An Er:YAG laser has a very special application in medicine. Its emission wavelength (2.94 micrometers) matches exactly the main water absorption peak in the IR range [1]. Therefore, the photothermic ablation threshold for the irradiated biological tissue is lower than for radiation of other wavelengths.

The Er:YAG laser system (Er:YAG crystal with a diameter of 4 mm and length equal to 100 mm) generating a radiation in a free-running, long-pulse mode regime with the output energy up to 610 mJ and a wavelength in a mid-infrared region was used for ophthalmic micro-surgery.

As a delivery system a cyclic olefin polymer-coated silver hollow glass waveguide was used [2]. The delivery system was ended either by focusing optics (CaF2 lens with a focusing length of 30 mm) or by a special sapphire tip (a diameter of 1 mm and a length of 15 mm), for a non-contact and contact treatment, respectively. The output energy from those systems was in the range of 5 mJ up to 280 mJ.

The experiment was divided into some parts connecting with the type of an eye tissue. The corneal ablation and perforation of a sclera tissue were examined for the energies from 5 up to 100 mJ, 10 pulses for each sequence in the case of non-contact preparation. When 1 pulse was used the threshold energies for the ablation of the sclera were measured to be 122 mJ (24.3 J/cm2). For the contact preparation the minimal energy was 270 J (34.4 J/cm2) for used 5 pulses. The threshold number of pulses for the contact perforation of the sclera tissue was 3-5 with the maximum possible energy 270 mJ. The threshold energy for the iris ablation was measured to be 10 mJ (1.3 J/cm2) for 1 pulse in the contact method. The lens was irradiated by the dose 50 mJ energy (4 pulses).

The results show the usability of this mid-infrared radiation in ophthalmic surgery.

References

[2] Shi,Y.W. - Hanamoto,K. - Wang,Y. - Miyagi,M.: New cyclic-olefin polymer-coated silver hollow glass waveguides for the near-infrared to mid-infrared BiOS Europe´98, Abstract book p.82, Paper No. 3570-01, Stockholm, 1998

This research has been supported by the grant of the Czech Grant Agency No. 102/99/1250.

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