Michael Mrochen

Clinical results of wavefront-guided laser in situ keratomileusis 3 months after surgery

Purpose: To investigate the visual and refractive outcome of wavefront‐guided laser in situ keratomileusis (LASIK) to correct myopic astigmatism. Setting: Departments of Ophthalmology of the Universities of Dresden, Dresden, Germany, and Zurich, Zurich, Switzerland. Methods: This prospective study comprised 35 eyes of 28 patients who had a mean preoperative spherical refraction of –4.8 diopters (D) ± 2.3 (SD) and a cylinder of –1.1 ± 0.9 D. Preoperative and postoperative wavefront analysis was performed with a Tscherning aberrometer. A scanning‐spot laser with a 1.0 mm spot size and a 200 Hz repetition rate was used. The eye‐tracking system had a response time of less than 6 milliseconds. The treatment area diameter ranged from 6.0 to 7.0 mm with a transition zone of 1.0 mm. Results: At 3 months, 68.0% of the eyes were within ±0.5 D of emmetropia and 93.5% were within ±1.0 D. Uncorrected visual acuity was 20/20 or better in 93.5% of eyes. No eye lost more than 1 line of low‐contrast, glare, and best spectacle‐corrected visual acuity (BSCVA). Supernormal vision (BSCVA of 20/10 or better) was achieved in 16.0% of eyes. The correction of higher‐order aberrations (spherical aberration, coma) was insufficient, with an increase factor of the overall root‐mean‐square wavefront error of 1.44 ± 0.74. Coma was better corrected than spherical aberration. Conclusions: Wavefront‐guided LASIK is a promising technique that offers the potential to correct refractive errors, to improve visual acuity, and to increase the quality of vision, especially under mesopic conditions. Studies that include selective overcorrrection of different Zernike components are needed to achieve better correction of the aberrations. Prospective controlled clinical studies must clarify the major benefits of wavefront‐guided LASIK.

Increased higher-order optical aberrations after laser refractive surgery: A problem of subclinical decentration

Purpose: To study the clinical and theoretical effects of subclinical decentrations on the optical performance of the eye after photorefractive laser surgery. Setting: Department of Ophthalmology, University of Dresden, Dresden, Germany. Methods: Ocular aberrations were determined before and 1 month after uneventful photorefractive keratectomy (PRK) with the Multiscan laser (Schwind) in 10 eyes of 8 patients. The corrections ranged from –2.5 to –6.0 diopters, and ablation zones of 6.0 mm and larger were used. The measured wavefront errors were compared to numerical simulations using the individually determined decentrations and currently used ablation profiles. Results: The PRK‐induced aberrations were significantly greater than the preoperative aberrations. The numerically calculated increase in the higher‐order optical aberrations correlated with the clinical results, demonstrating a major increase in coma‐ and spherical‐like aberrations. Subclinical decentration (less than 1.0 mm) was found to be a major factor in increased coma‐like and spherical‐like aberrations after corneal laser surgery. Conclusion. To minimize higher‐order optical errors, special efforts to center the ablation zone are necessary; for example, by eye‐tracking systems that consider the visual axis.

Improvement in photorefractive corneal laser surgery results using an active eye-tracking system

Purpose: To study the advantage of modern eye‐tracking systems for photorefractive surgery. Setting: Department of Ophthalmology, University of Zurich, Zurich, Switzerland. Methods: Photorefractive surgery (photorefractive keratectomy and laser in situ keratomileusis) for myopia and myopic astigmatism was performed in 40 eyes with a commercially available medical excimer laser system. The eyes were selected retrospectively from a larger group of patients treated at 1 clinic. In 20 eyes, the ablation was centered on the entrance pupil using the active, video‐based, eye‐tracking system (sampling frequency 50 Hz) of the laser. During laser treatment in the nontracker group (20 eyes), the active eye‐tracking system was switched off and centration was done manually by the surgeon. Preoperatively and 1 and 3 months after surgery, the patients had a standard ophthalmic examination as well as wavefront analysis by means of a custom‐designed wavefront analyzer. Results: After surgery, the visual acuity was significantly better (P < .05) in patients treated with the eye tracker. The increase in coma‐like (relative increase factor 0.4) and spherical aberrations (relative increase factor 1.1) was significantly smaller in these patients than in those in the nontracker group (spherical equivalents of 3.9 and 5.1, respectively; P < .05). The refractive outcome, however, was not significantly different in sphere and cylinder. Conclusion: The use of active eye tracking appeared to improve the optical and visual outcomes but did not affect the refractive outcome after photorefractive laser surgery.

