- Email: [email protected]
Optimal Er:YAG laser energy for preventing enamel demineralization
Journal of Dentistry (2006) 34, 62–66
www.intl.elsevierhealth.com/journals/jden
Optimal Er:YAG laser energy for preventing enamel demineralization Jeng-fen Liua, Yuanyuan Liub, Hsu Chin-Ying Stephenb,* a
National Yang Ming Medical University, Taipei, Taiwan Department of Preventive Dentistry, Faculty of Dentistry, National University of Singapore, 5 Lower Kent Ridge Road, 119074 Singapore
b
Received 12 May 2004; received in revised form 15 March 2005; accepted 18 March 2005
KEYWORDS Er:YAG laser; Enamel demineralization; Caries prevention; Polarized light microscopy
Summary Objectives: The purpose of this study was to identify the optimal laser energy range of Er:YAG laser irradiation for laser-induced caries prevention (LICP). Methods: Twenty-one human non-carious molars were selected. The teeth were covered with nail varnish, except two 4 mm!1 mm windows on both the buccal and lingual surfaces. The windows were randomly assigned to groups A, B, C and D, receiving no irradiation, 100, 200 and 300 mJ irradiation, respectively. The pulse width 10 pps (pulse per second) with a 1.0 mm spot size was used. After the laser treatment, each tooth was cut into two halves longitudinally. Then a two-day pHcycling was performed, with an 18-hour demineralization followed by a 6-hour remineralization. Sections of 120G20 mm in thickness were obtained from each window. Lesion depth was measured using polarized light microscope coupled with an image analysis software. One-way ANOVA and post-hoc Tukey tests were used for evaluation of treatment effects. Results: The laser treatments of 100 and 200 mJ have demonstrated significant protection of enamel demineralization (pZ0.01 and 0.001, respectively), but not the treatment with 300 mJ (pZ0.106). A smaller lesion depth was observed for the 200 mJ group (97.1 mm) than that of the 100 mJ group (105.6 mm). Compared with the control, a lesion reduction of 32.78 and 26.93% for the 200 mJ group and the 100 mJ group were obtained, respectively. Conclusion: Caries prevention may be achieved by using Er:YAG laser treatment if the optimal range of laser parameters for LICP can be employed. Q 2005 Elsevier Ltd. All rights reserved.
Introduction
* Corresponding author. Tel.: C65 6779 5555x1658; fax: C65 6773 2602. E-mail address: [email protected] (H.C.-Y. Stephen).
The Erbium-doped:Yttrium-aluminum-garnet (Er:YAG) laser, which emits at a wavelength of 2.94 mm in the mid-infrared region, has a potential for hard-tissue treatment due to its high
0300-5712/$ - see front matter Q 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.jdent.2005.03.005
Optimal Er:YAG laser energy for preventing enamel demineralization absorbability in water and hydroxyapatite.1,2 It is the first dental laser approved to be used for hard tissue ablation by the U.S. Food and Drug Administration (FDA) in 1997, and has also demonstrated significant effects on caries prevention.3,4 However, the optimal energy range for caries prevention by using Er:YAG laser has not yet been reported. When enamel was heated, the acid resistance increased from 100 to 200 8C but then decreased between 300 and 400 8C.5 If the photothermal effect is essential for laser-induced caries prevention (LICP), it is possible that LICP may decrease when the laser energy is beyond the optimal range. On the other hand, if the laser energy is below the optimal range, the LICP may not be significant.6 Therefore, the purpose of this study was to identify the optimal range of Er:YAG laser energy for LICP.
Materials and methods Tooth selection and grouping Twenty-one human non-carious molars, stored in 0.1% thymol solution, were selected and cleaned. The teeth were covered with nail varnish, leaving two 4 mm!1 mm windows on both the buccal and lingual surfaces at the middle third of the crown. The windows were randomly assigned to groups A, B, C and D.
