Nuss, Roger C. Infrared Laser Bone Ablation. 1988.

Lasers in Surgery and Medicine 8:381-391 (1988)
Infrared Laser Bone Ablation
Roger C. Nuss, BS, Richard L. Fabian, MD, Rajabrata Sarkar, BS, and
Carmen A. Puliafito, MD
Department of Otolaryngology and the Laser Research Laboratory, Department of
Ophthalmology, Massachusetts Eye and Ear Infirmary, Harvard Medical School, Boston
The bone ablation characteristics of five infrared lasers, including
three pulsed lasers (Nd:YAG, X = 1,064 pm; Hol:YSGG, X = 2.10 pm;
and Erb:YAG, X = 2.94 pm) and two continuous-wave lasers (NdYAG,
X = 1.064 pm; and C02, X = 10.6 pm), were studied. All laser ablations
were performed in vitro, using moist, freshly dissected calvarium of
guinea pig skulls. Quantitative etch rates of the three pulsed lasers
were calculated. Light microscopy of histologic sections of ablated
bone revealed a zone of tissue damage of 10 to 15 pm adjacent to the
lesion edge in the case of the pulsed Nd:YAG and the Erb:YAG lasers,
from 20 to 90 pm zone of tissue damage for bone ablated by the
Ho1:YSGG laser, and 60 to 135 pm zone of tissue damage in the case of
the two continuous-wave lasers. Possible mechanisms of bone ablation
and tissue damage are discussed.
Key words: COz, Erb:YAG, Hol:YSGG, Nd:YAG
I NTRO D U CTl ON
Previously published studies of lasers and
bone-cutting have largely centered upon the COZ
laser in both the continuous-wave and rapid superpulsed
modes to perform laser osteotomies [l-61.
These investigations have compared C02 laser osteotomies
performed in animals with similar lesions
produced by a rotating bur drill or a handheld
saw by radiographic, histologic, and mechanical
torsion testing methods. Healing studies have
been performed, demonstrating a delay in healing
of the C02 laser osteotomy as opposed to mechan-
ically-produced lesions [3,4,6,7]. The COZ laser has
been associated with a thermal mechanism of bone
ablation, with resulting coagulation, carbonization,
and vaporization of living tissues. The carbon
char produced may cause a foreign body-type reaction.
More recently, several investigators have
studied both the argon laser and the C02 laser as
a means of producing a small hole in the footplate
beneath the stapes of the middle ear (stapedotomy)
in the surgical treatment of otosclerosis [8-131.
With the recent development of several solidstate
crystal lasers operating in the infrared wavelengths,
it has become interesting to examine their
efficacy in ablating biologic materials. Already
the Ho1:YSGG and the Erb:YAG lasers have been
used to ablate cornea, sclera, and other ocular
structures [14]. Margolis TI, Farnath DA, Destro
0 1988 Alan R. Liss, Inc.
M, Puliafito CA: Erbium-YAG laser surgery on
experimental vitreous membranes in rabbits
(submitted for publication, 1987); Margolis TI,
Farnath DA, Puliafho CA: Mid-infrared laser sclerostomy
(submitted for publication, 1987) However,
there has been very little work done to
examine their bone cutting characteristics.
This study was performed to compare the in
uitro bone ablation characteristics of several infrared
lasers, including three pulsed lasers
(Nd:YAG, X = 1.064 pm; Hol:YSGG, X = 2.10 pm;
and Erb:YAG, X = 2.94 pm) and two continuouswave
lasers (Nd:YAG, X = 1.064 pm; and C02, X =
10.6 pm). The histologic appearance of bone ablations
from each of the lasers was compared, and
the quantitative cutting efficiency (etch rate) of
the three pulsed lasers was determined.
MATERIALS AND METHODS
Lasers
Five lasers with infrared light output were
studied. These included three pulsed lasers
(Nd:YAG, X = 1.064 pm; Hol:YSGG, A = 2.10 pm;
Accepted for publication April 29, 1988.
