Abstract
The objective is to characterize the impact of different ablation parameters on the thermal load during corneal refractive surgery by means of excimer laser ablation on porcine eyes. One hundred eleven ablations were performed in 105 porcine eyes. Each ablation was recorded using infrared thermography and analyzed mainly based on the two tested local frequencies (40 Hz, clinical local frequency; 1000 Hz, no local frequency). The change in peak corneal temperature was analyzed with respect to varying ablation parameters [local frequency, system repetition rate, pulse energy, optical zone (OZ) size, and refractive correction]. Transepithelial ablations were also compared to intrastromal ablations. The average of the baseline temperature across all eyes was 20.5°C±1.1 (17.7°C to 22.2°C). Average of the change in peak corneal temperature for all clinical local frequency ablations was 5.8°C±0.8 (p=3.3E-53 to baseline), whereas the average was 9.0°C±1.5 for all no local frequency ablations (p=1.8E-35 to baseline, 1.6E-16 to clinical local frequency ablations). A logarithmic relationship was observed between the changes in peak corneal temperature with increasing local frequency. For clinical local frequency, change in peak corneal temperature was comparatively flat (r 2 =0.68 with a range of 1.5°C) with increasing system repetition rate and increased linearly with increasing OZ size (r 2 =0.95 with a range of 2.4°C). Local frequency controls help maintain safe corneal temperature increase during excimer laser ablations. Transepithelial ablations induce higher thermal load compared to intrastromal ablations, indicating a need for stronger thermal controls in transepithelial refractive procedures.
Highlights
Laser-based refractive surgery techniques are constantly being developed with a common aim of optimizing the surgery outcomes in terms of visual acuity, contrast sensitivity and night vision
The change in peak corneal temperature for the ablations performed with clinical local frequency was on an average 5.8°C Æ 0.8 (p 1⁄4 3.3E − 53 to baseline); whereas 9.0°C Æ 1.5 was the average for the ablations performed with no local frequency (p 1⁄4 1.8E − 35 to baseline, 1.6E − 16 to clinical local frequency ablations)
For the clinical local frequency, the highest change in peak corneal temperature of 5.8°C was observed versus 11°C for no local frequency and 2.7°C for 10-Hz local frequency
Summary
Laser-based refractive surgery techniques are constantly being developed with a common aim of optimizing the surgery outcomes in terms of visual acuity, contrast sensitivity and night vision. Sophisticated algorithms and technological advancements help realize new features in refractive surgery like shorter treatment times, distributed thermal load with optimized spot patterns,[1,2] calibration systems,[1,3] precise eye tracking, and optimized ablation efficiency at non-normal incidence.[4,5] Spatial distribution of laser pulses on the cornea is controlled such that each pulse sequentially ablates a small amount of corneal tissue, collectively etching a very refined pattern calculated and designed to counterbalance the aberrations. Ultraviolet (UV) radiation commonly used in laser refractive surgeries is regarded as “cold” radiation. Even these UV laser pulses impart a certain thermal load to the cornea tissue seen as an increase in ocular surface temperature (OST), observed clinically[7,8,9] as well as in laboratory settings.[10,11] smaller laser spots are used along with the higher system repetition rates controlling the treatment time in most modern refractive surgery systems; each of these factors
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