- Email: [email protected]
enamel bond strength of human enamel following the application of the Nd:YAG laser and etching with phosphoric acid
Dent Mater 13:51-55, January, 1997
A comparison of surface roughness and compositel enamel bond strength of human enamel following the application of the Nd:YAG laser and etching with phosphoric acid Maria T. Ariyaratnam, Margaret A. Wilson, lain C. Mackie, Anthony S. Blinkhorn Department of Dental Medicine and Surgery, University Dental Hospital, Manchester, UNITED KINGDOM
ABSTRACT Objectives. This study was conducted to evaluate enamel morphology after laser etching and acid etching and to determine the shear bond strength of composite to acid-etched and laser-treated enamel. Methods. Enamel from freshly extracted permanent molar teeth was subjected to either laser treatment with an Nd:YAG laser in different laser parameters orwas exposed to 37% phosphoric acid for 60 s (Gluma Gel, Bayer Dental). Surface profile analysis of the enamel was undertaken with a Perthometer (SSP, Feinpruif). The results were analyzed by SPSS/PC multiple range test and Student-Newman Keuls procedure. Specimens were examined in a scanning electron microscope (SEM). Shear bond strengths of acid-etched and laseretched enamel/composite (Brilliant Dentin, Coltene AG and Pekalux, Bayer Dental) were also determined. These results were compared by SPSS/PC multiple range test. Results. The acid-etched specimens exhibited a qualitatively different type of enamel surface morphology when compared with the laser-treated specimens. Laser treatment at higher exposures resulted in the formation of microcracks and fissures. No significant difference in surface roughness was observed between laser-treated enamel in three different parameters (10 pps, O.S W; 15 pps, 1.0 W; 20 pps, 1.25 W) and acid-etched specimens. However, the mean bond strengths of all lasertreated specimens, regardless of the test parameters, were significantly lower (p < 0.05) than the acid-etched enamel specimens. Significance. Although the laser roughened the surface of the enamel, it did not provide a surface as retentive as a surface treated with conventional acid etching. It is concluded from this study that the Nd:YAG laser operated under the conditions described cannot be recommended as a viable alternative to acid etching.
INTRODUCTION Lasers have been suggested for use in clinical dentistry since the early 1960s. The manufacturers ofdental lasers claim that they can be used as an alternative to acid etching for preparing enamel surfaces for bonding with resin composite materials.
A number of researchers have investigated this application of lasers in terms ofchanges in enamel surface morphology (Hibst and Keller, 1989; Hess, 1990), surface texture analysis (Nelson et al., 1987; Arcoria et ai., 1991; 1993) and bonding to enamel (Melendez et al., 1991: White et al., 1991). These authors have shown that the effects of lasers on hard tissues are dependent upon wavelength specificity and energy density. However, to date, there are no reported studies which have assessed the surface oflased enamel with a profilometer and SEM examination and tried to correlate these effects with mechanical bond strength measurements. The objectives of this study were two-fold: 1) to evaluate enamel morphology after laser etching and acid etching using a profilometer and to correlate the results with a qualitative assessment by SEM, and 2) to determine the shear bond strength ofcomposite to acid-etched and laser-treated enamel.
