Lynn Cook Winterton

Oxygen Permeability of a New Type of High Dk Soft Contact Lens Material

Lotrafilcon A is a biphasic block-copolymer, comprising a highly permeable siloxane-based polymeric phase, coupled with a water phase (hydrogel phase). The high oxygen permeability of this material, and the fact that it is a hydrogel, places it outside the applicability of both the polarographic ISO standard and coulometric ISO draft standards for contact lens Dk determination. The oxygen permeability (Dk) and transmissibility (Dk/t) of lotrafilcon A lenses were determined by an adaptation of the standard coulometric method. Lenses with a thickness (t) range from 30 μm to over 300 μm were measured in a liquid-to-gas and a gas-to-gas configuration in an effort to combine features of the ISO (draft) standards to yield a valid measurement of the intrinsic materials oxygen transmission characteristics. The following results, for lotrafilcon A, are the mean values and SE for the oxygen permeability coefficient (Dk) determined at 34°C: 140 ± 2 barrer with water overlay (liquid-to-gas) at 2100 rpm stirring speed, 150 ± 5 barrer with water overlay (liquid-to-gas) at “infinite” stirring speed, and 170 ± 2 barrer without water overlay (gas-to-gas), where barrer=10-11 (mlO2 · cm)/(sec · cm2 · mm Hg). Clinical lenses produced from this material are expected to be in the 60 to 90 μm thickness range, with an average center thickness of 80 μm. Given that the Dkmaterial is greater than or equal to 140 barrers, for this material, a parallel sided lens of 80 μm central thickness would exhibit a lens transmissibility (Dk/t) of at least 170 x 10-9 mlO2/(sec · cm2 · mm Hg) at 34°C. This transmissibility is well in excess of the 87 x 10-9 mlO2/(sec · cm2 · mm Hg) value postulated to be sufficient to prevent overnight lens-induced corneal swelling and places it in the hyper-permeable material category.

Corneal oxygen distribution with contact lens wear.

Purpose: Since 1991, multilayer mathematical in vivo oxygenation models have been created to predict normal corneal oxygenation with contact lens wear. From these models, there have been assertions that most hydrogel contact lenses allow 97%-98% of normal corneal oxygenation compared to no contact lens wear. In light of hydrogel lens-induced neovascularization and limbal hyperemia, to clinicians, this finding seems counterintuitive. This work seeks to validate or refute those preexisting models and estimate the impact of contact lens wear on the oxygen distribution profile across the cornea. To this end, to estimate the impact of contact lens wear on the 3-dimensional (3-D) oxygen distribution profile within the cornea as a function of the oxygen permeability of the contact lens, a two-dimensional axisymetric finite element analysis (FEA) model was constructed for contact lenses, on the cornea, both having varying thickness profiles. Methods: A two dimensional (2-D) axi-symetric finite element analysis (FEA) model of a −3.00 D contact lens on eye was constructed. The model included the varying thickness profiles of the contact lens and cornea. By symmetry, this 2-D model is equivalent to a full 3-D model. The oxygen permeability, material thickness profile, and oxygen consumption coefficients from Brennan (Optometry and Vision Science, June 2005) were used for this validation. Several different oxygen consumption profiles were also considered. Oxygen partial pressure, flux, and consumption profiles were generated. Results: Profiles of the oxygen partial pressure, flux, and consumption were generated from the central cornea to the limbal junction. Conclusion: This FEA model reproduced Brennan 8-layer model (BEL model) results at the central cornea. However, BEL model parameters yielded regions of oxygen deficiency in the corneal periphery, even in the open eye with no contact lens. If the BEL model cannot account for oxygenation across the whole cornea, it may be incorrect or incomplete. This assertion calls into question any conclusions from the BEL model regarding the minimum contact lens transmissibility needed to fully oxygenate the eye.

Creating antimicrobial surfaces and materials for contact lenses and lens cases.

