concepts of enamel and dentin adhesion
TRANSCRIPT
-
7/29/2019 Concepts of Enamel and Dentin Adhesion
1/12
Bonding techniques allow more conservative tooth preparations, less reliance on
macromechanical retention, and less removal of unsupported enamel.
In dentistry, bonding of resins to tooth structure is a result of four possible mechanisms, as
follows:
1.Mechanicalpenetration of resin and formation of resin tags within the tooth surface2.Adsorptionchemical bonding to the inorganic component (hydroxyapatite) or organiccomponents (mainly type I collagen) of tooth structure
3.Diffusionprecipitation of substances on the tooth surfaces to which resin monomers canbond mechanically or chemically
4. A combination of the previous three mechanisms
The availability of new scientific information on the cause, diagnosis, and treatment of
carious lesions and the introduction of reliable adhesive restorative materials have
substantially reduced the need for extensive tooth preparations. Adhesive composite
restorations are used to replace carious dental tissues, to restore fractured teeth, and to
replace missing enamel or dentin in the cervical areas of teeth.With improvements in
materials, these indications have progressively shifted from the anterior segment only to
posterior teeth as well.
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 the shape and the color of anterior teeth
3. Improve retention for metallic crowns or for porcelain-fused-to-metal crowns
4. Bond all-ceramic restorations (Fig. 5-1)
5. Bond indirect resin-based restorations
6. Seal pits and fissures
7. Bond orthodontic brackets
8. Bond periodontal splints and conservative toothreplacement prostheses
9. Repair existing restorations (composite, amalgam, ceramic, or ceramometal)
10. Provide foundations for crowns
-
7/29/2019 Concepts of Enamel and Dentin Adhesion
2/12
11. Desensitize exposed root surfaces
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 (Fig. 5-2)
15. Bond prefabricated and cast posts
16. Reinforce fragile roots internally
17. Seal apical restorations placed during endodontic surgery
ADVANTAGES OF ENAMEL ADHESION
As measured in the laboratory, shear bond strengths of composite to phosphoric acidetched
enamel usually exceed 20 MPa (range 1-145 MPa).
Such bond strengths provide adequate retention for a broad variety of procedures and
prevent microleakage around enamel margins of restorations.
Adhesive restorations provide other benefits, such as cusp reinforcement after tooth
preparation.
Adhesive restorations substantially reinforce remaining enamel and dentin, making them
less susceptible to fracture.
Enamel Adhesion
Acid etching transforms the smooth enamel into an irregular surface (Fig. 5-3) 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 in the
material polymerize, and the material becomes interlocked with the enamel surface (Fig. 5-
4).16,62
The formation of resin microtags (micromechanical interlocking) within the enamel surfaceis the fundamental mechanism of resin-enamel adhesion.
Consequently, most current phosphoric acid gels in use have concentrations of 30% to 40%,
although some studies using lower concentrations have reported similar adhesion values.
-
7/29/2019 Concepts of Enamel and Dentin Adhesion
3/12
-
7/29/2019 Concepts of Enamel and Dentin Adhesion
4/12
-
7/29/2019 Concepts of Enamel and Dentin Adhesion
5/12
surface-free
energy to levels
compatible with a
more hydrophobic
restorative material
One-bottle
Total-Etch (E + [PB])
Removes the smear layer
Exposes intertubular and
peritubular collagen
Opens tubules in a funnel
configuration
Decreases surface-free
energy
Penetrates into the dentin tubules to form resin tags
The first coat applied on etched dentin works as a primerit increases
the surface-free energy of dentin
The second coat (and third, fourth, and so on) acts as the bonding agent
used in three-step
systemsit fills the spaces between the dense network of collagen
fibers
Two-Bottle
Self-etch ([EP] + B)
The SEP does not remove the smear layer, but fixes it
and exposes about 0.5-1 m of
intertubular collagen because of its acidity (pH = 1.2-
2.0)
The smear plug is impregnated with acidic monomers,
but it is not removed
When it impregnates the smear plug, the SEP
prepares the pathway for the penetration of
the subsequently placed fluid resin into the
microchannels that permeate the smear plug
Uses the same type of bonding agent
included in the three-step, total-etch
systems.