Laser treatment An Er:YAG laser (Opus 20, Sharplan 4020M, Israel) emitting at a wavelength of 2.94 mm was used in this study. Based on the results of pilot studies, group A was kept as the control group, and teeth of groups B, C, D were treated with the laser energies of 100, 200 and 300 mJ, respectively. Without water spray, the operation was performed by clamping the handpiece of the laser and moving the teeth manually with 10 pps (pulse per second), and 1.0 mm spot size. Since the size of window is 1 mm by 4 mm and the focal spot of the laser is 1.0 mm in diameter, the irradiation was carried out in a series of four steps of one-second irradiation each. This procedure was repeated three more times so that each ‘spot’ in a window is irradiated four times. Clinically, 600–700 mJ has been used to remove caries and for cavity preparation. Based on our pilot studies, the irradiation energy of 100, 200 and 300 mJ were chosen because these sub-ablative parameters are much lower than that for cavity preparation, and is very unlikely to damage the normal tooth structures. If we would use water
63
spray, a higher energy may be needed for significant LICP and thus increase the possibility of harming the peripheral dental tissues. The energy densities were calculated to be 12.7, 25.5 and 38.2 J/cm2 for the B, C and D groups, respectively.
pH-cycling process After the laser treatment, each tooth was cut into two halves longitudinally (disto-mesial) by an alloy grinder (DEMCOe, Dental Maintenance Co., Inc. Bonsall, CA, USA). The cut surfaces were covered with nail varnish and dried for 24 hours. A two-day pH-cycling scheme was performed, with an 18-hour demineralization followed by a 6-hour remineralization, with a stirring speed of 130 rpm at 37 8C.7 The demineralization solution at pH 4.5 contained 0.05 M acetic acid, 2.2 mM calcium, and 2.2 mM phosphate ions. The remineralization solution at pH 7.0 contained 0.15 M potassium chloride, 1.5 mM calcium, and 0.9 mM phosphate ions. A 5-min wash in the de-ionized and distilled water was done between the demineralization and remineralization phases and at the end of the process. Both demineralization and remineralization solutions were changed daily. All tooth halves were stored in plastic containers with 100% humidity before being sectioned.
Evaluation of demineralization A Silverstone-Taylore hard tissue microtome (series 1000 Deluxe, SciFab, Littleton, Co, USA) was used to obtain several sections of 120G20 mm in thickness from each window. Samples were characterized with stereomicroscope. Five teeth damaged during the procedures were discarded. With water as the imbibition solution, the demineralization patterns of selected slices were assessed using a polarized light microscope (Olympuse BH-2, Olympus Corp. of America, New Hyde Park, NY, USA). Using the Micro Image (Olympuse, Japan), a representative lesion area (200 mm in width) of each section was selected and measured as shown in Fig. 1. The “Lesion Area” value was divided by 200 mm to produce the average lesion depth for each section. The measurements of 4G2 sections from each window were averaged to represent the window lesion.
Statistical analysis The lesion depth (mm) obtained with the above method was used as the dependent variable. The independent variable was the laser energy of four levels, including 0, 100, 200 and 300 mJ.
64
J.-f. Liu et al. 200 mJ have demonstrated significant protection of enamel demineralization (pZ0.01 and 0.001, respectively) but not the treatment with 300 mJ (pZ0.106). The lesion depth of the C group (97.1 mm) is less than the B group (105.6 mm), although the difference has not reached the significant threshold. Compared with the lesion of the control windows, lesion reduction of 32.78 and 26.93% for the C and B group were obtained, respectively. Fig. 2 indicates representative lesions for each group.
Figure 1
Discussions
Illustration of lesion depth measurement.
The result of the Levene’s test (pZ0.860) confirmed the homogeneity of variances. Therefore, the parametric test (one-way ANOVA) was used for assessing the treatment effects. A post hoc test, the Tukey test, was chosen for evaluation of the statistical significance of the pair-wise comparisons between the four groups. A usual alpha level of 0.05 was used to determine the statistical significance of the results.
Results Qualitative analysis The enamel appears to be unaffected and remains indistinguishable between the laser-treated spots and untreated areas. When viewed under the stereoscope, enamel surfaces revealed no crater, crack or melting.