MATERIALS AND METHODS Freshly extracted, sound human permanent molar teeth were selected for this study. Following washing and removal of soft tissue remnants, the teeth were stored in 50% ethanol. The crowns were sectioned mesio-distally, with a water-cooled diamond disc (Bovis, Planomatic, England) to obtain a cut enamel surface. Four sections of enamel were obtained from each tooth. These sections were embedded in plaster blocks with the sectioned enamel surface uppermost. This surface was then abraded with 600 grit paper to create a smooth, uniform surface. A total of 80 specimens were prepared for surface roughness analysis. Eight groups (n = 10 for each group) were included in this study. Group 1 was used as a control, Groups 2A-7A were laser-etched using different parameters, and Group 8A was acid-etched. Sixty enamel specimens were randomly allocated to one of the six groups; these were air-dried for 60 s prior to laser treatment. A 4 mm diameter area of the sectioned enamel Dental Materials/January 199751
58
10
Acid-etching with 37% orthophosphoric acid
60
surface was painted with laser-enhancing gel (B.D.H. Formula, United Bristol Health Care, Bristol Royal Infirmary, Bristol, England). The contents of the laser-enhancing gel are: sterilized Indian ink, xanthan gum, methyl hydroxy benzoate and propyl hydroxy benzoate as preservatives, and water for mixing. This gel was gently spread over the enamel surface using an air syringe. Experimental laser treatment of enamel without the laser-enhancing gel did not produce any changes in enamel. The specimens were then exposed to a pulsed Nd:YAG laser (American Dental Laser, Birmingham, MI, USA) using six different energy levels and six different pulse rates (Table 1). The laser produces a near infrared wavelength of 1.06 pm with a pulse length of 150 J..lS delivered via a quartz optical fiber of 320 pm diameter. The optic fiber was used in a non-contact mode in a back and forth motion held at 3 mm from the surface to be treated. The laser treatment regimes chosen were recommended by the manufacturers. Experimental contact mode application of the optic fiber did not produce any effect on the surface of the enamel. For Group 8A specimens, a 37% orthophosphoric acid gel (Gluma Gel, Bayer Dental, Leverkusen, Germany) was applied to the enamel surface for 60 s. This was followed by thorough washing with water, and drying for 2 x 60 s.
52
Ariyaratnam et al.lA comparison of lased and etched enamel
The prepared surfaces were then tested for surface roughness using a profilometer (Perthometer S8P Feinpruif, Perthon, Germany) with a standard surface pickup range of ± 75 pm, using a conical diamond stylus at an angle of 90° ± 5° with a tip radius of5 pm ± 2 pm. The smallest feature the profilometer can measure is 1 pm. The vertical and horizontal resolutions were automatically calibrated. The surface roughness was measured in terms of Rm and Rt (in pm) where Rm, the maximum roughness depth, is the largest of five peak-to-valley values of surface roughness within the measuring length. Rt, the maximum roughness depth, is the greatest perpendicular distance between the highest and the lowest valley within the measuring length. Parallel tracings in each specimen were taken to include a 4 mm area of either the unlased enamel (Group lA); the lased enamel (Groups 2A, 3A, 4A, 5A, 6A and 7A); or the acid-etched enamel (Group 8A). These results were compared using SPSs/ PC multiple range tests and Student-Newman Keuls procedure. Two different composite materials,Brilliant Dentin, a highly filled hybrid composite (Batch No. DE738, Coltene AG, Altstatten, Switzerland) and Pekalux, a microfilled composite (Batch No. 0120F, Bayer Dental, Leverkusen, Germany), were used in this study. A total of 50 enamel specimens for each material (n = 10 for each group) were tested for bond strength between composite and enamel following the appropriate surface treatment of the enamel (Table 2). Three groups of enamel sections were treated with three different laser settings (Groups 2B, 3B and 4B). Orthophosphoric acid was applied to another group (5B) of enamel sections for 60 s, followed by washing and drying for 2 x 60 s. The remaining group (lB) served as a control. Bond strengths were determined following the application of the resin composite to the prepared sectioned plane surfaces of human enamel from recently extracted teeth. The protocol followed that recommended by the ISO (TR 11405, 1994). A shear test apparatus was utilized. A section of an orthodontic wire (0.7mm diameter) was formed into a U-shaped loop to wrap around the test cylinder of resin composite (Size 4 cellulose capsule). The wire was clamped to the shearing machine. Shear stress was applied with a crosshead speed of O.lmmls. Shear loads required to fracture the specimens were recorded and shear bond strengths were then calculated. The SPSS/PC multiple range test was used to analyze this data. Laser-treated and acid-etched specimens were examined under SEM. Longitudinal sections oflaser-treated enamel were also examined under SEM to determine the depth of penetration of the laser into the enamel.
RESULTS Mean surface roughness values for the different treatment groups were recorded (Table 3). A three-dimensional trace of lased enamel is shown in Fig. 1 and acid-etched enamel in Fig. 2.
