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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.

Purpose: Oxygen is essential for aerobic mammalian cell physiology. Oxygen tension (PO2) should reach a minimum at some position within the corneal stroma, and oxygen flux should be zero, by definition, at this point as well. We found the locations and magnitudes of this "corneal equilibrium flux" (xmin) and explored its physiological implications., Methods: We used an application of the Monod kinetic model to calculate xmin for normal human cornea as anterior surface PO2 changes from 155 to 20 mmHg., Results: We find that xmin deepens, broadens, and advances from 1.25 μm above the endothelial-aqueous humor surface toward the epithelium (reaching a position 320 μm above the endothelial-aqueous humor surface) as anterior corneal surface PO2 decreases from 155 to 20 mmHg., Conclusions: Our model supports an anterior corneal oxygen flux of 9 μL O2 · cm · h and an epithelial oxygen consumption of approximately 4 μL O2 · cm · h. Only at the highest anterior corneal PO2 does our model predict that oxygen diffuses all the way through the cornea to perhaps reach the anterior chamber. Of most interest, corneal oxygen consumption should be supported down to a corneal surface PO2 of 60 to 80 mmHg but declines below this range. We conclude that the critical oxygen tension for hypoxia induced corneal swelling is more likely this range rather than a fixed value.

Purpose: The main goal of this current work is to use an updated calculation paradigm, and updated boundary conditions, to provide theoretical guidelines to assist the clinician whose goal is to improve his or her scleral gas permeable (GP) contact lens wearing patients' anterior corneal oxygen supply., Methods: Our model uses a variable value of corneal oxygen consumption developed through Monod equations that disallows negative oxygen tensions within the stroma to predict oxygen tension at the anterior corneal surface of scleral GP contact lens wearing eyes, and to describe oxygen tension and flux profiles, for various boundary conditions, through the lens, tears, and cornea. We use several updated tissue and boundary parameters in our model. Tear exchange with GP scleral lenses is considered nonexistent in this model., Results: The majority of current scleral GP contact lenses should produce some levels of corneal hypoxia under open eye conditions. Only lenses producing the thinnest of tear vaults should result in anterior corneal surface oxygen tensions greater than a presumed critical oxygen tension of 100 mmHg. We also find that corneal oxygen tension and flux are each more sensitive to modification in tear vault than to changes in lens oxygen permeability, within the ranges of current clinical manipulation., Conclusions: Our study suggests that clinicians would be prudent to prescribe scleral GP lenses manufactured from higher oxygen permeability materials and especially to fit without excessive corneal clearance.

Objective: We calculated corneal surface oxygen tension under hybrid contact lenses (CLs) by extending existing models of CL oxygen diffusion to the circumstances of hybrid CL designs with tear vaults., Methods: Theoretical oxygen tensions at the corneal surface, if tear mixing and exchange are excluded, are calculated for hybrid CL (modern, high oxygen-permeable rigid center) designs with a single chamber corneal model using a computer software spreadsheet. Several specific in vivo instances of hybrid CL tear vaults are measured by both slitlamp biomicroscopy and optical coherence tomography (OCT)., Results: No significant difference was found between central thickness of hybrid CL rigid portions measured by Vigor gauge and OCT. The amount of central tear vault thickness was found to be significantly greater, however, when interpolated from biomicroscope images (500-1,554 μm, simulating a poorly fit lens) than from either OCT images (0-91 μm) or direct measurement through OCT software (0-96 μm). Using all measurements, excluding any potential contributions from tear exchange or mixing, we predict corneal surface tear pO2 values under several hybrid CLs to range from zero under a lens with an excessive vault to 112 mm Hg under a lens with a minimal vault., Conclusions: Tear vaults beneath the gas permeable portion of the hybrid CLs of 100 μm or less resulted in acceptable corneal surface oxygen values. Where tear vaults are greater than about 100 μm, predicted anterior corneal surface pO2 values are less likely to be adequate for optimal corneal physiology. Any significant tear exchange, however, should increase these values. Although measuring tear thickness, we noted that the photographic biomicroscope and the OCT methods provided inconsistent results, and thus limited the reliability of our primary result.

Purpose: To describe an interesting case in which use of standard corneal topography in isolation from clinical observations could have misdiagnosed keratoconus as regular astigmatism., Methods: Single patient case report., Results: Our patient received the correct diagnosis and appropriate contact lens treatment that resulted in optimal vision and tolerance. CONCLUSIONS. Use of either or both careful clinical slit lamp biomicroscopy and history or more sophisticated corneal topography analysis can detect keratoconus that standard corneal topography used in isolation from other clinical investigations may misdiagnose.

