Michael Bueeler

Maximum permissible lateral decentration in aberration-sensing and wavefront-guided corneal ablation

Purpose: To investigate the lateral alignment accuracy needed in wavefront‐guided refractive surgery to improve the ocular optics to a desired level in a percentage of normally aberrated eyes. Setting: Department of Ophthalmology, University of Zurich, Zurich, Switzerland. Methods: The effect of laterally misaligned ablations on the optical outcome was simulated using measured wavefront aberration patterns from 130 normal eyes. The calculations were done for 3.0 mm, 5.0 mm, and 7.0 mm pupils. The optical quality of the simulated correction was rated by means of the root‐mean‐square residual wavefront error. Results: To achieve the diffraction limit in 95% of the normal eyes with a 7.0 mm pupil, a lateral alignment accuracy of 0.07 mm or better was required. An accuracy of 0.2 mm was sufficient to reach the same goal with a 3.0 mm pupil. Conclusion: Procedures must be developed to ensure that the ablation is within a tolerance range based on each eyes original optical error. Rough centration based on the surgeons judgment might not be accurate enough to achieve significantly improved optical quality in a high percentage of treated eyes.

Correlation Between Corneal and Total Wavefront Aberrations in Myopic Eyes

PURPOSE Corneal topography data expressed as corneal aberrations are frequently used to report corneal laser surgery results. However, the optical image quality depends on all optical elements of the eye, including the human lens. We investigated correlations between corneal and total wavefront aberrations and the relevance of corneal aberrations for representing the optical quality of the total eye. METHODS Thirty-three eyes of 22 myopic patients were measured using a corneal topography system and a Tscherning-type wavefront analyzer. Pupils were dilated to at least 6 mm in diameter. All measurements were centered with respect to the line of sight. Corneal and total wavefront aberrations were calculated up to the 6th Zernike order in the same reference plane. RESULTS Statistically significant correlations (P<.05) between corneal and total wavefront aberrations were found for astigmatism (C3,C5) and all 3rd Zernike order coefficients such as coma (C7,C8). No statistically significant correlations were found for 4th, 5th, or 6th order Zernike coefficients. On average, all Zernike coefficients for corneal aberrations were larger than the Zernike coefficients for total wavefront aberrations. CONCLUSIONS Due to the lack of correlation between corneal and total wavefront aberrations in most of the higher order aberrations, measurement of corneal aberrations are of limited use for representation of the optical quality of the human eye, especially after corneal laser surgery. Corneal aberrations and optical elements within the eye are optically balanced. As a consequence, ideal customized ablations must take both corneal and total wavefront aberrations into consideration.

Simulation of Eye-tracker Latency, Spot Size, and Ablation Pulse Depth on the Correction of Higher Order Wavefront Aberrations With Scanning Spot Laser Systems

PURPOSE The aim of this theoretical work was to investigate the robustness of scanning spot laser treatments with different laser spot diameters and peak ablation depths in case of incomplete compensation of eye movements due to eye-tracker latency. METHODS Scanning spot corrections of 3rd to 5th Zernike order wavefront errors were numerically simulated. Measured eye-movement data were used to calculate the positioning error of each laser shot assuming eye-tracker latencies of 0, 5, 30, and 100 ms, and for the case of no eye tracking. The single spot ablation depth ranged from 0.25 to 1.0 microm and the spot diameter from 250 to 1000 microm. The quality of the ablation was rated by the postoperative surface variance and the Strehl intensity ratio, which was calculated after a low-pass filter was applied to simulate epithelial surface smoothing. RESULTS Treatments performed with nearly ideal eye tracking (latency approximately 0) provide the best results with a small laser spot (0.25 mm) and a small ablation depth (250 microm). However, combinations of a large spot diameter (1000 microm) and a small ablation depth per pulse (0.25 microm) yield the better results for latencies above a certain threshold to be determined specifically. Treatments performed with tracker latencies in the order of 100 ms yield similar results as treatments done completely without eye-movement compensation. CONCWSIONS: Reduction of spot diameter was shown to make the correction more susceptible to eye movement induced error. A smaller spot size is only beneficial when eye movement is neutralized with a tracking system with a latency <5 ms.

