- INTRODUCTION

- ADHESION

- MECHANISM OF ADHESION

CHEMICAL ADHESION

· IONIC

· COVALENT

· MECHANICAL

PHYSICAL ADHESION

MECHANICAL ADHESION

DIFFUSION

ELECTROSTATIC ADHESION

- VANDERWAAL’S FORCES

- CRITERIA OF ADHESION

SURFACE ENERGY

WETTING

CONTACT ANGLE

- FACTORS EFFECTING ADHESION TO TOOTH TISSUE

COMPOSITIONAL AND STRUCTURAL ASPECTS OF ENAMEL AND DENTIN

CHANGES IN DENTIN STRUCTURE

SMEAR LAYER

INTERNAL AND EXTERNAL DENTIN WETNESS

WETTING OF THE ADHESIVE

POLYMERIZATION CONTRACTION OF RESTORATIVE RESINS

- ENAMEL BONDING

- DENTIN BONDING

FIRST GENERATION BONDING AGENTS

SECOND GENERATION BONDING AGENTS

THIRD GENERATION BONDING AGENTS

FOURTH GENERATION BONDING AGENTS

FIFTH GENERATION BONDING AGENTS

SIXTH GENERATION BONDING AGENTS

SEVENTH GENERATION BONDING AGENTS

- CHEMICAL CONCEPT OF BONDING

- NEW CLASSIFICATION OF BONDING AGENTS

- Comparison of bond strength of different

materials and BONDING systems

- sealing ability of bonding agents

- clinical effectiveness of bonding agents

- acidity of the bonding systems

- antibacterial effects of bonding agents

- PERMEABILITY OF SIMPLIFIED DENTINAL ADHESIVES

- WATER TREEING PHENOMENON

MORPHOGENESIS OF WATER TREEING PHENOMENON

· REMNANT WATER THEORY

· WATER FLUX THEORY

- NANOLEAKAGE

- WET VS DRY BONDING

- Adhesion Strategies – A Scientific Classification of

Modern Adhesives

Smear Layer-Modifying Adhesives

Smear Layer-REMOVING Adhesives

Smear Layer-DISSOLVING Adhesives

- ADVANTAGES AND DISADVANTAGES OF CURRENT

ADHESIVES

- Clinical steps in adhesion

- AMALGAM BONDING

- ROLE OF BONDING AGENTS IN PULP CAPPING

- ROLE OF BONDING AGENTS IN DENTIN HYPERSENSITIVITY

- ROLE OF BONDING AGENTS IN ENDODONTICS

- GLASS IONOMER ADHESIVES

- BONDING OF RESIN TO CERAMIC

- SUCCESS AND FAILURES OF ADHESIVES

- References

INTRODUCTION

One major problem in restorative dentistry is the lack of proper union between the restorative material and the tooth surface. The process of invention over a period of time has led to the development of various techniques and modalities, which help in adhesion thereby reducing the tooth restoration gaps. Certain materials have also been developed which possess the property of bonding to the tooth surfaces. Bonding or adhesion may be physical, mechanical or chemical and such restorations are known as bonded restorations.

Bonding improves retention and stability of a restoration without excessive removal of sound tooth structure. Bonding also facilitates repair and replacement of deteriorated fillings with little or no additional removal of tooth structure. Adhesive techniques have greatly expanded the horizon of esthetic dentistry. Repair of fractured teeth can be carried out using fractured tooth fragments thereby maintaining original esthetics. Adhesive agents are also useful in other clinical situations as in luting, for treating dentin hypersensitivity, in direct pulp capping procedures and beneath amalgam restorations.

Buonocore in 1955, applied acid to teeth to “render the tooth surface more receptive to adhesion.” Buonocore’s pioneering work led to major changes in the practice of dentistry.

Adhesion is the force or the intermolecular attraction that exists between molecules of two unlike substances when placed in intimate contact with each other. The substance which is added to produce adhesion is known as the adhesive and material to which is applied is known as the adherend.

So, strong bond can be created only in case of intimate molecular contact. For a good bond the distance between the interacting molecules must be less than 0.7 nm/ 0.0007 u.

Adhesion in the dentistry usually involves the following mechanisms:

a) Chemical adhesion is based on primary valence forces such as covalent, ionic or metallic bonds.

· Ionic – mutual attraction of positively and negatively charged ions.

· Covalent – arises when atoms share their electrons.

· Metallic – positive ions surrounded by gas of electrons.

b) Physical adhesion relies on secondary valence forces. Such forces occur at molecular dipoles e.g. Vanderwaals forces.

c) Mechanical adhesion is based on surface roughness and irregularities.

d) Diffusion is the adhesion that occurs as the result of bonding between mobile molecules. Polymers from each side of an interface can cross over and react with molecules on the other side. Eventually, the interface will disappear and two parts will become one.

e) Electrostatic adhesion state that an electrical double layer forms at the interface between a metal and a polymer, making a certain, yet obscure, contribution to the bond strength.

Vanderwaal’s forces

When one end of molecule acquires temporary positive charge and other negative charge resulting in electric imbalance known as electric dipole. These dipoles allow molecules to interact with one another to form bonds called as Vanderwaals Bonds named after Dutch Physicist and Chemist Johannes Diderik Vanderwaal. Vaderwaals forces are intermolecular rather than intramolecular these forces although weaker are quite appreciable in magnitude. But these forces fall off rapidly in magnitude with increasing of separation. If interatomic distance between two objects is less than 0.0007u” Vanderwall’s forces occur between them.

Certain authors have wrong conception that adhesive bond is primarily a chemical bond. This is however not the case. Most adhesives rely on Vanderwaals forces plus whatever mechanical adhesion they can achieve. If the surface is rough and porous, the mechanical adhesion will predominate. If the surfaces are smooth and polished, we must expect the Vanderwaals forces to do the job.

BOND ENERGY AND BOND DISTANCE

Bond type	Bond energy (KJmol^-1)	Equilibrium length (A⁰)
Primary
Ionic	600-1200	2-4
Covalent	60-800	0.7-3
Secondary
Hydrogen	50	3
Vanderwaal	60	4

Criteria for adhesion

a) Surface energy: the energy of a solid on the surface is comparatively higher than its interior. Inside a crystal, each atom is equally surrounded by atoms on all sides and the interatomic distances are equal, hence the energy is minimal, whereas towards the periphery, the atoms are not equally distributed. The surface atoms are get strongly attracted to each other resulting in surface tension. Because of this energy substances are attracted to the substrate surface. Higher surface energy of substrate provides high adhesive properties. Hard solids have specific free surface energy values between 500-5000 ergs/m². Harder the surface, higher the surface energy and higher the adhesive properties.

b) Wetting: it can be illustrated by simple experiment in which two glass slides however polished they may be, when placed in close contact with each other do not exhibit attachment. This owes to the presence of micro irregularities on the slides and is responsible for the two surfaces contacting only at their high spots or hills. The total surface area in contact is decreased and hence no perceptible adhesion takes place. Whereas if a film of water is now placed between them, it becomes increasingly difficult to separate the two. For producing such type of adhesion, the liquid must easily flow over the entire surface and adhere to the solid. This characteristic is referred to as wetting. The wetting ability of the liquid adhesive depends upon the surface energy. Higher the surface energy, greater would be the wetting ability.

c) Contact angle: it is a measure of wettability and is the angle formed by the adhesive with the adherend at the interface. Smaller the contact angle greater is the wettability of the adhesive. Ideally the contact should be zero for complete wetting.

Factors Affecting Adhesion to Tooth Tissue

1. Compositional and structural aspects of enamel and dentin

The inorganic content of mature enamel is 95% to 98% by weight (wt%) and 86% by volume (vol %); the primary component is hydroxyapatite. The remainder consists of water (4wt% and 12 vol%) and organic material (1to2wt%and 2vol%). The major inorganic fraction exists in the form of submicron crystallites, preferentially oriented in three dimensions, in which the spread and contiguous relationship of the crystallites contribute to the microscopic unit, called the rod or prism. The natural surface of enamel is smooth, and the ends of the rods and exposed in what has been described as a keyhole pattern. Operatively prepared surfaces expose rods in tangential, oblique, and longitudinal planes. Enamel is almost homogeneous in structure and composition, irrespective of its depth and location, except for some aprismatic (prismless) enamel at the outer surface, in which the crystallites run parallel to each other and perpendicular to the surface.

Dentin contains a higher percentage of water (12 wt%) and organic material (18 wt%), mainly type I collagen and only about 70 wt% hydroxyapatite. Structurally more important to adhesion are the volumes occupied by the dentinal components. There is, combined as much organic material (25 vol %) and water (25%) as there is inorganic material (50 vol%). In addition, these constituents are unevenly distributed in intertubular and peritubular dentin, so the dentinal tissue is heterogeneous. High protein content is responsible for the low surface energy of dentin (44.8 dynes/cm²). Numerous dentinal tubules radiate from the pulp throughout the entire thickness of dentin, making dentin a highly permeable tissue. These dentinal tubules contain the odontoblastic processes and form a direct connection to the vital pulp. In contrast to enamel, dentin is a vital and dynamic tissue. Because of the fan-shaped radiation of dentinal tubules 96% of a superficial dentinal surface is composed of intertubular dentin only 1% is occupied by fluid in the dentinal tubules and 3% by peritubular dentin. Near the pulp, peritubular dentin represents 66% and intertubular dentin only 12% of the area is occupied by water. Similar data demonstrate that 3% of the area of a cut surface consists of dentinal tubules in superficial dentin and 25% in deep dentin. Hence, dentin is an intrinsically wet tissue. Dentinal fluid in the tubules is under a slight, but constant, outward pressure from the pulp. The intrapulpal fluid pressure is estimated to be 25 to 30 mm Hg or 34 to 40 cm water.

