Comparison of the coulometric and polarographic measurement of a high-Dk hydrogel.

BACKGROUND: International Organization for Standardization (ISO) document ISO 9913-1 details procedures for oxygen permeability coefficient (Dk) measurement of hydrogel contact lens materials up to 100 barrer. A draft ISO document 9913-2 describing the coulometric technique states that the recommended procedures are suitable for nonhydrogel materials. New generation hydrogel lens materials exceed 100 barrer, which is outside the scope of both documents. METHODS: A range of rigid gas-permeable materials from Dr. W. Benjamin's standard repository was used as a baseline to assess the accuracy of the polarographic and the coulometric methods. We also measured the permeability of a new generation hydrogel, lotrafilcon A, to explore the suitability of the polarographic and coulometric techniques for high-Dk hydrogel lenses. We modified equipment for both methods that incorporate several improvements including front surface masking to eliminate "edge effect." RESULTS: The coulometric technique provided results similar to those previously reported for the standard rigid gas-permeable materials and yielded relative standard errors typically <10%. The polarographic technique provided results similar to the accepted values for the standard rigid gas-permeable materials Dk <70 barrer only after the edge effect correction algorithm was applied. The polarographic results showed poor precision and significant, systematic differences from accepted values for the rigid gas-permeables with Dk > 70 barrer. CONCLUSIONS: The coulometric method is preferable for the measurement of contact lens materials with permeability >70 barrer. The coulometric method was successfully modified to include hydrogels.

A model of oxygen flux through contact lenses.

PURPOSE: Although oxygen transmissibility has been a favored index to describe the physiologic performance of contact lenses, it has been maintained by some that the flux through a contact lens would be a more useful guide. Here, a model is described that allows contact lens oxygen flux to be estimated under open and closed eye wearing conditions. METHODS: The equivalent oxygen potential (EOP) was used to approximate the oxygen concentration beneath a contact lens. A logarithmic relation between corneal oxygen consumption and this oxygen level was substituted into Fick's Law to provide a mathematical model. Paired data of EOP and oxygen transmissibility (Dk/t), from a previous empiric derivation, were entered into a nonlinear regression analysis of this model. RESULTS: The modelling procedure produces a good fit to the selected data. The estimated maximum flux during open eye wear is 7.5 microL/cm2 x h, consistent with previous determinations. Error estimates increased from 0 to 0.55 microL/cm2/h at Dk/t values of 0 and 200 x 10(-9) Barrer/cm, respectively, for the open eye. CONCLUSION: This study provides a workable model for estimating the oxygen flux through contact lenses. Varying the underlying relation between the oxygen tension beneath a lens and the oxygen flux produces minimal variation to the result. The model has a number of clinical applications, such as demonstrating the advantages of highly transmissible contact lenses and the limits to which increasing oxygen transmissibility can alter the corneal physiologic environment.

Proposed performance criteria for extended wear contact lenses.

The introduction of silicone-hydrogel materials has changed expectations for the response to contact lenses used in extended wear. In keeping with this technology shift, we have re-evaluated the criteria by which practitioners judge the performance of extended wear lenses. Attendees at the 1999 Annual Clinical Conference of the British Contact Lens Association were surveyed. The proportion of respondents who considered it appropriate to fit selected patients with conventional, disposable and silicone-hydrogel soft lenses for extended wear was 63%, 67% and 86%, respectively. Only 11% of respondents did not fit extended wear because of personal experience with infectious keratitis, and over 50% relied on educators' advice in avoiding this practice. Fifteen per cent of practitioners reported fitting extended wear on occasions. Ninety-five per cent of respondents thought that the relative risk of infectious keratitis with daily wear compared with no lens wear should be ten or less, which was reduced to 60% when it was revealed that the current relative risk is estimated to be 60. A strong preference for a relative risk of infection with extended wear compared to daily wear of less than five times was indicated, which is consistent with current estimates. A minority of respondents were prepared to accept an overnight oedema level of over 6% with extended wear, although silicone-hydrogel materials have been shown to induce less than 4%. A wide variety of microcyst responses were considered reasonable, with many respondents accepting up to 40 per cornea; again, this result contrasted with early research data suggesting a level of 10 microcysts or less occurring with silicone-hydrogel materials. Interpretation of these findings has led us to the following proposals for safe extended wear: (i) extended wear fitting should preferably be done with silicone-hydrogel materials, rather than conventional hydrogels, where the available parameter ranges overlap; (ii) the relative risk of corneal infection with extended wear compared to daily wear should be two to five times, providing the relative risk of daily wear compared to the no lens wear'situation is reduced to 20 times, and (iii) extended wear lenses should produce an average of no more than 4% overnight oedema and no more than 10 microcysts per eye in the long term.

