Dentin bonding agents includes history ,adhesion in enamel, adhesion in dentin, generations of dentin bonding etc

DENTIN BONDING
AGENTS
-PRESENTED BY
Dr RakshitaN Sheelvanth

CONTENTS
1.Introduction
2.History
3.Adhesion in dentistry
4.Bonding to enamel
5.Bonding to dentin
6.Classification
7.Dentin bonding agents
8.Evolution of dentin bonding agents

8.Role of water in bonding process
9.Additives for enamel-dentin adhesion
10.Bonding to interradicular dentin
11.Factors effecting success of adhesives
12.Water treeing phenomenon
13.Conclusion
14.References

Introduction
Adhesion has replaced traditional
restorative materials.
Thorough bonding:
•Improves retention and stabilization of
a restoration.
•Prevents excessive removal of sound
tooth structure and
•Reducesmicroleakage.
•Reduces post operative sensitivity,
marginal staining and recurrent caries

History
In 12
th
century : dental bonding:
•Forming undercutsin cavity
preparations for amalgam
restorations
•Luting using zinc phosphates and
other non adhesive cements

In 1856 prefabricated
ceramic inlays, used as
esthetic filling sealed
with gold foils
Herbst developed
fired ceramic inlays
in 1882
In 1888
fabrication of fired
ceramic inlays
over platinum foil
was developed by
hoffman-axthelm
Nyman in
1905,combined
resin adhesives and
porcelain etching ,
to bond the
restoration to tooth
Fletcher in 1871
developed
silicate
cements.

Advent of unfilled
resin: 1937, used for
esthetic fillings by
Blumenthal in 1945
Development
of epoxy
molecule by
swisschemist
castanin 1938
In 1949, Oskar Hagger
developed the first
bonding agent,
SevritonCavity Seal.
This system was based
on glycerophosphoric
acid dimethacrylateas
a self-adhesive or self-
etching component for
both enamel and
dentin bonding.
Demonstrated the interest in using
these molecules for bonding
restorative materials to tooth and at
the same time provided the first
description about ‘hybrid layer’ by
Mc lean and Kramer in 1952

•In 1955, Dr. Michael Buonocore, regarded as the
pioneer of ‘Adhesive Dentistry’.
•Observed previous fundamental works.
• Concept of acid-etching ,enhancing adhesion .
•Some automobile industries used phosphoric acid
to treat metal surfaces to obtain better adhesion of
the primer to metal .
•He applied this idea to enhance the adhesion of
resin to enamel surfaces
•Used 85% phosphoric acid
Michel Buonocore

•Bowen : BisGMA (bisphenol glycidyl methacrylate),
monomer used in modern composite resin materials .
•In 1965, ( a surface active comonomer)
NPG-GMA (N-phenylglycine-glycidyl methacrylate)
•Conclusion : NPG-GMA (coupling agent) improves bonding between
methacrylate resin and hard tooth tissues - primarily by interaction
with mineral phase rather than with organic phase of tooth
structures. This formed the basis of development of NPG-GMA origin
dentin bonding agents, (1st generation).
•Eg; Cervident (SS White)

1949 :Oskar Hagger:
first system of
bonding acrylic resin
to dentin
1955:Buonocore :
resin would bond
better to phosphoric
acid etched enamel
1956: Bowen : worked
on bisphenol A-
glycidyl methacrylate
resin and subsequently
formed 1st composite
material – 1st gen

1979: Fusayama: Total etch
concept of dentin and enamel
1982:
Nakabayashi:
concept of
hybrid layer
1992 :
Kanka: wet
bonding
technique
Self etching bonding systems developed
6
th
Gen: 1990
7
th
Gen : 2002
8
th
Gen : 2010

Classification of dentin
bonding agents
The bonding agents are classified as follows:
1. According to generations
2. Based on mode of application
3. Based on number of steps
4. Based on etching pattern

Classification according to generations
1.1
st
generation bonding agents
2.2
nd
generation bonding agents
3.3
rd
generation bonding agents
4.4
th
generation bonding agents
5.5
th
generation bonding agents
6.6
th
generation bonding agents
7.7
th
generation bonding agents
8.8
th
generation bonding agents

Classification - based mode of application
1.Adhesives which modify smear layer. They require one or two steps.
they use a single adhesive or primer and adhesive.
•One step - Prime and Bond 2.1
• Two step - Optec Universal Bond
2. Adhesives which completely remove smear layer .
Two step: involves dentin conditioning followed by combined primer
and adhesive.
Three step: involves separate conditioning priming and bonding
application.