M. Ogata et al. (2001) (JOD 26: 27-35) evaluated the influence of direction of tubules on bond strength to dentin. Four bonding systems namely Clearfill liner bond II, Imperva Fluoro Bond, Single bond and one step were used.

Results concluded that micro tensile bond strength of the groups with tubules parallel to the bonded interface (as is seen in the case of lateral walls of cavity) was higher that that of tubules cut perpendicular

3 reasons were given:

· The parallel group the percent of the total interface area occupied by the hybrid layer was greater due to absence of resin tags.

· The hybrid layer was thicker than perpendicular group because of the greater opportunity for radial diffusion of the adhesive from entire length of tubules into the intertubular dentin.

· Intrinsic wetness is less in the parallel group.

2. Changes in Dentinal Structure

Dentin undergoes physiologic dentinal sclerosis as part of the aging process and reactive sclerosis in response to slowly progressive or mild irritations, such as mechanical abrasion or chemical erosion. Tertiary or reparative, dentin is produced in the pulp chamber at the lesion site in response to insults such as caries, dentinal procedures, or attrition. Sclerosis is reported to result from the obstruction of dentinal tubules by apposition of peritubular dentin and precipitation of rhombohedral mineral crystals. Sclerotic dentin usually contains few, if any, patent tubules and therefore, has low permeability. Heavily sclerotic dentin has areas of complete hypermineralization without tubule exposure, even when etched with an acid.

All of these morphologic and structure transformations of dentin, induced by physiologic and pathologic processes, result in a dentinal substrate that is less receptive to adhesive treatments than is normal dentin.

3. The Smear Layer

When the tooth surface is altered by rotary and manual instrumentation during cavity preparation, cutting debris is smeared over the enamel and dentinal surfaces, forming what is termed the smear layer. This iatrogenically produced layer of debris has a great influence on any adhesive bond formed between the cut tooth and the restorative material.

It has been suggested that the burnishing action of the cutting instrument generates considerable amounts of frictional heat locally and shear forces, so that the smear layer becomes attached to the underlying surface in a manner that prevents it from being rinsed off or scrubbed away. The thickness of the smear later has been reported to vary from 0.5 to 5.0 µm.

4. Internal and External Dentinal Wetness

The variability in dentinal permeability makes dentin a more difficult substrate for bonding than enamel. Removal of the smear layer creates a wet bonding surface on which dentinal fluid exudes from the dentinal tubules. This aqueous environment affects adhesion, because water competes affectively, by hydrolysis, for all adhesion sites on the hard tissue. Early dentin bonding agents failed primarily because their hydrophilic substrate. In addition, bond strengths of several adhesive systems were shown to decrease as the depth of the preparation increased, because dentinal wetness was greater. No significant difference in bond strengths is observed between deep and superficial dentin when the smear layer is left intact. Bond strengths of more recent adhesive systems that remove the smear layer appear to be less affected by differences in dentinal depth, probably because they’re increased hydrophilicity provides better bonding to the wet dentinal surface.

In addition to internal dentinal wetness, external dentinal wetness, or environmental humidity, has been demonstrated to negatively affect bond strengths to dentin

Y Nakaoki et al. (2002) (J.O.D. 27: 563-568) studied the effect of residual water on dentin bond strength and hybridization of the one bottle adhesive system.

They used single bond and results showed tensile bond strengths at various levels of dryness and wetness of dentin as follows:

Wet/dry Condition	Bond Strength MPa
Overwet	5.2
Blot dry	12.6
One second dry	11.9
Desiccated	4.4

5. Wetting of the Adhesive

Intimate molecular contact between the two parts is a prerequisite for the development of strong adhesive joints. This means that the adhesive system must sufficient wet the solid surface, have a viscosity that is low enough to penetrate the micro-porosities, and be able to displace air and moisture during the bonding process. In one study, the wetting characteristics of six adhesives were compared and judged to be sufficient with contact angles of less than 15 degrees. Primers in currently available systems usually contain hydrophilic monomers, such as 2- hydroxyethyl methacrylate (HEMA), as surface- active agents to enhance the wettability of the hydrophobic adhesive resins. In addition, solvents in modern primers, such as ethanol or acetone, ensure adequate removal of air and liquid by rapid evaporation.

If an adhesive monomer has a polarity and solubility similar to those of a polymer substrate, the monomer may act as a solvent for the polymer and infiltrate it.

In dental adhesive technology, the collagen phase of dentin is a polymer, and both the primer and adhesive resin contain monomers that penetrate the exposed collagen layer to form a micro mechanical bond. If a given conditioner conveys to the dentinal surface a specific polarity and solubility, the primer must match these to achieve penetration. The same is true for the adhesive resin applied to the primed dentinal surface.

6. Polymerization Contraction of Restorative Resins

High filler loading of the restorative resin matrix reduces polymerization contraction, current resin composites shrink 2.9 to 7.1 vol% during free polymerization

Contraction stresses within resin of up to 7 MPa have been reported.

In clinical situation, the developing bond of the restorative material to the cavity walls restrains the curing contraction. This restriction induces polymerization contraction stress, which counteracts the developing resin- tooth bond by pulling the setting resin composite material away from the cavity walls.

Fast – setting light- curing resin composites exhibit less flow – related stress relief, while self- or autocuring resin composites give the developing adhesive bond to dentin more time to survive.

Restriction of flow is affected by the configuration of the restoration, known as the C-factor is the ratio of bonded ( flow- active) to unbonded or free ( flow- active) surfaces. An increase in the number of bonded surfaces results in a higher C-factor and greater contraction stress on the adhesive bond.

Enamel bonding

Enamel surfaces generally are inert fully reactive and under normal circumstances maintain homeostasis with the surroundings. The objective is to remove the contaminates and subsequently raise the energy and reactivity of enamel surfaces. The procedure that leads to this removal of organic layer, making the enamel surface more reactive is known as conditioning or etching. The importance and potential exploitation of this fact was first appreciated by Buonocore in 1955. He demonstrated a 100 fold increase in retention of small buttons of polymethyl methacrylate to incisors in vivo when enamel was etched with 85% phosphoric acid for 2 minutes.

Purpose of acid etching

· Increase the surface area

· Increase surface roughness or irregularities for micromechanical bonding

· Increase surface energy by removal of surface contaminates with creation of reactive polar sites.

· Removes residual pellicle to expose the inorganic crystallite component of enamel. Normally surface of enamel is covered by a layer of pellicle which has an extremely low surface energy (28mJ/m²). This layer is exposed and removed and layer with high surface energy and high reactivity is raised (42mJ/m²).

Mechanism of attachment

Buonocore suggested that formation of resin tags was primary attachment mechanism of resin to phosphoric acid etched enamel. Resin tags which form between enamel rod peripheries are called macrotags. Microtags are present at the cores of prisms.

Acid etching removes about 10 mm of the enamel surface and creates a microporous layer from 5 to 50 µm deep.

Factors on which etching depends apart from acid factors

· Enamel instrumentation: formation of surface layer

· Fluoride composition

· Prismatic or aprismatic nature of enamel

· Type of tooth being restored: primary or permanent

Complete isolation and polishing of surface

Highly polar nature of surface contaminates oil, dust blood will prevent non polar resin from closely adapting to enamel surface.

Use of rubber dam is preferred over cotton rolls.

Concentration of phosphoric acid

· Phosphoric acid is used in concentration between 30 and 50% with 37% being most commonly provided.

· Concentrations greater than 50% results in formation of monocalcium phosphate monohydrate on etched surface that inhibits further dissolution and can be rinsed off.

· Concentration less than 27% forms dicalcium phosphate dehydrate on etched surface that cannot be easily removed.

Etching patterns

Three kinds of etching patterns have been suggested by Gwinett AJ (1971)

Type I: Predominant dissolution of prism cores.

Type II: Predominant dissolution of prisms peripheries.

Type III: No prism structure evident.

ETCHING TIME:

Earlier 60 seconds with 30% - 40% phosphoric acid but various clinical and experimental studies have reduced it to a brief episode of 15 seconds as reported by Swift EJ (1995).

FORM

Acid etchants may be available in liquid or gel form. Gel etchants are preferred due to ease and control of placement. Gels are often made by adding colloidal silica, this will result in higher ph. They are applied to surface either with a brush or dispersed through syringe onto enamel.

Bond strength

Gilatrick RO (1991) reported in vitro bond strength to acid conditioned enamel in the range of 16-31 MPa.

Enamel bonding agents are commonly based on BIS–GMA, developed by Bowen in 1962, or urethane dimethacrylate (UDMA). Both monomers are viscous and hydrophobic and are often diluted with other monomers of higher hydrophilicity and lower viscous such as triethylene glycol dimethacrylate (TEG-DMA) and HEMA. The bond between enamel and the restorative material is established by polymerization of monomers inside the microporosites

Laser etching

Laser etching is a phenomenon of process of continuous vaporization and micro explosions due to vaporization of water trapped within hydroxapatite matrix. The amount of surface roughening is dependent upon type and wavelength of laser. CO and Nd:YAG lasers are proved be most effective. Laser etching has provided the same surface characteristics as of normal acid etching.

