Oxygen permeability of collagen shields.

The oxygen permeability (Dk) of ten 24-hr collagen shields was measured directly by polarographic methodology at approximately 2 hr of hydration. Edge and boundary effects were included in the calculations. Dk was found to be approximately 26 x 10(-11) cm ml O2/sec ml mmHg at 35 degrees C. Mean water content of the shields was 65.7% (SD = 1.0%) as measured by a hand refractometer. Therefore, the projected oxygen transmissibility of collagen shields is expected to be compatible with normal corneal metabolism.

Lipopolysaccharide in adherence of Pseudomonas aeruginosa to the cornea and contact lenses.

PURPOSE: To determine the role of smooth or rough lipopolysaccharide on adherence of Pseudomonas aeruginosa bacteria to the rat cornea in vitro and on contact lenses of differing types. METHODS: Adherence of a smooth (AK957) and isogenic rough strain (AK1012) of P. aeruginosa bacteria to rat corneas that were either normal, traumatized using a 20-gauge needle or treated for 15 min with 0.1N sodium hydrochloric acid was assessed by homogenization and viable counting. Adherence of these organisms to 43 unworn contact lenses representing the four Food and Drug Administration lens groups was also assessed using viable counts. RESULTS: Attachment to contact lenses was greater for the smooth strain for all four lens types (P < 0.001). No variation in adherence to the different lens types was observed. Smooth bacteria also adhered to the cornea to a greater extent than the rough strain, regardless of trauma type (P < 0.001). Adherence to traumatized corneas was greater than to nontraumatized corneas for both strains of P. aeruginosa bacteria (P < 0.01). Measurement of surface hydrophobicity of the two bacterial strains revealed that the smooth strain was more hydrophobic than the rough strain (P < 0.001), perhaps accounting for the adherence pattern. CONCLUSIONS: These results indicate that bacterial surface characteristics may be important determinants of adherence and could explain the propensity of certain bacterial strains to infect the cornea.

Corneal pathophysiology with contact lens wear.

Contact lens wear induces a wide spectrum of changes in the appearance and function of the cornea. The most salient effect of lens wear is the hypoxically induced reduction in the rate of metabolic activity of the corneal epithelium and its sequellae. Other important alterations to corneal health associated with contact lens wear may be caused by antigenic and toxic stimuli, mechanical forces, osmotic effects and carbon dioxide retention. Perhaps the most important task facing the contact lens clinician is to distinguish between an acceptable state of physiological modification and an anomalous or pathological state of hypofunction. In this article, we review the assortment of corneal changes primarily on the basis of the causative agents and time scale with reference to the physical and chemical processes leading to the observed signs or symptoms. This procedure allows a strong foundation for understanding the etiology and management principles for the variety of effects that contact lenses may have on the cornea.

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.

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.

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.

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.

Extended contact lens wear in the developing cynomolgus monkey eye. Recovery from adverse responses.

Soft extended wear contact lenses were designed for and fitted to 12 eyes of 11 infant semiwild cynomolgus monkeys. The lenses were satisfactorily worn over observation periods of up to 465 days, the longest continuous period of lens wear being 69 days. However, some of the corneas were subject to a pattern of complications analogous to that in human contact lens wear. Nine eyes showed adverse responses, and of these, the ocular complications were severe enough in four cases to cause abandonment of lens wear. The most remarkable feature of the complications was the rapid recovery of severe reactions to near normality in less than 24 h, indicating an impressive ability of the cornea to tolerate and survive insult. Another feature was the recurrent nature of mild complications in certain monkeys despite apparent resolution from previous episodes. There appeared to be no correlation between duration of lens wear and appearance of the first or recurrent episodes of corneal infiltration and vascularization.

Ocular surface temperature.

A wide-field color-coded infra-red imaging device was applied to the measurement of i) the temperature profile across the ocular surface and ii) the temporal stability of central corneal temperature, on 21 subjects. The thermographs showed a pattern of ellipsoidal isotherms (major axis horizontal) approximately concentric about a temperature apex (coldest point) which was slightly inferior to the geometric center of the cornea (GCC). The GCC had a mean temperature (+/- SD) of 34.3 +/- 0.7 degrees C (range 32.8 to 35.4 degrees C). Temperature increased towards the periphery of the cornea with the limbus being 0.45 degrees C warmer than the GCC (p less than 0.0001). Following a blink, the GCC cooled at a mean (+/- SD) rate of 0.033 +/- 0.024 degrees C/s (p less than 0.0001) over the first 15s. Subjects whose corneas cooled more slowly following a blink demonstrated a greater capacity to avoid blinking for a prolonged period (p less than 0.05). This improved method of measuring ocular surface temperature has important applications in modeling corneal physiology and pathology.

