Lana J. Nagy

Large-dynamic-range Shack-Hartmann wavefront sensor for highly aberrated eyes.

A conventional Shack-Hartmann wavefront sensor has a limitation that increasing the dynamic range usually requires sacrificing measurement sensitivity. The prototype large-dynamic-range Shack-Hartmann wavefront sensor presented resolves this problem by using a translatable plate with subapertures placed in conjugate with the lenslet array. Each subaperture is the same size as a lenslet and they are arranged so that they overlap every other lenslet position. Three translations of the plate are required to acquire four images to complete one measurement. This method increases the dynamic range by a factor of two with no subsequent change in measurement sensitivity and sampling resolution of the aberration. The feasibility of the sensor was demonstrated by measuring the higher order aberrations of a custom-made phase plate and human eyes with and without the plate.

Intratissue Refractive Index Shaping (IRIS) of the Cornea and Lens Using a Low-Pulse-Energy Femtosecond Laser Oscillator

PURPOSE To assess the optical effect of high-repetition-rate, low-energy femtosecond laser pulses on lightly fixed corneas and lenses. METHODS Eight corneas and eight lenses were extracted postmortem from normal, adult cats. They were lightly fixed and stored in a solution that minimized swelling and opacification. An 800-nm Ti:Sapphire femtosecond laser oscillator with a 27-fs pulse duration and 93-MHz repetition rate was used to inscribe gratings consisting of 20 to 40 lines, each 1-microm wide, 100-microm long, and 5-microm apart, 100 mum below the tissue surface. Refractive index changes in the micromachined regions were calculated immediately and after 1 month of storage by measuring the intensity distribution of diffracted light when the gratings were irradiated with a 632.8-nm He-Ne laser. RESULTS Periodic gratings were created in the stromal layer of the corneas and the cortex of the lenses by adjusting the laser pulse energy until visible plasma luminescence and bubbles were no longer generated. The gratings had low scattering loss and could only be visualized using phase microscopy. Refractive index changes measured 0.005 +/- 0.001 to 0.01 +/- 0.001 in corneal tissue and 0.015 +/- 0.001 to 0.021 +/- 0.001 in the lenses. The gratings and refractive index changes were preserved after storing the micromachined corneas and lenses for 1 month. CONCLUSIONS These pilot experiments demonstrate a novel application of low-pulse-energy, MHz femtosecond lasers in modifying the refractive index of transparent ocular tissues without apparent tissue destruction. Although it remains to be verified in living tissues, the stability of this effect suggests that the observed modifications are due to long-term molecular and/or structural changes.

Photorefractive keratectomy in the cat eye: Biological and optical outcomes

PURPOSE: To quantify optical and biomechanical properties of the feline cornea before and after photorefractive keratectomy (PRK) and assess the relative contribution of different biological factors to refractive outcome. SETTING: Department of Ophthalmology, University of Rochester, Rochester, New York, USA. METHODS: Adult cats had 6.0 diopter (D) myopic or 4.0 D hyperopic PRK over 6.0 or 8.0 mm optical zones (OZ). Preoperative and postoperative wavefront aberrations were measured, as were intraocular pressure (IOP), corneal hysteresis, the corneal resistance factor, axial length, corneal thickness, and radii of curvature. Finally, postmortem immunohistochemistry for vimentin and α‐smooth muscle actin was performed. RESULTS: Photorefractive keratectomy changed ocular defocus, increased higher‐order aberrations, and induced myofibroblast differentiation in cats. However, the intended defocus corrections were only achieved with 8.0 mm OZs. Long‐term flattening of the epithelial and stromal surfaces was noted after myopic, but not after hyperopic, PRK. The IOP was unaltered by PRK; however, corneal hysteresis and the corneal resistance factor decreased. Over the ensuing 6 months, ocular aberrations and the IOP remained stable, while central corneal thickness, corneal hysteresis, and the corneal resistance factor increased toward normal levels. CONCLUSIONS: Cat corneas exhibited optical, histological, and biomechanical reactions to PRK that resembled those previously described in humans, especially when the OZ size was normalized to the total corneal area. However, cats exhibited significant stromal regeneration, causing a return to preoperative corneal thickness, corneal hysteresis and the corneal resistance factor without significant regression of optical changes induced by the surgery. Thus, the principal effects of laser refractive surgery on ocular wavefront aberrations can be achieved despite clear interspecies differences in corneal biology.

