A comparison of two methods of evaluating cornea-to-contact lens base curve fluorescein patterns in keratoconus.

PURPOSE: The purposes of the study were as follows: (1) to compare the apical fitting relationship of habitual contact lens fluorescein patterns in keratoconus as determined by clinician assessment of on-eye patterns to those determined by photograph readers looking at slides of fluorescein patterns and (2) to determine the validity of the techniques used in assessing the apical fitting relationships of rigid corneal contact lenses on keratoconic corneas. METHODS: Central fluorescein patterns of rigid contact lens-wearing keratoconus patients enrolled in the Collaborative Longitudinal Evaluation of Keratoconus (CLEK) Study were graded as "definite touch," "touch," "clearance," or "definite clearance" by certified clinicians. Photographs of these patterns were evaluated independently by certified, masked photograph readers using the same grading scale. RESULTS: Agreement between "re-reads" of the same fluorescein pattern slides by the photograph readers was substantial (weighted kappa = 0.751). Agreement between assessments of habitual fit fluorescein patterns at the baseline vs. the repeat visits was poor for the photograph readers (weighted kappa = 0.254) and moderate for the clinicians (kappa = 0.480). Agreement between clinicians' and photograph readers' assessment of the habitual contact lens fluorescein pattern at the baseline visit was fair (weighted kappa = 0.382). CONCLUSIONS: Repeatability and validity of this technique were fair to excellent. Many factors influence fluorescein pattern interpretation, and improvement of the objective method of fluorescein pattern assessment by photograph readers will require improved methodology that takes these factors into consideration.

The thickness of the human precorneal tear film: evidence from reflection spectra.

PURPOSE: Interferometric methods have considerable potential for studying the thickness of layers of the human tear film and cornea because of their ability to make noninvasive, accurate, and rapid measurements. However, previous interferometric studies by Prydal and Danjo yielded tear thickness values near 40 and 11 microm, respectively, considerably greater than estimates made by invasive methods of 4 to 8 microm. Using a modified version of Danjo's method, interference effects from the tear film and cornea were studied, with the aim of correlation with known structure and optical properties of the cornea and hence determining the most probable value of tear film thickness. METHODS: Reflectance spectra from the human cornea were measured at normal incidence. These spectra show oscillations whose maxima correspond to constructive interference between light reflected from the air surface and from some deeper surface. The frequency of these spectral oscillations is proportional to the thickness of the layer between the air surface and the second surface. Therefore, Fourier analysis of reflectance spectra can be used to determine the thickness of layers of the tear film and cornea. In the main experiment, 36 low-resolution spectra were obtained from six normal eyes for measuring thickness up to 100 microm. Control experiments included measurements of the time course of thickness changes and high-resolution spectra for measuring thickness up to 1000 microm. RESULTS: For the main experiment, in the thickness range 1 to 100 microm, the strongest peak in the Fourier transform was near 3 microm (range, 1.5-4.7 microm) beneath the air surface. In the range 20 to 100 microm, the strongest peak was near 55 microm (range, 50-59 microm) for all 36 spectra; none were in Prydal's range near 40 microm. This 55-microm peak is consistent with a reflection from the basement membrane of the epithelium. Time course measurements after a blink show that the 3-microm peak is not an artifact. High-resolution spectra gave a peak near 510 microm, corresponding to the complete thickness of the cornea (plus tear film). This peak had a contrast similar to that of the 3-microm peak. CONCLUSIONS: These studies did not confirm Prydal's estimate of approximately 40 microm. Nor were there prominent peaks near Danjo's value of approximately 11 microm, except in cases of probable reflex tears. Because the reflection at the aqueous-mucus boundary would be expected to be weaker than that from the epithelial surface, the 3-microm peak is unlikely to correspond to the aqueous layer (rather than the complete tear film). The proposal that the 3-microm peak corresponds to a reflection from the front of the cornea is supported by the demonstration of a peak of similar contrast from the back of the cornea. Thus, the current evidence consistently supports a value of approximately 3 microm for the thickness of the human precorneal tear film.

Oxygen: how well is the closed eye being served?

