by Pulsed Lasers - Journal of Dental Research by shuifanglj


									         Journal of Dental Research

           Dye-enhanced Ablation of Enamel by Pulsed Lasers
E. Jennett, M. Motamedi, S. Rastegar, C. Frederickson, C. Arcoria and J.M. Powers
                           J DENT RES 1994 73: 1841
                     DOI: 10.1177/00220345940730120801

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J Dent Res 73(12):1841-1847, December, 1994

Dye-enhanced Ablation of Enamel
by Pulsed Lasers
E.Jennettl 2, M. Motamedil', S. Rastegar2, C. Frederickson3, C. Arcoria4, andJ.M. Powers5
'Biomedical Laser and Spectroscopy Program, Jennie Sealy Hospital, Route D56, University of Texas Medical Branch, 301 University
Boulevard, Galveston, Texas 77555; 2Bioengineering Program, Texas A&M University, College Station; 3MicroFab Technologies, Plano;
4Baylor College of Dentistry, Department of Restorative Sciences, Dallas; and 5University of Texas Houston Health Science Center, Dental
Branch, Department of Basic Sciences, Section Oral Biomaterials;*to whom correspondence and reprint requests should be addressed

Abstract. Laser removal of dental hard tissue has been                                       Introduction
proposed as a replacement for or augmented approach to the
dental handpiece. The main limitation for widespread usage                                   Laser ablation of the tooth has been reported in many
of lasers in dentistry has been inefficient ablation of dental                               articles since Stern and Sognnaes (1964) reported on the
hard tissue, accompanied by potential laser-induced damage                                   application of the ruby laser for removal of dental hard
to the surrounding tissue. The research focuses on a novel                                   tissue. The potential applications of lasers in dentistry have
approach for enhancement of tissue ablation and                                              been recognized by many groups, and much preliminary
confinement of laser interaction to a small tissue volume by                                 research has been conducted, including the ablation of
controlled placement of an exogenous dye on the enamel                                       dental soft and hard tissues as well as surface-processing
surface. Studies were done with both pulsed alexandrite and                                  (sealing fissures) (Stewart et al., 1985).
pulsed Nd:YAG lasers, with indocyanine green and India ink,                                      Stern and Sognnaes, in their early work, demonstrated
respectively, used as photo-absorbers. These dye-enhanced                                    that, in principle, ablation of enamel is possible. However,
laser processes demonstrated the feasibility of this technique                               extensive damage induced in the tooth discouraged further
for cavity preparation. While control studies produced little                                development of laser-based systems for the removal of
or no appreciable crater, average preparation depth for the                                  dental hard tissue. Since then, various continuous-wave
dye-enhanced ablation was from 1 to 1.5 mm, with a diameter                                  lasers-such as the CO2, Nd:YAG, and argon-and pulsed
of approximately 0.6 mm.                                                                     lasers-such as the Er:YAG, ArF excimer, ruby, CO2, and the
    Knoop hardness measurements show that, surrounding                                       Nd:YAG-have been tried on dental hard tissue in an effort
the crater, there is a small annular region slightly softened by                             to replace or complement the dental handpiece with an
the laser action. SEM studies of the interior structure of the                               effective alternative which permits the fine removal of
tooth did not show significant damage to the surrounding                                     tissue in a controlled fashion (Willenborg, 1989; Neev et al.,
tissue. Temperature measurement studies indicated that the                                   1991; Quintana et al., 1992; Pick, 1993; Wigdor et al., 1993).
pulsed nature of the laser, combined with the photo-                                             In recent years, there has been increasing optimism
absorbing dye, effectively prevented significant temperature                                 about the development of laser-based systems that can be
rise at the pulp.                                                                            used for the efficient and potentially painless removal of
    The remarkable effectiveness of this technique in creating                               dental hard tissue. The currently used dental handpiece is
cavity preparations and the absence of any notable collateral                                legendary for the discomfort and pain it causes during the
damage to the surrounding tissue suggest that dye-enhanced                                   vital tooth-preparation process. A laser that could remove
pulsed-laser ablation could be used as an alternative to the                                 dental hard tissue without producing patient trauma and
dental handpiece in selected procedures.                                                     fear would be a major benefit to both dentists and patients
Key words. Laser Ablation, Calcified Tissue, Hardness,                                           Most current medical lasers generate optical radiation in
Pulp Temperature, Temperature Measurement Studies.                                           the visible and near-infrared regions of the spectrum. For
                                                                                             this range, dental hard tissue is highly translucent and
Received February 10,1994; AcceptedJuly 20,1994                                              hence has lower optical absorption and does not allow for

