Proceedings Volume 5314

Ophthalmic Technologies XIV

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Proceedings Volume 5314

Ophthalmic Technologies XIV

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Volume Details

Date Published: 13 July 2004
Contents: 8 Sessions, 40 Papers, 0 Presentations
Conference: Biomedical Optics 2004 2004
Volume Number: 5314

Table of Contents

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Table of Contents

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  • Ophthalmic Diagnostics I
  • Optics of the Eye: Measurement, Correction, and Simulation
  • Laser and Light Interactions with Ocular Tissues and Implants
  • Ophthalmic Imaging
  • Ophthalmic Diagnostics II
  • Ocular Wavefront Aberrometry
  • Experimental Techniques in Ocular Surgery
  • Poster Session
  • Optics of the Eye: Measurement, Correction, and Simulation
  • Experimental Techniques in Ocular Surgery
  • Ocular Wavefront Aberrometry
Ophthalmic Diagnostics I
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Noncontact optical measurement of lens capsule thickness ex vivo
Purpose: To design a non-contact optical system to measure lens capsule thickness in cadaver eyes. Methods: The optical system uses a 670nm laser beam delivered to a single-mode fiber coupler. The output of the fiber coupler is focused onto the tissue using an aspheric lens (NA=0.68) mounted on a motorized translation stage. Light reflected from the sample is collected by the fiber coupler and sent to a silicon photodiode connected to a power meter. Peaks in the power signal are detected when the focal point of the aspheric lens coincides with the capsule boundaries. The capsule thickness is proportional to the distance between successive peaks. Anterior and posterior lens capsule thickness measurements were performed on 13 human, 10 monkey, and 34 New Zealand white rabbit lenses. The cadaver eyes were prepared for optical measurements by bonding a PMMA ring on the sclera. The posterior pole was sectioned, excess vitreous was removed, and the eye was placed on a Teflon slide. The cornea and iris were then sectioned. After the experiments, the lenses were excised, placed in 10% buffered formalin, and prepared for histology. Results: Central anterior lens capsule thickness was 9.4±2.9μm (human), 11.2±6.6μm (monkey), and 10.3±3.6μm (rabbit) optically and 14.9±1.6μm (human), 17.7±4.9μm (monkey), and 12.6±2.3μm (rabbit) histologically. The values for the central posterior capsule were 9.4±2.9μm (human), 6.6±2.5μm (monkey), and 7.9±2.3μm (rabbit) optically and 4.6±1.4μm (human), 4.5±1.2μm (monkey), and 5.7±1.7μm (rabbit) histologically. Conclusions: This study demonstrates that a non-contact optical system can successfully measure lens capsule thickness in cadaver eyes.
Mueller matrix of backscattered light of a chicken embryo
Dhiraj K. Sardar D.V.M., Raylon M. Yow
An in vivo investigation to determine instrinsic differences in optical activity of tissues of a three-day-old chicken embryo was performed. A homogenous flux of light at 514.4 nm from an argon ion laser was used to develop 13 of the 16 Mueller matrix components of backscattered light. The results show that all tissues backscatter light predominately in the same polarization state as that of the incident light. This effect manifests itself as large non-zero intensities in the diagonal terms of the Mueller matrix. Any change in polarization is distributed unbiased to the other polarization states as the off diagonal Mueller matrix elements each consists of a low intensity image. Depolarization by birefringent tissue structures such as collagen or actin-myosin fibers which would lead to non-zero intensities in the off diagonal Mueller matrix elements, is not observed. This may be due to the lack of such structures given the early age of the embryo.
Optical characterization of healthy and neovascularized human ocular tissues
Felipe S. Salinas, Dhiraj K. Sardar, Andrew T. C. Tsin
An in-depth characterization of optical properties of human retinal and retinal pigment epithelium (RPE)/choroidal tissues has been performed. The indices of refraction of these ocular tissues were determined by applying Brewster's law. The inverse adding doubling method, based on the diffusion approximation and radiative transport theory is applied to the measured values of the total diffuse transmission, total diffuse reflection, and collimated transmission to calculate the optical absorption, and scattering of the human retinal and retinal pigment epithelium/choroidal tissues. The scattering anisotropy coefficients were calculated using an independent method which relates scattering angles and intensities. The resulting values have been analyzed using appropriate statistical methods.
Optics of the Eye: Measurement, Correction, and Simulation
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Changes on the corneal thickness and curvature after orthokeratology
Iwane Mitsui, Yoshiya Yamada
To evaluate the corneal thickness and curvature changes after Orthokeratology contact lens wear, using the ORBSCAN II corneal topography system, corneal thickness and corneal curvature were measured on one hundred and twenty eyes of sixty patients before and after wearing the custom rigid gas permeable contact lenses for Orthokeratology. The contact lenses were specially designed for each eye. The subjects wore the orthokeratology lenses for approximately Four hours with their eyes closed. The corneal thickness of the subjects was increased on fifty-five eyes at not only the peripheral zone but also the center of the cornea. The average increase of central and peripheral corneal thickness was 18 micrometer and 22micrometer, respectively. The mean anterior curvature of corneal surface changed 1.25D. The mean posterior curvature of corneal endothelium side changed 0.75D.
Investigations for the correction of presbyopia by-fs-laser induced cuts
The most probable reason for presbyopia is an age-related loss of the elasticity of the lens. It develops through the whole life, but is first noticeable typically at the age of about 45. From that on it leads within 15 years to a total loss of the accommodation ability. However, both, the ciliary muscle and the lens capsule stay active and elastic, respectively. With respect to this, a possible treatment conception is to increase or regain the elasticity. The possibility to increase elasticity with ps-laser induced cuts inside the lens was already shown by Krueger. We made an improvement in cutting quality while using a fs laser with 5~kHz repetition rate emitting in the near infrared. Different fs-laser-induced μm smooth cuts inside fresh enucleated ex-vivo pig lenses will be presented.
Modelling the performance of accommodating intraocular lenses
Arthur Ho, Fabrice Manns, Therese Pham, et al.
