The "L" in Dk/L.

We measured the Dk (oxygen permeability) of one specific hydrogel contact lens material (phemfilcon A, nominally 38% water content and ionic) by use of the conventional single chamber polarographic technique at 35 degrees C. When corrected for the boundary and edge effect, Dk was found to be 13 x 10(-11) cm2 ml O2/s ml mm Hg. (These units will be used for this term throughout the remainder of this report.) We also used the polarographic cathode as a model cornea to measure the Dk/L (oxygen transmissibility) of 7 powered contact lenses (-5 to +15 D) made from the same material by the same manufacturer. We found values ranging from 5 to 15 x 10(-9) cm ml O2/s ml mm Hg. (These units will also be used for this term throughout this report.) From analysis of these data, we found that any reasonable model (of the several previously proposed) for L could be used to predict the polarographically measured oxygen transmissibility of the low minus (-5 to -1 D) powered contact lenses, but that plus powered lenses, at least on this measurement apparatus, give Dk/L values that are closer to predictions based on the observed Dk and central thickness rather than any version of average thickness.     

The critical Dk/L to avoid oedema for daily wear RGP contact lenses

Background : A study was conducted to investigate the critical Dk/L required to avoid open-eye oedema with rigid gas-permeable (RGP) contact lenses. Methods : RGP contact lenses in matched designs were fabricated from three materials of nominal Dk of 12, 30 and 45 times 10^?11 (cm².mlO₂)/(s.ml.mmHg), with a centre thickness of 0.15 mm. A randomised, double-masked study design was used. Over two lens-wearing sessions one week apart, nine young adult subjects wore one lens made of each of the three materials for four hours. In one session the non-lens-wearing eye served as a control. Corneal thickness was measured using an ultrasonic pachometer before and after lens wear and the extent of corneal oedema calculated. Results : Lenses with a Dk/L of 8 times 10^?9 (cm.mlO₂)/(s.ml.mmHg) induced statistically significant levels of corneal oedema (1.8 ± 1.5 per cent) after four hours of open-eye wear. No corneal oedema was detected with lenses above this critical Dk/L value. No evidence could be found to support the concept of ‘osmotic’ corneal oedema induced by reflex lacrimation in unadapted subjects. Conclusion : RGP lenses with a Dk/L of 20 times 10^?9 (cm.mlO₂)/(s.ml.mmHg) or greater worn for daily wear are unlikely to induce corneal oedema in the majority of patients.     

Effects of increasing Dk with rigid contact lens extended wear on rabbit corneal epithelium using confocal microscopy.

The effects of 24-h wear of various Dk-rigid gas-permeable (RGP) contact lenses on the rabbit corneal epithelium were studied by in vivo tandem scanning confocal microscopy (TSCM), and confirmed by scanning electron microscopy (SEM). Lenses used were polymethylmethacrylate (PMMA) (Dk/L = 0), RGP experimental A lens (siloxanylmethacrylate-fluoromethacrylate-methylmethacrylate , 33), experimental B (siloxanylmethacrylate-fluoromethacrylate, 56), and experimental C (siloxanylstyrene-fluoromethacrylate copolymer, 64 x 10(-9)) (cm/s) (ml O2/ml mm Hg) with 0.15-mm thickness (Dk/L measured by polarograph including boundary layer effect). After 24-h PMMA lens wear, TSCM showed no superficial epithelial cells but only exposed, underlying wing cells. The cornea with experimental A showed partial superficial epithelial desquamation. With experimental B wear, slight superficial epithelial cell swelling and desquamation were observed on the surface of the cornea. No changes were observed for the eye with experimental C and control. The observed severity of desquamation of superficial epithelial cells was dependent on the oxygen transmissibility (Dk/L) of RGP lenses worn. All in vivo findings were confirmed by SEM observations. Based on the results of this study, we conclude that (a) although Dk/L = 56 lens B shows no residual overnight corneal swelling, surface damage is still produced; (b) Dk/L = 64 lens C is best for epithelium showing the same corneal images as control; and (c) TSCM is a good way to evaluate the contact lens safety and efficacy in vivo at the cellular level noninvasively.     

Oxygen permeability (Dk) of thirty-seven rigid contact lens materials.