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1842     Jennett et al.                                                                                                                             J Dent Res 73(12) 1994
efficient tissue ablation (Vickers et al., 1992; Wigdor, 1993).                            poured into each sample cup and around the tooth, until the
However, promising results have been reported about the                                    resin was approximately 5 mm from the top of the facial
potential use of dyes to enhance tissue absorption in the                                  surface. The resin was allowed to cure for approximately 20
near-infrared range for several medical applications                                       min.
(Hillenkamp, 1989; Chuck et al., 1989; Oz et al., 1990; De                                     The samples were removed from the sample cups and
Coste et al., 1992; Brooks et al., 1992).                                                  prepared for surface grinding, which was extremely
   The application of photo-absorbing dyes in combination                                  important since a flat surface is necessary for accurate
with pulsed lasers causes effective tissue absorption to be                                Knoop hardness testing. During testing, the diamond
dramatically increased and energy penetration to be                                        indenter is loaded onto the tooth sample, and an indentation
confined to a small volume. This technique confines the                                    is made. Therefore, if the surface is uneven, an abnormally
deposited laser energy spatially while delivering a short                                  low Knoop hardness value will be obtained, because one
pulse of laser beam. Less thermal build-up occurs in the                                   end of the indenter had a greater surface area with which to
surrounding tissue, creating a safer and more efficient                                    indent. Therefore, each sample was ground individually on
means of tissue removal. Theoretically, any laser with an                                  the Buehler Polishmet Polisher. First, a Buehler 600-grit
appropriate dye may be useful for these types of                                           Carbimet Paper disk was used. The samples were ground for
applications. The wavelength range (from 700 to 1100 nm)                                   approximately one minute, or until there was a uniform flat
has the advantage of being delivered via flexible, reliable,                               surface on the facial side of the molar. The samples were
and inexpensive silica fibers. In addition, there is a very high                           then polished with a Mastertex Buehler Polishing Cloth and
potential for the development of reliable, inexpensive,                                    Alpha Micropolish II Alumina (1.0 gim). Polishing
diode-laser-based systems which operate in this range.                                     significantly reduced the lines on the flattened surface of
   The objective of this study was to establish the feasibility                            the tooth when examined microscopically. This simplified
of dye-enhanced pulsed-laser ablation for the cavitation of                                the Knoop Hardness measurements.
dental enamel. Studies were conducted by use of a                                              The lasers used for the cavitation of enamel were the
combination of Nd:YAG laser with India ink and                                             pulsed Nd:YAG (1064 nm) laser (Incisive Technologies, Inc.)
alexandrite laser with indocyanine green for the ablation of                               and alexandrite (720-790 nm) laser (1-2-3, Schwartz Electro-
extracted human molars.                                                                    optics, Inc.). The Nd:YAG laser is commonly used and well-
                                                                                           known in the medical/dental community; it generates laser