•Multiple bottle - eg : All-Bond 2, Scotchbond
multipurpose
• One bottle - eg : OptiBond SOLO, One step
3. Adhesives which dissolve the smear layer, rather than
remove it. The process is accomplished in two steps using a
combined conditioner and primer (self-etching primer)
followed by application of adhesive resin.
•Advantage: 1 no. rinsing: Quick application
4. Less postoperative sensitivity than total etch.9

Generation
1
st

2
nd
3
rd

4
th
5
th

6
th

7
th

Steps
3
3
3
2
2
1 or 2
1
Description
Etch enamel, apply
adhesive
Etch enamel, apply
adhesive
Etch enamel, apply
primer
Total etch, apply
primer
Total etch apply
adhesive
Apply self-etch
adhesive
Apply self-etch
adhesive
CLASSIFICATION BASED ON
NUMBER OF STEPS

Classification based on etching pattern
•the bonding agents could either be total-etch or self-etch.
1. TOTAL ETCHING TECHNIQUE
•Total etching technique may be as follows:
• Multibottle (fourth-generation, examples: All-Bond 2,
multipurpose ScotchBond)
•One bottle (fifth-generation, examples: Prime and Bond
NT, single bond).

2. SELF-ETCH SYSTEM
•In this method, there is no separate etching and
rinsing.
(i) decreases clinical application time
(ii) Reduces technique sensitivity.
A self-etch approach involves either a two or one-step
application procedure. These can be divided into
(i) self-etching primer
(ii) self-etching adhesives.

Adhesion in dentistry
• Adhesion : force or the intermolecular attraction that
exists between molecules of two unlike substances
when placed in intimate contact with each other
•Adhesive : substance added to produce the adhesion .
•Adherend : material to which adhesive is applied

Chemical
adhesion
Primary
valence
forces such
as covalent,
ionic or
metallic
bonds.
Physical
adhesion
Secondary
valence
forces.
Vander Waal
forces,
dispersion
forces ,
hydrogen
bonds
Mechanical
adhesion
Penetration at
microscopic level.
The formation of
hybridized dentin
Resin polymers
become entangled
with collagen fibrils.
Types of adhesion

FACTORS AFFECTING
ADHESION
•1. Surface energy:
energy created by
surface atoms of
substrate to strongly
attract adhesive
molecules.
•surface hardness α
surface energy α
adhesion

•2. wetting : the easy flowability of liquid over surface of
substrate and adhere to it.
•Wetting ability of liquid adhesive α surface energy and
cleanliness of adherend
•3.contact angle: measure of wettability and angle formed
by the adhesive with adherend at interface
•Smaller the contact angle ,greater is the wettability of the
adhesive.

Ideal requisite of bonding agent
•Biocompatibility.
•Effective bonding
•Fracture resistance against masticatory forces.
•Mechanical properties close to tooth structure
•Resistant to degradation in oral environment.
•Easy to handle.
•Ensure proper seal of restoration to prevent recurrent
decay, pulpal injuries and dentinal sensitivity

Bonding to enamel
In stagnant states, the proteinaceous organic smear layer which
covers enamel, attract micro organisms forming plaque.
Removal of this organic layer (10μm) ,expose inorganic layer,
making enamel more reactive is known as ‘conditioning’ .
Demineralization of superficial calcium ion is known as ‘etching’.
Adequate bond strength: prophylaxis , careful isolation.
Phosphoric acid : etching/conditioning of enamel.

Phosphoric acid as etchant
•50 % phosphoric acid for 60 seconds:
monocalcium phosphate monohydrate precipitate
•Concentrations below 25%:
dicalcium phosphate dihydrate precipitate
•30- 40% phosphoric aid for 15 sec is considered
appropriate.
•Alternative acids:2% sulphuric acid for 30 seconds,
pyruvic acid, 10% maleic acid ,2.5 nitric acid etc.

