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 C 0 2 , 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 super- pulsed modes to perform laser osteotomies [l-61. These investigations have compared C02 laser os- teotomies performed in animals with similar le- sions produced by a rotating bur drill or a hand- held saw by radiographic, histologic, and mechan- ical torsion testing methods. Healing studies have been performed, demonstrating a delay in healing of the C 0 2 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, carboniza- tion, and vaporization of living tissues. The carbon char produced may cause a foreign body-type re- action. More recently, several investigators have studied both the argon laser and the C 0 2 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 solid- state crystal lasers operating in the infrared wave- lengths, 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 scle- rostomy (submitted for publication, 1987) How- ever, 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 in- frared 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 (Nd:YAG, X = 1.064 pm; and C02 , X = 10.6 pm). The histologic appearance of bone abla- tions 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., Depart- ment of Otolaryngology, 243 Charles Street, Boston, MA 02114.

382 Nus et al

and Erb:YAG, h = 2.94 pm) and two continuous- wave lasers (Nd:YAG, h = 1.064 pm; and C 0 2 , 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.

Measurements Delivered pulse energy (pulsed lasers) or de-

livered power (continuous-wave lasers) was mea- sured with a Scientech Model 362 Power and Energy Meter (Scientech, Boulder, CO). At each energy or power setting, beam spot size was deter- mined by placing a piece of developed photo- graphic film into the focal point of the beam and measuring the etched spot under a calibrated ocu- lar. With these two measurements, radiant expo- sure (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 Euthana- sia solution (embutramide 200 mg/ml, mebezon- ium 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 be- hind 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

U

383 Infrared Laser Bone Ablation

100,

m.

- :: 80. A

= . 70.

5 60.

- .- <

5 E, 40. - 5 30.

" 20. iz 10

0

o! c

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 calvar- ium 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 phos- phate 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. Speci- mens 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 abla- tions 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 spec-

trophotometer was used to measure the infrared absorption of nondecalcified bone in the region of 1.0 to 3.2 pm wavelengths. The bone sample stud- ied was a piece of dehydrated nondecalcified com- pact 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 spectro- photometer.

. Ho1:YSGG Laser

. . _ . _ . . ~ _ . . . _

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 ad- dition, Figure 4 displays the etch rate versus ra- diant 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

300. " n - < 250.

2 a

" 5

m

200.

150.

= 100.

E, Y " a L " Y .-

so.

0 t 0 5 10 15 20 25 30

Radiant Expoaurr (J /c rn* )

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.

D

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.

01 * . , . . . . , . , . , . 0 20 40 60 80 I00

Radiant Exporurc (J tcmZ)

0

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-

384 Nus et al 120, t

I I

1 f Erb:VAG Laser; Dry bone specimen

80-

i 20/ 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 expo- sure (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 signifi- cance demonstrated a highly statistically signifi- cant difference ( P < 0.001) in the slope of the linear regression curve for each of the lasers ex- cept 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 ra- diant exposures, and such a statistical test would not be valid. It was not possible to accurately de- termine 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 ex- amined 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 alter- ation in the tissue staining characteristics. This generally was an increased basophilic staining character in the tissue region adjacent to the abla- tion 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 dam- age as the radiant exposure was increased for the Nd:YAG and the Erb:YAG lasers. Lesions pro- duced 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

Range of calvaria Laser 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

Infrared Laser Bone Ablation 385

TABLE 3. Pulsed Laser Bone Ablation Data

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

'Micron of bone ablated per pulse per J/cm2.

Erb:YAG (dry bone) 1.77 9-42 0.82 0.90

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 be- tween 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 appear- ance 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 correla- tion 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 dehy-

drated nondecalcified 20 pm-thick piece of com- pact 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 in- crease 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 abla- tion. 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 character- istics. Original magnification X 4 .

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 com- thick 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 N u s et al

its dry weight [ E l . The organic matrix of bone in adult mammals consists of about 95% collagen (predominantly type I), which lies in a highly or- dered arrangement. The collagenous fibers are embedded in a ground substance consisting of gly- cosaminoglycans (chondroitin sulfate, keratin sul- fate, 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 character- istic absorption qualities in the infrared region of the electromagnetic spectrum. H20 has its strong- est absorption peak at A = 2.7 to 3.2 pm (absorp- tion 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-', re- spectively [ 161.

Vertebrate collagen has four major absorp- tion 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, ab- sorbs most strongly in the infrared spectrum at wavelengths of 2.94 pm and 9.26 pm [19]. In addi- tion, calcium phosphate has strong infrared ab- sorption bands at wavelengths of 3.1 pm, 3.3 pm, 9.2 pm, and 9.7 pm[20].