BACKGROUND: While the environment of the naturally closed eye substantially challenges the oxygen-dependent processes of the cornea, the presence of a contact lens further exacerbates those stresses. The purposes of this study are: (1) to describe responses of the human cornea under closed eye conditions to a wide range of Dk/t environments as ratios of their corresponding normal, open-eye (physiological) baseline rates; (2) to describe the difference ("hypoxic gap") function that separates those closed-eye responses from parallel responses for the open eye; and (3) to present predictive models for all three (closed-eye, open-eye, and difference) response functions in both graphical and mathematical forms. METHODS: Oxygen uptake rates were measured polarographically: for the normal open eye and after both closed- and open-eye wear of six rigid contact lenses of transmissibilities ranging from 0 to 91 x 10(-9) (cm/sec)(ml O2/ml mmHg). RESULTS: Responses from the closed-eye and open-eye series to those six Dk/t levels were compared both graphically and statistically. Those response series were described by best-fit equations, from which a model for Dk/t effectiveness in meeting corneal oxygen needs under both closed- and open-eye conditions was developed. CONCLUSIONS: Response series for both closed and open eyes were found to be well described by natural log equations (R2 values > 0.99). Statistically, separation of those two series could be demonstrated (p < 0.05) by the Dk/t level of 12.8 x 10(-9) (cm/sec)(ml O2/ml mmHg). While, by the highest Dk/t level observed here of 91 x 10(-9) (cm/sec)(ml O2/ml mmHg), corneal responses for the open eye have already moderated down to just 1.5x the "ideal standard," closed-eye response rates are still averaging 2.7x that baseline. The difference ("hypoxic gap") function remaining between those closed- and open-eye curves appears to be stabilizing in its magnitude by 91 x 10(-9) (cm/sec)(ml O2/ml mmHg) as well.

Factors associated with corneal scarring in the Collaborative Longitudinal Evaluation of Keratoconus (CLEK) Study.

PURPOSE: The multicenter Collaborative Longitudinal Evaluation of Keratoconus (CLEK) Study is a prospective, observational study of 1,209 keratoconus patients. We report on the factors associated with corneal scarring at baseline. METHODS: We defined corneal scarring as scars that had been detected both by the clinician examining the patient with the slit-lamp biomicroscope and by masked readers of corneal photographs at the CLEK Photography Reading Center. We investigated associations between corneal scarring and patient variables including gender, ethnicity, a family history of keratoconus, a history of ocular trauma, eye rubbing, contact lens wear, rigid contact lens fitting relationships, and corneal findings (such as curvature, Vogt's striae, Fleischer's ring, and central/apical staining). Multiple logistic regression analysis using generalized estimating equations to adjust for the correlation between eyes was used for analysis. RESULTS: The following factors were found to increase the odds of corneal scarring at baseline in the CLEK Study: corneal staining (odds ratios (OR) = 3.40, 95% confidence interval 2.53-4.59), contact lens wear (OR = 3.51, 95% confidence interval 2.27-5.45), Fleischer's ring (OR = 1.63, 95% confidence interval 1.11-2.40), steeper first definite apical clearance lens base curve radius (per diopter, OR = 1.29, 95% confidence interval 1.25-1.33), and age (per decade, OR = 1.54, 95% confidence interval 1.35-1.75). CONCLUSIONS: These baseline data suggest that corneal scarring in keratoconus is associated with corneal staining, contact lens wear, Fleischer's ring, a steeper cornea, and increasing age. The factors that imply added risk for corneal scarring that may be affected by practitioner intervention are staining of the cornea, contact lens wear, and the contact lens fitting relationship.

Poland's syndrome in one identical twin.

Female twins were evaluated at the Shriners Hospital in Lexington, Kentucky. One twin was normal, and the other twin had the classic findings of Poland's syndrome, manifested by absence of the pectoralis major and symbrachydactyly of the right upper extremity. Buccal smears from each child were submitted for DNA testing. The test confirmed monozygosity with 99.9% probability. Some previous reports have stated that Poland's syndrome is an autosomal dominant, genetically determined trait, whereas others have maintained that there is no genetic association. The original case described by Poland in 1841 was his cadaver, and no family history was reported. This twin study provides strong evidence that the condition is not determined by gene transmission.

What is the oxygen environment under an encapsulated segment bifocal RGP contact lens?