In tenns ofsurface roughness, the lower power laser-treated enamel (Groups 2A and 3A) were not significantly different from the unlased enamel (Group lA). All the remaining lasertreated (Groups 4A, 5A, 6A and 7A) and the acid-etched enamel (Group 8A) were significantly different (p < 0.05) from the untreated enamel (Group lA). No significant difference could be found between laser-treated groups 4A, 5A, 6A and 7A and the acid-etched group 8A (Table 3). Mean bond strength values for the different treatment groups were calculated together with the standard deviations (Table 4). The bond strength values for Brilliant Dentin were higher than for Pekalux for all the experimental groups tested, but , v not statistically significant. The mean bond strengths of all laser parameters and acid-etched enamel tested were higher than the i VI'" ',HH H "H ,, unetched enamel for both the materials tested. All laser-treated specimens showed adhesive failures Fig. 1. A three-dimensional diamond contact stylus trace of surface roughness for lased enamel at 15 pps 1.0 W. at the bond sites. Some cohesive failures were evident for the acidtreated specimens. The mean bond strengths of all laser parameters tested were lower than of the acid-etched enamel. The Nd:YAG-treated groups in three different parameters (Groups 2B, 3B and 4B) were significantly different (p < 0.05) from the acid-etched group (Group 5B). Among the laser parameters tested, the bond strengths of Group 4B specimens were signifiG PROflL VCR 198.8 YH cantly different (p < 0.05) from the bond strength of specimens laseretched with other laser parameters tested. Fig.2. A three-dimensional diamond contact stylus trace of surface roughness for acid-etched enamel. The SEM observations of acidC
PW'UJ 11
I
I
Dental Materials/January 199753
Fig. 3. Scanning electron microscopic appearance of acid-etched enamel showing Type 2 etching pattern. Magnification, BOOx.
Fig. 4. Scanning electron microscopic appearance of laser-etched enamel at 10 pps O.B W. Magnification, 745x.
Fig. 5. Scanning electron microscopic appearance of laser-etched enamel at 20 pps 1.25 W. Magnification, BOOx.
Fig.6. Scanning electron microscopic appearance of sectioned laser-etched enamel. Magnification, BOOx.
etched enamel showed a typical Type 2 surface roughening where the prism cores projected toward the exposed enamel surface after preferential loss of the prism peripheries as described by Silverstone (1974) and Silverstone et 01. (1988). A typical example is shown in Fig. 3. The SEM photographs oflased enamel showed a wide variety of surface alterations: melted bubbles of enamel (Fig. 4), craters, cracking and micropores (Fig. 5). Longitudinal sections of laser-treated enamel showed that the average depth of penetration of the laser was confined to 30-40 pm of the superficial enamel (Fig. 6).
makes it possible to etch specific areas of the enamel (Morioka et 01., 1984; Hess 1990). The noncoated surface of the enamel remains unchanged when the beam is passed beyond the edge of the laser-enhanciIl.g gel. The near infrared Nd:YAG laser light transmits through water and penetrates wet tissues rapidly. It has been shown that the Nd:YAG laser works on a dark/light basis: the darker the material, the better the absorption (Hecht, 1992; Fortin, 1992). The use of the laser-enhancing gel thus enhanced the laser absorption into the enamel in the present study. Different laser parameters oflO pps and 0.8 W, 15 pps and 1. Ow, 20 pps and 1.25W resulted in similar surface roughness values, which compared closely with the values obtained from acid etching. Willems et aZ. (1991) have also shown that several roughness patterns with quite different geometries and properties could result in the same roughness values. Agden (1991) concluded that SEM photographs are required to give an accurate description and evaluation of surface roughness.
