Evaluation of Mineral Content and Photon Interaction Parameters of Dental Enamel After Phosphoric Acid and Er:YAG Laser Treatment

Abstract

Objective: The purpose of this study was to evaluate the effects of laser and acid etching on the mineral content and photon interaction parameters of dental enamel in human teeth. Background data: The composition of dental enamel may vary, especially at the surface, depending on the reactions that occur during dental treatment. Materials and methods: Forty maxillary premolars were divided randomly into 2 groups of 20 teeth. In the first group, half of teeth crowns were etched by using 37% phosphoric acid; in the second group, half of teeth crowns were etched by using an erbium:yttrium–aluminum–garnet (Er:YAG) laser. The remaining half crowns in each group were used as untreated controls. We characterized the calcium (Ca), phosphorus (P), magnesium (Mg), sodium (Na), and potassium (K) contents in each specimen by using wavelength dispersive X-ray fluorescence spectrometry. The total atomic cross-section ( \documentclass{aastex}\usepackage{amsbsy}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{bm}\usepackage{mathrsfs}\usepackage{pifont}\usepackage{stmaryrd}\usepackage{textcomp}\usepackage{portland, xspace}\usepackage{amsmath, amsxtra}\usepackage{upgreek}\pagestyle{empty}\DeclareMathSizes{10}{9}{7}{6}\begin{document}$$\sigma _t^{}$$ \end{document} ), effective atomic number ( \documentclass{aastex}\usepackage{amsbsy}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{bm}\usepackage{mathrsfs}\usepackage{pifont}\usepackage{stmaryrd}\usepackage{textcomp}\usepackage{portland, xspace}\usepackage{amsmath, amsxtra}\usepackage{upgreek}\pagestyle{empty}\DeclareMathSizes{10}{9}{7}{6}\begin{document}$${Z_{eff}}$$ \end{document} ), and electron density (N_e ) of the tooth samples were determined at photon energies of 22.1, 25, 59.5, and 88 keV by using a narrow beam transmission method. Data were analyzed statistically by using the Mann–Whitney U test. Results: The mineral contents after Er:YAG laser and phosphoric acid etching did not differ significantly (p > 0.05), and no significant variation in \documentclass{aastex}\usepackage{amsbsy}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{bm}\usepackage{mathrsfs}\usepackage{pifont}\usepackage{stmaryrd}\usepackage{textcomp}\usepackage{portland, xspace}\usepackage{amsmath, amsxtra}\usepackage{upgreek}\pagestyle{empty}\DeclareMathSizes{10}{9}{7}{6}\begin{document}$$\sigma _t^{}$$ \end{document} , \documentclass{aastex}\usepackage{amsbsy}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{bm}\usepackage{mathrsfs}\usepackage{pifont}\usepackage{stmaryrd}\usepackage{textcomp}\usepackage{portland, xspace}\usepackage{amsmath, amsxtra}\usepackage{upgreek}\pagestyle{empty}\DeclareMathSizes{10}{9}{7}{6}\begin{document}$${Z_{eff}}$$ \end{document} , or N_e was observed. Conclusions: Therefore, we conclude that the Er:YAG laser and phosphoric acid systems used in this study did not affect mineral composition or photon interaction parameters of dental enamel.

Introduction

Since the development of ruby lasers in the early 1960s, various laser treatments have been investigated in dentistry.¹ Recently, rather than acid etching, lasers have been used for etching of the enamel surface; in particular, erbium:yttrium–aluminum–garnet (Er:YAG) lasers have been used for this purpose.^2,3 Laser etching is typically performed by using erbium-based lasers with wavelengths of 2940 or 2780 nm.³ The acid-etching technique induces microporosity on the enamel surface, which facilitates micromechanical bonding. Etching of enamel using phosphoric acid results in loss of the superficial layer of enamel, as well as dissolution of the enamel subsurface.⁴ Er:YAG laser etching has several advantages, including the lack of vibration and heat; in addition, compared with acid etching, the resulting enamel surface is more resistant to acid attack due to the formation of less-soluble compounds and the alteration of the calcium (Ca) to phosphor ratio. These characteristics have made the use of Er:YAG lasers more popular in dentistry.⁵

Laser irradiation of dental hard tissues involves transformation of light energy to thermal energy for tissue resection. Laser irradiation of dental enamel induces changes within the enamel to a depth of 10–20 μm thermally, depending on the laser type and the energy applied to the dental enamel surface.⁶ Laser exposure causes physical changes in enamel, including recrystallization and melting, resulting in the creation of countless pores and very small bubble-like inclusions.⁷ Laser etching is, thus, a feasible method of etching enamel. Investigations of the surface roughness of etched enamel have shown that laser irradiation yields similar or less surface roughness than does acid etching.⁸

