Abstract
Purpose::
The primary aim of laser eye surgery is to correct refractive errors caused by an irregularly shaped eye. During photorefractive keratectomy (PRK), the cornea is reshaped using a 193nm Eximer laser which ablates micron-thick layers of tissue from Bowman’s layer and the anterior stroma. However, there are concerns about the practicality and safety of the Excimer laser. Here we compare the temperature changes and cellular outcomes of a solid state 213nm (5th Harmonic) Nd:YAG laser to those of the 193nm Excimer system.
Methods::
New Zealand White rabbits underwent PRK with -5 diopters, and a 6.5mm optical zone. The surface of the eye was filmed with a thermal camera throughout the procedure. Following sacrifice at 1 and 30 days (n=4-5 per group), corneas were dissected, fixed in 4% paraformaldehyde and cryosectioned at 10 µm. Total cells were counted using propidium iodide and cell death was detected by TUNEL. Myofibroblasts were identified by anti-smooth muscle actin at 30 days.
Results::
The mean temperature at the ablation site was significantly higher during irradiation with the 213nm laser (35.6oC +/- 0.90; n= 158) compared to the 193 nm laser (34.3oC +/- 0.77; n= 261). At 1 day, cell numbers were significantly increased in the ablation crater of 193nm (171.66 cells/0.5mm2 s.e. 2.86; p<0.05) compared to 213nm laser treated tissue (112.56 cells/0.5mm2 s.e. 0.86) and unlasered controls (137.22. cells/0.5mm2 s.e. 7.47). The number of apoptotic cells was similar, although greater variability was observed with the 193nm (9.54 - 217.83 cells/0.5mm2) compared to the 213nm laser (0-63.6 cells/0.5mm2). Cell numbers (total and apoptotic) were not significantly different from normal at 30 days for either laser. Myofibroblast infiltration was similar for both lasers at 30 days.
Conclusions::
Our results are consistent with minimal scarring and good healing described in previous studies. In addition, the reduced cell numbers in the ablation crater and smaller variability in cell death suggest that the 213nm laser may provide a more predictable visual outcome.
Keywords: refractive surgery: PRK • cornea: stroma and keratocytes • laser