9.smear layer and bonding considerations / orthodontic courses by indian dental academy

Bonding Considerations

Disturbed surface layers of dentin and enamel that are formed by cutting or

abrading instruments must be removed or altered to obtain strong adhesive

bonding between restorative materials and dentin and enamel. These layers can

be removed by acids, including formic and ascorbic acids or chelating compounds

both of which form soluble or insoluble reaction products (Bowen, 1978).

Dental material scientists have been concerned about the smear layer in so

far as it masks the underlying dentin matrix and may interfere with the bonding of

adhesive dental cements such as the polycarboxylates and glass ionomers, which

may react chemically with the dentin matrix. Dahl (1978) demonstrated that

simply pumicing the dentin surface produced a three-fold increase in the tensile

strength of the bond between dentin and polycarboxylate cement over that seen

with zinc phosphate cement, which relies strictly upon mechanical roughness for

retention.

Presumably allowing cements to react chemically with the smear layer,

rather than the matrix of sound intertubular dentin, produced a weaker bond due to

the fact that the smear layer can be torn away from the underlying matrix. Early

dentin bonding systems were hydrophobic and were bonded directly to the dentin

smear layer. Therefore, macroshear bond strengths were found to be less than 6

MPa, because that is the strength of the bond of the smear layer to sound dentin.

When cements were applied to dentin covered with a smear layer and then tested

for tensile strength, the failure was either adhesive (between cement and smear

layer) or cohesive (between constituents of the smear layer). If one wants to

increase the tensile strength of a cement-dentin interface there are several

approaches to the problem.

1. Remove the smear layer by etching with acid:

(Lee et al, 1971, 1973; Bowen, 1978; Brannstorm et al 1979a, 1980;

Pashley et al 1981). This seemingly extreme procedure does not injure the pulp

(Brannstrom, 1982), especially if dilute acids (Bowen, 1978) are used for short

periods of time.

50


Etching dentin with 6% citric acid for 60 seconds removed all of the smear

layer (and smear plugs) as does 15 seconds of etching with 37% phosphoric acid

(Pashley & others, 1981) .The advantages were that the smear layer was entirely

removed, the tubules were open and available for increased retention, and the

surface collagen was exposed for possible covalent linkages with primers for

cavities (Fusayama & others, 1979; Bowen, Cobb & Rapson, 1982; Bowen &

Cobb, 1983). Further, with the smear layer gone, one doesn't have to worry about

it slowly dissolving under a leaking restoration or being removed by acid

produced by bacteria, leaving a void between the cavity wall and the restoration,

which might permit bacterial colonization. AL-Helal et al (2003) investigated the

effect of smear layer on root demineralization adjacent to resin-modified glass

ionomer. 4 cavity surface treatments were carried out prior to the placement of

RMGI: no treatment (None), polyacrylic acid (PAA), phosphoric acid, and

Scotchbond Multi-Purpose adhesive (SMP). It was concluded that removal of the

smear layer with phosphoric acid provides significantly enhanced resistance to

secondary root caries formation adjacent to RMGI restorations.

The disadvantage of removing the smear layer was that, in its absence, there

was no physical barrier to bacterial penetration of the dentinal tubules. Further,

with nothing occluding the orifices of the tubules, the permeability of the dentin

increased 4 to 9 fold depending upon the size of the molecule (Pashley & others,

1978b; Boyer & Sware, 1981). It is clear why Brannstrom & others (1982)

would prefer to remove the smear layer over and between the tubules without

removing the smear plugs. However, this is very difficult to achieve clinically.

Another disadvantage is the possibility of an increase in the chemical toxicity of

restorative materials after smear layer removal. Meryon & Johnson (1988) found

that there was increase in the cytotoxicity of some dental restorative materials

after smear layer removal particularly if the intervening dentin is thin. Pezzoli &

Baldi (1997) investigated the cytotoxicity of the composite resins applied on

dentine samples with different smear layer removal. The results showed that the

composite resins are surely cytotoxic if directly applied on the dentine. The smear

layer is able to reduce the transdentinal diffusion of composite resin toxicity. On

51


the basis of the data obtained it is suggested that in vivo, being necessary to

eliminate the smear layer due to its bacterial contents, it is possible in the deep

cavities, to partially remove with EDTA maintaining the smear plugs after their

disinfection. Nevertheless EDTA application should not exceed 30 seconds.

