In Vitro Evaluation of Enamel Erosion After Nd:YAG Laser Irradiation and Fluoride Application

Abstract

Objective: Previous investigations have demonstrated improved enamel demineralization resistance after laser irradiation. Due to the possibility of a synergistic effect between laser and fluoride, this study investigated the effect of fluoridated agents and Nd:YAG irradiation separately and in combination on enamel resistance to erosion. Methods: One hundred bovine enamel blocks were randomly divided into 10 groups: G1, untreated (control); G2, acidic phosphate fluoride (APF) (1.23% F) for 4 min; G3, fluoride varnish for 6 h (NaF, 2.26%); G4, 0.5 W Nd:YAG laser (250 μm pulse width, 10 Hz, 35 J/cm², with uniform velocity for 30 sec in each application); G5, 0.75 W Nd:YAG laser (52.5 J/cm²); G6, 1.0 W Nd:YAG laser (70 J/cm²); G7, APF + 0.75 W Nd:YAG laser; G8, 0.75 W Nd:YAG laser + APF; G9, fluoride varnish + 0.75 W Nd:YAG laser; and G10, 0.75 W Nd:YAG laser + fluoride varnish. During 10 d the erosive cycle was conducted by immersion of the blocks in Sprite light^® for 1 min, followed by immersion in artificial saliva for 59 min. This procedure was consecutively repeated four times per day. In each day, during the remaining 20 h, the blocks were maintained in artificial saliva. The wear was evaluated by profilometry (days 5 and 10). Data were tested by two-way ANOVA and Bonferroni's tests (p < 0.05). Results: The mean wear at days 5 and 10 was, respectively: G1, 1.83 and 2.67 μm; G2, 1.04 and 2.60 μm; G3, 1.03 and 2.48 μm; G4, 1.13 and 2.47 μm; G5, 1.07 and 2.44 μm; G6, 1.0 and 2.35 μm; G7, 0.75 and 2.27 μm; G8, 0.80 and 2.12 μm; G9, 0.76 and 2.47 μm; and G10, 1.09 and 2.46 μm. At day 5, all the experimental groups presented significant lesser wear when compared to control group. However, at 10 d, only G7 and G8 were still different from control. Conclusions: The association between APF application and laser irradiation seems to be an alternative preventive measure against dental erosion.

Introduction

D ental erosion is a pathologic, chronic, localized loss of dental hard tissues that involves chemical removal of superficial hard tissue from the tooth surface by acids and/or chelation without bacterial involvement.¹ The acids can originate from soft drinks and fruit juices or from eating disorders and gastric reflux.¹

Dental erosion is regarded as a threat to oral health.^2,3 This fact highlights the importance of its primary prevention by the treatment and elimination of the causes before the lesions occur on teeth.⁴ As it is difficult to control possible etiological factors of dental erosion, such as the intake of acidic beverages or specific drinking habits, many strategies have been developed to prevent or arrest dental erosion, including topical fluoride applications.^4

–7 Although the preventive action of fluoride on dental caries is well known,⁸ its role in erosion is still controversial,⁸ since the deposited calcium fluoride-like material from topical fluoride application is supposed to dissolve readily in most acidic drinks.⁶ However, highly concentrated fluoride applications, such as gels and varnishes, have been shown to decrease the development of erosion in enamel.⁷

Another potential preventive measure for dental erosion could be application of high-intensity laser. Since the first study with ruby laser, it has been demonstrated that laser irradiation can increase the acid resistance of enamel with promising results.⁹ Other lasers, such as the Nd:YAG and CO₂, have also been used to enhance enamel resistance to demineralization.^10,11 Among these, the Nd:YAG laser was reported to be effective.^12,13 In addition, the effects of combined fluoride and laser treatment on the inhibition of enamel caries have been reported.^14,15 It is important to point out that these studies evaluated the use of laser to improve the enamel resistance to acids originated from caries and not from erosive challenge.^14,15 Taking these aspects into account, the ability of Nd:YAG laser with or without fluoride treatment to inhibit dental erosion remains to be elucidated.

Therefore, the objectives of the present study were to evaluate 1) the effect of Nd:YAG laser in inhibiting dental erosion, 2) the combined effect of laser irradiation and fluoride application and their most effective sequence of use, and 3) the maintenance of the effects of these treatments after successive erosive challenges.

