Hyperguides

Ophthalmology / Refractive surgery

The Erbium Laser

Neda Shamie, MD
Article


Introduction

Excimer lasers used in photorefractive keratectomy (PRK) and laser in situ keratomileusis (LASIK) emit a wavelength of 193 nm.1 The concerns over these ultraviolet (UV) laser systems include high investment and maintenance costs, as well as the risk of actinic damage and the use of toxic gases in a clinical environment.2 Advantages of a solid-state laser such as an Er:YAG laser include low maintenance and uncritical handling and therefore render its use appealing in a clinical environment, especially in photorefractive surgery. Er:YAG lasers that emit a wavelength of 2.9 mm were explored a few years after the ruby laser was invented by Maiman in 1960; however, technical advancements were made to the Er:YAG lasers only during the past 10 years.2

Clinical Data

The technically improved Er:YAG laser and its high absorption of water at a wavelength of 2.94 mm suggest the capability for mid-infrared photorefractive surgery. The first experiments on mid-infrared corneal photoablation were made using the Raman-shifted Nd:YAG laser at wavelengths of 2.80 mm and 2.92 mm and the hydrogen fluoride laser with wavelengths of 2.6 mm to 2.9 mm. In vivo experiments were performed in rabbits and primates to compare wound healing after excimer laser keratectomy and Er:YAG laser treatment. Peyman and colleagues reported a study with 10 albino rabbits.3 In 10 eyes (group 1) the epithelium was removed by mechanical abridgement to ablate the central anterior stoma (a 3.5-mm wide, 180-mm deep area) using an Er:YAG laser with a pulse duration of 200 msec. Laser treatment in the midstroma (300 mm deep) was performed in a second group (10 eyes) after a central corneal cap (thickness 0.15 mm) was removed. The epithelial wound healing in group 1 occurred in 2 to 3 days and a mild haze was observed with a slit lamp in both groups. Histological specimens were obtained immediately and at 2 and 6 months after the Er:YAG laser treatment. More prominent thermal damage (<40 mm) was reported at the borders of the ablated area. The epithelia in group 1 appeared normal except for the thickening at the margins of the ablated areas. The healing occurred with some degree of disorganization of the lamellae and an increase in the population of the keratocytes.

Bilateral PRK treatment was performed in monkeys by Kahle and colleagues.4 One eye was treated with an excimer laser and the fellow eye with a free-running Er:YAG laser. Immunohistology revealed no significant differences in wound healing pattern and there was a trend toward a faster healing after Er:YAG treatment.

In two blind eye studies, no adverse reactions were reported and the corneal haze was comparable with that after excimer laser treatment with the free-running Er:YAG. Jean and colleagues5 reported a rough surface with uneven but undelayed epithelialization after fundamental mode photoablation using a free-running Er:YAG laser. Subepithelial reactions were comparable to 3+ haze and showed bubble-like inclusions, which slowly disappeared. Four weeks postoperatively, a subepithelial 1+ haze was recorded. Thermal damage for the long free-running laser pulses ranged from 10 mm to 50 mm.

Conclusion

The solid-state Er:YAG laser emitting a wavelength of 2.94 mm appears to fulfill many, if not all, requirements for PRK. Currently, this technology is not approved for clinical use. Further experimental and clinical studies will help better delineate the predictability and the utility of the Er:YAG laser for refractive surgery.6,7

References

  1. Seiler T, McDonnell PJ. Excimer laser photorefractive keratectomy. Surv Ophthalmol. 1995;40:89-119.

  2. Seiler T, Mrochen M. Erbium Laser. In: Wu HK, et al, eds. Refractive Surgery. New York, NY: Thieme Incorporated; 1999: 461-466.

  3. Peyman GA, Badaro RM, Khoobehi B. Corneal ablation in rabbits using an infrared (2.9 micron) erbium:YAG laser. Ophthalmology. 1989;96:1160-1170.

  4. Kahle G, Zu Kaqun, Seiler T, Schroter-Kermani C, Wollensak J. Wundeheilung der kornea nach flachiger keratectomy: Er-YAG excimer. Fortschr Ophthalmol. 1991;88:380-385.

  5. Jean B, Kriegerowski M, Matallana M, Bende T. Correction of myopia with Er:YAG laser fundamental mode photorefractive keratectomy. J Refract Surg. 1995;11:392-396.

  6. Mrochen M, Meinhard FP, Semshichen V, Funk RW, Seiler T. Photoablation of the cornea with the erbium:YAG laser. Ophthalmology. 1999;96:387-391.

  7. Mrochen M. Semshichen V, Funk RH, Seiler T. Limitations of erbium:YAG laser photorefractive keratectomy. J Refract Surg. 2000;16:51-59.