Gene Hilmantel

Corneal First Surface Optical Aberrations and Visual Performance

PURPOSEnWavefront analysis has demonstrated that refractive surgery-induced corneal first surface aberrations are large, are dominated by symmetric aberrations (spherical-like aberrations), and are correlated to measures of visual performance. It is not clear whether the correlation between corneal first surface aberrations and visual performance can be generalized to other corneal conditions where large asymmetric aberrations (coma-like aberrations) may dominate the aberration structure. The purpose of the research reported here was to determine the general utility of corneal first surface wavefront analysis in predicting visual performance.nnnMETHODSnPatients were 13 normals and 78 patients with a variety of corneal conditions including surgically removed pterygia, penetrating keratoplasty, keratoconus, radial keratotomy, laser in situ keratomileusis, and others. Videokeratographs were taken for all patients and used to calculate corneal first surface wavefront variance for 3 and 7 mm pupils. Similarly, visual performance was quantified by measurements of contrast sensitivity and high and low contrast acuities through both 3 and 7 mm pupils.nnnRESULTSnStatistically significant correlations existed between all three measures of visual performance and the corneal wavefront variance. All relationships were stronger for the 7 mm diameter-pupil condition than the 3 mm pupil.nnnCONCLUSIONnRegardless of the cause, corneas with increased wavefront variance showed a quantifiable decrease in visual performance that was pupil size dependent.

Optics of aberroscopy and super vision

&NA; This paper (1) reviews the fundamental limits to visual performance imposed by optical imaging and photoreceptor sampling to determine the limits to the potential gains offered by ideal corrections; (2) examines the predicted losses in vision induced by chromatic aberration, phase shifts, typical ocular aberrations, and the gains possible by correcting the monochromatic aberrations of the eye; (3) discusses the principles of aberration measurement in the eye; and (4) presents methods for measuring and classifying monochromatic aberrations of the eye.

Standardized analyses of correction of astigmatism by laser systems that reshape the cornea.

PURPOSEnTo develop a minimum set of analyses and a format for presentation of outcomes of astigmatism correction by laser systems that reshape the cornea.nnnMETHODSnAn Astigmatism Project group was created under the auspices of the American National Standards Institute (ANSI) Z80.11 Working Group on Laser Systems for Corneal Reshaping. The Astigmatism Project Group was made up of experts in astigmatism analyses from academia, government, and industry. An extensive literature review was conducted to identify all currently available methodologies for the evaluation of astigmatic outcomes. Project Group members discussed the utility of each method and its specific parameters for evaluating the effectiveness of astigmatism-correcting devices. They gave consideration to unique terminology and analyses required for evaluation of correction of astigmatism by laser systems that reshape the comea.nnnRESULTSnThe Project Group defined a comprehensive list of analysis variables needed for the evaluation of astigmatism-correcting devices and generated a mathematical definition for each term. They developed a minimum set of analyses needed for evaluation of astigmatism treatments by laser systems that reshape the cornea. They established methods for calculating the refractive error analysis variables and constructed recommended table and graph formats for data presentation.nnnCONCLUSIONSnThis article contains the recommendations of the Astigmatism Project Group of the American National Standards Institute. We propose it as a standard reference for astigmatic refractive error analyses for the evaluation of safety and effectiveness of laser systems that reshape the cornea.

Corneal aberrations increase with the magnitude of radial keratotomy refractive correction

Background.Refractive surgery induces optically abrupt changes in shape in the midperiphery of the cornea. The abruptness of this change is in part dependent on the magnitude of the surgically induced refractive change. Therefore, the optical aberrations of the cornea, as quantified by wavefront variance (WFV), may be expected to increase as the surgically induced change in the refraction increases.Purpose.It is the purpose of this study to test the hypothesis that as the surgery-induced change in refraction increases, so does the WFV of the cornea.Methods.Fourteen radial keratotomy (RK) patients and seven normal patients served as subjects. Measurements were made before and 2 years after RK surgery. To quantify the WFV of the cornea, we used corneal topography measurements to calculate the surgically induced change in corneal WFV with respect to two different reference surfaces, a sphere and the presurgical cornea. To quantify the surgically induced change in the equivalent spherical correction (ESC), cycloplegic refractions were performed. The measurements were summarized by regressing the surgically induced change in the WFV against the surgically induced change in the ESC.Results.For large pupils (7 mm diameter), the correlation between the change in the WFV referenced to a sphere and the change in the ESC was significant (p < 0.0001, r2=0.745) and dominated by fourth order aberrations. Similar results were found for the surgical lens. For small pupils (3 mm diameter), the effects were markedly reduced.Conclusions.(1) As the magnitude of the surgically induced refractive change increases so does the WFV of the cornea, particularly for large pupils. (2) The increase in corneal WFV for large pupils is dominated by fourth order aberrations. (3) The increase in corneal WFV is consistent with reported decreases in visual function (contrast sensitivity and low contrast visual acuity), particularly for large pupil diameters in combination with large surgically induced changes in refractive error