Etchants ranked by demineralization potency

Etchant	Composition	Etch time (sec)	ph	Deminer-alization (um)
Clearfil CA Agent	10% Citric acid, 20% calcium chloride	15	0.10	0.5
Gluma 2000	1.6% oxalic acid, 2.6% aluminum nitrate	15	1.38	0.7
Mirage ABC conditioner	10% citric acid, 20% calcium chloride	15	0.42	1.7
Clearfil CA Agent	10% Citric acid, 20% calcium chloride	40	0.10	0.9
Amalgambond	10% Citric acid, 3% ferric chloride	10	0.59	1.3
Ultra-etch	10% phosphoric acid	15	1.31	1.7
Ultra-etch	35% phosphoric acid	15	0.02	1.9
Scotchbond Multi-Purpose	10% Maleic acid	15	0.87	2.1
Mirage ABC conditioner	2.5% nitric acid	60	0.42	2.2
Mirage ABC conditioner	10% phosphoric acid	15	-	2.2
Ultra-etch	10% phosphoric acid	30	1.31	2.2
All-etch	10% phosphoric acid	15	0.48	3.0
All-etch	10% phosphoric acid	15	0.78	3.0
All-etch	10% phosphoric acid	15	-	3.0
Scotchbond etching gel	35% phosphoric acid	15	0.28	3.0
Aqueous phosphoric acid solution	10% phosphoric acid	15	0.48	3.2
ESPE Etching gel	32% phosphoric acid	15	-	3.9
Uni-etch	32% phosphoric acid	15	-	4.0
De Trey etch	36% phosphoric acid	15	0.26	4.3
Mirage ABC conditioner	10% phosphoric acid	15	-	4.5
Etch-Rite	38% phosphoric acid	15	0.29	4.6
Uni-etch	32% phosphoric acid	15	0.17	4.8
Aqueous phosphoric acid solution	37% phosphoric acid	15	0.43	5.0
Kerr gel etchant	37.5% phosphoric acid	15	-	5.6

Adhesion to Dentin

Successful bonding to enamel was achieved with relative ease, but the development of predictable bonding to dentin has been more problematic. Only recently have dentin adhesive systems produced results that approach those of enamel bonding and achieve a predictable level of clinical success. The chronological method of classifying dentin bonding systems, the generational classifications system is described.

First Generation Adhesives

Imitating his enamel acid-etching technique, Buonocore et al. in 1956 reported that glycerophosphoric acid dimethacrylate (GPDM) could bond to hydrochloric acid-etched dentinal surfaces. They became bifunctional organic monomers with specific reactive groups that were claimed to react chemically with inorganic calcium hydroxyapatite or with organic collagen component of dentin. The bond strengths attained with this primitive adhesive technique were only 2 to 3 MPa. However, in contrast to the 15 to 20 MPa bond strengths obtained to acid-etched enamel and the bond was unstable in water. After the failures of this early dentin-acid-etching technique, numerous dentin adhesives with complex chemical formulas were designed. The development of N-phenlyglycine glycidyl methacrylate (NPG-GMA) was the basis of the first commercially available dentin bonding agent, Cervident (SS White).

Second Generation Adhesives

Clearfil Bond System F (Kuraray) introduced in 1978, was the first product of a large second generation of dentine adhesives, such as Bondlite (Kerr/Sybron), J&J VLC Dentin Bonding Agent (Johnson & Johnson Dental) and Scotchbond among others. These products were based on phosphorus esters of methacrylate derivatives. Their adhesive mechanism involved enhanced surface wetting as well as ionic interaction between negatively charged phosphate groups and positively charged calcium.

It composed of an ethyl alcohol solution containing tertiary amine as the activator. The catalyst liquid was Bis GMA monomer contamianing a phenyl phosphate ester, benzoyl peroxide and methyl methacrylate.

The second-generation systems had modest bond strengths, seldom exceeding (5 to 6 MPa). Clinical trials of these dentin-bonding agents commonly met with poor results. It was speculated that clinical failure was due to inadequate hydrolytic stability in the oral environment and because they’re primary bonding was to the smear layer rather than to the underlying dentin.

Drawbacks in the first and second generations:

The reasons for limited success of these generations of bonding agents include:

a. Lack of adequate bond strength that could overcome contraction stresses during polymerization.

b. Being hydrophobic in nature, close adaptation to the hydrophilic dentin could not be achieved.

c. Biocompatibility was not appropriate.

Lack of sufficient knowledge about the presence and nature of smear layer. Moreover, the adhesive bonded to the smear layer rather than the dentin. As a result the bond achieved was limited by cohesive failure in the smear layer or a break at the smear layer-dentin interface.

Third Generation Adhesives:

The concept of phosphoric acid: Etching of dentin before application of a phosphate ester type of bonding agent was introduced by Fusayama et al (1979).

It became evident that smear layer had a negative influence on the performance of adhesive systems. To overcome this third generation dentin bonding agents were introduced which differed from early materials in that an additional step was employed to either modify or remove the smear layer before the application of actual adhesive. The extra step comprised of conditioning and priming of dentin but made the procedure more complicated and time consuming.

Three steps

· Conditioning: acidic solution removes smear layer, rinsing is done.

· Primer: Applied and dried such that hydrophobic groups exposed to create favorable surface of bonding agent.

· Application of bonding agent.

First system of the third generation was known as oxalate bonding system. It utilizes a solution of acidic ferric oxalate (2.5% nitric acid + ferric oxalate) on the enamel and dentin as conditioner. This is followed by the application of an acetone solution of NTG GMA(reaction product of N p-toludiene glycine and glycidyl dimethacrylate) and then an another acetone solution of PMDM (Pyromellitic dianyhdride + 2 HEMA). This technique gave a bond strength of about 15 MPa to both enamel and dentin. The problem with this system is discoloration due to ferric ions.

Extensive research in Japan has demonstrated a favorable effect of 4-META on bonding to dentin. 4-META contains both hydrophobic and hydrophilic chemical groups. In 1982, Nakabayashi et al. used this system to describe the micromechanical bonding mechanism that is used by current adhesive systems. With this system, dentin is etched with an aqueous solution of 10% citric acid and 3% ferric chloride, followed by the application of an aqueous solution of 35% HEMA and a selfcuring adhesive resin containing 4-META, methylmethacrylate (MMA) and TBB, the last as a polymerization initiator. Based on this technology, adhesive systems such as C&B Metabond (Sun Medical), Super-Bond Di-Liner and Amalgambond Plus (Parkell) are commercially available and have been reported to yield consistent results in vitro experiment.

Removal of the smear layer with chelating agents such as EDTA was introduced with Gluma.

Another approach to smear layer treatment was the use of Scotchprep, an aqueous solution of 2.5% maleic acid and 55% HEMA, followed by the application of an unfilled Bis-GMA (62.5%) /HEMA (32.5%) adhesive resin. Scotchbond 2(3M) was in fact the precursor of current self-etching adhesives, though the self-etching primer Scotchprep at that time was advocated to be used solely on dentin. Supported by excellent clinical results in diverse clinical trials, Scotchbond 2 was the first product to receive Provisional Acceptance from the American Dental Association, which was followed by Full Acceptance.

Drawbacks of third generation

· Maintenance of longevity of bond

· Bond strength decreases with time

Fourth Generation dentin bonding agents :

The recent concept of bonding was proposed by Nakabayashi (1991) in which diffusion an dimpregnation of resin in to partially decalcified dentin followed by polymerization created a resin reinforced layer or the ‘hybrid layer’. The fourth generation bonding systems rely on this hybridization for attaining attachment.

When adhesive resin is applied a part of it penetrates into the microporous collagen scaffoled of the intertubular dentin known as the intertubular penetration. Here it polymerizes and co-polymerizes with the adhesion promoters of primer to forma intermingled layer of collagen and resin termed as hybrid layer.

Conditioning of Dentin

Conditioning of dentin can be defined as any chemical alteration of the dentinal surface by acids (or, previously, by the calcium chelator EDTA) with the objective of removing the smear layer and simultaneously demineralizing the dentinal surface. The use of the term conditioner found its origin in the early 1990s when the application of acid etchants to dentin, in particular in the United States and Europe, was taboo because of its alleged harmful effects on the underlying pulp. Conditioners are most commonly used as the initial step in the clinical application of total –etch systems and dentin following the so-called total-etch technique.

Various acids, in varying concentrations, such as citric, maleic, nitric, and phosphoric, are supplied with various adhesive systems. After clinical application, these conditioners are generally rinsed off to remove any acid remnants and dissolved calcium phosphates.

In addition to removing the smear layer, this superficial demineralization process exposes a microporous scaffold of collagen fibrils thus increasing the microporosity of intertubular dentin. Because this collagen matrix is normally supported by the inorganic dentinal fraction, demineralization causes it to collapse. The formation of a relatively impermeable amorphous gel on top of the exposed collagen scaffold has been ascribed to the combined effect of denaturation and collapse of residual smear layer collagen. This demineralization process also changes the surface-free energy of dentin. The high protein content exposed after conditioning with acidic agents is responsible for the low surface-free energy of etched dentin (44.8 dynes/cm), which differentiates it from etched enamel. Wetting of such a low-energy surface is difficult, and adhesion is hard to achieve if the dentinal surface energy is increased by the use of surface-active promoting agents, or primers.

Primers

Primers serve as the actual adhesion-promoting agents and contain hydrophilic monomers dissolved in organic solvents, such as acetone or ethanol. Because of their volatile characteristics, these solvents can displace water from the dentinal surface and the moist collagen network, they contain monomers with hydrophilic properties which have the affinity for the exposed collagen and hydrophobic properties for the copolymerization with adhesive resin.