Purpose. Pseudo-accommodating intra-ocular lenses (P-IOL) have been available for some time and the availability of accommodating IOL (A-IOL) is imminent. While these types of devices have been tested empirically, few studies have addressed the fundamental parameters governing their performance limits. We modelled the amplitude of accommodation of A-IOLs and P-IOLs to analyse parameters controlling their performance. Methods. Two types of two-element A-IOLs (those with a mobile anterior optical element, or a mobile posterior element) were modelled. Paraxial models were developed to identify key controlling parameters and potential optimal configurations, followed by finite modelling using computer assisted ray-tracing employing equi-convex/concave optical elements. A range of configurations representing varying focal lengths of front and back optical elements were tested. Degenerate cases representing P-IOLs were also tested. Results. P-IOLs have limited rate of pseudo-accommodation with axial shift (approximately 1.2D/mm). For A-IOLs, configurations with positive power front elements returned best rate of accommodation (up to approximately 3.0D/mm when the front element focal length is 25 mm). Conclusions. Considering the maximum potential amounts of axial shifts available, P-IOLs were predicted to provide less than 1D of accommodation whereas A-IOLs may provide up to 3-4D of accommodation, depending on design configuration.
Ex vivo accommodation simulator II: concept and preliminary results
A new instrument was developed to simulate the accommodation of excised human and animal crystalline lenses. The sclera is attached to 8 radially arranged translation stages and dissected into 8 segments. Change in accommodation is simulated by pulling and releasing the scleral segments, varying the load on the zonules and the capsular bag. Each translation stage is equipped with a sensitive force transducer for simultaneous recording of load and displacement. Additionally, changes in optical power, lens diameter, lens thickness and lens shape can be observed and quantified while the lens is stretched and released. The sensitivity and accuracy of the instrument was verified using steel springs and elastic bands. Preliminary results from sensitivity testing and instrument calibration are presented.
Laser and Light Interactions with Ocular Tissues and Implants
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Kinetics of corneal thermal shrinkage
David Borja, Fabrice Manns, William E. Lee, et al.
Purpose: The purpose of this study was to determine the effects of temperature and heating duration on the kinetics of thermal shrinkage in corneal strips using a custom-made shrinkage device. Methods: Thermal shrinkage was induced and measured in corneal strips under a constant load placed while bathed in 25% Dextran irrigation solution. A study was performed on 57 Florida Lions Eye Bank donated human cadaver eyes to determine the effect of temperature on the amount and rate of thermal shrinkage. Further experiments were performed on 20 human cadaver eyes to determine the effects of heating duration on permanent shrinkage. Data analysis was performed to determine the effects of temperature, heating duration, and age on the amount and kinetics of shrinkage. Results: Shrinkage consisted of two phases: a shrinkage phase during heating and a regression phase after heating. Permanent shrinkage increased with temperature and duration. The shrinkage and regression time constants followed Arrhenius type temperature dependence. The shrinkage time constants where calculated to be 67, 84, 121, 560 and 1112 (s) at 80, 75, 70, 65, and 60°C respectively. At 65°C the permanent shrinkage time constant was calculated to be 945s. Conclusion: These results show that shrinkage treatments need to raise the temperature of the tissue above 75°C for several seconds in order to prevent regression of the shrinkage effect immediately after treatment and to induce the maximum amount of permanent irreversible shrinkage.
A model of photothermally-induced damage to the retina during indocyanine green-assisted peeling of the inner limiting membrane
Intentional removal of the inner limiting membrane (ILM) in macular hole surgery is becoming a well-recognized procedure. It is usually performed with the assistance of Indocyanine Green (ICG), which selectively stains the membrane, in order to facilitate the visual control of surgery operations. In this theoretical study we investigate the possibility of heat damage to the retina being caused by the combination of ICG staining with the illumination provided by a standard light source for vitreo-retinal surgery, composed of a Xenon lamp and an optical fiber delivery system. For this purpose, we set up a bi-dimensional analytical model that describes light absorption and heat conduction in ICG-stained ILM and in retinal structures.
Noninvasive monitoring of the thermal stress in RPE using light-scattering spectroscopy
Georg Schule, Philip Huie, Alexander B. Vankov, et al.
Introduction: Light Scattering Spectroscopy has been a recently developed as a non-invasive technique capable of sizing the cellular organelles. With this technique, we monitor the heat-induced sub-cellular structural transformations in a human RPE cell culture. Material and Methods: A single layer of human RPE cells (ATCC) was grown on a glass slide. Cells are illuminated with light from a fiber-coupled broadband tungsten lamp. The backscattered (180 degree) light spectra are measured with an optical multichannel analyzer (OMA). Spectra are measured during heating of the sample. Results: We reconstructed the size distribution of sub-micron organelles in the RPE cells and observed temperature-related changes in the scattering density of the organelles in the 200-300nm range (which might be peroxisomes, microsomes or lysosomes). The sizes of the organelles did not vary with temperature, so the change in scattering is most probably due to the change in the refractive indexes. As opposed to strong spectral variation with temperature, the total intensity of the backscattered light did not significantly change in the temperature range of 32-49 °C. Conclusion: We demonstrate that Light Scattering Spectroscopy is a powerful tool for monitoring the temperature-induced sub-cellular transformations. This technique providing an insight into the temperature-induced cellular processes and can play an important role in quantitative assessment of the laser-induced thermal effects during retinal laser treatments, such as Transpupillary Thermal Therapy (TTT), photocoagulation, and Photodynamic Therapy (PDT).
Macular protection with IOLs
Per G. Soderberg, Stefan Lofgren, Marcelo Ayala, et al.
The clinical outcome within one month after phacoemulsification cataract extraction with implantation of the blue-blocking SN60AT IOL was examined prospectively and compared to a retrospectively examined material of implantations of the equivalent SA30AL without blue-blocker. There was no difference in best corrected visual acuity gain between the two lenses. In addition, the subjective color perception was examined for with a questionnaire after the first implantation of blue-blocking IOL and after the second implantation of blue-blocking IOL. Only one patient noted a changed color perception. There are thus strong theoretical reasons to block blue light in IOLs and no short term clinical inconvenience. But, it remains to be proven in long term follow up studies that the blue-blocking IOL protects against macular degeneration.
Development of fibrin-free intraocular lens with photochemical surface modification
Yuji Sato, Katsuya Tanizawa, Hiroyuki Anai, et al.