Address reprint requests to Richard L. Fabian, M.D., Department
of Otolaryngology, 243 Charles Street, Boston, MA
02114.

382 Nus et al
and Erb:YAG, h = 2.94 pm) and two continuouswave
lasers (Nd:YAG, h = 1.064 pm; and C02, X
= 10.6 pm). The laser type and model, operating
characteristics, and experimental parameters are
detailed in Table 1. For each laser, the beam was
focused to a circular spot with a spherical lens
(bench-mounted in all cases except for the C02
laser, which was a clinical unit). The size of the
focal spot was dependent on the focal distance of
the lens and the laser wavelength, and varied
with the energy or power level.
U
Measurements
Delivered pulse energy (pulsed lasers) or delivered
power (continuous-wave lasers) was measured
with a Scientech Model 362 Power and
Energy Meter (Scientech, Boulder, CO). At each
energy or power setting, beam spot size was determined
by placing a piece of developed photographic
film into the focal point of the beam and
measuring the etched spot under a calibrated ocular.
With these two measurements, radiant exposure
(J/cm2) or irradiance (W/cm2) was calculated.
Bone Tissue
All infrared laser bone ablation studies were
performed on guinea pig skull calvaria. Guinea
pigs (Hartley strain, 800-1,000 gm, female) were
sacrificed with a lethal injection of T-61 Euthanasia
solution (embutramide 200 mg/ml, mebezonium
iodide 50 mg/ml, tetracaine hydrochloride 5
mg/ml; Taylor Pharmacal Co., Decatur, IL). The
calvarium of the skull was immediately dissected,
wrapped in gauze moistened with normal saline,
and refrigerated until use within the next 12
hours. Care was taken to ensure that the bone was
kept moist during experimentation, except for one
series of ablations with the Erb:YAG laser in
which the bone was purposely dried in an oven at
55°C for 12 hr. This was done in order to study
laser ablation of bone in which unbound water had
been removed.
Etch Rate Calculation
For each of the three pulsed lasers, a series
of ablations were performed at several different
radiant exposures. The endpoint for laser ablation
was chosen to be perforation of the calvarium, as
detected by visualizing the laser beam etch a piece
of developed photographic film held directly behind
the bone specimen. At the pulse repetition
rates of 1 or 2 pulses per second used in this study,
the endpoint was quite distinct and was precise to
f 1 pulse. Following laser ablation of the bone

specimen, the thickness of the calvarium at the
point of ablation was measured with a micrometer
with a point-like contact surface (Starrett Model
210A-P micrometer, L.S. Starrett Co., Athol, MA).
With this information, the etch rate (micron bone
ablated per pulse) was calculated at each radiant
exposure.
Histology
Specimens were processed for histology in the
following manner. The ablation site and 1 to 1.5
mm of surrounding bone were cut from the calvarium
with a hand-held jigsaw. Individual ablation
specimens were fixed in modified Karnofsky 's fixer
(2% paraformaldehyde, 2.5% glutaraldehyde, 0.1
M sodium cacodylate buffer) for a minimum of 3
days. Bone specimens were then rinsed in phosphate
buffersd saline for 2 hours, and decalcified
in a commercially prepared solution (Decalcifier 11
Solution, Surgipath Medical Industries, Inc.,
Graystake, IL) for a period of 7 days, including at
least ten changes of decalcifying solution. Specimens
were then dehydrated through an extended
graded ethanol series, and embedded in JB-4
methacrylate embedding compound (Polysciences,
Inc., Warrington, PA). Sections of the bone ablations
were cut parallel to the axis of the beam
path at a thickness of 2 pm. Prepared slides were
stained with Stevenol's blue histologic stain and
examined under light microscopy.
Infrared Spectrophotometry
A Perkin-Elmer Lambda 9 WNISINIR spectrophotometer
was used to measure the infrared
absorption of nondecalcified bone in the region of
1.0 to 3.2 pm wavelengths. The bone sample studied
was a piece of dehydrated nondecalcified compact
bone that had been ground to a thickness of
20 pm and mounted on a glass slide. A similar
glass slide and mounting glue preparation was
used as a reference standard in the spectrophotometer.
