Biomaterials 23 (2002) 51–58

The effect of Nd : YAG radiation at nanosecond pulse duration

on dentine crater depth

A. McDonald

a,

*, N. Claffey

b

, G. Pearson

c

, W. Blau

d

, D. Setchell

a

a

Department of Restorative Dentistry, Eastman Dental Institute and Hospital for Oral Health Care Sciences, University of London,

256 Grays Inn Road, London WC1X 8LD, UK

b

Department of Periodontology, Restorative Dentistry, School of Dental Science, Trinity College, Dublin, Ireland

c

Department of Biomaterials, Eastman Dental Institute and Hospital for Oral Health Care Sciences, University of London, 256 Grays Inn Road,

London WC1X 8LD, UK

d

Physics Department, Trinity College, Dublin, Ireland

Received 20 December 1999; accepted 12 February 2001

Abstract

The effect of laser parameters on laser-dentine interaction is little known. The aim of this in vitro study was to determine the effect

on dentine crater depth of Nd : YAG laser radiation in relation to pulse repetition rate, total delivered energy, dentine site and the
presence or absence of a dye. One hundred and forty-four sound third molars were extracted and sectioned transversely to provide
288 upper and lower cut surfaces. The upper surfaces were painted with a layer of dye (IR5) suitable for absorption at 1064 nm. The
specimens were divided into 12 sub-groups each containing 12 upper and 12 lower specimens. These were exposed to a Nd : YAG
laser with a 30 nanosecond (ns) pulse duration. This laser operated in a non-contact mode (spot diameter 165 mm) with pulse
repetition rates of 2.5, 5.4 and 10.5 Hz. Four total energies were delivered at each repetition rate; 2.3, 3.63, 3.96, 4.29 joule (J) at
2.5 Hz repetition rate; 2.3, 2.64, 3.63, 4.29 J at 5.4 and 10.5 Hz repetition rates. Five outer and three inner sites were irradiated on each
specimen. Each dentine crater depth was measured five times using a Reflex Microscope and a three-dimensional centre of gravity
derived. An upper and lower specimen were taken from each sub-group and viewed under a scanning electron microscope (SEM).
ANOVA was applied: total delivered energy and dyed/undyed were found to have a statistically significant effect on crater depth
(p50:0001). In general increasing energy and the presence of dye produced deeper craters. Inner/outer dentine location and repetition
rate were not found to be statistically significant. All craters were carbonised. # 2001 Elsevier Science Ltd. All rights reserved.

Keywords:

Laser parameters; Nd : YAG; Dentine; Crater depth; Hard tissues

1. Introduction

The interaction of a laser with a target tissue has

stimulated great interest and a clear understanding is
essential in predicting the result. It is generally agreed
that there are three main types of laser-tissue interac-
tion; photochemical, photothermal and non-linear [1].
Non-linear interactions are primarily non-thermal and
include photodisruption and photoablation. However
photoablation and photodisruption require either high

photon energies or very short pulse durations in order to
eliminate any thermal component [2].

The ablative process can be explained by the following

example: two atoms are bound by a single common
electron. If a high energy photon is absorbed, the energy
gain is high enough to access an electronic state which
exceeds the bond energy. In this case the two atoms are
excited from an attractive to a repulsive state and
dissociate. Thus the photoablative process can be sum-
marised as a two stepprocess; excitation and dissociation.

When power densities are sufficiently high, a local

electric field can occur. If this electric field exceeds a
threshold value, ionisation of atoms and molecules
occur, optical breakdown is achieved and a plasma is
produced. Photodisruption is an ablation due to
mechanical breakdown and is described as a multi-
cause mechanical effect which starts with optical break-

*Corresponding author. Department of Conservation, Eastman

Dental Institute, Hospital for Oral Health Care Sciences, University of
London, 256 Grays Inn Road, London WC1X 8LD, UK. Tel.: +44-
20-1915-1028; fax: [phone].

E-mail address:

[e-mail] (A. McDonald).

