Oxygen permeability of contact lens materials and testing standards

Why is there a need for contact lens oxygen transport measurement? The importance of oxygen to the health of the cornea is well known; as a result contact lens users will prefer a product with proven high oxygen performance. Lens manufacturers advertise their various product specifications, including oxygen permeability and transmission, usually to Eye Care Professionals who can then advise patients on the most suitable choices for their particular needs. The permeability is referred to as Dk and the transmission Dk/t. These properties are derived from: D - the material diffusion coefficient k - the oxygen solubility t - the lens thickness. A given lens material, typically a hydrogel, will absorb an amount of oxygen governed by its k value, while a higher D will ensure rapid movement through the lens. The thicker the lens the further oxygen has to travel consequently reducing its transmission. A contact lens design using a particular material will require its oxygen transmission measured. Higher permeability materials will perform better, together with thinner lenses, but simply choosing the highest Dk material and making thin lenses is not an easy option, as will be explained later. The lens shape and thickness varies with prescription, a focal length of –3.00 Dioptres has been chosen as the standard for oxygen transmission measurement. The thickness in the central region of the lens is used because this covers the most critical area of the cornea. Types of lenses used today Conventional hydrogel soft lenses contain a high proportion of water which provides the main path for oxygen diffusion. So for these products the Dk value is directly proportional to the water content of the particular hydrogel used. However the highest Dk materials are difficult to manufacture into thin lenses, resulting in a range of designs, some medium Dk thinner varieties, and other higher Dk thicker ones. The aim is to produce the highest Dk/t to maximise corneal oxygenation, commensurate with comfort and visual performance. The latest silicone hydrogels do not have their oxygen permeability governed by water content alone, but also by their constituent siloxane groups. Contact lenses containing these products are some of the highest performing available today, with very high oxygen transport and the highest levels of comfort. Rigid gas permeable (RGP) contact lenses are still an attractive alternative to the soft varieties and probably offer better optical performance generally, combined with excellent corneal care. Although initially less comfortable than soft lenses, the Dk values for the RGP polymer materials are on a par with the best silicone hydrogels whilst offering much longer lifetimes with lower long term cost. In the early days of contact lens use, indeed up to the early 1980's the importance of corneal oxygenation was not well understood. However since that time international standards have been put in place to enable reliable oxygen permeation testing of lens materials. Early ISO standards 9913-1:1996 and 9913-2:2000 were put in place for polarographic (Fatt method) and coulometric determination of oxygen permeation through all types of contact lenses. However the high Dk Silicone based lenses proved difficult to measure accurately using these standards, in part due to lens dehydration during coulometric procedures. These standards have now been withdrawn and both replaced by ISO 18369-4:2006, which like the previous standards describes the measurement of Dk/t for finished lenses rather than a material sample. This oxygen transmission value is subsequently converted into the Dk value by measuring the lens thickness, t. Solutions from Systech Illinois Systech Illinois produce the 8000 range of oxygen permeation analysers, which are capable of measuring contact lens oxygen throughput.

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