Fig. 22

Smear layer (SL) in cross section. Smear plugs (SP) are formed from cutting debris forced into the tubules. The smear layer and plugs greatly reduce the permeability of cut dentin surface.

Fig 23

Most dentin bonding systems remove or solubilizes smear layer, allowing resin to penetrate and form “hybrid layer” with dentin structures. Ideally, smear plugs would not be removed.

52


Acids are among several agents that can remove the smear layer. For

enamel, phosphoric acid in gel or solution in a concentration ranging from 30-

65% is the most popular agent. The application of this agent to dentin removes the

smear layer and, by dissolution of the peritubular dentin, the lumen of the dentinal

tubules is significantly enlarged. Brannstrom and Nordenvall (1977) and

Gwinnett (1977) demonstrated that conditioning of dentin with phosphoric acid

facilitates penetration of resin into the dentinal tubules.

Fig. 24

SEM of resin which had penetrated deep into the dentinal tubules after conditioning with phosphoric acid and sodium hypochlorite. Resin was disclosed by tissue dissolution. X150

Such penetration probably contributes to the increased bond strengths of

resins employing acid conditioning of dentin (Fusayama & others, 1979). There

was equivocation as to whether the values decline or are stable with time in the

presence of water. It was clear from many studies that while phosphoric acid

removes the smear layer and enlarges the dentinal tubules, it also appears to

degrade the collagen matrix.

Some of the degradation products may be removed with water but the

surface of the acid-conditioned dentin appears relatively smooth with a gelatinous

quality even after a thorough lavage. Subsequent treatment of the same surface

with a solution of sodium hypochlorite brings about significant morphological

changes.

53


Fig. 25

Fig. 26

Fig. 25 & 26: Scanning Electron Micrograph show dentin etched for 10 seconds with 50% phosphoric acid. A significant morphological difference exists following additional treatment for

60 seconds with 5-25% sodium hypochlorite (Fig.26) X 1520.

The sodium hypochlorite dissolves the organic material to produce a rougher

texture to the surface, which is dependent upon the time of application of this

agent. When tubules are exposed in longitudinal section, lateral canals increase in

number with time of application of sodium hypochlorite.

54


Fig. 27

Fig. 28

]Fig. 27 & 28: SEM showing the tubules exposed in longitudinal section. After 60 sec of 50% phosphoric acid and 60 sec of 5-25 % NaOCl treatment, the surface appears smooth (fig 27). Increasing the time of application of NaOCl brings about a roughening of the surface and the

exposure of numerous lateral canals.

The biocompatibility of this method was contentious, and therefore, the

preparation of the dentin surfaces for bonding must take into account the viability

of this tissue and its morphological and physiological association with the pulp.

In addition the composition of dentin and its surface following instrumentation

also dictates the choice of treatment. Methods that raise the surface energy of

dentin by removing the smear layer while leaving the tubules plugged with cutting

debris are preferred.

55


Ozturk et al (2004) compared in vitro the sealing properties of five different

dentine adhesive materials (Prime&Bond NT (PBNT); Prompt L-Pop (PLP);

Clearfil SE Bond (CSEB); Scotchbond Multi Purpose Plus (SMPP); EBS-Multi

(EBSM)) inside the pulp chamber. The pulp chambers were treated with 5%

sodium hypochlorite (NaOCl) for 1 min prior to bonding agent application.

Leakage values of the materials were significantly different at different

measurement periods. In all groups, leakage values decreased with time. SEM

observation of pulp chamber walls demonstrated that the irregular dentine surface

without smear layer was present in the nontreated group. However, NaOCl

application removed the collagen fibrils leaving the dentine surface smooth. At

resin-dentine interfaces of specimens, no hybridization zone was observed. It was

concluded that none of the materials had created a perfect seal to the pulp

chamber walls. PBNT and PLP had better sealing over the short term, but over the

long term, there were no differences between the materials.

