Online optical coherence pachymetry to evaluate intraoperative ablation parameters in LASIK

Abstract

The ablation of corneal tissue with the excimer laser can be variable and can lead to miscorrections. The purpose of this study was to evaluate intraoperative ablation parameters during laser-assisted in-situ keratomileusis (LASIK) with online optical coherence pachymetry (OCP). In a prospective, nonrandomized, comparative clinical study, the ablation parameters were continuously assessed intraoperatively with online OCP (Heidelberg Engineering, Lübeck, Germany) in 45 myopic and 10 hyperopic LASIK treatments. The central intraoperative ablation values were compared with the calculated values of the excimer laser (ESIRIS, Schwind, Germany) and the postoperative refraction. The ablation process and the ablation rate in μm per layer, time, and dioptric correction were evaluated in myopic corrections. In myopic LASIK treatments, a linear ablation process was measured with a mean correlation coefficient of −0.968 ± 0.04. The intraoperative ablation rate was, on average, 0.59 ± 0.17 μm per layer, 1.45 ± 0.48 μm per second, and 24.63 ± 7.81 μm per corrected diopter. These values were 28.7% to 29.6% higher (P < 0.001) than the calculated values. There was a significant correlation (P < 0.001) for the ablation rate per layer (r = 0.823), per second (r = 0.869), and corrected diopter (r = 0.892), but no correlation (r = 0.21, P = 0.239) between the measured linear ablation process and the postoperative refraction. During hyperopic LASIK treatments, without ablation of the corneal center, there was a significant decrease (P = 0.005) of the stromal thickness by 18.34 ± 14.13 μm, which corresponded to a mean corneal dehydration rate of 0.27 μm per second. Online OCP allowed a clinical evaluation of intraoperative ablation parameters in LASIK. Further studies are needed to assess a possible active control of the excimer laser ablation from these continuous values, which could possibly improve current ablation nomograms.