PURPOSE: Oxygen permeability (Dk) was determined for 37 available rigid contact lens materials in a masked fashion. The results were compared with those of an earlier study that included different lots of 14 test materials assessed in the current study. METHODS: Six lenses of different thicknesses in each test and reference material were obtained. Test materials were arranged in sets of six to eight materials per set. Each set of materials, with inclusion of at least two reference materials for the purpose of simultaneous calibration, was measured to obtain preliminary amperages. Four preliminary measures were performed per thickness, resulting in 24 per material, in a schedule designed to spread the potential effects of machine drift and other factors. The mean preliminary amperages were used to derive corrected Dk values according to American National Standards Institute (ANSI) Z80.20-1998, and the values were linearly calibrated using the measured and established Dk values of the reference materials. RESULTS: The resistance (t/Dk) vs. thickness (t) plots for the 37 test and seven reference materials were approximated linearly. In 54 of 57 linear regressions, the coefficients of determination (R2) were >0.96, and in 48 instances were >0.98. Fourteen Dk values from the current study and an earlier study were linearly correlated (R2 = 0.9846), with a slope close to unity (+1.056) and intercept close to zero (-0.292). Ten of the current values fell within 10% of their corresponding earlier values. Only three current Dk values fell outside of the ANSI Z80.20-1998 tolerance for Dk (+/-20%). Two of these Dk values met the product tolerance when an obvious outlying point was graphically identified and omitted from the linear resistance (t/Dk) vs. thickness (t) regression. CONCLUSION: Omission of a single outlying point from a linear resistance vs. thickness regression can help provide a more valid Dk value. The ANSI Z80.20-1998 tolerance of +/-20% on Dk and the measurement reproducibility of +/-10% were achieved for the overwhelming majority of rigid contact lens materials up to at least 160 Dk units. The corrected, calibrated Dk values for the 37 test materials, in ANSI units, ranged from 13.8 to 175.1, having an overall mean of 43.2, median of 31.9, and standard error of 6.57 (N = 37).     

Comparison of two care systems when used with low Dk and high Dk silicone/acrylate materials

The Boston care system (a chlorhexdine preserved conditioning solution and cleaning solution) was compared to the Sherman care system (a benzyl alcohol preserved wetting/soaking solution and a cleaning/disinfecting solution) on two different silicone/acrylate lens materials. Two groups of 12 patients each were used, one group wearing a low Dk (12) material and the other a higher Dk (55) material. The patients used one care system on one lens and the other care system on the opposite lens. The study was double masked: The solutions were coded so the patients did not know the brand of the solutions, and the examiner did not know which care system was being used on which lens. Tear film break-up time on the front of the lens was measured, surface debris and peripheral corneal staining was graded at each visit over the 24 weeks. The patients completed a questionnaire at each visit. There was no significant difference between the care systems with the low Dk material, however, the Sherman system tended to perform better on the higher Dk material. Break-up time on the higher Dk material was shorter, and debris was slightly greater on the lower Dk material.     

Oxygen permeability of disposable soft contact lenses

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.     

Open-eye corneal swelling secondary to hydrogel contact lens wear

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.     

Oxygen transmissibility of disposable hydrogel contact lenses

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.     

Polarographic oxygen permeability measurement of silicone elastomer contact lens material.

The flexible silicone elastomer is a contact lens material of substantial potential, primarily because of its high theoretical oxygen permeability (Dk). Review of the previous literature indicates some difficulty in precisely determining the Dk of the silicone elastomer, and quantification of this permeability value ranges from 50 to 340 x 10(-11) cm2 ml 02/sec ml mmHg by various techniques. (The exponential term 10(-11) and the units of Dk will be omitted hereafter in this text). We herein report use of the single-chamber polarographic technique, with edge and boundary corrections, to arrive at a value of 190 with a standard deviation (SD) of 79 for the Dk of one silicone elastomer contact lens material. The excessive SD suggests that an improved method to evaluate contact lens materials with Dk values in excess of 30-50 should be determined.     

Stacking samples while measuring oxygen transmissibility of hydrogel contact lenses

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.     

Microcyst response to high Dk/t silicone hydrogel contact lenses.