Letter contrast sensitivity function of the eye.

PURPOSE: To provide empirical data of letter CSF for various levels of defocus under controlled conditions of luminance and age. METHODS: Corrected distance visual acuities were tested at different levels of contrast and defocus. An experiment was conducted using the Medmont visual acuity tester on 10 young subjects and under normal room lighting. RESULTS: Empirical data of visual acuity were obtained for 7 levels of contrast (5, 10, 15, 25, 40, 60, 80%) and defocus levels of 0, +1 and +2D. A mathematical model was derived (R2=0.995) and this can be used to estimate visual acuity at various contrast levels for defocus of < or =+2D. CONCLUSION: This information is useful for the clinician as normative data and for further development of optical models to predict visual performance of the eye.

Anatomically accurate, finite model eye for optical modeling.

There is a need for a schematic eye that models vision accurately under various conditions such as refractive surgical procedures, contact lens and spectacle wear, and near vision. Here we propose a new model eye close to anatomical, biometric, and optical realities. This is a finite model with four aspheric refracting surfaces and a gradient-index lens. It has an equivalent power of 60.35 D and an axial length of 23.95 mm. The new model eye provides spherical aberration values within the limits of empirical results and predicts chromatic aberration for wavelengths between 380 and 750 nm. It provides a model for calculating optical transfer functions and predicting optical performance of the eye.

Extended wear in perspective.

BACKGROUND: Convenience and safety are major motivating factors in the choice of a mode of correction for refractive errors. Extended wear of contact lenses is associated with greater convenience than daily wear, but questions remain concerning the safety and risks associated with this modality. This paper investigates the hurdles that have rendered extended wear an unfavorable option thus far and seeks to identify strategies to overcome these obstacles. METHOD: We review the history of extended contact lens wear, focusing on soft extended wear for cosmetic use. Results of incidence and relative risk studies for the major complications are scrutinized. The factors that are associated with increased risk together with mechanisms which have been proposed on based on scientific research are analyzed to derive strategies for preventing adverse reactions in extended wear. RESULT: Infectious keratitis associated with extended wear of contact lenses has been identified by the community as a major issue, although the degree of concern may be disproportionate. Current perceptions of the relative risk of extended wear versus daily wear in the context of the risk for nonwearers may be misplaced, and the role of eye closure versus consistency and duration of wear remains open to debate. In the minds of practitioners and patients, other adverse reactions may be of greater significance in rejecting extended wear. Nonetheless, the potential extent of injury from infection demands attention. Studies collectively attest that corneal hypoxia during extended wear is a potential causative mechanism in infectious keratitis, suggesting that the provision of higher oxygen levels to the cornea will reduce morbidity rates from this unwanted complication. However, the issues of stagnation of the post-lens tear film during eye closure, mechanical properties of the lens, and lens movement remain unresolved and may prove to be contributing factors to adverse responses. CONCLUSIONS: Attention to oxygen permeability and mechanical properties of soft contact lenses should solve the remaining problems with extended wear. Materials currently under development promise to meet at least some of the desired properties and may lead to the long-awaited advent of safe extended wear.

The axis of astigmatism in right and left eye pairs.

This study was designed to investigate the relationship between the astigmatic axes of right and left eye pairs, with particular attention given to determining the degree to which either direct or mirror symmetry (enantiomorphism) of the astigmatic axes exists. A sample of 192 "nonvisually" selected adults participated, with refractive error and corneal curvature data being measured using autokerato-refractive equipment. Total, corneal, and residual astigmatism were investigated, with residual astigmatism being taken as the vector difference between total and corneal astigmatism. There was no evidence for a predominance of either mirror or direct symmetry of the astigmatic axes within this sample. The patterns of astigmatic axis distribution of right and left eyes were remarkably similar but, within this context, there was no definite evidence for a definable association between the axis of the left and right pairs of individuals. These findings remained unchanged when the effect of the modulus of astigmatism was incorporated, either through weighting the frequency distributions or through analyzing a subgroup of the overall population (those individuals with greater than 0.50 D of astigmatism). We believe these findings contradict commonly held clinical impressions regarding the symmetry of astigmatic axes, and should be considered when performing statistical analysis of astigmatic data.

