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Purpose: To test the impact of professional management of soft contact lens wear on symptoms and ocular complications., Methods: Subjective symptoms and ocular complications of soft CL users who did not seek professional follow-up care (self-managed, SM), were compared to users who were prescribed CLs and their care professionally managed in optometry practices (PM), and to a control group of non-CL wearers. Habitual visual acuity, subjective dry-eye symptoms, and corneal abnormalities were assessed in all participants. CL wearers filled-out a usage habits questionnaire, and their CL fit was assessed. Outcomes were compared using Kruskal-Wallis and Chi Squared tests., Results: The SM, PM, and non-CL wearers cohorts included 127 (mean age:24.3±5.1, median:23, range:16-45 years,104 female), 132 (mean age:25.5±6.2, median:23, range:18-43 years,103 female), and 56 (mean age:22.3±3.5, median:21, range:18-39 years,36 female) participants, respectively. Meibomian gland dysfunction grade (p = 0.004, p<0.0001), limbal redness (both p = 0.04), corneal neovascularization (both p = 0.003), and papillary conjunctivitis (p<0.0001,p = 0.005) were significantly worse in SM CL wearers compared with both the non-CL wearers and PM CL wearers, respectively. Conjunctival staining was significantly worse in the SM cohort compared with the PM cohort (p = 0.01). 38.6% of the SM compared with 22.8% of the PM CL wearers, had an inappropriate refractive correction (p = 0.006). SM CL wearers wore CLs significantly more years (mean and median 1 year,p = 0.008), for more daily hours (mean and median of 2 hours,p<0.00001), and tended to nap or sleep with their CLs compared with the PM CL wearers (47 vs. 29,p = 0.02). The cohorts did not differ in their subjective symptoms., Conclusions: Complications are significantly more prevalent in SM CL wearers compared with PM CL wearers, and SM CL wearers tend to wear CLs with incorrect powers, and are less compliant with napping or sleeping with the CLs compared with PM CL wearers. These findings emphasize the importance of fitting, patient education and follow-ups in CL wearers., Competing Interests: Cooper-Vision Israel provided contact lens solutions that were provided to participants of the study. This does not alter our adherence to PLOS ONE policies on sharing data and materials. There are no patents, products in development, or marketed products associated with this research to declare., (Copyright: © 2024 Gantz et al. This is an open access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.)

Objectives: Limited literature supports the value of routine contact lens examinations. The purpose of this study is to document complications diagnosed when subjectively successful planned replacement soft contact lens patients are evaluated to renew their existing contact lens prescriptions., Methods: Asymptomatic soft contact lens patients who presented to the University Eye Center at Ketchum Health (Anaheim, CA) and the Eye and Vision Center at the Massachusetts College of Pharmacy and Health Sciences (Worcester, MA) for routine contact lens comprehensive exams to renew existing contact lens prescriptions were professionally evaluated and their complications documented. All subjects presented without complaint and had a history of subjectively successful planned replacement soft lens wear for at least 1 year., Results: A total of 202 subjects were recruited from both sites (age range 16-72 years; 76 male, 126 female; spherical contact lens optical power equivalent range: +7.75D to -19.25D). Considering health issues, one hundred and five (52%) patients were found to exhibit at least one undiagnosed complication (95% CI: 0.45-0.59): 70% were diagnosed with contact lens driven ocular complications; 54% were diagnosed with non-contact lens driven ocular health issues; and 4% showed signs of undiagnosed systemic disease. Of note, complication prevalence increases to 72% overall if both contact lens fit issues and contact lens care compliance problems are included as complications in analysis., Conclusions: A striking ocular health and contact lens complication rate in asymptomatic soft contact lens wearers is demonstrated across two different study sites. This data suggests that asymptomatic as well as symptomatic contact lens wearers require routine professional evaluations., (Copyright © 2020 British Contact Lens Association. Published by Elsevier Ltd. All rights reserved.)