0142-9612/02/$ - see front matter # 2001 Elsevier Science Ltd. All rights reserved.
PII: S 0 1 4 2 - 9 6 1 2 ( 0 1 ) 0 0 0 7 8 - 3

down. The primary mechanisms are shock-wave gen-
eration and cavitation.

In order to distinguish between photoablation and

thermal interaction the following explanation is given; a
single UV photon has a high photon energy and is
sufficient to dissociate a bound molecule. In thermal
interactions the photon energy is not high enough to
reach a repulsive state but is promoted only to a
vibrational state. By means of non-radiative relaxation,
the absorbed energy then dissipates to heat and the
molecule returns to its ground state. A multi-photon
process may also lead to a dissociated state, however in
the time needed for such a multi-photon absorbtion
process to occur, other tissue becomes vibrationally
excited, hence leading to a thermal effect. These
statements hold true unless pulse durations shorter than
100 ps are used at sufficiently high pulse energies and
induce plasma induced ablation or disruption [2].

In hard tissue irradiation it would be considered

beneficial if the thermal processes could either be
eliminated or, if present, limited to the target site. In
order to achieve localisation of heat the absorption
coefficient of the tissue must be high. The substrate has
an absorption coefficient which is wavelength specific,
but it may be possible to promote laser–tissue interac-
tion using surface agents [3,4].

If all the energy is delivered in a pulse shorter than the

thermal relaxation time, the tissue has time to recover
before the next pulse is delivered and damage is
effectively confined to the region penetrated by the
radiation [5–8]. In order to minimise thermal injury of
surrounding tissues control of two factors have been
recommended [6,9]. Firstly the pulse duration should be
kept shorter than the thermal relaxation time constant
of the tissue. Secondly, the repetition rate should be
adjusted so that the inter-pulse period is longer than the
thermal relaxation time and thus allow cooling to near
baseline between pulses. These recommendations corre-
lated well with the histological lack of thermally altered
tissue when using pulses of shorter duration than the
thermal relaxation time [6].

Decreasing the laser pulse duration will also increase

the power density of the laser beam. This will increase
the likelyhood of non-linear absorption and non-
thermal tissue removal. This study examined the effect
of Nd : YAG laser parameters (energy and repetition
rate) directed at inner or outer dentine sites on depth of
dentine removed. The effect of the presence or absence
of dentine dye was also evaluated.

2. Materials and method

One hundred and forty-four sound human third

molars were extracted and stored in saline. Each crown
was sectioned transversely midway between cusptip

s

and dentino–enamel junction (DEJ) using a low speed
diamond wheel saw (South Bay Technology, Temple
City, CA, USA). Eight target sites were outlined in
graphite and were divided into outer and inner sites; 5
outer sites placed within 4 mm of the DEJ and 3 inner
sites located centrally. Each specimen was orientated by
placement of a water resistant coloured spot at the
periphery of the specimen adjacent to the first target site.
The sites could then be numbered in a clockwise
direction in the upper specimens and in an anti-
clockwise direction in the lower, thus upper and lower
sites corresponded to a similar area in the same area of
the tooth. This allowed each site to be logged and
returned to when necessary. The 144 teeth were divided
into 12 sub-groups of 12 upper and 12 lower specimens
and were then stored in saline until exposure to a laser.
At the time of exposure each specimen was removed
from the saline and placed on a tissue to absorb excess
saline. The specimens were moist on exposure to the
laser.

A solution was made of IR 5 laser dye (Lambda

Physik, Gottingen, Germany) by dissolving 0.001 g of
powder in 1 cm

3

of trichloromethane (BDH Ltd., Poole,

UK). This absorbed well at a wavelength of 1064 nm.
Before each experimental session a layer of dye was
painted onto the upper cut surface of the specimen with
a sable brush. The layer of dye was measured and had a
mean thickness of 4 mm. The upper and corresponding
lower surfaces were exposed to the same laser para-
meters. The lower surface remained undyed.