Materials and methods                                                                      pulses of approximately 100 gsec. In these experiments, an
                                                                                           energy of 200 mJ/pulse was utilized, resulting in an energy
Ablation depths, temperature rise in the pulp, Knoop                                       density of 160 J/cm2. The laser beam was delivered to the
hardness, and SEM studies were measured and studied on                                     surface of the tooth via a quartz fiber, resulting in a 0.4-mm
the treated samples. These measurements were conducted                                     spot diameter. For most experiments, the laser was pulsed at
on the same tooth specimens so that more reliable and                                      1 Hz.
consistent data could be obtained. Extracted human molars                                      India ink (Black Sumi ink, Yasutomo Co.) was deposited
were used as the sample specimens throughout these                                         on the tooth surface by either a small drop through a 26-
experiments.                                                                               gauge needle, or by solid-state fluid-dispensing jet (Microjet,
   Twenty specimens were used for Knoop hardness testing                                   MicroFab Corp., Plano, TX). When the jet was used, trains of
(before and after laser irradiation), and three were used to                               5 pulses (1 Hz) at approximately 100 pL per pulse were used
determine average temperature rise in the pulp during laser                                for controllable and reproducible staining of tissue.
irradiation. They were stored in a formaldehyde solution                                       The alexandrite laser has a broad spectrum, with a peak
until prepared for use. The molars were rinsed with tap                                    at 750 nm and a pulsewidth of approximately 200 gsec,
water and brushed with a soft-bristled toothbrush for                                      composed of many spikes (Fig. 1). The raw alexandrite laser
removal of excess particles. After being dried, the molars                                 beam was focused through a convex lens to a 0.5-mm spot
were sterilized with ethylene oxide gas to protect the                                     size (diameter), with an energy of 700 mJ/pulse at the tooth
investigators from any diseases and organisms that might                                   surface, resulting in an energy density at the surface of
have been present on the tooth surfaces. In preparation for                                approximately 350 J/cm2. Laser radiation was delivered at
laser irradiation and hardness studies, the facial sides of the                            0.25 Hz. These parameters were chosen as a result of
molars were used for experimentation. The teeth were                                       preliminary experiments conducted to determine the
individually suspended in sample cups (Buehler Sampl-                                      threshold for dye-enhanced ablation of the enamel. The
kups) with utility wax and a wooden stirring stick, to                                     energy levels used in the study were chosen to ensure
ensure that the tops of the molars were not covered with                                   efficient removal of the enamel tissue at magnitudes
resin.                                                                                     sufficiently higher than the ablation threshold, which was
   Buehler Sampl-Kwick Powder was mixed with Buehler                                       determined to be about 200J/cm2.
Sampl-Kwick Liquid in a 2:1 ratio to form an                                                   Cavity preparations were created by use of this laser
isobutylmethacrylate resin, which was then immediately                                     with ICG (indocyanine green, Eastman Kodak Company,
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j Dent Res 73(12) 1994                        Dye-enhanced Ablation ofDental Hard Tissue

