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Biomedical subjects

V Portney

Publications and source records attributed to V Portney.

13 recordsLinked to original sources
PubMed2003

A dual optic accommodating foldable intraocular lens.

AIM: To design an accommodating intraocular lens with extended accommodative range that can be adapted to current standard phacoemulsification and endocapsular implantation technique. METHOD: Ray tracing analysis and lens design; cadaver eye implantation. RESULTS: Ray tracing analysis indicated that axial movement of an exaggerated converging anterior optic linked by spring loaded haptics to a compensatory static diverging posterior optic produced greater change in conjugation power of the eye compared to axial movement of a single optic lens. A dual optic one piece foldable silicone lens was constructed and implanted via a 4 mm corneal incision into the capsular bag of two cadaver eyes. CONCLUSION: A dual optic intraocular lens design can increase the optical effect of a given displacement and suggests improvements for accommodating intraocular lenses.

Accommodation, Ocular↗
PubMed1999

Analysis of edge glare phenomena in intraocular lens edge designs.

PURPOSE: To determine the image and relative intensity of reflected glare images from 4 commonly used intraocular lens (IOL) edge designs to assess the potential for noticeable postoperative edge glare. SETTING: University of Texas Medical School, Houston, Texas, USA. METHODS: The interaction of light rays from 4 common IOL edge designs were examined in an eye model using the OptiCAD 3-D radiometric ray-tracing program (Opticad Corp.). Comparison of the potential of the 4 edge designs to produce visual sensations was derived from plots of the spatial location and energy distribution of rays forming the retinal image. RESULTS: Edge designs with no anterior and posterior dioptric powers at the lens periphery (lenticular) and rounded corners distributed the edge glare rays over a large retinal area. Edge designs with sharp edges formed by "cropping" the anterior and posterior optic zones focused edge glare rays into distinct arc-shaped images. The peak intensity of the arc-shaped image was 8 to 10 times stronger than the peak intensity of the diffuse image formed by lenses with rounded edges. CONCLUSIONS: Rounded IOL edges distribute reflected glare image over a significantly greater area than sharp edges. Rounded edges reduce the potential for edge glare phenomena that appear to the patient as a thin crescent or partial ring.

Glare↗
PubMed1998

Determining the imaging quality of intraocular lenses.

PURPOSE: To validate the proposed optical requirements of a draft international standard for intraocular lenses (IOLs). SETTING: Eight optical testing laboratories in the United States, Germany, Japan, and The Netherlands. METHODS: The testing laboratories performed modulation transfer function (MTF) tests on various IOLs using a model eye and visual resolution tests in air. Each laboratory performed duplicate measurements on a set of 43 lenses that was circulated among the testing laboratories. RESULTS: The interlaboratory tests showed that the MTF measurements using a model eye had better repeatability and reproducibility than the more common industry practice of resolution testing in air with parallel light and the United States Air Force three-bar target. However, the two methods correlated well. The commonly applied criterion that an IOL resolve in air at least 60% of the Rayleigh cutoff spatial frequency corresponded to a minimum requirement of 0.43 MTF units at 100 mm-1 in a model eye. CONCLUSIONS: Either criterion may be applied in accordance with a proposed international standard for IOLs. The model eye method can be applied over a broader range of dioptric powers and is relevant for materials that interact with aqueous. Both tests appear to have a greater ability to detect unwanted surface aberrations than resolution testing of IOLs in a water cell using parallel light, a method described in the current American National Standards Institute standard.

Diagnostic Techniques, Ophthalmological↗
PubMed1996

Accuracy in determining intraocular lens dioptric power assessed by interlaboratory tests.