HEMA, described as essential to the promotion of adhesion because of its excellent wetting characteristics, is found in the primers of many modern adhesive systems. In addition to HEMA, primers contain other monomers, such as NTG-GMA, PMDM, biphenyl dimethacrylate (BPDM), and dispentaerythritol penta acrylate monophoshate (PE NTA)

Hybridization

Hybridization, or the formation of hybrid layer, occurs following an initial demineralization of the dentinal surface with an acidic conditioner, exposing a collagen fibril network with interfibrillar microporosities that subsequently monomers. This zone, in which resin of the adhesive system micromechanically interlocks with dentinal collagen, is termed the hybrid layer or hybrid zone.

Hybrid layer, three different layers or zones have been described, the top of the layer.

1. Consists of an amorphous electron-dense phase, which has been ascribed to denatured collagen.

2. The middle part of the hybrid layer contains cross-sectioned and longitudinally sectioned collagen fibrils separated by electron lucent spaces. These interfibrillar channels, which have typical dimensions of 10 to 20 nm, represent the areas wherein hydroxyapatite crystals had been deposited and have now been replaced by rein as a result of the hybrdization process.

3. Residual mineral crystals are sometimes scattered between the collagen fibrils .The base of the hybrid layer is characterized by a gradual transition to the underlying zone of dentin containing hydroxyapatite crystals enveloped by resin.

It has been suggested that excessive drying causes collapse of the collagen meshwork and prevents complete penetration of the subsequently applied primer and resin. A thin film of water present on the conditioned dentin helps suspend the collagen fibrils and create space between them for penetration. All Bond-2 and Scotchbond multipurpose are the earlier products.

All Bond-2 used an etchant of 35% phosphoric acid on dentin and enamel followed by the application of hydrophilic primer containing 2% NTG GMA (N Tolyglycine – glycidyl methacrylate) and 16% BPDM (biphenyl dimethcrylate) in ethanol or acetone. Subsequently, an unfilled resin containing BisGMA and HEMA is applied. Mean bond strength for this system is seen to be 21.4±7.8 MPa.

Scotchbond Multipurpose uses 10% maleic acid to etch both enamel and dentin. Primer is an aqueous solution of HEMA and poly alkenoate copolymers. The adhesive resin is a BisGMA containing HEMA.

Clearfil Liner Bond 2 (Kurarary) first used the concept of no-rinse self etching primer. The primer comprises of Phenyl-P, HEMA and 5 NMSA (N-Methacryloxyl 5 –amino salcyclic acid. The bonding resin contains MDP (10-methacryloxydecyldihydrogen phosphate) BisGMA and HEMA. 20-28 MPa bond strength was achived with this system.

Fifth Generation Dentin Bonding Agents :

This class of adhesives is also based on the concept of hybridzation and relies on the wet bonding technique. It differs from its predecessor in that it uses a one component resin i.e. after conditioning of enamel and dentin, the steps of priming and bonding are combined so that bonding is achieved with a one component formula.

Prime and Bond (Dentsply, Caulk) was the first system to be marketed amongst the fifth generation adhesives. Both Prime and Bond and its successor Prime and Bond 2.1 (with cetylamine fluorides) contain PENTA TEGDMA and an elastomeric UDMA resin in acetone. The system is highly sensitive to even mild dessication of acid conditioned dentin.

PENTA is multifuctional molecule and is believed to partially demineralize dentin, facilitating penetration of resin into it.

Opti Bond Solo (Kerr) is a recently developed one component version of Opti Bond and consists of HEMA, GPDM and BisGMA in an ethanol/water system.

Single Bond (3M) is also a recently developed one component version of Scotch Bond Multipurpose. The material contains HEMA, BisGMA, dimethacrylate resin and a unique methacrylate functions copolymer of polyacrylic and polyitaconic acid in water and ethanol solvent base.

Bond strengths for fifth generation systems are almost equal to those of fourth generation agents i.e 17 MPa-24 MPa.

Six Generation Adhesives

No need of etching rinsing and drying, so that risk of over etching and over drying eliminated. Self etching primers contain an acidic monomer and chemical polymerizing catalyst that successfully penetrates into mildly dematerialized collagen network.

In self etching primer, brief drying and then application of bonding resin. Therefore hybridized smear layer also becomes part of bond.

Mechanism of bonding of S. E primer (Chemical concept of bonding):

Hybridization, submicron hybrid layer, Resin tag less pronounced within such submicron hybrid layers, collagen fibres are not completely deprived of hydroxyapatite in contrast to total etch adhesives. This residual hydroxyapatite may serve as a receptor for additional inter molecular interaction with specific carboxyl or phosphate groups of functional monomers. This accounts for 15-20% of bonding.

6^th and 7^th Generation Bonding Systems

Ø 6^th and 7^th eliminate the need for etching with phosphoric acid by use of an acidic primer.

Ø 6^th and 7^th generation do not require rinsing.

The difference lies in that 7^th generation requires no mixing

i.e gn 6^th

Type I - S. E primer +Adhesive

Have components that are applied separately on to tooth

Type II- Self etching adhesives (3M) Prompt L. pop (Methacrylate phosphoric ester + HEAM + Bisphenyl A Diglucidyl ether), Xeno III (HEMA + Water + ethanol + 2,6 dietrbutyl p hydroxyl toluene) are first mixed and then applied.

Ø 7^th generation bonding agents (I- Bond) are self etching adhesives that require no mixing I- Bond – UDMA- 4META, acetone/ water solvent.

Newer classification of bonding

Type I: In this system three steps are involved. Etching is done separately and then primer is applied followed by application of bonding agent.

Type II: In this system two steps are involved. Etching is done separately followed by application of primer and bonding agent in one bottle.

Type III and Type IV: These are based on self etch concept. It may be a single component system containing etchant + primer + bonding agent or multicomponent.

Bond strength of the adhesive materials

De Munck et al (2003) reported that the bond strength with three step and two step etch and rinse adhesives was 39MPa and 40 MPa respectively. So when bonding to enamel and dentin etch and rinse adhesives still result in the highest bonding effectiveness.

A self etch approach has resulted in lowered bonding effectiveness. A bond strength value of around 30 MPa was obtained for a two step self etch adhesive. One step self etch adhesives produced significantly lower bond strength than two step self etch and etch & rinse adhesives. The bond strength was reported to be 16 MPa.

Comparison of bond strengths for different materials and systems

	Shear strength (MPa)
Enamel	90-200
Dentin	170
Composite	30-120
Enamel/enamel smear layer	4-6
Dentin/dentin smear layer	4-6
Dentin/bonding agent/composite	22-35
Enamel/bonding agent/composite	18-22
Enamel/bonding agent/amalgam	10-22
Dentin/smear layer/ GIC	6
Dentin/ no smear layer/ resin modified GIC	10-12
Enamel/ no smear layer/ resin modified GIC	8-12
Composite/ bonding agent/ resurfacing composite	10-27

Foong J et al (2006) [Aust Dent J. Sep;51(3):252-7] compared the microshear strength of self etch adhesives. One hundred and nineteen enamel specimens were bonded with either Clearfil Protect Bond (Kuraray) or Xeno III (Dentsply). Clearfil Protect Bond demonstrated higher and more consistent bond strengths than Xeno III. They concluded that All-in-one adhesive (Xeno III) seem to be less reliable than the two-step self-etching priming adhesive (Clearfil Protect Bond) when bonding to enamel.

Celik EU et al (2006) [J Adhes Dent. Oct;8(5):319-25] studied the shear bond strength of different adhesives to Er: YAG lasers prepared dentin. The samples were divided into seven groups. 1. Er:YAG laser (Key Laser 3, KaVo) + Clearfil Protect Bond (Kuraray); 2. Er:YAG laser + Clearfil tri-S Bond (Kuraray); 3. Er:YAG laser + 37% H3PO4 + Single Bond 2 (3M-ESPE); 4. Er:YAG laser + Single Bond 2; 5. conventional method + Clearfil Protect Bond; 6. conventional method + Clearfil tri-S Bond; 7. conventional method + 37% H3PO4 + Single Bond 2. Only the Er:YAG laser + Clearfil tri-S Bond group demonstrated significantly higher bond strengths vs conventionally prepared specimens. They concluded that Er:YAG laser irradiation did not adversely affect the shear bond strength of Single Bond 2 and Clearfil Protect Bond to dentin, whereas it increased the shear bond strength values of Clearfil tri-S Bond.

Atash R et al (2005) [Eur J Paediatr Dent Dec;6(4):185-90] studied to compare the shear bond and microleakage strength of four adhesive systems to the enamel and dentine. 120 bovine primary mandibular incisors were collected. The adhesives tested were Clearfil SE bond (SE), Adper Prompt L Pop (LP), Xeno III (XE), and Prime and Bond NT (PB). Shear bond strength values (MPa,) ranged from: on enamel 11.06 MPa to 5.34 MPa, in decreasing order SE, LP, XE and PB and on dentine 10.47 MPa to 4.74 MPa, in decreasing order SE, XE, LP and PB. The highest shear bond strength was achieved by Clearfil SE bond and the lowest by Prime and Bond NT. There was no significant difference concerning the sealing ability of the four adhesive systems.