Rochester, NY) to facilitate ablation of enamel tissue. The
dye is currently used in clinical applications such as the
measurement of cardiac output. To determine the influence
of dye concentration on ablation efficiency, we conducted
concentration studies on the ICG-alexandrite combination.
Indocyanine green was combined with distilled water at
concentrations of 0.5, 1.0, and 1.5% for various experiments
where ablation efficiency and the extent of tissue
involvement were studied. Directly before laser irradiation,
a spot of ICG (diameter approximately 1 mm) was topically
applied to the surface of the tooth by means of a 26-gauge
needle. This spot was located directly at the point of impact
of the alexandrite laser beam. A single pulse was then
delivered from the laser. This process was repeated until the
target number of ablation pulses was delivered. The amount
of dye placed on the surface of the tooth was not controlled
in the experiments where the alexandrite laser was used.
Control experiments were run without the topical
                                                                                                  ;- O *.

                                                                                            ~--1 0

                                                                                               -14- .
                                                                                                       4    ..


                                                                                                                             150 200

                                                                                         Figure 1. Temporal distribution of alexandrite laser pulse.
                                                                                                                                                                ...   ...

application of ICG.
    The diameters of the laser-induced craters as well as the
residual lesions were determined by use of a reticule within                             average temperature rise in pulpal tissue and to assess the
the microscope. The depth was measured by means of the                                   impact of the laser beam on the pulp of the tooth. This study
micrometer gauge on the microscope (Nikon Biological                                     utilized two extracted teeth that were fitted with
Microscope Labophot), focused on the surface surrounding                                 thermocouples (0.005 in, Omega type K) inserted into the
the crater. The focal depth was then changed to focus on the                             pulp chamber through holes drilled in the bottom of the
bottom of the hole (accuracy, + 0.7 ,um). Each measurement                               tooth root. In these experiments, the dye was applied via
was repeated three times. Based on repeated measurements,                                microsyringe, and the alexandrite laser was pulsed at 0.25
a maximum estimate of the error in measurement was                                       Hz.
determined to be 0.5 mm (SD). The change in focal length                                     To examine the surface morphology of the treated
was determined by the micrometer gauge on the fine-focus                                 groups, we removed the coronal portions of the teeth (both
knob of the microscope, and this change was manifested as                                enamel and dentin) from the remaining tooth structure
a depth measurement.                                                                     with a slow-speed diamond saw (Buehler, Ltd., Lake Bluff,
    To determine the effect of laser ablation on the                                     IL). These coronal remnants were air-dried for 24 h and
mechanical properties of surrounding tissue, we performed                                individually attached to mounted aluminum stubs with
Knoop hardness tests on tooth samples after the grinding                                 cyanoacrylate glue (Loctite Corp., Cleveland, OH) and silver
and polishing were complete. Determination of the sample                                 paint (Fullam, Inc., Latham, NY). Each remnant was lightly
site for hardness testing was based on the appearance and                                coated with gold (Hummer VII, Anatech, Ltd, Alexander,
orientation of the tooth surface. The Leco DM-400FT Knoop                                VA) to a 21-nanometer thickness, and the lased enamel
Hardness tester was used for hardness measurements. Once                                 surface was viewed under scanning electron microscopy
the indentation had been made, the distance between the                                  (JEOL JSM 35CF, Tokyo, Japan) at 130 and 600X
two end-points of the indentation was measured with                                      magnifications so thatthe differences in surface quality
calibrated pointers. Calibration was done once for every                                 among the laser-prepared cavitations could be observed.
tooth so that accuracy between teeth could be ensured. Prior
to laser irradiation, hardness was measured in at least five                              Results and Discussion
different spots within the area to be irradiated. The
measurements were then repeated in approximately the                                      Rather than compare Nd:YAG with alexandrite laser, this
 same area after irradiation. This allowed us to determine                                study tested the general feasibility of dye-enhanced pulse
 whether the laser treatment had effected a change in                                     laser ablation of dental hard tissue. Ablation depths were
 hardness on the enamel surface. Each sample was measured                                 measured for three concentrations of the ICG for the
 for hardness in five different locations on the polished                                 alexandrite laser ablation of the teeth. An energy level of
 surface of the tooth with a 200-g load. By measuring                                     approximately 700 mJ/pulse delivered at the tooth surface
 hardness several times on the surface of each tooth, we                                  was used in each case. Six different measurements were
 could obtain a more accurate average hardness value, since                               made for each concentration. The cavitations were typically
 enamel hardness is known to vary with tooth position.                                    0.6 mm in diameter and approximately i to 1.5 mm in depth,
    We conducted temperature studies to monitor the                                       with the variation due to the differing concentrations of dye
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1844          Jennett et al.                                                                                                                             J Decnt Res 7.3(12) 1994

             ..B                                                                    0**

 -7 1.5-                                                                                                                                                               
                          T                                                                                                                                                    !.u. -1i

  -     1-


                                      1.0%       1.5%

                               ICG Concentration
                                                                                                    Figure 3. Schematic representation of thc lasci clII cct on thc tooth
                                                                                                    surf ace.
Figure 2. The el fect of ICG    concentration      on       the total depth ol the
laser cavitation

