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Michael Mrochen

Publications and source records attributed to Michael Mrochen.

14 recordsLinked to original sources
PubMed2004

Wavefront-optimized ablation profiles: theoretical background.

PURPOSE: To describe a method for calculating wavefront-optimized ablation profiles to precompensate for the spherical aberration and higher-order astigmatism induced by myopic, hyperopic, and astigmatic corneal laser corrections. SETTING: IROC-Institut für Refraktive und Ophthalmo-Chirurgie, and Institute for Biomedical Engineering, Swiss Federal Institute of Technology, Zürich, Switzerland. METHODS: The basic ablation profile for myopic, hyperopic, and astigmatic correction is derived from the 2nd-order Zernike representation of wavefront aberrations. Including 4th-order spherical aberration and higher-order astigmatism in the theoretical calculation of the ablation profile allows precompensation for the expected amount of higher-order aberrations (HOAs). The shapes of wavefront-optimized ablation profiles are compared with the shapes of "classic" ablation profiles for myopic and astigmatic corrections. RESULTS: The introduction of precompensating spherical aberration and higher-order astigmatism leads to a more aspheric ablation profile with a significant increase in ablation depth (up to 35%) in the midperiphery of the optical zone. The central ablation depth remains unchanged in the myopic correction but increases by 3% in cylinder correction. CONCLUSIONS: Wavefront-optimized ablation profiles provide a simple method to precompensate for the expected 4th-order spherical aberration and higher-order astigmatism in the average eye. Further clinical studies must be performed to prove the theoretical results; demonstrate the reduction in HOAs; and predict safety, predictability, and stability of wavefront-optimized ablation profiles.

Algorithms↗
PubMed2004

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 surgeon's judgment may not be accurate enough to achieve significantly improved optical quality in a high percentage of treated eyes.

Adolescent↗
PubMed2003

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 eye's original optical error. Rough centration based on the surgeon's judgment might not be accurate enough to achieve significantly improved optical quality in a high percentage of treated eyes.

Adolescent↗
PubMed2002

Optical low coherence reflectometry for noncontact measurements of flap thickness during laser in situ keratomileusis.

OBJECTIVE: There is growing evidence that iatrogenic keratectasia after laser in situ keratomileusis (LASIK) for high corrections occurs more frequently than initially assumed, and that it may result from larger variation in flap thickness. DESIGN: Consecutive noncomparative case series PARTICIPANTS: Thirty-four patients who underwent LASIK for myopia and astigmatism (first treatment group) and 10 patients who received re-LASIK (retreatment group). METHODS: Central corneal thickness and thickness of the lamella during LASIK were determined by optical low coherence reflectometry (OLCR) and contact ultrasound pachymetry. MAIN OUTCOME MEASURES: Thickness of the flap and its standard deviation, as well as its correlation with age, sphere, cylinder, corneal thickness, intraocular pressure, and corneal refractive power (K-readings). RESULTS: The mean flap thickness of the first treatment group determined by OLCR was 130 +/- 29 microm; the 95 percentile was 169 microm and the 5 percentile was 86 microm. The flap thickness was not correlated with any of the investigated demographic or refractive parameters. The mean flap thickness of the retreatment group was 152 +/- 14 microm; the 95 percentile was 175 microm and the 5 percentile was 137 microm. Thus, the flap thickness of the retreatment group was significantly thicker compared with the first treatment group (P < 0.001). CONCLUSIONS: Optical low coherence reflectometry (OLCR) was shown to be an appropriate alternative to ultrasonic preoperative and intraoperative corneal pachymetry in laser assisted in situ keratomileusis. The lack of correlation between achieved flap thickness and preoperative clinical data, such as corneal thickness, corneal curvature, intraocular pressure, and refraction, emphasizes the importance of measuring flap thickness and corneal bed thickness during surgery.

Adult↗
PubMedDate not supplied

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.

Adult↗
PubMedDate not supplied

Anterior lamellar keratoplasty with a microkeratome: a method for managing complications after refractive surgery.

PURPOSE: To demonstrate a technique of anterior lamellar keratoplasty with standardized and automated preparation of surface-parallel cuts in both donor and recipient appropriate for addressing several problems after laser in situ keratomileusis (LASIK) and photorefractive keratectomy (PRK). METHODS: We report a noncomparative series of ten eyes with complications after LASIK and PRK. Lamellar cuts were performed in donor and recipient eyes by means of an automated microkeratome. Lamellar grafts were fixed by only four single sutures. In two eyes, a re-lift LASIK was performed after 6 months. RESULTS: Surgery was uneventful and visual acuity was improved in all eyes. Residual irregular astigmatism and refractive error were corrected in two eyes by means of excimer laser computer-assisted ablation and resulted in a further improvement of uncorrected and best spectacle-corrected visual acuity. CONCLUSIONS: Anterior lamellar keratoplasty with a microkeratome can be used for the management of certain complications of PRK and LASIK.

Adult↗
PubMedDate not supplied

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.

Adult↗
PubMedDate not supplied

From scattering to wavefronts--what's in between?

