LASER EFFECTS ON DENTAL HARD
TISSUES
J.D.B FEATHERSTONE AND D.G.A. NELSON
Department of Oral Biology, Eastman Dental Center, 625 Elmwood Avenue, Rochester, New York
14620
Adv Dent Res l(l):21-26, October, 1987
ABSTRACT
he use of lasers in dentistry has been considered for over 20 years. Higher-energy density lasers were
shown to fuse enamel but were potentially unsafe. Subsequently, low-energy density laser radiation
was shown to affect artificial caries lesion formation.
Recent studies have shown that carbon dioxide lasers can successfully be used at low-energy densities to
fuse enamel, dentin, and apatite. Our studies have shown that specific wavelengths are highly efficient. These
wavelengths are directly related to the infrared absorption regions of apatite. We have conducted studies with
enamel and dentin, using pulsed CO 2 laser radiation in the 9.32-jim to 10.49-|xm region with energy densities
in the 10 to 50 J.cm~2 range. This laser treatment caused surface fusion and inhibition of subsequent lesion
progression and markedly improved the bonding strength of a composite resin to dentin. Similar studies have
shown no pulpal damage or permanent deleterious effect on soft tissues.
This improved understanding of the scientific rationale for the interaction of CO 2 lasers with teeth can lead
to several clinical applications. This will depend, however, on the development of a technology to direct a
specific frequency laser beam precisely to a desired site.
T
INTRODUCTION
Studies on the possible application of lasers to dentistry began over 20 years ago. Many of the studies
that have been carried out have been based simply
on empirical use of available lasers and an examination by various techniques of their effect on dental
hard or soft tissues. Studies by Stern and Sognnaes
(1964) and Stern et ah (1966) utilized a ruby laser
(wavelength 693.4 nm) and did demonstrate that exposure of human enamel to the laser increased the
resistance of that enamel to subsurface demineralization in vitro. Vahl (1968) used electron microscopy
and x-ray diffraction to study the effects of laser radiation on enamel. These studies clearly demonstrated ultrastructural and crystallographic changes
in response to laser radiation.
Later studies by Yamamoto and Ooya (1974) and
Yamamoto and Sato (1980) showed the potential of a
Nd:YAG laser (wavelength 1060 nm) to fuse dental
Presented at the Conference on Diagnostic and Therapeutic Technology in Dentistry, September 29-October 1,1986, conducted by
the National Institute of Dental Research
This study was supported in part by a Fogarty International
Research Fellowship, F05 TW03162.
enamel and to make it highly resistant to subsequent
dissolution.
In the above studies and in other studies carried
out with visible light lasers during that same period,
non-specific laser radiation was utilized with very inefficient energy interaction with the tooth substance.
Often high-energy densities — for example, 104 J.cm~2
— were used together with relatively long interaction
times in the range of 50 msec to 2 sec. Later studies
utilizing the carbon dioxide laser utilized much lowerenergy density levels, with a much more efficient
transfer of energy, and these are described below.
An obvious possible utilization of lasers in dentistry is for the fusion of enamel for cavity preparation. Goldman et al. (1965) and Gordon (1967) studied
this possibility and came to the conclusion that the
enormous amount of heat generated locally caused
cracking and distortion of the enamel, and that the
technique did not warrant further study.
Interaction of lasers with the soft tissues is another
obvious possible application in terms of both surgery
and wound healing. Any surgical application of lasers which is currently in use in other parts of the
body has obvious application to the oral cavity. It is
beyond the scope of this paper to discuss that aspect.
However, studies have been carried out to investigate
the possibility of improved wound healing after the
21
22
FEATHERSTONE & NELSON
Adv Dent Res October 1987
use of lasers. Surinchak et al (1983) and Hunter et at.
(1984) utilized He:Ne laser irradiation at 632.8 nm and
low-energy densities (1-2 J.cm~2) but demonstrated
no advantage with respect to wound healing compared with conventional surgery. These aspects deserve further attention, but the potential does not
appear to be good.
on human dental enamel and dentin in the infrared
region from 9.3 through 10.6 jjim. Low-energy densities in the region 10-50 J.cm"2 and short interaction
times in the region of 100-200 nsec have been used.