Enamel is the most highly mineralized tissue in the human body. Dental enamel comprises 3% organic material, 1% water, and 96% inorganic material.⁹ The main elements in enamel are Ca, magnesium (Mg), and phosphorus (P), which build forms of hydroxyapatite. Other significant elements include sodium (Na) and potassium (K), which are important in the mineralization of enamel.¹⁰ The composition of dental enamel may vary, especially at the surface, depending on the reactions that occur during dental treatment. Thus, elemental analyses of dental hard tissues have long been recognized as important.¹¹ Due to the brittle and isolating properties of dental hard tissues, few methods are suitable for elemental analysis of enamel. One particularly useful approach is wavelength dispersive X-ray fluorescence (WDXRF) spectrometry.¹²

Photon interaction parameters (i.e., total atomic cross-section, electron density, and effective atomic number) are the main quantitative characteristics that determine the penetration of γ-rays and/or X-rays in material.¹³ The effective atomic number has a physical feature and provides many characteristics of material to be visualized with a number. The effective atomic number and electron density parameters can be derived from the total atomic cross-section at any given energy. The effective electron density is described as the numbers of electrons per unit mass. It differs depending on photon energy. These parameters may be assessed both experimentally and theoretically.¹⁴ When an γ- or X-ray beam is incident on a layer of material, it is absorbed, scattered, and/or transmitted, depending on the interactions of the photons with the atoms in the material.¹⁴ The attenuation of γ- and X-ray photons is related to the atomic number and density of the elements, and the photon interaction parameters for various materials have been previously described.^13,15,16 The attenuation of X-rays in dental materials (e.g., human teeth) is of interest for computed tomography applications.¹⁷ X-rays with various photon energies can be used to nondestructively characterize mineral concentrations in human teeth;^18,19 however, experimental or theoretical studies of the effective electron density and atomic number in human tooth samples after the application of different dental treatments are lacking. Mineral content and photon interaction parameters are important criteria in determining the physical and chemical changes in the structure of materials.¹⁵

The aim of this study was to evaluate the mineral content and photon interaction parameters of photons with energies of 22.1, 25, 59.5, and 88 keV after phosphoric acid and Er:YAG laser etching.

Materials and Methods

Sample preparation

Forty noncarious maxillary premolars that were extracted for orthodontic purposes were used in this present study. Teeth with cracks, hypoplastic areas, or gross irregularities of the dental enamel structure were excluded. Extracted teeth were stored in saline solution. To obtain suitable enamel surfaces for mineral analysis, the teeth were cleaned by using a pumice and to remove plaque, and calculus. Forty teeth were divided randomly into two groups and sectioned transversally at the cemento-enamel junction. The buccal enamel surfaces of all teeth were divided into two areas and to obtain enamel slices from these areas by using a low-speed diamond disk.

In the first group of 20 teeth, half of the buccal enamel surfaces were etched by using 37% phosphoric acid (3 M; ESPE, St. Paul, MN) for 30 sec, washed with water for 15 sec, and dried with air for 15 sec. In the second group of 20 teeth, half of the buccal enamel surfaces were etched by using an Er:YAG laser (Doctor Smile Erbium laser, 2940 nm; Lambda Scientifica, Vicenza, Italy) with an output power of 1.5 W for 30 sec (150 mJ pulse energy, 10 Hz repetition type, 250 μs a pulse duration, 795.7 W/cm² power density). The laser energy was delivered by using a sapphire tip terminal 400 μm diameter and 12 mm length. The distance from the hand-piece of the laser tip and the enamel surface was adjusted as 5 mm, and the laser beam was focused in noncontact mode and sweeping motion. During laser etching, a mixture of water and air (1:1 mass ratio, 2-cm³/min flow rate) was applied to prevent the enamel surface from overheating. The remaining buccal enamel surfaces in both groups served as untreated controls.

The tooth samples were compressed by using a 20-ton press, and ground by using a 25-mL ball mill (SPEX) to minimize the grain sizes, thereby minimizing artefacts in the X-ray intensity signal due to particle-size effects. The samples were sieved by using 37-μm (400 mesh) scaled sieves to ensure particle size homogeneity, and the resulting paste was pressed into pellets with diameters of 0.13 cm under a pressure of 10 tons/cm² by using a SPEX Pmax hydraulic press. This process resulted in smooth surfaces, minimizing artefacts due to surface irregularity.

Mineral content and photon interaction parameters

To determine the photon interaction parameters, elemental composition data for the tooth samples were required. We determined the Mg, P, K, Na, and Ca contents, as well as the Ca/P ratio, of each specimen by using WDXRF spectrometry (Fig. 1) (ZSX-100e; Rigaku, Tokyo, Japan). The mineral content was measured as a percentage of the weight of the tooth sample by using the SQX advanced semi-quantitative software package (Rigaku).²⁰

FIG. 1.

The experimental setup of WDXRF system. WDXRF, wavelength dispersive X-ray fluorescence.