Aberration-sensing and Wavefront-guided Laser in situ Keratomileusis: Management of Decentered Ablation

PURPOSE To clarify the feasibility of aberration-sensing and wavefront-guided laser in situ keratomileusis (LASIK) to manage grossly decentered ablation and to discuss the limitations of the technology. METHODS Three patients with previous decentrations of the ablation zone between 1.5 to 2.0 mm were scheduled for wavefront-guided LASIK. All patients reported monocular diplopia and halos. Wavefront aberrations were measured with a Tscherning-type aberrometer. Laser ablation was done with a WaveLight Allegretto in a one-step procedure with ablation profiles calculated only from the individual wavefront map. Decentrations were determined from corneal topography. RESULTS Three months after surgery, patient WM and patient SU had gained uncorrected and best spectacle-corrected visual acuity. The root mean square-wavefront error decreased up to 61% and 33%, respectively, for total and higher order aberrations (Zernike modes of 3rd order and higher). There was significant enlargement of the optical zone determined by corneal topography, and both patients no longer reported diplopia and halos at 3 months postoperatively. The optical aberration of the third patient (RE), after a 5.00-D overcorrection with a 2-mm decentration, was too high for aberration-sensing; retinal images obtained from the wavefront device were too smeared and not of sufficient contrast. In addition, this patient had a residual corneal thickness of 416 microm and thus wavefront-guided LASIK was not done. CONCLUSIONS Wavefront-guided LASIK offers a new way of managing grossly decentered laser ablations. Unfortunately, there are still patients who have aberrations too large for wavefront sensing or with other clinical limitations such as a residual corneal thickness too thin for further treatment.

Maximum permissible torsional misalignment in aberration-sensing and wavefront-guided corneal ablation

Purpose: To determine the maximum permissible torsional misalignment in wavefront‐guided refractive surgery. Setting: University of Zurich, Department of Ophthalmology, Zurich, Switzerland. Methods: The effect of torsionally misaligned ablations on the optical outcome was simulated using measured wavefront aberration patterns (2nd to 6th orders) in 130 normally aberrated eyes. The calculations were done for 3.0 mm, 5.0 mm, and 7.0 mm pupils. The optical quality of the simulated correction was rated by the root‐mean‐square residual wavefront error. Results: The required accuracy of torsional alignment is higher for the correction of higher‐order aberrations than for cylindrical treatments only. To improve the optical performance to the level of the best 10% of a normal, untreated population, ablation would have to occur within a tolerance range of 4.0 degrees for 7.0 mm pupils. Conclusions: The tolerance range for torisional alignment in wavefront‐guided higher‐order corrections depends on the amount of original optical error in each eye. Rough centration based on the surgeons judgment may not be accurate enough to achieve significantly improved optical quality in a high percentage of treated eyes.

Optimization Model for UV-Riboflavin Corneal Cross-linking

PURPOSE To develop a theoretical model for riboflavin ultraviolet-A cross-linking treatment that can predict the increase in stiffness of the corneal tissue as a function of the ultraviolet intensity and riboflavin concentration distribution, as well as the treatment time. METHODS A theoretical model for calculating the increase in corneal cross-linking (polymerization rate) was derived using Ficks second law of diffusion, Lambert-Beers law of light absorption, and a photopolymerization rate equation. Stress-strain experiments to determine Youngs modulus at 5% strain were performed on 43 sets of paired porcine corneal strips at different intensities (3-7 mW/cm²) and different riboflavin concentrations (0.0%-0.5%). The experimental results for Youngs modulus increase were correlated with the simulated polymerization increase to determine a relationship between the model and the experimental data. RESULTS This model allows the calculation of the one-dimensional spatial and temporal intensity and concentration distribution. The total absorbed radiant exposure, defined by intensity, concentration distribution, and treatment time, shows a linear correlation with the measured stiffness increase from which a threshold value of 1.7 J/cm² can be determined. The relative stiffness increase shows a linear correlation with the theoretical polymer increase per depth of tissue, as calculated by the model. CONCLUSIONS This theoretical model predicts the spatial distribution of increased stiffness by corneal cross-linking and, as such, can be used to customize treatment, according to the patients corneal thickness and medical indication.

Limitations of pupil tracking in refractive surgery: systematic error in determination of corneal locations.

PURPOSE The goal of this investigation was to show the theoretical limitations of pupil tracking in refractive surgery. The parallax error associated with localizing corneal positions by tracking the subjacent entrance pupil center was quantified. METHODS Optical ray-tracing in a schematic model eye was performed to determine the geometric parallax error. The calculations required several assumptions regarding ocular geometry, eye movements, and eye tracker position. Various parameter combinations were evaluated to assess the potential range of error to be expected in clinical practice. RESULTS Tracking error can amount to 30% (or more for eye trackers mounted closer than 500 mm to the eye) of the detected lateral shift. Thus, if the eye tracker registers a lateral shift of the entrance pupil of 0.2 mm away from the tracking reference axis, the point of interest located on the cornea would essentially be 0.26 mm away from this reference axis. A laser pulse fired at that moment would be systematically displaced by 60 microm. Our results depended on geometric parameters of the eye and the tracking device. Based on conservative assumptions regarding these geometric parameters, partial compensation could be realized by adding a certain percentage to the modulus of each eye tracker reading. CONCLUSIONS The fact that corneal displacement was generally underestimated by up to 30% of the measured entrance pupil shift demonstrates the severity of the parallax effect.