Objective: The purpose of this study is to determine the peripheral oxygen transmissibility (pDk/t) and respective central oxygen transmissibility (cDk/t) in soft contact lenses (SCLs) which might preclude SCL-driven corneal neovascularization (NV) in healthy myopic SCL users., Methods: Twenty subjectively successful SCL-wearing patients who presented with asymptomatic but active peripheral corneal NV (not ghost vessels) were recruited as study patients. Twenty-one patients who did not have NV were similarly recruited as controls. Demographic data were collected. Corneal NV was documented and photographed. Current habitual SCLs were collected and thicknesses measured to allow for the calculation of both pDk/t and cDk/t and estimation of local tear oxygen tensions., Results: No statistical differences between study and control groups in patient age, refraction, or the numbers of years, days per week, or hours per day patients reported SCL wear were identified. Statistically significant differences were found between the two groups for both pDk/t (P=0.006) and cDk/t (P=0.004): mean (±SD) pDk/t was 38.0±23.5 and 19.2±17.7 Fatt units for control and study corneas, respectively. Mean cDk/t were 80.0±54.4 and 36.8±33.1 Fatt units for control and study corneas, respectively. Peripheral tear oxygen tension that "protected" corneas from vascular filling was over 84 mm Hg., Conclusion: Maintaining a pDk/t above 30 to 40 Fatt units with daily wear SCLs should protect most normal corneas from NV as a complication of SCL wear.

Objectives: To quantify tear elimination rate (ER) underneath silicone hydrogel (Si-Hy) and scleral gas permeable (GP) contact lenses (CLs)., Methods: Subjects successfully using either well-fitting soft Si-Hy CLs or scleral GP CLs were recruited. Most scleral GP CL wearers had irregular corneas (e.g., keratoconus). An objective fluorometer measured decay of fluorescein isothiocyanate dextran dye signal (70 kD MW) from which the tear ER in %/min was calculated. For GP scleral lenses, the ER was determined for both the initial settling period and the 30- to 60-min period, and without lenses. All ERs were calculated from 5 to 30 min to avoid reflex tearing effects., Results: Fourteen soft Si-Hy CL and 12 scleral GP CL wearers completed the study. The ER for the scleral GP CL wearers averaged 0.57 (±0.6) %/min for the 0- to 30-min and 0.42 (±0.5) %/min for the 30- to 60-min period (P=0.515). Non-CL wear tear ER in these same subjects averaged 34.17 (±15.9) %/min and was significantly different versus both scleral GP wear periods (both P values <0.001). The ER for the soft Si-Hy CL wearers, 5 to 30 min, averaged 6.09 (±2.8) %/min., Conclusions: Our data demonstrate significantly less ER in well-fit scleral GP CL wearers compared with soft Si-Hy CL wearers for both the settling and longer wear periods (both P values <0.001). Moreover, slightly greater tear exchange was observed during the scleral GP CL settling period than later, which may reflect a change over time in tear vault thickness.

Background/purpose: Over the past decade, utilization of scleral gas permeable (GP) contact lenses has steadily increased. Scleral GP lenses offer yet another option for patients suffering from visually debilitating corneal and tear conditions. Oxygen delivery to the cornea in the presence of a contact lens system continues to be a subject of interest, and scleral GP lenses are no exception. This paper utilizes an existing model based on simultaneous two lens systems (piggyback lenses) as a resistance to oxygen in series, and applies this model to scleral GP lens systems., Methods: Theoretical oxygen tensions are calculated for tear layers trapped beneath scleral contact lens systems and the anterior corneal surface with a simple single chamber corneal model using a computer software spreadsheet., Results: Only the best case scenario for current scleral gas permeable lenses (thickness and Dk)/tear layer values allow sufficient tear layer oxygen tension (approximately 100 mmHg) to preclude corneal hypoxia., Conclusions: The results of the spreadsheet model suggest that clinicians would be prudent to prescribe scleral GP lenses manufactured in the highest Dk materials available and to fit without excessive corneal clearance to minimize anterior segment hypoxia., (Copyright © 2014 British Contact Lens Association. Published by Elsevier Ltd. All rights reserved.)

Background: Failure to comply with the manufacturers' recommended replacement frequency (MRRF) is expected to result in ocular complications. The goal of this study is to evaluate the empirical relationship between compliance with MRRF and the presence of ocular complications., Methods: All soft contact lens-wearing patients who presented at either of 2 clinics were evaluated prospectively for contact lens-related complications and for compliance with MRRF., Results: Compliance with MRRF varies significantly across replacement modality (P < 0.01) and lens solution type (P = 0.04). Among noncompliant patients, average days of overwear beyond MRRF significantly varies across age group (P = 0.02), lens material type (P = 0.01), and lens solution type (P = 0.02). Regression analysis controlling for demographic factors and lens and solution type found a marginally significant (P = 0.07) positive effect of days of overwear on average complications per eye., Conclusions: We find noncompliance broadly present across demographic groups and patient types and marginally positively related to complications., (Published by Elsevier Inc.)