Six samples from each group were evaluated to determine the photon interaction parameters. The total atomic cross-sections of samples were measured by performing transmission experiments using a narrow-beam geometry set up (Fig. 2). The beam path was arranged by using a laser, and emitted photons from radioactive point sources containing americium-241 at 100 mCi (3700 MBq) and cadmium-109 at 40 mCi (1480 MBq) strength were well collimated and properly detected. The radioactive sources were housed in the center of a 10-mm-diameter, 36-mm-long cylindrical shield. A high-resolution liquid nitrogen–cooled Si (Li) detector (with a full width at half maximum of 160 eV for the Mn-Kα line at 5.9 keV, an active area of 12 mm², a sensitive crystal depth of 3 mm, and a 25-μm-thick beryllium window) coupled to a 4k multichannel analyzer was employed to detect photons at 22.1, 25, 59.5, and 88 keV. The detector was shielded by a graded filter, with ∼4.2 mm lead (Pb), ∼1.1 mm iron, and ∼1 mm aluminum, to prevent radiation scattered by nearby objects, as well as radiation such as environmental background, L X-rays from the Pb mask, and background arising from scattered radiation due to the sample holder. The spectra were recorded by using a PC-based multichannel analyzer (Canberra, USA). The reproducibility and stability of the arrangement were checked both before and after each set of measurements. The measurements were done three times for each sample. The samples were set one by one between the detector and the source. The sample diameter was calculated by using a micrometer with a resolution of 20 μm.

FIG. 2.

Employed experimental setup for transmission experiments.

Errors resulted mainly from uncertainties in the peak area, counting statistics, and sample thickness. The estimated total uncertainty in the experimental values of total mass attenuation coefficients was ∼2%. Each spectrum was registered for an adequate time to measure a sufficient number of counts under the photon peak to limit the uncertainty to <1%; thus, the total uncertainty was estimated to be 3%. However, with a photon energy of 88 keV, the discrepancy was high because of the weak lines.

Theoretical basis and computational methods

The total atomic cross-section can be acquired by dividing the mass attenuation coefficient \documentclass{aastex}\usepackage{amsbsy}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{bm}\usepackage{mathrsfs}\usepackage{pifont}\usepackage{stmaryrd}\usepackage{textcomp}\usepackage{portland, xspace}\usepackage{amsmath, amsxtra}\usepackage{upgreek}\pagestyle{empty}\DeclareMathSizes{10}{9}{7}{6}\begin{document}${\mu / \rho} (cm^2 / {g})$ \end{document} by the total number of atoms present in 1 g of that compound as follows in Equation (1): \documentclass{aastex}\usepackage{amsbsy}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{bm}\usepackage{mathrsfs}\usepackage{pifont}\usepackage{stmaryrd}\usepackage{textcomp}\usepackage{portland, xspace}\usepackage{amsmath, amsxtra}\usepackage{upgreek}\pagestyle{empty}\DeclareMathSizes{10}{9}{7}{6}\begin{document} \begin{align*}{\sigma_a } = \frac {( \mu / \rho)_c} {N_A \sum \limits_i {\frac{w_i} {A_i}}} ( barns / atom ), \tag { 1 } \end{align*} \end{document}

The attenuation cross-section values of samples were interpolated at a given photon energy by using WinXCom to measure the effective atomic number \documentclass{aastex}\usepackage{amsbsy}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{bm}\usepackage{mathrsfs}\usepackage{pifont}\usepackage{stmaryrd}\usepackage{textcomp}\usepackage{portland, xspace}\usepackage{amsmath, amsxtra}\usepackage{upgreek}\pagestyle{empty}\DeclareMathSizes{10}{9}{7}{6}\begin{document}$${Z_{eff}}$$ \end{document} using the following logarithmic interpolation Equation (2) formula²¹: \documentclass{aastex}\usepackage{amsbsy}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{bm}\usepackage{mathrsfs}\usepackage{pifont}\usepackage{stmaryrd}\usepackage{textcomp}\usepackage{portland, xspace}\usepackage{amsmath, amsxtra}\usepackage{upgreek}\pagestyle{empty}\DeclareMathSizes{10}{9}{7}{6}\begin{document} \begin{align*}{ Z_ { eff } } = { \frac { { Z_1 } ( \log { \sigma _2 } - \log \sigma ) + { Z_2 } ( \log \sigma - \log { \sigma _1 } ) } { \log { \sigma _2 } - \log { \sigma _1 } } } , \tag { 2 } \end{align*} \end{document}