The laser used was the Raytheon SS500 laser

(Raytheon Laser Products, Burlington, MA) which
operated at a wavelength of 1064 nm, a maximum
average power output of 400 W and a maximum energy
of 50 joule (J) per pulse. The facility to Q-switch this
laser using an electro-optic crystal (a Pockels cell) was
utilised to alter the pulse duration to 30 nanoseconds
(ns). The active medium of the laser was a crystal of
yttrium aluminium garnet (YAG) doped with 1–3%
neodymium ions. This laser allowed independent
adjustment of 5 variables; average power, peak power,
pulse duration, repetition rate and power density.

Each specimen was mounted on a platform which

could move in x, y, and z axes. It was therefore possible
to bring the target site of the specimen into alignment
with the focus of the laser beam. A CCTV camera
(mounted along the optical path of the beam) and
a helium–neon guide light assisted alignment of the
laser with the centre of the target site. The laser was
focused by means of a lens of focal length 10 cm to
produce a spot size of diameter 165 mm. Prior to
experimentation the spot size was verified by recording
transmitted laser power when a metal edge was scanned
across the beam.

The 12 groups of 12 teeth were exposed to three

repetition rates of 2.5, 5.4 and 10.5 Hz at each of four

A. McDonald et al. / Biomaterials 23 (2002) 51–58

52

energy levels. The four total energies delivered at 2.5 Hz
repetition rates were 2.3, 3.63, 3.96, 4.29 J. The total
energy delivered at each of the energy levels was 2.3,
2.64, 3.63, and 4.29 J for the 5.4 Hz and 10.5 Hz groups
(Table 1). The maximum pulse energy was 300 mJ/pulse.
Thirty three shots were deposited at each target site.

An upper and lower specimen were taken from each

sub-group and ground on a lapping machine to produce
transverse crater sections. The final silicon carbide paper
used was 1000 grit. These 64 specimens were prepared
for scanning electron microscopy by dehydration in a
desiccator for 1 week followed by sputter coating at
20 mA for 3 min. All specimens were viewed under a
scanning electron microscope at an acceleration voltage
of 15 kV.

Measurement of crater depth was made with a Reflex

Microscope (Reflex Measurement Ltd., Somerset, UK).
In order to facilitate visualization of the dentine surface,
a thin layer of silver powder (Kerr Dental Manufac-
turers, Peterborough, UK) was applied to the surface of
all craters. This was placed with a short haired brush
and excess removed with a chipsyringe. All measuring
was performed using a 10 mm diameter measuring spot
and a field magnification of 20. A reference plane was
defined for each specimen by digitising 10 locations
widely distributed around the periphery of the flat
surface of each specimen. The depths of the craters were
determined by making 5 measurements at the deepest
point of each and calculating the offset of the three-
dimensional centre of gravity of these points from the
reference plane. An additional fibre-optic light source
was used in order to visualise the base of deeper craters.
All experimentation and evaluation was carried out by
one individual.

3. Results

In general the craters produced were rounded in

outline (Fig. 1). The rims of the craters were crazed with
carbonised material evident around the crater rim. The
base of the craters was lightly carbonised.

On SEM the margins of the craters varied in that

some had a fused irregular rim and some did not. Open
tubules were identified at some crater rims. The craters
were an inverted dome shape in transverse section
(Fig. 2).

The rim and the base of the crater were similar in

appearance except when fused irregular rims were
visible. The surface of the base had a fused irregular
porous appearance (Fig. 3). On viewing a section
through the crater wall, the fused layer was found to
be approximately 3 mm deep. The diameter of the craters
produced was in all cases greater than the spot size of
the laser beam.

In one SEM an inverted crater was observed in the

pulp chamber (Fig. 4) approximately 500 mm below the
surface crater. The tubules in this section were cut

Table 1
Experimental parameters used for nanosecond pulse duration group

Repetition rate

2.5 Hz

5.4 Hz

10.5 Hz

Energy level 1
Energy/pulse (J)

0.07

0.07

0.07

Total energy (J)

2.3

2.3

2.3

Fluence (J/cm

2

)

327

327

327

Energy level 2
Energy/pulse (J)

0.11

0.08

0.08

Total energy (J)

3.63

2.64

2.64

Fluence (J/cm

2

)

515

374

374

Energy level 3
Energy/pulse (J)

0.12

0.11

0.11

Total energy (J)

3.96

3.63

3.63

Fluence (J/cm

2

)

561

515

515

Energy level 4
Energy/pulse (J)

0.13

0.13

0.13

Total energy (J)

4.29

4.29

4.29

Fluence (J/cm

2

)

608

608

608

Fig. 1. Nanosecond crater on upper dentine surface at high magnifica-
tion.