Ernest CP et al(2004) [J Adhes Dent. Winter;6(4):293-9] determined the shear bond strength (SBS) of different established (Resulcin Aqua Prime & Monobond N: RA, Prompt L-Pop III: PLP) and experimental (AC-Bond: AC, AC-Bond + Desensitizer: ACD) self-etching adhesives in comparison to fourth (Optibond FL: FL) and fifth generation (Excite: EX, Gluma Comfort Bond: CB) adhesives. SBS in enamel: RA: 27.0+/-5.8 MPa, PLP: 15.9+/-3.4 MPa, AC: 28.1+/-4.4 MPa, ACD: 22.2+/-4.1 MPa, FL: 33.2+/-3.2 MPa, EX: 30.5+/-5.1 MPa, CB: 30.1+/-3.7 MPa. SBS in dentin: RA: 25.8+/-5.7 MPa, PLP: 20.7+/-2.9 MPa, AC: 27.0+/-4.5 MPa, ACD: 20.7+/-3.7 MPa, FL: 34.4+/-3.8 MPa, EX: 30.0+/-4.6 MPa, CB: 27.9+/-2.6 MPa. FL resulted in significantly (p < 0.002) higher SBS in enamel and dentin than RA, AC, ACD, and PLP, and in higher SBS to dentin than CB.

Koh SH et al (2001) [J Esthet Restor Dent.;13(6):379-86] compared the tensile bond strengths to extracted human dentin of four single-bottle (fifth-generation) and four multiple-bottle (fourth-generation) dentin bonding agents. Means and standard deviations (n = 10) of tensile bond strengths (MPa) for the single-bottle system was 21.3 MPa (6.7); for the multiple-bottle system it was 20.0 MPa (8.6).

Stalin A et al (2005) [J Indian Soc Pedod Prev Dent. Jun;23(2):83 -8] evaluated the tensile-bond strength, fracture mode (under SEM) and microleakage of total etching single bottle system (5^TH GENERATION) to self-etching adhesive system (6^TH GENERATION) in primary dentition. It was found that higher bond strength and less microleakage was found in self-etching adhesive system. It was concluded that the self-etching adhesive is better for bonding in primary dentition.

J Gorucu (2003) compared the fracture resistance.

Group	Fracture resistance (KgF)
Intact teeth	158.4
Bonding agent/composite	127.1
Amalgam bonding/amalgam	110.3

Marginal sealing effectiveness of adhesives

Marginal leakage has been defined as the clinically undetectable passage of bacteria, fluids, molecules or ions between a cavity wall and the restorative material.

Blunck and Roulet (2002) analyzed the marginal adaptation of various adhesives. After 1 year they reported that 93% of the restoration margins was gap free for three step etch & rinse adhesives (Optiobond FL) and 91% for two step self etch adhesives (Clearfil SE). Two step etch and rinse adhesives (Optibond Solo Plus) revealed significantly lower percentages of gap free margins of 80%. 48% of the margin length was gap free for one step self etch adhesives (Prompt L Pop).

Yazici R et al (2002) compared the microleakage of different generation of bonding systems.

Generation	Bonding agent	Total no. of specimens	Specimens showing leakage
Fourth (etch & rinse)	Optibond FL	15	4
Fifth (etch & rinse)	Gluma One Bond	15	3
Sixth (self etch)	Prompt-L- Pop	15	8

Ulker AE et al (2007) [J Contemp Dent Pract. Feb 1;8(2):60-9] compared the effectiveness of five self-etching and etch-rinse dentin-bonding agents in achieving a gap-free adaptation between the restorative material and the dentin in primary and permanent teeth. Statistical results of the SEM analysis revealed fewer gaps in the restorations made with self-etching dentin bonding agents than etch-rinse agents at the restoration-dentin interface in both primary and permanent teeth.

Gallo JR et al (2005) [Oper Dent. May-Jun;30(3):290-6] evaluated the clinical performance of a posterior resin composite used with a fourth- and fifth-generation bonding agent. Sixty-two Class I and II restorations were placed with half the restorations restored with Gluma Solid Bond (a fourth-generation bonding system, or total etch two-step system) and the other half restored with Gluma Comfort Bond and Desensitizer (a fifth-generation bonding system, or total etch one-step system). A modified USPHS scale was used to evaluate the restorations for marginal discoloration, recurrent caries, anatomic form, marginal adaptation and proximal contact. Statistical analysis revealed that at two years no significant differences were found between the two bonding agents.

Success rate/ Clinical effectiveness of dental adhesives

Clinical effectiveness of modern adhesives has significantly improved, allowing adhesive restorations to be placed with a high predictable level of clinical success.

Demunck et al (2003) studied the clinical performance of three step etch & rinse adhesives Optibond FL and Permaquist with a 100% and 96% retention rate at 5 years.

Dijken (2001) reported that two step etch and rinse performs clinically less favorably than conventional three step adhesives. Seven year retention rates of 84% and 79% with Clearfil Liner Bond and Optibond Dualcure.

Latta et al (2000) reported a still favorable 92% retention rate at three years for two step self etch adhesives.

One step self etch adhesives have favorable short term retention rates of 100% at 6 months and 96% at one year (Munoz, 2002).

Acidity of diverse adhesive solutions Van Meerbek (2003)

Adhesive	Classification	pH
Prompt L Pop	One step self etch	0.4
Prompt L Pop 2	One step self etch	0.8
XENO III	One step self etch	1.4
I Bond	One step self etch	1.6
Non Rinse Conditioner	Two step self etch	1.0
Adhesit SE primer	Two step self etch	1.4
Optibond Solo Plus	Two step self etch	1.5
Clearfil SE Bond primer	Two step self etch	1.9
Clearfil SE Bond plus primer	Two step self etch	2.0
Unifil Bond primer	Two step self etch	2.2
Panavia ED	Two step self etch	2.6
Optibond Solo Plus	Two step etch & rinse	2.1
Pime & Bond NT	Two step etch & rinse	2.2
Scotch Bond 1	Two step etch & rinse	4.7
OptiBond FL	Three step etch & rinse	1.8

Antibacterial activities of bonding agents

Meserret B (2005) studied the antibacterial activity efficacy of various bonding agents.

Diameters of zones of growth inhibition (mm)

Dentin bonding agent	pH	S.mutans	S.salivarius	L.acidophilus	L.casei
Optibond FL	2.0	18.4	15.0	21.4	18.6
Single Bond	4.5	-	-	-	-
Clearfil SE bond	2.0	24.5	12.4	13.7	24.2
Prompt L pop	1.0	20.4	14.3	14.6	20.6
Control		15.3	12.3	10.8	12.3

They reported that the pH determined the antibacterial efficacy of bonding systems.

Permeability of simplified Dentin Adhesives

Contemporary dentin adhesives are rendered very hydrophilic in order to improve their bonding to intrinsically wet substrates such as dentin. The incorporation of high concentrations of hydrophilic and / or ionic resin monomers in these adhesives increases their osmolalities and attraction of water, leading to increased water sorption. They behave as permeable membranes (Tay & others,2003) that permit rapid, through- and – through water movement across the polymerized adhesives.

Dentinal fluid transudation across simplified dentin adhesives has been shown to occur when they were bonded to non- carious, deep, vital human dentin.

The fact that fluid transudation rapidly occurs across polymerized dentin adhesives implies that there are interconnected porosities or channels within the adhesives that are responsible for rapid water movement within these adhesive membranes. Unfortunately, these very minute channels cannot be seen with standard electron microscopic techniques. As the water is lost after desiccation of the specimens and the channels collapse. This prompted the immersion of bonded specimens in electron-dense tracers, such as silver nitrate, before they were subsequently prepared for morphologic examination. Once silver nitrate is reduced to metallic silver grains, it remains trapped in those sites regardless of whether the specimen is dehydrated or not.

Water Treeing

Tay and pashly (2003) did not invent the term “water trees.” They borrowed this term from field of electrical engineering to reflect the similarities of the water channels seen in dentin adhesives with the microscopic tree-like channels that were identified in aged electrical insulation cables after water sorption. This well-known phenomenon was first reported in 1969 at the Electrical Insulation Conference, Boston, , USA, in a paper titled “ Deterioration of water-immersed polyethylene coating wire by treeing” ( Miyashita, 1969).water trees in polyethylene coated cables are submicroscopic, self-propagating, water-filled tracks that are formed electrochemically by the oxidation of the hydrophobic polymer into more hydrophilic moieties,

Tay and pashely (2003) hypothesize that water trees in dentin adhesives, together with nanoleakage within the hybrid layers, represent water-rich interfacial regions from which the leaching of hydrophilic resin components may occur readily and expedite the degradation resin-dentin bonds.

Morphogenesis of water Trees

1. Remanent water theory; It was initially thought that water trees were morphologic expressions of water that was incompletely removed from water contained in simplified dentin adhesives. It is also known that the inclusion of 2-hydroxymethacrylate (HEMA) in these adhesives makes it difficult to remove water completely from these adhesives. The addition of HEMA to water, lowered the rate of evaporation of water from the water-HEMA mixtures in a manner proportional to its effect on lowering the vapor pressure of water,

2. The “water Flux” Theory . Vital dentin, particularly, deep vital dentin, is highly permeable, because of the presence of relatively short and wide dentinal tubules and normal positive pulpal tissue pressure ( 14 cm H₂O OR 10.3 mm Hg;

Three types of fluid movement may occur through dentin: evaporative, osmotic and convective water fluxes. Both evaporative and osmotic fluxes may result in the permeation of water along regions within the adhesive where interchain segmental mobility is increased by the hydrogen bonding of retained bound water present in the adhesive mixture. Convective water flux is considerably more significant when smear plugs are removed by acid etching.