Treatment-induced shifts of ocular reference axes used for measurement centration

PURPOSE: To determine the shifts of the main corneal reference points in dependence of the chosen centration axis for the treatment. SETTING: Federal Institute of Technology Zurich, Institute of Biomedical Engineering, Zurich, Switzerland. METHODS: Computer simulations were performed on several variants of the Gullstrand‐Emsley schematic eye, which was modified by an off‐axis fovea. Refractive corrections were simulated by centering Munnerlyns formula on each of the 4 corneal reference points determined in the preoperative eye: the optical axis, the line of sight, the visual axis, and the first corneal reflex. Subsequently, the postoperative locations of these axes were determined and compared with the preoperative values. RESULTS: The postoperative line of sight was found to depend least on the choice of the preoperative centration axis for both myopic and hyperopic treatments. It undergoes a maximum movement of 0.040 mm when centering a +5 diopter correction on the preoperative line of sight, whereas the corneal reflex, which is used for centering most topography systems, can move by more than 0.10 mm. CONCLUSIONS: Centration of the correction on the preoperative line of sight enabled good comparability between preoperative and postoperative measurements that use the line of sight as a reference axis. Yet, centration of the treatment on the preoperative line of sight does not ensure comparability between preoperative and postoperative measurements that use the corneal reflex as a reference axis such as most corneal topography systems. Axis shifts might lead to misinterpretation of data such as a wrong diagnosis of a decentered ablation or changes in the Zernike representation.

Optimal visual perception and detectionof oral cavity neoplasia

The most common way to detect disease is by visual inspection of the suspect tissue. However, the human eye is not optimized for this task because the perceived spectrum of light is divided into three channels, all of which have overlapping spectral sensitivity curves. Here, we present new methods to optimize visually perceived contrast based on spectral differences between normal and abnormal tissue. We apply these methods to the perception of fluorescence emission from the oral cavity. Abnormalities in the oral cavity are optimally perceived when the excitation is between 420-440 nm. To optimally visualize fluorescence at 340-nm excitation, the emission should be observed through a blue bandpass filter transmitting light at 430 nm.

Effect of spot size, ablation depth, and eye-tracker latency on the optical outcome of corneal laser surgery with a scanning spot laser

Based on eye movement data, we present a study on the effect of various laser and eye-tracking parameters on the optical outcome after scanning spot refractive surgery. Numerical simulations of the entire ablation process were performed on a schematic model eye under variation of the following parameters: ablation depth per pulse, laser spot size, eye tracker latency and magnitude of refractive correction. Three-dimensional ray tracing through an analytical model eye featuring the ablated corneal front surface enabled evaluation of the resulting optical quality. The modulation transfer function (MTF) was calculated to rate the difference in optical quality between an ideal (movement-free) treatment, and treatments performed with an eye-tracker working with a certain latency. For all the calculations it was assumed, that the laser repetition rate remains constant at 250 Hz. It was shown, that the contrast transfer can decrease significantly with increasing latency of the eye-tracker. For constant laser and tracking parameters, this decrease was found to be more significant for higher myopic corrections. It was further shown, that treatments performed with smaller spot sizes and smaller ablation depths per pulse are more sensitive to tracking latency. Assuming a certain eye tracker latency, the most stable results are obtained for large beam diameters and high central ablation depths per pulse. Latencies below 10 ms would allow for a reduction of the beam diameter to 0.50 mm as well as for ablation depths as small as 0.50 microns.© (2003) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.Based on eye movement data, we present a study on the effect of various laser and eye-tracking parameters on the optical outcome after scanning spot refractive surgery. Numerical simulations of the entire ablation process were performed on a schematic model eye under variation of the following parameters: ablation depth per pulse, laser spot size, eye tracker latency and magnitude of refractive correction. Three-dimensional ray tracing through an analytical model eye featuring the ablated corneal front surface enabled evaluation of the resulting optical quality. The modulation transfer function (MTF) was calculated to rate the difference in optical quality between an ideal (movement-free) treatment, and treatments performed with an eye-tracker working with a certain latency. For all the calculations it was assumed, that the laser repetition rate remains constant at 250 Hz. It was shown, that the contrast transfer can decrease significantly with increasing latency of the eye-tracker. For constant laser and tracking parameters, this decrease was found to be more significant for higher myopic corrections. It was further shown, that treatments performed with smaller spot sizes and smaller ablation depths per pulse are more sensitive to tracking latency. Assuming a certain eye tracker latency, the most stable results are obtained for large beam diameters and high central ablation depths per pulse. Latencies below 10 ms would allow for a reduction of the beam diameter to 0.50 mm as well as for ablation depths as small as 0.50 microns.