substrate but also alters the surface free energy.
• When primer and bonding resins are applied to etched dentin, they
penetrate the intertubular dentin, forming a resin-dentin
interdiffusion zone, or ‘hybrid layer’.
• Examples : Scotch Bond Multi Purpose(3M), All Bond 2, Panavia 21
• These adhesives are a simplified version of the fourth generation
adhesives. Also known as ‘one bottle’ system.
• The primer and adhesive is combined in one bottle. A separate
etching step is still required.
• Though they require fewer steps to achieve dentin bonding, these
agents are inferior to the fourth generation bonding agents in
terms of bond strength.
• Example : Single Bond(3M), One-Step, Gluma Comfort Bond.
• Sixth generation dentin boding systems try to further simplify the
process of dentin adhesion by minimizing the clinical steps.
• Acids of lower concentration are generally used :10% Phosphoric,
2.5%nitric, 10% citric, 10% maleic acids.
• They are also known as SEP – Self Etching Primers.
• Commercially they are available in two forms :
1. Self etching primers : etchant and primer is
in one bottle while adhesive is in a
separate bottle. First the etchant and
primer are applied on the tooth surface
which is then followed by application of
example : Clearfil SE bond, Xeno
2. Self etching adhesives : in these, the etchant, primer and
adhesive are all in one package but require mixing before
application on the tooth surface
example : Prompt-L-Pop(3M)
• Also known as All-in-one adhesive system.
• These are the most recent generation of dentin bonding agents.
• They combine etchant, primer, and adhesive in one bottle. They do
not require any mixing prior to application.
• Primarily these agents are intricate mixes of hydrophilic and
hydrophobic components in one bottle.
• Their bond strength is less than fourth and fifth generation
• Example : Cleaefil S3 Bond, G-Bond, Xeno IV
Adhesive restorative dentistry originated with the work of Buonocore
in 1995 who developed the concept of acid etching of enamel. Since
then, adhesive materials and techniques have developed at a rapid
rate. Dentin bonding agents have advanced greatly over the last two
decades and can now provide predictable bonding to enamel and
dentin. The mechanism of bonding is primarily micromechanical by
formation of a hybrid layer. The various advances in adhesive
technology have expanded the applications of dentin bonding agents
to include bonding of composite, ceramic as well as metallic
restorations successfully to tooth structure.
Sturdevant’s Art and Science of Operative Dentistry – Fifth edition
Clinical Operative Dentistry Principles and Practice –Remya Raghu