PURPOSE: To describe a testing program conducted by a standards group as a guide for setting international tolerances for intraocular lens (IOL) dioptric power. SETTING: Multicenter study. METHODS: Seven biconvex, poly(methyl methacrylate) IOLs ranging in power from 10.00 through 30.00 diopter (D) were circulated among nine participating laboratories experienced in IOL optical measurements. Each laboratory performed repeated optical tests to determine dioptric power. These results were analyzed for repeatability and reproducibility in accordance with methods specified by the International Organization for Standardization. RESULTS: Intralaboratory repeatability was less than 0.5% of the dioptric power, and interlaboratory reproducibility, when following a normalized procedure for correction and conversion, was less than 1.0% of the dioptric power. CONCLUSION: Tolerance limits of +/0 0.30 D in the range 0 to 15.00 D, +/- 0.40 D for 15.50 to 25.00 D, and +/- 0.50 D for 25.50 to 30.00 D have been proposed as an international standard for IOLs. The contribution of IOL power error within the limits of the standard are estimated to contribute less than 1.0% to the total error in postoperative refractive prediction.

Lenses, Intraocular↗
PubMed1995

The "Expected Visual Outcome" (EVO) model: methodology and clinical validation.

PROBLEM: If changes are made to the optics of the eye (e.g., intraocular lens implants, contact lenses, etc.) how will they affect performance on clinical tests of vision? METHOD: A phenomenological method is presented based on in vitro optical transfer function (OTF) and a simple model of human threshold detection. RESULTS: The model is used to predict and the results are compared with clinical data (acuity, contrast sensitivity) obtained from pseudophakic patients implanted with a multifocal intraocular lens (IOL). A good qualitative agreement is found with the clinical data. SIGNIFICANCE: This model predicts the relative change in clinical performance for a given change in the optical components of the human eye. This simple phenomenological model permits numerical prediction of clinical tests and is easily calculable.

Cataract Extraction↗
PubMed1993

Interpreting multifocal intraocular lens modulation transfer functions.

The optical performance of new multifocal intraocular lens designs is frequently assessed using the modulation transfer function (MTF). We discuss the relationship between the MTF and clinical measures of human visual function, such as threshold visual acuity and contrast sensitivity. Using in vitro MTF measurements of a human eye model containing a multifocal or monofocal intraocular lens, we predict relative changes in acuity and contrast sensitivity and outline the techniques using a simple model of human retinal threshold detection. Specific concepts introduced include the visual acuity graph, predicted visual acuity graph, and predicted contrast sensitivity function.

Contrast Sensitivity↗
PubMed1993

Phenomenological model for interpreting the clinical significance of the in vitro optical transfer function.

We describe a methodology to predict the outcome of clinical tests caused by changes made to the optical elements of the human eye. This formalism, called the expected visual outcome model, is based on in vitro measurements of the optical transfer function and takes into account a simple model of human threshold performance. The clinical tests under consideration are high-contrast visual acuity and contrast sensitivity. Using the expected visual outcome, we describe a useful performance index called the predicted visual acuity graph, which can be measured clinically. The theoretical results are compared with visual function measured in patients with pseudophakic (multifocal and monofocal) implants.

Cataract Extraction↗
PubMed1992

Optical testing and inspection methodology for modern intraocular lenses.

A method of in vitro optical performance evaluation of intraocular lenses (IOLs) is discussed. The IOLs are tested in the eye cell, which is designed to be optically equivalent to the theoretical eye model. The eye cell is a combination of a wet cell and the optical element (the "conjugation lens") that converges a beam of light, similar to the action of the cornea in the theoretical eye model. The eye cell is designed so the spatial frequency scale (modulation transfer frequency [MTF] testing), expressed in terms of line pairs per millimeter (linear scale), equals the scale produced by the theoretical eye model. The most common way to express spatial frequencies in visual science is in line pairs per degree or cycles per degree (angular scale). The problem with in vitro testing is that the corresponding angular scale varies with the dioptric power of the conjugation lens. This paper provides a simple method of mapping the angular scale of the eye cell onto the theoretical eye model, allowing a comparison with clinical results as well as with MTFs measured in different eye cells. All corresponding formula for calculating the eye cell optical characteristics are provided.