Having substituted the hydrophilic and hydrophobic groups alternately on the soft acrylic resin intraocular lens (IOL) surface by using an ArF excimer laser and a Xe2 excimer lamp, we have developed the IOL that is free from fibrin. Acrylic resin or PMMA lens has been used as an intraocular lens for 50 years. However, protein and fat are stuck onto the IOL surface after a long implantation, which opacifies the surface (after-cataract). Thus, we designed the micro domain structures of hydrophilic and hydrophobic groups on the IOL surface for fibrin-free. Firstly, the IOL was irradiated with the Xe2 excimer lamp in the presence of perfluoropolyether in order to make it hydrophobic. By this photochemical reaction, the CF3 functional groups were substituted on the IOL surface. Secondly, the ArF laser was projected on the IOL through the mask pattern in reduced size in the presence of water in order to be hydrophilic. With the photochemical reaction, the OH groups were substituted at the part exposed. The fibrin adsorption test of the modified IOL surface was carried out with FT-IR; which revealed that the fibrin-sticking rate of the treated sample has decreased by 23% compared with that of the non-treated sample. As a result, the fibrin-free IOL has been made by modifying the surface of the IOL to have the micro domain structures of the hydrophilic and hydrophobic groups that are arrayed alternately. In conclusion, the ideal intraocular lens has been demonstrated.
Ophthalmic Imaging
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Visualizing morphological changes of clear corneal cataract incisions with optical coherence tomography
Bin Rao, Jun Zhang, Mehran Taban, et al.
Dynamic morphological changes of clear corneal cataract incisions are studied with Optical Coherence Tomography (OCT). Two opposite types of dynamic incision wound behaviors are documented. A stable incision angle range is found to be existent for single-planed, clear corneal cataract incisions. When well pressurized, incision angles within this stable range result in well-apposed incision edges that resist gapping while incision angles falling outside this range have a larger tendency for wound leakage. It is also shown that a two-planed incision can effectively expand the stable range. For incision angles outside the stable range, the farther the incision angle is away from stable range, the larger the gap between incision wound edges when well pressurized. Thus, incision construction method has a major impact on the self-sealing capability of the incision wounds. In this investigation, OCT has been demonstrated as an effective modality for imaging and monitoring corneal surgery.
Mapping retinal thickness and macular edema by high-speed three-dimensional optical coherence tomography
Qienyuan Zhou, Peter K. Trost, Pak-Wai Lo, et al.
Conventional OCT generates one or few cross-sections of the retina and requires predetermination of measurement location and geometry. Because retinal pathologies are usually irregular and 3-dimensional in nature, a retinal imaging device with both high depth resolution and high lateral resolution is desired. The lateral resolution of the conventional OCT system is limited by sampling density, which in turn is limited by the speed of the system. In this paper, we present a three-dimensional optical coherence retinal tomograph (3D-OCT) which combines the rapid transversal imaging mode of a confocal scanning laser ophthalmoscope (cSLO) with the depth resolution of optical coherence tomography (OCT) to achieve high speed 3-D imaging. In contrary to the conventional OCT which performs adjacent A-scans to form a cross-section image (B-scan) perpendicular to the retinal surface, 3D-OCT acquires section images (C-scan) parallel to the retinal surface at defined depths across the thickness of the retina. Three-dimensional distribution of light-remitting sites within the retina is recorded at a depth resolution of ~12 μm (in eye) and lateral resolution of 10μm x 20μm within 1.2 seconds. In this paper, we present the results of in vivo retinal imaging of healthy volunteers and diabetic patients, retinal thickness mapping, and macular edema detection with the 3D-OCT device. Reproducibility of retinal thickness mapping ranges from 16 μm ~ 35 μm for different study subjects. Detailed retinal thickness map allows ready identification of location and area of macular thickening. C-scan images and continuous longitudinal cross section images provide visualization of pathological changes in the retina, such as presence of cyst formation and hard exudates. The need to predetermine measurement location and geometry is eliminated in 3D-OCT, in contrast to conventional OCT.
Real-time and static in vivo ophthalmic imaging by spectral optical coherence tomography
Fast Spectral Optical Coherence Tomography (SOCT) technique is used to perform cross sectional and three-dimensional ophthalmic images. Static, real-time and 3-D in vivo images of the human cornea, lens, iris, corneo-scleral junction, retinal layers, optic disc and macula lutea are presented. The ophthalmic application of SOCT is promising because this technique ensures fast acquisition with relatively low optical power of incident light. All demonstrated images are obtained with the aid of SOCT instrument, which was constructed in the optical laboratory of medical physics group at Nicolaus Copernicus University (Torun, Poland). What is to our knowledge there are the first good quality (>90dB sensitivity) ophthalmic OCT images obtained by technique, which is different than time domain OCT.
Quantification of photoreceptor layer thickness in different macular pathologies using ultrahigh-resolution optical coherence tomography
In vivo ultrahigh resolution ophthalmic OCT has been performed in more than 300 eyes of 200 patients with several retinal pathologies, demonstrating unprecedented visualization of all major intraretinal layers, in particular the photoreceptor layer. Visualization as well as quantification of the inner and outer segment of the photoreceptor layer especially in the foveal region has been acvhieved. In normal subjects the photoreceptor layer thickness in the center of the fovea is about of 90 μm, approximately equally distributed to the inner and the outer photoreceptor segment. In the parafoveal region this thickness is reduced to ~50 μm (~30 μm for the inner and ~20 μm for the outer segment). This is in good agreement with well known increase of cone outer segments in the central foveal region. Photoreceptor layer impairment in different macular pathologies like macular hole, central serous chorioretinopathy, age related macular degeneration, foveomacular dystrophies, Stargardt dystrophy as well as retinitis pigmentosa has been investigated. Photoreceptor layer loss significantly correlated with visual acuity (R2 = 0.6, p < 0.001) and microperimetry findings for the first time in 22 eyes with Stargardt dystrophy. Visualization and quantification of photoreceptor inner and outer segment using ultrahigh resolution OCT has the potential to improve early ophthalmic diagnosis, contributes to a better understanding of pathogenesis of retinal diseases as well as might have impact in the development and monitoring of novel therapy approaches.
Ophthalmic Diagnostics II
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Hyperspectral imaging analysis for ophthalmic applications
Gilberto Zamora, Paul W. Truitt, Sheila C. Nemeth, et al.
