Principles and designs of the iseikonic lenses for near vision.

PURPOSE: The purposes of this study were to find the problems of afocal iseikonic lenses as used for near vision and to elaborate the principles and designs of the ideal near iseikonic lenses. METHODS: By analyzing the image-object relationship and imaging processes, the formulae were derived to quantitatively and qualitatively describe two errors of afocal iseikonic lenses as used for near vision: 1) inequality of accommodative demands to both eyes and 2) the difference between the actual and the nominal magnification. Formulae were derived to calculate and design the ideal near iseikonic lenses. RESULTS: For iseikonic lenses now used for near vision, inequality of accommodative demands to both eyes and difference between the actual and nominal magnification increased as the magnification increased and the viewing distance decreased. Afocal iseikonic lenses have significant inequality of accommodative demands to both eyes with 3% magnification at a viewing distance of 25 cm or with 5% magnification at 40 cm. The ideal near iseikonic lenses were found to be flatter than the afocal ones. A series of the ideal near iseikonic lenses were designed with some practical considerations. The central thickness and the refractive index of the lenses affect magnification and lens shape. The thicker the lenses, the higher the magnification. The higher refractive index also gives higher magnification. CONCLUSIONS: Iseikonic lens design is much different for distance and near vision. When the iseikonic lenses are used at a distance other than the designated distance, they will cause blurred vision and artificial aniseikonia as a result of inequality of accommodative demands to both eyes. In research as well as clinical settings, it is necessary to apply the ideal near iseikonic lenses for near vision.     

Iris sutures for refixation of decentered intraocular lenses

BACKGROUND: Most revisions after intraocular lens (IOL) implantation are due to an insufficiency of the zonular apparatus. Frequently, an inferior decentration can be seen ("sunset syndrome"). In this clinical study, suture refixation of the haptics to the iris was assessed. Functional and morphological results were considered. METHOD: 21 eyes with subluxation of the IOL were treated with iris sutures for refixation. The operation was performed under topical and intracameral anaesthesia. Refixation was achieved by suturing one or both haptics to one or two fixation points in the outer periphery of the iris. Complete zonular dialysis made it necessary to suture at two fixation spots. All eyes were examined preoperatively, at one day and three months postoperatively considering functional results and postoperative IOL centration. RESULTS: In 15 eyes (71%), the IOL was optimally centred postoperatively. In 5 eyes (24%), a revision had to be performed due to instable centration. In one further case the IOL was minimally decentred, but its position was stable. Best corrected visual acuity was 0.3 +/- 0.2 preoperatively and 0.5 +/- 0.2 after three months. The mean refraction was stable in the postoperative course and astigmatism did not change significantly. There were no major complications intraoperatively or postoperatively, but a localised iris atrophy at the haptic fixation points was noted. CONCLUSIONS: In the presence of a partially intact zonular apparatus, iris sutures are a safe and minimally invasive method for fixing a decentred IOL. Postoperative centration and functional results were stable after 3 months.     

Enzymatic distribution patterns of rat lenses and the changes that occur during naphthalene cataract development.

The topographic distribution of enzyme activities in normal rat lenses and their changes occurring during naphthalene cataract development were investigated. Cataract formation was documented by slit images according to the Scheimpflug principle. Before the onset of visible changes and after the development of early visible opacities, the animals were sacrificed, their lenses removed and biochemically analyzed. The lenses were divided reproducibly into 6 parts--the equatorial ring and 5 layers of central cylinders from the anterior to the posterior cortex. The enzyme activity spectra (AR, SDH, GR, GPX, PFK, ALD, GAP-DH, and LDH) showed region-related patterns, which could not be detected with analysis of the total lens. Image analysis of the first signs of opacity showed that changes in the activities of several enzymes correlate with density changes in the corresponding lens layers.     

Sulcus fixation of posterior chamber lenses by transscleral sutures

Sulcus fixation of posterior chamber lenses by transscleral sutures was performed in 23 cases. We used new techniques to be sure that the sulcus was pierced accurately and the suture was placed in the posterior chamber instead of the ruptured capsule. This paper describes these techniques and the clinical results of 23 cases.     

