Morphology of resin–dentin interfaces after Er,Cr:YSGG laser and acid etching preparation and application of different bonding systems
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- Beer, F., Buchmair, A., Körpert, W. et al. Lasers Med Sci (2012) 27: 835. doi:10.1007/s10103-011-0979-x
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The goal of this study was to show the modifications in the ultrastructure of the dentin surface morphology following different surface treatments. The stability of the adhesive compound with dentin after laser preparation compared with conventional preparation using different bonding agents was evaluated. An Er,Cr:YSGG laser and 36% phosphoric acid in combination with various bonding systems were used. A total of 100 caries-free human third molars were used in this study. Immediately after surgical removal teeth were cut using a band saw and 1-mm thick dentin slices were created starting at a distance of 4 mm from the cusp plane to ensure complete removal of the enamel. The discs were polished with silicon carbide paper into rectangular shapes to a size of 6 × 4 mm (±0,2 mm).The discs as well as the remaining teeth stumps were stored in 0.9% NaCl at room temperature. The specimens were divided into three main groups (group I laser group, group II etch group, group III laser and etch group) and each group was subdivided into three subgroups which were allocated to the different bonding systems (subgroup A Excite, subgroup B Scotchbond, subgroup C Syntac). Each disc and the corresponding tooth stump were treated in the same way. After preparation the bonding composite material was applied according to the manufacturers’ guidelines in a hollow tube of 2 mm diameter to the disc as well as to the corresponding tooth stump. Shear bond strength testing and environmental scanning electron microscopy were used to assess the morphology and stability of the resin–dentin interface. The self-etching bonding system showed the highest and the most constant shear values in all three main groups, thus enabling etching with phosphoric acid after laser preparation to be avoided. Thus we conclude that laser preparation creates a surface texture that allows prediction of the quality of the restoration without the risk of negative influences during the following treatment steps. This can easily and repeatedly be achieved.
KeywordsDentinal tubulesEr,Cr:YSGG laserHybrid-layerPhosphoric acidTagsUltra structureShear bond strength
The dentin–resin interface is the key to a high-quality, durable composite restoration . The method of surface treatment as well as the choice of bonding agent influences the development of resin tags and a hybrid layer. Dentinal tubules are the only porosities in mineralized dentin that may permit resin infiltration to achieve mechanical retention. Van Meerbeek et al.  concluded that the bond between dentin and composite is more micromechanical than chemical. Preparation with rotating instruments produces a smear layer which contains hard particles, blood, bacteria and saliva . Acid etching dissolves the smear layer, demineralizes the peritubular and intertubular dentin and exposes the collagen matrix . The demineralized dentin is infiltrated by resin monomers which create a hybrid layer after their polymerization . To create the hybrid layer, it is necessary to remove the smear layer and demineralize the superficial dentin layer. Etching exposes the collagen fibre network of the dentin matrix, thereby permitting infiltration of bonding agents into the spaces between the fibres . The fibres are engulfed and the complex fibre–resin is polymerized providing improved micromechanical retention of resin polymers [2, 4, 5].
Air-drying causes a dramatic collapse of the collagen network. The dentin matrix may shrink by up to 65% in volume when allowed to dry . With new self-etching bonding systems this problem does not arise because the collagen matrix is stabilized through the synchronized process of etching and bonding, so that air-drying between the two treatment steps is not required. Acid etching produces a smooth dentin surface due to the almost complete removal of intertubular and peritubular dentin resulting in a demineralized zone in the subsurface region. Recent observations have cast doubt on the ability of resin monomers to fully infiltrate the demineralized zone [2, 7].
In the 1990s erbium lasers were introduced for the preparation of hard tissue. The Er,Cr:YSGG laser (emitting at a wavelength of 2.79 μm) and the Er:YAG laser (emitting at a wavelength of 2.94 μm) are effective tools for the removal of dental hard tissues [8, 9]. Furthermore, the erbium lasers selectively remove carious lesions [10–12]. Different studies have shown that, despite differences in wavelength, pulse duration and energy , the morphological features of the dentin surface irradiated with these two erbium lasers are comparable  and they have given promising results in comparison with other systems [9, 15]. Dentin irradiated with the Er,Cr:YSGG laser shows a microscopically rough surface  without demineralization, open dentinal tubules [9, 15], no smear layer and satisfactory sterilization of the cavity . These characteristics are considered an advantage of laser preparation if composite resins are to be applied as filling materials . Acid etching produces a hybrid layer and characteristic funnel shaped resin tags regardless of the type of surface preparation (bur/laser). When laser preparation is performed without subsequent acid etching no particular hybrid layer equivalent can be detected, and tags appear in a cylindrical shape .
We investigated the development of different dentin–resin interfaces created by three different surface treatments and three different bonding systems. The aim of the study was to compare the ultrastructure of the laser-irradiated dentin and conventional phosphoric acid-etched dentin to create an ideal morphology for composite restorations. The stability of the restoration was investigated by shear bond strength testing to find the optimum combination of preparation technique and bonding system.