Infrared Laser Bone Ablation
100,
m.
A
-: 80.
=
. 70.
-
.-
5 60.
<
5
E, 40.
-
5 30.
o!
c
" 20.
iz
10
0
300.
" n
-
< 250.
2
a
200.
" 5
m
E,
Y
150.
"
a
= 100.
L
.-
"
Y
so.
Ho1:YSGG Laser
383
0 t
0 5 10 15 20 25 30
Radiant Expoaurr (J/crn*)
Fig. 1. Pulsed NdYAG (A = 1.064 pm) bone ablation: mean
etch rate ( pm bone ablatedpulse) versus radiant exposure (J/
cm'). Error bars indicate standard error of the mean. Linear
regression was performed on all individual data points in the
radiant exposure range of 8.0 to 22.5 J/cm2.
. . _ . _ . . ~ _ . . . _
Fig. 2. Ho1:YSGG (A = 2.10 pm) bone ablation: etch rate
(pm bone ablatedpulse) versus radiant exposure (J/cm2). All
data points are indicated.
.
D
RESULTS
Etch Rates
Plots of etch rate (micron of bone ablated per
pulse) versus radiant exposure (J/cm2) for the three
pulsed lasers are presented in Figures 1-3. In addition,
Figure 4 displays the etch rate versus radiant
exposure plot of Erb:YAG laser ablation of
dry guinea pig calvaria. The average number of
pulses delivered until perforation at each radiant
exposure and the range of thicknesses of calvaria
used with the three pulsed lasers is summarized
01 *. , . .. . , . , . , .
0 20 40 60 80 I00
Radiant Exporurc (JtcmZ)
Fig. 3. Erb:YAG (A = 2.94 pm) bone ablation: etch rate ( pm
bone ablatedpulse) versus radiant exposure (J/cm2X All data
points are indicated.
in Table 2. The slope of the linear regression curve
(reported as micron of bone ablated per pulse per
J/cm2), the range of radiant exposures over which
the linear regression was performed, the coeffi-
0

384 Nus et al
f
120, t
I
I
1
80-
i 20/
Erb:VAG Laser; Dry bone specimen
i
0 5 10 15 20 25 30 35 40 45 SO
Radiant Cxposurc (J/crn*)
Fig. 4. Erb:YAG (A = 2.94 pm) bone ablation of desicated
bone: etch rate (pm bone ablatedlpulse) versus radiant exposure
(J/cm2). All data points are indicated.
cient of determination (r2), and the correlation
coefficient (r) for the four sets of data points are
summarized in Table 3. Tests of statistical significance
demonstrated a highly statistically significant
difference (P < 0.001) in the slope of the
linear regression curve for each of the lasers except
when comparing the Nd:YAG and the
Ho1:YSGG lasers. However, it was not possible to
test these two lasers over the same range of radiant
exposures, and such a statistical test would
not be valid. It was not possible to accurately determine
an etch rate for the two continuous-wave
lasers.
Optic Fiber Delivery
It was possible to pass the Ho1:YSGG laser
output (A = 2.10 pm) down a silica optic fiber (300
pm core diameter, 60 cm length) with sufficient
delivered pulse energy (800 mJ) to easily ablate
bone. However, the Erb:YAG laser (A = 2.94 pm)
was extremely attenuated, such that only 45 mJ
could be transmitted through a 18 cm length of
silica optic fiber. This was insufficient to ablate
bone.
Histology
Histologic sections of ablated bone were examined
with light microscopy. Bone specimens
ablated by a range of radiant exposures for the
three pulsed lasers and by a range of irradiances
for the two continuous-wave lasers were studied.
Zones of tissue damage were identified by an alteration
in the tissue staining characteristics. This
generally was an increased basophilic staining
character in the tissue region adjacent to the ablation
edge. Lesions produced by the pulsed Nd:YAG
and the Erb:YAG lasers had smooth edges and a
10 to 15 pm zone of tissue damage over the whole
range of radiant exposures studied (Figs. 5, 7).