Carbon Dioxide Laser
The abovementioned studies all utilized lasers with
energy frequency values that do not interact efficiently with enamel. In those cases, the energy density of the laser irradiation was large enough not only
to cause changes in the enamel surface structurally
and morphologically but also to cause an apparent
large change in the physical properties of the enamel.
Pitting and melting of the surface enamel and fusion
of the crystals occurred. Dental enamel, cementum,
and dentin contain carbonated apatite which has absorption bands in the infrared region due to phosphate, carbonate, and hydroxyl groups in the crystal
structure. The carbon dioxide laser produces radiation which falls in the infrared region and coincides
closely with some of the absorption bands of apatite
(Nelson and Featherstone, 1982; Nelson and Williamson, 1982).
Stern et al (1972), Kantola et al (1973), and Lenz et
al (1982) carried out studies utilizing carbon dioxide
lasers and demonstrated ultrastructural crystallographic effects with this laser. In most previous studies using the carbon dioxide laser, continuous wave
lasers were used, and typical interaction times varied
from 50 msec to 2 sec. Kuroda and Fowler (1984) and
Fowler and Kuroda (1986) reported structural and
phase changes in surface enamel when treated with
the carbon dioxide laser or when heated. All of these
studies suggest that the carbon dioxide laser could
be the system of choice for possible dental applications.
In studies carried out in our laboratories during the
last five years, we have considered that the wavelength of the laser light is extremely important. Dental enamel absorbs very little light in the visible region,
so the use of visible light lasers requires high-energy
densities as discussed above. Further, pulsed lasers
rather than continuous lasers provide a way for increasing the peak power density while keeping the
pulse energy density constant. This means that fusion, melting, and recrystallization of enamel crystals
are confined to a 5-10-|i,m thin surface region without
affecting the underlying enamel at depths of 50 |xm
or greater and — very importantly — the underlying
dentin or pulp. Dental hard tissue has intense absorption bands in the 9.0 through 11.0 juim region
because of the apatite crystals, and this implies that
infrared radiation from the carbon dioxide laser should
be much more efficiently absorbed than radiation from
the visible spectrum. We summarize here some of the
recent investigations of our group (Nelson et al, 1986a;
Nelson et al 1987). These studies have concentrated
on the use of low-energy pulsed CO2 laser radiation
Full experimental details have been presented elsewhere (Nelson et al, 1986a; Nelson et al, 1987). The
present paper is designed to summarize these studies
and to present the principal conclusions from them.
MATERIALS AND METHODS
Artificial Caries-like Lesion Formation
Non-carious crowns of human molars or pre-molars
were removed from the roots, cleaned according to
previously reported methods (Featherstone et al, 1983),
and painted with acid-resistant varnish, leaving two
1 x 3-mm rectangular windows side by side at equal
heights from the cemento-enamel junction. The teeth
were then cut in half so that one window could be
retained as an unlased control, with the other exposed to laser treatment (see below). A total of 106
teeth was utilized in the studies. The control teeth,
or the lased teeth (with the windows re-painted), were
immersed in artificial caries-producing buffer, pH 5.0,
at 37°C, containing 0.04 mol/L lactic acid, 0.1 mmol/
L methane hydroxy diphosphonate (MHDP), 40 mL
per tooth, for periods from two hr up to 14 days. The
demineralizing buffers were analyzed for calcium and
phosphate at the end of the dissolution period. The
teeth were cut in half through the window for subsequent examination by microhardness testing, polarized light microscopy, and scanning electron
microscopy (see below).
Laser Treatment
We used a Lumonics TEA103-1 Multimode Line
Tunable CO 2 Gas Laser (Lumonics Research Ltd., Ottawa, Canada) that produced 100-200 nsec pulses with
a maximum pulse energy of 5J. Four specific lines
were chosen by means of an Optical Engineering
Model 16A carbon dioxide laser spectrum analyzer
(Optical Engineering, Inc., Santa Rosa, CA) at the
following wavelengths (wave numbers): 9.32 |im (1073
cm- 1 ), 9.57 |mm (1045 cm" 1 ), 10.27 \im (973 cm" 1 ),
and 10.59 |xm (945 cm" 1 ). The pulses were ellipticallypolarized with unequal components in the perpendicular and parallel polarization directions. The spot
size was varied from 2-5 mm in diameter with a concave mirror so that pulse energy densities of approximately 10-50 J.cm~2 and peak power densities of
107-108 Won- 2 could be obtained.