(Fig. 2). The deepest crater was obtained with the 1.5% ICG                                        the pulsed Nd:YAG laser. The role of the dye in pulsed laser
concentration. These craters did not show any evidence of                                          ablation is important in the localization of temperature and
carbonization under the microscope, later verified by SEM.                                         heat from the laser light. The combination of the short
    Specific to thisexperiment, one of the absorption peaks                                        pulsed laser and the dye absorption on the tooth surl'ace
of ICG lies directly in the alexandrite wavelength spectrum.                                       appears to provide clean and efIficient ablation ofenamel.
This maximizes the confinement of the laser light in the                                               Hardness tests were performed on every tooth sample
enamel at the site of ICG placement. Similar effects are seen                                      (Table), including means and standard deviations on both
with the India ink dye-enhancement in combination with                                             the lased and non-lased sides of the tooth. Each hardness

Table. Knoop hardness mneasurementsfor samples before and afterlaser irradiation
Sample Number                       Mean Base Hardness(+ SD)                                     Mean Hardness afterl                   aser(+   SD)         Percent       Chanige
                                            337     +
                                                             6.69                                               324        +   16.97                                     3.9
        2                                   337         +    21.48                                              307        +   2001                                      8.7
        3                                    359    +
                                                             29.37                                              326    +        19.11                                8.9
        4                                   320         +  22.10                                                341        +    17.78                               +
         5                                  331     +
                                                           20.74                                                322    +       22.87                                 2.7
        6                                   332         + 29.52                                                 306        +   26.67                                 7.8
        7                                   319         + 14.90                                                 306    +        6.38                                 4.0
        8                                    335    +
                                                              5.22                                              314        +    6.69                                6.0
        9                                    359    +
                                                             17.92                                              326    +       12.50                                     9.1
        10                                  327     +
                                                             24.83                                              319    +      16.76                                      2.3
        11                                  322    +
                                                              6.28                                              302    +      1277                                   6.3
        12                                  346    +
                                                              [2.54                                             314    +      16.54                                  9.2
        13                                  348    +
                                                             22.10                                              308        + 12.92                              -116(
        14                                  324    +
                                                             1620                                               295    +       6.31                               9.2
         15                                 325    +
                                                             19.58                                              299        + 18.35                                 7.9
        16                                  353    +
                                                              5.63                                              303    +       9.04                             -14.0
        17                                   361        +     1345                                               311   +       8.35                             -13.9
        18                                   314        +     6.88                                              318    +       19.46                                +1.1
         19                                  318        +     13.01                                             295        + 13.46                                  7.5
        20                                   327   +
                                                              11.84                                             334    +        13.60                           +    2.0
      Average                                335   +
                                                             1601                                               314        +   14.83                            -62
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jDcnt Res 73(12)1994                               Dye-enhanced Ablation of Dental Hard Tissue                                                                           1845

                   .           .               .
     5.-1.         I
                               I               I                 I                 I-

                             without dye
                   I           I               I                 I
      1*I 4.                                                                       I.
                  5t)        1()o      -l()
                            Time (seconds)
                                                              2Y00              2S0

Figure 4. Temperature measurements of the dye enhanced
alexandrite showing eflective elimination of temperature rise at
the pulpal cavity. Similar results were seen with the Nd.YAG laser
enhanced with India ink.