In general, there are three possible mecha- nisms 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 (im- plying a thermal bone ablation mechanism), 2) absorption of the laser energy by the organic col- lagen matrix and/or the inorganic calcium salts of the bone (also a thermal mechanism), and 3) an optical breakdown and plasma cutting phenome- non 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 pro- posing a mechanism of laser bone ablation.

At wavelengths where bone has a large ab- sorption coefficient (a) and shallow penetration

5

4

Abiorbance

A = 109 lo 3

2

I

. . . . . . . . . . . . 1 0 1 2 1 1 1 6 1 8 2 0 2 2 2 4 2 6 2 8 3 0 3 2

Wavelenpth, urn

Fig. 10. Spectrophotometry of nondecalcified 20 pm-thick piece of ground compact bone. Absorbance plotted in near- infrared 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 nonde- calcified 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 in- organic calcium salts themselves are strong ab- sorbers of infrared irradiation from 2.9 pm to 3.3 pm [17-201. Based on this information, it is ex- pected that a laser operating at a 2.94 pm wave- length 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 op- tical breakdown will occur. This has been de- scribed extensively with reference to the pulsed Nd:YAG laser [21,22]. Optical breakdown is ac- companied 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

Infrared Laser Bone Ablation 389

ionized state in which electrons have freely disso- ciated 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 pro- duced sufficiently high irradiance to achieve opti- cal breakdown. It is important to note the phenomenon of plasma shielding which occurs with optical breakdown. Once formed, plasma ab- sorbs and scatters incident light. This has the ef- fect of shielding underlying targets in the beam path [21-231. This may actually result in a de- crease 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 plasma (f ‘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 pro- duced 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 mech- anism of ablation. The very narrow zone of ther- mal 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 expo- sure 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 simi- lar 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

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 spal- lation through the ejection of chunks of target tissue as part of the explosive vaporization process

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 un- bound 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 associ- ated 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 tis- sue) 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, pre- vious 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 compa- rable to those determined in the present study, though the lower radiant exposure threshold for bone ablation is likely due to a difference in meth- odology 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. Histo- logic examination confirmed a large lateral spread of thermal injury (Figs. 8, 9). There was no consis- tent threshold irradiance for bone ablation by the two continuous-wave lasers. There is strong ab-

~ 7 1 .

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 abla- tion 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 com- ponent of bone that is a strong absorber of the 1.064 pm CW-Nd:YAG laser. There was much var- iability in this laser’s ability to initiate an abla- tion site in the bone, as if a target chromophore had to be hit before ablation could proceed.

The results of this study suggest several bone- cutting 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 mini- mal 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 cer- tainly 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 localiza- tion and depth of cut is required. In the field of otolaryngology, procedures such as endoscopic na- sal sinus surgery and optic canal decompression may be facilitated by such an instrument. As pre- viously 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 un- desirable 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 mi- croscopic 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 abla- tion characteristics. In addition, healing studies with these two lasers will be important to evalu- ate the tissue response to laser ablation.

CONCLUSIONS

The bone ablation characteristics of five in- frared 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 (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. Histo- logic evidence of thermal tissue damage adjacent to the lesion edge extended 10 to 15 pm for abla- tions created by the pulsed Nd:YAG and Erb:YAG lasers, from 20 to 90 pm for Ho1:YSGG laser abla- tions, and from 60 to 135 pm for lesions created by the two continuous-wave lasers.

The components of bone, including type I col- lagen and inorganic calcium salts resembling hy- droxyapatite, have broad spectrophotometric ab- sorption bands in the range of 2.9 to 3.3 pm. This correlates well with the observed increase in in- frared absorbance at 2.7 pm of a piece of de- hydrated nondecalcified bone analyzed by spectro- photometry. In addition, H2O has its strongest absorption peak in this range.

Observed and theoretical considerations lead to the following proposed bone ablation mecha- nisms for the five infrared lasers. The Q-switched Nd:YAG laser is operating at sufficiently high ir- radiance to ablate bone in a plasma-cutting man- ner, with little or no thermal component. The other four lasers, however, ablate bone by a thermal mechanism. The Erb:YAG laser delivers a wave- length 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

Infrared Laser Bone Ablation 391

amounts of thermal damage to adjacent bone tis- sue and cut in a nonlinear and nonpredictable manner.

The ability of the Ho1:YSGG laser to be transmitted through a silica optical fiber with suf- ficient 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.

REFERENCES

1.

2.

3.

4.

5.

6.

7.

8.