Effect of laser-treatment on demineralization A 32.78% lesion reduction was found in this study using 200 mJ Er:YAG laser. This is in agreement with previous studies,3,4,8 revealing a 5–67% reduction of enamel demineralization after irradiation with the Er:YAG laser. A marked (40%) caries inhibition was achieved by Fried et al.4 using sub-ablative laser parameters. Apel et al.8 have also reported a significant decline in enamel solubility after laser irradiation. Water-cooling was not used in our study, although irradiation with and without water-cooling appeared to be both significantly effective in caries prevention.3 In a previous study, when teeth samples were treated using pulse energy of 400 mJ, 2 Hz, and a spot size of 0.63 mm, the result revealed a statistically significant preventive effect, which was more evidenced without water mist (67%) than with water mist (15%).3 If water-cooling was used in our study, the LICP might be less.
Quantitative analysis Potential mechanisms for observed LICP The means of lesion depths and standard deviations of the four groups are listed in Table 1. Evaluated by using ANOVA, the enamel lesion depth of all four groups are not equal (pZ0.001). In comparison with the control, the laser treatments with 100 and Table 1
This preventive effect may be due to the laserinduced heating of the samples and the subsequent crystallographic9 and compositional changes.10 These changes have been reported to attributed to
Comparison of mean lesion depths of four experimental groups.
Group
Energy (mJ)
Na
Mean (mm)
Std. deviation (mm)
Rankingb
A B C D
0 100 200 300
16 16 16 16
144.46 105.56 97.10 116.66
35.10 39.82 29.40 30.41
I II II I
a
Sample size. The ranking order was obtained from the post-hoc Tukey multiple comparisons. Groups with defferent numerals are statistically different at a level of p!0.05 or lower. b
Optimal Er:YAG laser energy for preventing enamel demineralization
Figure 2
65
Representative lesions for A(0 mJ), B(100 mJ), C(200 mJ) and D(300 mJ) groups.
the decomposition of carbonate15 and removal of water etc.11 In previous studies, the reduction of carbonate was observed when enamel was treated using argon laser12 at an energy density of 67 J/cm2 and Er:YAG laser13 at 5.1 J/cm2. The quantity of carbonate will influence the enamel susceptibility to in vitro demineralization because they fit less well in the lattice and therefore give rise to less stable and more acid-soluble apatite-phase.14,15 It was found that the mineral loss started preferentially from the central core and then the outer core of the crystallites of enamel.16 It is likely that the existence of carbonate causes some crystal defect and remains as the highly strained area. Laser treatment may remove the carbonate and increase crystalline stability leading to the enamel less vulnerable to acid attack. Laser-induced temperature rise may have changed the enamel structure. Zuerlein et al.17 inferred that carbonate decomposes only from 400 8C onwards. However, Fowler and Kuroda11 indicated that there was substantial loss of carbonate and water at temperatures between 100 and 400 8C, which were sufficient to change the crystallinity of the enamel mineral. As for the water, loosely bounded water losses occur when enamel is heated up to 200 8C10 while tightly bounded water is totally eliminated only at high temperatures (1300 8C).18 Study demonstrated that the loss of water between 200 and 400 8C is irreversible and causes a contraction in the a-axis dimensions.19 In a recent study, Bachmann et al.20 has discussed the water loss of heated enamel powder by using IR measurement. They stated that the removal of water depends largely on the temperature generated at the irradiated spot and surrounding regions, i.e.
