Are you fitting comfortably?

The fact that a recent run of roadshows organised by Johnson & Johnson were fully-booked just goes to show how popular these events are in the current CET climate. Event organiser David Ruston had put together a programme with some big name speakers and, more laudably, was of use in all aspects of contact lens work, not just those relating to the sponsor's product.

Material choice

Associate Professor Lyndon Jones (Waterloo, Canada) started by summarising some familiar facts about material developments relating primarily to silicone hydrogel material, and managed along the way to still include much that was new and of interest.

He began by recalling how traditional HEMA, once the basis of all soft lenses fitted, had a Dk of around just nine units, which compares poorly with the 350 or so units of silicone hydrogel. With silicone hydrogel, the Dk drops as the water content increases, though a notable exception to this is the forthcoming CooperVision Biofinity lens which is to become available later this year.

He then touched upon the sometimes thorny subject of how manufacturers stipulated the oxygen performance of a material, and suggested how oxygen flux might be a better model. It is long known that as transmissibility reaches a certain (high) value, the flux plateau, meaning that increasing the value further offered diminishing returns. It is also important to remember that most quoted Dk/t values are based upon a -3.00D lens and, as such, do not reflect differences in the oxygen actually reaching the cornea at the centre or the periphery, or the reduced oxygen availability through a positive lens.
Jones is one of the many experts now stating that hypoxic problems have been resolved by silicone hydrogel, 'but it is not just about oxygen', he stated.

'As I get older, I realise just how important stiffness is!' The first generation silicones hydrogels were not well adapted to by many patients because of the high modulus. Bearing in mind that Jones felt that the lens was the first choice for daily as well as extended wear, discomfort is an important problem that needed solving. Twenty per cent of new wearers cease wear within one year and quote discomfort/dryness as the main reason in 55 per cent of cases. In 2004, there were 850,000 new fits and 800,000 existing wearers stopped use.

Other factors are, of course, at play. Lens design is clearly important as, whatever the material, edge thickness may cause discomfort, especially with higher moduli. Furthermore, improving surface wettability and reducing lens deposition is also key. Treatments have started to address this, Acuvue Oasys, for example, having a lower co-efficient of friction than other silicone hydrogels currently available. Group 4 conventional hydrogel materials, such as Acuvue, tend to attract protein (and lipid is not a problem), while Group 2 (non-ionic silicone hydrogel materials) tend to attract lipid. If a silicone lens is noticeably deposited it is likely to be lipid and not protein.

As new lenses become available, Jones suggested that we had a duty to keep our patients, even apparently happy ones, up to date with developments.