Stress wave amplitudes during laser surgery of the cornea

PURPOSE To determine the stress wave amplitudes generated during photoablation of the cornea using an argon fluoride excimer laser. DESIGN Experimental study using porcine eyes. METHODS Profiles of the stress wave amplitudes and enucleated human eyes along the axis of symmetry of porcine eyes and enucleated human eyes were measured using a miniature piezoelectric transducer. The ablation parameters, fluence, and ablation diameters were varied within the range of clinical application. MAIN OUTCOME MEASURES Stress wave amplitudes generated during photoablation. RESULTS The stress waves pass through a pressure focus located in the posterior lens and anterior vitreous, where amplitudes of up to 100 atm were measured with a 6-mm or larger ablation zone. Posterior to this focus, the stress wave amplitudes rapidly decrease to less than 10 atm at the retinal site. Small diameter excimer laser spots (< or =1.5 mm) produce a declining stress wave with no pressure focus at the lens and anterior vitreous. CONCLUSIONS Stress waves may be potentially hazardous to anterior structures of the human eye, including the corneal endothelium, lens and anterior vitreous face. They peak at the lens and vitreous with a broad beam, but not with small spot laser ablation. At posterior retinal and subretinal structures, they may be considered harmless.

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.

Automated ocular wavefront analyzer for clinical use

Higher-order optical errors of the human eye are often responsible for a reduced visual acuity in spite of an optimal spherical or cylindrical refraction. These optical aberrations are of natural origin or can result from operations in the eye involving optical structures. The presented wavefront analyzer bases on Tschernings aberroscope. A collimated laser beam (532 nm, 10 mW) illuminates a mask with a regular matrix of 0.3 mm diameter holes which forms a bundle of thin parallel rays. These rays are focused by a lens in front of the eye that their intraocular focus point is located in a certain distance in front of the retina generating a corresponding pattern of light spots on it. According to the existing ocular optical errors, this spot pattern is more or less distorted in comparison to the mask matrix. For a 6 mm pupil diameter 68 retinal spots are plottable for the assessment of optical aberrations. The retinal spot pattern is imaged onto the sensor of a low-light CCD video camera by indirect ophthalmoscopy. The deviations of all spots from their ideal regular positions are measured by means of a PC, and from these values the intraocular wave-front aberration is computed in the form of Zernike polynomials up to the 6th order.

Light beam shaping and homogenization (LSBH) by irregular microlens structure for medical applications

Purpose: The increasing interest in a homogeneous Gaussian light beam profile for applications in ophthalmology e.g. photorefractive keratectomy (PRK) requests simple optical systems with low energy losses. Therefore, we developed the Light Shaping Beam Homogenizer (LSBH) working from UV up to mid-IR. Method: The irregular microlenses structure on a quartz surface was fabricated by using photolithography, chemical etching and chemical polishing processes. This created a three dimensional structure on the quartz substrate characterized in case of a Gaussian beam by random law distribution of individual irregularities tilts. The LSBH was realized for the 193 nm and the 2.94 micrometer wavelengths. Simulation results obtained by 3-D analysis for an arbitrary incident light beam were compared to experimental results. Results: The correlation to a numerical Gaussian fit is better than 94% with high uniformity for an incident beam with an intensity modulation of nearly 100%. In the far field the cross section of the beam shows always rotation symmetry. Transmittance and damage threshold of the LSBH are only dependent on the substrate characteristics. Conclusions: considering our experimental and simulation results it is possible to control the angular distribution of the beam intensity after LSBH with higher efficiency compared to diffraction or holographic optical elements.