Purpose: To identify baseline demographic and clinical factors associated with undergoing penetrating keratoplasty (PK) in a prospective cohort of 1,065 keratoconus patients followed for eight years in the Collaborative Longitudinal Evaluation of Keratoconus (CLEK) Study., Design: Multicenter, prospective, observational cohort study., Methods: We report the rate of PK over eight years and baseline factors predictive of PK in 1,065 patients who, at the time of study enrollment, had not undergone PK in either eye., Results: Eighty-two percent of patients completed the eight-year close-out visit. Twelve percent (126 of 1,065) had PK in one (9.3%) or both eyes (2.5%). Baseline factors associated with increased likelihood of PK included younger age, steeper keratometric values, worse visual acuity, corneal scarring, poorer contact lens comfort, and poorer vision-related quality of life. The percent of eyes undergoing PK was 15% for patients 40 years old and younger, 28% for eyes with a steep keratometric value greater than 52 diopters, 33% for visual acuity less than 20/40, and 24% for eyes with corneal scarring., Conclusions: The CLEK Study confirmed previous reports of the increased likelihood of PK associated with corneal scarring, steeper keratometry values, poorer visual acuity, and poorer contact lens comfort. The CLEK Study is among the first to report an increased risk of PK associated with younger age, worse vision-related quality of life, and flatter contact lens fits. Knowledge of these factors is beneficial to clinicians in patient education and may be useful in disease management.

Purpose: Progressive corneal astigmatism occurring at least 10 years after penetrating keratoplasty for keratoconus is a late-phase complication of surgery. This report characterizes this condition in a series of patients from three corneal referral centers in the United States., Methods: Charts were retrospectively reviewed which met the following criteria: penetrating keratoplasty performed for keratoconus at least 10 years ago, keratometry or simulated keratometry from topography as well as manifest refraction recorded at least 6 months after the last suture removal ("baseline"), and an increase in corneal astigmatism of at least three D over baseline recorded at least 5 years later. Patients who had any other corneal or intraocular surgery performed were excluded., Results: Data from 15 patients (11 males and 4 females) who had penetrating keratoplasties performed by 8 different surgeons are included in this descriptive series. Postoperative follow-up averaged 17.27 years (range 11-24 years). The average donor button size was 7.83 mm (range 7.25-8.5 mm). Baseline corneal astigmatism was obtained an average of 5.2 years after penetrating keratoplasty (range 1.5-16 years) and was on average 3.57 +/- 1.8 D (10 with-the-rule [WTR], 3 against-the-rule [ATR], 2 oblique). Corneal astigmatism significantly increased to an average of 11.23 +/- 3.56 D (range 8.00-19.37 D, P < 0.0001) and most astigmatism was regular and WTR (9 WTR, 3 ATR, 3 oblique) 15.3 years (range 10-22 years) after surgery. Inferior steepening on topography was often noted, even those with oblique and ATR axes., Conclusions: High, late-stage, regular astigmatism after penetrating keratoplasty for keratoconus is described in a series of patients occurring at least 10 years after surgery. Possible mechanisms of this progressive astigmatism are recurrence of keratoconus in the graft, progressive corneal thinning of the host cornea, or progressive misalignment of the graft-host interface over time.

Purpose: To quantify and describe the complexity of contact lens correction on corneas altered by refractive surgery., Design: A case-control study involving postrefractive surgery patients and controls from one multidisciplinary contact lens practice., Methods: The contact lens care of one randomly selected eye of 30 postrefractive surgery patients (15 postincisional and 15 postlamellar refractive surgery cases) was retrospectively analyzed and compared with that of 30 age-and-gender matched nonrefractive surgery controls. We quantified the complexity by the number of diagnostic lenses used to establish the initial lens order, the number of ordered lenses required to complete the fit, and the number of office visits during the first 4 months of lens care. Final contact lens optical powers, corrected visual acuities, complications, and lens success/failure were also studied. The nonparametric Mann-Whitney U test was used for statistical analysis., Results: Postrefractive surgery eyes utilized statistically more diagnostic lenses (P =.048), ordered lenses (P =.008), and office visits (P <.0001). There were no statistical differences in the number of office visits, number of diagnostic contact lenses used, and number of ordered contact lenses between the postincisional and postlamellar refractive surgery patients. Although final contact lens optical powers were not significantly different (P =.06) between the two groups, contact lens corrected LogMAR acuities were statistically worse (P =.013) in the postrefractive surgery eyes than in the control group. Contact lens failures occurred in four postrefractive surgery eyes, while no eyes in the control group failed to tolerate contact lenses., Conclusions: Corneas altered by either incisional or lamellar refractive surgery techniques are more challenging to fit with contact lenses than nonsurgically altered eyes, requiring more lenses and chair time, while resulting in slightly poorer contact lens corrected visions and more failures.