BACKGROUND: A bifocal rigid gas permeable (RGP) contact lens is now available that totally encapsulates an add segment of a different material within its inferior aspect. The purposes of this study were to determine the effects of the major lens and of its encapsulated segment region on the oxygen uptake rates of underlying cornea. METHODS: Corneal oxygen uptake rates of 10 human corneas were measured--both centrally and inferiorly--for non-wearing conditions and then immediately after 300-second wearing periods of a polymethylmethacrylate (PMMA) lens and of this RGP-encapsulated segment bifocal contact lens. Corneal responses associated with these three environmental conditions were compared statistically (i.e., without the influence of blink-driven tear flow). RESULTS: Under the nonblinking conditions maintained (i.e., with oxygen availability restricted to lens transmissibility alone), central corneal hypoxia was found for the RGP bifocal lens to be only 47% of that induced by the PMMA (Dk/L = 0) condition, while the cornea under the encapsulated segment of the bifocal lens averaged 78% of the hypoxic response induced by that same condition. Central site responses were found to be significantly different for the bifocal and the PMMA lens (p < 0.001), as they were for the two inferior sites with these two lenses (p < 0.001). All lens-related responses were found to be significantly different from the no lens condition (p < 0.001). CONCLUSIONS: Using Smith's model (JAOA, 1997) to estimate lens transmissibility, the major lens of this RGP bifocal design appears to meet approximately 80% of the Holden-Mertz daily-wear oxygen requirement--even without post-lens tear exchange, but only about 25% of that requirement appears to be met beneath the encapsulated segment region. Efficient supplementary infusion of oxygen under this lens by blink-driven tear exchange, then, is necessary to maintain optimal corneal health.

Three interferometric methods for measuring the thickness of layers of the tear film.

The thickness of different layers of the tear film has been measured by three types of interference method, namely, wavelength-dependent fringes (WDFs), thickness-dependent fringes (TDFs), and angle-dependent fringes (ADFs). This review begins with a discussion of characteristics which are common to all these methods--high-, intermediate-, and low-index layers, phase, optical path difference, and contrast. For each of the three methods, we present a figure showing constructive and destructive interference, derive equations for calculating tear layer thickness, describe a typical optical system, and show representative results. The particular advantages and limitations of each method are discussed. Given the clinical importance of the tear film in dry eye syndrome and contact lens wear, it is unfortunate that there are considerable discrepancies among the results of interferometric and other methods for measuring tear film thickness; further development of these noninvasive, interferometric methods should help to provide a clearer picture of the thickness of different layers of the tear film, in normal and dry eyes, and in contact lens wear.

Corneal scarring in the Collaborative Longitudinal Evaluation of Keratoconus (CLEK) Study: baseline prevalence and repeatability of detection.

PURPOSE: The multicenter Collaborative Longitudinal Evaluation of Keratoconus (CLEK) Study is a prospective, observational study of 1,209 keratoconus patients. We report on the prevalence of corneal scarring in these patients. We also report on the test-retest repeatability of corneal scar documentation at the slit-lamp biomicroscope by trained clinicians and by masked photograph readers and on the scarring-status agreement at baseline between clinicians and photograph readers. METHODS: Clinicians and masked photograph readers graded each cornea as to scarring status. Patients were examined by clinicians, and their corneas were photographed at baseline (2,297 nongrafted eyes of 1,209 patients) and at a repeated visit (258 nongrafted eyes of 138 patients). These photographs were evaluated by the masked readers at the CLEK Photography Reading Center (CPRC). Clinicians reported corneal scars in right eyes at baseline as "definitely not present" in 53.9%, "probably not present" in 8.4%, "probably present" in 8.2%, and "definitely present" in 29.4% of patients. A weighted kappa statistic of 0.83 (95% confidence interval from 0.78 to 0.88) indicates that agreement is excellent between baseline and repeated assessments for the presence of a corneal scar by clinicians. RESULTS: Agreement is very good between baseline and repeated photograph-reader assessments for the presence of a scar, with a weighted kappa of 0.77 (95% confidence interval, 0.72-0.82). The kappa statistic comparing photograph-reader scarring assessments with clinician results was 0.69 (95% confidence interval, 0.66-0.71). CONCLUSION: The data also suggest better agreement between clinicians and readers when Vogt's striae and corneal nerves were observed. The data also suggest better agreement when corneal staining was not observed by the photograph readers. The CLEK Study protocol for determining the presence of scars is highly repeatable.

Effects of rigid contact lens prism ballasting on oxygen access to the cornea under static and dynamic conditions.