Acid Etch Technique
•Acids used: 30-40% phosphoric acid, sulphuric acid for 30 seconds,
pyruvic acid, 10% maleic acid ,2.5 nitric acid etc.
• Conditioning : Less concentration of acid for smaller period of time .
•Etching: higher concentration of acid for longer duration of time.
•Effectively etched surface on drying gives a matt white or frosted
appearance.
•Care should be taken to avoid ruining of enamel tags formed.
•Resin is applied , it flows into microporosities (5- 50 μm), and
polimerizes to form resin tags.

Acid Etch Technique
Isolation ,
surface
treatment
•Rubber Dam
Etchant
application
•37% phosphoric
acid for 15 sec
Rinse
with
water
Dry
White
frosted
appearance
confirms
proper
etching
•Rinse with stream
of water for 20 sec
•Dried with
warm air
•Cleaned with pumice

Permanent teeth -15sec Primary teeth - 45-60sec
Teeth with fluorosis - 60sec
Etching time

Patterns of etching in enamel
•Type I: removal of enamel
prism core with prism
peripheries remaining intact.
•Honeycomb like
appearance.
•Type II: removal of
peripheries of enamel prism,
leaving the cores intact.
•Gives cobblestone
appearence

Patterns of etching in enamel
•Type III: etching pattern
resembles type I and type II,
associated with presence of
prismless enamel.
•Type VI: pattern has patches of
pitted surface seen in cervical
third of enamel.
•It displayed only a random
distribution of depressions with
no preferential destruction of
either cores or peripheries.

•Type V: the regions of enamel
are flat and smooth, lack micro
irregularities for penetration of
resin.
•seen in high flouride areas.

•Macrotags : Resin tags form in enamel rod peripheries
•Microtags : finer network of small tags across the end
of each rod where hydroxyapetite crystals have been
dissolved leaving crypts outlined by residual organic
matter

Bonding to dentin
Constituents
of dentin

•Treatment of dentin for optimum bonding: successful
bonding of resin composite to dentin: micromechanical
attachment between resin and demineralized, primed
layer of intertubular dentin : reffered as hybrid layer.
3--6μms

Role of Smear layer
in bonding
Tissues→ rotary
instrument→ layer of
organic film is formed.
(smear layer)
During RCT, In dentin, the
smear layer burnished into
the underlying dentinal
tubules→ smear plug
(lowers dentin
permeability, protective
effect)
It is also a very
weak cohesive
material and
interferes with
strong
bonding.

Application of
acid→ removes the
smear layer→
enhance its cohesive
strength.
In dentin :if smear
layer→ left in
place →barrier for
resin penetration;
bacteria entrapped
multiplies beneath
restoration

Step 1 :Conditioning of dentin
Types:
1. Chemical conditioning
2.Thermal conditioning
3.Mechanical conditioning.
Alteration
of smear
layer of
dentin
surface
Obtaining a
substrate
capable of
micro -
mechanical
and
chemical
bonding to
adhesive
Physical
effect:
altering
thickness
and
morphology
of smear
layer and
dentinal
tubules
Chemical
effect:
modifying
organic matter
and
decalcification
of inorganic
portion

Chemical conditioning
1. Acid conditioners: demineralize superficial dentin.
Expose microporous collagen scaffold for resin to penetrate.
Amorphous layer covered on collagen fibers exposed on
intertubular dentin→ Denaturation and collapse of residual smear
layer collagen.
collagen smear layer→ reduces permeability of underlying dentin
Peritubular dentin is completely dissolved at the tubule orifice to
form a funnel shaped structure
Collagen fibrils at tubule wall gets exposed as they aid as retentive
sites.
Wet bonding technique maintains moist dentin surface.

•2. Chelators:
Remove smear layer
without decalcification,
without physical changes
on dentin substrate.
Chelate :compound with
a central metal ion
surrounded by covalently
bonded atoms/ions/
molecules called ligands
posessing additional
bonds for chemical
reaction.

Thermal conditioning
Mechanical conditioning
Lasers cause recrystallization of dentin resulting in fungi
like appearance .
This increases micro- retension and chemical adhesion of
restorative material to tooth structure
Microabrasion

Step 2: Priming
•Primers contain monomers with
1.Hydrophilic properties which bond with exposed
collagen fibrils and
2.Hydrophobic properties for co-polymerization with
adhesive resin.
•Primer promotes resin diffusion into moist
demineralized dentin.
•Aim : complete resin penetration.