Clauser C: Comparison of depth and profile of osteoto- mies performed by rapid superpulsed and continuous- wave C02 laser beams at high power output. J Oral Maxillofac Surg 1986; 44:425-430. Biyikli S, Modest MF: Energy requirements for osteo- tomy of femora and tibiae with a moving CW COO laser. Lasers Surg Med 1987; 7:512-519. Small IA, Osborn TP, Fuller T, Hussain M, Kobernick S: Observations of carbon dioxide laser and bone bur in the osteotomy of the rabbit tibia. J Oral Surg 1979; 37:159- 166. Clayman L, Fuller T, Beckman H: Healing of continuous- wave and rapid superpulsed, carbon dioxide, laser-in- duced bone defects. J Oral Surg 1978; 36:932-937. Clauser C, Panzoni E: Comparison between rapid super- pulsed and continuous-wave C02 laser for osteotomies. In: Atsumi K, Nimsakul K, eds. Laser Tokyo ’81. Tokyo: Japan Inter Group Corp, 1981:4-19. Gertzbein SD, deDemeter D, Cruickshank B, Kapasouri A: The effect of laser osteotomy on bone healing. Lasers Surg Med 1981; 1:361-373. Horch HH, McCord RC, Keiditsch E: Histological and long-term results following laser osteotomy. In: Proceed- ings of the Second International Surgical Laser Sympo- sium, 1977. Coker NJ, Ator GA, Jenkins HA, Neblett CR Carbon dioxide laser stapedotomy: A histopathologic study. Am J Otolaryngol 1986; 7:253-257.

investigations on the suitability of the carbon dioxide laser for stapedotomy. Ann Otol Rhino1 Laryngol 1986;

10. Gardner G, Robertson JH, Tomoda K, Clark WC: C02 laser stapedotomy: Is it practical? Am J Otolaryngol 1984;

11. McGee TM: The argon laser in surgery for chronic ear disease and otosclerosis. Laryngoscope 1983; 93:l 177- 1182.

12. DiBartolomeo JR, Ellis M: The argon laser in otology. Laryngoscope 1980; 90:1786-1796.

13. Perkins RC: Laser stapedotomy for otosclerosis. Laryn- goscope 1980; 90:228-241.

14. Wolbarsht ML, Foulks GN, Esterowitz L, %an DC, Levin K, Storm M: Corneal surgery with an Er:YAG laser at 2.94 pm. In: ARVO Abstracts. Supplement to Invest Ophthalmol Vis Sci. Philadelphia: J.B. Lippincott, 1986; 93.

15. Fawcett DW: A extbook of Histology. Philadelphia: W.B. Saunders, 1986:199-208.

16. Bayly JG, Kartha VB, Stevens WH: The absorption spec- tra of liquid phase H20, HDO and D20 from 0.7 pm to 10 pm. Infrared Physics 1963; 3:211-223.

17. Yannas IV: Collagen and gelatin in the solid state. J Macromol Sci Rev Macromol Chem 1972; 1:49-104.

18. Doyle BB, Bendit EG, Blout E R Infrared spectroscopy of collagen and collagen-like polypeptides. Biopolymers

19. Beaton JD, Charlton TL, Speer R Identification of soil- fertilizer reaction products in a calcareous Saskatchewan soil by infra-red absorption analysis. Nature 1963;

20. Miller FA, Wilkins CH: Infrared spectra and character- istic frequencies of inorganic ions. Anal Chem 1952;

21. Steinert RF, Puliafito CA: “The Nd-YAG Laser in Oph- thalmology.” Philadelphia: W.B. Saunders, 1985:22-35.

22. Mainster MA, Sliney DH, Belcher CD, Buzney SM: Laser photodisruptors: Damage mechanisms, instrument de- sign and safety. Ophthalmology 1983; 90:973-991.

23. Docchio F, Dossi L, Sacchi C A Q-switched Nd:YAG laser irradiation of the eye and related phenomena: An exper- imental study. 11. Shielding properties of laser-induced plasmas in liquids and membranes. Lasers Life Sci 1986;

24. Tobin JM: Laser-induced plasma ablation of biologic tis- sue. Massachusetts Institute of Technology, 1985 (doc- toral thesis).

25. Sinofsky E: Comparative thermal modeling of Er:YAG, Ho:YAG and COP laser pulses for tissue vaporization. SPIE Lasers in Medicine 1986; 712:188-192.

26. Bonner RJ?, Smith PD, Leon M, Esterowitz L, Storm M, Levin K, Tran D: Quantification of tissue effects due to a pulsed Er:YAG laser at 2.9 pm with beam delivery in a wet field via zirconium fluoride fibers. SPIE Optical Fi- bers in Medicine 1986; 713:2-5.

27. Sartori MP, et al. Chemical laser interactions with hu- man cardiovascular tissues. SPIE Laser Interaction with

95: 126-131.

5:108-117.

1975; 14:937-957.

19~1329-1330.

24:1253-1254.

1: 105-116.

9. Thoma J, cowinsk i D, Kastenbauer E R Experimental Tissue 1988; 908:l-9.