There was no increase in the zone of tissue damage
as the radiant exposure was increased for the
Nd:YAG and the Erb:YAG lasers. Lesions produced
by the Ho1:YSGG laser had a histologic
TABLE 2. Summary of the Range of Thickness of Calvaria and the Average Number of
Pulses Required to Perforate the Calvaria at the Various Radiant Exposures Used With
the Three Pulsed Lasers
Laser
Range of calvaria
thicknesses (um)
Nd:YAG 380-710
Ho1:YSGG 410-1,070
Erb:YAG 810-1,320
(wet bone)
Erb:YAG 330-640
(dry bone)
Radiant
exposure (J/cm2)
8
11
17
23
27
18
27
33
44
78
135
8
15
23
46
102
9
10
16
20
22
31
42
Average no. of pulses
required for perforation
145
93
64
29
53
57
34
36
25
11
11
161
24
14
8
3
18
10
7.6
8
7
6
A

TABLE 3. Pulsed Laser Bone Ablation Data
Infrared Laser Bone Ablation 385
Slope of linear Range of radiant Coefficient of Correlation
regression curve' exposures (J/cm2) determination (r2) coefficient (r)
Nd:YAG 0.74 8-22.5 0.43 0.66
Ho1:YSGG 0.67 18-135 0.92 0.96.
Erb:YAG (moist bone) 2.70 8-102 0.90 0.95
Erb:YAG (dry bone) 1.77 9-42 0.82 0.90
'Micron of bone ablated per pulse per J/cm2.
appearance that varied with the radiant exposure:
at a radiant exposure of 18 Jlcm2, the ablated
lesion had smooth edges and a 20 pm zone of tissue
damage, while at a radiant exposure of 135 J/cm2,
the lesion edge had a more fibrillar appearance
and a wider 90 pm zone of tissue damage. Radiant
exposures between these two extremes resulted in
an intermediate amount of tissue damage (Fig. 6).
There was a moderate degree of correlation between
radiant exposure and the extent of tissue
damage for this laser (r = 0.70).
Lesions produced by the two continuous-wave
lasers had jagged edges with a fibrillar appearance
and had a 60 to 135 pm zone of tissue damage
(Figs. 8, 9). In the case of the continuous-wave
lasers, there was also a narrow zone (5 to 10 pm)
of decreased stain uptake immediately adjacent to
the lesion edge. There was no significant correlation
between irradiance and the size of the area of
tissue damage for the continuous-wave lasers.
There also was no significant correlation between
total delivered energy and the area of tissue
damage.
Infrared Spectrophotometry
The absorbance characteristics of a dehydrated
nondecalcified 20 pm-thick piece of compact
bone in the infrared wavelengths from X =
1.0 through 3.2 pm are presented in Figure 10.
Absorbance is defined as follows: A = loglo IoA,
where 10 is the intensity of incident laser light
and 10 is the intensity of transmitted light. Of
special note is the greater than 4 log-scale increase
in infrared absorbance as the wavelength
increases from 2.7 to 2.8 pm.
DISCUSSION
It is useful to consider the composition and
structure of bone prior to discussing possible
mechanisms of bone ablation. Bone is a biologic
material with an inherent nonhomogeneity and
Fig. 5. Photomicrograph of pulsed Nd:YAG (A = 1.064 pm) lesion edge and narrow (10 to 15 pm) zone of altered staining
bone ablation. Radiant exposure = 16.5 J/cm2, 10 nsec pulse characteristics. Original magnification x4.
width, 130 pulses delivered at 1 pulse per sec. Note smooth

386 Nuss et al
Fig. 6. Photomicrograph of HoI:YSGG (A = 2.10 pm) bone ance of lesion edge and wide (60 to 90 pm) zone of altered
ablation. Radiant exposure = 44 J/crn2, 250 psec pulse width, staining characteristics. Original magnification x4.
25 pulses delivered at 2 pulses per sec. Note rough appear-
Fig. 7. Photomicrograph of Erb:YAG (A = 2.94 pm) bone ablation.