DISCUSSION Laser etching produced a qualitatively different surface profile and roughness which was significantly different from untreated enamel. This agrees with previous studies of Arcoria et aZ. (1991; 1993). Laser etching of the enamel was not possible without a laser-enhancing gel. The application of a laser-enhancing gel 54
Ariyaratnam et aUA comparison of lased and etched enamel
It can be seen from the SEM results that the pattern of roughness for acid etching and laser etching (Figs. 3,4 and 5) was quite dilferent. Acid etching produced a Type 2 etching pattern. With laser etching, the melted bubble-like openings are thought to be "beads" of hydroxyapatite which have resulted from a superheated enamel surface that has undergone subsequent cooling at room temperature. Although surface roughness values were similar for the laser parameters of 10 pps 0.8 W; 15 pps 1.0 W and 20 pps 1.25 W, bond strength results showed that at 20 pps and 1.25 W, bond strength was significantly greater than the other laser parameters tested. Studies byWhiteet al. (1991) to detennine orthodontic bracket bond strength to Nd:YAG laser-etched enamel have also shown that the shear bond strength increased proportionately to power and energy per pulse, and maximum bond strength was achieved at the frequency of20 pps. In the present study, bond strength results have shown that despite similar surface roughness values, acid-etchingproduced significantly greater bond strength than all the laser parameters tested for both materials used. This agrees with studies of Jamjoum et al. (1995). It is thus evident that the bond strength results do not correlate to the measured surlace roughness. It is also evident that the enamel surface following laser-etching does not provide a favorable etching surface when compared with acid-etching for composite bonding. In this study, SEM evaluation showed evidence of cracks and fissures at 20 pps and 1.25 W laser ablation. The formation of microcracks, fissures or chipped surfaces may occur because of rapid thermal cycling of the surface enamel during pulsed laser irradiation. Since the thickness of the enamel varies in different parts of the tooth, laser-propagated cracks could easily penetrate to the amelD-dentinal junction. However, SEM observations of sections of laser-treated enamel have shown that the effects of the laser did not penetrate into deeper enamel and were confined to the superficial layers of 30-40 J.lIll. The differences found in this study are of clinical significance, as the shear bond strength of resin composite to enamel was significantly lower (p < 0.05) for laser-treated enamel than for acid-etched enamel. Thus, the dental laser operated under the conditions described cannot be recommended as a viable alternative to the conventional acidetch technique.
Received May 29, 19961 Accepted October 19, 1996 Address for correspondence and reprint requests to: Anthony S. Blinkhorn Oral Health and Development University Dental Hospital Higher Cambridge Street Manchester M15 6FH, UNITED KINGDOM
Phone: +44-161-275-6610 Fax: +44-161-275-6610 em: [email protected]
REFERENCES Agden A (1991). Evaluation of finishing and polishing teclmiques on surface roughness ofCrCo casting.J Prosthet Dent 65:762-769. Arcoria CJ, Steele RE, Wagner MJ (1991). Enamel surface roughness and dental pulp response to coaxial carbon dioxide-neodymium:YAG laser irradiation. J Dent 19:8589. Arcoria CJ, Lippas MG, Vitasek BA (1993). Enamel surface roughness analysis after laser ablation and acid etching. J Oral Rehabi120:213-224. Fortin RH (1992). Choosing fibres for medical beam delivery. Lasers and Optro 4:29-32. Hecht J (1992). Nd:YAG lasers prove versatile over three decades. Laser Foe World 4: 126-132. Hess B (1990). SEM study of laser induced morphological changes ofa coated enamel surface. Lasers Surg Med 10:458462. Hibst R, Keller U (1989). Experimental studies of the application of the Er:YAG laser on the dental hard substances. Measurement of ablation rates. Lasers Surg Med 9:338-344. ISO Technical Report 11405 (1994). Dental materials guidance on testing of adhesion to tooth structure. International Standards Organisation, Geneva. Jamjoum H, Pearson GH, McDonald AV (1995). A comparative study of etching enamel by acid and laser. Lasers Med Sci 10:37-42. Melendez EJ, Arcoria CJ, Wagner MJ (1991). Composite! enamel bond strength using multiple lasers versus acid etching. J Dent Res 70:571,Abstr. No. 2444. Morioka TJ, Suzuki K, Tagamuri S (1984). Effect of beam absorption mediators on acid resistance of surface by Nd:YAG laser. J Dent Health 34:44. Nelson DG, Wefel JS, JongehloedWL, Featherstone JD (1987). Morphology, histology and crystallgraphs of human dental enamel treated with pulsed low energy infrared sedation. Caries Res 21:411-426. Silverstone M (1974). Fissure sealants. Caries Res 8:2-26. Silverstone M, Hicks M, Featherstone M (1988). Dynamic factors affecting lesion initiation and progression in human enamel. Surface morphologies ofsound and carious enamel. Quintessence Inter 19:773-780. White JM, Goodis HE, Asbill SR, Watanabe LG (1991). Orthodontic bracket bond strength ofNd:YAG laser etched enamel. J Dent Res 70:297,Abstr. No. 252. Willems H, Lambrechts P, Braem M, Vanherle G (1991). The surface roughness of enamel-to-enamel contact areas compared with the intrinsic roughness of dental resin composites. J Dent Res 70: 1299-1301.