where \documentclass{aastex}\usepackage{amsbsy}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{bm}\usepackage{mathrsfs}\usepackage{pifont}\usepackage{stmaryrd}\usepackage{textcomp}\usepackage{portland, xspace}\usepackage{amsmath, amsxtra}\usepackage{upgreek}\pagestyle{empty}\DeclareMathSizes{10}{9}{7}{6}\begin{document}$${ \sigma _1}$$ \end{document} and \documentclass{aastex}\usepackage{amsbsy}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{bm}\usepackage{mathrsfs}\usepackage{pifont}\usepackage{stmaryrd}\usepackage{textcomp}\usepackage{portland, xspace}\usepackage{amsmath, amsxtra}\usepackage{upgreek}\pagestyle{empty}\DeclareMathSizes{10}{9}{7}{6}\begin{document}$${ \sigma _2}$$ \end{document} are the elemental cross-sections \documentclass{aastex}\usepackage{amsbsy}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{bm}\usepackage{mathrsfs}\usepackage{pifont}\usepackage{stmaryrd}\usepackage{textcomp}\usepackage{portland, xspace}\usepackage{amsmath, amsxtra}\usepackage{upgreek}\pagestyle{empty}\DeclareMathSizes{10}{9}{7}{6}\begin{document}$$( barns / atom )$$ \end{document} above and below the atomic cross-section \documentclass{aastex}\usepackage{amsbsy}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{bm}\usepackage{mathrsfs}\usepackage{pifont}\usepackage{stmaryrd}\usepackage{textcomp}\usepackage{portland, xspace}\usepackage{amsmath, amsxtra}\usepackage{upgreek}\pagestyle{empty}\DeclareMathSizes{10}{9}{7}{6}\begin{document}$$\sigma$$ \end{document} of the material, respectively, and Z₁ and Z₂ are the respective atomic numbers of the elements corresponding to the cross-sections \documentclass{aastex}\usepackage{amsbsy}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{bm}\usepackage{mathrsfs}\usepackage{pifont}\usepackage{stmaryrd}\usepackage{textcomp}\usepackage{portland, xspace}\usepackage{amsmath, amsxtra}\usepackage{upgreek}\pagestyle{empty}\DeclareMathSizes{10}{9}{7}{6}\begin{document}$${ \sigma _1}$$ \end{document} and \documentclass{aastex}\usepackage{amsbsy}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{bm}\usepackage{mathrsfs}\usepackage{pifont}\usepackage{stmaryrd}\usepackage{textcomp}\usepackage{portland, xspace}\usepackage{amsmath, amsxtra}\usepackage{upgreek}\pagestyle{empty}\DeclareMathSizes{10}{9}{7}{6}\begin{document}$${ \sigma _2}$$ \end{document} .

The effective electron density is given as follows¹⁶: \documentclass{aastex}\usepackage{amsbsy}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{bm}\usepackage{mathrsfs}\usepackage{pifont}\usepackage{stmaryrd}\usepackage{textcomp}\usepackage{portland, xspace}\usepackage{amsmath, amsxtra}\usepackage{upgreek}\pagestyle{empty}\DeclareMathSizes{10}{9}{7}{6}\begin{document} \begin{align*}{ N_E } = { N_A } { \frac { n { Z_ { eff } } } { \mathop \sum \limits_i { { n_i } { A_i } } } } = { N_A } { \frac { { Z_ { eff } } } { \langle A \rangle } } ( electrons / g ) , \tag { 3 } \end{align*} \end{document}

Results

Table 1 lists the mean percentage weights of the elements Ca, P, Mg, Na, and K in the dental enamel both before and after laser and acid etching. The Mann–Whitney U test showed no significant difference between pre-treatment and post-treatment values in either treatment group (p > 0.05). Although the five element levels increased after etching with phosphoric acid, none of these changes was significant (p > 0.05). After Er:YAG laser etching, the Ca, Na level, and Ca/P ratio were decreased but these differences were not significant (p > 0.05). Also, Mg and P levels were unchanged after Er:YAG laser irradiation.

Table 1.

Elemental Analyses Results (wt%) of Five Elements in Dental Enamel Before and After Treatment with Er:YAG Laser and Phosphoric Acid as Measured By Wavelength Dispersive X-Ray Fluorescence

Group	Na, mean (SD)	Mg, mean (SD)	P, mean (SD)	K, mean (SD)	Ca, mean (SD)	Ca/P, mean (SD)
Before acid etching	0.0061 (0.0003)	0.0040 (0.0004)	0.1474 (0.0021)	0.0003 (0.0001)	0.3396 (0.0073)	2.3043 (0.0207)
After acid etching	0.0062 (0.0006)	0.0044 (0.0004)	0.1479 (0.0011)	0.0004 (0.0001)	0.3412 (0.0037)	2.3069 (0.0113)
Before Er:YAG laser	0.0062 (0.0004)	0.0046 (0.0005)	0.1481 (0.0011)	0.0003 (0.00004)	0.3428 (0.0021)	2.3155 (0.0145)
After Er:YAG laser	0.0061 (0.0004)	0.0046 (0.0007)	0.1481 (0.0014)	0.0003 (0.00004)	0.3413 (0.0044)	2.3050 (0.0137)

Ca, calcium; Er:YAG, erbium:yttrium–aluminum–garnet; K, potassium; Mg, magnesium; Na, sodium; P, phosphorus; SD, standard deviation.