100–400 8C mainly for absorbed water, 700–1000 8C mainly for structural water. Another contributing factor may be porosity changes.21–24 Our pervious data showed that the surface area and pore volume of normal enamel decreased significantly after laser treatment.23 It was indicated that laser-induced melting of organic matrix may block the diffusion pathway and result in a reduced efficiency in ion diffusion and subsequent enamel demineralization.21,22,24
Effect of laser power The laser treatments using 100 and 200 mJ have demonstrated significant protection of enamel demineralization but not the treatment with 300 mJ. The significant role of organic matrix in the laser-induced retardation of enamel demineralization has been elucidated.22,24 The organic blocking effect has resulted in a 25 and 57% inhibition of mineral loss and lesion progress, respectively. Protein in enamel was found to decompose at about 350–400 8C.10 Our previous study suggested the organic blocking effect may reach a maximum and decrease after the decomposition of organic matrix above 400 8C.22 Other PLM studies showed that the majority of enamel structures became positively birefringent when heated to 400 8C.5,25 Based on aforementioned studies, laser treatment used for the group D may have destroyed the organic matrix and thus reduced the organic blocking effect. Therefore, our results have substantiated the notion that greater laser energy does not always provide more LICP. Furthermore, the insignificant or negative LICP shown in previous
66 studies may be due to their laser parameters falling either below or above the optimal range.6,8,26 In summary, this study has identified an optimal energy range (around 100 and 200 mJ) for LICP of Er:YAG laser treatment without water cooling. Using laser energy greater or lower than the optimal range may decrease the LICP. The optimal ranges of other Er:YAG laser parameters (e.g. frequency) are currently under investigation.
Conclusion An optimal energy range of Er:YAG irradiation has been established for inhibition of enamel demineralization. Caries prevention may be achieved by using Er:YAG laser treatment if the optimal range of laser parameters for LICP can be employed.
J.-f. Liu et al.
9.
10. 11.
12.
13.
14.
Acknowledgements 15.
This project is financially supported by grants (TCVGH 925603, from the Ministry of Administration, Taiwan), grants R-222-000-013-112 and R-222-000-015305 (both from the Ministry of Education and the Biomedical Research Council, Singapore).
References 1. Paghdiwala A. Er:YAG laser hard tissue effects. In: Westford, editor. Lasers in dentistry. Massachusetts: Penn Well Publishing Co; 1991 p 63–75. 2. Wigdor H, Walsh JJ, Featherstone J, Visuri S, Fried D, Waldvogel J. Lasers in dentistry. Lasers in Surgery and Medicine 1995;16(2):103–33. 3. Hossain M, Nakamura Y, Kimura Y, Yamada Y, Ito M, Matsumoto K. Caries-preventive effect of Er:YAG laser irradiation with or without water mist. Journal of Clinical Laser in Medicinal Surgery 2000;18(2):61–5. 4. Fried D, Featherstone J, Visuri S, Seka W, Walsh J. The caries inhibition potential of Er:YAG and Er:YSGG laser radiation. Proceedings of SPIE-International Society Of Optical Engineearing 1996;2672:73–8. 5. Sato K. Relation between acid dissolution and histological alteration of heated tooth enamel. Caries Research 1983; 17(6):490–5. 6. Delbem A, Cury J, Nakassima C, Gouveia V, Theodoro L. Effect of Er:YAG laser on CaF2 formation and its anti-cariogenic action on human enamel: an in vitro study. Journal of Clinical Laser in Medicinal Surgery 2003;21(4):197–201. 7. Damato F, Strang R, Stephen K. Comparison of solution- and gel-prepared enamel lesions—and in vitro pH-cycling study. Journal of Dental Research 1988;67(8):1122–5. 8. Apel C, Meister J, Schmitt N, Graber H, Gutknecht N. Calcium solubility of dental enamel following sub-ablative
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Er:YAG and Er:YSGG laser irradiation in vitro. Lasers in Surgery and Medicine 2002;30(5):337–41. Lin C, Lee B, Kok S, Lan W, Tseng Y, Lin F. Treatment of tooth fracture by medium energy CO2 laser and DP-bioactive glass paste: thermal behavior and phase transformation of human tooth enamel and dentin after irradiation by CO2 laser. Journal of Materials Science: Materials in Medicience 2000; 11(6):373–81. Holcomb D, Young R. Thermal decomposition of human tooth enamel. Calcified Tissue International 1980;31(3):189–201. Fowler B, Kuroda S. Changes in heated and in laser-irradiated human tooth enamel and their probable effects on solubility. Calcified Tissue International 1986;38(4):197–208. Oho T, Morioka T. A possible mechanism of acquired acid resistance of human dental enamel by laser irradiation. Caries Research 1990;24(2):86–92. Deng Y, Hsu C. Combined effect of fluoride and laser on the crystalline structure of human enamel—a pilot study. SPIE 2005 (In press). Bachra B, Trautz O, Simon S. Precipitation of calcium carbonates and phosphates. 