Scanning Electron Microscopy
The SEM examination was carried out by Dr. Dennis Nelson and Dr. W. Jongebloed (University of
Groningen) and is reported in detail elsewhere (Nelson et al, 1987), but is included here for completeness. Individual enamel specimens were treated with
one of four different laser wavelengths as above, with
Vol. 1 No. 1
LASER EFFECTS ON DENTAL TISSUE
an energy density of 50 J.cm 2 for 400 pulses. This
part of the study was carried out so that we could
assess the surface enamel changes which occurred
under the conditions utilized in the lesion formation
experiments. Dentin was also irradiated with 1073
cm 2 line for 20 pulses. These samples were sputtered with a layer of gold 10-12 nm thick and examined in a JEOL 35C Scanning Electron Microscope
(JEOL, Japan) at 25 kV.
Microhardness Testing
The lased and control halves of each tooth were
embedded and polished with the cut face exposed,
according to standard procedures in our laboratory
(Featherstone et al., 1983). The hardness profiles were
determined at 25-|a.m intervals across the lesion section into the underlying normal enamel, and converted via the empirical relationship previously
reported (Featherstone et al., 1983). A computer program was used for calculation of the average profiles
for each group, based on the means at each depth.
Polarized Light Microscopy
Thin sections were cut to a thickness of approximately 80 |xm and examined by polarized light microscopy in water, Thoulet's solution, and quinoline,
according to standard procedures. [This portion of
the study was conducted by Dr. James Wefel (University of Iowa), and the details are reported elsewhere (Nelson et al., 1987).] The thin sections used
were immediately adjacent to the hemi-sections used
for the microhardness testing.
100
100
150
200
250
300
Depth (microns)
Fig. 1 —Composite mineral profile of 14-day-old lesions obtained
from cross-sectional microhardness data for enamel lased with the
9.32-n.m (1073 c m 1 ) line ( • ) compared with the non-lased controls (x). Each point is the average of 15 microhardness indentations, with the p value (paired t test at each depth) shown above
each set of points. Bars show standard deviations (from Nelson et
al., 1986a; published courtesy of Karger AG).
| low energy
[ j high energy
Chemical Analysis
The demineralization solutions were analyzed for
calcium by atomic absorption, and phosphate by the
molybdate method.
RESULTS
Hardness Profiles
The irradiation of intact human dental enamel by
carbon dioxide laser inhibited subsequent artificial
caries-like lesion formation. This was, however, variable according to the wavelength used and according
to the energy density used. The most effective laser
line at the pulse energy1 density of 10 J.cm"2 was the
R12 line at 1073 cm" . This produced significantly
less demineralization and shallower lesions than did
the controls (Fig. 1). Although the 1045-, 973-, and
945-cm ' lines also produced effects, they were not
as pronounced, with
pre-treatments consisting of 400
pulses of 10 J.cm 2 energy density laser beam.
For each of the experiments performed, the area of
mineral loss was calculated from the data between
25-200 |xm depth and expressed as a percentage of
the total demineralization in the control calculated in
the same way. This gives an estimate of the average
reduction in artificial lesion formation given by a particular laser pre-treatment. Fig. 2 presents the results
1073
1045
973
945
wavenumber (cm'^l
Fig. 2 —Mean percentage reduction in artificial lesion formation
(14-day immersion) in enamel when compared with the control
group for 400-pulse laser pre-treatment at 10 J.cm 2 (dark shading)
and 50 J.cm 2 (single cross-hatching). Error bars are standard deviations (from Nelson ct al., 1986a; published courtesy of Karger
AG).
expressed using this2 parameter for the low-energy
treatment (10 J.cm"
) and the higher energy treatment of 50 J.cm 2 at each
of the four wavelengths
studied. The 50 J.cm 2 energy treatment was more
effective than the low-energy treatment at each wavelength. The most effective wavelength at reducing
subsequent subsurface caries-like lesion progression
24
Adv Dent Res October 1987
FEATHERSTONE & NELSON
was the 9.32 |xm (1073 cm"1) line, with lesions approximately 50 percent less demineralized than those
of the controls. Each of the four wavelengths tested
did, however, have an effect, as illustrated in Fig. 2.