Material and Methods

Enamel block preparation

One hundred bovine enamel blocks (4 × 4 × 3 mm) were prepared from extracted bovine incisors that were previously stored in 2% formaldehyde solution (pH 7.0) for 30 d at room temperature. One block was cut from each crown using an ISOMET low speed saw cutting machine (Buehler Ltd., Lake Bluff, IL, USA) and two diamond disks (Extec Corp., Enfield, CT, USA), which were separated by a 4 mm diameter spacer. The enamel surface was ground flat with water-cooled carborundum discs (320, 600, and 1200 grades of Al₂O₃ papers; Buehler) and polished with felt paper wet by diamond spray (1 μm; Buehler). This procedure resulted in a removal of about 100 μm of enamel, which was controlled by micrometer. The surface microhardness was determined by performing five indentations (Knoop diamond, 25 g, 5 sec, HMV-2000; Shimadzu Corporation, Tokyo, Japan) for selection purpose. Enamel blocks with a microhardness ranging from 320 to 378 knoop hardness number (350.89 ± 13.87) were randomly distributed into 10 groups (n = 10): G1, untreated (control); G2, acidic phosphate fluoride (APF; 1.23% F) for 4 min; G3, fluoride varnish for 6 h (NaF, 2.26%); G4, 0.5 W Nd:YAG laser (250 μm pulse width, 10 Hz, 35 J/cm²); G5, 0.75 W Nd:YAG laser (52.5 J/cm²); G6, 1.0 W Nd:YAG laser (70 J/cm²); G7, APF + 0.75 W Nd:YAG laser; G8, 0.75 W Nd:YAG laser + APF; G9, fluoride varnish + 0.75 W Nd:YAG laser; and G10, 0.75 W Nd:YAG laser + fluoride varnish.

Experimental procedure

In G1, the enamel blocks were not treated and served as control. In G2, the APF gel (pH 3.5, 1.23% F, Flugel, DFL, Rio de Janeiro, Brazil) was applied on each enamel surface with a microbrush for 4 min, after which the excess gel was removed by cotton swabs. In G3, the fluoride varnish (5% NaF, 2.26% F, pH 4.5; Duraphat, Colgate, Brazil) was applied in a thin layer with a microbrush on each enamel surface. After 6 h, the varnishes were carefully removed using acetone and a scalpel blade.^16
–18

In G4–G6, the samples were irradiated with Nd:YAG laser (Twinlight; Fotona Medical Lasers, Slovenia) by using contact mode applied at a perpendicular angle through a quartz fiber with a diameter of 300 μm. This laser system presents a pulse width of 250 μsec and repetition rate of 10 Hz. The laser device was fixed on a horizontally moving table, which allowed the scanning movements in four directions (mesial–distal and apical–occlusal, 5 × 5 mm) with uniform velocity for 30 sec in each application. For laser application, the blocks were shifted by movement of the table allowing the fixed laser beam to scan the whole surface. The irradiation conditions for laser groups were: G4, 0.5 W Nd:YAG laser (35 J/cm²); G5, 0.75 W Nd:YAG laser (52.5 J/cm²); and G6, 1.0 W Nd:YAG laser (70 J/cm²).

In G7, the blocks were first treated with APF, as described above, and then irradiated with 0.75 W Nd:YAG laser; in G8, the blocks were irradiated first (0.75 W Nd:YAG) followed by APF application. G9 was first treated with F varnish, as described above, and then irradiated with 0.75 W Nd:YAG laser; in G10 the sequence was switched, i.e., first the blocks were irradiated and then the F varnish was applied. After the treatments, two layers of nail varnish were applied on half of the surface of the enamel in order to maintain reference surfaces for lesion depth determination.

During 10 days the erosive cycle was conducted by immersion of the blocks in Sprite light^® (citric acid; Coca-Cola Company, Spal, Porto Real, RJ, Brazil; 30 mL/block) for 1 min, followed by immersion in artificial saliva (30 mL/block) for 59 min. This procedure was consecutively repeated four times per day.¹⁸ Each day, during the remaining 20 h, the blocks were maintained in artificial saliva (25°C). The Sprite light (pH 2.87) presented a buffering capacity of 0.375 ± 0.01, which is equivalent to 0.375 mL of 0.2 M NaOH/3 mL beverage to increase by one pH unit.¹⁸ The composition of the artificial saliva (pH 7.0) was 1.5 mM Ca(NO₃)₂ · 4H₂O, 0.9 mM NaH₂PO₄ · 2H₂O, 150 mM KCl, 0.1 M Tris buffer, and 0.03 ppm F.¹⁹

Wear assessment

The enamel wear was determined by profilometry in relation to the reference surface that was protected by nail varnish (Hommel Tester T1000, VS, Schwenningen, Germany) at the 5th and 10th experimental days. The nail varnish on the reference surfaces was carefully removed with acetone-soaked cotton wool.²⁰ Five readings were performed on each block by scanning from the reference to the exposed surface. The mean values of five readings for each group were averaged.

