Modeling Corneal Oxygen with Scleral Gas Permeable Lens Wear.

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.

Calculated in situ tear oxygen tension under hybrid contact lenses.

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.

Baseline factors predictive of incident penetrating keratoplasty in keratoconus.

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.

Longitudinal changes in corneal curvature in keratoconus.

PURPOSE: This study was designed to identify factors that predict longitudinal increases in corneal curvature as measured by the First Definite Apical Clearance Lens (FDACL) and flatter keratometric reading (Flat K) in keratoconus. METHODS: The Collaborative Longitudinal Evaluation of Keratoconus (CLEK) Study is a long-te rm evaluation of the natural history of keratoconus involving 1209 patients. This report uses 8 years of follow-up data from 1032 patients who had penetrating keratoplasty in neither eye at baseline and who provided enough data to compute the slope of the change with time in the FDACL or the Flat K. Outcome measures included the aforementioned slopes and whether the FDACL or the Flat K increased by 3.00 or more diopters (D) in at least 1 eye. RESULTS: At CLEK Study entry, patients were aged 38.9 +/- 10.8 years. Overall, 44.3% of them were women, and 69.3% of them were white. The slope of the change in FDACL (0.18 +/- 0.60 D/y) and Flat K (0.20 +/- 0.80 D) during 8 years translates into expected 8-year increases of 1.44 D in FDACL and 1.60 D in Flat K. Increases of > or = 3.00 D in either eye had an 8-year incidence of 24.8% for FDACL and 24.1% for Flat K. Independent predictors of increased FDACL included younger age, poorer baseline high-contrast manifest refraction visual acuity, and non-white race. Younger age and poorer high-contrast manifest refraction visual acuity were independent predictors of a >3.00-D increase for both FDACL and Flat K. CONCLUSION: CLEK patients exhibited a slow but clear increase in corneal curvature. Younger age and poorer high-contrast manifest refraction visual acuity at baseline predicted the rate of change in corneal curvature.

Predicting scleral GP lens entrapped tear layer oxygen tensions.

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.

Characteristics of corneal ectasia after LASIK for myopia.

PURPOSE: There are numerous reports of corneal ectasia after laser in situ keratomileusis (LASIK) for myopia without a consistent definition of this condition or a definitive etiology. We conducted a retrospective analysis of published case reports to describe common characteristics of this postoperative event and compared them with findings from a group of successful LASIK patients. METHODS: A MEDLINE search for "LASIK" and "ectasia" yielded 21 relevant articles published before May 2003 (n = 86 eyes, 59 patients). A comparison group (n = 103 eyes, 63 patients) was selected from a clinic-based sample of successful LASIK patients with 12 months of follow-up after treatment. Descriptive statistics are reported as median and interquartile range. Comparisons were performed using the Wilcoxon rank sum, Wilcoxon signed rank, and chi-square tests. RESULTS: Time to diagnosis of ectasia after LASIK was 13 months (6 to 20 months). Residual myopia in the ectasia group was -3.69 D (-6.00 to -2.13 D) and was significantly greater than the comparison group, -0.38 D (-0.75 to 0.00 D), P < 0.001. After surgery, eyes with ectasia had increased corneal toricity 2.87 D (2.00 to 4.9 D) with increased oblique astigmatism 1.3 D (0.23 to 2.89 D) relative to eyes in the comparison group 0.00 D (0.00 to 0.08 D), and a loss of 2 lines (-0.5 to -6 lines) of best spectacle-corrected visual acuity (all P < 0.001). Thirty-five percent of reported cases resulted in subsequent corneal transplantation. CONCLUSIONS: Preoperative characteristics of corneal ectasia include worse visual acuity, less corneal thickness, greater residual myopia, and greater corneal toricity than nonectatic eyes. Treatment factors associated with corneal ectasia after LASIK are greater stromal ablation and less residual stromal bed thickness. Postoperative characteristics of corneal ectasia are myopic refractive error with increased astigmatism, worse spectacle-corrected visual acuity, increased corneal toricity with topographic abnormality, and progressive corneal thinning.

A literature review: the impact of rotational stabilization methods on toric soft contact lens performance.