3. The effect of magnesium and fluoride ions on the spontaneous precipitation of calcium carbonates and phosphates. Archives of Oral Biology 1965; 10(5):731–8. Robinson C, Shore R, Brooks S, Strafford S, Wood S, Kirkham J. The chemistry of enamel caries. Critical Reviews Oral Biology and Medicine 2000;11(4):481–95. Arends J. Mechanism of dental caries. In: Nancollas, editor. In: Biological mineralization and demineralization: life sciences research report 23. Berlin: Springer-Verlag; 1982. p. 303–24. Zuerlein M, Fried D, Featherstone J. Modeling the modification depth of carbon dioxide laser-treated dental enamel. Lasers in Surgery and Medicine 1999;25(4):335–47. Little M, Casciani F. The nature of water in sound human enamel. A preliminary study. Archives of Oral Biology 1966; 11(6):565–71. LeGeros R, Bonel G, Legros R. Types of “H2O” in human enamel and in precipitated apatites. Calcified Tissue International 1978;26(2):111–8. Bachmann L, Gomes A, Zezell D. Bound energy of water in hard dental tissues. Spectroscopy Letters 2004;37(6):565–79. Yamamoto H, Sato K. Prevention of dental caries by acoustooptically Q-switched Nd:YAG laser irradiation. Journal of Dental Research 1980;59(2):137. Hsu C, Jordan T, Dederich D, Wefel J. Effects of low-energy CO2 laser irradiation and the organic matrix on inhibition of enamel demineralization. Journal of Dental Research 2000; 79(9):1725–30. Ying D, Chuah G, Hsu C. Effect of Er:YAG laser and organic matrix on porosity changes in human enamel. Journal of Dentistry 2004;32(1):41–6. Stern R, Sognnaes R, Goodman F. Laser effect on in vitro enamel permeability and solubility. Journal of American Dental Association 1966;73(4):838–43. Palamara J, Phakey P, Rachinger W, Orams H. The ultrastructure of human dental enamel heat-treated in the temperature range 200–600 8C. Journal of Dental Research 1987;66(12):1742–7. Apel C, Schafer C, Gutknecht N. Demineralization of Er:YAG and Er,Cr:YSGG laser-prepared enamel cavities in vitro. Caries Research 2003;37(1):34–7.
www.intl.elsevierhealth.com/journals/jden
Optimal Er:YAG laser energy for preventing enamel demineralization Jeng-fen Liua, Yuanyuan Liub, Hsu Chin-Ying Stephenb,* a
National Yang Ming Medical University, Taipei, Taiwan Department of Preventive Dentistry, Faculty of Dentistry, National University of Singapore, 5 Lower Kent Ridge Road, 119074 Singapore
b
Received 12 May 2004; received in revised form 15 March 2005; accepted 18 March 2005
KEYWORDS Er:YAG laser; Enamel demineralization; Caries prevention; Polarized light microscopy
Summary Objectives: The purpose of this study was to identify the optimal laser energy range of Er:YAG laser irradiation for laser-induced caries prevention (LICP). Methods: Twenty-one human non-carious molars were selected. The teeth were covered with nail varnish, except two 4 mm!1 mm windows on both the buccal and lingual surfaces. The windows were randomly assigned to groups A, B, C and D, receiving no irradiation, 100, 200 and 300 mJ irradiation, respectively. The pulse width 10 pps (pulse per second) with a 1.0 mm spot size was used. After the laser treatment, each tooth was cut into two halves longitudinally. Then a two-day pHcycling was performed, with an 18-hour demineralization followed by a 6-hour remineralization. Sections of 120G20 mm in thickness were obtained from each window. Lesion depth was measured using polarized light microscope coupled with an image analysis software. One-way ANOVA and post-hoc Tukey tests were used for evaluation of treatment effects. Results: The laser treatments of 100 and 200 mJ have demonstrated significant protection of enamel demineralization (pZ0.01 and 0.001, respectively), but not the treatment with 300 mJ (pZ0.106). A smaller lesion depth was observed for the 200 mJ group (97.1 mm) than that of the 100 mJ group (105.6 mm). Compared with the control, a lesion reduction of 32.78 and 26.93% for the 200 mJ group and the 100 mJ group were obtained, respectively. Conclusion: Caries prevention may be achieved by using Er:YAG laser treatment if the optimal range of laser parameters for LICP can be employed. Q 2005 Elsevier Ltd. All rights reserved.