Dental Materials/January 1997 55
A comparison of surface roughness and compositel enamel bond strength of human enamel following the application of the Nd:YAG laser and etching with phosphoric acid Maria T. Ariyaratnam, Margaret A. Wilson, lain C. Mackie, Anthony S. Blinkhorn Department of Dental Medicine and Surgery, University Dental Hospital, Manchester, UNITED KINGDOM
ABSTRACT Objectives. This study was conducted to evaluate enamel morphology after laser etching and acid etching and to determine the shear bond strength of composite to acid-etched and laser-treated enamel. Methods. Enamel from freshly extracted permanent molar teeth was subjected to either laser treatment with an Nd:YAG laser in different laser parameters orwas exposed to 37% phosphoric acid for 60 s (Gluma Gel, Bayer Dental). Surface profile analysis of the enamel was undertaken with a Perthometer (SSP, Feinpruif). The results were analyzed by SPSS/PC multiple range test and Student-Newman Keuls procedure. Specimens were examined in a scanning electron microscope (SEM). Shear bond strengths of acid-etched and laseretched enamel/composite (Brilliant Dentin, Coltene AG and Pekalux, Bayer Dental) were also determined. These results were compared by SPSS/PC multiple range test. Results. The acid-etched specimens exhibited a qualitatively different type of enamel surface morphology when compared with the laser-treated specimens. Laser treatment at higher exposures resulted in the formation of microcracks and fissures. No significant difference in surface roughness was observed between laser-treated enamel in three different parameters (10 pps, O.S W; 15 pps, 1.0 W; 20 pps, 1.25 W) and acid-etched specimens. However, the mean bond strengths of all lasertreated specimens, regardless of the test parameters, were significantly lower (p < 0.05) than the acid-etched enamel specimens. Significance. Although the laser roughened the surface of the enamel, it did not provide a surface as retentive as a surface treated with conventional acid etching. It is concluded from this study that the Nd:YAG laser operated under the conditions described cannot be recommended as a viable alternative to acid etching.
INTRODUCTION Lasers have been suggested for use in clinical dentistry since the early 1960s. The manufacturers ofdental lasers claim that they can be used as an alternative to acid etching for preparing enamel surfaces for bonding with resin composite materials.
A number of researchers have investigated this application of lasers in terms ofchanges in enamel surface morphology (Hibst and Keller, 1989; Hess, 1990), surface texture analysis (Nelson et al., 1987; Arcoria et ai., 1991; 1993) and bonding to enamel (Melendez et al., 1991: White et al., 1991). These authors have shown that the effects of lasers on hard tissues are dependent upon wavelength specificity and energy density. However, to date, there are no reported studies which have assessed the surface oflased enamel with a profilometer and SEM examination and tried to correlate these effects with mechanical bond strength measurements. The objectives of this study were two-fold: 1) to evaluate enamel morphology after laser etching and acid etching using a profilometer and to correlate the results with a qualitative assessment by SEM, and 2) to determine the shear bond strength ofcomposite to acid-etched and laser-treated enamel.
MATERIALS AND METHODS Freshly extracted, sound human permanent molar teeth were selected for this study. Following washing and removal of soft tissue remnants, the teeth were stored in 50% ethanol. The crowns were sectioned mesio-distally, with a water-cooled diamond disc (Bovis, Planomatic, England) to obtain a cut enamel surface. Four sections of enamel were obtained from each tooth. These sections were embedded in plaster blocks with the sectioned enamel surface uppermost. This surface was then abraded with 600 grit paper to create a smooth, uniform surface. A total of 80 specimens were prepared for surface roughness analysis. Eight groups (n = 10 for each group) were included in this study. Group 1 was used as a control, Groups 2A-7A were laser-etched using different parameters, and Group 8A was acid-etched. Sixty enamel specimens were randomly allocated to one of the six groups; these were air-dried for 60 s prior to laser treatment. A 4 mm diameter area of the sectioned enamel Dental Materials/January 199751
58
10
Acid-etching with 37% orthophosphoric acid
60
surface was painted with laser-enhancing gel (B.D.H. Formula, United Bristol Health Care, Bristol Royal Infirmary, Bristol, England). The contents of the laser-enhancing gel are: sterilized Indian ink, xanthan gum, methyl hydroxy benzoate and propyl hydroxy benzoate as preservatives, and water for mixing. This gel was gently spread over the enamel surface using an air syringe. Experimental laser treatment of enamel without the laser-enhancing gel did not produce any changes in enamel. The specimens were then exposed to a pulsed Nd:YAG laser (American Dental Laser, Birmingham, MI, USA) using six different energy levels and six different pulse rates (Table 1). The laser produces a near infrared wavelength of 1.06 pm with a pulse length of 150 J..lS delivered via a quartz optical fiber of 320 pm diameter. The optic fiber was used in a non-contact mode in a back and forth motion held at 3 mm from the surface to be treated. The laser treatment regimes chosen were recommended by the manufacturers. Experimental contact mode application of the optic fiber did not produce any effect on the surface of the enamel. For Group 8A specimens, a 37% orthophosphoric acid gel (Gluma Gel, Bayer Dental, Leverkusen, Germany) was applied to the enamel surface for 60 s. This was followed by thorough washing with water, and drying for 2 x 60 s.