A continuing clinical need exists to find diagnostic tools that will detect and characterize the extent of retinal abnormalities as early as possible with non-invasive, highly sensitive techniques. The objective of this paper was to demonstrate the utility of a Hyperspectral Fundus Imager and related analytical tools to detect and characterize retinal tissues based on their spectral signatures. In particular, the paper shows that this system can measure spectral differences between normal retinal tissue and clinically significant macular edema. Future work will lead to clinical studies focused on spectrally characterizing retinal tissue, its diseases, and on the detection and tracking of the progression of retinal disease.
Compact single-channel Raman detector for macular pigments
Igor V. Ermakov, Maia R. Ermakova, Werner Gellermann
Raman detection of macular pigments (MP) holds promise as a novel noninvasive technology for the quantification of lutein and zeaxanthin carotenoids, which are thought to prevent or delay the onset of age-related macular degeneration. Using resonant excitation in the visible, we measure the Raman signals that originate from the double-bond stretch vibrations of the p-conjugated carotenoid molecule's carbon backbone. In this paper we describe the construction and performance of a new, compact, and low-cost MP Raman instrument using dielectric, angle-tuned band-pass filters for wavelength selection and single-channel photo-multiplier detection of carotenoid Raman responses. MP concentration measurements are fast and accurate, as seen in experiments with model eyes and living human eyes. The ease and rapidity of Raman MP measurements, the relative simplicity of the instrumentation, the high accuracy of the measurements, and the lack of significant systematic errors should make this technology useful for widespread clinical research.
Hand-held digital line-scanning laser ophthalmoscope (LSLO)
Scanning laser ophthalmoscopy is a powerful research tool with specialized but, to date, limited use in ophthalmic clinics due in part to the size, cost, and complexity of instruments. Conversely, low-cost retinal imaging devices have limited capabilities in screening, detection, and diagnosis of diseases. To fill the niche between these two, a low-cost, hand-held, line-scanning laser ophthalmoscope (LSLO) was designed, constructed, and tested on normal human subjects. The LSLO has only one moving part, multiple imaging modes, and uses low-cost but highly sensitive complimentary metal oxide semiconductor (CMOS) linear arrays for imaging with a detector dynamic range of 12-bits. The line-scanning approach produces high contrast quasi-confocal images with nearly the same performance as a flying-spot SLO. Imaging modes include simultaneous dual wavelength illumination and live stereoscopic imaging with a split aperture. Image processing and display functions are controlled with two stacked prototype compact printed circuit boards using field-programmable gated arrays (FPGA) and other digital electronic elements. With near shot-noise limited performance, the digital LSLO camera requires low illumination power (~ 100 μW) at near-infrared wavelengths. Wide field fundus images with several imaging modes have been obtained from several human subjects. The LSLO will significantly enhance confocal scanning laser ophthalmoscopy for routine use by ophthalmologist, optometrists, general practitioners and also non-specialized emergency medical personnel and technicians in the field for retinal disease detection and other diverse applications.
Wide-field stabilized Doppler flowmetry with the tracking scanning laser ophthalmoscope (TSLO)
R. Daniel Ferguson, Daniel X. Hammer, Ann E. Elsner, et al.
Real time, high-speed image stabilization with a retinal tracking scanning laser ophthalmoscope (TSLO) enables new approaches to established diagnostics. Large dynamic range, wide-field stabilized Doppler flowmetry imaging was demonstrated in initial human subject testing with a large frequency range and up to 40 deg fields of view. The retinal tracking system uses a confocal reflectometer with a closed loop optical servo system to lock onto features in the ocular fundus. The fundus imaging method is a quasi-confocal line-scanning laser ophthalmoscope (LSLO). By locking to the retina and performing a slow scan of the laser line imager taking up to 30 seconds, frequency-resolved retinal perfusion and vascular flow images were obtained. The tracking system and auto-relock algorithms after blinks allowed the full image to be captured free of eye motion artifacts. Image cubes comprised of 512 by 512 spatial pixels by 512 or more temporal pixels were captured with a line rate of up to 20 kHz. The line sensor integrates, as opposed to conventional SLO flowmeters, thus minimizing aliasing that contributes to velocity errors. Fast Fourier Transform (FFT) power spectra at each image pixel were computed. Image reconstruction accuracy was typically better than one pixel width. Doppler frequency resolution from 14 Hz to 3.675 kHz was demonstrated in this work. Pulsatile flow effects were observed. Normal adult subjects and patients were tested with or without mydriasis to characterize flow imaging performance.
Analysis of birefringent image in the retinal nerve fiber layer by polarization-sensitive optical coherence tomography
Phase retardation per unit depth (PR/UD) is a physiologically significant parameter which correlates with the orderly arrangement of neurons and neurofibrils within the retinal nerve fiber layer (RNFL) and can vary in glaucoma. The objective of this study is to use Polarization Sensitive Optical Coherence Tomography (PSOCT), to detect both RNFL thickness and depth-resolved birefringence and produce thickness and PR/UD maps of the primate RNFL. RNFL thickness is obtained from the PSOCT intensity image with boundary detection using imaging processing methods. Analysis of PR/UD begins with calculating the Stokes parameters from the horizontal magnitude, vertical magnitude and relative phase difference in the interference fringes of light back scattered from the retina. Computed Stokes parameters are used to determine the fast axis in the RNFL and local phase retardation. PR/UD is calculated by dividing local birefringence by the corresponding RNFL thickness. A three-dimensional map of PR/UD and RNFL thickness is constructed by combining registered B-scans. Three-dimensional maps of thickness and PR/UD from the primate RNFL show that PSOCT is can be used for detecting thickness and PR/UD of the peripapillary RNFL and a very effective modality to diagnose glaucoma.
Thickness and birefringence of retinal nerve fiber layer of healthy and glaucomatous subjects measured with polarization-sensitive optical coherence tomography
Barry Cense, Teresa C. Chen, B. Hyle Park, et al.