The effects of rigid contact lens material [polymethyl methyacrylate (PMMA) and itabisfluorofocon A] and prism ballasting (0, 1.5, 2.25, and 3 delta) on tear pump efficiency were studied by measuring corneal oxygen uptake rates on the right eyes of six human subjects under three conditions: (1) normal open eye; (2) after 5 min of static (without blinking) wear of the contact lens; and (3) after 5 min of dynamic (with blinking once every 5 s) wear of the same lens. As expected, corneal oxygen uptake rates (i.e., demand for oxygen) measured with the itabisfluorofocon A material were significantly lower (p < 0.0001) than those measured with PMMA under both static and dynamic conditions. Under static conditions, no significant differences were found across materials among the corneal oxygen uptake rates associated with the four amounts of prism ballasting (p = 0.0514). However, under dynamic conditions, significant differences (p < 0.0001) were found across materials among corneal oxygen uptake rates associated with the four amounts of prism ballasting, with the lower amounts of prism ballasting being associated with lower corneal oxygen uptake rates. The changes in the measured corneal oxygen uptake rates from static to dynamic conditions, relative to those measured for the normal open eye, served as an index of tear pump efficiency. There were no significant differences in these changes for the prism amounts studied; however, static condition data were significantly higher than dynamic condition data for the 0 delta and 1.5 delta lenses only, whereas greater amounts of prism ballasting resulted in no reduction in oxygen uptake under dynamic conditions. In addition, significantly greater differences between static and dynamic condition data were found for the PMMA material than for itabisfluorofocon A.

Corneal responses to lens transmissibility.

BACKGROUND: Lens transmissibility (Dk/L) and corneal relief from hypoxia are not linearly related. We describe graphically and by equations the relationship between corneal relief (scaled in oxygen shortfall units) and Dk/L across the transmissibility range of 1.2 to 189 x 10(-9) (cm/sec)(ml O2/ml mmHg). METHODS: Corneal responses to 28 Dk/L levels across the transmissibility range were expressed in oxygen shortfall units (osús), a relative scale of 100 steps bounded by each cornea's normal oxygen uptake rate with no contact lens in place (set = to 0 osús), and its maximum uptake rate for the open eye due to static PMMA lens wear (set = to 100), thus allowing the degree of hypoxic stress exhibited by all corneas to be compared on the same relative scale. RESULTS: Responses of 11 human corneas to each of those 28 Dk/L steps were recorded across that entire transmissibility range, and their collective relationship described graphically. CONCLUSION: The graphic composite of all corneal responses to transmissibility steps across the Dk/L range of 1.2 to 189 X 10(-9) (cm/sec)(ml O2/ml mmHg) was found to be monotonic and continuous. While no single equation was found that optimally described (predicted) corneal responses across that entire spectrum, equations of the quadratic form fit limited domains of Dk/L (r2 values exceeding 90%) adequately for most laboratory and clinical applications.

Contact lens transmissibility: effects on delivery of oxygen to the cornea.

The purpose of this study was to determine the effects of rigid contact lens materials of various permeabilities and identical design on the oxygen shortfall of the human cornea and to explore differences in oxygen delivery to the cornea associated with contact lens materials of different modulus of elasticity values. Measurement of corneal oxygen uptake was performed for six human subjects under three conditions: (1) normal open eye; (2) after the static wear of six materials; and (3) after the dynamic wear of the same six materials. The 6 materials had Dk values that ranged from 0.02 to 127 (cm2/s)(ml O2/ml x mm Hg) and modulus of elasticity values from 1300 to 2200 MPa. Major results include: (1) that the oxygen shortfall of the cornea decreased with increasing lens transmissibility under both static and dynamic conditions, with significant differences (p < 0.001) being demonstrated among materials; and (2) significantly larger differences (p < 0.001) between the static and dynamic condition data were seen for materials of low Dk but were not predictable by lens modulus (p = 0.43, r2 = 0.018). The corneal demand differences between high and low Dk materials are possibly related to changes in corneal oxygen dynamics rather than to differences in tear exchange.

Using corneal oxygen demand to optimize rigid contact lens design.