Influence of across-pupil sutures on the optical quality of intraocular lenses

Purpose

In cases of pupillary capture after previous transscleral fixation of an intraocular lens (IOL), an across-pupil suture technique is sometimes used to stabilize the IOL. We investigated the optical effects of 10-0 polypropylene sutures placed across the pupil.

Methods

Optical performance was evaluated using the optical bench metrology system, and a single-piece IOL was used in an optical bench system consisting of a model eye, a high-resolution Hartmann–Shack wave front sensor, and an image capturing device with 2.0, 3.0, 4.0, and 4.5 mm apertures with distance focus. To simulate across-pupil sutures, two 10-0 polypropylene sutures were placed 2 mm apart across the pupil. The focus image, spherical aberration, and image quality were measured and compared with and without sutures.

Results

When pupil size increased, spherical aberration increased, irrespective of sutures. No difference was found in spherical aberration with and without sutures, and image qualities at 17.6 and 35.2 cycles per degree were not affected by the across-pupil sutures.

Conclusions

The differences in optical quality using across-pupil 10-0 polypropylene sutures were clinically negligible.

     

A method for using relay lenses with display monitors for vision testing at far and near.

Resolution limitations preclude the use of display monitors for near testing of acuity and contrast sensitivity. Relay lenses can form minified aerial images of the display at any given near viewing distance, but the image will differ in spatial frequency from the display. Equations are presented that can be used to specify the far- and near-viewing distances and the necessary focal length of a lens so that the display and its near aerial image have identical spatial frequencies when viewed by a subject at a fixed location. Modulation transfer function (MTF) calculations show that achromatic doublets will not degrade the resolution across a 300-mm wide display, thereby providing the versatility of display monitors for near vision testing.     

Impact testing of ophthalmic lenses: stress distribution and the "search" theory.

Elementary concepts in fracture mechanics, stress analysis, and impact are discussed as they pertain to fracture of lenses in drop ball testing. Quasi-static estimates for stress distributions during impact are obtained. These profiles show a sharp peak in the tensile stress under the impact point. Identical groups of lenses were impact tested in two different modes: 1) 25 impacts at a single spot on the lens, 2) 1 impact at each of 25 different spots. Much more breakage was observed in the latter group. Both the experimental and analytical considerations lend support to the "search" theory of drop ball testing--fracture occurs when the ball strikes a weak spot (where there exists a sufficiently severe flaw) on the lens. Lenses which pass an impact test may well have dangerous flaws away from the impact point.     

Prevalence and distribution of corrective lenses among school-age children.

BACKGROUND: No population-based data are available regarding the proportion of school-age children who have corrective lenses in the U.S. The objective of this study was to quantify the proportion of children who have corrective lenses (glasses or contact lenses) and to evaluate the association of corrective lenses with age, gender, race/ethnicity, health insurance status, and family income. METHODS: Children 6 to 18 years of age were identified in the 1998 Medical Expenditure Panel Survey. National estimates were made of the proportion with corrective lenses. Logistic regression modeling was used to assess factors that were associated with corrective lenses. RESULTS: Based on the 5,141 children in the 1988 Medical Expenditure Panel Survey, an estimated 25.4% of the 52.6 million children between 6 and 18 years had corrective lenses. Girls had greater odds than boys of having corrective lenses (odds ratio, 1.41; p < 0.001). Insured children, regardless of race/ethnicity, and uninsured nonblack/non-Hispanic children had similar odds of having corrective lenses. Compared with uninsured black or Hispanic children (odds ratio, 1), greater odds of corrective lens use was found among uninsured nonblack/non-Hispanic children (odds ratio, 2.29; p = 0.002) and black or Hispanic children with public (odds ratio, 1.67; p = 0.005) or private health insurance (odds ratio,1.77; p = 0.004). Among families with an income > or =200% of the federal poverty level, the odds of having corrective lenses increased with age (p < or = 0.04). In contrast, among those families <200% of the federal poverty level, the odds of having corrective lenses at 12 to 14 years was similar to 15- to 18-year olds (p = 0.93). CONCLUSIONS: The use of corrective lenses suggests that correctable visual impairment is the most common treatable chronic condition of childhood. Income, gender, and race/ethnicity, depending on insurance status, are associated with having corrective lenses. The underlying causes and the impacts of these differences must be understood to ensure optimal delivery of eye care.     