                                                                                                Figure 5. SEM of the interior surf ace                   of the dyc-cnhan(ccd
value represented in the Table is an average of the five                                        alexandrite laser preparations (mag. 600X)
measurements taken on each tooth. Fig. 3 shows
schematically the regions of ablation, an annular 'soft'
region, and the region where hardness tests were performed                                     show that the superficial surf ace of the annulus
on the lased samples. A statistical analysis was performed (t                                  surrounding the ablated enamel appears to be a
test of dilfferences) on the hardness data before and after                                    recrystalized enamel beaded surface, which could be
laser irradiation. This analysis suggests that the hardness of                                 removed with a hand-held dental instrument. The interior
the enamel surrounding the irradiated zone decreased                                           sides of the crater contain what appears to be recrystalized
significantly. The 95% confidence interval for these data did                                  molten material which has re-precipitated into the crater
not include zero, and therefore, the difference between                                        after irradiation. The SEM studies reinforce the idea that
mean values was statistically significant (p = 0.00001). The                                   irradiation with the alexandrite laser does not cause any
percent change in the mean values was a decrease of 6.16%                                      apparent thermal damage to the surrounding tissue. It is
in the lased enamel as compared with the non-lased enamel.                                     not currently known how these changes would affect
However, the clinical importance of this decrease in enamel                                    resin-bonded restorations in clinical practice. Since this
hardness is uncertain. Generally, since enamel hardness can                                    study was meant to demonstrate a new technique in hard-
vary between 250 and 500 on any given tooth, a variation                                       tissue 'optical drilling', resin-bonding studies have not yet
decrease of 6.16% in a bounded area around the irradiated                                      been conducted. Future studies with these types of dye-
cavitation may not be clinically significant. Long-term                                        laser combinations with more clinical relevance are being
studies will be needed to substantiate the clinical                                            plan ned.
significance of these findings.                                                                   The presence of re-precipitated crystals has been
    Cavitations resulting from the dye-enhanced alexandrite                                    previously reported in laser ablation of dental hard tissue
laser preparations were from about I to 1.5 mm deep. During                                    (Featherstone and Nelson, 1987; Feuerstein et al., 1992).
these preparations, temperature recordings showed no                                           Vahl (1968) had shown with x ray diffraction that the
significant change in temperature at the pulp cavity (Fig. 4).                                 material that re-precipitated onto the surrounding surface
The control test shown was a tooth in the same situation;                                      of the irradiated crater was tricalcium phosphate or other
however, no dye was applied. Without dye, no cavitation                                        high-temperature phases. This new material is a
occurred; however, there was a temperature rise of from 3 to                                   dehydrated form of hydroxyapatite (Vahl, 1968). It is
4 C at the pulp cavity. These temperature measurements                                         possible that the re-precipitated crystals seen with the dye
give an estimate of the average temperature at the pulp                                        enhanced alexandrite preparations are also tricalciuin
cavity. Transient temperature at the pulp could have more                                      phosphate (Kuroda and Fowler, 1984). This would explain
variations, however, these experiments were conducted in a                                     the softer area in the surrounding annulus and would
Iworst case scenario', i.e., in air. Without-the surrounding                                   indicate that temperatures above the tooth surface were
heat dissipaters (blood, gingiva), as in these experiments, the                                extremely high.
average and transient temperature increases should                                                An SEM photograph (Fig. 6) is typical of the results
theoretically be much higher than if the same experiment                                       obtained in these dye-assisted ablations by India ink and
were conducted under in vivo conditions. However, in vivo                                      the pulsed NdLYAG laser. Application of 50 pulses generally
experiments will be needed to verify this.                                                     produced cone-shaped craters about 0.5 to 1 mm deep. The
    SEM studies were done for both alexandrite and                                             cut surfaces of the tooth were free of shattering or cracking,
Nd:YAG experiments. Alexandrite SEM studies (Fig. 5)                                           but there was a fine "spray" of material (presumably a
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1846     jennctt et al.                                                                                                                           J Dent Res 73(12)1994