The experimental values of total atomic cross-sections ( \documentclass{aastex}\usepackage{amsbsy}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{bm}\usepackage{mathrsfs}\usepackage{pifont}\usepackage{stmaryrd}\usepackage{textcomp}\usepackage{portland, xspace}\usepackage{amsmath, amsxtra}\usepackage{upgreek}\pagestyle{empty}\DeclareMathSizes{10}{9}{7}{6}\begin{document}$$\sigma _t^{}$$ \end{document} ) are enlisted in Table 2. Figure 3a and b show \documentclass{aastex}\usepackage{amsbsy}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{bm}\usepackage{mathrsfs}\usepackage{pifont}\usepackage{stmaryrd}\usepackage{textcomp}\usepackage{portland, xspace}\usepackage{amsmath, amsxtra}\usepackage{upgreek}\pagestyle{empty}\DeclareMathSizes{10}{9}{7}{6}\begin{document}$$\sigma _t^{}$$ \end{document} as a function of the photon energy. In general, \documentclass{aastex}\usepackage{amsbsy}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{bm}\usepackage{mathrsfs}\usepackage{pifont}\usepackage{stmaryrd}\usepackage{textcomp}\usepackage{portland, xspace}\usepackage{amsmath, amsxtra}\usepackage{upgreek}\pagestyle{empty}\DeclareMathSizes{10}{9}{7}{6}\begin{document}$$\sigma _t^{}$$ \end{document} decreased with increasing photon energy. In these figures, the lines indicating the exponential fits are of second order.

FIG. 3.

(a) Total atomic cross-sections for human teeth treated with acid at different energies. (b) Total atomic cross-sections for human teeth treated with laser at different energies. (c) Effective atomic numbers for human teeth treated with acid at different energies. (d) Effective atomic numbers for human teeth treated with laser at different energies. (e) Effective electron densities for human teeth treated with acid at different energies. (f) Effective electron densities for human teeth treated with laser at different energies.

Table 2.

Total Atomic Cross-Sections of Teeth Samples Etching with Phosphoric Acid and Er:YAG Laser

	Energy (keV)
	22.1		25		59.5		88
S. No	Exp	Theo	Exp	Theo	Exp	Theo	Exp	Theo
Phosphoric acid
1	151.3^a	154.6	110.5	109.4	13.4	13.7	7.2	8.0
	158.8^b	154.8	119.0	109.6	13.8	13.7	8.0	8.0
2	148.0	155.6	110.7	110.0	14.1	13.7	8.7	8.0
	165.6	157.3	123.9	111.2	14.0	13.8	8.4	8.0
3	152.9	157.6	113.3	111.5	14.5	13.8	6.2	8.0
	161.6	157.3	116.2	111.1	14.2	13.8	8.0	8.0
4	158.5	154.5	118.8	109.4	13.8	13.7	8.7	7.9
	170.9	161.4	127.0	113.8	14.2	14.1	7.7	8.1
5	159.9	155.9	118.9	110.2	14.8	13.7	8.3	8.0
	141.6	149.1	107.8	105.6	13.1	13.3	8.2	7.9
6	159.2	151.7	117.6	107.2	14.3	13.5	8.6	7.9
	148.8	152.7	109.9	108.1	13.5	13.5	9.4	7.9
Er:YAG laser
1	151.2^c	156.0	112.1	110.3	13.4	13.8	8.3	8.0
	156.1^d	155.7	115.2	110.1	13.5	13.8	8.3	8.0
2	158.2	158.2	118.2	111.7	14.2	13.9	8.4	8.0
	165.3	156.3	123.3	110.6	13.3	13.8	7.3	8.0
3	160.6	151.6	117.8	107.4	13.9	13.5	9.3	7.9
	157.8	154.9	117.0	109.4	13.7	13.7	8.3	7.9
4	162.4	156.6	120.3	110.5	13.1	13.8	8.7	8.0
	153.2	156.4	110.7	110.3	14.4	13.8	8.4	8.0
5	134.8	154.7	102.3	109.5	12.3	13.7	9.0	8.0
	132.5	157.2	100.2	111.1	13.8	13.8	9.1	8.0
6	175.7	157.9	129.5	111.3	13.9	13.9	7.3	8.0
	143.7	158.3	109.9	112.1	13.3	13.9	7.6	8.0

Effective atomic numbers of samples etching with phosphoric acid.

Effective atomic numbers of samples consisting of control group.

Effective atomic numbers of samples etching with Er:YAG laser.

Effective atomic numbers of samples consisting of control group.

Exp, experimental group; S. No., subject number; Theo, theoretical group.

Table 3 lists the effective atomic numbers ( \documentclass{aastex}\usepackage{amsbsy}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{bm}\usepackage{mathrsfs}\usepackage{pifont}\usepackage{stmaryrd}\usepackage{textcomp}\usepackage{portland, xspace}\usepackage{amsmath, amsxtra}\usepackage{upgreek}\pagestyle{empty}\DeclareMathSizes{10}{9}{7}{6}\begin{document}$${Z_{eff}}$$ \end{document} ) for tooth samples at different photon energies; these data are shown graphically in Fig. 3c and d. Table 4 lists the effective electron density (N_E ), calculated using Equation (3). Figure 3e and f shows (N_E ) as a function of the incident photon energy.

Table 3.