•Incomplete surface coverage
•Incomplete interfibrillar
penetration
•Incomplete penetration to full
depth of demineralized dentin
Microscopic
examination
shows
deficiencies:
•Applied in multiple coats.
•Conditioning of dentinal
surface
•Retaining adequate water
content
•Time of conditioning ;15
seconds
Improve
surface
coverage and
penetration of
primer:

•Water helps in penetration of primer and adhesive
resin
I.Excessive drying of conditioned dentin collapses
the collagen network and prohibits the primer entry.
II.Over wetting results in emulsion polymerization of
adhesive resin due to separation of primer
components
III.blot drying with damp cotton pledget, ideal, moist
bonding surface of dentin without any excessive
water can be maintained.

Dentin bonding agents
Formation and
stabilization of
hybrid layer
Production of
resin tags in
unplugged
dentinal
tubules
Effectively
sealing them, to
reduce
permeability
Major role of
adhesive resin

The rest of adhesive resin enters into dentinal
tubules to a limited depth of 10-20µms forming
resin tags known as intratubular penetration .

Resins also penetrate and hybridize the walls of lateral
tubule branches forming sub micron resin tags, and the
process is called as lateral tubule hybridization

•When the resin is not completely
infiltrated into the collagen
meshwork ,the microspaces are
open to microleakage and this is
referred as ‘nanoleakage’.

TESTS TO DETECT MICROLEAKAGE
•Dye penetration
•Fluid filtration
•Dye extraction
•Air pressure method
•Electrochemical method
•Radioisotope method

Mechanism of dentin bonding
•Intervening layer of fluid :creates intimate microscopic
contact of composite resin with dentin.
•It fills the microscopic space, polymerizes and combines
with composite resin and components of dentin

Bonding to inorganic portion of dentin
•Occurs by ionic interaction
between Ca2+ ions on dentin
surface and negative charges on
group X of adhesive
•The bond strengths of Ca2+
bonding adhesives: when used
alone wont exceed >6Mpa
when used with dentin
conditioning 10-15Mpa
•Eg: bondite , scotch bond
clearfil ,prisma universal bond.

Bonding to organic portion of dentin
•Involves interaction with amino,
hydroxyl or carboxyl groups
present in collagen of dentin.
•Removal of H2 from any of these
groups allows combination with
chemicals present in bonding
agents.
•Eg: Dentin Adhesit, Gluma etc.

1
st
generation of dentin bonding agents
•These contained N-phenylglycine
and glycidyl methacrylate (NPG-
GMA)- a bifunctional molecule
•One end of this molecule bonds to
dentin while the other bonds
(polymerizes) to composite resin.
•As Ca2+ ions are mediators , it
bond stronger to enamel than in
dentin.
•First commercially available was
CERVIDENT.
Bonding of NPG portion to Ca2+
ions by chelation

•DISADVANTAGES
•Low bond strengths of only 2-3 MPa.
•Being hydrophobic in nature ,close adaptation to
hydrophilic dentin could not be achieved.
•Had low biocompatibility

2
nd
generation of dentin bonding agents
•Phosphate esters dentin bonding agents were
introduced
•containing unfilled resin like phenyl-P /BisGMA/
HEMA in ethanol.
•Its mechanism of action was based on polar
interactions between negatively charged phosphate
groups in resin and positively charged Ca ion in
smear layer.
•Bond strength: 2- 7 Mpa

•Examples:
•Clearfil bond system F.
•Bondlite ( Kerr).
•Prisma Universal Bond( Johnson and
•Johnson).
•DISADVANTAGES
•Low bond strength of 2-7 Mpa
•Hydrolysis of bonds between phosphate esters and
dentin on contact with moisture at dentin surface ,
liberating hydrochloric acid

3
rd
generation of dentin bonding agents
•Three step systems: BOWEN
•Was known as oxalate bonding system; acid ferric
oxalate /phosphoric acid as conditioner
•GLUMA( bonding system) utilizes EDTA Required
either removal, modification or dissolution of the
smear layer.
•Dentin etching (Introduced by Fusuyama)
•Primer application
•Application of an unfilled resin.
• Bond strength: 9-15 mpa

•Drawbacks:
•there was decrease in retention with time
•Technique sensitive and time consuming

4
th
generation of dentin bonding agents
•Total etch concept/Moist Bonding
•Hybrid layer :Nakabayashi 1982
•Impregnation of resin into demineralized dentin
followed by polymerization.
•Good Bond strength: 17-25 Mpa.
•No reduction in bond strength when applied to moist
surface.
•Can bond to mineralized tissue as well as metal,
amalgam, composite and porcelain