The effect of an interrupted daily period of normal visual stimulation on form deprivation myopia in chicks.

The aim of this study was to determine whether an integrator of neural activity influences the amount of myopia and axial elongation resulting from deprivation of form vision. The effects on ocular parameters of a continuous period of 30 min per day of normal vision was compared to two exposures of 15 min duration each, or three exposures of 10 min each. For the remaining time, chicks had monocular translucent occlusion in a 12 hr light/12 hr dark diurnal cycle, for either 2 or 3 weeks. Fellow eyes and the eyes of bilaterally unoccluded chicks were used as controls. We found that several short periods of normal visual stimulation per day were more effective in preventing the development of form deprivation myopia and axial elongation than was one single period of the same total duration, after both 2 and 3 weeks of treatment. This study suggests that the level of neural activity in the retina may have a cumulative effect in influencing ocular growth.

Bulbar conjunctival fluorescein staining in hydrogel contact lens wearers.

PURPOSE: To evaluate the clinical usefulness of bulbar conjunctival staining in assessing hydrogel contact lens patients. METHODS: Overall staining, as well as staining at five separate sites (limbal, nasal band, temporal band, superior, and inferior) was graded on an analog scale in 48 contact lens wearing subjects and 50 controls. The degree to which subjects experienced sensations of dryness, wateriness, itchiness, grittiness, and comfort was also assessed using analog scales. RESULTS: Some measure of conjunctival staining was noted in 98% of subjects, with contact lens wearers showing statistically significant greater staining than controls. Only 12% of controls showed staining of greater than grade 1 (equivalent), whereas 62% of contact lens wearers were above this level. Regression analysis showed that overall staining was a function of whether contact lenses were worn, the degree of dryness, and the amount of itchiness. CONCLUSIONS: Conjunctival fluorescein staining appears to serve some clinical usefulness as a composite indicator for certain factors and symptoms and, in addition, provides information which is unique for each individual.

The prediction of spherical aberration with schematic eyes.

Many model eyes have been proposed; they differ in optical characteristics and therefore have different aberrations and image quality. In predicting the visual performance of the eye, we are most concerned with the central foveal vision. Spherical aberration is the only on-axis monochromatic aberration and can be used as a criterion to assess the degree of resemblance of eye models to the human eye. We reviewed and compiled experimental values of the spherical aberration of the eye, calculated the spherical aberration of several different categories of model eyes and compared the calculated results to the experimental data. Results show an over-estimation of spherical aberration by all models, the finite schematic eyes predicting values of spherical aberration closest to the experimental data. Current model eyes do not predict the average experimental values of the spherical aberration of the eye. A new model eye satisfying this assessment criterion is required for investigations of the visual performance of the eye.

Distribution of astigmatism in the adult population.

We have quantified statistically the astigmatic frequency distribution. Vectorial analysis was used, as it enables formal multivariate statistical techniques to be applied to astigmatic data, allowing the simultaneous inclusion of both modulus and axis in the analytical procedure. These methods were applied to population data for each of total, corneal, and residual astigmatism from a sample of 198 adults. Right and left eyes were analyzed separately. All the distributions were found to depart significantly from a normal distribution. All the distributions were significantly leptokurtic (p < 0.005), and the distributions of total right eye, corneal right eye, and residual left eye astigmatism were also found to be significantly skewed (p < 0.05). Significant mild correlations were found between total and corneal astigmatism (p < 0.05). These findings add to the database of knowledge of astigmatic refractive error and may be of interest to those investigating refractive-error development.

Clinical evaluation of refractive techniques.