To evaluate the effects of base curve radius, overall diameter, and axial edge lift on rigid contact lens tear pump efficiency, corneal oxygen uptake rates were measured on six eyes under three conditions: normal open eye, after 5 min of static (without blinking) wear, and after 5 min of dynamic (with blinking once every 5 s) wear. The three parameters were varied one at a time from a standard lens design. Differences in corneal oxygen demands between the static and dynamic rates provided quantitative measures of the tear pump efficiency for each lens design. Tear exchange was found to be most sensitively related to changes in base curve radius, followed by overall diameter changes (66.9% as effective) and axial edge lift changes (64.6% as effective). Design equivalencies (to produce identical tear exchange to a 0.05-mm flattening of base curve toward alignment) were 0.07-mm steepening of the base curve toward alignment, 0.35-mm decrease in overall diameter, or 0.037-mm increase in axial edge lift.

Rigid contact lens design equivalencies of overall diameter and axial edge lift: individual variations.

We compared the effects on rigid contact lens tear pump efficiency of changes in lens overall and optic zone diameter and of changes in lens axial edge lift by measuring corneal oxygen uptake on the right eyes of six human subjects after 5 min of both static and dynamic contact lens wear. Two sets of five lenses were worn by each subject. In set 1, overall diameter varied from 8.2 to 9.4 mm in 0.3-mm steps, with the optic zone diameter being 1.4 mm smaller and a 0.09 mm axial edge lift. In set 2, axial edge lift varied from 0.05 to 0.13 mm in 0.02-mm steps; overall diameter was 8.8 mm, and optic zone diameter was 7.4 mm. For the pooled data, a 0.3-mm decrease in overall diameter was approximately equivalent to a 0.03-mm increase in axial edge lift as a means of enhancing tear pump efficiency. However, considerable intersubject differences were exhibited, with the following increases in axial edge lift being equivalent to the 0.3-mm reduction in overall diameter: 0.01, 0.01, 0.04, 0.06, 0.09, and > 0.20 mm. This illustrates that subject factors (i.e., palpebral aperture size, lid-to-cornea geometry, corneal toricity, peripheral corneal topography, lid tension) influence lens positioning, movement, and tear pump efficiency.

Influence of lens design on the tear pump efficiency associated with plus-power rigid contact lenses: a comparison of single cut, lenticular, and minus-carrier designs.

In order to determine the tear pump efficiencies associated with three plus-power rigid contact lens designs (single cut, lenticular, and minus-carrier), we measured corneal oxygen uptake rates on the right eyes of six human subjects under three conditions: (1) normal open eye, (2) after 5 min of static (without blinking) wear of the contact lens, and (3) after 5 min of dynamic (with blinking once every 5 s) wear of the same lens. The differences among the corneal oxygen uptake rates measured under static and dynamic conditions, relative to those measured for the normal open eye, served as an index of tear pump efficiency. Under both static and dynamic conditions, no significant differences were found among the corneal oxygen uptake rates associated with the three plus-power lens designs. In addition, no significant differences were found in the differences among the static and dynamic condition data, an indication of tear pump efficiency, associated with the three plus-power lens designs. This was true for both the population data and for the data from each subject. Nevertheless, there were significant differences among subjects in the static condition, dynamic condition, and difference data.

Influence of power changes in single cut rigid contact lenses on tear pump efficiency.

Differences between corneal oxygen uptake rates measured after 5 min of static (without blinking) and 5 min of dynamic (with blinking once every 5 s) non-gas permeable (polymethyl methacrylate) contact lens wear, referenced to the oxygen uptake rates of the normal open eye, were used as indications of tear pump efficiencies associated with each of seven contact lens back vertex powers (-9.00 to +9.00 D, in 3.00 D steps). Measurements were made in vivo on six human corneas showing with-the-rule (0.25 to 0.87 D) toricity. Lens overall diameter was 8.8 mm, optic zone diameter was 7.4 mm, and base curve radius was fitted "on K." Lens center thickness was 0.14 mm for all minus-powered lenses, and 0.19, 0.26, 0.34, and 0.41 mm for the plano, +3.00, +6.00, and +9.00 D lenses, respectively, maintaining an edge thickness of 0.11 mm. Under both static and dynamic wearing conditions, no significant differences were found among the corneal oxygen uptake rates associated with the various contact lens back vertex powers; however, the difference between the static and dynamic condition data, an indication of tear pump efficiency, was greatest for the -3.00 D lens. The difference values associated with the +9.00 and +6.00 D lenses were significantly lower than those associated with the minus power lenses.

Influence of rigid contact lens base curve radius on tear pump efficiency.

