Femtosecond laser corneal damage thresholds at 1540 nm and 2000 nm

Abstract

Recently, pulsed lasers with ultrashort pulse durations have become ubiquitous in a variety of applications, including medical procedures such as laser eye surgery. These sources are capable of generating extremely high peak powers that can cause laser-induced tissue breakdown upon exposure. However, current laser safety standards do not provide exposure limits for wavelengths longer than 1400 nm and pulse durations shorter than 1 ns due to a lack of biological data. Instead, the recommendation is to limit the peak irradiance to the maximum permissible exposure (MPE) limits applicable to 1 ns pulse durations. We applied femtosecond laser pulses of varying energies at 1540 nm and 2000 nm to corneas of anesthetized rabbits. We used slit lamp biomicroscopy and optical coherence tomography to examine the exposure sites and determine the presence or absence of visible lesions 1 h and 24 h post-exposure. The dose-response data correlating the presence or absence of any alteration of the corneal surface to the pulse energy 1 h post-exposure was evaluated using probit analysis to extract the median effective dose (ED₅₀) corresponding to the cornea damage threshold. We compared our results to the MPE limits applicable to 1 ns pulse durations and determined that the current safety standard procedures are not adequate to evaluate small diameter single pulse femtosecond exposures at 1540 nm and 2000 nm. The results of this study contribute to the knowledge base used for setting laser safety standards in the near infrared range for ocular exposure to ultrashort pulses.