BACKGROUND: The accuracy of two commercially available auto kerato-refractometers (Shin-Nippon Automatic Ref-Keratometer model RC380 and Topcon Auto Kerato-Refractometer model KR-3100) and one autokeratometer (Alcon Systems Auto-Keratometer) was compared to that of subjective refraction and conventional keratometry. METHODS: Refractive error and corneal curvature were measured on 20 subjects. Measurements were converted from the standard clinical format of sphere, cylinder and axis to a vector format to assess the contribution of spherical and cylindrical errors simultaneously. RESULTS: For measurements of refractive error taken in immediate succession, the Shin-Nippon instrument was found to be more repeatable than the Topcon instrument. This trend reversed when subjects were realigned between measurements. The 95 percent confidence limit for precision for subjective refraction was considerably greater: 93 percent of subjective refractions resulted in visual acuities better than or equal to 6/6, compared with 85 percent for the Topcon instrument and 45 percent for the Shin-Nippon instrument. Each of the methods for measuring corneal curvature showed minimal bias and comparable precision. CONCLUSIONS: Although subjective methods of determining refractive error generally achieved the same or better visual acuity as the automated methods, they displayed considerably poorer precision. Each of the methods of measurement of corneal curvature produced similar results.

The effects of short-term contact lens wear on adherence of Pseudomonas aeruginosa to human corneal cells.

BACKGROUND: Ulcerative keratitis is the most serious complication of contact lens wear, with the majority of infections being attributed to Pseudomonas aeruginosa. The degree to which bacteria adhere to the cornea may indicate its susceptibility to infection. It has been found that long-term contact lens wear leads to increased adherence of Pseudomonas aeruginosa to corneal epithelial cells. This study examines the effect of short-term contact lens wear. MEANS: Cells were collected using a non-contact corneal irrigation system from a total of 16 subjects fitted with tight and loosely-fitted soft contact lenses. The control group consisted of 10 subjects with no recent contact-lens-wearing experience, eight of whom were later fitted with lenses to form a neophyte group. Six contact lens wearers formed the experienced group. After wearing lenses for a 30-hour period, including overnight, the collected corneal cells were incubated with Pseudomonas aeruginosa, differentially stained with Acridine orange and examined under fluorescence microscopy. RESULTS: The number of bacteria adhering to the cells was determined for the control group, neophyte and experienced groups. The frequency distribution profiles were similar for all groups and did not vary significantly between lens-wearing and non-wearing conditions, between neophyte and experienced wearers, or with lens fit. CONCLUSIONS: Using the technique reported, short-term extended contact lens wear produced minimal detectable changes in the potential of corneal epithelium cells to bind bacteria.

The duration of normal visual exposure necessary to prevent form deprivation myopia in chicks.

The aim of this study was to determine the minimum daily period of exposure to normal visual stimulation required to prevent occlusion induced myopia in chicks. Chicks were treated with monocular translucent occlusion in a 12 hr light/12 hr dark cycle. Occluders were removed for 0 (constant occlusion), 15, 20, 30, 40, 60, 75, 90, 120, 150, 240 or 720 (no occlusion) minutes each day for either 2 or 3 weeks. Fellow eyes and the eyes of normal chicks (bilaterally unoccluded) were used as controls. Occlusion-induced myopia and axial elongation were found to decrease significantly (P < 0.01) with increasing daily exposure to normal visual stimulation. Application of a time series equation to the data estimates that 30 and 130 min of normal visual exposure per day reduces myopia by 50 and 95% respectively. This study demonstrated that the regulation of ocular growth is affected strongly by short periods of normal visual stimulation in the presence of long periods of abnormal stimulation.

Soft lens movement: effects of humidity and hypertonic saline on lens settling.

Hydrogel contact lenses require a settling period before lens fit stabilizes, but the process underlying the initial reduction in lens mobility is poorly understood. Explanations for this phenomenon include base curve steeping with dehydration, expulsion of postlens tear fluid, and osmotic flow of hypotonic tears into the cornea. We conducted two randomized, single-masked interocular comparisons for ten subjects wearing HEMA lenses for 1 h. We investigated potential mechanisms of dehydration-dependent lens tightening, by limiting dehydration in one eye using a high humidity environment (experiment 1), and of a hypotonic lacrimation-dependent decrease in lens movement, by presoaking lenses in isotonic (0.9%) or hypertonic (1.5%) saline (experiment 2). Lens mobility profiles were not significantly affected by modification of environment; lenses in both normal and high humidity environments displayed a significant reduction in lens movement during the first 15 min of wear (ANOVA, p = 0.0001), and no change in lens mobility thereafter. Lens mobility profiles were identical for lenses presoaked in isotonic and hypertonic saline, with initial lens movement being significantly greater than for subsequent measurements (ANOVA, p < 0.05). These studies find no evidence to support dehydration-dependent steepening of base curve, or osmotic-dependent binding from hypotonic lacrimation as mechanisms for the initial postinsertion decrease in lens movement.