PURPOSE: To investigate the microcyst response to extended wear (EW) with high oxygen transmissible (Dk/t) silicone hydrogel lenses. METHODS: Microcysts were monitored for 12 months in subjects wearing low Dk/t hydrogel lenses on a 6-night EW schedule or high Dk/t hydrogel lenses on a 30-night EW schedule. Subjects wearing low Dk/t lenses transferred to the high Dk/t EW lenses and schedule after 12 months and were monitored for a further 6 months. RESULTS: The mean number of microcysts did not deviate from baseline in the high Dk/t group. Microcysts in the low Dk/t group increased over 12 months, and more microcysts were observed in low Dk/t lens wearers compared with high Dk/t lens wearers after 3 months. Microcysts increased in 50% of subjects 1 week after transfer to high Dk/t lenses and returned to baseline levels seen with high Dk/t lens wear within 3 months. CONCLUSIONS: EW with high Dk/t silicone hydrogel lenses did not cause an increase in microcyst numbers. It is not necessary to discontinue lens wear with patients who transfer from low to high Dk/t lenses because the increase in microcysts is transitory. This result has implications for practitioners when fitting and assessing the success of high Dk/t hydrogel lenses.     

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.     

The ocular response to extended wear of a high Dk silicone hydrogel contact lens

Purpose: A four‐month extended wear clinical trial was conducted to compare die ocular effects of a high Dk Balafilcon A silicone hydrogel lens and a low Dk HEMA 38.6 per cent H²O soft lens. Method: Twenty‐four subjects who were adapted to daily wear of soft lenses wore a high Dk lens in one eye and a low Dk HEMA lens in the other eye for four months on an extended wear basis after one week of daily wear. Thirteen progress evaluations were conducted using standard clinical procedures. Results: Eighteen subjects (75 per cent) completed the study. The high Dk lens induced significantly less bulbar and limbal injection and corneal vascularisation dian the low Dk HEMA lens (p < 0.05). Epithelial microcysts were observed only in the eyes wearing the low Dk lens. A significant increase in myopia was found in die eyes wearing die low Dk HEMA lens (mean = 0.50 D, p < 0.01) compared to die insignificant myopic increase of 0.06 D in the eyes wearing the high Dk lens. Three subjects developed small infiltrates in the high Dk lens wearing eyes and significantly more post‐lens debris was observed under the high Dk lens. Six subjects developed papillary conjunctivitis in die eye wearing silicone hydrogel lenses but only two of those were discontinued from the study. Conclusion: No hypoxia‐related effects were observed with extended wear of the high Dk Balafilcon A silicone hydrogel lens.     

Critical corneal oxygen values: a summary

Data from several previous studies were collected and analyzed by remeasuring test hydrogel contact lens oxygen transmissibilities (Dk/L) at ocular temperature to arrive at summary critical human pre-corneal oxygen tension (30 mmHg), and required Dk/L (18 X 10(-9) cm ml O2/sec ml mmHg) under open-eye conditions. Estimations for closed-eye (extended wear) conditions were also performed.     

Corneal response to different oxygen levels during extended wear

In this study we explored the relationship between hypoxic exposure level and corneal response by assuming that the partial pressure of oxygen (PO2) under a contact lens during eye closure is directly related to oxygen transmissibility (Dk/L). To study this relationship, we monitored a group of subjects who wore RGP lenses of various Dk/L values on an extended wear basis. The results revealed that as Dk/L increases, there is a substantial decrease in overnight corneal edema and epithelial microcysts. However, other responses seemingly related to purely mechanical properties (e.g., lens adherence, corneal topographical changes, and 3-9 limbal superficial punctate keratitis) appear to be independent of Dk/L. Results also suggest that sufficient PO2 levels under a contact lens can minimize endothelial morphological changes associated with hypoxia. We conclude that metabolically driven complications accompanying RGP extended wear can be substantially eliminated with lenses having Dk/L values of 80 x 10(-9) (cm x mL O2)/(sec x mL x mm Hg) or greater.     

Tear lactate dehydrogenase levels. A new method to assess effects of contact lens wear in man.

