In the first of three articles looking at contact lens material properties, Dr Philip Morgan and Dr Noel Brennan review the measurement of the oxygen performance of soft contact lenses and the clinical relevance of these measures for currently available contact lenses. CET Module C4836c
A SUCCESSFUL CONTACT LENS needs to provide acceptable levels of vision, comfort, handleability/usability and physiological response. The first three parameters are of primary concern to the wearer and are usually immediately apparent. The final requirement – an acceptable physiological response – is usually of greater concern to the contact lens practitioner and is usually of medium and long-term concern.
Very broadly, the various forms of adverse physiological ocular response to contact lenses can be attributed to mechanical or lens surface effects, and hypoxia-related phenomena.1 Mechanical effects include papillary conjunctivitis, superior epithelial arcuate lesions and conjunctival staining. Hypoxia-related adverse responses include limbal redness, some types of corneal staining and, most seriously, microbial keratitis.
One of the measures, therefore, of the physiological performance of a contact lens, relates to the amount of oxygen which reaches the ocular surface from the atmosphere. This is most important in the central and anterior parts of the cornea. The corneal periphery may derive some of its oxygen supply from the local vasculature, whereas the posterior layers are supplied by the aqueous humour. However, the central anterior cornea is very dependent on atmospheric supply.
Given this, it is desirable to have some measure to indicate the oxygen performance of a contact lens. For more than 30 years, the best-known measures in this area have been the oxygen permeability of the materials from which lenses are fabricated, and oxygen transmissibility – the oxygen performance of a finished, manufactured lens.
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