Fig. 2. SEM of transversely sectioned crater ( 210).

A. McDonald et al. / Biomaterials 23 (2002) 51–58

53

longitudinally so that both craters appeared to be in
communication via the dentinal tubules and the inferior
crater lay vertically below the surface crater. The surface
layer of the inferior crater appeared fused and was quite
different in structure to the surrounding roof of the pulp
chamber (Fig. 5).

Certain sites did not crater when exposed to the laser.

Under high magnification these sites appeared similar to
the surrounding dentine. These were labelled ‘‘no result’’
areas and included 253 sites (Tables 2 and 3). The no-
result areas were excluded from the statistical analysis.
Means and standard deviations of crater depths were
calculated for the different repetition rates and energy
levels (Tables 4–6). The data was further subdivided by
inner/outer sites and by upper (dyed) and lower
(undyed) specimens.

The data were found to be bimodal due to the

presence of no-result areas. These were removed from
the statistical analysis following which the data were still
not entirely normal. The presence of occasional deep
craters (1–2.5 mm deep) were responsible for skewness
of the data. However, biostatistical opinion confirmed
the suitability and flexibility of the ANOVA model used
to analyse the data. The independent variables used
were

1. Inner/outer.
2. Dyed/undyed.
3. Repetition rate.
4. Energy.

The dependent variable was crater depth. The

statistical package used was jmp version 3.1 (Apple
Macintosh), SAS Institute Inc., 1996, SAS Circle, Box
8000, Cary, NC 27512-8000.

Two hundred and fifty three sites (11%) did not crater

and these occurred in 81 of the 144 teeth. The
distribution of sites not showing cratering for both
dyed and undyed surfaces can be seen in Tables 2 and 3.

Chi squared test for independence of the data revealed

that there was an association between the incidence of
no-result areas, energy levels and repetition rates for the
undyed sites (chi squared value=76.63, p50:0001). Of
the total number of no-result areas, 107 (42.3%)
occurred in inner sites and 146 (57.7%) in outer sites.
The means and standard deviations (mm) for crater
depths for the 2.5, 5.4, 10.5 Hz groups are presented in
Tables 4–6.

The overall model proved to be significant F ratio

49.63, P50:001. The main effects, energy and dyed/
undyed, were statistically significant (Table 7). Deeper

Fig. 3. SEM of crater base ( 1000).

Fig. 4. SEM of section showing surface and inverted craters ( 65).

Fig. 5. SEM of inverted crater at higher magnification ( 513).

Table 2
Frequency of no-result areas by energy level and repetition rate for
dyed surfaces

Repetition rate
Total energy

2.5 Hz

10.5 Hz

Total

1

5

1

6 (N ¼ 185)

3

4

0

4 (N ¼ 179)

4

2

0

2 (N ¼ 191)

Total

11 (N ¼ 285)

1 (N ¼ 270)

12 (N ¼ 555)

A. McDonald et al. / Biomaterials 23 (2002) 51–58

54

craters were noted in dyed sites (p50:0001) and with
increasing total energy (p50:0001). Significant interac-
tions were also found (Table 7 and Figs. 6–9).

On examination of the energy by repetition rate

column chart (Fig. 6), interaction of energy with
repetition rate would appear to vary with each energy

level. Energy levels 1 and 4 demonstrated a trend
towards increasing crater depths with increasing repeti-
tion rates. This trend was reversed for energy levels 2
and 3. The different profiles for the four total energy
groups were responsible for the interaction between
energy and repetition rates.

