Christian Wuellner

Effect of time sequences in scanning algorithms on the surface temperature during corneal laser surgery with high-repetition-rate excimer laser.

PURPOSE: To investigate the influence of temporal and spatial spot sequences on the ocular surface temperature increase during corneal laser surgery with a high‐repetition‐rate excimer laser. SETTING: Institute for Refractive and Ophthalmic Surgery, Zurich, Switzerland, and WaveLight AG, Erlangen, Germany. METHODS: An argon–fluoride excimer laser system working at a repetition rate of 1050 Hz was used to photoablate bovine corneas with various myopic, hyperopic, and phototherapeutic ablation profiles. The temporal distribution of ablation profiles was modified by 4 spot sequences: line, circumferential, random, and an optimized scan algorithm. The increase in ocular surface temperature was measured using an infrared camera. RESULTS: The maximum and mean ocular surface temperature increases depended primarily on the spatial and temporal distribution of the spots during photoablation and the amount of refractive correction. The highest temperature increases were with the line and circumferential scan sequences. Significant lower temperature increases were found with the optimized and random scan algorithms. CONCLUSIONS: High‐repetition‐rate excimer laser systems require spot sequences with optimized temporal and spatial spot distribution to minimize the increase in ocular surface temperature. An ocular surface temperature increase will always occur depending on the amount of refractive correction, the type of ablation profile, the radiant exposure, and the repetition rate of the laser system.

Influence of spatial and temporal spot distribution on the ocular surface quality and maximum ablation depth after photoablation with a 1050 Hz excimer laser system

PURPOSE: To investigate the effect of temporal and spatial distributions of laser spots (scan sequences) on the corneal surface quality after ablation and the maximum ablation of a given refractive correction after photoablation with a high‐repetition‐rate scanning‐spot laser. SETTING: IROC AG, Zurich, Switzerland, and WaveLight AG, Erlangen, Germany. METHODS: Bovine corneas and poly(methyl methacrylate) (PMMA) plates were photoablated using a 1050 Hz excimer laser prototype for corneal laser surgery. Four temporal and spatial spot distributions (scan sequences) with different temporal overlapping factors were created for 3 myopic, 3 hyperopic, and 3 phototherapeutic keratectomy ablation profiles. Surface quality and maximum ablation depth were measured using a surface profiling system. RESULTS: The surface quality factor increased (rough surfaces) as the amount of temporal overlapping in the scan sequence and the amount of correction increased. The rise in surface quality factor was less for bovine corneas than for PMMA. The scan sequence might cause systematic substructures at the surface of the ablated material depending on the overlapping factor. The maximum ablation varied within the scan sequence. CONCLUSIONS: The temporal and spatial distribution of the laser spots (scan sequence) during a corneal laser procedure affected the surface quality and maximum ablation depth of the ablation profile. Corneal laser surgery could theoretically benefit from smaller spot sizes and higher repetition rates. The temporal and spatial spot distributions are relevant to achieving these aims.

Safety, efficacy, predictability and stability of laser in situ keratomileusis (LASIK) with a 1000-Hz scanning spot excimer laser.

Purpose:  To evaluate the safety, efficacy, predictability and stability of laser in situ keratomileusis (LASIK) with a 1000‐Hz scanning spot excimer laser (Concept System 1000; WaveLight GmbH, Erlangen, Germany).

Experimental setup to determine the pulse energies and radiant exposures for excimer lasers with repetition rates ranging from 100 to 1050 Hz.

PURPOSE: To evaluate the feasibility of surface profiling for central ablation depth measurements and determine experimentally the required single‐pulse energies and radiant exposures to achieve equivalent central ablation depths on bovine corneas for a myopic correction of −6.00 diopters (optical zone 6.5 mm) performed with laser repetition rates ranging from 100 to 1050 Hz. SETTING: Institute for Refractive and Ophthalmic Surgery, Zurich, Switzerland, and WaveLight AG, Erlangen, Germany. METHODS: Freshly enucleated bovine corneas and poly(methyl methacrylate) (PMMA) plates were photoablated. The shot pattern for the myopic correction was maintained during all experiments; the pulse laser energy was adjusted to achieve equal ablation depths for all repetition rates. Pulse energy, radiant exposure, and pulse duration were monitored to determine the required laser parameter. RESULTS: The variations (standard deviation) of the profile measurements were ±0.45 μm or less for PMMA and ±1.50 μm or less for bovine corneas. Measurements with bovine corneas should be performed within 3 minutes or less to avoid larger variations in profile measurements. Increasing the repetition rate from 100 Hz to 1050 Hz required an increase in peak radiant exposure from 400 mJ/cm2 to 530 mJ/cm2 to achieve equal ablation for the myopic correction. The required increase in the mean radiant exposure ranged from 190 to 260 mJ/cm2. CONCLUSIONS: Higher‐repetition‐rate excimer lasers require increased radiant exposure. Further experimental studies should be performed to determine the relevance of spatial and temporal spot positioning, ablation‐plume dynamics, and temperature increases during high‐repetition‐rate laser treatments.

Initial surface temperature of PMMA plates used for daily laser calibration affects the predictability of corneal refractive surgery.

PURPOSE To investigate the relevance of initial temperature of the polymethylmethacrylate (PMMA) plates used as a target for photoablation during calibration of excimer lasers performed in daily clinical routine. METHODS An experimental argon fluoride excimer laser with a repetition rate of 1050 Hz, a radiant exposure of 500 mJ/cm², and single pulse energy of 2.1 mJ was used for photoablation of PMMA plates. The initial plate temperature varied from 10.1°C to 75.7°C. The initial temperature was measured with an infrared camera and the central ablation depth of a myopic ablation of -9.00 diopters (D) with an optical zone of 6.5 mm was measured by means of a surface profiling system. RESULTS The ablation depth increased linearly from 73.9 to 96.3 μm within a temperature increase from 10.1°C to 75.7°C (increase rate of 0.3192 μm/K). The linear correlation was found to be significant (P<.05) with a coefficient of determination of R²=0.95. Based on these results and assuming a standard room temperature of 20°C, optimal plate temperature was calculated to be 15°C to 25°C to maintain an ablation within 0.25 D. CONCLUSIONS The temperature of PMMA plates for clinical laser calibration should be controlled ideally within a range of approximately ±5°C, to avoid visually significant refractive error due to calibration error. Further experimental investigations are required to determine the influence of different initial corneal temperatures on the refractive outcome.