52
Ariyaratnam et al.lA comparison of lased and etched enamel
The prepared surfaces were then tested for surface roughness using a profilometer (Perthometer S8P Feinpruif, Perthon, Germany) with a standard surface pickup range of ± 75 pm, using a conical diamond stylus at an angle of 90° ± 5° with a tip radius of5 pm ± 2 pm. The smallest feature the profilometer can measure is 1 pm. The vertical and horizontal resolutions were automatically calibrated. The surface roughness was measured in terms of Rm and Rt (in pm) where Rm, the maximum roughness depth, is the largest of five peak-to-valley values of surface roughness within the measuring length. Rt, the maximum roughness depth, is the greatest perpendicular distance between the highest and the lowest valley within the measuring length. Parallel tracings in each specimen were taken to include a 4 mm area of either the unlased enamel (Group lA); the lased enamel (Groups 2A, 3A, 4A, 5A, 6A and 7A); or the acid-etched enamel (Group 8A). These results were compared using SPSs/ PC multiple range tests and Student-Newman Keuls procedure. Two different composite materials,Brilliant Dentin, a highly filled hybrid composite (Batch No. DE738, Coltene AG, Altstatten, Switzerland) and Pekalux, a microfilled composite (Batch No. 0120F, Bayer Dental, Leverkusen, Germany), were used in this study. A total of 50 enamel specimens for each material (n = 10 for each group) were tested for bond strength between composite and enamel following the appropriate surface treatment of the enamel (Table 2). Three groups of enamel sections were treated with three different laser settings (Groups 2B, 3B and 4B). Orthophosphoric acid was applied to another group (5B) of enamel sections for 60 s, followed by washing and drying for 2 x 60 s. The remaining group (lB) served as a control. Bond strengths were determined following the application of the resin composite to the prepared sectioned plane surfaces of human enamel from recently extracted teeth. The protocol followed that recommended by the ISO (TR 11405, 1994). A shear test apparatus was utilized. A section of an orthodontic wire (0.7mm diameter) was formed into a U-shaped loop to wrap around the test cylinder of resin composite (Size 4 cellulose capsule). The wire was clamped to the shearing machine. Shear stress was applied with a crosshead speed of O.lmmls. Shear loads required to fracture the specimens were recorded and shear bond strengths were then calculated. The SPSS/PC multiple range test was used to analyze this data. Laser-treated and acid-etched specimens were examined under SEM. Longitudinal sections oflaser-treated enamel were also examined under SEM to determine the depth of penetration of the laser into the enamel.
RESULTS Mean surface roughness values for the different treatment groups were recorded (Table 3). A three-dimensional trace of lased enamel is shown in Fig. 1 and acid-etched enamel in Fig. 2.