Introduction
* Corresponding author. Tel.: C65 6779 5555x1658; fax: C65 6773 2602. E-mail address: [email protected] (H.C.-Y. Stephen).
The Erbium-doped:Yttrium-aluminum-garnet (Er:YAG) laser, which emits at a wavelength of 2.94 mm in the mid-infrared region, has a potential for hard-tissue treatment due to its high
0300-5712/$ - see front matter Q 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.jdent.2005.03.005
Optimal Er:YAG laser energy for preventing enamel demineralization absorbability in water and hydroxyapatite.1,2 It is the first dental laser approved to be used for hard tissue ablation by the U.S. Food and Drug Administration (FDA) in 1997, and has also demonstrated significant effects on caries prevention.3,4 However, the optimal energy range for caries prevention by using Er:YAG laser has not yet been reported. When enamel was heated, the acid resistance increased from 100 to 200 8C but then decreased between 300 and 400 8C.5 If the photothermal effect is essential for laser-induced caries prevention (LICP), it is possible that LICP may decrease when the laser energy is beyond the optimal range. On the other hand, if the laser energy is below the optimal range, the LICP may not be significant.6 Therefore, the purpose of this study was to identify the optimal range of Er:YAG laser energy for LICP.
Materials and methods Tooth selection and grouping Twenty-one human non-carious molars, stored in 0.1% thymol solution, were selected and cleaned. The teeth were covered with nail varnish, leaving two 4 mm!1 mm windows on both the buccal and lingual surfaces at the middle third of the crown. The windows were randomly assigned to groups A, B, C and D.
Laser treatment An Er:YAG laser (Opus 20, Sharplan 4020M, Israel) emitting at a wavelength of 2.94 mm was used in this study. Based on the results of pilot studies, group A was kept as the control group, and teeth of groups B, C, D were treated with the laser energies of 100, 200 and 300 mJ, respectively. Without water spray, the operation was performed by clamping the handpiece of the laser and moving the teeth manually with 10 pps (pulse per second), and 1.0 mm spot size. Since the size of window is 1 mm by 4 mm and the focal spot of the laser is 1.0 mm in diameter, the irradiation was carried out in a series of four steps of one-second irradiation each. This procedure was repeated three more times so that each ‘spot’ in a window is irradiated four times. Clinically, 600–700 mJ has been used to remove caries and for cavity preparation. Based on our pilot studies, the irradiation energy of 100, 200 and 300 mJ were chosen because these sub-ablative parameters are much lower than that for cavity preparation, and is very unlikely to damage the normal tooth structures. If we would use water
63
spray, a higher energy may be needed for significant LICP and thus increase the possibility of harming the peripheral dental tissues. The energy densities were calculated to be 12.7, 25.5 and 38.2 J/cm2 for the B, C and D groups, respectively.
pH-cycling process After the laser treatment, each tooth was cut into two halves longitudinally (disto-mesial) by an alloy grinder (DEMCOe, Dental Maintenance Co., Inc. Bonsall, CA, USA). The cut surfaces were covered with nail varnish and dried for 24 hours. A two-day pH-cycling scheme was performed, with an 18-hour demineralization followed by a 6-hour remineralization, with a stirring speed of 130 rpm at 37 8C.7 The demineralization solution at pH 4.5 contained 0.05 M acetic acid, 2.2 mM calcium, and 2.2 mM phosphate ions. The remineralization solution at pH 7.0 contained 0.15 M potassium chloride, 1.5 mM calcium, and 0.9 mM phosphate ions. A 5-min wash in the de-ionized and distilled water was done between the demineralization and remineralization phases and at the end of the process. Both demineralization and remineralization solutions were changed daily. All tooth halves were stored in plastic containers with 100% humidity before being sectioned.
