الفهرس 
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Abstract
Enamel demineralization is not only the initial step dental caries and white spot lesions, but also it is the first stage of another serious dental problem which is dental erosion. This study was conducted to assess and compare the influence of Er,Cr-YSGG (2780 nm) laser, Nd-YAG (1064 nm) nanosecond laser conditioning and acidulated phosphate fluoride, on the acquired acid resistance of human enamel in vitro. Sixty non-carious human premolars, extracted for orthodontic reasons, were selected randomly from unknown persons. Each enamel surface was examined under stereomicroscope and teeth with any defects, erosions, micro-cracks or visible stains were excluded from the study. An in-vitro study was done and the specimens were randomly allocated and assigned to six groups of (10 specimens each); group I with no treatment done (control), group II in which samples were treated with topical APF gel, group III (treated with Er,Cr:YSSG laser only), group IV (treated with Er,CR:YSGG laser followed by topical APF gel application), group V (treated with Nd:YAG laser only) and group VI (treated with Nd:YAG laser followed by topical APF gel application. The specimens from all groups were then subjected to pH cycling (demineralization). Topical fluoride application was done by single application of acidulated Phosphate Fluoride gel (APF) containing 1.23٪ F, pH 3.5 using a cotton swab for 4 minutes. Samples were then washed with deionized water for 1 min then dried with absorbent paper. Laser conditioning was done for group III and IV by pulsed Er,CR: YSGG laser at 2780 nm wavelength with the following parameters: 0.75 W, power, 12.5 mJ , pulse energy, 20 Hz, repetition rate, 140 μs, pulse width and 10 s average exposure time. The air pressure and water level were set at: 11٪ and 0٪ respectively. Laser conditioning was done for group V and VI by pulsed nanosecond Nd-YAG laser at 1064 nm wavelength with the following parameters: 800 mw power, 10 Hz Repetition rate,7 ns pulse width and 10 seconds average exposure time. Morphological assessment was done by examination of samples by scanning electron microscope (SEM), at baseline (sound untreated enamel), after enamel surface treatment and after pH cycling. Only specimens from group I (untreated control) were analyzed first at baseline then after exposure to pH cycling. Elemental analysis for calcium and phosphorus of enamel surface in all groups was performed by both Energy Dispersive Analysis of X-ray Spectroscopy (EDAX) and Laser Induced Breakdown Spectroscopy (LIBS) at baseline of sound untreated enamel then after exposure to pH cycling. The present investigation revealed the following results: I- SEM results: SEM examination revealed the following: Sound enamel surface at the baseline of all groups showed a relatively smooth homogenous surface with a uniform rodless enamel surface layer while SEM examination after exposure of sound enamel surface to pH cycling (group I) revealed irregular porous dissoluted enamel surface, areas of distinct hollowing of rod core surrounded by relatively intact peripheries at some areas and raised high peripheries at other areas. Following treatment with topical application of APF gel (group II) revealed discrete granular surface coating. Unevenly distributed globular deposits were identified. However, when enamel was examined after exposure to pH cycling, it showed partially dissoluted surface with honeycomb pattern. This pattern was not clearly evident in some regions as enamel surface appeared smooth. Following treatment with Er,Cr:YSGG laser (group III) showed minute disruption in the morphology of enamel surface. The surface appeared relatively smooth except for some alterations including scattered unhomogenous patches. After exposure to pH cycling the treated enamel surface examination revealed porous enamel structure intermixed with areas of rodless enamel that covered the underlying structure. Following treatment with Er,Cr:YSGG laser followed by topical APF gel (group IV) showed relatively smooth surface with multiple scattered microspaces. Deposition of globular particle with uneven distribution was also evident throughout the enamel field. Following exposure of enamel to pH cycling, SEM examination revealed relatively smooth enamel surface intermingled with areas of exposed enamel rods as well as areas of microporosities. Enamel surface treated with nanosecond Nd:YAG laser (group V) revealed smooth surface that was relatively unhomogenous. Irregularly shaped cracks were observed scattered in most of enamel field. After exposure to pH cycling it revealed partially dissoluted enamel surface. Most of enamel field demonstrated eroded rod peripheries with relatively intact rod core. Enamel surface treated with nanosecond Nd: YAG laser followed by topical APF gel (group VI) revealed unhomogenous enamel surface intermixed with smooth areas with deposition of globular particles with uneven distribution. Some of these particles were found to obscure some of underlying microspaces. After being subjected to pH cycling, SEM showed partial dissolution of enamel surface with some areas of smooth enamel surface. II- EDAX results: Regarding normality testing, all Ca and P data collected from EDAX, fulfilled normality criteria, and were all normally distributed. 1-Comparison between mean Ca and P wt ٪ values at baseline and after demineralization cycle within each group revealed a statistically significant decrease in calcium and phosphorus wt ٪ after exposure to demineralization cycle. Mean difference between Ca and P wt ٪ at baseline and after demineralization within each group was obtained. 2- By comparing all values of percentage decrease in both Ca and P wt ٪ among all groups, it was shown that there was a statistically significant difference between all groups. The highest percentage decrease was recorded in group I (untreated control), whereas the lowest value was recorded in group IV (Er, Cr: YSGG laser+APF. 3- Multiple pairwise comparisons between percentage decreases in Ca wt ٪ across different groups revealed a statistically significant pairwise difference among different groups. 4- Multiple pairwise comparisons between percentage decreases in P wt ٪ across different groups revealed a statistically significant pairwise difference among different groups. However, non-statistically significant differences were observed between (group II and V), (group III and V), (group III and VI) and (group V and VI). III- LIBS results: Regarding normality testing, all Ca and P data collected from LIBS, fulfilled normality criteria, and were all normally distributed. 1- Comparison between mean Ca and P line intensities values at baseline and after demineralization cycle within each group revealed a statistically significant decrease in Ca and P line intensities after exposure to demineralization cycle. Mean difference between Ca and P line intensities at baseline and after demineralization within each group was obtained. 2- Comparison of all values of percentage decrease in Ca line intensities of all groups showed that there was a statistically significant difference among different groups. The highest percentage decrease was recorded in group I (untreated control), whereas the lowest value was recorded in group IV (Er, Cr: YSGG laser+APF). 3- Multiple pairwise comparisons between percentage decreases in Ca line intensities across different groups revealed a statistically significant pairwise difference among different groups. However, nonsignificant difference was observed between group II and V. 4- Multiple pairwise comparisons between percentage decreases in P line intensities across different groups revealed a statistically significant pairwise difference among different groups. However, non-significant difference was observed between group V and VI.