Correlations between corneal and total wavefront aberrations

Purpose: Corneal topography data expressed as corneal aberrations are frequently used to report corneal laser surgery results. However, the optical image quality at the retina depends on all optical elements of the eye such as the human lens. Thus, the aim of this study was to investigate the correlations between the corneal and total wavefront aberrations and to discuss the importance of corneal aberrations for representing corneal laser surgery results. Methods: Thirty three eyes of 22 myopic subjects were measured with a corneal topography system and a Tschernig-type wavefront analyzer after the 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 < 0.05) between the corneal and total wavefront aberrations were found for the astigmatism (C3,C5) and all 3rd Zernike order coefficients such as coma (C7,C8). No statistically significant correlations were found for all 4th to 6th order Zernike coefficients except for the 5th order horizontal coma C18 (p equals 0.003). On average, all Zernike coefficients for the corneal aberrations were found to be larger compared to Zernike coefficients for the total wavefront aberrations. Conclusions: Corneal aberrations are only of limited use for representing the optical quality of the human eye after corneal laser surgery. This is due to the lack of correlation between corneal and total wavefront aberrations in most of the higher order aberrations. Besides this, the data present in this study yield towards an aberration balancing between corneal aberrations and the optical elements within the eye that reduces the aberration from the cornea by a certain degree. Consequently, ideal customized ablations have to take both, corneal and total wavefront aberrations, into consideration.

Optical aberrations induced by subclinical decentrations of the ablation pattern

Purpose: The aim of this work was to study the effect of currently used ablation profiles along with eccentric ablations on the increase of higher order aberrations observed after PRK. Material and Methods: The optical aberrations of 10 eyes were tested before and after PRK. Refractive surgery was performed using a ArF-excimer laser system. In all cases, the ablation zone was 6 mm or larger. The spherical equivalent of the correction was ranging from -2.5 D to -6.0 D. The measured wavefront error was compared to numerical simulations done with the reduced eye model and currently used ablation profiles as well as compared with experimental results obtained from ablation on PMMA balls. Results: The aberration measurements result in a considerable change of the spherical- and coma-like wavefront errors. This result was in good correlation with the numerical simulations and the experimental results. Furthermore, it has been derived that the major contribution on the induced higher order aberrations are a result of the small decentration (less than 1.0 mm) of the ablation zone. Conclusions: Higher order spherical- and coma-like aberrations after PRK are mainly determined by the decentration of the ablation zone during laser refractive surgery. However, future laser systems should use efficient eye-tracking systems and aspherical ablation profiles to overcome this problem.

Required accuracy of lateral and torsional alignment in aberration-sensing and wavefront guided treatments

The effect of lateral and torsional misalignments of the ablation on the postoperative optical outcome was theoretically investigated based on measured wavefront aberration data from 130 normal eyes. Simulations included lateral decentrations and rotations around the longitudinal axis of the eye (torsion). The optical quality of the simulated refractive correction was rated by means of the root-mean-squared residual wavefront error. The accuracy for lateral centration in order to achieve the diffraction limit at a pupil size of 8 mm in 95% of the investigated eyes should be 50 microns or better. However, an accuracy of 450 microns was found to be enough to guarantee that none of the investigated eyes would suffer from a decreased optical performance after surgery. Alignment would have to be performed with a torsional precision of approximately 1 deg or better in order to achieve the diffraction limit in 95% of the measured normal eyes for an 8-mm pupil, whereas an accuracy of 15 deg is required to obtain at least some improvement of the optical quality in all the examined eyes. The accuracy needed for torsional alignment increases compared to pure sphero-cylindrical treatments when additional correction of the higher-order aberrations is aspired.