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Lasers in Surgery and Medicine 37:144148 (2005)

In Vivo Animal Trials With a ScanningCO2 Laser Osteotome

Mikhail Ivanenko,1* Robert Sader,2,3 Said Afilal,1 Martin Werner,1 Martina Hartstock,3

Christian von Hanisch,5 Stefan Milz,4 Wolf Erhardt,5 Hans-Florian Zeilhofer,2,3 and Peter Hering1,6

1center of advanced european studies and research, 53175 Bonn, Germany2Division of Cranio-Maxillo-Facial Surgery, Clinic for Reconstructive Surgery, University Hospital,

4031 Basel, Switzerland3Center of Advanced Studies in Cranio-Maxillo-Facial Surgery, Klinikum Rechts der Isar,

University Hospital, 81675 Munich, Germany4Anatomical Institute, LMU University of Munich, 80336 Munich, Germany5Institute of Experimental Oncology and Therapy Research, Klinikum Rechts der Isar,

University Hospital, 81675 Munich, Germany6Institute of Laser Medicine, University of Duesseldorf, 40225 Duesseldorf, Germany

Background and Objectives: We report first results of

animaltrialsusing an improved laser osteotomy technique.

This technique allows effective bone cutting without the

usual thermal tissue damage.

Study Design/Materials and Methods: A comparative

invivostudy onmandibles ofseven canines was donewitha

mechanical saw and a CO2 laser based osteotome with a

pulse duration of 80 microseconds. The laser incisions were

performed in a multipass mode using a PC-controlled

galvanic beam scanner and an assisting water spray.

Results: A complete healing through a whole bony rear-

rangement of the osteotomy gap with newly build lamellar

Haversian bone was observed 22 days after the laseroperations under optimal irradiation conditions.

Conclusions: An effective CO2 laser osteotomy without

aggravating thermal side effects and healing delay is

possible using the described irradiationtechnique. It allows

an arbitrary cut geometry and may result in new advanta-

geous bone surgery procedures. Lasers Surg. Med. 37:144

148, 2005. 2005 Wiley-Liss, Inc.

Key words: laser osteotomy; CO2 laser; animal trials

INTRODUCTION

Noncontact cutting of bone tissue without mechanical

stress is an old dream of surgeons and a laser beam is an

instinctive choice for this. It can be tightly focused and itsposition controlled electronically, so that precise narrow

cuts with sophisticated geometry can be exactly planned

and performed. Early attempts to cut bones with lasers

have failed, however, because of strong thermal sideeffects

in case of continuous wave (cw) and long-pulsed carbon

dioxide (CO2) lasers [15] or very low cutting rate with

excimer lasers [6 9]. One third of the compact bone tissue

volume consists of hard minerals, which melt only above

1,0008C [10,11]. The minerals are embedded on the other

side in a sensitive collagen matrix, which will be charred

after evaporation of the internal bone liquid and tempera-

ture rise above 1508C. Living bone cells will be irreparably

damaged at even lower temperatures. The common laser

cutting, as known from material processing, is therefore

unsuitable for bone surgery.

A successful laser osteotomy has to be based on an effec-

tive tissue ablation process, which does not takes place at

very high temperatures and is much faster than the heat

diffusion in the bone. Little thermal damage has been

demonstrated, for example, in ex vivo studies with short

CO2 laser pulse durations of 0.12 microseconds [1214].

With an additional use of an air-water spray, the collateral

thermal damage remains small even after prolonged tissue

irradiation with such laser systems [15]. The ablation rateis, however, not high for such short pulses (510 mm/pulse,

see refs. above).

A relatively fast and clean tissue removal (up to 100mm/

pulse) is possible with 100500 microseconds pulses of an

Er:YAG laser at the wavelength of 2.94mm [1620]. A zone

of thermal necrosis after Er:YAG laser incision in cortical

bone is small, 2050 mm, according to [2123], so that

healing observed in animal trials is similar to one after the

use of a mechanical saw [22].

