In clinical practice, laser ablation usually works under liquid environment such as water, blood or their
mixture. Laser-induced vapor bubble or bubble formation and its consequent dynamics were believed to have
important influence on tissue ablation. In the paper, the dynamic process of vapor bubble formation and
consequently collapse induced by pulsed Ho:YAG laser in static water was investigated by using high-speed
camera. The results showed that vapor channel / bubble can be produced with pulsed Ho:YAG laser, and the
whole dynamic process of vapor bubble formation, pulsation and consequently collapse can be monitored by
using high-speed camera. The dynamic characteristics of vapor bubble, such as pulsation period, the
maximum depth and width were determined. The dependence of above dynamic parameters on incident
radiant exposure was also presented. Based on which, the influence of vapor bubble on hard tissue ablation
was discussed.
The aim of this study is to evaluate the demineralization process in human dentine using Raman spectroscopy (RS). Nine
human molars from Chinese subjects were cut into twelve tooth sections. The sections were painted with an acidresistant
varnish leaving one dentine surface exposed and then immersed into 0.3% citric acid to simulate the oral natural
demineralization. According to the acid-etch time, specimens were randomly divided into four groups: Ⅰ5 min, Ⅱ 10
min, Ⅲ 20 min and Ⅳ 40 min. Raman spectra were measured before and after each treatment. The result showed that no
new bands, bands shifts, or disappearance of bands occurred in the whole process. However, the peak intensities of
inorganic constituents decreased with the increase of acid-etch time, while the trends of organic constituents were
opposite. In conclusion, RS could be able to efficiently monitor the demineralization status of human dentine.
Water plays an important role in laser ablation. There are two main interpretations of laser-water interaction:
hydrokinetic effect and vapor phenomenon. The two explanations are reasonable in some way, but they can’t explain the
mechanism of laser-water interaction completely. In this study, the dynamic process of vapor channel formation induced
by pulsed CO2 laser in static water layer was monitored by high-speed camera. The wavelength of pulsed CO2 laser is
10.64 um, and pulse repetition rate is 60 Hz. The laser power ranged from 1 to 7 W with a step of 0.5 W. The frame rate
of high-speed camera used in the experiment was 80025 fps. Based on high-speed camera pictures, the dynamic process
of vapor channel formation was examined, and the threshold of vapor channel formation, pulsation period, the volume,
the maximum depth and corresponding width of vapor channel were determined. The results showed that the threshold of
vapor channel formation was about 2.5 W. Moreover, pulsation period, the maximum depth and corresponding width of
vapor channel increased with the increasing of the laser power.
To examine the temperature changes in the pulp chamber during cavity preparation in dentin with the Er:YAG laser
(2940 nm), a total 20 intact premolars teeth were divided into 4 groups for dentin ablation with different radiant
exposures at 4Hz and 8Hz with and without water spray. A K-type thermocouple was used to monitor the temperature
changes in pulp chamber during laser treatment. The total time of irradiation was 70 sec. the water spray rate was 3
mL/min. It showed that maximum temperature rise increases with the increasing of radiant exposure and pulse
repetition rate and the additional water cooling during laser ablation can significantly reduce the temperature rise in
pulp chamber which will benefit to avoid or reduce thermal damage to tooth structure and dental pulp. The highest
rise of temperature in the pulp was achieved with 20 J/cm2 and 8 Hz (19.83°C ). For all sample without water spray, the
rise of temperature was exceed 5 °C . In contrast, with water spray, the temperature rise in the pulp can be firmly
controlled under 1°C. The results also indicated that ablation rate and efficiency can be enhanced by increasing the
incident radiant exposure and pulse repetition rate, which simultaneously producing more heat accumulation in
dental tissue and causing thermal damage to dental tissue. By applying an additional water spray, thermal damage
can be significantly reduced in clinical application.
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