The resin tags form on resin penetrationinto the microchannels of the primer-
impregnated smear plug
All-in-one
Self-etch
(EPB)
Etches enamelIncorporates the smear layer into interfaceBeing an aqueous solution of a phosphonated monomer, it demineralizes and penetrates dentinsimultaneously, leaving a precipitate on the hybrid layerForms a thin layer of adhesive, leading to low bond strengths; a multi -coat approach isrecommended
Incompatible with self-cure composite resinsIncorporates the smear layer into interface
Being an aqueous solution of a phosphonated monomer, it demineralizes and penetrates dentin
simultaneously, leaving a precipitate on the hybrid layer.
Forms a thin layer of adhesive, leading to low bond strengths; a multi-coat approach is recommended
Incompatible with self-cure composite resins
*The term etch and rinse is an alternative term for the total-etch systems.
-
7/29/2019 Concepts of Enamel and Dentin Adhesion
6/12
**For good adhesion, close contact must exist between the adhesive and the
substrate (enamel or dentin). The surface tension of the adhesive must be
lower than the surface energy of enamel and dentin.
Bonding to Tooth Structure
Mechanical adhesion: Penetration of resin into tooth to form tags
Adsorption adhesion : Chemical bonding to HA or collagen
Diffusion adhesion : Precipitation of substances onto tooth surface that resins can attach to
(mechanical or chemical)
Electrostatic adhesion : Involved in metal bonding
Bond to Enamel
Macrotags : Interprism resin penetration
Microtags : Intraprism resin penetration
Microtags provide the major retention
Challenges in Dentin Bonding
(1)Composition of dentin:*50 % V= HA (less mineral)
*25 % V= collagen
*25 % V= water includes tubules-(more water)
(2) Dentin Structure and Organization:
**dentin is not uniformly mineralized: peritubular dentin is more mineralized than intertubular
dentin**tubule density changes with location: More holes and more hole area as pulp is approached
**Dentin changes with time: Sclerotic dentin -more mineralless collagen
(3) Dentin covered by Smear Layer**adherent layer of tooth debris, saliva and bacteria
**covers surface and plugs tubules to reduces dentin permeability
**weak cohesion and adhesion (cannot be removed with air/water syringe)
(4) Smear layer and Bonding
**interferes with resin bonding
-
7/29/2019 Concepts of Enamel and Dentin Adhesion
7/12
**newer dentin bonding systems (DBS) either remove smear layer (total-etch) OR alter
smear layer (self-etch)
When primer and bonding resins are applied to etched dentin, they penetrate the
intertubular dentin, forming a resin-dentin interdiffusion zone, or hybrid layer. They also
penetrate and polymerize in the open dentinal tubules, forming resin tags.
Current Dentin Bonding Systems Available
3 step total-etch technique
2 step total-etch technique
2 step self-etch technique
1 step self-etch technique
(1)Total-Etch 3 step Technique
Step 1: Total etch (35% H3PO4)
Smear layer removed
Surface dentin demineralized
Dentin vulnerable to desiccation
Step 2: Hydrophilic Primer applied
Re-wets dentin
Establishes Hybrid layer
Step 3: Adhesive
Composite placement
Limitations:
Too many steps
dentin moisture variable
some sensitivity reported
(2)Total-Etch 2 step Technique
Step 1: Total etch (35% H3PO4)
Smear layer removed
Surface dentin demineralized
Dentin vulnerable to desiccation
Step 2: Primer/Adhesive mixture applied Re-wets dentin (not as effective)
Establishes Hybrid layer
Composite placement
Limitations:
1-Too many steps 2-dentin moisture variable3-most sensitivity reported
Ways to minimize post-op sensitivity when using total etch techniques
-
7/29/2019 Concepts of Enamel and Dentin Adhesion
8/12
1- Do Not desiccate dentin
Keep dentin moist
2-Rewet with wetting agent
Water, Gluma, dentin primers, multiple adhesive layers,
3-RMGI liner or base prior to etching
(3)Self-Etching 2 Step Technique
Step 1: Apply Self-Etching Primers
Smear layer modified
Surface dentin demineralized
Hybrid layer developed
Step 2: Apply Adhesive
More Complete seal and betterpenetration
Composite placement
1-Lower sensitivity reported
2-Concerns about stability
3-Seal does not appear to be as good
(4)Self-Etching 1 Step Technique
Step 1: Apply Self-Etching Primer/Adhesive mixture
Smear layer modified
Surface dentin demineralized
Hybrid layer developed
-
7/29/2019 Concepts of Enamel and Dentin Adhesion
9/12
Composite placement
1-Lower sensitivity reported
2-Concerns about stability
3-Seal does not appear to be as good
Advantages Self-etch adhesives:
Extremely easy to apply, no etch, no rinsing! In vitro, they result in high dentin bond strengths
Major Disadvantages of Self-etch adhesives:
Acidity - 2.5 > pH > 1.1, therefore they do not etch enamel to the same depth as H3PO4
May result in clinical signs of enamel leakage at 1 year (Opdam et al., 1998)
Thermal cycling (30,000X) decreases enamel bond strengths (Miyasaki et al., 2000).