Toric soft contact lenses have been in the marketplace for more than 30 years. Over this period of time, substantial improvements have been made in the areas of lens reproducibility, lens material permeability, frequency of lens replacement, and availability of lens design and parameter options. This article will review the methods used to stabilize lens rotation and their effectiveness as reported in the literature.

A pre-application drop containing carboxymethylcellulose can reduce multipurpose solution-induced corneal staining.

PURPOSE: Use of polyhexanide based multipurpose solutions (MPSs) for contact lens disinfection has been linked to low-grade corneal staining. In vitro data suggest that carboxymethylcellulose (CMC) may neutralize polyhexanides. The purpose of this investigation was to determine whether a pre-application drop of CMC reduces polyhexanide staining in vivo. METHODS: Thirty adapted soft contact lens (SCL) wearers participated in this investigator-masked, randomized, two-way cross-over study. Subjects wore a new Group II lens (alphafilcon A, 66% water) daily for 4 weeks and disinfected lenses using a MPS containing 0.0001% polyaminopropyl biguanide. A lens lubricant containing either CMC or povidone as the primary viscolyzer was applied to the lens each day before lens wear. Biomicroscopic signs and symptomatology were assessed. The difference in scores, 0 to 4 weeks and the difference between lubricants were analyzed. RESULTS: The cumulative fluorescein staining scores for combined eyes demonstrated a significant increase over time (e.g., cumulative staining score; p=0.004 and p<0.001 for CMC and povidone, respectively, matched pairs t-test, two-tailed), suggesting that for both lubricants the staining worsened with wear. This effect was expected and likely driven by the MPS. However, the mean cumulative staining scores for CMC and povidone were 2.8 and 2.6 out of 20 possible at baseline, increasing to 4.9 and 7.1 at 4 weeks, respectively. The increases were significantly different (p=0.003, matched pairs t-test, two-tailed) suggesting a greater increase in corneal staining for the povidone lubricant. The symptom scores were not significantly different, 0 to 4 weeks by regimen or between preinstillation drops. CONCLUSIONS: These results suggest that a CMC-containing preapplication drop can reduce corneal staining resulting from disinfection with a polyhexanide MPS. This result is consistent with a proposed mechanism for CMC to neutralize cationic disinfectants and may offer clinicians another means to reduce this type of corneal staining.

Comparison of flat and steep rigid contact lens fitting methods in keratoconus.

PURPOSE: The purpose of this article is to compare the safety and efficacy of flat- and steep-fitting rigid contact lenses in keratoconus. METHODS: The Collaborative Longitudinal Evaluation of Keratoconus (CLEK) Study is a 16-center observational study. Cross-sectional results at baseline were generated for 1091 subjects with longitudinal results from the 871 subjects who completed 8 years of follow up. RESULTS: Of the 761 rigid contact lens-wearing patients at baseline, 41% had a scar at baseline compared with 24% of the nonrigid contact lens wearers (odds ratio [OR], 2.15; 95% confidence interval [CI], 1.35-3.43; p = 0.001). Eighty-seven percent were fitted with flat-fitting lenses, whereas 13% were fitted with steep-fitting lenses. Rigid lens fitting method was also associated with incident corneal scarring. A greater proportion of the corneas wearing flat-fitting contact lenses were scarred (43% compared with 26% for the steep-fitted eyes; OR,= 2.19; 95% CI, 1.37-3.51; p = 0.001). After controlling for corneal curvature, the association of rigid contact lens fit and corneal scarring at baseline did not persist (adjusted OR, 1.20; 95% CI, 0.70-2.06; p = 0.52). Thirty-two percent of unscarred eyes at baseline fitted flat had developed an incident corneal scar by the eighth year follow-up visit compared with 14% of eyes fitted steep (OR, 2.93; 95% CI, 1.34-6.42; p = 0.007). CONCLUSIONS: The data reported here indicate that, after controlling for disease severity in the form of corneal curvature, keratoconic eyes fitted with a rigid contact lens resulting in an apical touch fluorescein pattern did not have an increased risk of being scarred centrally at baseline. This "natural history" sample cannot determine causal proof that one method of fitting lenses is safer than another. To achieve this, a randomized clinical trial is needed.