Introduction: The goal of this study was to apply a theoretical analysis of the relationship between contact lens oxygen transmissibility and tear layer oxygen tension to silicone hydrogel toric lenses and thereby model the success of such lenses in decreasing corneal hypoxia., Method: Lens thickness was measured at different points along the vertical meridian of one prism-ballasted, one dynamic-stabilized silicone hydrogel toric lens design, and one control "traditional" hydrogel toric lens design. Using lens thickness measurements and the nominal oxygen permeability values of these three lens designs, the subsequent oxygen tension in the tear layer trapped between the contact lens and the anterior cornea (P2) were calculated for both open- and closed-eye conditions using a single corneal chamber model with a personal computer program., Results: We found that cylindrical powers, regardless of lens materials, did not have a statistically significant effect on lens thickness (F=0.30, p=0.5834) to the limitation of our measurements, while contact lens type, spherical power, location on lens, and axis of cylindrical power were all found to do so. When multi-factor ANOVA was applied to the lens thickness and P2 data, contact lens location had a statistically significant effect on P2 values for both prism-ballasted (F=640.16, p<0.0001) and dynamic-stabilized toric lenses (F=352.85, p<0.0001). When the same statistical methodology was used to compare the relative performance of all three lenses on P2 during daily wear, the average P2 values for the three different lens brands were all statistically significantly different, while under closed-eye conditions the average P2 values for both silicone hydrogel toric lenses were no longer statistically significantly different (F=1.00, p=0.3178)., Conclusion: Assuming that the critical P2 was 100 mmHg, we predicted that silicone hydrogel toric soft lenses would provide reasonable anterior corneal oxygenation, certainly much enhanced over the environment predicted under traditional hydrogel designs, especially during daily (e.g. open eye) wear. Substantial corneal hypoxia continued to be predicted during extended wear of all lenses, but especially so with the use of traditional hydrogels.

Purpose: To determine whether contact lens, related corneal pannus regressed when eyes were refitted with rigid gas-permeable contact lenses and to investigate the role of hypoxia as the putative origin of this complication., Methods: Twenty-three eyes of 14 patients with corneal pannus were identified retrospectively from a referral practice population and were separated into two subgroups. One subgroup accepted refitting with rigid gas-permeable contact lenses or discontinued contact lens wear, and the other elected to continue wearing hydrogel or polymethylmethacrylate contact lenses. The response of pannus was observed by slit-lamp biomicroscopy and photographs and was compared between the two subgroups. One eye of each of six patients identified with bilateral corneal pannus secondary to daily wear of hydrogel contact lenses was selected at random. Duplicates of the hydrogel contact lenses worn by these eyes were obtained, and their peripheral oxygen transmissibility (Dk/Lp) values were compared with those of a control group of hydrogel contact lenses identical to those worn by another group of patients, matched by age, gender, and laterality to the study group, but who did not have corneal vascularization., Results: Vascularization regressed in 14 eyes of nine patients refitted with rigid gas-permeable contact lenses and also in both eyes of one patient who discontinued contact lens wear. All seven eyes of five patients who declined refitting showed persistence or increase of pannus. The mean Dk/Lp of the hydrogel contact lenses for six eyes with pannus was 3.8 X 10(-9) cm ml O2/s ml mm Hg; mean Dk/Lp was 8.6 X 10(-9) cm ml O2/s ml mm Hg for the lens of the control eyes. This difference was found to be statistically significant (P = .028)., Conclusion: Peripheral, contact lens-induced hypoxia is a risk factor for corneal pannus, and conversion from hydrogel or polymethylmethacrylate contact lenses to daily wear rigid gas-permeable contact lenses in these eyes successfully reversed this complication.