Effective Atomic Numbers of Teeth Samples Etching with Phosphoric Acid and Er:YAG Laser

	Energy (keV)
	22.1		25		59.5		88
S. No.	Exp	Theo	Exp	Theo	Exp	Theo	Exp	Theo
Phosphoric acid
1	13.9^a	14.0	14.0	14.0	13.3	13.4	11.9	12.6
	14.1^b	14.0	14.3	14.0	13.5	13.5	12.6	12.6
2	13.8	14.0	14.0	14.0	13.6	13.5	13.3	12.6
	14.2	14.0	14.4	14.0	13.6	13.5	13.0	12.7
3	13.9	14.0	14.1	14.0	13.8	13.5	10.6	12.6
	14.1	14.0	14.2	14.0	13.7	13.5	12.7	12.7
4	14.1	14.0	14.3	14.0	13.5	13.4	13.3	12.6
	14.3	14.1	14.5	14.1	13.7	13.6	12.3	12.7
5	14.1	14.0	14.3	14.0	13.9	13.5	13.0	12.6
	13.7	13.8	13.9	13.9	13.2	13.3	12.9	12.5
6	14.1	13.9	14.2	13.9	13.7	13.4	13.2	12.6
	13.8	13.9	14.0	13.9	13.4	13.4	13.9	12.6
Er:YAG laser
1	13.9^c	14.0	14.1	14.0	13.3	13.5	13.0	12.6
	14.0^d	14.0	14.2	14.0	13.4	13.5	13.0	12.6
2	14.1	14.1	14.3	14.1	13.6	13.5	13.0	12.7
	14.2	14.0	14.4	14.0	13.3	13.5	11.9	12.6
3	14.1	13.9	14.2	13.9	13.5	13.4	13.9	12.6
	14.0	14.0	14.2	14.0	13.4	13.5	13.0	12.6
4	14.1	14.0	14.3	14.0	13.2	13.5	13.3	12.6
	13.9	14.0	14.0	14.0	13.7	13.5	13.0	12.6
5	13.5	14.0	13.7	14.0	12.9	13.4	13.6	12.6
	13.4	14.0	13.7	14.0	12.9	13.4	13.6	12.6
6	14.4	14.0	14.6	14.0	13.5	13.5	11.9	12.7
	13.7	14.0	14.0	14.1	13.3	13.5	12.3	12.7

Effective atomic numbers of samples etching with phosphoric acid.

Effective atomic numbers of samples consisting of control group.

Effective atomic numbers of samples etching with Er:YAG laser.

Effective atomic numbers of samples consisting of control group.

Table 4.

Effective Electron Densities of Teeth Samples Etching with Phosphoric Acid and Er:YAG Laser

	Energy (keV)
	22.1		25		59.5		88
S. No.	Exp	Theo	Exp	Theo	Exp	Theo	Exp	Theo
Phosphoric acid
1	3.2^a	3.2	3.2	3.2	3.1	3.1	2.7	2.9
	3.2^b	3.2	3.3	3.2	3.1	3.1	2.9	2.9
2	3.2	3.2	3.2	3.2	3.1	3.1	3.0	2.9
	3.2	3.2	3.3	3.2	3.1	3.1	3.0	2.9
3	3.2	3.2	3.2	3.2	3.1	3.1	2.4	2.9
	3.2	3.2	3.2	3.2	3.1	3.1	2.9	2.9
4	3.2	3.2	3.3	3.2	3.1	3.1	3.0	2.9
	3.2	3.2	3.3	3.2	3.1	3.1	2.8	2.9
5	3.2	3.2	3.3	3.2	3.2	3.1	3.0	2.9
	3.2	3.2	3.2	3.2	3.1	3.1	3.0	2.9
6	3.2	3.2	3.3	3.2	3.2	3.1	3.1	2.9
	3.2	3.2	3.2	3.2	3.1	3.1	3.2	2.9
Er:YAG laser
1	3.2^c	3.2	3.2	3.2	3.0	3.1	3.0	2.9
	3.2^d	3.2	3.2	3.2	3.1	3.1	3.0	2.9
2	3.2	3.2	3.2	3.2	3.1	3.1	3.0	2.9
	3.2	3.2	3.3	3.2	3.0	3.1	2.7	2.9
3	3.2	3.2	3.3	3.2	3.1	3.1	3.2	2.9
	3.2	3.2	3.3	3.2	3.1	3.1	3.0	2.9
4	3.2	3.2	3.3	3.2	3.0	3.1	3.0	2.9
	3.2	3.2	3.2	3.2	3.1	3.1	3.0	2.9
5	3.1	3.2	3.1	3.2	2.9	3.1	3.1	2.9
	3.1	3.2	3.1	3.2	3.1	3.1	3.1	2.9
6	3.3	3.2	3.3	3.2	3.1	3.1	2.7	2.9
	3.1	3.2	3.2	3.2	3.0	3.1	2.8	2.9

Effective atomic numbers of samples etching with phosphoric acid.

Effective atomic numbers of samples consisting of control group.

Effective atomic numbers of samples etching with Er:YAG laser.

Effective atomic numbers of samples consisting of control group.