Soft lens movement: temporal characteristics.

The time-course of on-eye hydrogel lens movement has not been carefully scrutinized, despite the importance of lens movement in optimizing lens fit and corneal physiology. We conducted a study to define the time-course of soft lens movement using 26 subjects. Video slitlamp recordings were made of lens movement at 5-min intervals for 30-min wear and after 8-h wear of 38 or 67% water content lenses (N = 14 and 12, respectively). Lens mobility profiles were statistically indistinguishable for high and low water content lenses, and for experienced and neophyte lens wearers. Lens movement displayed biphasic temporal characteristics, decreasing significantly over the first 25 min from a median of 0.6 to 0.3 mm (Wilcoxon matched-pairs signed-rank test, p = 0.002), then increasing significantly to 0.5 mm after 8 h of wear (p = 0.03). Although some subjects exhibited little alteration in lens movement, 31% showed a decrease in lens movement > 0.25 mm during the first half-hour of wear. Optimal predictability of lens mobility after 8-h wear was achieved 5 min after insertion, with 77% of subjects displaying lens movement within +/- 0.25 mm of the final value. In-office assessment of lens movement is best achieved 5 min after insertion, although clinical and real world lens mobility will differ significantly in about one in four patients.

Comparison of clinical diagnostic tests in hydrogel extended wear.

We compared the relative value of biomicroscopy, pachometry, esthesiometry, and corneal oxygen flux measurements for the detection of corneal hypoxia in myopic patients wearing hydrogel extended wear lenses. Measurements were standardized by relating the change in each parameter to its test-retest reliability. After 3 months of extended wear the greatest change occurred in biomicroscopic signs (microcysts and vacuoles, fluorescein staining, inferior conjunctival hyperemia) and in stromal edema measured with pachometry. These four signs were poorly correlated to each other (Pearson's r < 0.3), indicating that none could be considered redundant. Stromal striae, endothelial edema (blebs), esthesiometry, and corneal oxygen flux measurements showed relatively little change during the period of lens wear.

X-ray and scanning electron microscopic analysis of the structural composition of tear ferns.

Dried tears from keratoconjunctivitis sicca eyes fail to exhibit the fern-like crystallization patterns observed with tears from eyes with normal tear function. To test our hypothesis that the extent of ferning depends on the ratio of salts to protein and mucin in the tear sample, dried tears from six normal subjects were subjected to scanning electron microscopy (SEM) and X-ray analyses. X-ray diffraction identified sodium chloride and potassium chloride as the major components of tear fern crystals. X-ray fluorescence detected the elements potassium, chlorine, calcium, and sulfur in the dried tear samples, with sulfur indicating the presence of protein and/or mucin. As well as confirming the presence of cubic fern nuclei, SEM revealed two kinds of material, having crystalline and globular appearances, that are hypothesized to be composed of salts and protein/mucin, respectively. Globular material appeared to block extension of crystal fern arms or to coat crystalline material, but did not crystallize. These findings suggest that tear fern crystals are composed of sodium and potassium chloride, with proteinaceous material controlling crystallization indirectly by coating crystal faces and blocking fern extension. This structural composition is consistent with the hypothesis that the ratio of salt to macromolecular species is an important determinant of tear ferning.

The role of pili in the attachment of Pseudomonas aeruginosa to unworn hydrogel contact lenses.

Contamination of contact lenses is thought to increase the risk of infectious keratitis, yet factors promoting attachment of bacteria to contact lenses are not fully understood. It has been suggested that strains of Pseudomonas aeruginosa attach to mucosal surfaces via pili which are appendages found on some strains. This study investigated the role of pili and the effect of incubation time on the attachment of P. aeruginosa to 20 unworn hydrogel lenses representative of each of the four FDA categories. Ten lenses were incubated for 15 minutes and another ten for 180 minutes. Lenses were incubated with either PAK + P. aeruginosa which possessed pili or its isogenic mutant pair, PAK-, which was genetically similar except for the absence of pili. Bacteria were quantified, following homogenization of the contact lens, by viable counts. Non-piliated bacteria were significantly more likely to adhere to the lenses (p < 0.001). A significant interaction between lens type and incubation time was observed (p < 0.05); thus it is difficult to generalize about either of these effects in isolation. These results show that surface characteristics may confer an attachment advantage to bacteria.