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It has been shown that the laser can increase acid resistance of
enamel. This study planed to compare the effects of 2 kinds of
lasers (Nd:YAG laser and CO2 laser) on the acid resistance of
primary teeth and young permanent teeth.Thirty primary teeth and
30 permanent premolars were used in this study. Three of 2x4 mm
surfaces were obtained from the polished labial and lingual side
of each tooth . Each surface was treated by one of the following
：(1) Nd:YAG laser, 6W, 50pps, spot size 0.6 mm, moving rate 12
mm/s, energy density 83.33 J/cm2. (2) CO2 laser: 1W, 750pps ,
spot size 0.3 mm, moving rate 4 mm/s , energy density 83.33 J/
cm2. (3)control . After treatment, each specimen was coated with
nail varnish leaving a rectangle window (1x3 mm) and immersed in
10 ml acid solution ( pH=4.5 lactate buffer). The time for acid
challenge was 24 hours , 72 hours, and 168 hours. There were 18
different treated conditions and 10 specimens in each condition.
To study the acid resistance of the enamel specimens, 4 methods
were used : (1) scanning electron microscope. (2) polarized
light microscope: observing the longitudinal surface. (3)
polarized light microscope: observing erosion depth and (4)
atomic absorption spectroscopy: measuring the [Ca] in the acid
solution.Under polarized light microscope , the surfaces of the
control groups that had not been through acid erosion is smooth.
The surfaces of the laser groups that had not been through acid
erosion had many pits with fissures and craters . After 24 hours
of acid challenge , even Type II etching patterns emerged in the
control groups , while not only uneven Type II etching patterns
, but also many fissures and unrecognizable disorderly patterns
appeared in the laser groups . After 72 hours of acid challenge
, permanent teeth of the control groups and primary teeth had
surfaces that appeared smooth with wide fissures , but
disorderly enamel rods could be observed on the permanent teeth
of the laser groups . After 168 hours of acid challenge , no
noticeable difference could be found between the laser groups
and control groups. Resulting observations under the scanning
electron microscope were unable to conclude that laser can
increase resistance of enamel against acid challenge.Results
using the polarized light microscope showed that after laser
treatment, enamel surfaces gained 5 mm ~ 20 mm of dark zones.
After 24 hours of acid challenge, the dark zones of the laser
groups expanded, especially in the CO2 laser groups. After 72
hours of acid challenge, the erosion of laser groups deepened
and the depths of dark zones decreased, but were still more
prominent than that of the control groups. After 168 hours of
acid challenge, other than the surface of the laser groups being
uneven, no significant difference was found. The dark zones that
appeared after acid challenge of the laser groups were probably
due to a result of demineralization and remineralization, but
this conclusion needed further experimental proof.There were
significant differences between the permanent teeth and the
primary teeth in the results of depths of acid erosion or
amounts of calcium releasing. The erosion depths and amounts of
calcium releasing in primary teeth were more than that of
permanent teeth. There were also significant differences in the
duration of immersed times. The longer the immersion was, the
deeper the acid erosion and the more the calcium released into
the acid solution.The depths of acid erosion and the amounts of
calcium releasing of the control groups were more than that of
the Nd:YAG laser groups, which, in turn, were more than that of
the CO2 laser groups.The increasing acid resistance was
according to the reduced erosion depths of laser groups compared
with their respective controls. The resulting findings were that
primary teeth performed better than permanent teeth. Laser
irradiation could reduce acid erosion depths and calcium
releasing of enamel surfaces. This was most noticeable in the
first 24 hours, and CO2 laser groups were more effective than
the Nd:YAG laser groups.In conclusion, laser treated enamel
decreased erosion depth and calcium releasing into acid solution
as compared with nontreated groups upon challenging by acid. The
effect of CO2 laser on the acid resistance of surface enamel was
better tha that of Nd:YAG laser. The erosion depths and calcium
releasing into acid solution of laser treated primary teeth were
more than which of laser treated young permanent teeth. The
increasing acid resistance was better in primary teeth than in
young permanent teeth. The longer the immersed time in lactate
buffer was, the lesser the acid resistance of the lased enamel
surfaces was seen.
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