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.

Background: Ophthalmic evaluation of patients with keratoconus (KC) often reveals highly myopic and irregular astigmatic refractive corrections. Irregular corneal astigmatism and central corneal scarring in patients with KC often result in a loss of best-corrected spectacle acuity. Rigid gaspermeable contact lenses generally optimize visual acuities for patients with KC., Case Reports: Two cases are discussed of patients who manifested clinically diagnosed KC but unusually good unaided Snellen visual acuities (20/25+ or better) in both eyes., Conclusion: Good unaided visual acuities are not necessarily inconsistent with the diagnosis of KC.

If there is little lateral diffusion of oxygen in an optically powered hydrogel contact lens, then point-to-point differences in the thickness of the lens can result in different areas of the cornea receiving different amounts of oxygen. Specifically, areas under the thicker portions of the lens will receive less oxygen. Cells in these oxygen-deprived areas may produce metabolic products that spread laterally in the cornea resulting in cornea-wide edema. The fact that most of a contact lens transmits adequate oxygen to the cornea does not lead to satisfactory lens performance, if some part of the lens is too thick. We address the problem of lateral diffusion of oxygen in a contact lens and describe the methods for quantifying the point-to-point thickness of optically powered hydrogel lenses. Areal differences in oxygen supply to the cornea are found to be almost totally dependent on optical power; water content and lens design have far less effect.

A patient presented with central corneal scarring and neovascularization associated with elevated deposits that were shown to be keratin and calcium. Giant papillary conjunctivitis (GPC) was noted at a corresponding location in the palpebral conjunctiva. The lid reaction resolved after the elevated corneal deposits were debrided. A rigid gas-permeable contact lens was then fitted for visual rehabilitation. Either foreign bodies or elevated corneal deposits may cause GPC.

No oxygen was detected on the corneal surface of human subjects breathing air or almost pure oxygen. These results suggest that the epithelium obtains all its oxygen fro the air or the palpebral conjunctiva and that the endothelium is supplied with oxygen solely by the aqueous humor.

Aqueous humor oxygen tension of the in vivo rabbit eye was estimated by a relatively atraumatic procedure. The anterior corneal surface of the eye was first scraped free of its epithelial layer. A polarographic oxygen electrode was then used to measure the oxygen tension at the bare stromal surface. Aqueous humor oxygen tension can be estimated from this measured steady-state stromal surface oxygen tension by correcting the measured oxygen tension for the tension drop across the stroma and endothelium. When animals breathed room air (155 mm Hg oxygen tension), the oxygen tension of the aqueous humor was 13 mm Hg; this rose to 150 mm Hg when the inspired oxygen tension was 713 mm Hg.

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 permeability (Dk) of 15 rabbit and 19 human cornea stromal samples was measured by conventional polarographic means at 36 C. Hydration and thickness were also measured. Rabbit stromal permeability was found to be about 26 X 10(-11) ml O2 cm2/sec ml mmHg at normal in vivo hydration; in vivo human cornea stromal permeability is probably less than 29 X 10(-11) ml O2 cm2/sec ml mmHg.

Optical pachometry was used to monitor thickness increases in six human corneas exposed to an anoxic external environment. Corneal swelling was observed to occur to an extent consistent with an earlier study (5%/hr or 25mu m/hr). Use of Darcy's law allows analysis of the "pump-leak" equilibrium, which leads to the conclusion that any direct interference by surface anoxia affecting corneal hydration must be at the endothelial side of the cornea.

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.

In vivo corneal oxygen consumption of whole human corneas is mathematically estimated to be 4.85 X 10(-5) ml O2/ml/s, or 0.8 microliter O2/mg dry weight/hr, from data gathered by a noninvasive technique.

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.