The Clinical Value of The Measure of Oxygen Transmissibility

Craig Woods PhD, FAAO Research Manager, Centre for Contact Lens Research Craig is currently the CCLR's Research Manager. He graduated from The City University and after a period of working in private practice in London, joined the staff at the Institute of Optometry, in London as Assistant Clinical Director. He then moved to Manchester, where he obtained his PhD whilst Clinic Manager for the Department of Optometry and Vision Science, UMIST. In 1999 he moved to Melbourne to become the Deputy Clinic Director at the Victorian College of Optometry. Craig is a therapeutically accredited optometrist, a Fellow of the Victorian College of Optometry, the American Academy of Optometry and a member of the College of Optometry (UK).

Physiological compromise of the ocular adnexa, particularly the cornea, is a significant consequence of wearing contact lenses for refractive correction. While innovation and development of the contact lens has been ongoing, the majority of research investment has centred on addressing, understanding and resolving the physiological changes induced by lens wear.

Lenses that deliver low levels of oxygen induce acute and chronic ocular changes during daily wear, and more so during extended or continuous wear.  These changes include corneal swelling, epithelial microcysts, increased myopia, limbal hyperemia, corneal vascularisation, endothelial polymegethism, epithelial thinning, increased bacterial binding to epithelial cells and suppressed epithelial proliferation rates.^(1-3) It has been shown that these ocular changes decrease as oxygen transmissibility increases^(1,4-6) and it would be reasonable to assume that this continues with an increasing supply of oxygen. Some of these physiological changes resulting from decreased oxygen supply (e.g. epithelial oxygen consumption, ie stromal pH and corneal swelling) can be used to clinically assess the oxygen delivery of a lens.^(3-7)

A fundamental requirement of a contact lens is that it allows sufficient levels of oxygen to reach the cornea for normal aerobic metabolism activity. The permeability (Dk) of a material to oxygen has been described by the relationship of that material’s diffusion coefficient to the gas (D) and the solubility of that gas in the polymer (k)⁽⁸⁾ .  The higher the value of oxygen permeability, the more oxygen is transported through that material - for example, silicone hydrogel lens materials.  The rate that oxygen is transported through a lens is influenced by the material permeability (Dk) and the lens thickness (t); where the thinner the lens the higher the rate of oxygen flow. This is known as the oxygen transmissibility (Dk/t) of the lens.

The value for material permeability is dependent on the method of measurement. The Gas to Gas method or the Polarographic technique requires an adjustment due to the boundary effect⁽⁹⁾.  As these values differ, it is important to reference which method was used when quoting a value. This issue was addressed with the introduction of the international standard ISO 8321-2 (2000). However; the matter was further complicated somewhat by the recommendation of using pascal units rather than mmHg for the unit of pressure in determining the material permeability, also necessitating an adjustment of the Dk value.

Oxygen transmissibility as a performance indicator of a contact lens with respect to oxygen delivery has been described as the rate of flow of oxygen through a lens for a given thickness⁽¹⁰⁾. This parameter is often used to characterise and differentiate between different lenses and designs. Thinner lenses and/or lenses manufactured from  materials with higher oxygen permeabilities have a higher oxygen transmissibility and thus anticipated superior physiological performance.

Harvitt and Bonanno⁽¹¹⁾ suggested that an oxygen transmissibility value of 125 x 10-9 would avoid stromal anoxia under closed eye conditions while Holden and Mertz⁽¹²⁾ found that an oxygen transmission value of 87 ±3.3 x 10-9 is necessary to avoid contact lens induced corneal oedema during overnight wear. It has since been suggested that Holden and Mertz underestimated the critical oxygen transmission value for overnight wear⁽¹³⁾ . Oxygen transmissibility has been and continues to be a very useful tool for clinicians to determine which lens design or material can provide the necessary level of oxygen for their patient’s corneas. 

These values relate to the average patient, so assuming a normal distribution in the cohort under investigation, 50% of these patients will need a higher level of oxygen delivery; while we may conclude that 125 x 10-9 is our target for delivering optimum care, half of our patients will need more. So lenses offering the highest oxygen transmissibility may be more appropriate for some patients.  It has also been suggested that older patients have a higher oxygen demand. Conversely those people who do not need such high levels of oxygen may benefit from the advantages of lenses with a lower oxygen transmissibility. 

Furthermore, there are different levels of oxygen transmissibility across lens designs.  Myopic corrective lenses increase in thickness towards the lens edge, so while a lens may have a high central oxygen transmissibility, the cornea’s oxygen demand may not be met by averaging the oxygen transmission across the lens. Bruce⁽¹⁴⁾  reported that Focus Night and Day, currently the lens with the highest reported oxygen transmissibility, was only able to deliver oxygen to match the Holden and Mertz criterion across the whole lens with lenses ranging from -6.00D to +3.00D; other lens designs would obviously have a reduced power range over which this criterion was met.  So not only do we have patient variability in corneal oxygen demand but also a variation in oxygen transmissibility across the lens.

The clinical determination of  an individual’s corneal oxygen needs is very difficult and monitoring the ocular state over time is the preferred clinical option. This unfortunately is not a preventive measure. A safer one to avoid oxygen deficiency is to prescribe the highest oxygen transmissibility available, but that is not always practical. Central oxygen transmissibility is acknowledged as the standard by which practitioners judge how much oxygen the lens is providing and it would be even better if the average oxgen transmissibility were provided from the measurement of the thickness profile of the lens. I suspect oxygen transmissibility will continue to be used as a guide by clinicians for many years to come yet.

Reference List

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