Another View of Oxygen Transmission
BY NOEL A BRENNAN, MSCOPTOM, PHD, FAAO, & PHILIP B. MORGAN, PHD, MCOPTOM, FAAO

The oxygen transport characteristics of silicone hydrogel lenses have recently become a topic of much discussion. Unfortunately, the important message about the value of high-Dk/t lenses is overshadowed by the marketing hype that surrounds these lenses. We'll answer some common questions that practitioners ask us about contact lenses and oxygen.

Contact Lens and Oxygen FAQs

Is 125 the new Dk/t criterion for continuous wear? Absolutely not. There are scientific flaws in the arguments that promote this value. While the details of this science are beyond the scope of this article, future publications in the literature will address these flaws. In reality, all current silicone hydrogel materials provide enough oxygen for the purpose for which they're recommended, certainly across the power ranges and circumstances under which the lenses are normally worn.

Do better criteria exist for how much oxygen a contact lens should provide? Dozens of Dk/t criteria have evolved over the years; for example, continuous wear criteria range in values from 34 to 350, even as high as infinity. But Dk/t itself isn't a particularly useful index because large chan-ges in Dk/t above a value of about 20 for daily wear and 40 for continuous wear produce relatively small changes in oxygenation of the cornea.

Surely doubling the Dk/t doubles the amount of oxygen the cornea gets? Not at all. It's true for low Dk/t lenses that oxygenation is linearly related to Dk/t. But at higher values, the law of diminishing returns applies — a lens can have 10 times the Dk/t of another lens and actually deliver no more oxygen to the cornea.

How do we know that large changes in Dk/t produce small changes in total corneal oxygenation? Different authors have attempted to calculate the relationship between the laboratory-
measured oxygen properties of a contact lens (Dk/t) and physiology-based estimates of the amount of oxygen consumed by the cornea during contact lens wear. The conclusions of these various models all imply the same effect — that beyond a certain threshold, large changes in Dk/t have little effect on corneal oxygenation. The only question of debate is where, not whether, the oxygen vs. Dk/t curve levels off.

How do we know the models are right? You can model extreme cases and they show the theory to hold. Also, clinical data support the theory.

Are there Dk/t guidelines? We could suggest that above about 20 is the desirable range for daily wear, and above about 50 for continuous wear. But this should apply across the whole lens, not just its center. The latter value of 50 for continuous wear arises from clinical research of the Dk/t levels to avoid microcysts, as well as experiences with certain silicone hydrogel lenses.

On this basis, do traditional hydrogel lenses supply enough oxygen for daily wear? Most values for hydrogel lenses give a false impression of their oxygen capabilities. Remember, these values should apply across the entire lens. Most of the hydrogels available in the market today fail to provide a Dk/t of 10 all the way across the lens. Only one hydrogel lens on the market, and then only in a few low powers, provides a Dk/t of 20 across the entire lens. The models and some scientific data suggest that the cornea is more hypoxic during daily wear of hydrogels than during continuous wear of silicone-hydrogels. To avoid hypoxia in daily wear, we recommend using silicone hydrogels.

But there's no evidence that corneas under daily wear suffer any significant hypoxic-related complications. The biggest cause of infectious corneal ulceration in the developed world is lens wear, and more cases result from daily wear hydrogels than from any other modality. The epidemiological data do not show that lens Dk/t influences the number of cases of infiltrative keratitis. But severity of keratitis might be related to lens oxygen transmission — it seems so in continuous wear — and this is the reason to go with silicone hydrogels.

Doesn't hypoxia cause dryness? This is another myth. There's absolutely no evidence in the medline-abstracted literature to back this claim. Some high Dk/t materials may have properties that also cause less dryness — but hypoxia isn't the common factor, and lower Dk/t lenses can also have similar properties.

But, in the end, isn't more oxygen just better? Shouldn't we prescribe lenses with higher Dk/t values as a measure of safety? Absolutely — if everything else is equal. But lenses with different Dk/t values have different modulus values, surface treatments and designs. As stated above, all current silicone hydrogel materials provide enough oxygen for their recommended purposes. Modulus, surface performance and design are the important clinical differences among materials. We believe these lens characteristics will emerge as the important factors for better clinical performance long after the various unfounded claims about oxygen are forgotten. CLS

Dr. Brennan is director of Brennan Consultants Pty Ltd, a consultancy group based in Melbourne, Australia, that performs scientific and clinical research for the contact lens industry. Dr. Morgan is the research manager of Eurolens Research at the University of Manchester, UK.