Statistical analysis

Graph Pad Prism software 4 version 4.0 for Windows (Graph Pad Software, San Diego, CA, USA) was used. The assumptions of equality of variances (Bartlett's test) and normal distribution of errors (Kolmogorov–Smirnov test) were checked for all the variables tested. Since the assumptions were satisfied, two-way repeated measures ANOVA and Bonferroni post hoc test were carried out for statistical comparisons and the significance limit was set at p < 0.05.

Results

Table 1 shows the mean wear (μm ± SE) for the studied groups on the 5th and 10th days. There was a significant difference among the groups (F = 6.758, p < 0.0001), as well as between days 5 and 10 (F = 576.6, p < 0.0001). There was no interaction between the criteria (F = 1.57, p = 0.137). At day 5, all the treated groups presented a significant lesser wear when compared to control. However, at day 10, only G7 (APF + laser) and G8 (laser + APF) significantly differed from control (p < 0.05). On the 5th day, the Nd:YAG laser irradiation promoted nearly 41% reduction of the erosion (G4, 38%; G5, 41%; G6, 45%) and on the 10th day, this reduction was only about 9% (G4, 7%; G5, 9%; G6, 12%). The combined effect of laser irradiation and fluoride application resulted in about 54% erosion reduction (G7, 59%; G8, 57%; G9, 58%; G10, 40%) on the 5th day and on the 10th day, the reduction decreased to around 13% (G7, 15%; G8, 21%; G9, 7%; G10, 8%).

Table 1.

Mean Enamel Wear for Each Experimental Group

Groups	5th day wear	10th day wear
G1, Control	1.83 ± 0.41 a	2.67 ± 0.17 a
G2, APF gel	1.04 ± 0.27 bc	2.60 ± 0.31 ab
G3, F varnish	1.04 ± 0.46 bc	2.48 ± 0.32 ab
G4, 0.5 W Nd:YAG	1.13 ± 0.41 b	2.47 ± 0.30 ab
G5, 0.75 W Nd:YAG	1.07 ± 0.38 bc	2.44 ± 0.43 ab
G6, 1.0 W Nd:YAG	1.00 ± 0.34 bc	2.35 ± 0.53 ab
G7, APF gel + 0.75 W Nd:YAG	0.75 ± 0.27 c	2.27 ± 0.39 b
G8, 0.75 W Nd:YAG + APF gel	0.79 ± 0.28 bc	2.12 ± 0.51 b
G9, F varnish + 0.75 W Nd:YAG	0.76 ± 0.34 bc	2.47 ± 0.49 ab
G10, 0.75 W Nd:YAG + F varnish	1.09 ± 0.43 bc	2.45 ± 0.53 ab

Experimental periods were significantly different (p < 0.0001). Values followed by different lowercase letters showed significant difference among the treatments at each experimental period (p < 0.0001).

Discussion

The present study examined the effect of laser therapy using three different laser doses with or without two different fluoride agents as preventive measures against enamel erosion. The laser treatment and the type of fluoride were varied to evaluate the best parameters and associations to use Nd:YAG laser for erosion. The 0.75 W laser was used for fluoride association because it was an intermediate value.

One possible limitation of the present study was the reference area for profilometric measurement, which was established after laser or fluoride treatment, respectively. That means that possible alterations due to the pretreatment are not considered in the measurements. However, a previous study showed that fluoride varnish did not promote enamel wear or gain.²¹ However, we do not know the possible effects of the laser pretreatment on enamel wear.