Evaluation of demineralization A Silverstone-Taylore hard tissue microtome (series 1000 Deluxe, SciFab, Littleton, Co, USA) was used to obtain several sections of 120G20 mm in thickness from each window. Samples were characterized with stereomicroscope. Five teeth damaged during the procedures were discarded. With water as the imbibition solution, the demineralization patterns of selected slices were assessed using a polarized light microscope (Olympuse BH-2, Olympus Corp. of America, New Hyde Park, NY, USA). Using the Micro Image (Olympuse, Japan), a representative lesion area (200 mm in width) of each section was selected and measured as shown in Fig. 1. The “Lesion Area” value was divided by 200 mm to produce the average lesion depth for each section. The measurements of 4G2 sections from each window were averaged to represent the window lesion.
Statistical analysis The lesion depth (mm) obtained with the above method was used as the dependent variable. The independent variable was the laser energy of four levels, including 0, 100, 200 and 300 mJ.
64
J.-f. Liu et al. 200 mJ have demonstrated significant protection of enamel demineralization (pZ0.01 and 0.001, respectively) but not the treatment with 300 mJ (pZ0.106). The lesion depth of the C group (97.1 mm) is less than the B group (105.6 mm), although the difference has not reached the significant threshold. Compared with the lesion of the control windows, lesion reduction of 32.78 and 26.93% for the C and B group were obtained, respectively. Fig. 2 indicates representative lesions for each group.
Figure 1
Discussions
Illustration of lesion depth measurement.
The result of the Levene’s test (pZ0.860) confirmed the homogeneity of variances. Therefore, the parametric test (one-way ANOVA) was used for assessing the treatment effects. A post hoc test, the Tukey test, was chosen for evaluation of the statistical significance of the pair-wise comparisons between the four groups. A usual alpha level of 0.05 was used to determine the statistical significance of the results.
Results Qualitative analysis The enamel appears to be unaffected and remains indistinguishable between the laser-treated spots and untreated areas. When viewed under the stereoscope, enamel surfaces revealed no crater, crack or melting.
Effect of laser-treatment on demineralization A 32.78% lesion reduction was found in this study using 200 mJ Er:YAG laser. This is in agreement with previous studies,3,4,8 revealing a 5–67% reduction of enamel demineralization after irradiation with the Er:YAG laser. A marked (40%) caries inhibition was achieved by Fried et al.4 using sub-ablative laser parameters. Apel et al.8 have also reported a significant decline in enamel solubility after laser irradiation. Water-cooling was not used in our study, although irradiation with and without water-cooling appeared to be both significantly effective in caries prevention.3 In a previous study, when teeth samples were treated using pulse energy of 400 mJ, 2 Hz, and a spot size of 0.63 mm, the result revealed a statistically significant preventive effect, which was more evidenced without water mist (67%) than with water mist (15%).3 If water-cooling was used in our study, the LICP might be less.
Quantitative analysis Potential mechanisms for observed LICP The means of lesion depths and standard deviations of the four groups are listed in Table 1. Evaluated by using ANOVA, the enamel lesion depth of all four groups are not equal (pZ0.001). In comparison with the control, the laser treatments with 100 and Table 1
This preventive effect may be due to the laserinduced heating of the samples and the subsequent crystallographic9 and compositional changes.10 These changes have been reported to attributed to
Comparison of mean lesion depths of four experimental groups.