HYBRID LAYER FORMATION:

Acid treatment of dentin removes the smear layer and hydroxyapatite from

the treated dentin and exposes the fibrillar collagen matrix thereby providing a

substrate for the resin monomers to infiltrate and form a hybrid layer. Briefly, the

substitution of resin in the subsurface of mineralized tissues is the essence of the

creation of the hybrid layer. When primer and bonding resins are applied to etched

dentin, they penetrate the intertubular dentin, forming a resin-dentin interdiffusion

zone, or “hybrid layer”. They also penetrate and polymerize in the open dentinal

tubules, forming “resin tags”. Thus hybridized dentin is a molecular-level

mixture of collagen and resin polymers. It has a concentration gradient structure,

because it is prepared by diffusion of monomers that have been placed on the

conditioned dentinal surface and subsequently polymerized in situ. The bond

strengths achieved by the “total-etch dentin adhesive systems” is between 17 to 30

MPa.

E.g.: 4th generation bonding agents: All-Bond 2 (Bisco, Illinois), Optibond FL

(Kerr, California), Scotchbond Multipurpose (3M, ESPE).

56


5th generation bonding systems: One-Step (Bisco, Illinois), Prime & Bond, Prime

& Bond 2.1 (Dentsply Caulk, Delaware), Optibond Solo Plus (Kerr), Gluma One

Bond (Haraeus Kulzer).

Fig. 29

Bonding of resin to dentin, using a “total-etch” technique

Mitchem & Gronas (1991) determined the adhesion to dentin with and

without smear layer under varying degrees of wetness. The bond strengths of

glass ionomer cements to prepared dentin ranged between 2.1 and 4.7 MPa for all

test conditions and did not appear to be adversely affected by the presence of fluid

under physiologic pressure or by the presence or absence of a smear layer. On the

other hand, the resin dentin adhesive was adversely affected by the presence of

moisture (1 +/- 1.3 MPa when the tubules were full of fluid and under pressure;

9.4 +/- 10.6 MPa when the tubules were full of fluid but not under pressure; and

18.3 +/- 7.6 MPa when the tubules were empty.

Mineralized dentin usually does not permit much monomer diffusion into its

substance. Therefore, dentin must be suitably conditioned to permit diffusion of

monomers, which should have a good affinity for demineralized dentin, into the

substrate. Prepared dentinal surfaces are covered with a smear layer that adheres

57


weakly to the underlying dentin. Dentin conditioning involves the removal or the

modification of the smear layer to permit monomer diffusion into the

demineralized collagen matrix. Phosphoric acid denatures the peptides exposed

during the removal of the smear layer (Scott & Leaver, 1974; Mizunuma, 1986;

Okamoto, 1991). The degree of denaturation depends on the phosphoric acid

concentration and time of exposure. Similar results were obtained with 10% citric

acid.

The demineralized dentinal matrix, composed mainly of collagen, can

collapse easily upon air-drying, causing a decrease in the interfibrillar spacing and

loss of permeability to resin monomers. The challenge is to maintain the spaces

between the demineralized collagen fibrils after the hydroxyapatite crystals have

been removed.

Fig. 30

Collapse of etching dentin by air-drying

One approach to overcome this challenge is by adding 3% ferric chloride to

10% citric acid. Ferric chloride prevents the collapse of the demineralized dentin

by getting adsorbed on the demineralized dentin and cross-link peptides, thereby

immobilizing them and preventing collapse when air-dried.

58


Cupric ions added to 10% phosphoric acid also cross-link collagenous and

non-collagenous proteins, thereby stabilizing the structure of interfibrillar spaces

that are necessary for monomer permeation during bonding.

Kato & Nakabayashi (1997) added 1% calcium phosphate to 10%

phosphoric acid and obtained very high quality hybrid layers. Presumably the

calcium ions stabilized the collagen network and immobilized non-collagenous

proteins and glycosaminoglycans (Kuboki, 1979).