Stereoacuity at distance and near.

PURPOSE: Because previous studies have reported conflicting evidence, we examined a possible difference in stereoacuity between distance and near, in particular using a random-dot display. We compared distance and near stereoacuities using identical presentation formats at the two distances. METHODS: Twelve young adults with low, stable refractive errors and apparently normal binocular vision participated. Stereoacuity was determined with a Mentor B-VAT II using Random Dot E (BVRDE) and Contour Circles (BVC) stereograms presented on a standard monitor (25 x 19.3 cm) at 518 cm (distance-habitual) and a small monitor (2.0 x 1.4 cm) at 40 cm (near-habitual). To examine whether accommodation-convergence influenced stereoacuity, testing at 40 cm was repeated with the addition of +2.50 DS lenses and base-in prisms (near-compensated) that created the same accommodation and convergence demands as for distance-habitual viewing. RESULTS: The two stereotests produced similar findings. Stereoacuity was not significantly different for distance-habitual and near-habitual viewing of the BVRDE (p = 0.43) and BVC (p = 0.79) stereotests. Near-compensated stereoacuity was worse than near-habitual (BVRDE, p = 0.005; BVC, p = 0.004) and distance-habitual (BVRDE, p = 0.05; BVC, p = 0.003) for both stereotests. Because near stereoacuity with yoked prisms (control condition) was the same as without prism (near-habitual), prism-induced optical distortions cannot account for the difference. CONCLUSIONS: Stereoacuity was not different at distance and near under normal viewing conditions. The conflict between subject knowledge of target proximity and the optically-induced relaxation of accommodation and convergence, or an inaccurate accommodative-convergence response, might have caused poor near-compensated stereoacuity.     

Changes in accommodative and vergence responses when viewing through near addition lenses.

It has been proposed that near addition lenses reduce the lag of accommodation and may slow myopia progression. In this study, we investigated the differences in accommodative response and near phoria in subjects with and without near addition lenses. Fourteen subjects (7 emmetropic and 7 myopic) participated in the study. Monocular and binocular accommodative responses to a target at 40 cm were measured with and without +2.00 diopter (D) lenses using a Canon R-1 optometer (Canon Europe N.V., Amsterdam, The Netherlands). Near dissociated phoria was measured using 3 testing methods: Maddox rod, cover test, and Von Graefe technique. The differences in accommodative response and near phoria between the 2 viewing conditions with and without the near addition lens were significant (P < 0.0001). No significant differences were revealed in accommodative response and near phoria between refractive error groups. The average accommodative responses of all subjects were 2.03 +/- 0.06 (SE) D (without +2.00 D lens) and 0.60 +/- 0.07 D (with +2.00 D lens) under monocular viewing conditions and 2.11 +/- 0.06 D (without +2.00 D lenses) and 0.77 +/- 0.07 D (with +2.00 D lenses) under binocular viewing conditions. The average near phorias of all subjects were -2.08 +/- 0.69 prism diopters (PD, without +2.00 D lenses) and -7.90 +/- 0.68 PD (with +2.00 D lenses). We discuss the effect of near addition lenses on the defocus of the retinal image, accommodative response, and near phoria. In addition, we propose that the observed difference between binocular and monocular accommodation, when viewing through near addition lenses, is caused by an increased vergence accommodation.     

The concentric distribution of protein on patient-worn hydrogel lenses.

We investigated the concentric distribution of protein on patient-worn hydrogel lenses. We measured patient-worn lenses for diameter, sagittal depth, and thickness so that the surface area of each lens could be calculated. The lenses were then cut with a circular blade to give an inner circular core and an outer concentric ring having approximately equal lens surface area. Total protein was then determined for each lens half using a modified ninhydrin procedure. The protein deposition on the inner core was compared to the amount on the outer ring for each of the four FDA lens groups. Lenses of group III and group IV were found to deposit significantly more protein on the outer portion while the other groups showed essentially the same amount on both halves. This finding indicates that the type of lens has an effect on the distribution pattern of protein.     