Nanoleakage. Effective dentin bonding depends upon the formation of a hybrid layer that is optimally infiltrated with adhesive resins. Incomplete resin penetration in the hybrid later permits nanoleakage to occur ( Sano et al, 1995a). “ Nanoleakage” was originally used to describe microporosities within hybrid layers that allow silver nitrate penetration to occur in the absence of gap formation between resins.When bonding to sound dentin, the extensive nanoleakage that was seen in the hybrid layers of the latest simplified self-etc adhesives may be attributed to the combined adverse affect of evaporative, osmotic and possibly convective water fluxes that result in an outward fluid movement from both the intertubular dentin and the dentinal tubules.. As the inward diffusion of acidic resin monomers demineralize intact dentin and dissolve smear layers and smear plugs, the outward, osmotically- induced water fluxes generated may dilute or even flush out the partially neutralized but still acidic, resin monomers from the partially demineralized dentin. The presence of dilute water in partially acidic adhesive may retard the polymerization of resin within the already resin-sparse interfibrillar spaces. This is probably the mechanism responsible for the manifestation of heavily silver-impregnated hybrid layers after silver tracer immersion.

W.W. Brackett et al. (2005) (J.O.D. 30-6, 733-738) they studied the effect of adding a hydrophobic layer on the single component self etching adhesive resin systems like Adper prompt L pop, i Bond G.I and XenoIII. The results concluded that addition of more hydrophobic resin layer significantly improved the bond strengths of single component systems.

Reason: Possible explanation could be that these hydrophobic layer limits diffusion of water in hybrid layer (nanoleakage), this nanoleakage could have otherwise resulted in inhibition of polymerisation and weakening of bond.

Wet-Vs Dry Bonding

After conditioning, the enamel and dentin surfaces should be properly treated to allow full penetration of adhesive monomers. On the enamel surface, a dry condition is theoretically preferred. On the dentin site, a certain amount of moisture is needed to avoid collapse of the exposed collagen scaffold, which impedes effective penetration of adhesive monomers. Consequently, in the treatment of enamel and dentin, it is difficult to achieve the optimal environment for both substrates. One way to achieve this goal is to keep the substrate field dry and to use adhesive systems with water-based primers to rehydrate and thus re-expand, the collapsed collagen network, enabling the resin monomer to interdiffuse efficiently. The alternative is to keep the acid etched dentin surface moist and to rely on the water chasing capacity of acetone-based primers. This clinical technique commonly referred to as “wet bonding”, was introduced by Kanca and Gwinnett in the early 1990s.

Clinically a shiny, hydrated surface is seen with moist dentin. Pooled moisture should be removed by blotting or be wiped off with a slightly damp cotton pellet. Excess water might dilute the primer and render it less effective. Hydrophilic primer monomers are dissolved in volatile solvents, such as acetone and ethanol. These solvents may aid in displacement of the remaining water as well as carrying the polymerizable monomers into the opened dentinal tubules and through the nanospaces of the collagen web. The primer solvents are then evaporated by gently air drying, leaving the active primer monomers behind. These monomers have hydrophilic ends with an affinity for the exposed collagen fibrils and hydrophobic ends that form receptors for co polymerization with adhesive resin.

When the water inside the collagen network is not completely displaced, the polymerization of resin inside the hybrid layer may be affected or, at least, the remaining water will compete for space with resin inside the demineralized dentin.

In such overwet condition, excessive water that was incompletely removed during priming appeared to cause phase separation of the hydrophobic and hydrophilic monomer components, resulting in blister and globule formation at the resin-dentin interface.

Even gentle drying of the dentin surface, for times as short as 3 seconds prior to the application of a water free, acetone-based primer, has been shown to result in incomplete intertubular resin infiltration resin infiltration. Ineffective resin penetration due to collagen collapse has been observed ultra morphologically as the formation of a so-called hybridoid zone. Wet-bonding technique appears technique-sensitive, especially in terms of the precise amount of moisture that should be kept on the dentin surface after conditioning.

Acetone quickly evaporates from the primer bottle so that, after the primer bottle should be immediately closed and the primer solution immediately applied to the etched surface.

Another disadvantage of keeping the cavity walls wet after conditioning is that one cannot observe the white, frosted appearance of the enamel that indicates that it has been properly etched.

Adhesive systems that provide water-dissolved primers have been demonstrated to bond equally effectively to dry or wet dentin.

Water based primers were capable of displacing the water that remained as part of the wet-bonding technique as well as the additional amount of water that was introduced with the primer, which evidently provides sufficient water to re-expand the air-dried and collapsed collagen scaffold, has been advanced as a reasonable explanation as to why these systems perform equally well in wet or dry conditions. Air-drying of demineralized dentin reduces its volume by 65%, but the original dimensions can be regained after rewetting.

In contrast to adhesive systems that provide acetone-based primers, adhesive systems that provide water-based primers appear less technique-sensitive and bond equally well to varying degrees of surface dryness and wetness.

Adhesion Strategies – A Scientific Classification of Modern Adhesives

The most common classification of adhesives is based on the time of their release on the dental market. Typically, generations are distinguished. However, this classification in generations lacks scientific basis and thus does not allow the adhesives to be categorized on objective criteria. A more logical classification of adhesives would be based on the number of clinical application steps and more importantly, on their interactions with dentinal substrate. Three adhesion strategies, distinguished by how they interact with the smear layer, are currently in use with modern dentin adhesive systems.

Smear Layer-Modifying Adhesives

Dentin adhesives that modify the smear layer are based on the concept that the smear layer provides a natural barrier to the pulp, protecting it against bacterial invasion and limiting the outflow of pulpal fluid that might impair bonding efficiency.

The interaction of these adhesives with dentin is very superficial, with only a limited penetration of resin into the dentinal surface. This shallow interaction of the adhesive system with dentin without any collagen fibril exposure, confirms the weak acidity of these smear layer-modifying primers. The dentinal tubules commonly remain plugged by smear debris.

Eg: Pro Bond, Prime & Bond 2.1, Compoglass.

Smear Layer-Removing Adhesives

Most of today’s adhesive systems opt for a complete removal of the smear layer, using a total-etch concept. Their mechanism is principally based on the combined effect of hybridization and formation of resin tags. These systems are, in their original configuration, applied in three consecutive steps and subsequently categorized as three-step smear layer-removing adhesives. With the newest generation of one-bottle or single-bottle adhesives, the conventional three-step application procedure of smear layer-removing systems has been reduced to two steps by combining the primer with the adhesive resin in one solution.

Smear Layer-Dissolving Adhesives

A simplified application procedure is also a feature of the smear layer-dissolving adhesives or “self-etching adhesives”, which use slightly acidic primers or so-called self-etching primers. These primers partially de-mineralize the smear layer and the underlying dentin surface without removing the dissolved smear layer remnants or unplugging the tubule orifices.

The current two-step smear layer-dissolving adhesives provide self-etching primers for simultaneous conditioning and priming of both enamel and dentin. Simplification of the clinical application procedure is obtained not only by reduction of application steps, but also by omission of a post-conditioning rinsing phase.

Systems	Smear layer modifying	Smear layer removing	Smear layer dissolving
One step	· Hytac OSB · Pertec universal bond · Prime & Bond 2.1 · Solist · Tokusu light bond
Two step	· Optec Universal Bonding · Pro Bond · Tokuso light bond (two step) · Tripton	· Fuji Bond LC · Gluma 2000 · Optibond Solo · Prime & Bond 2 · Scotch Bond 1 · Syntac Single Componenent	· Clearfil Liner Bond 2 · Denthesive 11 · Opti Bond · Imperva Bond · Scotch Bond 2 · Syntac · XR Bond
Three step	· All Bond 2 · Amalgam Bond Plus · Clearfil Liner Bond · Gluma · Imperva Bond · Mirage Bond · optiBond · Scotchbond Multipurpos · ScotchBond Mutipurpos

Advantages and disadvantages of current dental adhesiveS

Type of dentin adhesive	Advantages	Disadvantages
Self-etching multicomponent (Clearfil SE Bond, Clearfil Liner, Bond 2V, and Experimental ABF (Kuraray Co., Osaka, Japan), Etch & Prime 3.0 (Degussa, AG, Dusseldorf, Germany)	· No rinsing, · quick application · Less postoperative · sensitivity than total-etch adhesives · Results of clinical studies support use on sclerotic dentin · Bond well to dentin etched with phosphoric acid	· Clinically, may result in enamel microleakage due to a deficient enamel etch · Slight degradation of the hybrid layer over 1 year , in vivo
Self-etching “all in one”(Prompt L-Pop(3M ESPE), One-up Bond (Tokuyama Co.,Tokyo, Japan)	· No rinsing, very quick application · Results in an enamel etch pattern similar to that of phosphoric acid if enamel is instrumented · No bottles, disposable container (for Prompt L-Pop)	· Does not bond well to unprepared enamel · Has resulted in a wide range of bond strength value · Prompt L-Pop bonds better with compomers than with composites · One-Up Bond still requires mixing of two components with a brush · Needs multiple coats to bond effectively to dentin · Not indicated for indirect restorations
Total-etch multibottle (fourth generation) (All-Bond 2 (Bisco), EBS Multi (3M ESPE), OptiBond FL(Kerr Co.), Scotchbond Multi-Purpose (3M ESPE)	· Several research reports support its use on different substrates, including metals and porcelain · The highest dentin bond strengths among all dentin adhesives · Generally contain a dual-cure option for indirect restorations and bonded amalgams	· Multiple bottles make utilization cumbersome · Some bottles in the kit may never be used · Possibility of running out of primer A before primer B (or vice versa) · Because primer and adhesive resin are dispensed into the same plastic container, their sequential application may be inverted ·
Total-etch one bottle (fifth generation ) (Bond 1(Pentron Clinical Tenhnologies LLC, Wallingford , CT), Dentastic Uno/Duo (Pulpdent Co, Watertown,MA), Ecite (Ivoclar Vivadent, Sehaan, Liechtenstein), Integra Bond (Premier Dental Products, King of Prussia, PA) One Step (Bisco), OptiBond SOLO Plus (Kerr Co.), PQ1 (Ultradent, South Jordan, UT), Prime & bond NT (Dentsply) Single Bond (3M ESPE)	· Laboratory research supports use on enamel and dentin · Clinical studies up to 3 years show positive results · The one-bottle concept makes it extremely user-friendly	· For some one-bottle adhesives, lower bond strengths than their multibottle adhesive counterparts · Acetone-based adhesives may lose their efficacy with constant use · Acetone-based adhesives may need more coats than those recommended by the manufacturer · Thick adhesives may pool easily around the preparation margin · Some one-bottle adhesives are not compatible with self-cured or resin luting cements)