Another approach to over the disadvantage of collapse of demineralized

dentinal matrix is to re-expand the collapsed collagen network to regain the

permeability of the inter-tubular dentin for hybridization by the primer. If water

or an aqueous primer were added to dried dentin, the water molecules would

form hydrogen bond with the collagen peptides, breaking intermolecular hydrogen

bonds. Residual stresses that were created in the network when it shrank could

permit elastic recoil, allowing the network to actively re-expand (Balooch et al,

1996).

Combination of 10% citric acid and HEMA (Hydroxy Ethyl MethAcrylate)

priming provided good bonding of the resin to dentin even in the absence of ferric

ions, though it took 1 hour to re-expand the demineralized dentin (Nakabayashi

& Takarada, 1992). The collapsed demineralized dentin re-expanded 50% when

it was treated with 30% HEMA in water for 1minute and 100% when the primer

was allowed to diffuse for 10 minutes. Application of 1% Phenyl-P in 30%

HEMA in water for 1 minute to etched, air-dried, collapsed dentin caused much

more re-expansion than did application of 30% HEMA for the same time

(Igarashi & Nakabayashi, 1996).

One recent approach to preventing the collapse of demineralized dentin is to

leave the smear layer in place but to use acidic monomers to etch through the

smear layer into the underlying dentin and to avoid rinsing the conditioned

surface. This prevents loss of dentin mass but solubilizes enough apatite crystals

59


from around collagen fibrils to permit infiltration of adhesive monomers

(Watanabe, 1992; Toida, 1995). Self-etching conditioners and primers

eliminate several bonding steps at which errors can occur. In conventional multi-

step bonding procedures, an acidic conditioner is applied and rinsed off with

water, the water is blown off (at the risk of drying or collapsing the demineralized

dentinal mesh) and then the primer is applied. If too little rinsing occurs residual

acid may over-etch the dentin or residual reaction products may block the narrow

channels around the collagen fibrils. Self-etching and self-priming systems may

avoid these problems. The liquid is applied, allowed to react for 30 seconds and

then air-dried.

2. Use of a resin that would infiltrate through the entire thickness of the

smear layer and either bond to the underlying matrix or penetrate into the

tubules:

The impressive tensile strength for Scotchbond (3M) may be due to such a

process. Results indicate stronger bonds between the resin and pumiced dentin

than between the resin and etched dentin. Etching with acid, in addition to

removing the smear layer and exposing the surface collagen, also removed the

peritubular dentin from the top 5-l0 m of the tubules, yielding a tubule with a

funnel shaped orifice which provides less retention since the walls are divergent

as compared to normal parallel walls of unetched tubules. Additionally, etching

with acid demineralized the surface, which would lower the adhesive bond

between cements and the mineralized dentin.

Yu et al (1991) examined teeth treated with smear layer-mediated dentinal

bonding agents and restored with composite resin, at the dentin-restoration

interface. Results indicated that these dentinal bonding agents actually bonded to

the smear layer, and samples demonstrated delamination of the smear layer from

underlying dentin following thermocycling. This finding suggests that the stresses

developed within the composite resin exceeded the adhesive strength of the smear

layer to dentin. Thus, the bond strength for dentinal bonding agents that require

60


the presence of the smear layer cannot exceed the adhesion of the smear layer to

dentin.

Smear layers on deep dentin may have more organic material in them than

those on superficial dentin. This may be due to the number of odontoblastic

process or to the greater amount of proteoglycans lining the tubules (Thomas &

Paine, 1983). Etching with acid, i.e., removal of the smear layer increased the

adhesive strength of composite resins (Adaptic, Clearfil) to superficial dentin by

800-1000% over that to deep dentin even though far more tubules were available

for penetration of resin in deep dentin than in the superficial dentin. This indicates

that composite resins probably do not derive their adhesiveness from penetration

of resin into the tubules, but rather by interacting with mineralized intertubular

dentin.