Radiant exposure = 46 J/cm2, 250 psec pulse width, 8
pulses delivered at 2 pulses per sec. Note smooth lesion edge
and narrow (10 to 15 pm) zone of altered staining characteristics.
Original magnification X4.

Infrared Laser Bone Ablation 387
Fig. 8. Photomicrograph of CW-Nd:YAG (A = 1.064 pm) bone
ablation. Irradiance = 2,700 Wkm2,lOO msec pulse duration,
5 pulses delivered at 1 pulse per sec. Note jagged, fibrillar
appearance of lesion edge and wide (60 to 135 pm) zone of
altered staining characteristcs. Original magnification x2.
Fig. 9. Photomicrograph of CW-COP (A = 10.6 pm) bone abla- appearance of lesion edge and wide (60 to 135 pm) zone of
tion. Irradiance = 880 Wkm2, 50 msec pulse duration, 5 altered staining characteristics. Original magnification X2.
pulses delivered at 1 pulse per sec. Note jagged, fibrillar
consists of compact (substantia compacta) and them is a layer of substantia spongiosa of varying
spongy (substantia spongiosa) forms. In the flat thickness that is occupied by bone marrow.
bones of the skull, the substantia compacta forms The interstitial substance of bone is comthick
layers on each surface, which are referred to posed of two major components, an organic matrix
as the inner and outer tables of the skull. Between and inorganic salts, each comprising about 50% of

388 Nus et al
its dry weight [El. The organic matrix of bone in
adult mammals consists of about 95% collagen
(predominantly type I), which lies in a highly ordered
arrangement. The collagenous fibers are
embedded in a ground substance consisting of glycosaminoglycans
(chondroitin sulfate, keratin sulfate,
and hyaluronic acid) [15]. The inorganic
portion of bone consists of submicroscopic deposits
of a form of calcium phosphate that is very similar
to the mineral hydroxyapatite (Calo[P04]6[OH],).
Also present are significant amounts of the citrate
ion C6H507-3 and the carbonate ion C03-3 [15].
Water molecules are closely associated with the
organic matrix and the inorganic salts of bone.
Each of these components has its characteristic
absorption qualities in the infrared region of
the electromagnetic spectrum. H20 has its strongest
absorption peak at A = 2.7 to 3.2 pm (absorption
coefficient a = 7700cm-l) [16]. The
absorption coefficient (a) is equal to 2.3/L, where
L is the extinction length of the material being
analyzed. For the wavelengths of interest in this
study, the absorption coefficient of H20 at A =
1.064 pm, 2.10 pm, 2.94 pm, and 10.6 pm is a =
0.4 cm-l, 40 cm-',7700 cm-', and 600 cm-', respectively
[ 161.
Vertebrate collagen has four major absorption
bands in the infrared spectrum. These occur
at wavelengths of 3.03 pm, 6.06 pm, 6.54 pm, and
8.06 pm [ 17,181. The mineral, hydroxyapatite, absorbs
most strongly in the infrared spectrum at
wavelengths of 2.94 pm and 9.26 pm [19]. In addition,
calcium phosphate has strong infrared absorption
bands at wavelengths of 3.1 pm, 3.3 pm,
9.2 pm, and 9.7 pm[20].
In general, there are three possible mechanisms
of infrared laser bone ablation and tissue
damage. These include 1) absorption of infrared
laser energy by H20 molecules associated with the
organic and inorganic components of the bone (implying
a thermal bone ablation mechanism), 2)
absorption of the laser energy by the organic collagen
matrix and/or the inorganic calcium salts of
the bone (also a thermal mechanism), and 3) an
optical breakdown and plasma cutting phenomenon
producing bone ablation at sufficiently high
laser irradiance. The absorption characteristics of
the laser wavelengths by the components of the
bone, the pulse duration, and the radiant exposure
or irradiance are relevant considerations in proposing
a mechanism of laser bone ablation.
At wavelengths where bone has a large absorption
coefficient (a) and shallow penetration
Abiorbance
A = 109 lo
5
4
3
2
I
. . . . . . . . . . . .