Table 3
Frequency of no-result areas by energy level and repetition rate for undyed surfaces

Repetition rate
Total energy

2.5 Hz

5.4 Hz

10.5 Hz

Total (N ¼ 1136)

1

21

12

16

49 (N ¼ 285)

2

4

17

47

68 (N ¼ 281)

3

40

16

16

72 (N ¼ 285)

4

13

31

8

52 (N ¼ 285)

Total

78 (N ¼ 377)

76 (N ¼ 379)

87 (N ¼ 380)

241 (N ¼ 1136)

Table 4
The means and SD (mm) of crater depths for dyed and undyed inner and outer sites at four energies in the 2.5 Hz sub-group

a

Energy

Dyed

Undyed

Total (all dyed/undyed)

Inner

Outer

Inner

Outer

1 (2.3 J)

0.385 0.127

0.39 0.076

0.291 0.082

0.275 0.075

0.339 0.103 (N ¼ 165)

2 (3.63 J)

0.481 0.071

0.47 0.101

0.365 0.07

0.33 0.061

0.411 0.103 (N ¼ 182)

3 (3.96 J)

0.484 0.092

0.501 0.125

0.348 0.179

0.394 0.127

0.45 0.14 (N ¼ 144)

4 (4.29 J)

0.399 0.101

0.416 0.112

0.483 0.117

0.417 0.142

0.424 0.123 (N ¼ 176)

Total

0.438 0.109
(N ¼ 136)

0.444 0.113
(N ¼ 232)

0.375 0.132
(N ¼ 114)

0.352 0.118
(N ¼ 185)

0.405 0.124 (N ¼ 667)

a

J=joule.

Table 5
The means and SD (mm) of crater depths for dyed and undyed, inner and outer sites at four energies in the 5.4 Hz sub-group

a

Energy

Dyed

Undyed

Total (all dyed/undyed)

Inner

Outer

Inner

Outer

1 (2.3 J)

0.379 0.063

0.380 0.053

0.324 0.089

0.292 0.06

0.344 0.075 (N ¼ 178)

2 (2.64 J)

0.427 0.078

0.420 0.048

0.376 0.107

0.316 0.067

0.385 0.085 (N ¼ 173)

3 (3.63 J)

0.44 0.076

0.450 0.06

0.342 0.107

0.317 0.082

0.391 0.099 (N ¼ 172)

4 (4.29 J)

0.507 0.06

0.508 0.104

0.446 0.129

0.424 0.109

0.477 0.107 (N ¼ 160)

Total

0.438 0.083
(N ¼ 141)

0.440 0.084
(N ¼ 239)

0.366 0.114
(N ¼ 104)

0.334 0.094
(N ¼ 199)

0.397 0.103 (N ¼ 683)

a

J=joule.

Table 6
The means and SD (mm) of crater depths for dyed and undyed, inner and outer sites at four energies in the 10.5 Hz sub-group

a

Energy

Dyed

Undyed

Total (all dyed/undyed)

Inner

Outer

Inner

Outer

1 (2.28 J)

0.412 0.064

0.404 0.076

0.297 0.094

0.275 0.063

0.348 0.097 (N ¼ 168)

2 (2.64 J)

0.386 0.041

0.383 0.043

0.323 0.106

0.246 0.104

0.345 0.09 (N ¼ 142)

3 (3.6 J)

0.492 0.06

0.47 0.054

0.344 0.133

0.336 0.101

0.411 0.111 (N ¼ 165)

4 (4.2 J)

0.507 0.041

0.477 0.051

0.406 0.062

0.37 0.056

0.438 0.076 (N ¼ 181)

Total

0.448 0.073
(N ¼ 128)

0.434 0.07
(N ¼ 235)

0.347 0.107
(N ¼ 101)

0.313 0.094
(N ¼ 192)

0.388 0.103 (N ¼ 656)

a

J=joule.

A. McDonald et al. / Biomaterials 23 (2002) 51–58

55

All

energy

levels

produced

deeper

craters

in

dyed specimens when compared to undyed (Fig. 7).
The

smallest

difference

in

crater

depth

between

dyed and undyed was observed for energy level 4. The
interaction effect was primarily attributable to energy
level 4.