Another variable interfering with the adhesive of substances to dentin was

the presence of dentinal fluid, a fluid much like other interstitial fluids (Pashley,

1979 ) both within the dentinal tubules and within the smear layer. Brannstrom

el al (1979a) indicated that, in dentin etched with acid, dentinal fluid could be

removed by blasts of air and replaced by tags of resin extending deep into the

tubules. Bowen's (1982) approach was to treat the dentin with solutions of resins

in acetone which was miscible with dentinal fluid yet compatible with

hydrophobic polymers.

The recently introduced self-etching primers (SEPs) perform two functions

simultaneously-etching and priming of dentin and enamel. They are applied to the

smear layer-covered dentin for a designated period of time. Without further

rinsing, a layer of adhesive resin is then applied to the treated dentin. In these

systems, the goal is to incorporate the smear layer into the hybrid layer and the

smear plugs into the resin tags.

61


Fig 31

Bonding to dentin using a self-etching primer

The use of self-etching, self-priming adhesives (6th & 7th generation bonding

agents) is attractive because they are used on dry dentin and require only one

primer application which is subsequently air-dried rather than rinsed. Self-etching

systems are less technique sensitive compared with systems that utilize separate

acid conditioning and rinsing steps, thus reducing the possibility of overwetting or

overdrying (i.e. collapse of air-dried demineralized collagen), which can have a

negative influence in adhesion. The susceptibility of moisture contamination of

the adhesive through transudation of dentinal fluid is reduced.

The disadvantage of self-etching systems is that they must diffuse by

demineralizing though the smear layer and into sound underlying dentin if they

are to give high bond strengths. There is the danger that if the smear layer is thick,

the SEP may not be ale to penetrate through it. The acidity of the primer may also

be buffered by the mineral components of the dentin smear layer (Wang &

Hume, 1988), thereby reducing the potential for demineralization and hybrid

layer formation within the subsurface intact dentin. This property, in addition to

the tight packing of the smear layer particles to each other, seem to limit the depth

of penetration of monomer to about 2 m. If the smear layer is 1.5 m thick, then

62


the acidic co-monomer mixture may only penetrate 0.5 m into the underlying

intact dentin (Yoshiyama et al, 1996). Smear layer represents a diffusion barrier,

depending on how they have been prepared and their thickness. A smooth smear

layer, 0.5 m thick offers much less resistance to diffusion than does a rough

smear layer that is 2 m thick. However, it is also seen that despite the short resin

tags, a good seal is achieved as the smear plugs are left intact (Ferrari et al,

1997).

Tay & Pashley (2001) examined the aggressiveness of three self-etching

adhesives systems in penetrating dentin smear layers of different thickness. Teeth

with thin and thick smear layers were bonded using one of the following: Clearfil

Mega Bond (Kuraray), Non-Rinse Conditioner and Prime & Bond NT (Dentsply

DeTrey) and Prompt L-Pop (ESPE). For Mega Bond, thin authentic hybrid layers

between 0.4-0.5 m were found and smear layer and plugs were retained as part

of the hybridized layer. For the NRC group the hybrid layers were about 1.2-2.2

m thick, with the smear layer and plugs completely dissolved in dentin with thin

smear layers and partially retained in dentin with thick smear layers. For Prompt

L-Pop, hybrid layers were 2.5-5 m thick and smear layer and plugs were

completely dissolved even in dentin with thick smear layers. Thus contemporary

self-etching systems may be classified as mild, moderate and aggressive based on

their ability to penetrate dentin smear layers and their depth of demineralization

into the subsurface dentin.

The ideal self-etching self-priming bonding system is one that can penetrate

2 m of smear layer and engage underlying dentin to a depth of about 1 m. This

should provide sufficient retentive strength and an adequate seal even if the

infiltrated smear layer fails. Whether this ideal can be achieved remains to be

determined through clinical experience. However, bond strengths ranging between

20 to 28 MPa have been reported with “self-etching dentin adhesive systems”

which are similar to those strengths obtained with phosphoric acid-etching of

enamel.

63


E.g.: Clearfil Liner Bond 2 (Kuraray, Japan), Prompt L-Pop (3M, ESPE) i.e.: 6 th

generation bonding system, NRC Non Rinse Conditioner (Dentsply DeTrey,

Germany), iBond (7th generation bonding system).

