Principles of Adhesion (Operative Dentistry)

PRINCIPLES OF ADHESIONDR. SARANG SURESH HOTCHANDANIDentist, Bibi Aseefa Dental College, SMBBMU.Larkana, Sindh, [email protected]

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05/02/2023 07:35 PM SARANG SURESH HOTCHANDANI 2

Mechanisms of Adhesion

Mechanical adhesion

Adsorption adhesion

Diffusion adhesion

Electrostatic

adhesion


Mechanical Adhesion

In this type, solidified adhesive interlock micro mechanically in surface roughness and irregularities of adherend by formation of resin tags within tooth surface.


Adsorption Adhesion

In this type, adhesive makes chemical bonding with substrate or adherend (inorganic hydroxyapatite & organic collagen fibers of tooth).

Contain all types chemical bonds like; Primary bonds (ionic & covalent) Secondary valence bonds

Hydrogen bonds, Van der waals forces (dipole interaction, London dispersion)


Diffusion Adhesion

In this type, adhesion or bonding occurs between mobile molecules.

Polymers from one surface come out and react with other surface and eventually adhesive disappears and both parts become one. Precipitation of substance on tooth surface to which

resin can bound mechanically or chemically.


Electrostatic Adhesion

In this two surfaces are joined by electro static forces.

Usually one surface is resin and other is metal.


Requirements of Good Adhesion The materials which are being joined should be close as much as

possible.

Adhesive should have Sufficient wetting of the adhesive means spread of liquid.

Measured by contact angle of droplet placed on adherend. Low contact angle.

High the contact angle less the wetting, complete wetting means 0-degree contact angle. Low surface tension of adhesive.

affinity for one another that causes them to stay together rather than interact with the surface they contact

Adherend must be rough Increases the surface area & increase the potential for adhesion.

Adhesive should not be too viscous nor too fluid.



Requirements of Good Adhesion Adherend should have high surface energy.

In nature, there is a desire for all objects to seek a reduced energy state, simply because reduced energy is the most stable condition.

Surfaces, in general, are of higher energy than the internal aspects of an object because molecules present at the surface have unsatisfied bonds.

In other words, molecules on surface would prefer to be covered” by other molecules to satisfy their bond complexes and reduce their overall energy state.

This covering can occur by; oxygen, water, or other molecules.

The higher the energy of the surface, the more receptive it is to being bonded to by another material, such as an adhesive.

Methods of Increasing Surface Energy Surface cleaning by pumice or prophylactic paste Etching with acids Cleaning with solvents to remove contaminants


Adhesion to Enamel is easy than Adhesion to Dentine!!

Enamel has high surface energy due to presence of only hydroxyapatite

Dentine has low surface energy and is difficult because it contains hydroxyapatite of high surface energy while collage fibers of low surface energy.

We need isolation of tooth for adhesion because if cavity is contaminated by saliva it will decrease the surface energy of surface and prevents the adequate wetting by adhesive.


Adhesion to Enamel is easy than Adhesion to Dentine!!

And also during cavity preparation smear layer is produce which is also of low surface energy which will prevent good adhesion as described above, that’s why we remove this smear layer after cavity preparation.

GIC is only dental cement to possess intrinsic adhesive property and does not require intermediate resin to bond restoration with tooth material, while composite requires an intermediate bonding agent to micromechanically bond with etched enamel or dentine.


FACTORS AFFECTING ADHESION TO TOOTH SURFACE


Strength and durability of adhesive bond depends on several factors.

Physicochemical properties of adhesive and adherend

External stresses which reduce the process of bonding.

Mechanism of transmission & distribution of applied load through bond.

Changes in oral environment.


Strength and durability of adhesive bond depends on several factors.

Structural properties of adherend (enamel & dentine)

Hypo mineralized & eroded tooth structure is difficult to bond because of weakened tooth structure.

Hyper mineralized teeth will require longer time of etching because they are resistant to demineralization.

Dentine sclerosis Fluorosis Fluoride treatment

Surface contaminants during cavity preparation (smear layer)


COMPOSITIONAL AND STRUCTURAL ASPECTS OF ENAMEL AND DENTINE.


