Ian G. Cox

Causes of spherical aberration induced by laser refractive surgery

Purpose: To develop a corneal model to better explain how refractive surgery procedures induce spherical aberration. Setting: Department of Ophthalmology and Center for Visual Science, University of Rochester, Rochester, New York, USA. Methods: The preoperative cornea was modeled as a rotationally symmetric surface with various radii of curvature and asphericities. The postoperative cornea was defined as the difference between the preoperative cornea and an ablation thickness profile computed based on the Munnerlyn equation. A ray‐tracing program and Zernike polynomial fitting were used to calculate the induced amount of spherical aberration assuming a fixed ablation depth per pulse or a variable ablation depth depending on the incidence angle of each pulse on the cornea. A biological eye model of the corneal surface change after laser refractive surgery was also developed to explain the induced spherical aberrations after myopic and hyperopic treatments. Results: The clinical data showed that positive spherical aberration was induced after myopic correction and negative spherical aberration increased after hyperopic correction. In contrast, assuming a fixed ablation depth per pulse, the theoretical prediction was that negative spherical aberration with myopic treatment and positive spherical aberration with hyperopic treatment would increase. However, when assuming a variable ablation depth per pulse caused by non‐normal incidence of laser spot on the cornea, the theoretically predicted induction of spherical aberration tends to fit better with the myopic and hyperopic clinical data. The effect of a variable ablation depth accounted for approximately half the clinically observed amount of spherical aberration. The biological model of the corneal surface change used to explain this remaining discrepancy showed the magnitude of the biological response in myopic correction is 3 times smaller than in hyperopic correction and that the direction of the biological response in hyperopic treatment is opposite that in myopic treatment. Conclusions: This nontoric eye model, which separates the effects of differences in ablation efficiency and biological corneal surface change quantitatively, explains how spherical aberration is induced after myopic and hyperopic laser refractive surgery. With the corneal topographic data, this model can be incorporated into the ablation algorithm to decrease induced spherical aberrations, improving the outcomes of conventional and customized treatments.

Aberrations induced in wavefront-guided laser refractive surgery due to shifts between natural and dilated pupil center locations

PURPOSE: To determine the aberrations induced in wavefront‐guided laser refractive surgery due to shifts in pupil center location from when aberrations are measured preoperatively (over a dilated pupil) to when they are corrected surgically (over a natural pupil). SETTING: Center for Visual Science and Department of Ophthalmology, University of Rochester, Rochester, New York, USA. METHODS: Shifts in pupil center were measured between dilated phenylephrine hydrochloride (Neo‐Synephrine [2.5%]) and nonpharmacological mesopic conditions in 65 myopic eyes treated with wavefront‐guided laser in situ keratomileusis (Technolas 217z, Bausch & Lomb). Each patients preoperative and 6‐month postoperative wave aberrations were measured over the dilated pupil. Aberrations theoretically induced by decentration of a wavefront‐guided ablation were calculated and compared with those measured 6 months postoperatively (6.0 mm pupil). RESULTS: The mean magnitude of pupil center shift was 0.29 mm ± 0.141 (SD) and usually occurred in the inferonasal direction as the pupil dilated. Depending on the magnitude of shift, the fraction of the higher‐order postoperative root‐mean‐square wavefront error that could be due theoretically to pupil center decentrations was highly variable (mean 0.26 ± 0.20 mm). There was little correlation between the calculated and 6‐month postoperative wavefronts, most likely because pupil center decentrations are only 1 of several potential sources of postoperative aberrations. CONCLUSIONS: Measuring aberrations over a Neo‐Synephrine‐dilated pupil and treating them over an undilated pupil typically resulted in a shift of the wavefront‐guided ablation in the superotemporal direction and an induction of higher‐order aberrations. Methods referencing the aberration measurement and treatment with respect to a fixed feature on the eye will reduce the potential for inducing aberrations due to shifts in pupil center.

Surgeon offsets and dynamic eye movements in laser refractive surgery

PURPOSE: To determine the amount of static and dynamic pupil decentrations that occur during laser refractive surgery. SETTING: The Center of Visual Science and the Department of Ophthalmology, University of Rochester, Rochester, New York, USA. METHODS: The surgeons accuracy in aligning the pupil center with the laser center axis was measured when engaging the eye‐tracker in 17 eyes receiving conventional laser in situ keratomileusis (LASIK) procedures (Technolas 217z; Bausch & Lomb). Eye movements were measured subsequently during the treatment in 10 eyes using a pupil camera operating at 50 Hz. Temporal power spectra were calculated from the eye movement measurements. RESULTS: The mean pupil misalignment by the surgeon at the beginning of the procedure was 206.1 μm ± 80.99 (SD) (with respect to the laser center). The laser center was typically misaligned below (inferiorly) and to the left (nasally and temporally in left and right eyes, respectively) of the laser center. Small amounts of cyclotorsion were observed during the ablation (<2 degrees). The mean magnitude of dynamic pupil decentration from the laser center during treatment was 227.0 ± 44.07 μm. The mean standard deviation of eye movements was 65.7 ± 25.64 μm. Temporal power spectra calculated from the horizontal and vertical changes in eye position during the ablation were similar. Ninety‐five percent of the total power of the eye movements was contained in temporal frequencies up to 1 Hz, on average, in both directions. CONCLUSIONS: Most eye movements during LASIK are slow drifts in fixation. An eye‐tracker with a 1.4 Hz closed‐loop bandwidth could compensate for most eye movements in conventional or customized ablations.

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.

Quantitative optical inspection of contact lenses immersed in wet cell using swept source OCT

We demonstrate swept source optical coherence tomography (OCT) imaging of contact lenses (CLs) in a wet cell and comprehensive quantitative characterization of CLs from volumetric OCT datasets. The approach is based on a technique developed for lens autopositioning and autoleveling enabled by lateral capillary interactions between the wet cell wall and the lens floating on the liquid surface. The demonstrated OCT imaging has enhanced contrast due to the application of a scattering medium and it improves visualization of both CL interfaces and edges. We also present precise and accurate three-dimensional metrology of soft and rigid CLs based on the OCT data. The accuracy and precision of the extracted lens parameters are compared with the manufacturers specifications. The presented methodology facilitates industrial inspection methods of the CLs.

The why and wherefore of soft lens visual performance

Although significant advances have been made over the past decades in improving the physiological performance of hydrogel lenses, little has been done to understand and improve the quality of vision experienced by soft lens wearers. Yet, almost since their inception, the literature has revealed numerous reports claiming that soft lenses provide inferior visual performance compared to spectacles or rigid contact lenses. This paper outlines those physical, physiological and optical factors which may be responsible for less than optimum soft lens vision, and describes the results of a series of investigations designed to pinpoint the most probable culprit in soft lens designs typically used by todays clinician. The results of these studies will provide the clinician with a better understanding of the contact lens/ocular environment interactions which affect soft lens visual performance and a guide to solving their soft lens patients day-to-day vision problems.