Van Meerbeek et al (1999) compared the hybridization effectiveness of two

adhesive systems that are applied in respectively three (OptiBond Dual-Cure) and

two (OptiBond Solo) steps. It was demonstrated that some collapse of the exposed

collagen fibril network, due to gentle postconditioning air-drying of the dentin

surface, may not have been totally recovered through hybridization by the two-

step adhesive formulation as opposed to the three-step precursor. The findings

suggest that simplifying the application procedure of adhesives by combining the

primer and adhesive resin into a single application step may reduce hybridization

effectiveness.

Brunton et al (1999) found that with the possible exception of excellent

color match, Coltene ART Bond/Brilliant restorations may be found to perform

favorably in mixed Class V lesions in selected adult patients over a period of at

least three years. The findings indicated that resin composite restorations placed

with a smear-layer mediated dentin bonding agent may be found to have a

favorable clinical outcome in non-carious cervical lesions.

Toledano M et al (2004) evaluated the effect of the hydration status of the

smear layer on the wettability and bond strength of a self-etching primer to dentin.

Differences were found between contact angles on completely and briefly air-

dried smear layer when the primer was used and micro tensile bond strength was

greater when the adhesive was applied on the completely air-dried smear layer.

Chan KM et al (2003) compared the microtensile bond strength (microTBS)

and the ultrastructure of resin-dentin interfaces of four self-etching systems that

were applied to dentin with thick smear layers. For each adhesive, agitation

produced significantly higher microTBS than passive application. With passive

application, all systems diffused through thick smear layers and formed thin

hybrid layers in intact dentin. With continuous agitation, smear layers were

64


completely dispersed or dissolved, and thicker hybrid layers with upstanding

collagen fibrils were produced.

3. Dissolution of the smear layer:

The dissolved smear layer plays a part in the chemical attachment of the

dentin bonding agent to dentin. E.g.: Scotchbond 2 (3M)-i.e.: 3rd generation

bonding system, Mirage Bond (Chameleon Dental, Kansas).

The 3rd generation of bonding agents as coined by Retief (1987) contained

cleansers or mordants for removal or modification of the dentinal smear layer.

Wendt et al (1990) evaluated the shear bond strengths of a light-cured composite

to dentin treated with one proprietary (0.5 M EDTA) cleanser and one

experimental (1:1polyacrylic/maleic acid solution) cleansing agent using Gluma

(Bayer Dental, Germany) bonding adhesive. It was found that bond strengths with

the experimental cleanser were significantly greater than bond strengths of teeth

cleansed with EDTA.

4. Fixing the smear layer:

Glutaraldehyde (Hoppenbrouwers, Driessens and Stadhouders, 1974) or

tanning agents such as tannic acid or ferric chloride (Powis et al, 1982)87are the

agents used for this approach. The idea was to increase the cross linking of

exposed collagen fibers within the smear layer and between it and the matrix of

the underlying dentin to improve its cohesion.

5. Removal of the smear layer by etching with acid and replacing it with an

artificial smear layer composed of a crystalline precipitate:

This is one of the most convenient approaches to the problem (Causton and

Johnson, 1982). Bowen used this approach by treating dentin with 5% ferric

oxalate (in acidic solution), which replaced the original smear layer with a new

complex permitting extremely high bond strengths to be produced between resin

and dentin (Bowen & others, 1982; Bowen & Cobb, 1983). Solutions of ferric

oxalate (Tenure, DenMat Corp, Santa Maria, CA) dissolved the smeared surface

layer yet form insoluble reaction products that apparently occlude the openings of

65


the dentinal tubules (Bowen, Cobb & Rapson, 198284). These solutions also

remove the smeared layer on cut enamel, revealing typical patterns of enamel

prisms. When the ferric oxalate was followed by treatment with solutions of a

specific surface active compound and then a polymerizable coupling agent, strong

adhesive bonds with composites are possible on dentin and enamel in vitro

(Bowen et al, 1982). Greenhill and Pashley (1981) have produced similar

artificial smear layer with a variety of chemicals as a method of desensitizing

hypersensitive radicular dentin.