Temperature of the hyperemic bulbar conjunctiva.

The relationship between bulbar conjunctival hyperemia and temperature was investigated. Conjunctival redness was induced in eighteen volunteers by instilling hypertonic saline into the conjunctival sac. The degree of redness was estimated using a subjective grading scale. The subsequent changes in temperature of the nasal bulbar conjunctiva were monitored using an infra-red bolometer. Conjunctival hyperemia was significantly correlated with conjunctival temperature; the maximum response of a 3-grade change in redness was accompanied by an increase of 0.5 degrees C in temperature. These findings confirm the classic association between inflammation, rubor and calor.

Corneal thickness changes following sleep and overnight contact lens wear in the primate (Macaca fascicularis).

Corneal thickness was monitored on seven cynomolgus monkeys (Macaca fascicularis) over a 76 hour period. During this time, corneal thickness measurements were also made on six monkeys after overnight wear of a hydrogel contact lens in one eye. Mean corneal thickness was 417 +/- 12 microns. An overall diurnal variation of 16 +/- 5 microns (3.8 +/- 1.2%) was found. During the first half hour after waking, corneal thickness changes ranged from -9 microns to +14 microns. After overnight contact lens wear, the cornea had swelled an average of 42 +/- 24 microns (9.1 +/- 4.8%). Following lens removal, the cornea returned to normal thickness within approximately 90 minutes. These results are similar to those found in humans and indicate that with respect to contact lens induced corneal thickness changes, the cynomolgus monkey is a suitable model for the physiological response to contact lens wear.

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.

Oxygen permeability of hard gas permeable contact lens materials.

Oxygen transmissibility (Dk/L) data of contact lenses enables prediction of the ocular response during wear. However, there has been difficulty in relating the data from highly permeable rigid lens materials to models predicting corneal swelling based upon the Dk/L of soft lenses. We have examined the methodology used to determine the oxygen permeability (Dk) of hard gas permeable (HGP) materials, and have applied a measurement technique that overcomes certain deficiencies of previous methods. A representative range of HGP and hydrophilic lens materials was measured. The Dk values reported here for hydrophilic materials are in close agreement with those published elsewhere. However, the Dk values of HGP materials were found to be substantially less than those reported previously, although the relative ranking appears to be independent of measurement technique. Possible explanations of these findings are presented.

The effects of fenestrating soft contact lenses on corneal swelling: a re-examination.

Previous studies have suggested that central corneal oedema during hydrogel lens wear can be reduced by placing peripheral fenestrations in the lens. In this study, optical pachometry was used to monitor central and peripheral corneal swelling in response to a peripherally fenestrated hydrogel lens. The experiment was conducted in a double-masked, randomized manner on 10 unadapted subjects. Fenestrations did not alter central corneal oedema, but peripheral corneal oedema was reduced by 34% (p > 0.001). Fenestrations may therefore provide a useful strategy for minimizing lens-induced corneal oedema. Clinical trials with fenestrated lenses are now indicated.

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.

Average thickness of a hydrogel lens for gas transmissibility calculations.

Several attempts have been made to calculate the average thickness of hydrogel lenses for the purpose of calculation of oxygen transmissibility. The deficiencies of these equations are discussed and a new equation is given. Results of average thickness calculations with variation in different parameters are presented. These results indicate that the effect of thickness variation in higher powered lenses has been seriously underestimated. The parameters which cause a major difference in the average thickness compared to the center thickness are power, chord radius, and water content.

Application of hydrogel lens average thickness.

The equation for the average thickness of a hydrogel lens is a complex function. This report gives a simple method of estimating this parameter. By structuring the information into nomograms, an accurate system is derived which facilitates the design of a contact lens to provide a required average oxygen transmissibility. Several examples of the accuracy of the estimated value compared to the calculated value are shown. Details of the use of these nomograms are given with a discussion on the worth of an average thickness value as compared to a local thickness value.

Esthesiometry as an indicator of corneal health.

We review the literature regarding changes in the structure and function of the cornea as an index of the physiological status of the tissue. Accurate measurement of corneal sensitivity may be the most reliable test of long-term corneal compromise inasmuch as the edema response, the most popular test of corneal physiological integrity, shows "adaptation". Although reduced sensitivity has been associated with corneal hypoxia, decreased corneal pH may also play a major role. Certainly, small departures of pH from normal levels in the body can lead to major depression of neural function. Traditionally, mechanical sensitivity of the cornea has been measured with procedures based on the von Frey hair. More recently, esthesiometry has been performed with the use of a puff of air, a chemical stimulant, and heat. Results from early trials with these procedures are encouraging in terms of the development of a viable clinical method for assessing metabolic effects on the cornea from contact lens wear.

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.