Further details of the effect on the subsurface profiles
are presented elsewhere (Nelson et al., 1986a).
Scanning Electron Microscopy
Scanning electron microscopy (for details, refer to
Nelson et al., 1987) revealed that extensive surface
roughening of enamel occurred when laser irradiation at 50 J.cm~2 was used. This surface roughening
was wavelength-dependent, with the 9.32- and 9.57ixm lines producing the greatest effect (Figs. 3 a,b),
and the 10.59-|xm producing the smoothest effect (Figs.
3 c,d). At higher magnifications, it was shown that
there was a central melt region (at the center of beam
contact with the surface), which, although rough, was
covered by a thin, smooth, fused surface layer (Figs.
3 d,b). Fractured cross-sections (Figs. 3 e,f) revealed
a region, up to 5 |xm deep from the enamel surface,
where the melting and fusion of the crystals had occurred.
Polarized Light Microscopy
Polarized light microscopy confirmed the considerable reduction in lesion depth as a result of preradiation by the laser treatment. It also confirmed
thin well-fused surface zones up to 10 |xm thick as a
result of laser treatment with the 973-, 1045-, and
1073-cm1 lines (see Nelson et al, 1987, for further
details). This effect at the surface was not shown in
the control, unlased lesions.
Chemical Analysis
The mineral loss as determined by chemical analysis was of the same order as the change in mineral
profiles recorded by microhardness testing. The details are given elsewhere (Nelson et al., 1986a).
DISCUSSION
The studies summarized here, together with the
earlier studies utilizing carbon dioxide lasers, summarized in the "Introduction", clearly show that lowenergy infrared laser irradiation of enamel markedly
inhibits the progress of subsurface caries-like lesions.
Fig. 3 —(a) Low-magnification SEM of human dental enamel lased with an energy density of 50 J.cm~2 using the 1073-cm ' line. Bar =
300 |xm. (b) Higher magnification of (a). Bar = 50 |im. (c) Enamel lased with the 945-cm 1 line. Bar = 300 p.m. (d) Higher magnification
of (c). Bar = 50 jim. (e) Cross-fracture through lased enamel (1073-cm ] line). Bar = 3 |j.m. (f) Higher magnification of cross-fracture.
Notice completely fused crystallites (fc) near the surface, partially fused crystallites (pf), and normal crystallites (nc). Bar = 1 jjtm. (from
Nelson et al., 1987; published courtesy of Karger AG)
Vol. 1 No. 1
LASER EFFECTS ON DENTAL TISSUE
The present studies have shown that this inhibition
is wavelength-dependent and that the carbon dioxide
laser is a very much more efficient treatment system
than the visible wavelength lasers previously utilized
by other workers. The effect of the laser on the enamel
surface was to produce a surface melt which one would
normally associate with momentary temperature rises
in the region of 800-1100°C. This is sufficient for fusing and melting of enamel crystals, which are carbonated-apatite. The pulsed laser utilized in the
present studies, although having pulses only 100-200
nsec in length, was sufficient to cause this surface
temperature rise. In order to confirm this, we subsequently carried out experiments to measure this
temperature rise, using a Modline Temperature Measuring System (Ircon, Inc., Niles, IL) which showed
that temperatures rose to approximately 1050°C at the
enamel surface for the 1073-cm 1 and approximately
800°C for the 945-cm"1 line. We have subsequently
carried out infrared analysis of the surface enamel
after laser treatment with the 1073-cm 1 line for 400
pulses at 50 J . c m 2 (Nelson et ah, 1987). The carbonate content was dramatically reduced, and the surface
phases were hydroxyapatite and tetracalcium diphosphate monoxide. This is comparable with effects
reported after heat treatment or laser treatment (Fowler and Kuroda, 1986). This surface melt zone is no
deeper than 5 \im, beneath which there is a region
of interaction 10-40 jim deep, where the temperature
rise is insufficient for the sintering process but sufficient for some compositional changes to the crystals. Hargreaves et ah (1984) reported experiments that
demonstrated a minimal temperature rise in the pulp
cavity as a result of laser irradiation of the surface
enamel. Hence, it appears possible for surface enamel,
or dentin, or carious enamel, or carious dentin, to be
treated with specifically directed low-energy pulsed
carbon dioxide laser without biological damage to the
inner tooth.