The results of the present study showed that both tested products with high fluoride concentrations promoted enamel resistance against erosion, which is in accordance with the literature.^17,22,23 It is well established that the application of fluoride results in the precipitation of F-rich mineral layers, especially when the preparations have a low pH and a high fluoride concentration.²⁴ In the present study, it is supposed that fluoride was incorporated into and deposited onto the enamel, positively affecting the enamel erosion.²⁵ However it is important to point out that at the 10th day this protective effect was low, showing the importance of more intensive professional fluoride applications for patients with erosion risk.

For the laser irradiation groups all the laser doses used resulted in less enamel erosion when compared to the control group on the 5th day. In addition, the preventive effect of isolated laser irradiation was similar to that caused by fluoride application only. It is important to point out that these results could be different if CO₂ laser was used, since Tsai et al.²⁶ showed that after 24 h in lactate buffer, CO₂ laser–treated tooth enamel was more resistant to acid challenge than was Nd:YAG laser–treated enamel, but after 72 h neither type of laser increased acid resistance of enamel. In the present study, Nd:YAG laser was tested to evaluate its use on patients at risk of dental erosion.

Vlacic et al.²⁷ also found that Nd:YAG irradiation provides protection to dental enamel against an erosive challenge. However, it is difficult to compare the results of that study with ours, since the response variable (Vicker's hardness number changes), the energy density (15 J/cm²), and the erosive challenge used were different. The mechanism by which Nd:YAG laser enhances enamel acid resistance is not clarified. It has been demonstrated that lasers can significantly increase the acid resistance of enamel by altering crystallinity, acid solubility, and permeability of enamel.²⁸ The Nd:YAG laser can diminish the permeability of enamel because the laser irradiation induces enamel melting and resolidification and the melted enamel surface can show a crystal growth that can reduce the interprismatic spaces and consequently, the diffusion of acids during an acid challenge.²⁹

Heating enamel in an oven produces chemical changes in the matrix such as those related to water and carbonate losses.³⁰ When the surface temperature increases to 100–650°C the major carbonate component in the phosphate position decreases and the acid phosphate ions condense to form pyrophosphate ions.^29,31 It is important to point out that carbonated apatite has a higher solubility than hydroxyapatite and pyrophosphate concentrates can reduce the hydroxyapatite dissolution rate to zero.³² At 650–1100°C, the main changes are thermal recrystallization and crystal size growth, and pyrophosphate reacts with apatite to form PO₄ along with the formation of β-tri-calcium-phosphate (β-TCP).³¹ The main change at temperatures above 1100°C is that the β-TCP is converted to α-TCP.³¹ Fowler and Kuroda³³ hypothesized that heating to temperatures in excess of 1200°C may increase the susceptibility of dental enamel to acid dissolution because the β-TCP and α-TCP phases are more soluble than hydroxyapatite and dental enamel.

In addition, Nd:YAG is expected to induce morphological alterations in the enamel at temperatures over 1100°C.²⁹ At this temperature the morphologically changed surface presents a greater amount of acid-soluble compounds such as β-TCP and α-TCP than does the sound enamel.²⁹ On the other hand, as long as the laser beam is absorbed by the tissue, its effects on the underlying surface differ in relation to the temperature decrease towards the enamel-dentin structures.³³ Therefore Castellan et al.²⁹ suggested that the Nd:YAG-treated enamel subsurface presents fewer soluble compounds and can be more resistant during pH cycling. However, our results on day 10, when the wear was near 2.4 μm and the laser irradiation reduced erosion by only 9%, indicate that the effect of laser irradiation is limited to the surface area.

Laser irradiation associated with the application of high fluoride–containing products was also able to increase the enamel resistance against dental erosion. Several physico-chemical changes have been suggested to occur during combined laser irradiation and fluoride treatment, including deposition of calcium fluoride, formation of TCP, and phase transformation of hydroxyapatite to fluorapatite.³⁴ The latter is more resistant to both strong (corrosive/erosive) acids as well as to weaker acids than the carbonated hydroxyapatite predominantly found in dental enamel.²⁷ Tagomori and Morioka¹⁴ have reported the enhanced uptake of fluoride after laser irradiation, while Goodman and Kaufman³⁵ found that laser irradiation of enamel results in superficial melting or dissolution of crystals, followed by cooling and recrystallization and incorporation of fluoride to form less soluble fluorapatite. These physico-chemical changes would help to explain the erosive inhibition found in our study when the laser and fluoride application were associated. At day 5, despite the association between laser and fluoride, which seemed to numerically decrease the enamel wear in comparison to laser alone and fluoride alone, the difference was not significant (except for G4 compared to G7). At day 10, however, the combinations APF + laser and laser + APF were the only treatments able to significantly reduce the wear when compared to control group. The same was not valid for the combinations laser + varnish and varnish +laser. One possible explanation for this finding is the low pH of APF gel. It is possible that this low pH may have increased the enamel porosity and when followed by laser irradiation enhanced its penetration, thus allowing deeper changes in enamel structure. On the other hand, when APF was applied after laser irradiation its low pH may have degraded the weaker mineral phases formed during laser application, allowing the formation of more resistant F-rich crystals. These changes allowed the maintenance of the effects of these treatments after successive erosive challenges. Similar to Tagomori and Morioka¹⁴ who showed that fluoride application after laser irradiation produces a greater fluoride uptake in the smooth enamel surface than fluoride application before laser irradiation, we showed similar effects for the different sequences of use (groups 7, 8, 9, and 10).