Changes in retinal nerve fiber layer thickness and birefringence may both precede clinically detectable glaucomatous vision loss. Early detection of retinal nerve fiber layer changes may enable treatment to prevent permanent loss of vision. Polarization sensitive optical coherence tomography (PS-OCT) can provide objective information on retinal nerve fiber layer thickness and birefringence. PS-OCT scans around the optic nerve head (ONH) of two healthy young volunteers were made using 10 concentric circles of increasing radius. Both the mean retinal nerve fiber layer thickness and mean retinal nerve fiber birefringence for each of 48 sectors on a circle were determined with data analysis. Birefringence of healthy RNFL is constant as a function of scan radius but varies as a function of position around the ONH, with higher values occurring superior and inferior to the ONH. Measured double pass phase retardation per unit depth values around the ONH range between 0.10 and 0.35°/μm, equivalent to birefringence values of 1.2•10-4 and 4.1•10-4 respectively, measured at a wavelength of 840 nm. Consequently, conversion of phase retardation measurements (as obtained with scanning laser polarimetry) to RNFL thickness measurements, assuming a constant birefringence value, will yield thickness values that are incorrect.
Ocular Wavefront Aberrometry
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Finite element modelling of soft contact lens flexure and aberrations
Arthur Ho, Simon Evans
Background. Contact lenses may be optimised to correct for the aberrations of the eye and provide superior vision. However, due to various requirements, soft contact lenses (SCL) are deliberately fitted to be different from the shape of the eye. As a consequence, SCL will distort when placed on the eye in the clinical phenomenon known as the Lens Flexure Effect (LFE). The defocus effect of LFE has only been studied previously but little is known about its effect on aberrations. With the current intentions to correct aberrations using SCL, the effect of LFE on higher order aberrations becomes important. Method. We used finite element analysis (ANSYS) to investigate the shape change of SCL placed on eye. We transferred the output as sixteenth order even polynomials into a ray-tracing program (Zemax) to evaluate the optical performance of the pre-flexed and post-LFE SCL. A range of SCL base-curves (BOZR, 7.0 mm to 9.0 mm in seven steps) and back vertex powers (BVP, -12D to +3D in seven steps) were studied to look at the impact of these parameters on LFE. Results. Effect of LFE of BVP increases with BVP and departure of BOZR from corneal shape. However, within clinically practical values, the impact is small (around 0.5D). LFE also has an impact on spherical aberrations although its impact is varied according to BVP and BOZR. Conclusion. The optical impact of SCL LFE can be predicted using computation modelling. LFE may limit the performance of some designs of aberration-corrected SCL.
Effect of the internal optics on the outcome of custom-LASIK in an eye model
Purpose. The purpose of this study was to evaluate if changes in the aberration-contribution of the internal optics of the eye have a significant effect on the outcome of wavefront-guided corneal reshaping. Methods. The Navarro-Escudero eye model was simulated using optical analysis software. The eye was rendered myopic by shifting the plane of the retina. Custom-LASIK was simulated by changing the radius of curvature and asphericity of the anterior corneal surface of the eye model. The radius of curvature was adjusted to provide a retinal conjugate at infinity. Three approaches were used to determine the postoperative corneal asphericity: minimizing third-order spherical aberration, minimizing third-order coma, and maximizing the Strehl ratio. The aberration contribution of the anterior corneal surface and internal optics was calculated before and after each simulated customized correction. Results. For a 5.2mm diameter pupil, the contribution of the anterior corneal surface to third-order spherical aberration and coma (in micrometers) was 2.22 and 2.49 preop, -0.36 and 2.83 postop when spherical aberration is minimized, 5.88 and 1.10 postop when coma is minimized, and -0.63 and 2.91 postop when Strehl ratio is maximized. The contribution of the internal optics of the eye to spherical aberration and coma for the same four conditions was: 0.43 and -1.13, 0.37 and -1.10, 0.37 and -1.10 and 0.37 and -1.10, respectively. Conclusion. In the model eye, the contribution of the internal optics of the eye to the change in the ocular aberration state is negligible.
Relationship between total aberrations (RMS) and pupil size in ametropic eyes with the LADARWAVE system
Ezra Maguen M.D., Jasmine Wong, Jay Schlanger, et al.
Purpose: To evaluate the relationship between pupil size and total aberrations as represented by the root mean square value (RMS) in ametropic otherwise normal eyes. Method: 22 eyes (11 patients) had wavefront measurements with both undilated and dilated pupils. Total, and spherical aberrations, and coma were recorded using the RMS. Software integrated within the LadarWave system allowed simulation of all of the above aberrations for increments of 0.5mm of pupil dilation. Results: A linear relationship was found in all eyes between pupil diameter and total aberrations as expressed in RMS, with a high coefficient of determination (R2). The mean R2±SD was 0.9515±0.0334 (range: 0.886-0.9835). The slope was variable. Conclusions: This data shows the existence of a direct relationship between pupil diameter and the total amount of higher order aberrations as expressed by RMS. The variable slope needs to be further investigated as it may correlate with symptoms related to the presence of higher order aberrations both in the normal state and postoperatively.
Measuring peripheral wavefront aberrations in subjects with large central visual field loss
Linda Lundstrom, Peter Unsbo, Jorgen Gustafsson
Introduction: In a previous study we have shown that correction of peripheral refractive errors can improve the remaining vision in the preferred retinal location (PRL) of subjects with large central visual field loss (CFL). Measuring peripheral refractive errors with traditional methods is often difficult due to the low visual acuity and large aberrations. Therefore a Hartmann-Shack (HS) sensor has been designed to measure peripheral wavefront aberrations in CFL subjects. Method: The HS sensor incorporates an eyetracker and analyzing software designed to handle large wavefront aberrations. To ensure that the measurement axis is aligned with the subject's PRL, a special fixation target has been developed. It consists of concentric rings surrounding the aperture of the HS together with a central fixation mark along the measurement axis. Results: Some initial measurements on subjects with CFL have been performed successfully. As a first step in improving the peripheral optics of the eye, the wavefront data have been used to calculate the subject's optimal eccentric refraction. Conclusion: Measuring the wavefront aberrations is a fast and easy way to assess the details of the optics in subjects with CFL. The wavefront data can then be used to better understand the problems of eccentric correction.