                                                                                             and superheating steam trapped within the matrix of the
                                                                                             hydroxyapatite. This theory is based on the large disparity
                                                                                             between the vaporization temperature of trapped water and
                                                                                             that of hydroxyapatite. Here, when the pressure exceeds the
                                                                                             tensile strength of the tooth, material is ablated.
                                                                                                 With these theories, one can only estimate the actual
                                                                                             method of ablation occurring with the dye-enhanced
                                                                                             pulsed lasers. Further work will determine the mechanism
                                                                                             of ablation and the overall effects on the tooth. Preliminary
                                                                                             work has begun on the effects of acoustic and plasma
                                                                                             waves generated during dye-enhanced laser ablation of
                                                                                             enamel. Further experiments will determine the effects of
                                                                                             laser-induced acoustic waves on the structural integrity of
                                                                                             the tooth.
                                                                                                 This series of experiments has shown that the use of
                                                                                             the dye-enhanced pulsed laser for tooth cavitation is
                                                                                             feasible and could be developed into a practical method
Figure 6. SEM of the dye-enhanced Nd YAG laser preparations.                                 for cavity preparation. We demonstrate that the pulsed
The interior surl ace of this preparation is bumpy and rough,                                nature of the laser is primarily responsible for clean and
similar to that seen in Fig 5 (mag 130X).
                                                                                             efi icient ablation. Furthermore, preliminary studies do
                                                                                             not indicate a significant rise in temperature within the
recondensed residue of previously molten hydroxyapatite)                                     tooth structure during such ablation. There was a small
that coated the inner surface of the crater and made a very                                  measurable decrease (about 6%) in hardness in an
fine (100 pm) rim on the enamel surface surrounding the                                      annulus surrounding the crater; however, the clinical and
crater.                                                                                      medical importance of such a decrease is yet to be
    For a clinically acceptable system to be created, injury                                 determined. Work is in progress to determine the actual
must be minimized and eff'iciency of the laser drilling                                      mechanism of ablation and optimization. Development of
process maximized. The mechanism or combination of                                           a dye-laser handpiece is currently under consideration.
mechanisms contributing to ablation of enamel must be                                        The advent of such systems would allow for easy
understood. Several explanations are being explored. In                                      integration within the dentist's office by utilization of
dye-enhanced pulsed ablation, explosive vaporization of                                      suctions and water sprays to remove excess material
water may contribute. It is theorized that the water in the                                  during the preparation process, much like those used
ICG solution superheats with exposure from the laser                                         today in conventional drilling. These results demonstrate
light, explosively expands in a plume that recoils, causes                                   clinical relevance and offer a major reason to pursue this
a counterforce, and, in return, removes material from the                                    novel approach in dental research.
tooth (Hibst and Keller, 1993). Theoretically, this recoil
effect would not cause thermal damage to the adjacent                                        Acknowledgments
tissue, since ablation occurs above the surface of the
tooth.                                                                                       This work was supported in part by a grant from the
   Cavitations due to vaporization effects seen with other                                   Department of Energy (DOE), and in part by NIH grant
high-powered lasers can also be produced with the dye-                                       #1R43 DE10687.
enhanced pulsed laser (Keller and Hibst, 1989; Vickers et al,                                    The authors would like to thank David Ladd of the
1992; Hibst and Keller, 1993; Venugopalan et al, 1993). If the                               University of Texas Houston Health Science Center, Dental
rate of' vaporization becomes sufficiently high, then the                                    Branch, for the use of the mechanics laboratory and Knoop
resulting shock waves can introduce another significant                                      hardness tester for preparation of samples and hardness
ablation mechanism. As the ICG is ablated, an upwardly                                       testing. Thanks also to Michael Coy of the Electron
active force is complemented by a reactive force down onto                                   Microscopy Center, Texas A&M University, for his expertise
the tooth. The magnitude of this reactive force is dependent                                 and advice with the SEM studies.
on the mass removal rate and the speed at which the                                              Appreciation is also extended to Dr.Edward Mallia for
vaporized dye leaves the surface. The pressure exerted by                                    providing tooth samples for experimentation and to Dr.
the shock wave can be estimated by assuming that the                                         Duncan MacFarlane for useful discussion on the theory
temperature of the ablated material is the vaporization                                      behind ablation mechanisms.
temperature. When this pressure exceeds the compressive                                          This paper is based partially on a thesis submitted to the
strength of the tooth, material is ablated.                                                  graduate faculty, Texas A&M University, in partial fulfillment
   A third contributory mechanism could involve heating                                      of the requirements for the Master of Science degree.
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j Dent Res 73(12) 1994                              Dye-enhanced Ablation of Dental Hard Tissue                                                                  1847

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