Composition of Enamel

Inorganic content (Hydroxyapatite)

95% - 98% by weight (wt. %)

86% by volume (vol. %) Organic content

1% - 2% by weight 2% by volume

Water 4% by weight 12% by volume

The inorganic portion is arranged in crystallites form arranged in three dimensional way called as prism or rods whose ends are shaped as keyhole pattern.

Enamel is homogenous while dentine is heterogeneous.

A prismatic or prism less enamel is present on outer surface in which crystals are parallel to each other and perpendicular to the surface.


Composition of Dentine

Inorganic (hydroxyapatite)

70% by weight 50% by volume

Organic (type 1 collagen) 18% by weight 25% by volume

Water 12% by weight 25% by volume

Dentine is permeable because it contains dentinal tubules which radiate from pulp to enamel-dentin junction.

These tubules contain odontoblastic processes and establish direct connection to pulp.

The diameter and number of dentinal tubules decreases from pulp to enamel-dentine junction.

2.5 micron (diameter) & 45,00o/mm2 (number) near pulp

0.8 micron (diameter) & 20,000/mm2

(number) near Enamel-dentine junction. Average 30,000 tubules/mm2 in middle part

of dentine.



Each tubule is made of peritubular dentine containing odontoblastic process inside it and these dentinal tubules are separated by intertubular dentine.

Intertubular dentine is less mineralized and contain more organic content.

In the deepest 1/3rd of dentinal tubules it contains; Dentinal fluid Organic membrane called lamina limitans Intra-tubular collagen fibrils



Dentinal tubules are arranged in fan shaped manner. 96% superficial dentine is composed of intertubular dentine, 1% of dentinal fluid,

3% peritubular dentine because number and diameter of dentinal tubule decreases as they move from pulp to EDJ.

Near the pulp, 12% intertubular dentine, 66% peritubular dentine 22% dentinal fluid.

Dentine is intrinsically wet tissue. Dentinal fluid is under outward pressure from pulp.

Intra-pulpal fluid pressure is 35-30 mmHg or 34-40 cm of water.


Changes in Dentine Structure

Dentinal sclerosis

Tertiary or reparative dentine formation

Hyper mineralization & obstruction of dentinal tubules


SMEAR LAYER


The Smear Layer

It is iatrogenically produced layer of debris calcific in nature, produced by reduction or instrumentation of dentine, enamel or cementum that prevent interaction with underlying pure tooth tissue.


Characteristics of Smear Layer

Smear layer is produced by rotary or hand instruments in cavity preparation, root canal preparation, root planning, so to obtain good bonding tooth surface should be cleaned.

The burnishing action of instrument produces frictional heat and shear forces which makes the smear layer to get attached to tooth surface and prevent it being rinsed off.

Following conditioners are being used for removing smear layer; (Conditioners are chemicals which help in removal of smear layer)

EDTA (most effective) Acidic Conditioners

Citric acid Polyacrylic acid Lactic acid Phosphoric acid

Cavity cleansers (slight effect) Tubulicid Hydrogen peroxide



Morphologic features, composition and thickness of smear layer depends upon;

Instrument used Method of irrigation/ instrumentation Site of tooth

Smear layer seen in SEM to be of 0.5 to 2 micro meter in thickness, granular in structure with irregular surface.

The orifices of dentinal tubules are obstructed by debris tags of smear layer called as smear plugs which may extend in dentinal tubule up to 1 – 10 micro meter.



The smear layer is made of; Crushed Hydroxyapatite Denatured collagen Bacteria Saliva

The smear layer reduces dentinal permeability by 86%.

Smear layer thickness increases with increasing the size & roughness of bur.



Smear layer have weak attachment to tooth and is brittle in nature and decrease the effective bonding. To overcome this effect two procedures are used as mentioned under;

Etch and rinse approach In this procedure, smear layer is removed before application of adhesive or

bonding. Self-etch approach

In this procedure, bonding agent is applied without removal of smear layer which penetrate it.

After removal of smear layer dentinal permeability increases by about 90%.



Adhesive techniques that require removal of smear layers lead to post-operative sensitivity, pulp damage, and post-operative pain due to opening of dentinal tubules, micro gaps & micro leakages.