The percentage deviations in \documentclass{aastex}\usepackage{amsbsy}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{bm}\usepackage{mathrsfs}\usepackage{pifont}\usepackage{stmaryrd}\usepackage{textcomp}\usepackage{portland, xspace}\usepackage{amsmath, amsxtra}\usepackage{upgreek}\pagestyle{empty}\DeclareMathSizes{10}{9}{7}{6}\begin{document}$${Z_{eff}}$$ \end{document} for the laser-etched and acid-etched groups were 0.5–1.7% and 0.3–1.6%, respectively, compared with the control groups. The variation in N_e was in the range of 0.6–4.1% for the laser-etched group, and in the range of 0.6–2.4% for the acid-etched group, compared with the controls.

Discussion

Etching with phosphoric acid is used commonly to aid bonding of dental materials to the enamel surface. Phosphoric acid etching is effective, but it leads to demineralization of the enamel surface.² The development of dental lasers has enabled the use of laser ablation as an alternative, and the effects of laser etching on enamel have been investigated.²² Compared with acid etching, Er:YAG laser treatment produces a surface that is more resistant to acid attack, which means that it may prevent dental caries.⁵ The shear strength of bonds formed after Er:YAG laser treatment has been shown to be greater than that of bonds formed after etching with phosphoric acid.^23,24 However, higher bond strength values have also been reported after acid etching.² Both etching processes may alter the mineral content of the enamel surface. Although the shear bond strength of comparing the effects of phosphoric acid laser and Er:YAG has been widely investigated, the mineral composition and photon interaction parameters of the enamel have not been reported. We analyzed the photon interaction parameters and mineral content of dental enamel after phosphoric acid and Er:YAG laser-etching processes by using WDXRF spectrometry.

Chemical and morphological changes may result from the heating of enamel during laser etching. The inhibition has been hypothesized to occur due to the fusion and melting of hydroxyapatite and/or the subsequent sealing of the dental enamel surface.²⁵ The enamel temperature increases rapidly to 10,000°C during laser irradiation. The organic matrix in the enamel evaporates and decomposes at temperatures in the range of 400°C–800°C; the protective effect of the organic matrix may diminish or even disappear when the enamel is heated above 1000°C. Melting of the enamel may occur at temperatures in the range of 900°C–1200°C.²⁶ Changes in the structure and composition of apatite crystals may cause alterations in the Ca/P ratio. The application of cooling during laser irradiation limits these effects. We used air and water cooling, and we observed that the Ca/P ratio did not differ significantly among the laser-etched, acid-etched, and control groups.

Various analytical methods, including atomic absorption spectrophotometry, proton-induced X-ray emission, electrothermal vaporization, laser ablation inductively coupled plasma mass spectrometry, and energy-dispersive X-ray analysis, have been used to characterize the mineral composition of dental enamel.^27,28 WDXRF spectroscopy characterizes each element on the basis of oxides, and, thus, it is suitable for mineralogical samples, such as hydroxyapatite matrix. The technique is also accurate for heavy elements.¹² In our study, we used the WDXRF analyzing system because of these characteristics.

We characterized the composition of dental enamel after phosphoric acid etching and Er:YAG laser etching by using WDXRF spectroscopy. We found that the mean percentage weights of Ca, P, Mg, Na, and K, as well as the Ca/P ratio of the groups, were not affected significantly by the laser and acid etching. Sazak et al.²⁹ reported that neodymium:yttrium–aluminum–garnet (Nd:YAG) laser irradiation reduced the mean percentage of P in dental enamel. They used energy-dispersive spectrometry and scanning electron microscopy to determine mineral changes. Dankner et al.³⁰ reported that these changes are indicative of melting and re-crystallization processes. Differences in the results of these studies may be due to the use of different laser systems and detection methods.

