Please cite this article in press as: Papas EB. The significance of oxygen during contact lens wear. Contact Lens Anterior Eye (2014), http://dx.doi.org/10.1016/j.clae.2014.07.012 ARTICLE IN PRESS G Model CLAE-723; No. of Pages 11 Contact Lens & Anterior Eye xxx (2014) xxx–xxx Contents lists available at ScienceDirect Contact Lens & Anterior Eye jou rn al h om epa ge : w ww.e l sevier.com/locate/clae Review The significance of oxygen during contact lens wear Eric B. Papas a,b,c,∗ a Brien Holden Vision Institute, Sydney, Australia b School of Optometry and Vision Science, University of New South Wales, Sydney, Australia c Vision CRC, Sydney, Australia a r t i c l e i n f o Article history: Received 26 March 2014 Received in revised form 19 July 2014 Accepted 22 July 2014 Keywords: Oxygen Contact lens Oxygen transmissibility Oxygen permeability Normoxia Silicone hydrogel a b s t r a c t In order to establish the relevance of oxygen to contemporary contact lens practice, a review of the literature was conducted. The results indicate that there are a number of processes occurring in the normal healthy eye where oxygen is required and which are potentially affected by the presence of a contact lens. These activities appear to take place at all corneal levels, as well as at the limbus. Evidence from laboratory, clinical and modelling studies indicates that what constitutes normal oxygenation (normoxia) depends on, among other things, the physiological system under consideration, corneal location and the state of eye closure. This diversity is reflected in the wide range of minimum lens oxygen transmissibility (Dk/t) requirements that are present in a literature. © 2014 British Contact Lens Association. Published by Elsevier Ltd. All rights reserved. 1. Introduction Since the discovery on the early 1950s that oxygen was neces- sary to prevent corneal oedema [1], it has been incumbent upon contact lens clinicians to take steps to improve the cornea’s access to the atmosphere. During the early years when lenses were large, scleral and made from glass or PMMA, there was little that could be done beyond the introduction of an air bubble to the post-lens optic region. The invention of micro-corneal lenses [2] changed all that by being smaller than the cornea and mobile. So while still made from gas impermeable PMMA, these lenses allowed oxygenated tears to bathe the previously anoxic cornea. This opened the door for contact lenses to become a widely used vision correction choice. The advent of materials with intrinsic oxygen permeability, first hydrogels [3], then rigid gas permeables [4] and most recently sil- icone hydrogels [5], has provided an increasing range of options to help clinicians avoid the consequences of hypoxia. The last 60 years have seen considerable research conducted into the way con- tact lenses interact with the ocular surface, including both direct laboratory and clinical studies, as well as increasingly sophisti- cated modelling approaches to understanding the key physiological systems. In that time it has become evident that oxygen, or the lack of it, is an important factor determining how several systems ∗ Correspondence to: Level 4 Rupert Myers Building, Gate 14 UNSW, Barker St, Kensington, NSW 2052, Australia. Tel.: +61 2 9385 7489; fax: +61 9385 7401. E-mail addresses: e.papas@brienholdenvision.org, ebpapas@hotmail.com function. An understanding of the role of oxygen in corneal health and how this is modified by contact lenses is essential to give clinicians a platform on which to base their efforts to optimize per- formance. The purpose of this article is to assist that process by providing a review of oxygen related interactions between contact lenses and the cornea together with its associated tissues. We begin by considering how contact lenses may impede the eyes’ access to oxygen and then move on to review the potential consequences of such interference. 2. Diffusion kinetics There are only two routes whereby oxygen can reach the ocu- lar surface beneath a contact lens. The first is by dissolving in the tears and passing around the lens edge into the post-lens space and the second is by diffusing through the material of the lens itself. Soft lenses have large diameters, move relatively little and closely follow the ocular surface contour, all of which limit the scope for significant tear exchange to occur [6–9]. While rigid lenses are better placed in this regard, being smaller and considerably more mobile [10], the route of oxygen supply by tear exchange has proven to be insufficient by itself to prevent clinical signs of hypoxia occurring [11]. For both rigid and soft lenses then, intrinsic oxygen permeability is a necessary requirement. Oxygen passes through a lens by diffusion. This is passive pro- cess whereby oxygen molecules move from regions of high to low concentration in a manner that, in the steady state, is described by Fick’s first law. In the case of oxygen flow through a contact lens http://dx.doi.org/10.1016/j.clae.2014.07.012 1367-0484/© 2014 British Contact Lens Association. Published by Elsevier Ltd. All rights reserved.