The reasons for the observed inhibition of lesion
formation by laser pre-treatment are still not clear,
although there are several possible explanations. It
has been suggested by other authors (Stern et ah,
1966; Lenz et ah, 1982) that the surface is sealed by
the laser and is less permeable for the subsequent
diffusion of ions into and from the enamel. We believe that the laser-induced compositional changes of
the enamel surface reduce its solubility and that
changes below the surface, not detected by methods
utilized to date, will reduce the solubility also at the
subsurface. [Many of these changes have been discussed by Fowler and Kuroda (1986).] There may also
be an effect of laser irradiation on the organic matrix
(Yamamoto and Sato, 1980) which may affect the solubility. With regard to permeability, a recent report
by Borggreven et ah (1980) suggested that laser irradiation actually increased the permeability of bovine
dental enamel. At this stage, the relative importance
of these various aspects has yet to be determined.
However, what is known is that specific-frequency
25
carbon dioxide laser irradiation is efficiently transformed from light into heat at the enamel surface and
in the immediate subsurface, causing crystal transformation and subsequent inhibition of subsurface lesion formation.
It is of considerable interest to compare the infrared
absorption spectrum of dental enamel and the Raman
spectrum of dental enamel with the laser lines used
in the present study (Fig. 4). It can be seen that three
of the lines used in the present study nearly coincide
with the phosphate and carbonate absorption bands,
but that one (the 945-cm 1 line) is slightly displaced.
This overlap of frequencies of absorption should theoretically coincide with highly efficient uptake of energy by the apatite crystals, followed by transformation
into other forms of energy. This phenomenon deserves further theoretical consideration (Lin and
George, 1984).
On the basis of the above data, it may be expected
that the carbon dioxide laser would also have a dramatic effect on dentin. This has indeed been shown
to be the case, in that lased dentin readily melted at
the surface, with low-energy pulsed carbon dioxide
irradiation similar to that reported here for enamel
(Nelson et ah, 1986b). Cooper et ah (1986) from our
group recently reported studies where they irradiated
the surface of dentin and followed this by bonding
of an appliance. The bond strength was increased by
300 percent compared with that to non-lased dentin.
Other possible applications relating to dentin have
been investigated in a preliminary manner by Zakariasen (1986), who looked at the fusion of apatite or
enamel into root canal preparations. These are all
preliminary studies which deserve further experimentation to look at the potential of the carbon dioxide laser at specific frequencies for clinical applications.
2000
1600
1200
Wavenumber (cm" 1 )
800
400
Fig. 4-Diagrammatic representation of the infrared spectrum of
dental enamel (A), the Raman spectrum of dental enamel (B, Nelson and Williamson, 1982), and the tunable infrared laser spectrum
(C) superimposed. The dotted lines represent the four laser lines
used in the present study (from Nelson et al., 1986a; published
courtesy of Karger AG).
26
FEATHERSTONE & NELSON
For clinical applications, the optimal wavelength of
carbon dioxide laser radiation must be determined,
together with the optimal energy density and optimal
pulse frequency. It would also be necessary that the
beam be directed very precisely by means of a fiberoptic system which is currently in its developmental
stages (Beckman and Fuller, 1983). The treatment of
caries lesions, the treatment of pits and fissures
(Stewart et ah, 1985), and the pre-treatment of occlusal surfaces are all possibilities which could be explored further in the future. If the technology becomes
available at a reasonable cost to take advantage of
these conditions which can be determined in the laboratory experimentally, then there may well be a future for the clinical application of lasers in dentistry.
ACKNOWLEDGMENTS
The input and assistance of the following people
into these reported studies are gratefully acknowledged: L.F. Cooper, R. Glena, M. Shariati, C.P.
Shields, J. Mclntyre, W.L. Jongebloed, J. Wefel, A.
Mowery, T.F. George, and J.T. Lin. The Laboratory
for Laser Energetics, University of Rochester, is
gratefully thanked for the loan of the laser equipment.
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