The experimental data listed in Tables 2 –4 show good agreement with the theoretical data. Figure 3a and b show clearly that the mass attenuation coefficient decreased with increasing photon energy. According to Hine's³¹ expressions, the effective atomic number of a material that is composed of several elements cannot be expressed by using a single number, and it should be considered an energy-dependent parameter due to the different partial photon interaction processes with matter for which the various atomic numbers in the material have to be weighted differently. From Fig. 3c and d, it can be concluded that the effective atomic numbers of tooth samples vary with photon energy. The effective atomic numbers were found to decrease with increasing photon energy for the given samples. The \documentclass{aastex}\usepackage{amsbsy}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{bm}\usepackage{mathrsfs}\usepackage{pifont}\usepackage{stmaryrd}\usepackage{textcomp}\usepackage{portland, xspace}\usepackage{amsmath, amsxtra}\usepackage{upgreek}\pagestyle{empty}\DeclareMathSizes{10}{9}{7}{6}\begin{document}$${Z_{eff}}$$ \end{document} varies from higher values at lower energies to lower values at higher energies, which may be due to a partial photon interaction process, namely the photoelectric effect. This process becomes dominant at low photon energies for materials having high Z elements as constituents. Also, the photoelectric effect differs with \documentclass{aastex}\usepackage{amsbsy}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{bm}\usepackage{mathrsfs}\usepackage{pifont}\usepackage{stmaryrd}\usepackage{textcomp}\usepackage{portland, xspace}\usepackage{amsmath, amsxtra}\usepackage{upgreek}\pagestyle{empty}\DeclareMathSizes{10}{9}{7}{6}\begin{document}$${Z^{4 - 5}}$$ \end{document} ; it follows that the photoelectric absorption cross-section is larger for high-Z elements, which can explain the larger \documentclass{aastex}\usepackage{amsbsy}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{bm}\usepackage{mathrsfs}\usepackage{pifont}\usepackage{stmaryrd}\usepackage{textcomp}\usepackage{portland, xspace}\usepackage{amsmath, amsxtra}\usepackage{upgreek}\pagestyle{empty}\DeclareMathSizes{10}{9}{7}{6}\begin{document}$${Z_{eff}}$$ \end{document} values at low energies. The electron density varied with the photon energy in a manner similar to the effective atomic number. We found no significant variation in \documentclass{aastex}\usepackage{amsbsy}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{bm}\usepackage{mathrsfs}\usepackage{pifont}\usepackage{stmaryrd}\usepackage{textcomp}\usepackage{portland, xspace}\usepackage{amsmath, amsxtra}\usepackage{upgreek}\pagestyle{empty}\DeclareMathSizes{10}{9}{7}{6}\begin{document}$${Z_{eff}}$$ \end{document} or \documentclass{aastex}\usepackage{amsbsy}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{bm}\usepackage{mathrsfs}\usepackage{pifont}\usepackage{stmaryrd}\usepackage{textcomp}\usepackage{portland, xspace}\usepackage{amsmath, amsxtra}\usepackage{upgreek}\pagestyle{empty}\DeclareMathSizes{10}{9}{7}{6}\begin{document}$${N_{e}}$$ \end{document} among the different treatments and the controls, and all deviations were within the estimated experimental errors. \documentclass{aastex}\usepackage{amsbsy}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{bm}\usepackage{mathrsfs}\usepackage{pifont}\usepackage{stmaryrd}\usepackage{textcomp}\usepackage{portland, xspace}\usepackage{amsmath, amsxtra}\usepackage{upgreek}\pagestyle{empty}\DeclareMathSizes{10}{9}{7}{6}\begin{document}$${Z_{eff}}$$ \end{document} , \documentclass{aastex}\usepackage{amsbsy}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{bm}\usepackage{mathrsfs}\usepackage{pifont}\usepackage{stmaryrd}\usepackage{textcomp}\usepackage{portland, xspace}\usepackage{amsmath, amsxtra}\usepackage{upgreek}\pagestyle{empty}\DeclareMathSizes{10}{9}{7}{6}\begin{document}$$\sigma _t^{}$$ \end{document} , and \documentclass{aastex}\usepackage{amsbsy}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{bm}\usepackage{mathrsfs}\usepackage{pifont}\usepackage{stmaryrd}\usepackage{textcomp}\usepackage{portland, xspace}\usepackage{amsmath, amsxtra}\usepackage{upgreek}\pagestyle{empty}\DeclareMathSizes{10}{9}{7}{6}\begin{document}$${N_{e}}$$ \end{document} parameters are very important for understanding physical properties of materials. There were no differences between the phosphoric acid and Er:YAG laser-etching groups in terms of experimental values. Therefore, both phosphoric acid and Er:YAG laser-etching methods can be used with confidence clinically for etching dental enamel.

Conclusions

We investigated the elemental compositions of dental enamel after phosphoric acid etching and Er:YAG laser etching by using WDXRF spectroscopy. We may draw the following conclusions:

(1) Phosphoric acid and Er:YAG laser etching did not significantly affect the mineral composition of dental enamel. Er:YAG laser etching is, thus, an important alternative to phosphoric acid etching.

(2) The electron densities and effective atomic numbers varied with the incident photon energy; both values were larger at lower photon energies, which may be due to a partial photon interaction process (i.e., the photoelectric effect).

(3) No significant variation in \documentclass{aastex}\usepackage{amsbsy}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{bm}\usepackage{mathrsfs}\usepackage{pifont}\usepackage{stmaryrd}\usepackage{textcomp}\usepackage{portland, xspace}\usepackage{amsmath, amsxtra}\usepackage{upgreek}\pagestyle{empty}\DeclareMathSizes{10}{9}{7}{6}\begin{document}$${Z_{eff}}$$ \end{document} or \documentclass{aastex}\usepackage{amsbsy}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{bm}\usepackage{mathrsfs}\usepackage{pifont}\usepackage{stmaryrd}\usepackage{textcomp}\usepackage{portland, xspace}\usepackage{amsmath, amsxtra}\usepackage{upgreek}\pagestyle{empty}\DeclareMathSizes{10}{9}{7}{6}\begin{document}$${N_{e}}$$ \end{document} was observed after the different treatments, and all deviations were within the estimated experimental errors.

Footnotes

Acknowledgments

The authors would like to thank Professor L. Gerward of the Department of Physics, Technical University of Denmark, for providing them with the WinXCom program.

Author Disclosure Statement

No competing financial interests exist.

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