Their acidity may inhibit the polymerization of self-cured composite resins (Tay et al., 2002).
==========
Clinical Application of Bonding:
Direct anterior composites
Direct posterior composites
Indirect composites
Porcelain veneers, inlays, crowns
Fiber-reinforced posts
Periodontal splints
Adhesive amalgam restorations
Endodontic sealer
Reattachment of fractured tooth fragments
Pit and fissure sealants
-
7/29/2019 Concepts of Enamel and Dentin Adhesion
10/12
Orthodontic brackets
Root desensitization
Cuent Strategies for Adhesion of Resins to Dentin* Current Strategies for Adhesion of Resins tontin*p
Challenges in Dentin Bonding Substrate.
Bonding to enamel is a relatively simple process, without major technical requirements or
difficulties.
Bonding to dentin presents a much greater challenge.
Several factors account for this difference between enamel and dentin
bonding.
(1)Dentin contains a substantial proportion of water and organic material, primarily type I
collagen.
(2)Dentin also contains a dense network of tubules that connect the pulp with the
dentinoenamel junction (DEJ) .
(3) Movement of fluid from the pulp to the DEJ is a result of a slight but constant pulpalpressure. Dentinal tubules enclose cellular extensions from the odontoblasts and are in direct
communication with the pulp.
(4)Bond strengths are generally less in deep dentin than in superficial dentin.
(5)Whenever tooth structure is prepared with a bur or other instrument, residual organic and
inorganic components form asmear layer.The smear layer fills the orifices of dentinal
tubules, forming smear plugs.
The removal of the smear layer and smear plugs with acidic solutions results in an increase
flow of fluids from the pulp through the tubules.
When primer and bonding resins are applied to etched dentin, they penetrate the
intertubular dentin, forming a resin-dentin interdiffusion zone, or hybrid layer. They also
penetrate and polymerize in the open dentinal tubules, forming resin tags.
-
7/29/2019 Concepts of Enamel and Dentin Adhesion
11/12
From PickardsCompositesThe majority ofresin composites are inherently hydrophobic (water-hating) materials and so
some form of intermediary bonding resin needs to be applied to the tooth.
Bonding to enamel
The bonding of composites to enamel is dependent upon the mechanical retention of resin tagswithin the highly mineralized enamel.These tags created by the etching action of acids
(phosphoric acid has been most widely used).
Currently, the etching times with phosphoric acid vary between 1530 seconds dependingon the state of the tooth.
Enamel is a substrate that can be dried relatively easily and the use of intermediate hydrophobic
resins dictates such an approach, even though vigorous post-etch drying for 1020 seconds will
cause collapse of the exposed hydroxyapatite crystals. In this way, the ideal enamel etch pattern
would show clinically as a frosty appearance awaiting these resin intermediaries.
Bonding to dentine
Bonding to dentine presents a more difficult problem because it is a living, wet tissue and
contains less mineralized tissue than enamel. Resin-based adhesives have been developed forbonding to dentine and these generally work by removing the surface smear layer and infiltrating
the exposed dentine. Whilst it is possible to dry out dentine after etching, in the same way as
enamel, the net result may prevent satisfactory resin penetration for some of the bonding agentsin use. This is because the drying causes the fragile, exposed collagen network to collapse and
the resin cannot penetrate this collapsed structure.