In the repetition rate by dyed/undyed column chart

(Fig. 8) similar crater depths were observed for either
dyed or undyed sites at all three repetition rates. All
repetition rates produced deeper craters in dyed speci-
mens. The interaction of repetition rate with dyed/
undyed was attributed to the differing profiles of each
repetition rate for dyed and undyed. There was a
statistically significant interaction between inner/outer
and dyed/undyed (Fig. 9). However this was less
significant than the other interactions in the nanosecond
pulse duration.

4. Discussion

Macroscopically crater outlines were generally round

(Fig. 1) and the crater walls close to the rims were either
irregular or smooth (Fig. 2). These differences in
topography did not seem to be associated with any of
the factors in this study but the irregularity in rim
morphology may be explained by interaction of the

Table 7
The results of the effect tests

a

DF

Sum of
squares

F

ratio

Probability

Energy

3

0.973

42.14

50.0001

Dyed/undyed

1

0.556

72.26

50.0001

Energy repetition rate

6

0.855

18.503

50.0001

Energy dyed/undyed

3

0.424

18.36

50.0001

Repetition rate dyed/

undyed

2

0.151

9.82

50.0001

In/out dyed/undyed

1

0.087

11.28

0.0008

a

DF=degrees of freedom.

Fig. 6. Interaction effects: mean crater depths for four total energies at
three repetition rate}nanosecond pulse duration (for all means
6.5 mm5SE57.8 mm).

Fig. 7. Interaction effects: mean crater depths at four total energies for
dyed and undyed specimens}nanosecond pulse duration (for all
means 8.1 mm5SE59.7 mm).

Fig. 8. Interaction effects: mean crater depths for dyed and undyed
specimens at three repetition rates}nanosecond pulse duration (for all
means 8 mm5SE58.8 mm).

Fig. 9. Interaction effects: mean crater depths for dyed and undyed
specimens in inner and outer sites}nanosecond pulse duration (for all
means 9 mm5SE511 mm).

A. McDonald et al. / Biomaterials 23 (2002) 51–58

56

escaping plume with the successive pulse. A lack of
carbonisation was noted at the base of the craters and
may reflect either a dissipation of energy by the time the
laser reaches the depth of the crater, or vaporisation of
tissue without carbonisation.

The incidence of no-result areas was (N ¼ 253) 11%

of all sites. The vast majority of these occurred in
undyed specimens (N ¼ 241) and would seem to
indicate that dye may have acted as an initiator. It
was also noted in undyed specimens that there was no
particular pattern to their distribution. However they
tended to occur at lower repetition rates in the dyed
specimens and therefore may have been related to a
threshold phenomenon.

The large incidence of no-result areas in this group

was not attributed to fluctuation in laser operation as
consistency of laser output was verified at the start of
and during each experimental session. The large number
of target sites (8 target sites in 144 specimens i.e. 2304
target sites) should have been sufficient to avoid any bias
due to variability in substrate composition. The
incidence of no-result areas was most likely to be
attributable to the interaction of the laser photons with
the substrate at this pulse duration.

All energy levels produced deeper craters in dyed

specimens when compared to undyed specimens. As
radiation must be absorbed before any effect can occur,
the dye may promote interaction by increasing absorp-
tion thus acting as an initiator. Although dyed craters
were generally deeper, little difference was noted on
micrographs between the topography of dyed and
undyed craters. The dye was therefore most likely to
have had an initiation effect and minor propagation
effects. This would appear to be in contrast to those who
supported the concept that surface dye may alter the
deposition of a laser within the tissue [10].