Contact lenses are a safe and effective mode of vision correction used by more than 100 million people worldwide, yet some adverse responses to microbial contamination of contact lenses still occur. Various medical devices, including contact lens cases, currently use antimicrobial agents to eliminate or reduce microbial contamination at the surface. The application of antimicrobial surface technologies to contact lenses and lens cases is being explored. This article describes agents that hold promise for antimicrobial surfaces for contact lenses or lens cases.

Ex vivo protein deposition on bi-weekly silicone hydrogel contact lenses.

We would like to comment on the article by Boone et al. as we take issue with some of the methodology, the manner in which some of the data are presented, and with some aspects of the Discussion. In this study, the authors found the Bausch & Lomb balafilcon A lenses to have the largest amount of protein deposit, most of which the authors claim to be active or not denatured. The two CIBA VISION lenses, lotrafilcon A and lotrafilcon B, deposited the least amount of protein, most of which the authors claim to be denatured. Finally, for the two Johnson & Johnson Vision Care lenses (galyfilcon A and senofilcon A), the authors claim to have protein adsorption and denaturation percentages midway between the other examples. The researchers describe the use of an extraction process in which they used 10 times lower amount of trifluoroacetic acid in the solvent to extract the two Johnson & Johnson Vision Care lenses than is used with the other lenses. This deviation in procedure could certainly create less denaturation of the extracted proteins. In addition, the method used to determine the reduction in the activity of the lysozyme is highly time dependent and cannot reliably be used to calculate the actual micrograms per lens at an arbitrary time point. Be that as it may, allow us to look at their data and the way in which it is presented to the reader. Table 2 presents a summary of lens deposition. Almost any way one looks at this, the lotrafilcon lenses show the least amount of total protein, total lysozyme, and denatured lysozyme in micrograms per lens. In fact, for the lotrafilcon B lenses, the denatured lysozyme is 0.2 0.2 g/lens, indicating that the actual amount could well be 0.0. It is only by taking the curious turn of calculating a percentage of lysozyme that is claimed to be denatured that the lotrafilcon A and lotrafilcon B lenses are made to appear to have performed poorly (see Fig. 3). There is no precedent or justification for this manipulation. With the miniscule amount of total lysozyme on the lotrafilcon materials, of course any amount of denatured protein (except, of course, 0) will drive the percentage up. It may be important to remind readers that Johnson & Johnson Vision Care’s etafilcon A lenses have been successfully worn, for decades, for both daily and extended wear. This same laboratory found the amount of lysozyme deposited on etafilcon A lenses to be about 1,000 g/lens after 2 week of wear and about 22% 5% of the lysozymes were denatured (about 242 g/lens). This would suggest that a lens with this much denatured lysozyme can be worn without major complications. It would seem absurd to suggest that protein deposits over a thousand times less than occurs with successful HEMA lenses would have any clinical significance. The authors have cited an analysis of 317 wearers of lotrafilcon A lenses over a 3-year period, which showed only low levels of clinically significant palpebral changes, suggesting that denatured lysozyme on lotrafilcon A lenses is not an issue at all for most patients. Finally, if the authors’ desire is to warn practitioners away from denatured protein because of issues related to GPC, one needs to be reminded that immunological responses, like GPC, are a function of repeated exposures to the immunogenic entity. This is a function of total entity, not a percentage. In an earlier article coauthored by one of the authors of this article, discussion of percent of denatured lysozyme led to the following statement “However, given the small amount of lysozyme deposited on these new materials, this may not be a critical factor in the development of lid changes in patients who use these materials on a continuous wear basis.” We agree. With all due respect, these are challenging laboratory procedures, and we understand the authors’ desire to present their findings. However, ascribing or suggesting a clinical relevance when there is no evidence for one is an extrapolation beyond the data at hand. Lynn C. Winterton, PhD Manal M. Gabriel, DDS, PhD Peter D. Bergenske, OD, FAAO CIBA VISION Corporation Duluth, Georgia