Essential details of the bone ablation mechanism are

probably similar for Er:YAG and CO2 lasers. At high in-

tensity of the light pulse, the energy will be accumulated

very quickly in the tissue and confined initially in the form

of heat in a very thin absorption layer. That overheatedlayer explodes after high internal pressure is built up

(ultimate tensile strength is about 1,000 bar for compact

Robert Saders present address is University of Frankfurt,Klinikum fur Kiefer- und Plastische Gesichtschirurgie, Theodor-Stern-Kai 7, 60596 Frankfurt am Main, Germany.

*Correspondence to: Dr. Mikhail Ivanenko, caesar, Ludwig-Erhard-Allee 2, 53175 Bonn, Germany.E-mail: [email protected]

Accepted 20 May 2005Published online 29 August 2005 in Wiley InterScience(www.interscience.wiley.com).DOI 10.1002/lsm.20207

2005 Wiley-Liss, Inc.


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bone [24]). If the pressure built-up time is shorter than

thermal relaxation time of the tissue, then most of the

energy is consumed for this thermomechanical ablation

process and removed from the tissue together with the hot

ablation products. The water spray prevents tissue parch-

ing and cools it additionally.

A very promising way to avoid an accumulation of a restheat in the bone tissue is the multipass cutting. A fast

repeated motion of the focused beam along a predefined

cut trajectory can be realized conveniently using a PC-

controlled galvanic beam scanner. We use this technique,

since it allows an effectivecleanosteotomywith relatively

long (about 100 microseconds) powerful CO2 laser pulses,

at pulse repetition rates of several hundreds hertz and

average laser power of several tens of watts [25]. For

example, at the pulse energy of 68 mJ, every laser pulse

removes up to 500 mm of compact tissue, so that at

the average laser power of 40 W, we reach a cut rate of

40 mm/minute for a 6-mm deep and 0.2-mm broad incision.

Ex vivo incisions with such a laser system have previously

been examined histologically [26]. The incisions were notaccompanied with carbonization. At thecut border, a 50-mm

broad zone with in part empty cell lacunae and damaged

osteocytes was presented. Intact osteocytes were, however,

observed also very near to the cut surface. Polarized light

microscopy showed no alterations in the inorganic struc-

ture of the bone at the cut borders.

In the present report, we describe first experience of

in vivo application of this scanning laser osteotomy tech-

nique and preliminary results of the operations on seven

beagle canines. A complete report on the healing process

and a detailed evaluation of the histological examinations

will be given later.

MATERIALS AND METHODS

In the animaltrials, a prototype CO2 laser osteotome was

used, which was developed by the center of advanced

european studies and research (caesar) in Bonn. The

laser osteotome is based on a RF-excited slab CO2 laser

system with wavelength of 10.6 mm. Laser pulses of 80 mJ

energyand of 80 microseconds duration aredeliveredto the

application site through an articulated mirror-arm with an

action radius of 1.5 m. The preoperative planning of

the incision and control over all the functions of the

osteotome are done with a PC. Exact positioning and

motion of the beam is fulfilled with a PC-controlled galvanic

X-Y-scanner. The scanner is mounted on a 5-axis mechan-ical adjustment unit, which is fixed at the operation stand

near patient. The unit is used for initial manual focus

positioning andcut orientation relative to thebone.Two red

pilot beams are helped by the adjustment: one of them is

collinearwith theCO2 laser beam; theotherone crosses it in

the focus plane 159 mm below the scanner. The focusing

optic provides a spot diameter of about 200mm on the tissue

(1/e2 intensity level) and is protected against ablation

products, blood, and water droplets with a pressurized

airflow. Two fine spray-nozzles are mounted sideward to

irrigate the incision continuously with isotonic NaCl solu-

tion during the osteotomy (throughput about 3 ml/minute

by every spray). The velocity of the beamfocus on the tissue

in the multipass osteotomy was 480 mm/second.

To assess the in vivo healing of bone in response to the

laser osteotomy versus a conventional mechanical saw, we

carried out a comparative study on the mandibles of seven

beagle canines. The mechanical incisions were done with aOsseoscalpel micro saw with oscillating hub according to

Sachse and blades of 0.35-mm thickness (Medicon eG,

Tuttlingen, Germany).