In tenns ofsurface roughness, the lower power laser-treated enamel (Groups 2A and 3A) were not significantly different from the unlased enamel (Group lA). All the remaining lasertreated (Groups 4A, 5A, 6A and 7A) and the acid-etched enamel (Group 8A) were significantly different (p < 0.05) from the untreated enamel (Group lA). No significant difference could be found between laser-treated groups 4A, 5A, 6A and 7A and the acid-etched group 8A (Table 3). Mean bond strength values for the different treatment groups were calculated together with the standard deviations (Table 4). The bond strength values for Brilliant Dentin were higher than for Pekalux for all the experimental groups tested, but , v not statistically significant. The mean bond strengths of all laser parameters and acid-etched enamel tested were higher than the i VI'" ',HH H "H ,, unetched enamel for both the materials tested. All laser-treated specimens showed adhesive failures Fig. 1. A three-dimensional diamond contact stylus trace of surface roughness for lased enamel at 15 pps 1.0 W. at the bond sites. Some cohesive failures were evident for the acidtreated specimens. The mean bond strengths of all laser parameters tested were lower than of the acid-etched enamel. The Nd:YAG-treated groups in three different parameters (Groups 2B, 3B and 4B) were significantly different (p < 0.05) from the acid-etched group (Group 5B). Among the laser parameters tested, the bond strengths of Group 4B specimens were signifiG PROflL VCR 198.8 YH cantly different (p < 0.05) from the bond strength of specimens laseretched with other laser parameters tested. Fig.2. A three-dimensional diamond contact stylus trace of surface roughness for acid-etched enamel. The SEM observations of acidC
PW'UJ 11
I
I
Dental Materials/January 199753
Fig. 3. Scanning electron microscopic appearance of acid-etched enamel showing Type 2 etching pattern. Magnification, BOOx.
Fig. 4. Scanning electron microscopic appearance of laser-etched enamel at 10 pps O.B W. Magnification, 745x.
Fig. 5. Scanning electron microscopic appearance of laser-etched enamel at 20 pps 1.25 W. Magnification, BOOx.
Fig.6. Scanning electron microscopic appearance of sectioned laser-etched enamel. Magnification, BOOx.
etched enamel showed a typical Type 2 surface roughening where the prism cores projected toward the exposed enamel surface after preferential loss of the prism peripheries as described by Silverstone (1974) and Silverstone et 01. (1988). A typical example is shown in Fig. 3. The SEM photographs oflased enamel showed a wide variety of surface alterations: melted bubbles of enamel (Fig. 4), craters, cracking and micropores (Fig. 5). Longitudinal sections of laser-treated enamel showed that the average depth of penetration of the laser was confined to 30-40 pm of the superficial enamel (Fig. 6).
makes it possible to etch specific areas of the enamel (Morioka et 01., 1984; Hess 1990). The noncoated surface of the enamel remains unchanged when the beam is passed beyond the edge of the laser-enhanciIl.g gel. The near infrared Nd:YAG laser light transmits through water and penetrates wet tissues rapidly. It has been shown that the Nd:YAG laser works on a dark/light basis: the darker the material, the better the absorption (Hecht, 1992; Fortin, 1992). The use of the laser-enhancing gel thus enhanced the laser absorption into the enamel in the present study. Different laser parameters oflO pps and 0.8 W, 15 pps and 1. Ow, 20 pps and 1.25W resulted in similar surface roughness values, which compared closely with the values obtained from acid etching. Willems et aZ. (1991) have also shown that several roughness patterns with quite different geometries and properties could result in the same roughness values. Agden (1991) concluded that SEM photographs are required to give an accurate description and evaluation of surface roughness.