Using wavefront measurements to predict visual acuity
The traditional means of measuring visual acuity in human eyes relies on eye charts and the patient's perceptions. With the advent of wavefront-based technologies, it is now feasible to objectively determine optical resolution. This paper proposes a technique using a resolution spoke to accurately predict visual acuity based on wavefront measurements. Resolution rings are constructed using Rayleigh's criterion for the determination of optical acuity; subsequent cross correlation of the blurred resolution spoke with the un-blurred spoke is used to estimate decentration of the PSF. After laser refractive surgery, the visual acuity of 11 eyes (formerly myopic) was estimated using this technique. The predicted visual acuity was compared to the corresponding subjective measurements using 100% contrast. The correlation variance between predicted and measured acuity was about 74%, which shows that the optical acuity of human eyes can be measured objectively.
Simulated retinal images of Landolt rings in human eyes including asymmetric aberrations on the point spread function analysis system
To simulate the retinal images of the human eye including asymmetric aberrations is very important and interesting with using a new point spread function analysis system (PSFAS). The point light source (SLD 840 nm) was projected onto the subject's eye and the reflected image at the retina was captured by the charge coupled device which was in the conjugated point with the retina (double-pass formula). To obtain the modulation transfer function (MTF) of the optical system, equal sized apertures were used as entrance and exit pupils. To obtain the phase transfer function (PTF), unequal sized apertures were used. To obtain the simulated retinal images, the Fourier spectrum of the original chart was multiplied by the MTF and the PTF as the phase term of the original chart was added. The inverse Fourier transformation of the Fourier spectrum term and the phase term was the simulated retinal image. The simulated retinal images of the Landolt rings in human eyes might sufficiently involve asymmetric aberrations without losing the high-frequency range of spatial frequency. The PSFAS can simulate retinal images, which are based on not only the information from the symmetric aberrations and the scattering and absorption of haze but also on the information from the asymmetric aberrations. The PSFAS can objectively evaluate the characteristics of the human optical system and therefore is useful in an ophthalmology clinic setting.
Experimental Techniques in Ocular Surgery
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Diode laser welding for cornea suturing: an experimental study of the repair process
Roberto Pini, Francesca Rossi, Luca Menabuoni, et al.
We present an experimental study, performed in vivo, to evaluate the application of laser welding of the cornea. The welding technique is based on controlled irradiation provided by a diode laser (810 nm) operating at low power (60-90 mW), in association with the chromophore Indocyanine Green which was applied locally in the corneal wound to be repaired. Thirty-four rabbits were selected to undergo both conventional and laser-induced suturing of corneal wounds, to simulate cataract surgery and penetrating keratoplasty. A follow-up study 7-30 days after surgery was carried out by means of histological examinations of cornea slices, in order to investigate the healing process induced by laser welding. In the laser group, the analyses evidenced a faster and more effective restoration of the architecture of the stroma, with re-organization of both epithelium and endothelium. We also present measurements of the spectral absorption of Indocyanine Green and of the temperature rise in the corneal surface during laser welding.
Deep sclerectomy using erbium:YAG laser in pig’s eyes
Yehia A. Badr D.D.S., Ibrahim M. Taher M.D., Mostafa M. Bahgat M.D., et al.
The potential benefits of using pulsed Erbium: YAG laser in removing the deep lamella of the sclera during the procedure of deep sclerectomy was studied. Thirty porcine eyes were divided into 3 groups. A superficial lamellar scleral flap with an area of 5x5 mm as for trabeculectomy was surgically prepared. Using an Erbium: YAG laser (2.94 micron), the deep lamella with an area of 3x1.8 mm was removed. Group I was subjected to an energy level of 40-60 m.J, Descemet's membrane was preserved and trabecular meshwork was left intact and no thermal damage on the contiguous structures in all eyes, group II to 60-80 m.J Descemet's membrane was ruptured in 30% (3 eyes), thermal damage was 20% (2 eyes) on superficial structures, while group III to 80-100m.J there was a high risk of rupture of Descemet's membrane 50% (5 eyes), thermal damage was 30%(3 eyes) on superficial structures & 20%(2 eyes) on deep & superficial structures. Eyes were analyzed histologically by electron microscopy to study Descemet's membrane & the trabecular meshwork & the thermal damage on contiguous structures. Eyes with rupture of Descemet's membrane had total energy power of 11.75 J +/- 6.39, average power was 0.58W +/- 0.07 & power density 1155W/cm2 +/- 144, compared to eyes with no rupture 24.23J +/- 11.77 total energy power, 0.46W average power & 916.4W/cm2 +/- 227 power density. Thermal damage changes occurred at total energy power of 10.14 J, average power was 0.59W & power density 1180W/cm2, compared to eyes with no rupture 24.17J total energy power, 0.46W average power & 919.1W/cm2 power density.
First in vivo animal studies on intraocular nanosurgery and multiphoton tomography with low-energy 80-MHz near-infrared femtosecond laser pulses
Karsten Konig, Bagui Wang, Oliver Krauss, et al.
We report on a method for refractive laser surgery based on low-energy femtosecond laser pulses provided by ultracompact turn-key non-amplified laser systems. An additional excimer laser is not required for ablation of the stroma. The novel method has the potential to be used for (i) optical flap creation as well as stroma ablation and (ii) for non-invasive flap-free intrastromal ablation. In addition, 3D multiphoton imaging of the cornea can be performed. In particular, we used sub-nanojoule near infrared 80 MHz femtosecond laser pulses for multiphoton imaging of corneal structures with ultrahigh resolution (< 1μm) as well as for highly precise intraocular refractive surgery. Imaging based on two-photon excited cellular autofluorescence and SHG formation in collagen structures was performed at GW/cm2 intensities, whereas destructive optical breakdown for nanoprocessing occurred at TW/cm2 light intensities. These high intensities were realized with sub-nJ pulses within a subfemtoliter intrastromal volume by diffraction-limited focussing with high NA objectives and beam scanning 50 to 140 μm below the epithelial surface. Multiphoton tomography of the cornea was used to determine the target of interest and to visualize intraocular post-laser effects. Histological examination with light- and electron microscopes of laser-exposed porcine and rabbit eyes reveal a minimum intratissue cut size below 1 μm without destructive effects to surrounding collagen structures. LASIK flaps and intracorneal cavities could be realized with high precision using 200 fs, 80 MHz, sub-nanojoule pulses at 800 nm. First studies on 80 MHz femtosecond laser surgery on living rabbits have been performed.