The canines were chosen, because their mandible con-

tains, like in humans, a central artery with centripetal

nutrition and homogenous spongiosa architecture. Fromits

size, form, and hardness, the bone is comparable with the

human one. Also the physiological reaction is similar, as

secondary osteons can be detected during bone healing

process.

At the beginning of the study, four male and three

female canines, from breeding facilities of the German

National Research Center for Environment and Health,

were between 2 and 8 years old and 12.519 kg heavy.The canines were accommodated in groups of up to three

animals in 12 m2 large ventilated double boxes. Also an

access to walking area was granted to them. The tempera-

ture in the areas was 19238C and relative air humidity

50 70%. The illumination changed automatically between

day and night phases. The animals were inoculated,

dewormed, and free from specific pathogens. Feeding took

place once daily. There was no feeding 18 hours prior to the

operation.

The animal head was fixed during the operation with

non-invasive headrest pad and tape. A 58 mm long laser

incision and similar in geometry saw incision were made in

pairs in the ventral aspect of the lower margin of both leftand right mandibles of each canine under general anesthe-

sia. Thedistance between thelaser andsaw incision at each

mandible side was about 10 mm. To avoid laser damage

of the soft tissue, a metal shield was placed behind

the bone during the irradiation. Two animal groups were

built randomly to evaluate different irradiation settings. A

special scan procedure [27] has been applied to dilate the

laser cut and so to increase the cutting efficiency in the first

group. The beamfocus was positioned 6 mmbelow the front

bone surface in this case. The pulse repetition rate was

400 Hz and the duration of the irradiation varied between

45 and 90 seconds, depending on the cut length and

thickness of mandibles (710 mm). In the second group,

narrow cuts without the dilatation at the repetition rate of200 Hz and the focus position 2 mm below the bone surface

were done. The cut duration was 60155 seconds. Seven

laser incisions were done in every group: three canines

belong to the first group, three to the second and one animal

became dilated laser incision at the left mandible side

and narrow incision at the right one (e.g., belongs to both

groups). Both types of laser cutting procedure were at first

carefully tested and optimized in ex vivo models. The used

slit dilation technique allows to shorten the cut time about

1.5 times for 8-mm thick mandible and to fulfill deeper

incisions. The optimal positioning of the beam focus

SCANNING CO2 LASER OSTEOTOME 145


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beneath the bone surface brings further improvement in

the cut rate. There was no difference between the two

animal groups in relation to saw incisions.

During the healing period, three different intra-

vital stains were injected one after the other to mark

the surface of new-bone growth for fluorescence micro-

scopy. By this, after the animals were sacrificed 22 daysfollowing the surgery, the tissue response was examin-

ed by polychrome sequential labeling and undecalcifi-

ed paragon-stained ground sections. In addition, contact

radiographic and micro-radiographic examinations were

performed.

The study met the requirements of the German law for

the animal protection and was approved from the District

Government of Upper Bavaria.

RESULTS

The laser osteotomy was in general unproblematic. Small

bleeding did not prevent the cutting, while the blood was

blown out of the incision with the spray and due to theexplosion-like action of repetitive laser pulses. Only a

relativelystrong bleeding hasled once to the fail of thelaser

procedure. In one other case, the lasing has been inter-

rupted, because the canine twitched.

In the first group, the dilated incisions, like in Figure 1a,

with an entry width of 1 mm were performed. They were

wedge-shaped, that is, the cut slit at the exit site was 3

4 times narrower as at the entry site. By the very first

incision, a carbonization at the medial end of the cut

channel occurred. The thickness of the carbonized zone

amounted to about 300 mm. It was caused by a strong back

light reflection from the dorsal metal shield, which was

positioned in direct contact with the jaw. To avoid this, weused in further operations, in both animal groups, metal

plates with enhanced absorption and more homogeneous

light scattering, which were positioned 2 3 mm apart from

the bone. No carbonization was found in this case. As an

interesting subsidiary result, it was observed that it did not

come to a callus formation by the first laser group. It can be

interpreted as a missing induction of a secondary fracture

healing and associated with relatively large width of the

dilated laser incisionsas compared to thesaw incisions with

0.5 mm width.