It must be remembered that, in general, diamonds, through the introduction

of grooved anomalies produce a greater surface area than burs. This has

implications in bonding where differences in the bond strength of resin attached to

enamel have already been reported to be higher for diamonds compared to burs

(Aker, Aker & Sorensen, 1979). The increased surface area probably offered a

larger number of reaction or retentive sites. These sites in enamel are primarily

micromechanical and the retention mechanism for this tissue lies in the multitude

of superficial micropores enhanced following acid conditioning of the tissue.

Several studies have been carried out which either support removal or

retention of the smear layer for improvement in bonding:

REMOVAL OF SMEAR LAYER IS BETTER FOR BONDING was

demonstrated by Nakabayashi (1993) who determined the long-term durability of

photocured phenyl-P in TEGDMA to smear layer retained bovine dentin. He

found that long-term water-immersion weakened the bonds between the adhesive

resin and the smear layer-retained dentin because there was insufficient diffusion

of the adhesive resin through the retained smear layer. Davis et al (1992)

compared the effect on bond strength of smear layer removal (40% polyacrylic

acid or 10% phosphoric acid) versus smear layer conditioning for dentin bonding

agents requiring conditioning. Results indicated no difference in shear bond

strength for groups in which the smear layer was conditioned or removed with

phosphoric acid. Smear layer removal with polyacrylic acid resulted in lower

66


bond strengths between DBAs and dentin than either phosphoric acid removal or

conditioning.

Santini & Mitchell (1998) investigated the effect of conditioners (Gluma

CPS etchant and 20% phosphoric acid liquid) on smear layers produce by

different bur types and speeds and the interaction of applied primer (Gluma CPS)

with these surfaces. It was found that Gluma CPS conditioner did not completely

remove the smear layer but a zone of demineralization did occur beneath it which

was only partly filled with the primer. Smear layers were completely removed by

the 20% phosphoric acid to expose a delicate collagen network. Thus, with the

Gluma CPS system, partially filled demineralization zones may contribute to

microleakage and bond failure. Chen X & Duan H (1998) determined whether

removing of smear layer and using of self-cured adhesive can increase retentive

strength of amalgam restoration. Removing smear layer had little effect on

retentive force of amalgam, while removing smear layer and applying self-cured

adhesive improved the retentive strength of amalgam fillings significantly. They

also (1997) evaluated the effect of removing smear layer and applying adhesive

on the marginal adaptation of amalgam fillings to cavity walls. Results indicated

that removing smear layer and using adhesive resin significantly improved the

adaptation of amalgam to cavity walls.

Abdalla (2000) evaluated the micromorphological interface between dentin

and several hybrid ionomer restoratives with and without smear-layer removal.

The three poly-acid-modified composite resins showed the formation of hybrid

layers and resin tags at the interface to the dentin. Removal of the smear layer

significantly improves hybridization of these materials. Also, among the resin-

modified glass ionomers, Fuji II LC produced a hybrid layer while the Photac-Fil

showed no evidence of hybridization.

Chaves P et al (2002) evaluated the tensile bond strength of two self-etching

adhesive systems and one one-bottle system applied on dentin surfaces after

different smear layer treatments, i.e.: direct application over smear layer (no

67


treatment), sandblasting for 10 s, etching with 36% phosphoric acid for 15 s, or

conditioning with 0.5 M EDTA for 2 min. Mean bond strength for Prime & Bond

was significantly higher than self-etching systems. No significant differences were

observed among smear layer treatments within the same dentin adhesive.

Baghdadi (2003) determined the shear bond strengths (SBS) of Non-Rinse

Conditioner (NRC) combined with Prime & Bond NT (PBNT) and SBS of

conventional Phosphoric acid (PA) etching and bonding application to permanent

and primary dentin. He found that SBS were remarkably greater in the groups

etched with PA in comparison with those conditioned with NRC.