Drawbacks
•Multiple steps: Technique sensitive.
•Unless the primer and adhesive are applied
consecutively, the overlying composite resin will
not bond to the surface.
•Eg : imperva bond , solid bond,opti bond C
,Metabond

5
th
generation of dentin bonding agents
•Totally removes smear layer
•Based on Total etch concept
•2 step system
1. Etching
2. Priming + Bonding
•Moist bonding technique
•Good bond strength. 17-24Mpa
•Time saving and relatively simpler to use.
•Some agents have incorporated fluoride and
elastomeric components to improve marginal integrity

•Disadvantages:
•They lack many of the components to perform
multi-substrate bonding.
•Post operative sensitivity
(most common in situations that magnify effects of
composite polymerization shrinkage.)

6
th
generation of dentin bonding agents
•Primer and etchant →one step.( Self etching
primers)
•Phosphate derivatives of hydrophilic monomer such
as phenyl-P. and
•50% HEMA or other hydrophilic monomer, so they
both etch and prime the dentin
•Type I: Two step/ Non rinsing conditioner/ Self etch
primer systems
•Type II : One step/ Self etching adhesives/ All in one
systems(Requires mixing)
•Examples: Clearfil® SF Bond
(Kurarray),Simplicity (Apex)

•ADVANTAGES: bond strength: 32Mpa
•No need to acid etch with phosphoric acid.
•No post conditioning rinsing required.
•Reduced post operative sensitivity
•Simultaneous demineralization and resin infiltration.
•Less sensitive to degree of wetness and dryness.
•Low technique sensitivity
•DISADVANTAGES
•Less effective bonding of enamel
•Initial bond might deteriorate with aging, →premature
failures.
•Bonding to Sclerotic and caries affected dentin → May
inhibit set of self cure or dual cure resin materials.

Prompt L-PoP
Xeno III
Clearfil S3 bond

Prompt L-PoP
•No rinsing
•Quick apply
•No bottles
•No cross-
contamination

7
th
generation of dentin bonding agents
•Similar to the 6th gen but a desensitizing agent added
to overcome problem of hypersensitivity .
•Single-component, no-mix, one-step application
dental adhesive with an etchant , adhesive,
desensitizer, and photo-initiator
•They combine conditioning, priming & application of
adhesive resin but unlike 6th gen don’t require
mixing.
•Contains both hydrophilic & hydrophobic
components.

•Disadvantages
•complex nature of mixed
solutions - more prone to phase
separation & formation of
droplets within adhesive layer.
•Lack of Polymerization-thin
film
•Bond strength : 31Mpa
: i-Bond

8
th
generation of dentin bonding agents
•Introduced by voco America as futurabond DC, the
modified version of 7
th
generation
•It is one step dual-cured, non-filled ,self etch in single
use
•Bond strengtg: 34Mpa

Evolution of dentin bonding agents
1st
generation
•Cavity primers with low bond strength
2nd
generation
•Dentin-enamel bonding agents with improved bond strength to
etched enamel
3rd
generation
•Etching of dentin and partially removal of smear layer
4th
generation
•Total-etch technique and formation of the hybrid layer and
resin tags
5th
generation
•Simplification of clinical procedures: one-bottle systems and
self-etching primers
6th
generation
•One-step bonding systems with proper bond to enamel and
dentin; Type 1 are compatible with selfcured composites and
Type 2 are not
7th
generation
•Single bottle no mix self-etching adhesive, not compatible with
selfcured composites

Role of water in bonding process
Solutions used for etching contain water to ionize acids and
dissolve extracted minerals.
Rinsing removes this dissolved mineral and leaves 70% of
demineralized dentin covered with water → acts as
plastisizer and maintains collagen in expanded state &
preserve spaces needed for infiltration of resin
If dentin is dried or exposed to air, water evaporates leaving
collagen in collapsed stiffened state (surface tension forces).
This reduces the ability of applied agents to penetrate
collagen web.