In the second group, we have reduced the laser cut width

down to 0.20.3 mm at the entry side (Figs. 1b and 2a).

The duration of the cuts increased up to 70%. There were

no visible traces of tissue carbonization immediately afterthe irradiation. Theincision boarders were clean and plain.

Twenty-two days post-operative, the histological speci-

mens did not show any noticeable thermal damage at the

border of the laser cuts (Fig. 2). A complete healing through

a whole bony rearrangement of the osteotomy gap with

newly build lamellar Haversian bone was observed in this

laser group. Scanning acoustic microscopy of the laser

incision (Fig. 1c) confirmed that the newly built bone pos-

sessed elastomechanical properties, which are very similar

to the neighboring bone tissue. Contrary to this, the sawed

mandibles demonstrated persistent superficial defect at

Fig. 1. a: Examples of 1-mm broad laser incisions in canine

mandible (ex vivo) with a special cut slit dilatation procedure.

b: Narrow (0.25 mm) in vivo incision in canine mandible with

the scanning CO2 laser osteotome. c: Scanning acoustic

microscopy of the laser incision (b) 22 days postoperative.

Arrowsindicate the cut margins. The incision gap is completely

bridged, its acoustic properties indicate that the newly built

bone is similar to neighboring bone tissue.

146 IVANENKO ET AL.


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the impact site and steady delay of the healing process. The

saw-osteotomy gaps were not completelybony bridged after

the 22 days.

DISCUSSION

The results of these trials prove that effective osteotomywithout aggravating thermal side effects is possible with

a pulsed CO2 laser under optimal irradiation conditions.

These conditions include pulse duration of 80 microse-

conds, use of an air-water spray, and fast multipass beam

scanning along the cut trajectory. The scanning velocity

hastobehighenoughtoshiftthebeamfocusonthetissueat

half of its diameter or more in the time between consequent

laser pulses. Such a multipass cutting cannot be done per

hand because of the fastness and very high demands on the

reproducibility of the beam focus position and beam orien-

tation at the every pass.

In most other in vivo experiments with CO2-lasers re-

ported so far, carbonization and an extended necrosis zone

in the treated bones have been described. For example

Horch et al. stated early that the CO2 laser osteotomy is not

promising because of the collateral thermal damage [4,28].

They worked with a continuous cw CO2 laser with 34 W of

average power. A gas jet has been used for cooling. Severecarbonization has led to an impairment of the healing

process of more than 12weeks. The bad results of these and

other authors [15] were responsible for the fact that most

research groups were looking for a new laser system for

osteotomy. Actually, more understanding of the ablation

process and careful optimization of the irradiation para-

meters were necessary. By this, the average laser power

used before was quite similar to that we used now. The

reported irradiation technique provides, however, more

effective ablation and prevents accumulation of the rest

heat in the tissue.

The experiences of this trial also show that further

improvements are necessary to grant an acceptance of the

laser in the field of osteotomy. One important goal will be aprevention of the laser action on tissue behind the bone. In

the present study, it was done with a small shielding metal

plate. An optimal way, however, is recognition of a bone-

soft-tissue interface by changes in the acoustical signal

[29,30] or by changes in optical emission accompanying the

ablation process. Theentire laser systemhas also to become

more surgeon-friendly and compact. Special surgeon-

oriented software and combination with robotic or haptic

guidance and computer-assisted navigation will make the

positioning of the beam on the tissue much easier. The

absence of a tactile feedback with the present laser tech-

nique makes this especially important. If these improve-

ments are done, the laser osteotomy may result in new

Fig. 2. Typical histological slices of (a) laser incision (10) and

(b) saw osteotomy (2.5). Canine 4, right mandible side,

undecalcified, paragon-stained.

Fig. 3. An example of a self-stabilizing laser incision in

compact bone tissue (ex vivo).

SCANNING CO2 LASER OSTEOTOME 147


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advanced bone surgery techniques, for example, a self-

stabilizing osteotomy in Figure 3. Such a complicated and

precise incision would never be possible with a hand-held

system.

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