Electro-adhesive forceps for tissue manipulation
Alexander B. Vankov, Philip Huie, Mark S. Blumenkranz, et al.
Capturing, separation and removal of thin, evasive, and often transparent membranes attached to the underlying tissue is typically a very difficult task in vitreoretinal surgery. The most challenging part of such procedures is in initial separation of the membrane, which then allows for a strong grip of the micro-tweezers holding it from two sides. Attempts of performing this procedure often lead to piercing and otherwise damaging the underlying tissue. Accordingly, there is a need for devices that could attach to tissue in a minimally-traumatic manner approaching it from only one side. It is desirable that such a device would attach to a tissue on a push of a button and release it on demand. We developed a technique that allows for strong attachment of an electrode to tissue with a single electrical pulse, and disconnection of it from the tissue with a different pulse. Adhesion does not require any electrical support after the pulse, and the adhesive forces generated on a wire electrode of 50 micrometer in diameter are sufficient for manipulation of all types of cellular and non-cellular intraocular tissues. To reduce electroporation-related tissue damage the bipolar train of pulses is applied with burst duration 50-200 microsecond. At optimal pulse parameters the tissue damage is limited to a single layer of cells adjacent to the surface of electrode. Electrically-induced adhesion is very convenient for lifting and manipulation of vitreoretinal membranes. It can also be used for attachment of a needle to a membrane and injection of liquid into the sub-membrane space, thus separating the membrane from the underlying tissue without peeling. Similarly, injection of medication into small retinal blood vessels can be performed without insertion of the needle inside the blood vessels.
Optoacoustic online dosimetry during selective RPE treatment
Georg Schuele, Hanno Elsner, Hans Hoerauf, et al.
Introduction: The selective RPE treatment (SRT) is a new method, which targets retinal diseases associated with disorders in the retinal pigment epithelium (RPE). By applying a train of μs laser pulses, it is possible to selectively damage RPE cells while sparing the adjacent photoreceptors and the neural retina. Due to the ophthalmoscopic invisibility of the RPE effects we investigated an optoacoustic (OA) on-line dosimetry system to monitor RPE damage non-invasively. Material and Methods: For in vitro experiments porcine RPE was irradiated with a Nd:YLF laser pulse train (527nm, 1.7μs, 5-40μJ, 30 pulses, 100 Hz). Pressure waves (optoacoustic transients) generated at the RPE were measured with a piezoelectric transducer. The RPE cell damage was visualised by fluorescence microscopy by means of the vitality stain CalceinAM. During 27 patient treatments (527nm, 1.7μs, 50-150μJ, 30 pulses, 100 Hz) the optoacoustic signals were measured with an ultrasonic transducer embedded in the contact lens. The RPE leakage was visualized by fluorescein and ICG angiography. Results: In vitro: Below the RPE cell damage threshold, the optoacoustic transients from each single pulse are almost similar. With RPE damage, fluctuations of the individual transients are observed during the pulse train. These fluctuations can be explained by statistical irregular microbubble formation around the strong light absorbing melanosomes inside the RPE cells, which occur after the temperature exceeds the vaporization threshold. The transient microbubbles probably lead to RPE cell disruption. An optoacoustic value (OA-value) calculated from the fluctuations was defined in order to assess RPE damage. Patient treatment: If optoacoustic pulse-to-pulse fluctuations were measured, RPE leakage was observed in fluorescein and ICG angiography. In 96% of the irradiated areas, RPE-leakage in fluorescein angiography and OA-value correlated. The stronger the optoacoustic pulse-to-pulse fluctuations, thus the higher the OA-value, the more intense angiographic leakage was observed in ICG-angiography. Conclusion: A non-invasive optoacoustic on-line dosimetry control to monitor RPE damage during SRT was developed. In order to avoid invasive angiography, it is currently evaluated in a multicenter clinical SRT study.
Optical combing to align photoreceptors in detached retinas
Shizhuo Yin, Thomas W. Gardner, Fei Wu, et al.
In this paper, we presented a novel micro-manipulating method, called 'optical combing', that could improve the retina reattachment surgery results. Optical combing adopts the working principle of optical tweezers (i.e., focused Gaussian beam produces a trapping force when it incidents onto a micro-object. The trapping force can pull the micro-object to the central point of focused laser beam. Optical combing is implemented by scanning a focused laser beam on the misaligned micro objects (such as misaligned photoreceptors). In our preliminary experiment, a set of misaligned micro glass rods was re-aligned by applying this optical combing technology, which verified our theory. In the future, this technique will be used to re-align misaligned photoreceptors in real retina.
Attracting retinal cells to electrodes for high-resolution stimulation
Daniel V. Palanker, Philip Huie, Alexander B. Vankov, et al.
Development of the electronic retinal prosthesis for restoration of sight in patients suffering from the degenerative retinal diseases faces many technological challenges. To achieve significant improvement in the low vision patients the visual acuity of 20/80 would be desirable, which corresponds to the pixel size of 20μm in the retinal implant. Stimulating current strongly (quadratically) depends on distance between electrode and cell. To achieve uniformity in stimulation thresholds, to avoid erosion of the electrodes and overheating of tissue, and to reduce the cross-talk between the neighboring pixels the neural cells should not be separated from electrodes by more than a few micrometers. Achieving such a close proximity along the whole surface of an implant is one of the major obstacles for the high resolution retinal implant. To ensure proximity of cells and electrodes we have developed a technique that prompts migration of retinal cells towards stimulating sites. The device consists of a multilayered membrane with an array of perforations of several (5-15) micrometers in diameter in which addressable electrodes can be embedded. In experiments in-vitro using explants of the whole retina of P7 rats, and in-vivo using adult rabbits and RCS rats the retinal tissue grew into the pores when membranes were positioned on the sub-retinal side. Histology has demonstrated that migrating cells preserve synaptic connections with cells outside the pores, thus allowing for signal transduction into the retina above the implant. Intimate proximity of cells to electrodes achieved with this technique allows for reduction of the stimulation current to 2μA at the 10μm electrode. A 3mm disk array with 18,000 pixels can stimulate cells with 0.5 ms pulses at 50Hz while maintaining temperature rise at the implant surface below 0.3°C. Such an implant can, in principle, provide spatial resolution geometrically corresponding to the visual acuity of 20/80 in a visual field of 10°.