Conclusion

In conclusion, these in vitro results suggest that the association between APF application and laser irradiation seems to be a promising preventive measure against dental erosion, which should be tested in further in situ and clinical studies.

Footnotes

Acknowledgments

The authors thank Prof. Rafael Lia Mondelli for the use of the profilometer and the undergraduate student Mardoqueu Martins Costa (EESC-USP) for the help with laser irradiation.

Disclosure Statement

No competing financial interests exist.

References

ten Cate

J.M.

, Imfeld

1996. Dental erosion, summary. Eur. J. Oral. Sci., 104:241–244.

Jaeggi

, Lussi

2006. Prevalence, incidence and distribution of erosion. Monogr. Oral Sci., 20:44–65.

Lussi

2006. Erosive tooth wear—a multifactorial condition of growing concern and increasing knowledge. Monogr. Oral Sci., 20:1–8.

Lussi

, Hellwig

2006. Risk assessment and preventive measures. Monogr. Oral Sci., 20:190–199.

Johansson

A.K.

, Lingström

, Imfeld

, Birkhed

2004. Influence of drinking method on tooth-surface pH in relation to dental erosion. Eur. J. Oral. Sci., 112:484–489.

Wiegand

, Attin

2003. Influence of fluoride on the prevention of erosive lesions—a review. Oral Health Prev. Dent., 1:245–253.

Ganss

, Klimek

, Brune

, Schumann

2004. Effects of two fluoridation measures in erosion progression on enamel and dentine in situ. Caries Res., 38:561–566.

ten Cate

J.M.

1997. Review on fluoride, with special emphasis on calcium fluoride mechanisms in caries prevention. Eur. J. Oral. Sci., 105:461–465.

Stern

R.H.

, Sognnaes

R.F.

1965. Laser effect on dental hard tissue. J. So. Cal. Dental Assoc., 33:328–329.

10.

Yamamoto

, Ooya

1974. Potential of yttrium-aluminum-garnet laser in caries prevention. J. Oral Pathol., 3:7–15.

11.

Powell

G.L.

, Yu

, Higuchi

W.I.

, Fox

J.L.

1993. Comparison of three lasers on demineralization of human enamel. SPIE Proc., 1880:188–192.

12.

Hashimoto

1990. Effects of Nd:YAG laser irradiation on acid resistance of defective rat enamel. Shoni Shikaguzu Zasshi, 28:956–967(In Japanese.)

13.

Kimura

, Wilder-Smith

, Arrastia-Jitosho

A.M.A.

, Liaw

L.H.L.

, Matsumoto

, Berns

M.W.

1987. Effects of nanosecond pulsed Nd:YAG laser irradiation on dentin resistance to artificial caries-like lesions. Lasers Surg. Med., 20:15–21.

14.

Tagomori

, Morioka

1989. Combined effects of laser and fluoride on acid resistance of human dental enamel. Caries Res., 23:225–231.

15.

Tagliaferro

E.P.S.

, Rodrigues

L.K.A.

, Nobre dos Santos

, Soares

L.E.S.

, Martin

A.A.

2007. Combined effects of carbon dioxide laser and fluoride on demineralized primary enamel: an in situ study. Caries Res., 41:74–76.

16.

Delbem

A.C.B.

, Bergamaschi

, Sassaki

K.T.

, Cunha

R.F.

Effect of fluoridated varnish and silver diamine fluoride solution on enamel demineralization: pH-cycling study. J. Appl. Oral. Sci., 14:88–92.