Critical steps in adhesion

Isolation

Before any bonding procedure is begun, adequate isolation and moisture control of the substrate to be bonded to must be achieved. Contamination of the substrate with external fluids, impeding effective contact between adhesive and bondong substrate. Salivary contamination is detrimental because saliva contains proteins that may block adequate infiltration of resin.

Dentin and pulp protection

Adhesive material such as glass ionomer cement can be used. In a deep cavity with a remaining dentin thickness of less than 0.5 mm and very permeable dentin such as in youg teeth, calcium hydroxide remains the material of choice. Its major disadvantage is that it rapidly dissolves if the cavity is not adequately sealed. Therefore when calcium hydroxide is used it must be covered by a less soluble conventional glass ionomer or resin modiofied GIC.

Universal enamel and dentin etching

Etching is done with phosphoric acid in concentration of 30%-40% for 15 seconds. In smear layer removing systems the conditioner should be thoroughly rinsed before application of the primer. If self-etch approach is used i.e. self etch primers containing acidic monomers. Self etch primers should be applied for 30 seconds and actively applied by rubbing the dentin surface. No rinsing is required.

Primer application

A primer application of at least 15 seconds should be respected to allow monomers to interdiffuse to the complete depth of surface demineralization. The primer should be actively rubbed into dentin surface with disposable brushes.

Adhesive resin application

In the final step of the bonding process the adhesive layer is placed. Spreading of adhesive resin should be done with a brush so as to have an optimal thickness of about 100 um. When placed in a sufficiently thick layer the adhesive resin may act as a stress relaxation buffer to counteract the polymerization contraction stress of resin composite and also aid in absorbing masticatory forces. For light cured bonding agents, the adhesive resin should always be cured before the application of composite to have a stable resin tooth bond. Because of the oxygen inhibition, the top 15um of the adhesive resin will not polymerize, but will provide sufficient double methacrylate bonds for copolymerization with restorative resin.

Amalgam bonding

The use of adhesive technology to bond amalgam to tooth tissue is an application of universal or multipurpose adhesive systems. Adhesive systems such as All Bond 2, Amalgambond Plus, Panavia and Scotch Bond Multipurpose Plus have been advocated for bonding amalgam to enamel and dentin. The nature of bond between resin and amalgam is yet unclear but appears to involve at least micromechanical as amalgam interlocks with the fluid resin during condensation. Because amalgam does not allow light transmission these amalgam bonding systems must have autopolymrizing capability. In vitro bond strength of amalgam to dentin are generally less than10 MPa which is less than bond strengths of resin composite to dentin.

Dental amalgam is strongly hydrophobic whereas enamel is hydrophilic, hence a wetting agent needs to be incorporated into the bonding resin that can wet both hydrophobic and hydrophilic surfaces.

The use of amalgam bonding techniques has potential benefits. Retention gained by bonding lessens the need for removal of tooth structure to gain retention or for retentive devices such as dovetails, groves and pins. Bonded amalgam may increase the fracture resistance of restored teeth and adhesive resin liners may seal the margin better than traditional cavity varnishes with decreased risk for post operative sensitivity and caries recurrence.

For the use of bonding resin in amalgam restorations, current research suggest the improved efficacy of filled resin compared with unfilled or minimally filled resins. The method of incorporation of filer rein varies. One system (Amalgambond Plus) uses very fine methyl methacrylate powder, added to the liquid resin as the filler. Another system (All Bond 2) uses a filled flowable resin composite liner to provide the filled resin. With both types of system, amalgam is condensed into the filled resin while the rein is in a viscous liquid form. Microscopic fingers of resin are incorporated into the amalgam at the interface. Because light cannot penetrate to the resin underlying enamel it is important to use a self cure or dual cure bonding resin.

Belcher (1997) reported that no doubt there is decreased leakage of fluids when an amalgam bonding systems is used, but the mechanical properties such as compressive strength may be compromised.

Shetty V et al (2004) studied the bond strength of amalgam bonding agents using chemical light and dual cure agents.

Group	Mean bond strength (MPa)
Dual cure	16.6
Light cure	14.7
Chemical cure	10.1

Neme AL et al (2000) studied the microleakage of various bonding systems bonded to amalgam and composites.

Adhesive	Amalgam %age microleakage	Composite %age microleakage
Prime and Bond	3.4	1.4
One Step/ Resinomer	4.1	2.5
Clearfil LB 2	5.6	5.1
Scotchbond MP Plus	6.3	2.6
Amalgambond	7.0	9.1
Optibond FL	8.3	0.4
Tenure Quick	11.2	10.0
Control	18.5	56.6

Muniz M (2005) [Oper Dent. Mar-Apr;30(2):228-33] evaluated the tensile bond strength (BS) and microleakage (MI) of bonded amalgam restorations to dentin when an unfilled and a filled system are used under three application modes. Adhesive system (Scotchbond Multi-Purpose Plus [SBMP], Optibond dual cure [OPTB]) and Application mode (light-LC, chemical-C and combination of light and chemical curing-LCC). the highest BS mean was obtained using the LCC technique and the OPTB system. Regarding the MI test, only the application mode was significant: lower dye infiltration was observed for LC and LCC.

Summit JB (2004) [Oper Dent. May-Jun;29(3):261-8] compared the performance of complex amalgam restorations retained with self-threading pins or bonded with a filled, 4-META-based resin. Sixty amalgam restorations (28 pin-retained and 32 bonded), each replacing at least one cusp, were placed. At six years, 11 restorations had failed; eight of which were pin-retained and three bonded.-

Role of Bonding agents in pulp capping

A major short coming of calcium hydroxide preparations is their lack of adhesion to hard tissues and resultant inability to provide an adequate seal against microleakage. Currently hybridizing bonding agents represent the state of art in the mechanical adhesion to dentin with resultant microleakage control beneath restorations. Cox et al (1993) demonstrated that pulps sealed with 4 META showed reparative dentin deposition without sub adjacent pulp pathosis.

A number of investigators have proposed that sealing vital pulp exposures with hybridizing dentin bonding agents may provide a superior outcome to calcium hydroxide direct pulp capping techniques. Because of their superior adhesion to peripheral hard tissues, an effective seal against microleakage can be expected.

Kashiwada and Takagi (1991) demonstrated 60 of 64 teeth to be vital and free of any clinical and radiographic signs of pulp degeneration 12 months after pulp capping with a resin bonding agent and composite resin. Teeth receiving this treatment were histologically studied and demonstrated dentin bridge formation below the area of exposure.

Kanca (1996) reported a 4 year clinical and radiographic success with dentin bonding agent application following etching material applied directly to a fracture-induced exposed pulp and dentin in rebonding a tooth fragment.

Stanley (1998) has stated that acid conditioning agents can harm the pulp when placed in direct contact with exposed tissues. In a primate tooth sample with pulp exposed tissues. In a primate tooth sample with pulp exposure treated with total etch followed by application of bonding agent. They found that 45% became non vital and 25% exhibited bridge formation after 75 days. In no etch calcium hydroxide pulp capping sample, 7% became non vital and 82% exhibited bridge formation.

Gwitnnet and Tay (1998) reported that some specimens after bonding agent application showed signs of initial repair with dentin bridge along the exposed site and reparative dentin dentin adjacent to the exposed site. Other specimens demonstrated persistence of inflammation of chronic inflammation with a foreign body response.

Silva GA (2006) [Oper Dent. May-Jun;31(3):297-307] evaluated the pulpal response in human dental pulp to direct pulp capping with the Single Bond Adhesive System (SBAS) (Group I) and after capping with Calcium Hydroxide (CH) (Grop II). After 1, 3, 7 and 30 days, the teeth were extracted and processed for light microscopical examination. The histological response in Groups I was without signs of cellular differentiation and dentin neoformation up to 30 days. In the group II (CH) at day 7, the pulps exhibited cells with high synthetic activity (Ag-NOR-positive) underneath the area of coagulation necrosis. Dentin bridging was observed at the thirteenth day. In conclusion, SBAS should be avoided for vital pulp therapy, while CH remains the capping agent of choice for mechanically exposed human dental pulp.