10 12 11 16 18 20 22 24 26 28 30 32
Wavelenpth, urn
Fig. 10. Spectrophotometry of nondecalcified 20 pm-thick
piece of ground compact bone. Absorbance plotted in nearinfrared
range from X = 1.0 through 3.2 pm. Note the greater
than 4 log-scale increase in absorbance as the wavelength
increases from X = 2.7 to 2.8 pm.
expected that the laser energy will be absorbed in
a relatively small volume and would ablate bone
more efficiently than a laser wavelength that is
scattered and absorbed over a larger volume. The
absorbance characteristics of a dehydrated nondecalcified
bone (Fig. 10) demonstrates relatively
moderate absorption of 1.064 pm and 2.10 pm
wavelengths. However, bone becomes essentially
impervious to the transmission of infrared light
for wavelengths greater than 2.7 pm. Apart from
the H2O molecules normally associated with the
bone constituents, the organic matrix and the inorganic
calcium salts themselves are strong absorbers
of infrared irradiation from 2.9 pm to 3.3
pm [17-201. Based on this information, it is expected
that a laser operating at a 2.94 pm wavelength
would be more highly absorbed by bone
than one operating at a 2.10 pm wavelength, which
in turn would be absorbed more highly than a
1.064 pm wavelength laser.
When laser energy is sufficiently condensed
in time and space to achieve an extremely high
irradiance, a nonlinear phenomenon known as optical
breakdown will occur. This has been described
extensively with reference to the pulsed
Nd:YAG laser [21,22]. Optical breakdown is accompanied
by a spark and an audible snap. It
depth (inverse of absorption Coefficient, l/d, it is involves the creation of a plasma, which is an

ionized state in which electrons have freely dissociated
from their atoms. Optical breakdown will
occur when an irradiance on the order of lo9 to
10l2 W/cm2 is achieved [21,22]. Only the
Q-switched Nd:YAG laser used in this study produced
sufficiently high irradiance to achieve optical
breakdown. It is important to note the
phenomenon of plasma shielding which occurs
with optical breakdown. Once formed, plasma absorbs
and scatters incident light. This has the effect
of shielding underlying targets in the beam
path [21-231. This may actually result in a decrease
in energy transmission to the target as the
laser irradiance is increased and a corresponding
decrease in target ablation. Just as the creation of
plasma during optical breakdown is a nonlinear
process, the ablation of biologic tissues with a
(f plasma ‘plasma-cutting”) also behaves nonlin-
early [23,24].
Based on the above discussion, a mechanism
of bohe ablation for each of these lasers will be
proposed. The Q-switched Nd:YAG laser had a
threshold radiant exposure for bone ablation of 8.1
J/cm2. The laser and delivery system was limited
to a maximum radiant exposure of 27 J/cm2. This,
however, correlates with an irradiance of 2.7 x
lo9 W/cm2. Each pulse of delivered energy produced
a spark and clear snap, correlating with the
expected optical breakdown. Because there is very
little absorption of this wavelength by bone tissue,
a plasma-cutting process is the most likely mechanism
of ablation. The very narrow zone of thermal
tissue injury around the ablation site (Fig. 5)
makes any thermal component to bone ablation
very unlikely. As seen in Figure 1, there was a
linear increase in etch rate as the radiant exposure
increased from 8.1 to 22.5 J/cm2, which is
then followed by a drop in etch rate at higher
radiant exposures. A plasma shielding effect is the
likely explanation for this observation.
The Ho1:YSGG and Erb:YAG lasers are similar
in all aspects except for their wavelength. They
both are capable of an irradiance on the order of
lo5 W/cm2 and so are clearly not in the range of
optical breakdown and plasma formation. These
lasers are more likely associated with a thermal
mechanism causing vaporization of bone tissue.