Poster Session
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Near infrared Nd:YAP laser radiation transmission through the human eye tissue structure
Helena Jelinkov, Jiri Pasta, Jan Sulc, et al.
Comparison of the eye tissue transmission for two (1.08 μm and 1.34 μm) Nd:YAP laser wavelengths was done. The lasers were working in pulsed free-running regime. The interaction energy of ~100 mJ and spot diameter of 5 mm was used for both wavelengths yielding in the radiation fluence ~0.51 J/cm2 (8500 W/cm2). From the step by step transmission measurement of the human eye globe layers (in vitro) was recognized that the value of the absorbed energy in particular segments was different for monitoring wavelengths yielding in the two orders fluence difference on the retina. From the results follows that the 1.34 μm radiation is absorbed mainly by the first segments of the eye in contrast to 1.08 μm for which the main part of the radiation going through all eye layers.
Optics of the Eye: Measurement, Correction, and Simulation
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Simulation of optical path and volume-under-test calculation in an integrated dynamic light scattering and autofluorescence device
We have developed a non-invasive diagnostic instrument based on a scanning fluorophotometer integrating autofluorescence and dynamic light scattering techniques. The device makes a scan along the optical axis of the eye. In a DLS measurement, it is extremely important to precisely define the position of the volume-under-test inside the eye and its effective volume. In this work, a specific optical computer aided design tool based on the Navarro's model of the human eye is used to simulate the optical path of the DLS laser beams and to calculate position and size of the volume-under-test.
Computational modeling of corneal refractive surgery
Delia Cabrera Fernandez, Abdel-Salam M. Niazy, Ronald M. Kurtz, et al.
A finite element method was used to study the biomechanical behavior of the cornea and its response to refractive surgery when stiffness inhomogeneities varying with depth are considered. Side-by-side comparisons of different constitutive laws that have been commonly used to model refractive surgery were also performed. To facilitate the comparison, the material property constants were identified from the same experimental data, which were obtained from mechanical tests on corneal strips and membrane inflation experiments. We then validated the resulting model by comparing computed refractive power changes with clinical results. The model developed provides a much more predictable refractive outcome when the stiffness inhomogeneities of the cornea and nonlinearities of the deformations are included in the finite element simulations. Thus, it can be stated that the inhomogeneous model is a more accurate representation of the corneal material properties in order to model the biomechanical effects of refractive surgery. The simulations also revealed that the para-central and peripheral parts of the cornea deformed less in response to pressure loading compared to the central cornea and the limbus. Furthermore, the deformations in response to pressure loading predicted by the non-homogeneous and nonlinear model, showed that the para-central region is mechanically enhanced in the meridional direction. This result is in agreement with the experimentally documented regional differences reported in the literature by other investigators.
Experimental Techniques in Ocular Surgery
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Computer-assisted 3-D design software for teaching neuro-ophthalmology of the oculomotor system and training new retinal surgery techniques
Carl Glittenberg M.D., Susanne Binder M.D.
Purpose: To create a more effective method of demonstrating complex subject matter in ophthalmology with the use of high end, 3-D, computer aided animation and interactive multimedia technologies. Specifically, to explore the possibilities of demonstrating the complex nature of the neuroophthalmological basics of the human oculomotor system in a clear and non confusing way, and to demonstrate new forms of retinal surgery in a manner that makes the procedures easier to understand for other retinal surgeons. Methods and Materials: Using Reflektions 4.3, Monzoom Pro 4.5, Cinema 4D XL 5.03, Cinema 4D XL 8 Studio Bundle, Mediator 4.0, Mediator Pro 5.03, Fujitsu-Siemens Pentium III and IV, Gericom Webgine laptop, M.G.I. Video Wave 1.0 and 5, Micrografix Picture Publisher 6.0 and 8, Amorphium 1.0, and Blobs for Windows, we created 3-D animations showing the origin, insertion, course, main direction of pull, and auxiliary direction of pull of the six extra-ocular eye muscles. We created 3-D animations that (a) show the intra-cranial path of the relevant oculomotor cranial nerves and which muscles are supplied by them, (b) show which muscles are active in each of the ten lines of sight, (c) demonstrate the various malfunctions of oculomotor systems, as well as (d) show the surgical techniques and the challenges in radial optic neurotomies and subretinal surgeries. Most of the 3-D animations were integrated in interactive multimedia teaching programs. Their effectiveness was compared to conventional teaching methods in a comparative study performed at the University of Vienna. We also performed a survey to examine the response of students being taught with the interactive programs. We are currently in the process of placing most of the animations in an interactive web site in order to make them freely available to everyone who is interested. Results: Although learning how to use complex 3-D computer animation and multimedia authoring software can be very time consuming and frustrating, we found that once the programs are mastered they can be used to create 3-D animations that drastically improve the quality of medical demonstrations. The comparative study showed a significant advantage of using these technologies over conventional teaching methods. The feedback from medical students, doctors, and retinal surgeons was overwhelmingly positive. A strong interest was expressed to have more subjects and techniques demonstrated in this fashion. Conclusion: 3-D computer technologies should be used in the demonstration of all complex medical subjects. More effort and resources need to be given to the development of these technologies that can improve the understanding of medicine for students, doctors, and patients alike.
Ocular Wavefront Aberrometry
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Study of the effects caused by displacement of wavefront measurement
Qi Li, Qiushi Ren
Purpose: To study how test results of wavefront aberrations are affected by displacement during wavefront measurement. Methods: A Shack-Hartmann based wavefront sensor was used to measure an artificial eye at controlled positions along axial and lateral directions. We use deviation amplitude and relative deviation amplitude to study the changes of reconstructed wavefront aberrations during the displacement within 1.00 mm. The analysis was performed with a 4th-order Zernike polynomial expansion with the 0th-order (piston) and 1st-order (tip and tilt) removed. RMS of the whole and each order are studied. Results: Effects caused by movement along axis within 1.00 mm are not apparent. A dramatic change was observed in relative deviation amplitude during the movement off the center, although the absolute value of changes are small. The changes are highly polynomials depended. RMS stays relatively stable during both procedures. Conclusions: When wavefront aberration is introduced into customized refractive surgery, it is important for physicians to align the device during measurements, particularly in lateral direction.