When collagen fibrils are close, secondary forces
start becoming active between adjacent peptide
chains in collagen triple helix and thus increase
stiffness or modulus of elasticity for collagen
On addition of water ,collagen network
reexpands over a period of 10- 30 seconds

Schematic representation of
dentin matrix:
Mineralised
dentin
matrix;
Demineralised
dentin matrix
filled with
water, i.e. in a
plasticized state;
Collapsed,
stiffened, air
dried
demineralized
dentin matrix ;
Demineralized
dentin matrix
stiffened by
organic solvent
prior to air
drying

When water based primers are applied to dry ,
shrunken and demineralized dentin.
•low water conc of primer ;Water soluble resin will stiffen
the collagen and it will not completely re expand .
•when water concof primer is large enough to plastisize
the collagen faster than the resin solvent stiffens it ;
hydrophilic resin monomer (acetone /ethanol) will
infiltrate the network as it expands .
•Thus, complete penetration is expected with ‘self
wetting’ effect of water based primers used on dentin.
Dry bonding

•When water free acetone based
primers are used, they do not
effectively infiltrate the exposed
collagen network because of
absence of water, forming the so
called ‘hybridoid regions’.
And the layer is called as
ghost hybrid layer.

Ghost hybrid layer
•When the resin is not completely infiltrated into the
collagen meshwork ,the microspaces filled with air
are formed at the resin- substrate interface.
•This is refered as ghost hybrid layer.

Reverse hybrid layer
•Hybrid layer is formed by penetration of monomer in
dentin and its polymerization which is essential for good
bonding.
•When the hybrid layer is surrounded by more inorganic
layer , the collagen is no longer infiltrated by the resin-
based materials, but ratherthe resin monomers instead
occupy the space where the collagen was originally
present, it is termed as reverse hybrid layer.

Wet bonding
•An effective wet bonding uses
hydrophilic resin monomers
dissolved in water miscible organic
solvents like acetone and ethanol. On
application of such primers on wet
dentin, the water from it diffuses into
the organic solvent, while the latter
diffuses into the demineralized dentin
matrix and tubules carrying with it the
polymerizable monomers.

Over wetting phenomenon
Occurs when the water from dentin is not
completely removed by hydrophilic monomer.
The monomers are present in the form of globules
over the water layer and dentinal tubules.
They may present as ‘blister like’ structures on the
dentin surface with water being trapped beneath
the resin layer.

This decreases concentation of organic solvents
in the primer, thereby lowering the solubility of
monomers, bond strength by lack of resin tags
formation.

Additives for dentin-enamel adhesion
The addition of fillers : silica, improves bond strength→
thickener to increase viscosity, →adequate film thickness.
Prevents incomplete polymerization due to oxygen
inhibition.
Polysiloxane: Fluoride releasing, anticariogenicity → in
increased resistance to acid attacks; inhibits the carbohydrate
metabolism of dental plaque. The addition of fluoride→
improves the bond strength and opacity

Dyes: indicates the proper mixing of the components
by color change from yellow to pink. When the
adhesive is light cured, the color fades. Eg : One-up
bond and Tyrian SPE
Glutaraldehyde : disinfectant in earlier multi bottle
systems such as Syntac or Gluma-bond & iBond
MDPB, (methacryl-oyloxydodecyl pyridinium
bromide) : antibacterial component in various
adhesives

•It is different from
coronal dentin in luting
fiber posts to
intraradicular dentin
within the endodontic
space.
Bonding to intraradicular dentin

•Intraradicular dentin : non-homogeneous tissue
characterized by the presence of tubules extending
from the pulp to the tooth periphery
Intraradicular
dentin

•Classified depending on the density and distribution of the tubules:
•Peritubular dentin is constitutes a collar of hyper-mineralized tissue
and low content of type I-collagen fibrils
•Intertubular dentin is mainly composed of mineralized type I-
collagen fibrils

•As the number of tubules reduces toward
the apical region of intraradicular dentin,
the ratio between peritubular and
intertubular dentin greatly varies from the
apical to the coronal third.
•Moving toward the apex, the substrate
modification will induce changes in the
impregnation pattern of the adhesive
system, thus reducing peritubular dentin
infiltration and resin tag formation, while
increasing intertubular dentin
impregnation.

Fiber
post
Bonded
luting
cement
Internal
root
dentin
Dentinal
tubules
Type I
collagen
fibrils

•Studies report that bond strength to intraradicular dentin decreases
from coronal to apical third of canal and thickness of the hybrid
layer is reduced in apical third.
•This is responsible for the lower bond strength due to reduced
impregnation of the adhesive system at this level of the root canal