PURPOSE: Some optical errors are too localized and random to be detected by commercial wavefront devices and Zernike polynomial expression. We looked beyond aberrations defined by Zernike expression to discuss implications of fine irregularities associated with highly aberrated corneal surfaces and complex surface roughness that can lead to light scattering. METHODS: Most fine irregularities are related to postoperative surface roughness, complexities of corneal ablation, and the laser in situ keratomileusis (LASIK) flap. These can be characterized mathematically by a random function that includes local surface tilts, the correlation radius of irregularities (Ic), surface roughness, and other terms. The Kirchoff method of scatter analysis characterizes fine surface irregularities by replacing each point on the surface with a tangential plane, allowing it to be governed by Snellen and Fresnel laws. RESULTS: The joint action of the continuum of microbeams defines a complex point spread function that can be expressed by the Strehl ratio. Small, highly irregular steep central islands and flap striae may not be adequately detected by Zernike expression and may have a surface irregularity diameter of 0.1 to 2.0 mm and height of 10 to 20 microm that results in a reduced Strehl ratio below 0.8. Laser ablation inhomogeneities may have dimensions of 1 to 10 microm, resulting in a root mean square tilt value approaching 1.0 and a Strehl ratio below 0.5. CONCLUSION: Corneal surface irregularities after laser vision correction may induce significant optical aberrations and distortions apart from classical wavefront or scattering errors. As these may not be detected by commercial wavefront devices, and yet contribute to the degradation of optical performance, alternate techniques should be evaluated to detect and describe these surface irregularities.

Cornea↗
PubMedDate not supplied

Laser intrastromal keratoplasty--case report.

PURPOSE: To evaluate the feasibility of correcting high hyperopia by means of intrastromal implantation of a laser shaped corneal lenticule prepared from a human donor eye. METHODS: A female patient with high hyperopia and irregular astigmatism resulting from multiple laser in situ keratomileusis procedures and lamellar keratoplasty underwent laser intrastromal keratoplasty. Her preoperative uncorrected visual acuity (UCVA) was 20/300 and best spectacle-corrected visual acuity (BSCVA) was 20/100 with a refraction of +8.00 -1.00 x 130 degrees. Corneal topography showed a highly irregular corneal surface. Central corneal thickness was 398 microm. Lenticule preparation included mechanical de-epithelialization of a human donor eye, keratectomy with a microkeratome, user-designed software combining a photorefractive keratectomy (PRK) treatment for +8.00 D sphere, an ablation zone of 7.0 mm, and a circumferential cut (internal diameter of 6.5 mm) for tissue ablation. Implantation involved re-lifting the flap, positioning the lenticule onto the corneal bed, and repositioning of the flap. RESULTS: The operation was uneventful as was the early postoperative follow-up. BSCVA improved to 20/50 with +1.00 -2.25 x 120 degrees at 2 months postoperatively. Corneal topography showed a more regular cornea with increased curvature in all meridians. Central corneal thickness increased to 600 microm. CONCLUSION: Laser intrastroma keratoplasty may be an option for correcting high hyperopa and irregular astigmatism in eyes with a thin corneal bed.

Adult↗
PubMedDate not supplied

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.

Cornea↗
PubMedDate not supplied

Clinical photoablation with a 500-Hz scanning spot excimer laser.

PURPOSE: The aim of this study was to use a 500-Hz scanning spot laser (Concept500, WaveLight Laser Technologie AG, Erlangen, Germany) to investigate potential side effects that might be associated with the use of a high repetition rate laser platform. METHODS: Seven eyes were treated using a 500-Hz scanning spot laser for laser in situ keratomileusis (LASIK). The local frequency of the ablation was kept below 40 Hz to avoid local heating of corneal tissue. With the exception of the high repetition rate (500 Hz), all other laser parameters such as fluence, algorithm, ablation profile, and spot diameter were identical to a standard WaveLight Allegretto laser system. Patients were examined at 1 month and 1 year after initial treatment. Preoperative and postoperative examination included manifest sphere and cylinder, uncorrected and best spectacle-corrected visual acuity (BSCVA). RESULTS: All eyes were treated for myopia or myopic astigmatism. Five eyes received spherocylindrical and two eyes spherical ablation only. No adverse events correlated with the use of a high repetition rate laser system were observed during surgery or at any point during follow-up. All eyes maintained or had improved BSCVA at 12 months after treatment when compared to preoperative values. CONCLUSION: The use of an excimer laser with a maximal repetition rate of 500 Hz and a local repetition rate of less than 40 Hz was free of any specific side effect that might be associated with the use of such a high repetition rate.

Adult↗
PubMedDate not supplied

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.

Computer Simulation↗
PubMedDate not supplied

Transferring wavefront measurements into corneal ablations: an overview of related topics.

We give an overview of possible side effects that are specific for, or of particular relevance in, customized treatments. Certain processes involved in customized ablations have the potential to alter the quality of the optical correction. Professionals associated with customized treatment should be informed and trained with respect to possible sources of error.

Algorithms↗