New etch and rinse adhesive use 30% to 40% phosphoric acid gel for conditioning

Alternative conditioners are; maleic acid, nitric acid, citric acid, tannic acid.

Polyaleknoic acid is used in GIC.



In Endo, Chelating agent EDTA is used to remove smear layer while sodium hypochlorite is applied to remove organic remnants of bacteria..


Internal & External Dentinal Wetness

Internal dentinal wetness refers to wetness of dentine or cavity surface from fluid of dentinal tubules, while external dentinal wetness occurs due to environmental humidity.

Dentine is highly permeable and high permeability results in easy penetration of bacteria and toxins to pulp and easy exudation of dentinal fluid.


Internal & External Dentinal Wetness

When removal of smear layer occur it makes the bonding surface wet which will have following effects; Fluid competes at all hard tissue sites by hydrolysis Interfere with polymerization of adhesives or

bonding agents.

That’s why Dentine bonding agents should be hydrophilic for effective bonding.


Dentinal Permeability & Dentinal Wetness Depends Upon;

Diameter and length of tubules Deep the cavity more the wettability

because in deep cavity diameter of dentinal tubule increase while length is decreased.

Occlusal dentine is more permeable over pulp horns than at center

Viscosity & molecular size of dentinal fluid

It is inversely proportional to internal dentinal wetness

Pressure gradient It is directly proportional

Surface area available for diffusion

Wider the cavity more IDW Proximal dentine is more permeable

than occlusal dentine Coronal dentine is more permeable

than root dentine.

Patency of tubules Sclerotic dentine or tubules blocked

by smear layer has less permeability

Rate of removal of substance by pulpal circulation.


Wetting of Adhesive

For good bonding the adhesive must sufficiently wet the surface and should have low viscosity and be able to displace air and moisture during bonding process.

Minimum contact angel for good adhesion should be less than 15 degrees.

Primers in current system contains; (Primers are bonding promoters)

Surface active agent which enhances the wettability of hydrophobic adhesive bonding agent

HEMA (2-hydroxyethyle methacrylate) Solvents for removal of air and moisture

Ethanol or acetone


Polymerization Contraction

The linking of monomer into polymer during polymerization leads to contraction which may de-bond the restoration.

It creates the force which pull the adhesive from adherend.

High filler content decreases the contraction.

Current resins shrink up to 2.9 – 7.1 vol%.

Polymerization contraction induces contraction/ shrinkage stress.


COMPENSATION FOR POLYMERIZATION SHRINKAGE


Flow or Plastic Deformation

Plastic deformation of set resin may relax the contraction stresses.

As the setting proceeds, contraction and plastic deformation decreases due to increase in stiffness or modulus of elasticity of material.

Fast setting light curing resins exhibit less relaxation of contraction stress by plastic deformation.



While self-curing or auto curing shows relaxation with plastic deformation due to availability of time.

That’s why marginal adaption is better with self-curing resin due to high time for deformation to occur for relaxation and also due to presence of air bubbles which provide space for flow.

So for better marginal adaption curing rate should be low and formation of bond should be rapid.



Flow or plastic deformation depends on C Factor (configuration factor)

It is ration of bonded surfaces to un-bonded surfaces in restoration. Example; in Class-1 cavity there are 5 bonded surfaces and one un-bonded

surface so the C factor will be 5. Increase in contraction factor lead to increase in contraction stress in

restoration. Only the un-bonded surface is responsible for plastic deformation.

Other methods to compensate for polymerization shrinkage. Placement of glass ceramic blocks into soft resin composite before light curing

Displace the resin and provide space like air bubbles. Use of mega-filled resin Use of pre-polymerized resin Use of inlays


Hygroscopic Expansion

It is swelling of resin due to absorption of fluid.

It my decrease polymerization shrinkage and marginal leakage by expanding the resin.

Micro-filled resin shows 2 & ½ times more water than macro filled because of greater volume of resin in micro filled.


THE ENDReference: Summit’s Fundamentals of Operative Dentistry 4th Edition

Education

Principles of Adhesion (Operative Dentistry)