Objective: To evaluate the effectiveness and safety of daily wear contact lenses in the treatment of infantile aphakia., Design: A study of prognosis using a cohort followed up for a mean of 58 months., Setting: All operations, contact lens care, and follow-up were conducted at a university referral center., Patients: Of 111 children undergoing surgery for cataract between 1980 and 1990, 51 (68 eyes) met the criteria of age younger than 2 years at the time of surgery and of cataract of nontraumatic origin. Patients were evaluated for visual acuity, complications, number of contact lenses worn per year, and changes in contact lens refractive power per month. A subgroup of 28 eyes of patients undergoing surgery before age 7 months and followed up for at least 24 months was studied., Intervention: Following cataract surgery, all patients were fitted with daily wear contact lenses., Main Outcome Measure: Final visual acuity and complications were studied. The distribution of mean contact lens power for each month of age was determined., Results: During the follow-up period, no serious complications were encountered. The visual acuity outcome was better following bilateral cataract surgery than unilateral surgery (P < .001 using chi 2 analysis) and was comparable with that achieved with extended wear contact lenses., Conclusion: Daily wear contact lenses were found to be safe and effective in the treatment of infantile surgical aphakia. The daily care was easily learned by the parents.

Stipulation of the oxygen transmissibility (Dk/L) of a contact lens based upon oxygen permeability (Dk) and nominal lens thickness (L) can be misleading since center thickness, average thickness, and hence Dk/L of a contact lens vary with lens power. While earlier confusion regarding determination of Dk has now been resolved, this data has yet to be fully incorporated into a clinically applicable model. We developed a graphical solution describing variations in Dk/L with power for a representative sample of currently available hydrogel and rigid contact lenses. The Dk/L is found to be optimal for lenses of low minus power (approximately -2.50 D). The model presented here will assist practitioners in assessing the potential oxygen performance of different lens designs with respect to a given power. We advocate that lens manufacturers print the average Dk/L on lens packaging, according to the model described in this paper, along with the parameters that are usually stated.

We measured the central and mid-peripheral thicknesses of three "disposable" hydrogel contact lens designs of varying powers. These measurements allowed calculation of the areal average lens thicknesses (over a central 6 mm chord), which in turn allowed calculation of predicted oxygen transmissibility (Dk/L) for the lenses studied (using previously reported values for measured oxygen permeability [Dk] of these materials). Dk/L values for the disposable hydrogel lenses tested were found to range from 9 to 17 x 10(-9) cm mL O2/sec mL mmHg.

Disposable contact lenses are inexpensive hydrogel lenses that are approved for both daily and extended wear. Confusion may exist regarding the physical properties of disposable contact lenses. We used the single-chamber polarographic oxygen permeability measurement method, corrected for both boundary and edge effects, to determine objectively the oxygen permeability of three brands of disposable contact lenses. The oxygen permeability values determined for each lens material are as follows: Acuvue, 18 x 10(-11) cm2 ml O2/sec ml mm Hg (Dk); NewVues, 15 x 10(-11) Dk; and SeeQuence, 9 x 10(-11) Dk. This demonstrates that the inexpensive production techniques, which confer a relatively low unit expense, do not change the physical properties of the hydrogel materials as they relate to oxygen permeability and transmissibility. We concluded that hypoxic stress to the cornea is just as likely when using a disposable contact lens as it is when using a conventional reusable soft hydrogel lens of similar composition and water content.

The use of contact lenses for extended wear appears to be accompanied by an increase in several complications, the most serious of which is corneal infection. It has been proposed that infection is due to corneal epithelial layer microtrauma and lens care system contamination, but noncompliance, contamination of care systems, and microtrauma [especially among keratoconics and polymethylmethacrylate (PMMA) lens users, for example] are so common in daily wear that we believe they are insufficient to explain the approximately sixfold increase in the rate of corneal infection with extended wear. Perhaps the chronic hypoxic stress of extended wear use of contact lenses substantially reduces the resistance of the epithelium to either injury or microbial infection. Until evidence is provided that new contact lenses have sufficient oxygen transmissibility (Dk/L) to fully satisfy corneal oxygen needs under extended wear conditions, and the extended wear use of such lenses is thoroughly tested in a study population, we believe that the majority of contact lens patients should be maintained on daily wear schedules.

Mathematical statements are made for the various hypotheses describing the change in radius of curvature of the back surface of a flexible contact lens as it flexes. These hypotheses may now be tested for validity in predicting both optical and physiological behavior of real contact lenses in situ.