Optical effects of anti-TGFβ treatment after photorefractive keratectomy in a cat model

PURPOSE To assess the contribution of corneal myofibroblasts to optical changes induced by photorefractive keratectomy (PRK) in a cat model. METHODS The transforming growth factor (TGF)-beta-dependence of feline corneal keratocyte differentiation into alpha-smooth muscle actin (alphaSMA)-positive myofibroblasts was first tested in vitro. Twenty-nine eyes of 16 cats were then treated with -10 D PRK in vivo and divided into two postoperative treatment groups that received either 100 microg anti-TGFbeta antibody for 7 days, followed by 50 microg dexamethasone for another 7 days to inhibit myofibroblast differentiation, or vehicle solution for 14 days (control eyes). Corneal thickness and reflectivity were measured by optical coherence tomography. Wavefront sensing was performed in the awake-behaving state before surgery and 2, 4, 8, and 12 weeks after surgery. Wound healing was monitored using in vivo confocal imaging and postmortem alphaSMA immunohistochemistry. RESULTS In culture, TGFbeta caused cat corneal keratocytes to differentiate into alphaSMA-positive myofibroblasts, an effect that was blocked by coincubation with anti-TGFbeta antibody. In vivo, anti-TGFbeta treatment after PRK resulted in less alphaSMA immunoreactivity in the subablation stroma, lower corneal reflectivity, less stromal regrowth, and lower nonspherical higher order aberration induction than in control eyes. However, there were no intergroup differences in epithelial regeneration or lower order aberration changes. CONCLUSIONS Anti-TGFbeta treatment reduced feline corneal myofibroblast differentiation in vitro and after PRK. It also decreased corneal haze and fine-grained irregularities in ocular wavefront after PRK, suggesting that attenuation of the differentiation of keratocytes into myofibroblast can significantly enhance optical quality after refractive surface ablations.

Potentiation of Femtosecond Laser Intratissue Refractive Index Shaping (IRIS) in the Living Cornea with Sodium Fluorescein

PURPOSE To assess the effectiveness of intratissue refractive index shaping (IRIS) in living corneas and test the hypothesis that it can be enhanced by increasing the two-photon absorption (TPA) of the tissue. METHODS Three corneas were removed from adult cats and cut into six pieces, which were placed in preservative (Optisol-GS; Bausch & Lomb, Inc., Irvine, CA) containing 0%, 0.25%, 1%, 1.5%, or 2.5% sodium fluorescein (Na-Fl). An 800-nm Ti:Sapphire femtosecond laser with a 100-fs pulse duration and 80-MHz repetition rate was used to perform IRIS in each piece, creating several refractive index (RI) modification lines at different speeds (between 0.1 and 5 mm/s). The lines were 1 mum wide, 10 microm apart, and approximately 150 microm below the tissue surface. The RI change of each grating was measured using calibrated, differential interference contrast microscopy. TUNEL staining was performed to assess whether IRIS or Na-Fl doping causes cell death. RESULTS Scanning at 0.1 mm/s changed the RI of undoped, living corneas by 0.005. In doped corneas, RI changes between 0.01 and 0.02 were reliably achieved with higher scanning speeds. The magnitude of RI changes attained was directly proportional to Na-Fl doping concentration and inversely proportional to the scanning speed used to create the gratings. CONCLUSIONS IRIS can be efficiently performed in living corneal tissue. Increasing the TPA of the tissue with Na-Fl increased both the scanning speeds and the magnitude of RI changes in a dose-dependent manner. Ongoing studies are exploring the use of IRIS to alter the optical properties of corneal tissue in situ, over an extended period.

The effect of the asphericity of myopic laser ablation profiles on the induction of wavefront aberrations.

PURPOSE To compare the effects of laser profile asphericity on the induction of wavefront aberrations, susceptibility to decentration, and depth of focus in a polymethylmethacrylate (PMMA) model. METHODS Four PMMA lenses received an excimer laser ablation of -6 D with a 6-mm optical zone and different amounts of primary spherical aberration (Z(4)(0)): 0, -0.346, -1.038, and -2.076 microm. The curvature of each lens was measured by using surface profilometry, and wavefront changes were computed from curvature differences. Changes in optical quality were compared by treatment simulation of 13 real myopic eyes. The influence of pupil diameter, ablation decentration, and defocus on retinal image quality was measured by using the optical transfer function-based visual Strehl ratio (VSOTF). RESULTS Aspheric ablation profiles induced significantly less primary but higher secondary spherical aberration (Z(6)(0)) than did the standard profile; however, Z(4)(0) compensation was incomplete. Simulated treatments with aspheric profiles resulted in significantly better retinal image quality and higher decentration tolerance than did the standard profile. Optical depth of focus was not affected with a 3-mm pupil, whereas with a 6-mm pupil, there was a small but statistically significant decrease in depth of focus. CONCLUSIONS Aspheric laser profiles showed theoretical optical benefits over standard ablation profiles for the treatment of myopia, including terms of decentration tolerance. However, there remained profound induction and thus, undercorrection of Z(4)(0), due to loss of laser ablation efficiency in the lens periphery.

Enhancement of intra-tissue refractive index shaping (IRIS) of the cornea by two-photon absorption

Nanojoule femtosecond laser pulses were used to modify the refractive index of living corneas. By doping with Sodium Fluorescein to increase two-photon absorption, laser scanning speed could be greatly increased while inducing large RI modifications.

High resolution measurement of sodium fluorescein distribution in doped live corneal tissue

Two-photon fluorescence was used for the first time to measure sodium fluorescein distribution in live corneal tissue. The diffusion depth was determined to be 350 µm under study conditions with 11µm axial resolution.