The effects of rigid contact lens base curve radius changes on tear pump efficiency were determined by measuring oxygen uptake on six with-the-rule corneas (with toricities between 0.37 and 1.00 D) under three conditions: (1) normal open eye, (2) after 5 min of static (without blinking) wear of polymethyl methacrylate (non-gas permeable) contact lenses, and (3) after 5 min of dynamic (with blinking once every 5 s) wear of the same lenses. The difference between the oxygen uptake rates measured under static and dynamic conditions provides an index of tear pump efficiency. Each subject wore a series of five lenses: fitted on K and 0.05 or 0.10 mm steeper and flatter than the flattest corneal meridian. The overall diameter of all lenses was 8.8 mm, the optic zone was 7.4 mm, the axial edge lift was 0.09 mm, and all other parameters were constant. The contact lens fitted on K was associated with the greatest central corneal oxygen debt under static conditions, whereas the same lens also provided for the best tear exchange with the blink. In addition, better tear exchange was found to be associated with larger palpebral aperture sizes.

Effects of rigid contact lens edge lift changes on tear pump efficiency.

The effects of rigid contact lens axial edge lift changes on corneal oxygenation and tear exchange with the blink were studied for lenses of five axial edge lifts (0.05 to 0.13 mm in 0.02-mm steps). The overall and optic zone diameters were maintained at 8.8 and 7.4 mm, respectively, and all other parameters were held constant. Oxygen uptake rates were measured for the normal open eye after 5 min of static (without blinking) and dynamic (with blinking once every 5 s) contact lens wear. As expected, no significant differences were found in the oxygen uptake rates associated with the static wear of the contact lenses; however, under dynamic conditions, increasing edge lift was associated with a reduction in oxygen demand. The differences between data obtained under static and dynamic conditions increased with increasing axial edge lift, and subjects with small palpebral aperture sizes seemed to benefit more from increasing axial edge lift.

Rigid lens tear pump efficiency: effects of overall diameter/base curve combinations.

The effect of changes in rigid contact lens overall and optic zone diameters on tear pump efficiency were studied using a polarographic technique. Six overall diameters ranged from 7.6 to 10.6 mm in 0.6-mm steps, with the optic zone diameter always being 1.4 mm smaller. Base curve radii were made flatter as lens overall/optic zone diameters increased in order to maintain a constant tear layer thickness. All other parameters were held constant. Oxygen uptake rates were measured for the normal, open eye, as well as after 5 min of static (without blinking) and dynamic (with blinking once every 5 s) wear of each lens design. Under both static and dynamic conditions, oxygen uptake rates increased with increase in lens overall/optic zone diameter. The differences between the static and dynamic condition data served as an index of tear pump efficiency for each lens. These differences decreased with increase in lens overall/optic zone diameter.

Rigid contact lens design: effects of overall diameter changes on tear pump efficiency.

The effects of rigid contact lens overall diameter changes on tear pump efficiency were studied for lenses of 7 overall diameters (8.2 to 10.0 mm in 0.3-mm steps) while maintaining all other parameters (including a 7.4-mm optic zone diameter) constant. Oxygen uptake rates were measured in the normal, open eye and after static (without blinking) and dynamic (with blinking once every 5 s) wear of oxygen-impermeable (polymethyl methacrylate) contact lenses. Differences between uptake rates obtained under static vs. dynamic conditions served as an index of tear pump efficiency for each lens. Tear pump efficiency increased with the use of progressively smaller lens diameters and subjects with large palpebral aperture sizes were also found to have better tear exchange (r = 0.934, p = 0.006 for the 9.1-mm overall diameter reference lens).

Responses to oxygen deprivation: variations among human corneas.

The cornea's interface with the atmosphere makes it uniquely accessible among avascular tissue for the study of local oxygen demand and proximal reservoir dynamics. Corneal oxygen-uptake rates obtained under normoxic vs hypoxic conditions are not predictive of each other. Proximal tissue reservoirs appear to play a role in moderating oxygen demands following periods of hypoxic stress: as the ratio of epithelia/stromal thickness rises, so does oxygen demand (r = +0.932, P = 0.01). The thickness of the stroma was most closely related to the oxygen uptake rates associated with the normal, open eye (r = +0.773, P = 0.07), whereas the thickness of the epithelium was more closely related to rates obtained after the cornea had been deprived of oxygen (r = +0.783, P = 0.07).