Group
Energy (mJ)
Na
Mean (mm)
Std. deviation (mm)
Rankingb
A B C D
0 100 200 300
16 16 16 16
144.46 105.56 97.10 116.66
35.10 39.82 29.40 30.41
I II II I
a
Sample size. The ranking order was obtained from the post-hoc Tukey multiple comparisons. Groups with defferent numerals are statistically different at a level of p!0.05 or lower. b
Optimal Er:YAG laser energy for preventing enamel demineralization
Figure 2
65
Representative lesions for A(0 mJ), B(100 mJ), C(200 mJ) and D(300 mJ) groups.
the decomposition of carbonate15 and removal of water etc.11 In previous studies, the reduction of carbonate was observed when enamel was treated using argon laser12 at an energy density of 67 J/cm2 and Er:YAG laser13 at 5.1 J/cm2. The quantity of carbonate will influence the enamel susceptibility to in vitro demineralization because they fit less well in the lattice and therefore give rise to less stable and more acid-soluble apatite-phase.14,15 It was found that the mineral loss started preferentially from the central core and then the outer core of the crystallites of enamel.16 It is likely that the existence of carbonate causes some crystal defect and remains as the highly strained area. Laser treatment may remove the carbonate and increase crystalline stability leading to the enamel less vulnerable to acid attack. Laser-induced temperature rise may have changed the enamel structure. Zuerlein et al.17 inferred that carbonate decomposes only from 400 8C onwards. However, Fowler and Kuroda11 indicated that there was substantial loss of carbonate and water at temperatures between 100 and 400 8C, which were sufficient to change the crystallinity of the enamel mineral. As for the water, loosely bounded water losses occur when enamel is heated up to 200 8C10 while tightly bounded water is totally eliminated only at high temperatures (1300 8C).18 Study demonstrated that the loss of water between 200 and 400 8C is irreversible and causes a contraction in the a-axis dimensions.19 In a recent study, Bachmann et al.20 has discussed the water loss of heated enamel powder by using IR measurement. They stated that the removal of water depends largely on the temperature generated at the irradiated spot and surrounding regions, i.e.
100–400 8C mainly for absorbed water, 700–1000 8C mainly for structural water. Another contributing factor may be porosity changes.21–24 Our pervious data showed that the surface area and pore volume of normal enamel decreased significantly after laser treatment.23 It was indicated that laser-induced melting of organic matrix may block the diffusion pathway and result in a reduced efficiency in ion diffusion and subsequent enamel demineralization.21,22,24
Effect of laser power The laser treatments using 100 and 200 mJ have demonstrated significant protection of enamel demineralization but not the treatment with 300 mJ. The significant role of organic matrix in the laser-induced retardation of enamel demineralization has been elucidated.22,24 The organic blocking effect has resulted in a 25 and 57% inhibition of mineral loss and lesion progress, respectively. Protein in enamel was found to decompose at about 350–400 8C.10 Our previous study suggested the organic blocking effect may reach a maximum and decrease after the decomposition of organic matrix above 400 8C.22 Other PLM studies showed that the majority of enamel structures became positively birefringent when heated to 400 8C.5,25 Based on aforementioned studies, laser treatment used for the group D may have destroyed the organic matrix and thus reduced the organic blocking effect. Therefore, our results have substantiated the notion that greater laser energy does not always provide more LICP. Furthermore, the insignificant or negative LICP shown in previous
66 studies may be due to their laser parameters falling either below or above the optimal range.6,8,26 In summary, this study has identified an optimal energy range (around 100 and 200 mJ) for LICP of Er:YAG laser treatment without water cooling. Using laser energy greater or lower than the optimal range may decrease the LICP. The optimal ranges of other Er:YAG laser parameters (e.g. frequency) are currently under investigation.
Conclusion An optimal energy range of Er:YAG irradiation has been established for inhibition of enamel demineralization. Caries prevention may be achieved by using Er:YAG laser treatment if the optimal range of laser parameters for LICP can be employed.
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This project is financially supported by grants (TCVGH 925603, from the Ministry of Administration, Taiwan), grants R-222-000-013-112 and R-222-000-015305 (both from the Ministry of Education and the Biomedical Research Council, Singapore).
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