Materials and methods
Group I (laser group)
Group II (etch group)
Group III (laser/etch group)
1/A (n = 10)
II/A (n = 10)
III/A (n = 10)
1/B (n = 10)
II/B (n = 10)
III/B (n = 10)
1/C (n = 10)
IIa/C: phosphoric acid 37% (n = 10)
IIIa/C: laser + 37% phosphoric acid (n = 10)
IIb/C: self etching (n = 10)
IIIb/C (= I/C): laser + self etching (n = 10)
Laser group: In subgroups I/A, 1/B and I/C, the dentin surfaces were prepared with the Er,Cr:YSGG laser (Biolase, Waterlase YSGG, San Clemente, CA) equipped with a sapphire tip (600 μm diameter). The power settings were 2 W, 55% H2O, 65% air at 13.4 J/cm2.
Etch group: In subgroups II/A, II/B and IIa/C, the dentin surfaces were etched with 37% phosphoric acid (3 M, St. Paul, MN) for 15 s, followed by rinsing with a water spray for 30 s. In subgroup IIb/C, the dentin surfaces were only etched with the self-etching bonding system.
Laser/etch group: In subgroups III/A, III/B and IIIa/C, the dentin surfaces were prepared with the Er,Cr:YSGG (Biolase, Waterlase YSGG, San Clemente, CA) equipped with a sapphire tip (600 μm diameter). The power-settings were 2 W, 55% H20, 65% air at 13.4 J/cm2), and additionally were etched with 37% phosphoric acid (3 M, St. Paul, MN) for 15 s, followed by rinsing with a water spray for 30 s. In the subgroup IIIb/C, the dentin surfaces were prepared with the laser and etched only with the self-etching bonding system (therefore this subgroup is identical to laser subgroup I/C).
Excite (Ivoclar Vivadent, Schaan, Liechtenstein): a one-bottle universal dentin adhesive, containing HEMA, dimethacrylate, phosphonic acid acrylate, highly dispersed silicon dioxide, initiators and stabilizers in an alcohol solution. The application time is 10 s on dentin, followed by 15 s air-drying and 10 s light-curing (blue phase, Ivoclar Vivadent).
Scotchbond Multipurpose (3 M ESPE, St. Paul, MN): a two-bottle system (primer/bonding). The primer is applied and rubbed in for 20 s, followed by air-drying for 5 s. The adhesive is light-cured for 10 s after application.
Syntac Classic (Ivoclar Vivadent, Schaan, Liechtenstein): a three-bottle system (primer/bonding/adhesive). After rubbing the primer into the dentin for 15 s it is dried thoroughly with air syringe. Adhesive is then applied for 10 s, followed by air-drying. Heliobond is applied and blown to a thin layer followed by light-curing for 20 s.
After the bonding procedure composite material (Z100 MP Restorative A3; 3 M ESPE, St. Paul, MN) was applied according to the manufacturer’s guidelines in a hollow tube of 2 mm diameter to the disc as well as to the corresponding tooth stump. Each disc and the corresponding tooth stump were treated in the same way. After application of the composite, the discs were broken in half with a chisel (to avoid artefacts at the dentin/resin interface), gold-sputtered and assessed with environmental scanning electron microscopy (ESEM XL30; Philips, Preston, VA) in GSE mode. Following the identical preparation procedure and application of the composite the tooth stumps were embedded in a prosthetic plastic mass (Paladur; Heraeus Kulzer, Hanau, Germany) and the stability of the adhesive compound was proved by shear strength testing. The data were analysed using the t-test for mean value distribution.
Group I: dentin preparation with Er,Cr:YSGG Laser
Shear bond strength testing
Group II: conventional dentin preparation and phosphoric acid etching
Shear bond strength testing
Group III: laser and phosphoric acid etching
Shear bond strength testing
Preparation of dentin with rotary instruments leaves a smear layer on the surface. The smear layer consists primarily of pulverized enamel and dentin, caries debris and bacteria. The smear layer inhibits impregnation of the enamel and dentin with the adhesive agent, and thus prevents adequate adhesion . Acid etching is recommended for conventional cavity preparation to remove the smear layer and to demineralize the subsurface. Thereby the dental tissue is prepared for the creation of a hybrid layer  which is produced by the diffusion of the resin into the demineralized zone. One concern regarding the efficacy of the hybrid layer as a bonding mechanism for adhesive resin to dentin is that the resin monomers would not reach the bottom of the demineralized zone. Strong acids such as phosphoric acid create demineralization deeper than the diffusion capacity of the resin monomers thus leaving collagen fibres in the deep tissue layers unprotected . This might be a problem and compromise the quality of bonding.