It was interesting to find open tubules associated with

some of the crater rims of the nanosecond pulse
duration group(Fig. 2). The lack of fusion of these
open tubules may therefore reflect the rapid rate of
deposition of energy at shortening pulse durations which
resulted in a different mode of laser–tissue interaction
(partially non-thermal). However the likelyhood of
photoablation at this pulse duration is small. The lack
of fusion may reflect rapid vaporization of dentine as
part of a thermal reaction. The presence of these open
tubules would appear to suggest that the surface may be
suitable for adhesive bonding. These patent tubules
occurred adjacent to fused dentine which may compli-
cate the choice of bonding system. If reliable production
of open tubules could be achieved with this laser it may
be possible to simplify the bonding process. The open
tubular pattern differs from the smear layer produced by
a bur.

All groups demonstrated a trend towards deeper

craters with increasing energy level. Repetition rate was

not found to have a statistically significant effect.
However, any effect of increase in repetition rate may
have been negated by interference of the escaping plume
with the subsequent laser pulse. It has been proposed
[11] that decreasing crater depth could occur in craters
where the absorbing tissue vapour remained within the
crater.

There was no statistically significant difference in

crater depth for inner/outer sites. In the case of a wholly
thermal reaction, the short pulse duration may have
resulted in insufficient temperature rises for preferential
organic tissue removal, the pulse duration being shorter
than the thermal relaxation time.

The craters were less carbonized at the base.

This may be explained by the energy being expended
in rapid vaporization of fluid with little energy
remaining for carbonization of surrounding tissues.
The surface temperature of the crater would therefore
have been insufficient to produce a charring effect.
However when vaporization of tissue has previously
been proposed as a means of tissue removal a soft tissue
substrate was used which would have had a compara-
tively greater water content [7]. It may also be
postulated that the interaction was not wholly thermal
and that there may have been a non-thermal component
to the interactive process. In this way the tissue may
have been partly removed by photoablation or photo-
disruption.

An unusual finding was noted during SEM (Fig. 4).

An inverted crater was observed in the pulp chamber
approximately 500 mm below the surface crater. The
surface layer of the inferior crater appeared fused and
was quite different in structure to the surrounding roof
of the pulp chamber (Fig. 5). It was rarely found that the
plane of section was parallel to the long axis of the
tubules and as a result this observation is somewhat
anecdotal. However, the laser irradiation may have been
transmitted through the tooth in this case and caused
cratering deepto the dentine surface. This finding is of
concern as transmission of radiation into pulpal tissue
along dentinal tubules may cause uncontrolled effects.
Factors which may have caused cratering at this site
include light refraction as the light entered the pulp
chamber, reflection of light from the opposing internal
pulp chamber wall or the presence of differing material
on the roof of the pulp chamber. A difference in light
transmission due to orientation of dentinal tubules has
not been demonstrated and Fried et al. [12] reported
that light was unlikely to travel more than 300 mm before
it was scrambled by the curvature of the tubules. A
slight change in thermal conductivity between parallel
and transverse sections of the dentinal tubules has been
reported [13]. As the possible transmission through
dentine of laser irradiation in the present study was
based on a single incident, no firm conclusions can be
drawn. Diffusion of Nd : YAG radiation through

A. McDonald et al. / Biomaterials 23 (2002) 51–58

57

dentine into pulp has been described [14]. Similarly
adverse pulpal effects have been reported following
enamel or dentine irradiation with Nd : YAG [1,10,
15–17]. Further work is needed to explore this phenom-
enon.

5. Conclusions

*

Craters appeared fused and carbonised but this was
less apparent at the crater base. In transverse section
they had an inverted dome shape.

*

11% of sites did not crater.

*

No-result areas occurred most frequently in undyed
sites (N ¼ 241) as opposed to dyed (N ¼ 12).

*

Energy and dyed/undyed were found to have a
statistically significant effect on crater depth.

*

Repetition rate and inner/outer were not found to
have a statistically significant effect on crater depth.

*

A large number of specimens were included within
this pulse duration group. Consequently, factors may
have been found to be statistically significant which
may have been insignificant with a smaller sample
size. The clinical practicality of their significance must
be interpreted cautiously.

Acknowledgements

I would like to thank Dr. Alan Kelly for his helpwith

the statistical analysis. I am also grateful to Miles
Maidment and Stephen Blake in the Media Centre,
Eastman Dental Institute.

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