It is necessary to measure several samples with different thicknesses of the same material to be able to determine a value for the oxygen permeability (Dk incm2 ml O2/s ml mm Hg) of that material. Some current contact lens materials are not available in multiple thicknesses, but it might be possible to "stack" several samples of the same thickness as an alternative procedure. This study demonstrates that measurements of Dk for one particular midwater-content hydrogel (Methafilcon, a nominally 55% water-content ionic material) give statistically indistinguishable results whether single samples of various thicknesses (Dk = 20.52 x 10(-11)) or thinner stacked samples to attain similar thicknesses (Dk = 20.05 x 10(-11)) are used in this measurement.

Young children occasionally suffer from cataracts. Etiology may be trauma or heredity, but surgical removal and proper contact lens care are now considered imperative for visual development. This paper describes the author's method for fitting and caring for young aphakic children.

Fifteen patients discontinued use of extended-wear hydrogel contact lenses. The primary cause was neovascularization (60%), although lens soilage, giant papillary conjunctivitis and ulceration were factors in several patients each. All patients were successfully refitted with daily-wear rigid or thin, hydrogel contact lenses following simple clinical techniques.

Both clinical impressions and theoretical analysis suggest a slight loss of power with flexure of low-plus-power hydrogel contact lenses. Low-minus-power hydrogel lenses do not appear to change power. This study confirms this effect.

Extended wear contact lenses are now a reality. Unfortunately, the physiological basis for their apparent success is less based on knowledge than wishful thinking. This paper discusses both what we know and what we don't know with the goal of giving the practitioner in the field a global view of this important and controversial procedure.

Oxygen tensions were calculated under two different oxygen-permeable, flexible contact lenses (daily wear and extended wear) both with and without consideration of tear layers (but without tear exchange). Tear layers do not have significant effects on precorneal oxygen tension.

Twenty patients with corneal disease were treated with a recently released soft lens specifically designed for use as a bandage. Patients wore their lenses continuously 24 hr a day. The results were similar to those of previous studies conducted with other lens designs. Improvement was frequent (10 of 12 cases) for bullous keratopathy and corneal graft, burn, or neuroparalytic lesion; limited (1 of 4 cases) for dry-eye syndrome; and absent (4 cases) for irregular cornea and vernal keratoconjunctivitis. No difficulties were found with the concurrent use of medication.

Surveys of contact lens practice, comparing the use of daily wear and extended wear cosmetic soft contact lenses, were distributed to almost 3,000 members of the Contact Lens Section of the American Optometric Association. There were 759 responses received of which 440 were considered valid for analysis. The results of our analysis suggest that slightly more physiological complications are encountered with extended than with daily wear of contact lenses. Most optometrists (96%), however, are satisfied with this modality and continue prescribing extended wear lenses, although many have decreased the number of days over which patients are advised to use the lenses before removal and cleaning. Most of the respondees now favor medium water content, thin lens designs for extended wear use, with 70% of them suggesting weekly removal, cleaning and aseptization. Peroxide cleaning regimens are most popular for this function.

We examined two soft contact lens wearers who developed keratitis associated with Bacillus contamination of their contact lens care systems. Patient 1 developed a corneal ulcer caused by B. subtilis, and Patient 2 demonstrated multiple, diffuse, punctate corneal epithelial opacities associated with B. cereus contamination in the contact lens and lens case compartment. The contact lens cases of both patients contained Bacillus spores that survived multiple heat disinfection treatments. Three different contact lens chemical disinfection systems used for the minimum recommended time failed to kill the Bacillus organisms.

Many models have been proposed to describe the simultaneous changes which occur in front and back radii of curvature of a hydrogel contact lens flexing or "wrapping" onto the surface of a human eye. This study first discusses the concept that the change in front radius (delta r1) may be related to the change in back radius (delta r2) by a simple constant which may change with lens design, and then develops a separate new general relation from observed lens behavior: delta r1/delta r2 = 1 - 0.05 F, where F is unflexed lens back vertex power.

This study consists of clinical experience with the recently produced lathe cut Bausch & lomb SOFTLENS. Sixteen patients were fitted with the spin cast and then lathe cut lens. Visual acuities were better with the lathe cut lens. Comfort was good with both lenses. Some degree of circum-corneal injection was noted with the larger lathe cut design.