The dentin surface after Er,Cr:YSGG laser irradiation (2 W, 20 Hz, 55% water/65% air) shows no smear layer, dentin tubules are open and the subsurface is not demineralized. Irradiation of dentin with an Er,Cr:YSGG laser creates a rough surface with chimney-like formations due to the preferential removal of intertubular dentin. The erbium laser is mainly absorbed by water and other hydrated organic components of the tissue. The different contents of water in peri- and intertubular dentin is responsible for the selective ablation of intertubular dentin by this wavelength. Due to rapid heating of the embedded water, the internal pressure increases in the tissue until explosive removal of the inorganic components of the surface occurs. Since intertubular dentin contains more water and has a lower mineral component than peritubular dentin, it is selectively ablated to a higher extent than the peritubular dentin, leaving protruding dentinal tubules with a cuff-like appearance [8, 15, 22, 23]. This may contribute to an increase in the adhesive area . Open tubules and the absence of a smear layer are additional factors that may enhance bonding to laser-treated dentin. Improved adhesion to laser-treated dentin may be explained by resin tag formation and the infiltration of adhesive resin into the microirregularities in lased demineralized dentin . Phosphoric acid etching after preparation with the Er,Cr:YSGG laser leads to partly giving up the advantages of this ideal surface morphology. Acid application dissolves the intertubular dentin too, thus destroying the chimney-like formations and widening the orifices of the dentinal tubules. Furthermore consecutive acid etching involves the problems of unpredictable depths of the demineralization zone  and a deficit in the diffusion depth of resin monomers .
Not only does the sensitivity of the bonding system to differently moisturized surface affect the bonding results, but also the sensitivity of the collagen fibres to dehydration . Air-drying after water rinsing at the end of the etching procedure might cause a collapse of the collagen fibres at the top of the demineralized zone, resulting in a layer with largely reduced porosity which inhibits resin permeation into the spaces around the collagen fibres. This enhances the likelihood following acid etching of the problem of a varying rate of unprotected collagen fibres in the deep layers of the demineralized zone, which the resin monomers cannot reach . This is confirmed by the findings of our study where Syntac (mean 14.07 MPa, range 13.09–15.2 MPa) achieved the best results when applied after laser preparation without subsequent phosphoric acid etching. The predictability was better than in all the nonlaser groups and the most constant results were achieved.
Laser preparation also leaves a moist dentin surface that needs drying before priming and bonding, but due to the nonexistent demineralization zone, there is no risk of collagen fibre collapse. Weak acids, like those used in self-etching systems (Syntac classic Primer), achieve bonding to dentin by etching and simultaneous infiltration of adhesive monomers into the dentin surface. The advantage is that no discrepancies occur between the extension of resin and etching depth . This avoids the unwanted side effect of unprotected collagen fibres in deep tissue layers as can be seen after phosphoric acid etching, where the created demineralization is deeper than the diffusion capacity of the resin monomers. A limiting factor for the penetration depth of self-etching systems with weak acids after conventional preparation is the remaining smear layer. Weak acids modify the smear layer . The modified products are integrated into the adhesive compound, thus inhibiting the formation of tags. The absence of a smear layer after Er,Cr:YSGG laser preparation avoids this zone of altered material. Conditioning agents such as weak acids remove the smear layer but do not demineralize the inorganic matrix of the tooth .
There is variability in the dentin bond strengths found in previous studies. Lee et al.  found no significant difference in the bond strength between bur-cut/acid-etched and Er,Cr:YSGG laser-ablated (3.5 W, 20 Hz) acid-etched human dentin. Sung et al.  found high dentin shear bond strengths in Er,Cr:YSGG laser-treated (4 and 5 W for preparation and 0.75 W for etching) human primary teeth. Recent studies have confirmed that enamel and dentin surfaces prepared with the Er,Cr:YSGG laser for direct composite restorations may provide comparable sealing . Türkmen et al. found that the Er,Cr:YSGG laser “etches” the enamel surface more effectively than 37% phosphoric acid for subsequent attachment of composite material . Our study showed that laser preparation produced the most constant results of all three main groups concerning shear bond strength. This might have been the result of greater predictability than with etching, rinsing and drying of the dentin. With regard to the choice of bonding system, Syntac classic showed the highest stability in the etching group as well as in the laser group, especially if it was not combined with phosphoric acid etching. The self-etching bonding system showed almost equal results in all main groups (mean shear bond strength 14.07, 14.06 and 14.07 MPa, respectively) together with the least variation. Thus the use of this system may allow phosphoric acid etching with all its risks (nonselective dentin dissolution, unpredictable penetration depth of acid, volume reduction by collagen fibre collapse) to be avoided without loss of stability of the restoration.
Laser preparation of dentin creates a retentive surface for composite restoration, avoiding the problems created by a smear layer after conventional preparation. The disadvantages of acid etching can be avoided. This leads to the conclusion that the laser preparation creates a surface texture which allows prediction of the quality of the restoration without the risk of negative influences during the following treatment steps. The formation of fine cylindrical tags with microbranches provides an ideal morphology of the resin–dentin interface. This can easily and repeatedly be achieved.
Conflicts of interest
All authors state that they have no conflicts of interest.