THE RETENTION OF THE SMEAR LAYER during dentin bonding

procedures is supported by the following studies: Ishioka & Caputo (1989)

studied the interaction between dentinal smear layer removal with various agents,

a dentinal adhesive, and the shear bond strength of a posterior composite resin to

dentin. The use of Scotchbond dentinal adhesive in conjunction with the

composite P-30, with the smear layer intact, produced the highest shear bond

strengths. Removal of the smear layer produced bond strengths similar to those

obtained with the smear layer intact. Application of ferric oxalate, and to a lesser

degree 17% EDTA, resulted in diminished bond strengths. It was concluded that

optimal bond strengths with the adhesive-composite resin systems tested may be

obtained with an intact smear layer. White et al (1989) examined the effect of

smear layer removal on the bond strengths of a glass-ionomer cement and three

representative dentin bonding agents. For all but one dentin bonding agent

(Gluma), treatment with 17% EDTA caused a reduction in bond strength. While

Gluma probably bonds via dentinal collagen, the other materials interact primarily

with dentinal calcium. Removal of smear layer for adhesives reliant on the

presence of calcium is therefore undesirable. Hayakawa et al (1995) measured

the adhesion of composite to dentin retaining the smear layer. The aqueous

bonding agent tested was capable of penetrating the smear layer and the bond

strength exceeded the values obtained by bonding agents that remove the smear

layer. Konuma-M (1996) found that the bond strength of dental cements using

three surface conditioners was not stronger than that of no treatment.

68


Tay et al (2000) evaluated the effect of absence and presence of smear layer

on bonds made to dentin using a self-etching primer system, Clearfil SE Bond. SE

Bond produced high bond strengths to both smear layer-free and smear layer-

covered dentin. It was found that self-etching primers create thin hybrid layers

that incorporate the smear layer and that formation of true hybrid layers occurs

irrespective of smear layer thickness. In another study Tay et al (2000)

determined the depth of demineralization into intact dentin using several self-

etching primer systems with different pH values and whether hybridization by

Clearfil SE Bond may be affected by variation in the thickness of the smear layer.

All three systems etched beyond the smear layer to from true hybrid layers with

the thickest (1.2-1.4 m) one occurring with Clearfil Liner Bond II and thin layers

(0.5 m) formed by Clearfil Liner Bond 2V and Clearfil SE Bond. Application of

SE Bond to dentin of different surface roughness produced hybridized smear

layers of variable thickness. However, the thickness of the underlying true hybrid

remained consistent for the 4 groups (0.4 -0.5 m). Thus, the suspicion that thick

smear layers may interfere with the diffusion of self-etching primers into the

underlying intact dentin was not confirmed. Tani & Finger (2002) investigated

the effect of dentin smear-layer thickness on the bond strength of three all-in-one

adhesives of different acidity. Smear layer thickness (SLT) increased with

decreasing SiC grit numbers and increasing diamond bur roughness. In spite of

the widely differing acidity, all three adhesives tested were equally effective over

the range of SLT from 2.6 micron through 0.9 micron. Shear bond strengths for

the individual adhesives did not differ significantly by SiC grit size.

Koibuchi et al (2001) investigated the effect of smear layers on the tensile

bond strength (TBS) to dentin. Significantly different TBS of approx. 10 MPa and

28 MPa were found for #180 and #600 abrasive paper prepared dentin

respectively. The former specimens fractured within the hybridized relatively

coarse smear layer, while the latter demonstrate adhesive failure between the

composite and the attached PMMA rod, not between dentin and applied adhesive

agent. Thus, the presence and quality of smear layer yields significantly different

bond strengths and a TBS of approx. 10 MPa is evidently adequate. Oliveira et al

69


(2003) determined the effect of dentinal smear layers created by various abrasives

on the adhesion of a self-etching primer (SE) and total-etch (SB) bonding

systems. It was found that overall, the shear bond strength (SBS) was lower when

SB was used than when SE was used. However, SBS decreased with increasing

coarseness of the abrasive (which caused an increase in thickness of the smear

layer) in the SE group. The higher SBS and thin smear layer of the carbide bur

group, suggests its use when self-etching materials are used in vivo.

70

9.smear layer and bonding considerations / orthodontic courses by indian dental academy

Documents