DISCUSSION Laser etching produced a qualitatively different surface profile and roughness which was significantly different from untreated enamel. This agrees with previous studies of Arcoria et aZ. (1991; 1993). Laser etching of the enamel was not possible without a laser-enhancing gel. The application of a laser-enhancing gel 54
Ariyaratnam et aUA comparison of lased and etched enamel
It can be seen from the SEM results that the pattern of roughness for acid etching and laser etching (Figs. 3,4 and 5) was quite dilferent. Acid etching produced a Type 2 etching pattern. With laser etching, the melted bubble-like openings are thought to be "beads" of hydroxyapatite which have resulted from a superheated enamel surface that has undergone subsequent cooling at room temperature. Although surface roughness values were similar for the laser parameters of 10 pps 0.8 W; 15 pps 1.0 W and 20 pps 1.25 W, bond strength results showed that at 20 pps and 1.25 W, bond strength was significantly greater than the other laser parameters tested. Studies byWhiteet al. (1991) to detennine orthodontic bracket bond strength to Nd:YAG laser-etched enamel have also shown that the shear bond strength increased proportionately to power and energy per pulse, and maximum bond strength was achieved at the frequency of20 pps. In the present study, bond strength results have shown that despite similar surface roughness values, acid-etchingproduced significantly greater bond strength than all the laser parameters tested for both materials used. This agrees with studies of Jamjoum et al. (1995). It is thus evident that the bond strength results do not correlate to the measured surlace roughness. It is also evident that the enamel surface following laser-etching does not provide a favorable etching surface when compared with acid-etching for composite bonding. In this study, SEM evaluation showed evidence of cracks and fissures at 20 pps and 1.25 W laser ablation. The formation of microcracks, fissures or chipped surfaces may occur because of rapid thermal cycling of the surface enamel during pulsed laser irradiation. Since the thickness of the enamel varies in different parts of the tooth, laser-propagated cracks could easily penetrate to the amelD-dentinal junction. However, SEM observations of sections of laser-treated enamel have shown that the effects of the laser did not penetrate into deeper enamel and were confined to the superficial layers of 30-40 J.lIll. The differences found in this study are of clinical significance, as the shear bond strength of resin composite to enamel was significantly lower (p < 0.05) for laser-treated enamel than for acid-etched enamel. Thus, the dental laser operated under the conditions described cannot be recommended as a viable alternative to the conventional acidetch technique.
Received May 29, 19961 Accepted October 19, 1996 Address for correspondence and reprint requests to: Anthony S. Blinkhorn Oral Health and Development University Dental Hospital Higher Cambridge Street Manchester M15 6FH, UNITED KINGDOM
Phone: +44-161-275-6610 Fax: +44-161-275-6610 em: [email protected]
REFERENCES Agden A (1991). Evaluation of finishing and polishing teclmiques on surface roughness ofCrCo casting.J Prosthet Dent 65:762-769. Arcoria CJ, Steele RE, Wagner MJ (1991). Enamel surface roughness and dental pulp response to coaxial carbon dioxide-neodymium:YAG laser irradiation. J Dent 19:8589. Arcoria CJ, Lippas MG, Vitasek BA (1993). Enamel surface roughness analysis after laser ablation and acid etching. J Oral Rehabi120:213-224. Fortin RH (1992). Choosing fibres for medical beam delivery. Lasers and Optro 4:29-32. Hecht J (1992). Nd:YAG lasers prove versatile over three decades. Laser Foe World 4: 126-132. Hess B (1990). SEM study of laser induced morphological changes ofa coated enamel surface. Lasers Surg Med 10:458462. Hibst R, Keller U (1989). Experimental studies of the application of the Er:YAG laser on the dental hard substances. Measurement of ablation rates. Lasers Surg Med 9:338-344. ISO Technical Report 11405 (1994). Dental materials guidance on testing of adhesion to tooth structure. International Standards Organisation, Geneva. Jamjoum H, Pearson GH, McDonald AV (1995). A comparative study of etching enamel by acid and laser. Lasers Med Sci 10:37-42. Melendez EJ, Arcoria CJ, Wagner MJ (1991). Composite! enamel bond strength using multiple lasers versus acid etching. J Dent Res 70:571,Abstr. No. 2444. Morioka TJ, Suzuki K, Tagamuri S (1984). Effect of beam absorption mediators on acid resistance of surface by Nd:YAG laser. J Dent Health 34:44. Nelson DG, Wefel JS, JongehloedWL, Featherstone JD (1987). Morphology, histology and crystallgraphs of human dental enamel treated with pulsed low energy infrared sedation. Caries Res 21:411-426. Silverstone M (1974). Fissure sealants. Caries Res 8:2-26. Silverstone M, Hicks M, Featherstone M (1988). Dynamic factors affecting lesion initiation and progression in human enamel. Surface morphologies ofsound and carious enamel. Quintessence Inter 19:773-780. White JM, Goodis HE, Asbill SR, Watanabe LG (1991). Orthodontic bracket bond strength ofNd:YAG laser etched enamel. J Dent Res 70:297,Abstr. No. 252. Willems H, Lambrechts P, Braem M, Vanherle G (1991). The surface roughness of enamel-to-enamel contact areas compared with the intrinsic roughness of dental resin composites. J Dent Res 70: 1299-1301.
Dental Materials/January 1997 55