INSTITUTE OF LASER MEDICINE

Prof. Dr. P. Hering   

I. Ablation of bonetissue with Q-switch CO₂ laser

II. Ablation of tooth enamel and dentin with Q-switch CO₂ laser

III. Laser transmyocardial revascularisation (TMLR)
 
 

CO₂ laser beam (l = 9.2-10.8 mm) evaporates very effectively biological tissues with a high water content. Therefore it is widely used in surgery of soft tissues and in the dermatology. So-called "super-" or "ultra-pulsed" laser systems are nowadays applied to make this ablation process even more efficient and to reduce a thermal damage of the surrounding tissue. These systems emit pulses of about 1 ms duration with several times higher peak power, as compared to continuous (cw) operation mode. At the high laser light intensity the evaporation front moves into the tissue faster than the heat spreads out, so that the thermal damage is minimal. Nevertheless much shorter than 1 ms laser pulses are necessary in some cases to provide "clean" ablation. Sub-ms laser pulses can be supplied with a special transversal organization of a very fast electrical discharge (TE systems). The TE CO₂ lasers are however not very reliable due to extremely intense electrical discharge.

 

We use a mechanically Q-switched CO₂ laser instead. It is based on an industrial cw CO₂ laser with very long life-time and service intervals. The system is modernized by us with a fast mechanical Q-switch (Fig.1). The resulting laser pulses are of 250-400 ns duration (FWHM). The pulse repetition rate, f, could be adjusted from 100 Hz up to several hundreds of kHz. The pulse energy, E, amount up to 80 mJ at f = 300 Hz in a nearly Gaussian mode (M² = 1.05-1.1). Even better output parameters are conceivable adapting the Q-switch to a modern RF-excited CO₂ laser with ultra-pulsing option. Obvious advantages of our system comparing to TE CO₂ laser are very high reliability and repetition rate, very good mode quality and absence of an electrical interference.
 

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I. Ablation of bones with Q-switch CO₂ laser. 

A heavy thermal damage happens usually by attempts to cut bones with a CO₂ laser. Because of the small water content most of the CO₂ laser energy is absorbed by very hard hydroxiapatite crystallites (melting temperature is of 1500°C !). Moreover the very short thermal relaxation time (several m s) leads to a fast heat diffusion out of the initial absorption volume. Uncarbonized ablation of the hard tissues with CO₂ laser is conceivable only by very intense and short (t £ 1 m s) pulses.

Our in vitro experiments show, that a direct application of even such short pulses does not allow to produce really deep uncarbonized ablation craters in bones. We succeed, however, to cut bones very clean and effective introducing additionally a water-spray system to maintain an optimal H₂O content in the ablated layer and a fast beam scanning system. Basic ablation characteristics have been measured: the ablation threshold of 2-3 J/cm², specific ablation energy density of 20 J/mm³ at the optimal pulse energy density of 25 J/cm². With the pulse energy of 15-20 mJ and f = 10 kHz we produce a clean cut of 60-90 m m depth per pass in a hard bone compacta moving the beam with a velocity of 30 cm/sec. The ablation is accompanied with a strong acoustic signal, which could be used for the treatment control. These results are very stimulating and surpass the best published till now data (Er:YAG laser, l = 2.94 mm).

M. M. Ivanenko, P. Hering, "Wet bone ablation with mechanically Q-switched high-repetition-rate CO₂ laser.", Appl. Phys. B 67, 395-397 (1998). You can download this paper here: bone_abl.pdf (144K). 

M. M. Ivanenko, P. Hering, "Hard tissue ablation with a  mechanically Q-switched CO₂ laser.", Thermal Therapy, Laser Welding and Tissue Interaction, SPIE Proc. 3565 28, pp. 110-115 (1998).
You can download this paper here: hard_tis.pdf (66K). 

O. Kuhne, "Photoablation an Hartgewebe", Diploma thesis, Fachhochschule Aachen Abteilung Jülich, Germany, 1998.
You can download this paper here: kuhne.pdf (1176K). 

T. Mitra "Ablation biologischen Hartgewebes mit gepulsten IR-Lasern", PhD thesis, University Düsseldorf, Germany, 2002.
You can download this paper here: mitra.html (13000K).

(The free Adobe(R) Acrobat(R) Reader allows you to view, navigate, and print PDF files across all major computing platforms.) 

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II. Ablation of tooth enamel and dentin with Q-switched CO₂ laser.  
in cooperation with
Westdeutsche Kieferklinik 
Poliklinik für zahnärztliche Chirurgie, Heinrich-Heine-Universität.
Tooth enamel is most hard human tissue and extremely difficult object for laser ablation. Preliminary experiments with 400 ns pulses of mechanically Q-switch CO₂ laser and use of the water-spray/scanner system demonstrate free from charring and mechanical fissures ablation. A systematic study of tooth material ablation with sub-ms CO₂ laser is under progress.


 

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III. Laser transmyocardial revascularization (TMLR).   in cooperation with
Klinik für Thorax- und kardiovaskuläre Chirurgie   
During the TMLR procedure 30-50 thin channels are "drilled" with a laser beam through the myocardium. These channels supposedly promotes the blood perfusion into the damage myocardium region and relief angina symptoms. A very powerful (800 W) quasi-cw CO₂ laser is usually applied to drill the transmyocardial channels at a beating heart. Several disadvantages of this system have become evident during last time. It is very large and cumbersome. A surplus of the laser pulse energy is necessary to "drill" the channel with certainty. The extent of the thermally damaged zone around the channel is at least 0.2 – 0.3 mm, and can be essentially larger according to some reports.

Smaller thermal damage and more efficient ablation could be attained presumably with a burst of short sub-msec pulses of the Q-switched CO₂ laser. Current experiments with soft biological tissues are intended to clarify, how much the ablation efficiency can be improved and the thermal damage reduced, as compared to quasi-cw or super-pulsed CO₂ laser systems.
 
M. M. Ivanenko, P. Hering, M. Klein, E. Gams, "Transmyocardial laser revascularization: Are new approaches with new lasers possible?",  In: "TMLR: Management of Coronary Artery Disease", Berlin, Heidelberg: Springer Verlag ed. M. Klein, H. D. Schulte, E. Gams, 1998, pp. 153-164 

You can download this paper here:  tmlr.pdf (81K). (The free Adobe(R) Acrobat(R) Reader allows you to view, navigate, and print PDF files across all major computing platforms.) 
 

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