Accuracy of the tomey topographic modeling system in measuring surface elevations of asymmetric objects.

BACKGROUNDnMost studies have assessed the accuracy of videokeratographic systems using spheres, ellipsoids, or toric surfaces. Most human corneas are asymmetric to some degree and many pathological corneas are markedly asymmetric. To date, little work has been done to ascertain the accuracy of videokeratographic systems for measuring the shape of asymmetric objects.nnnPURPOSEnThe purpose of this study is to determine the accuracy with which the Tomey Topographic Modeling System can evaluate the topography of asymmetric surfaces.nnnMETHODSnCalibrated ellipsoidal test objects were tilted with respect to the videokeratometric axis to create asymmetric test surfaces with known characteristics.nnnRESULTSnRoot mean squared error of all the measured surface elevations varied from 0.7 microm to 11.3 microm. Although there was a trend for greater error with the more asymmetric surfaces, the trend was not statistically significant. Accuracy was not dependent on apical radius. Measurement error increased toward the periphery.nnnCONCLUSIONSnFor the Tomey Topographic Modeling System, the accuracy in measurement of smooth, asymmetric surfaces is comparable the accuracy in measurement of symmetric surfaces.

Entoptic foveal avascular zone measurement and diabetic retinopathy.

BACKGROUNDnEntoptic visualization of the foveal avascular zone (FAZ) provides a noninvasive method for measurement of the FAZ. To determine if repeatability of measurement with this technique is good enough to monitor changes in diabetic retinopathy, we quantify (1) the repeatability of entoptic FAZ measurement in healthy subjects and (2) the relationship between measured FAZ diameter and the severity of retinopathy.nnnMETHODSn(1) To determine FAZ measurement repeatability, 10 healthy adults entoptically measured their FAZ diameters in 11 separate testing sessions. (2) In a separate experiment, 53 patients with varying levels of diabetic retinopathy and 21 control subjects used a vascular entoptoscope to measure their FAZ diameters.nnnRESULTSnThe disease-related increase in FAZ diameter is large (approximately 400 microm) compared to the repeatability of the entoptic measurement within a subject (SD approximately 35 microm).nnnCONCLUSIONnEntoptic measurements have the repeatability necessary to reliably monitor increases in FAZ diameter of the magnitude of those induced by diabetes.

The U. S. Food and Drug Administration's role in contact lens development and safety.

Purpose. To provide information concerning the U. S. Food and Drug Administration’s (FDA) role as a regulatory agency in the development and safe use of contact lenses. Methods. The FDA’s organizational structure and regulatory classification of medical devices are explained. Specific regulatory authorities are described and examples provided with respect to the FDA’s role in the regulation of the new hyper-oxygen permeable continuous wear contact lenses. Results. The application of medical device regulations addresses the rights and safety of subjects in preapproval clinical studies and provides a basis to generate valid scientific evidence to demonstrate the safety and effectiveness of a medical device. Once on the market, there are postmarket requirements with which a manufacturer must comply. Conclusion. For continuous wear 30-day lenses, the FDA is using additional regulatory authorities new to the contact lens area to assure that product safety is maintained and consumer information is improved. These include postapproval studies, more informative labeling, and advertising restrictions. Whereas it is the FDA’s mission to promote and protect the health of the public by ensuring the safety and effectiveness of medical devices, the agency works in partnership with other stakeholders to maintain the safe use of medical devices.