Humans↗
PubMed1990

Optical performance of multifocal intraocular lenses.

The optical performance of one monofocal and five multifocal lenses was evaluated in the laboratory and photographically. The laboratory testing included determination of the modulation transfer function (MTF), through focus response (TFR), resolution efficiency, and Strehl ratio of each lens. The photographic testing included photographs of the Regan high contrast acuity chart at ten feet with clearest focus and 18 additional photographs in which the image was defocused using minus trial lenses in 0.25 diopter increments. A color photograph of the Kodak color chart was also taken using each lens. All testing was conducted using a 3 mm artificial pupil under ideal implant conditions with no decentration or tilt. The laboratory and photographic results demonstrate that all the multifocal lenses had a two- to three-fold increase in the depth of field with at least a 50% lower contrast in the retinal image. The photographic testing revealed a one to two line better resolution limit with the monofocal lens, which corresponded to the 12% to 41% better MTF cut-off value with the monofocal lens by laboratory testing. The measured resolution efficiencies of all six lenses were comparable. The color photographs revealed color mixing of adjacent colors with the multifocal lenses, whereas the colors appeared unchanged from the original with the monofocal lens.

Equipment Design↗
PubMed1988

Silicone intraocular lens resolution in air and in water.

The resolution efficiencies of 31 biconvex silicone intraocular lenses, ranging in power from 16.0 to 23.5 diopters, were tested in air and in water to see if a predictable relationship existed as previously reported with polymethylmethacrylate lenses. Resolution efficiency is defined as the percentage ratio of the actual resolving power of a lens to that of a perfect lens of the same focal length which is only limited in resolution by diffraction. The lenses ranged from 29% to 58% resolution efficiency in air. No lenses exhibiting multiple images were included. All 31 lenses achieved at least 73% resolution efficiency in water, and one lens achieved 82%. Based on these findings, a biconvex silicone lens that exceeds 30% resolution efficiency in air and does not produce multiple images can perform near its diffraction limit when implanted in the eye.

Air↗
PubMed1987

Intraocular lens resolution in air and water.

The resolution of 96 polymethylmethacrylate intraocular lenses with convexo-plano optics, ranging in power from 13 to 27 diopters, was measured in air and water. The resolution of each lens was expressed in linear units of resolving power, which is the maximum number of line-pairs that can be resolved per millimeter, as described in the current ANSI Standard Z80.7-1984. There was no clearly defined relationship between linear resolving power measured in air and that measured in water. Measurements on high power lenses (greater than 20 diopters) indicate that it is possible for an intraocular lens to meet the current 100 line-pair per millimeter standard for resolution and still be a limiting factor in a patient's best attainable visual acuity. An alternative method for evaluating lens resolution is to determine the resolution efficiency (the relative percentage performance of a lens compared to a diffraction-limited lens of the same dioptric power). Using these units, a consistent and predictable relationship from air to water was demonstrated. Our findings confirm that if a minimum standard of 30% resolution efficiency in air is established, in contrast to linear resolving power, the lens will perform near its diffraction limit when implanted in the eye. For intraocular lenses of materials other than polymethylmethacrylate, a minimum resolution efficiency in air other than 30% may be required.

Air↗
PubMedDate not supplied

Evaluation and implantation of a teledioptric lens system for cataract and age-related macular degeneration.

The teledioptric lens implant, with a high-minus central zone, was evaluated on an optical bench and implanted in the eyes of 50 patients with both cataract and age-related macular degeneration. In vitro testing demonstrated slightly reduced contrast and modulation-transfer-function as compared with a standard intraocular lens. The lens implant functions ideally with a 1.9-millimeter diameter on the high-minus portion. Lens implantation was proven safe in the clinical study. When used with spectacles as a part of a teledioptric system, the resulting visual field was 2.6 times larger than that achieved with an external telescope. Postoperative visual acuity initially improved in more than 65% of patients. After 1 year, due to disease progression, only 20% had improved acuity.

Aged↗