The absorption of laser irradiation and conversion
into thermal energy results in a local deposition
of heat. As energy is added and the water in the
tissue is raised to its boiling point, an explosive
vaporization of the tissue will occur [25,26]. It is
interesting to note that both the Ho1:YSGG and
the Erb:YAG lasers produced an audible crack and
Infrared Laser Bone Ablation 389
a yellow-orange flame 1 to 2 cm in length at the
target site for the higher radiant exposures. The
dissipation of pulse energy in a thermal ablation
process occurs not only as the grossly observed fire
and thermalacoustic waves but also in a process
known as spallation. Energy is dissipated in spallation
through the ejection of chunks of target
tissue as part of the explosive vaporization process
~71.
The Ho1:YSGG and the Erb:YAG lasers had
threshold radiant exposures for bone ablation of
18 and 8.0 Jlcm2, respectively. The higher bone
ablation rate observed with the 2.94 pm laser
wavelength correlates with the higher absorbance
of this wavelength by collagen, inorganic salts,
and H2O. Even in a dried bone in which all unbound
water (which may increase infrared laser
energy absorption) has been removed, the
Erb:YAG laser ablates bone more efficiently than
the Ho1:YSGG laser. Because the Erb:YAG laser
is more highly absorbed over a smaller volume of
bone tissue, it is expected that there would be both
more efficient bone ablation as well as less associated
thermal damage than that produced by the
Ho1:YSGG laser (Figs. 6, 7).
These predictions have been supported by
computer modeling [25]. A comparative thermal
modeling of Erb:YAG and Ho1:YAG laser pulses
for tissue vaporization (of a “typical” biologic tissue)
predicts that the Erb:YAG laser will produce
more efficient vaporization with a smaller rim of
thermal damage (12 pm), while the Ho1:YAG laser
will produce a greater margin of thermal damage
(500 pm) because of deeper penetration into the
tissue and because a higher energy is needed to
reach vaporization threshold [25]. In addition, previous
work done with the Erb:YAG laser reported
that the tissue thermal damage zone was confined
to a region of 3 to 5 pm from the edge of the
ablated zone, and that the ablation threshold in
bone was 1.8 J/cm2 [26]. These values are comparable
to those determined in the present study,
though the lower radiant exposure threshold for
bone ablation is likely due to a difference in methodology
for determining ablation threshold.
The two continuous-wave lasers both clearly
ablated bone in a thermal mechanism. Impact of
the laser beam with the bone tissue produced a
whitish glow of the target and subsequent gross
charring surrounding the laser ablation. Histologic
examination confirmed a large lateral spread
of thermal injury (Figs. 8, 9). There was no consistent
threshold irradiance for bone ablation by the
two continuous-wave lasers. There is strong ab-

390 Nus et al
sorption of 10.6 pm laser energy by H2O. The COz
laser was observed to initially vaporize the target
bone tissue, but would frequently cease bone ablation
as the tissue became desicated and a char was
produced. This “stall-out” phenomenon with char
formation has been observed by others [3,8]. It has
been suggested that cooling of the ablation site
with a jet of nitrogen gas may reduce the heat of
lasing and decrease the amount of carbon char
formed [3]. Unlike the C02 laser, there is no component
of bone that is a strong absorber of the
1.064 pm CW-Nd:YAG laser. There was much variability
in this laser’s ability to initiate an ablation
site in the bone, as if a target chromophore
had to be hit before ablation could proceed.
The results of this study suggest several bonecutting
applications for these lasers. Both the
Ho1:YSGG and the Erb:YAG lasers efficiently
ablated bone in a precise and controlled fashion.
Moreover, the Erb:YAG laser produced very minimal
thermal damage to adjacent tissue, while the
Ho1:YSGG laser produced a noticeably greater
amount. However, the 2.10 pm wavelength of the
Ho1:YSGG laser can be efficiently transmitted
through a silica optic fiber with little attenuation
of laser energy, such that it was possible to ablate
bone with this laser using a fiber optic delivery
system. At this time there is no optic fiber that
offers sufficient transmissibility and flexibility to
be useful with the Erb:YAG laser for the purpose
of bone ablation. The development of such a fiber
to carry the 2.94 pm laser wavelength will certainly
increase the versatility and usefulness of
this laser.