Effects of sutures on rat corneas]

Cellular infiltration of the suture line was studied immunohistologically after rat corneas were sutured with various types of suture materials. The suture line was invaded by helper/inducer T lymphocytes, cytotoxic/suppressor T lymphocytes, and Ia-antigen-positive cells. Among the invading cells, Ia-antigen-positive cells were the most prominent over an extended period of time from immediately after suture placement. The helper/inducer T lymphocytes consisted predominantly of cytotoxic/suppressor T lymphocytes. The 9-0 silk suture produced a higher degree of cellular infiltration than the 10-0 nylon suture or the 10-0 prolene suture. There was no marked difference in the infiltration produced by the 10-0 nylon and 10-0 prolene sutures. After the sutures were removed, the cellular infiltration appreciably decreased. More marked invasion occurred with interrupted sutures than after the inter-lamellar insertion of suture material. These findings indicate that suture materials on the cornea promote infiltration of immunocompetent cells and that mechanical stimulation of the suture notch enhances such cellular response, which may be involved in graft rejection of the corneal transplantation.     

A two-year trial of intraocular lenses at the Wilmer Institute.

Using the Binkhorst four-loop intraocular lens, we began a trial of intraocular lens implantation in 1975. We compared the operative and postoperative complications and the visual results of 100 implant cases during a two-year period to those of 100 age-matched aphakic control cases. Although the additional steps of lens implantation resulted in more operative and postoperative complications in the intraocular lens group, the visual results were good and were the same for both groups at an average follow-up of 12 months. Because the long-term follow-up results of implant cases are unknown, we recommend a conservative approach to selection of patients for intraocular lens implantation.     

Oxidation enhances calpain-induced turbidity in young rat lenses.

PURPOSE: To determine if oxidation enhances turbidity after proteolysis of rat lens crystallins by the calcium-activated protease calpain (EC 3.4.22.17). METHODS: Total soluble proteins from young rat lens were hydrolyzed for 24 hr by endogenous lens calpain, and the proteins were further incubated with the oxidant diamide for up to 7 days. Turbidity was measured daily at 405 nm. To measure proteolysis and turbidity in cultured lenses, rat lenses were cultured for 6 days in low calcium medium and diamide. The lenses were then photographed to assess transmission of light. SDS-PAGE and immunoblotting assessed proteolysis of crystallins, alpha-spectrin, and activation of calpain. RESULTS: Appreciable in vitro turbidity occurred in soluble proteins from young rat lenses after proteolysis of crystallins by endogenous calpain. Calpain inhibitor E64, or anti-oxidants DTE and GSH, inhibited this turbidity. On the other hand, the oxidant diamide markedly enhanced calpain-induced turbidity. Cultured rat lenses showed elevated intralenticular calcium and proteolysis of crystallins by calpain, but no nuclear cataract. Addition of diamide to the culture medium caused development of nuclear cataract. CONCLUSIONS: Diamide enhanced turbidity only when crystallins were proteolyzed. Oxidation may be one of the factors promoting light scatter and insolubilization after proteolysis. These data are consistent with the hypothesis that proteolysis of crystallins from young rat lens may expose cysteine residues, which are then oxidized, become insoluble and scatter light.     

Prismatic effect in bifocal lenses and the location of the near optical centre*

A matrix expression is derived for the prismatic effect in the reading portion of a bifocal lens. Using this expression the problem of finding the location of the optical centre of the reading portion is examined. Matrix methods lead to a complete solution that holds in all circumstances within the usual limitations of first-order optics and thin lenses Both the main and segment lenses may be astigmatic. In the usual case there is a unique near optical centre but situations can occur in which there is no near optical centre or an infinite number of them. These situations are examined systematically. Numerical examples illustrate the method.     

Optimal postoperative refraction for good unaided near and distance vision with monofocal intraocular lenses.

Forty five eyes with up to 2 dioptres of myopic astigmatism and up to 1 dioptre sphere either plus or minus following cataract extraction and implantation of a monofocal intraocular lens were examined to assess their unaided visual acuities. Forty three percent were able to see 6/12 and N8, and 60% were able to see 6/12 and N10. Subjects with between 1 and 2 dioptres of myopic astigmatism and virtually no sphere were able to see 6/12 and N10 in 82% of cases. This study confirms the benefits to both distance and near vision of myopic astigmatism as an alternative to multifocal intraocular lenses.