Lactate dehydrogenase (LDH) activity in tears was measured in 202 myopic rigid gas permeable (RGP) and 79 hydrogel human contact lens wearers and 48 normal controls by noninvasive microcapillary sampling. The oxygen permeabilities (Dk) of five selected RGP contact lenses ranged between 0 and 230 x 10(-11) (cm2/s) ml O2/ml mm Hg), and the water content (WC) of the hydrogel lenses was 38, 72, and 80%. When normal diurnal variation of tear LDH activity and tear sample volume (0.3-0.6 microliters) were carefully controlled, the tear LDH activity of RGP lens wearers in daily and extended wear correlated as an inverse function of Dk/L, with the highest enzyme activity observed in wearers of polymethylmethacrylate daily wear lenses (347.4 U/L). The tear LDH activity in Menicon EX (Dk 108) lens wearers was higher in the extended wear (192.7 U/L) than in the daily wear group (161.9 U/L). There was no significant difference in tear LDH activity between the Menicon SF-P (Dk 230) extended wear group (132.0 U/L) and controls (133.5 U/L). In the hydrogel lens daily wear group, tear LDH activity of Experimental 72 (WC 72%) and Experimental 80 (WC 80%) was higher than that of hydroxyethylmethacrylate (HEMA) despite having high Dk value. Experimental 80 extended wearers showed lower LDH activity in tears sampled between 0.3 and 0.6 microliters than that of HEMA wearers. These results suggest that sequential measurement of LDH levels in tears may offer a new and unique method for the assessment of the physiologic effects of contact lens wear on the ocular surface, and provide a new clinical paradigm for the interaction of the contact lens with the cornea.     

Re-evaluation of the oxygen diffusion model for predicting minimum contact lens Dk/t values needed to avoid corneal anoxia.

PURPOSE: (1) To update Fatt's mathematical model of the distribution of oxygen tension (pO2) across the cornea and contact lens (CL) to include the recent finding that corneal oxygen consumption increases with the acidification that occurs with CL wear. (2) To estimate the minimum transmissibility (CL Dk/t) to avoid epithelial anoxia or to avoid stromal anoxia. METHODS: A five-layer static and one-dimensional mathematical model of oxygen diffusion through the cornea based on Fatt's models was used. The relationships between acidosis and increased QO2, and acidosis and CL Dk/t were used to estimate corneal QO2 for a given CL Dk/t. RESULTS: (1) Revised model predictions are in agreement with direct tear pO2 measurements beneath CLs in the rabbit. (2) For the human eye, the minimum CL Dk/t for oxygen delivery to the basal epithelial cells was determined to be 23 for the open eye and 89 for the closed eye. To prevent anoxia throughout the entire corneal thickness the Dk/t requirements are 35 for the open eye and 125 for the closed eye. CONCLUSIONS: (1) Model predictions of the oxygen distribution beneath contact lenses are significantly lower than previous models that did not include the effect of acidosis on corneal QO2. (2) Minimum Dk/t values that allow oxygen delivery to the basal epithelium are in agreement with the Dk/t needed to avoid corneal edema.     

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.     

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.     

Effect of rigid contact lens oxygen transmissibility on stromal pH in the living human eye

Corneal stromal pH was measured fluorometrically in nine human subjects after 1.5 hours of eye closure while wearing hard gas permeable contact lenses. Six lens types providing a wide range in oxygen transmissibilities (Dk/L)O2 from 0.15 to 55.0 X 10(-9) (cm ml O2/seconds of ml mmHg) were used. Stromal pH upon opening the eyes was directly related to (Dk/L)O2 and ranged from 7.01 to 7.26. These pH values were all significantly lower than the control (no lens), which had a pH of 7.38 (P less than 0.005). After eye opening (lens still on) stromal pH increased and reached a new steady-state (range, 7.11-7.45) in approximately 30 minutes; however, these pH levels were still below the control (no lens) (pH 7.54; P less than 0.005). The rate of pH change after eye opening also increased directly with (Dk/L)O2. The authors conclude that the lenses presently available provide insufficient gas exchange to prevent alteration of corneal pH and suggest that the measurement of stromal pH may provide a sensitive index for evaluating the metabolic effects of contract lens wear.