Clinical applications for the Ho1:YSGG laser
with a fiber optic delivery system include those
circumstances where precise control of localization
and depth of cut is required. In the field of
otolaryngology, procedures such as endoscopic nasal
sinus surgery and optic canal decompression
may be facilitated by such an instrument. As previously
described, both the Ho1:YSGG and the
Erb:YAG lasers were noted to produce both a flame
and audible crack at higher radiant exposures.
This thermoacoustic shock wave may produce undesirable
effects when ablating bone near delicate
structures.
The continuous-wave lasers will ablate bone,
especially if used in a continuous mode at a high
irradiance, and may be useful in situations where
the gross removal of large amounts of bone is
desired and where there is no concern of thermal
injury and charring to adjacent tissue. Of note is
that the CW-Nd:YAG laser may be delivered by a
fiber optic delivery system. Both the CW-Nd:YAG
and the CW-CO2 lasers are much less precise than
the pulsed lasers and would not be useful when
control of bone ablation depth is of importance.
Future investigation of infrared laser bone
ablation will involve a transmission electron microscopic
examination of the lesions created by
these five lasers. In uivo studies with both the
Ho1:YSGG and the Erb:YAG laser will be useful
to assess other factors such as blood flow through
marrow spaces, which may affect their bone ablation
characteristics. In addition, healing studies
with these two lasers will be important to evaluate
the tissue response to laser ablation.
CONCLUSIONS
The bone ablation characteristics of five infrared
lasers, including three pulsed lasers
(Nd:YAG, X = 1.064 pm; Hol:YSGG, X = 2.10 pm;
and Erb:YAG, X = 2.94 pm) and two continuouswave
lasers (Nd:YAG, X = 1.064 pm; and C02, X
= 10.6 pm), were studied. Quantitative etch rates
could be determined for the three pulsed lasers.
The Erb:YAG laser was the most effective bone
ablater, followed by the Ho1:YSGG laser. Histologic
evidence of thermal tissue damage adjacent
to the lesion edge extended 10 to 15 pm for ablations
created by the pulsed Nd:YAG and Erb:YAG
lasers, from 20 to 90 pm for Ho1:YSGG laser ablations,
and from 60 to 135 pm for lesions created by
the two continuous-wave lasers.
The components of bone, including type I collagen
and inorganic calcium salts resembling hydroxyapatite,
have broad spectrophotometric absorption
bands in the range of 2.9 to 3.3 pm. This
correlates well with the observed increase in infrared
absorbance at 2.7 pm of a piece of dehydrated
nondecalcified bone analyzed by spectrophotometry.
In addition, H2O has its strongest
absorption peak in this range.
Observed and theoretical considerations lead
to the following proposed bone ablation mechanisms
for the five infrared lasers. The Q-switched
Nd:YAG laser is operating at sufficiently high irradiance
to ablate bone in a plasma-cutting manner,
with little or no thermal component. The other
four lasers, however, ablate bone by a thermal
mechanism. The Erb:YAG laser delivers a wavelength
that is highly absorbed by bone, and is the
most effective ablater. The Ho1:YSGG laser is not
absorbed by bone as well, and it cuts bone less
effectively and with greater thermal damage. Both
of the continuous-wave lasers create large

amounts of thermal damage to adjacent bone tissue
and cut in a nonlinear and nonpredictable
manner.
The ability of the Ho1:YSGG laser to be
transmitted through a silica optical fiber with sufficient
radiant exposure to ablate bone suggests
applications for this laser in such procedures as
endoscopic nasal sinus surgery and optic canal
decompression procedures. The development of an
optic fiber that is able to efficiently transmit the
Erb:YAG wavelength will increase this laser’s
usefulness. Future investigations will include
transmission electron microscopic studies of the
lesions created by these lasers, as well as in uiuo
and healing studies for the Ho1:YSGG and
Erb:YAG lasers.
ACKNOWLEDGMENTS
The authors wish to thank Jeff Mani of
Schwartz Electro-Optics, Inc. for his help with bone
ablation experiments.
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