Tziafas D et al (2005) [J Dent. Sep;33(8):639-47] evaluated the pulpal responses following direct pulp capping of mechanically exposed teeth with new dentine adhesive systems. The cavities were assigned to five experimental groups, representing one control group treated with a Ca(OH)2-based material and four experimental groups where the adhesive systems Clearfil SE Bond, Prompt-L-Pop, Etch & Prime 3.0 and Single Bond were tested. The pulpal tissue responses to dentine adhesives were assessed at post-operative periods of 7, 21, 65 days. Variable responses were recorded, which were characterized by moderate to severe inflammatory reactions, progressive extension of tissue necrosis with time and total absence of continuous hard tissue bridge formation after pulp capping with each of the four adhesive systems. Application of a Ca(OH)2-based material was characterized by inflammatory cell infiltration, limited tissue necrosis as well as partial to complete hard tissue bridging.

Scarano et al (2003) [J Endod. Nov;29(11):729-34] studied the histological analysis of four different materials. The samples were randomly divided into four groups of six specimens each: group I: dental-bonding agent (Solist) followed by resin composite (Ecusit); group II: dental adhesive (Prompt) and resin composite (Pertac II); group III: traditional calcium hydroxide (Dycal) plus resin composite (Ecusit); group IV: light-curing calcium hydroxide (Ultrablend Plus) and amalgam (Dentsply). In the specimens of all groups, there were active odontoblasts near the composite resins and no newly formed dentin. Small quantities of inflammatory cells were present. A 1- to 3-microm layer zone of necrosis was present. In conclusion, all materials tested in this study induced similar tissue responses.

Role of bonding agents in Dentin hypersensitivity

Best explained by hydrodynamic theory, patients complain of teeth sensitivity when subjected to temperature changes, osmotic gradient sweet or salty food.

Dental adhesives

Resin tags + hybridization + Precipitation of proteins e.g. Multi bottle afhesive All Bond 2

Gluma Adhesive (5% gluteraldehyde + 35% HEMA). Felton JP (1995) reported that this Gluma adhesive decreases dentin permeability and cause protein denaturation, thus decreasing the dentin sensitivity.

Kakaboura (2005) [Am J Dent. Aug;18(4):291-5] investigated the desensitizing ability of a one-bottle bonding agent and a glutaraldehyde-based HEMA formulation on sensitive tooth cervical areas for a period up to 9 months. Three sensitive teeth per patient were treated; one received One-Step (one-bottle bonding agent), the other Gluma Desensitizer (glutaraldehyde-based agent) and the third distilled water (control group). The hypersensitivity level was determined before, immediately after the desensitizing session, at 8 weeks, and 9 months post-treatment. No significant differences were recorded between One-Step and Gluma Desensitizer at immediate and 8-week examinations, whereas Gluma Desensitizer produced lower hypersensitivity than One-Step at the 9-month assessment. In general, a lower level of reduction was found for the 9-month interval compared to the 8-week hypersensitivity score for both agents tested.

Lorenzi R et al (1999) [Am J Dent. Jun;12(3):103-6] evaluate the effects of topical applications of Gluma Alternate, a Gluma Desensitizer version with reduced glutaraldehyde content, Health-Dent Desensitizer and Scotchbond Multi-Purpose (SMP) on hypersensitive erosion/abrasion lesions. Sensitivity was recorded as response to cold air stimulus prior to treatment, immediately after the topical application of the agents, and after 1 week, 1 month and 6 months. Gluma Desensitizer reduces the sensitivity at all the time period than Health-Dent Desensitizer and Scotchbond Multi-Purpose.

Role of bonding agents in endodontics

It seems quite probable that dentin bonding agents play a major role in sealant endodontics. Their ability to halt microleakage. Zidan (1987) reported the efficacy of the four different dentin bonding agent used as root canal sealer was tested. No leakage was measurable in 75% of the canals sealed with Scotchbond, in 70% of canals sealed with Restodent, in 60% of the canals with Dentin Adhesit and only 30% of canals sealed with Gluma.

Ferrai M (1998) found that dentin bonding agents along with AH26 sealer and gutta percha laterally condensed, to obturate canals for leakage tests. It was found that less leakage in those cases in which bonding agents were used along with AH26 versus AH26 alone.

Omar and M.Eldeeb in 1985(JOE 28(10): 684-686) studied the use of a dentinal bonding agent as a root canal sealer. The quality of the apical seal obtained with laterally condensed gutta percha and an experimental sealer Scotchbond as compared with a commonly used sealer (Tubliseal) was assessed in 40 recently extracted maxillary canines. The apical seal was assessed by measuring the linear penetration of methylene blue dye into the root canals. It was found that Scotch bond produces an apical seal that was significantly better than Tubliseal.

Chistos Gogos in 2003(Jour. Of Dent. 31: 321-328) studied the bond strength of AH-26 root canal sealer to dentin using bonding agent. They found that shear bond strength without bonding agent was 3.67 MPa and it was increased to 6.9 MPa with Clearfill SE bonding agent. They concluded that use of dentin bonding agent improved significantly the adhesion of AH-26 sealer with root canal dentin.

Michael D (2006) reported that Endorez root canal sealer with a dual cure two step self etch adhesive (Liner Bond 2V) gave a better bond strength (6.8 MPa) as compared when used alone (0.7 MPa).

Fabricio (2004) reported that Resilon a new thermoplastic, synthetic root canal filling material which is used in same manner as most bonding systems. A self etch primer is first introduced into the canal to condition it and to prepare them for bonding to resin core material. After that a dual cure resin based sealer is placed followed by the Resilon core material. It gives a good sealing ability. A gap of 26-10 um was found between gutta percha and AH26 sealer and no gap was found in case of Resilon and Resilon sealer. Root canals filled with Resilon were more resistance to fracture than roots filled with Gp and AH plus.

The fixation of fiber post using an adhesive technique allows passive cementation and provides increased post retention in the root canal compared with the conventional methods. The combination of fiber post and Bis-GMA based resin cement has been described as a homogeneous structure leading to a better absorption of stresses.

K.Bitter et al (2004) conducted a study to determine the efficacy of various bonding systems according to the thickness of hybrid layer formation.

Bonding systems	Category	Hybrid layer thickness
Clearfil core (New Bond)	Etch and rinse	5.45um
Multilink (Mutilink Primer A & B)	Self etch	0.8um
Panavia 21 (ED Primer)	Self etch	0.5um
Permoflo (Permaflo DC Primer A& B)	Etch and rinse	3.36um
Variolink (Excite DSC)	Etch and rinse	4.33um

They concluded that the conditioning of root canal dentin with phosphoric acid and the use of one and two bottle bonding systems gave a thicker and uniform hybrid layer for a more durable bond than self etch adhesive.

They also reported that dual cure systems gave a better bond with root canal dentin as compared to light cure systems.

Hale Ari et al (2003) [J Endodon 29(4): 248-251] studied the bond strength of various bonding agents to root canal dentin. Root canal dentin was treated with four bonding systems.

Mean bond strength to root canal dentin

Material	Bond strength (MPa)
C & B Meta Bond	27.7
Panavia F	20.1
Variolink II	19.3
Rely X	16.8

Glass ionomer adhesives

A totally new evolution in glass ionomer adhesive was introduced with Fuji Bond LC, an adhesive material based on resin modified glass ionomer technology. It is a resin diluted version of resin modified glass ionomer restorative material.

· Mechanism - micromechanical + chemical bonding.

· A short polyalkenoic acid pretreatment cleans the tooth surface and removes the smear layer

· Exposes collagen fibrils to a depth of about 5 um

· Resin interdiffuses following concept of hybridization

· Polyalkenoic acid pretreatment is much less severe than traditional phosphoric acid treatment in that the exposed collagen fibrils are not completely denuded of hydroxyapatite

· Chemical bonding is obtained by ionic interaction of carboxyl groups of polyalkenoic acid with calcium of hydroxyapatite attached to the collagen fibrils

Advantages:

· Fast and simple application

· Viscous particle filled adhesive cariostatic

· Two fold bonding mechanism ionic to hydroxyapatite micromechanical through hybridization

Disadvantages:

· Adequate adhesion to enamel requires smear layer removal

· No long term clinical research

Luting of cast restorations via bonding agents

The resin based (bonding systems) cements both self cured and dual cured have been used for cementation of cast restorations. They principle advantages include the good adhesion to the tooth structure and provide good retention (Micromechanical retention). There is reduction in microleakage. Voorde A (1999) compared C& B Metabond with GIC as luting material. It was found that bonding systems provide a better seal and retention as compared to GIC.

Bonding of resin to Ceramic

Bonding resin to a ceramic surface whether porcelain or glass ceramic is based on the combined effects of micromechanical interlocking and chemical bonding. Porcelain and glass ceramic surface are generally etched with hydrofluoric acid and ammonium biflouride, respectively, to increase the surface area and create porosities. The adhesive resin flows into the porosities and interlocks, forming strong micromechanical bonds.

Success/ Failures of adhesives

Several factors can effect the clinical performance of adhesive systems.

· Material factors

· Substrate

· Size and shape of the lesion

· Patient’s age

· Dentin wetness

· Polymerization shrinkage

REFERENCES

1. Fundamental of Operative Dentistry James B.Summit (2^nd Edition) Page no. 178-229.

2. Fundamental of Operative Dentistry Studervant (4^th Edition) page no. 250-265.

3. F.R. Tay J.O.Dent 2005 20(5) Pg. 561-579