Corneal staining in hydrogel lens wearers.

PURPOSE: The purpose of this study was to determine the factors that contribute to corneal staining in hydrogel lens wearers by examining the following variables: type of lens worn, wearing time and lens replacement schedule, lens care system, and topical and systemic medication use. METHODS: Five hundred full-time, successful hydrogel contact lens wearers were evaluated for corneal fluorescein staining at 20 clinical sites. Corneal staining was evaluated in five regions and was graded on a scale of 0 (no staining) to 4 (severe staining) in 0.5 steps for severity. RESULTS: Corneal fluorescein staining was present in at least one eye in 55.7% of the hydrogel lens wearers examined in this study, and 8.0% had moderate-to-severe staining (defined as a cumulative staining score of > or = 3 with at least one quadrant score > or = 2). Staining was most severe for the inferior portion of the cornea (mean +/- SD; OD: 0.34 +/- 0.57, OS: 0.30 +/- 0.54). All other areas of the cornea showed lesser amounts of staining, all of which were approximately equal (mean range, 0.13 to 0.16). Staining was observed in two or more quadrants in 24% of right eyes and 22.4% of left eyes. When questioned about care system compliance, 81.1% of subjects reported compliance. Patients who were noncompliant with their lens care system (chi 2 p = 0.0037), used rewetting drops (chi 2 p = 0.0005), or wore conventional lenses without a planned replacement schedule (chi 2 p = 0.0008) were more likely to have some degree of corneal staining. Noncompliance with care system (chi 2 p = 0.0147), replacement schedule (chi 2 p = 0.0039), and lens power (chi 2 p = 0.0224) were associated with moderate-to-severe staining. The following factors were not significantly associated with corneal staining: patient age, gender, medication use including oral contraceptives, type of wear (extended, flexible, or daily), average wearing time, wearing time at the examination, lens type (spherical or toric), water content of lenses (high or low water content), type of care system, use of enzymes, and pinching the contact lens directly off the cornea. CONCLUSIONS: Staining occurs to some extent in many hydrogel contact lens patients, and is influenced by many factors. Moderate-to-severe staining, which may be more clinically significant, is associated with noncompliance with care system, a conventional replacement schedule, and lens power.

Variables affecting rigid contact lens comfort in the collaborative longitudinal evaluation of keratoconus (CLEK) study.

PURPOSE: To identify factors associated with rigid contact lens comfort in keratoconus. METHODS: Baseline data from the 16 Collaborative Longitudinal Evaluation of Keratoconus (CLEK) Study clinical sites were analyzed for all patients wearing a rigid contact lens in their more severely keratoconic eye (as determined by steep keratometry). Corneal transplant patients, patients who did not wear a rigid contact lens in either eye, patients who did not wear a rigid lens in their worse eye, and patients with missing contact lens comfort data were excluded from the sample. A total of 751 eyes were included. Variables assessed included measures of disease severity, visual acuity through the patients' habitual rigid contact lenses, contact lens wearing time, the apical fitting relationship of the contact lens, the degree of peripheral clearance, and the presence of corneal scarring and staining. Comfort was measured by asking the patients "In general, how comfortable are your contact lenses?" (1 = very comfortable through 5 = very irritating). RESULTS: Measures of disease severity (steep keratometry and the first definite apical clearance lens) were not associated with lens comfort. There was no difference in self-reported contact lens comfort between patients fitted with apical touch vs. apical clearance. Patients with a peripheral clearance rating of "minimal unacceptable" (more common among patients with milder keratoconus) were approximately half as likely to report good contact lens comfort compared with patients with "average" peripheral clearance (unadjusted odds ratio, 0.39; 95% confidence interval, 0.19 to 0.79). There was no association between contact lens comfort and the other peripheral clearance ratings compared with ratings of average. CONCLUSIONS: There does not appear to be an association between decreasing patient-reported rigid lens comfort and increasing disease severity as measured by steep keratometry or first definite apical clearance lens in this sample. The apical fitting relationship (flat vs. steep) does not appear to be associated with patient-reported comfort. Minimal peripheral clearance may contribute to decreased rigid contact lens comfort in keratoconus.