17.

Vieira

, Lugtenborg

, Ruben

J.L.

, Huysmans

M.C.

2006. Brushing abrasion of eroded bovine enamel pretreated with topical fluorides. Caries Res., 40:224–230.

18.

Magalhães

A.C.

, Rios

, Machado

M.A.A.M.

et al. 2008. Effect of Nd:YAG irradiation and fluoride application on dentin resistance to erosion in vitro. Photomed. Laser Surg., 26:559–563.

19.

Vieira

A.E.M.

, Delbem

A.C.B.

, Sassaki

K.T.

, Rodrigues

, Cury

J.A.

, Cunha

R.F.

2005. Fluoride dose response in pH-cycling models using bovine enamel. Caries Res., 39:514–520.

20.

Attin

, Buchalla

, Gollner

, Hellwig

2000. Use of variable remineralization periods to improve the abrasion resistance of previously eroded enamel. Caries Res., 34:48–52.

21.

Magalhães

A.C.

, Kato

M.T.

, Rios

, Wiegand

, Attin

, Buzalaf

M.A.

2008. The effect of an experimental 4% Tif4 varnish compared to NaF varnishes and 4% TiF4 solution on dental erosion in vitro. Caries Res., 42:269–274.

22.

Attin

, Deifuss

, Hellwig

1999. Influence of acidified fluoride gel on abrasion resistance of eroded enamel. Caries Res., 33:135–139.

23.

Magalhães

A.C.

, Stancari

F.H.

, Rios

, Buzalaf

M.A.R.

2007. Effect of an experimental 4% titanium tetrafluoride varnish on dental erosion by a soft drink. J. Dent., 35:858–861.

24.

Saxegaard

, Rølla

1988. Fluoride acquisition on and in human enamel during topical application in vitro. Scand. J. Dent. Res., 96:523–535.

25.

Lussi

, Jaeggi

, Gerber

, Megert

2004. Effect of amine/sodium fluoride rinsing on toothbrush abrasion of softened enamel in situ. Caries Res., 38:567–571.

26.

Tsai

C.L.

, Lin

Y.T.

, Huang

S.T.

, Chang

H.W.

2002. In vitro acid resistance of CO2 and Nd-YAG laser-treated human tooth enamel. Caries Res., 36:423–429.

27.

Vlacic

, Meyers

I.A.

, Walsh

L.J.

2007. Laser-activated fluoride treatment of enamel as prevention against erosion. Aust. Dent. J., 52:175–180.

28.

Featherstone

J.D.B.

, Fried

, Bitten

E.R.

1997. Mechanism of laser-induced solubility reduction of dental enamel, in Lasers in Dentistry III. Bellingham, SPIE, 2973:112–116.

29.

Castellan

C.S.

, Luiz

A.C.

, Bezinelli

L.M.

et al. 2007. In vitro evaluation of enamel demineralization after Er:YAG and Nd:YAG laser irradiation on primary teeth. Photomed. Laser Surg., 25:85–90.

30.

Bachmann

, Craievich

A.F.

, Zezell

D.M.

2004. Crystalline structure of dental enamel after Ho:YLF laser irradiation. Arch. Oral. Biol., 49:923–929.

31.

Widgor

H.A.

, Walsh

J.T.

, Featherstone

J.D.B.

, Visuri

S.R.

, Fried

, Waldvogel

J.L.

1995. Lasers in dentistry. Lasers Surg. Med., 16:103–133.

32.

Christofferson

, Christofferson

M.R.

1981. Kinetics of dissolution of calcium hydroxyapatite IV. The effect of some biologically important inhibitors. J. Crystal Growth, 53:42–54.

33.

Fowler

B.O.

, Kuroda

1986. Changes in heated and in laser-irradiated human tooth enamel and their probable effects on solubility. Calcif. Tissue Int., 38:197–208.

34.

Meurman

J.H.

, Hemmerle

, Voegel

J.C.

, Rauhamaa-Makinen

, Luomanen

1997. Transformation of hydroxyapatite to fluorapatite by irradiation with high-energy CO₂ laser. Caries Res., 31:397–400.

35.

Goodman

B.D.

, Kaufman

H.W.

1977. Effects of an argon laser on the crystalline properties and rate of dissolution in acid of tooth enamel in the presence of sodium fluoride. J. Dent. Res., 56:1201–1207.