The main problem of dentine bonding is that a hydrophobic resin composite is required to stick
to a watery substrate.
Resin technology has now developed such that it is possible to produce a bonding agent in a
solvent that will infiltrate and stick to wet dentine (Fig. 6.10) and maintain the bond over a
significant time period (Fig. 6.10h).
Bonding to wet dentine (and enamel)Most dentine bonding agents require smear layer removal with weak acids. There are two ways
of doing this. The more established systems use a separate etch stage with phosphoric acidfollowed by the bonding agent, whilst others combine acid and the bonding agent and are
therefore called self-etching.
Separate etching stage
The smear layer is removed with acid (Fig. 6.10ac) and then the bonding agent is applied (Fig.6.10d and e). This dentine bonding system uses alcohol and water as the solvent base for the
hydrophilic resin: these materials will be capable of re-wetting a dried collagen surface, although
their penetration time into the substrate may be slightly longer than the more volatile solvent
bases such as acetone.
Solvents such as acetone mix freely with water and therefore require a moist tooth surface forthorough dentinepenetration in the technique described as wet bonding. This does not mean
bonding through a pool of blood and saliva, but it does mean that the tooth surface should not bedried out too much as this will cause collapse of the exposed collagen networkand enamel
crystalliteswith grossly inadequate penetration of the bonding agent. Judging the correct
degree of wetness in these situations is a problem. Some suggest that surplus water should beremoved by the use of a cotton-wool pledget. For these materials, the desiccating effect of an air
blast and the frosted appearance of etched enamel should be avoided (cf., Fig. 6.10c).
-
7/29/2019 Concepts of Enamel and Dentin Adhesion
12/12
A low-molecular-weight hydrophilic resin (e.g. Hydroxy Ethyl MethAcrylate: HEMA), in a
suitable solvent (e.g. water, alcohol, acetone), is flooded over the altered surface of the
dentine (Fig. 6.10df). It then flows into the dentinal tubules and the porous intertubular dentine
between them. The type of solvent in use is critical for the method of handling the bonding
agent.
Data sheets may give some indication of content or, alternatively, it is a reasonable test to smellthe bonding systembefore use (although dont make a habit of it) to determine the type of
solvent in use.
Having acted as a good wetting agent on the dentine and enamel, layers of increasinglyresinous hydrophilic material are applied and the relative stiffness of the resin component
is increased by gentle evaporation of solvent. Many systems therefore involve either a
lightcuring stage or a self-curing reaction. The dentine and patient are consequently embeddedin an increasingly hydrophobic and stiff resin matrix which acts as a firm foundation on which to
build a composite restoration. The region between the unaffected dentine and the restoration is a
mixture of both and so is called the hybrid zone (Fig. 6.10f).
Another name for it is the interdiffusion zone which also indicates the 5
the resins into the intertubular dentine.After curing, all resin-based materials will have a thin layer of slippery, greasy, unpolymerized
material on the surface where they are exposed to the airthe air-inhibited layerandthis allows increments of material to be bonded to each other.
The most important consideration for achieving a sound, reliable bond is the avoidance of anycontamination between these layers.
Self-etching adhesivesA problem with bonding agents that have a separate etching stage is that successful bonds will
only be achieved if the adhesive resin penetrates to the depth of demineralization caused by the
acid-etching. Materials that use a self-etching combination will not suffer from this problembecause the two components are already combined (Fig. 6.12). They will also not require a
separate washing and drying stage before the application of the bonding agent, so helping to
reduce the risk of contamination. Self-etching adhesives would therefore appear to offer the ideal
solution of simple handling and technique insensitivity, giving excellent dentine bonds, but theevidence for successful enamel bonding with these materials has been less convincing (Fig.
6.12).
These materials may be applied as two separate stages (Fig. 6.12) or alternatively as onecomponent (Fig. 6.13b).
New techniques for cavity preparation and finishing will offer huge scope for the development of
simpler bonding agents.