Ten familiar spin cast and ten lathe cut Bausch & Lomb SOFLENS contact lenses were measured as to their power on a lensometer and on an eye. Both quality of the optics and quantitative measurements were considered. Lens flexure and the presence of a fluid lens between the posterior surface of the contact lens and the anterior cornea is indicated for both lenses to explain differences between power of the lens in air and on the eye. The spin cast lens design appears to create a quantitatively larger fluid lens, and one which will add positive optical power to the lens/eye system. Either from this and/or additional factors, the lathe cut lens appears to give improved optical performance both in air and on the eye.

Three hydrogel contact lenses of measured oxygen transmissibility (Dk/L = 20, 12, and 6 x 10(-9) cm ml O2/s ml mm Hg, respectively) were used to induce central corneal swelling as measured by optical pachometry in five human subjects under open-eye conditions (8 h wear). Both contact lenses with higher Dk/L values induced mean central corneal swelling of about 1.5%. The lower Dk/L lens induced a mean central corneal swelling of 2.2%. However, statistical analysis of the differences in central corneal swelling between the lens-wearing and the control eyes indicates that swelling induced by the higher Dk/L lenses is not identical, and indicates that even a Dk/L of 20 x 10(-9) is insufficient to reduce corneal swelling to zero. This suggests that the "critical oxygen tension" (COT) could be in excess of 20 to 40 mm Hg, but not necessarily as high as 70 mm Hg.

Predictions of effective powers of flexible contact lenses including tear lenses, or assuming alignment, are compared with actual equivalent sphere effective powers of 12 low-minus and 2 high-plus lenses. The assumption of a tear lens improves accuracy of prediction.

Mathematical formulas are used to infer volumetric tear exchange from power changes of flexible contact lenses in situ. These calculations are developed from changes along the radius, and must be considered approximate due to several underlying assumptions.

We treated nine myopic and nine aphakic eyes in patients who developed corneal ulcers while wearing extended-wear soft contact lenses. Bacteria were recovered from the corneal ulcers of 13 of 18 patients with Pseudomonas aeruginosa being the pathogen most frequently isolated. Chemical disinfection was used by seven of the nine patients with myopia and eight of the nine with aphakia. Two of nine patients in the myopic group had visual loss to 20/200 and hand movements; five of nine patients in the aphakic group had visual loss to 20/50 or worse. Invasion of the corneal stroma by bacteria may occur after breakdown of the epithelial barrier by contact lens manipulation or after chronic overnight anoxic stress. Thus, the use of soft contact lenses on an extended-wear basis may be complicated by the development of corneal ulcers. Both aphakic and myopic eyes are at risk.

We use a new model for describing the flexure of hydrogel lenses in situ to evaluate toric hydrogel lenses. The mathematical development of the model precludes use of prism ballasted lenses (because of differential thickness in the vertical meridian and possible lenticulation in the midperiphery), and so double-thin zone lenses were used here. Our results suggest that such lenses (low-minus power) align almost perfectly with the anterior corneal surface creating minimal tear layers.

We have developed a computer program which allows analysis of soft contact lens flexure without assuming a priori that the posterior surface of the lens aligns with the central anterior cornea. Such analysis leads to the conclusion that thin low-minus lenses entrap tear layers of low volume (about 5.5 microliter) and minimum power, whereas low-plus lens systems can develop tear layers of greater volume (about 9.5 microliter), and significant power (about -2.00 D). The later finding supports clinical impressions that plus lenses "lose" power when placed on the eye. In some cases our data suggest that the lens posterior surface closely aligns with the anterior cornea, whereas in other cases such alignment is unlikely. It also appears that mathematical models for the simultaneous flexure of front and back radii of soft lenses may be described for specific lens designs. This current study suggests that delta r2 congruent to 2 delta r1 describes the relation for low-plus lenses, and delta r2 congruent to 0.75 delta r1 is the relation for low-minus lenses.

The quantification of tear volume trapped under a flexible contact lens in situ is important for an understanding of both optical and physiological lens behavior. Mathematical treatment is used to calculate the flexed back radii of flexible contact lenses in situ from the various hypotheses available. For each hypothetical radius the tear volume under the lens is calculated. The method is limited in accuracy due to several underlying assumptions. Twelve lens situations are analyzed, and the results indicate a tear volume of the order of 10 microliters.

Central corneal thickness of four human subjects was monitored by optical pachometry during wear of selected hydrophilic (polymacon) contact lenses of known parameters. Simple algebraic techniques were used to calculate anterior corneal surface oxygen flux using central corneal thickness change as an indication of tear layer oxygen tension.