Local oxygen transmissibility of disposable contact lenses

Despite global standardisation in measurement of oxygen transmissibility (Dk/t), in our clinic we continue to see limbal vasculature changes in patients wearing some disposable lenses. The use of central Dk/t as a simple designator of a contact lens oxygen performance has clearly failed, it being acknowledged as often unrelated to the oxygen performance of the lens periphery. This study investigates a new criterion for contact lens oxygen transmissibility, the local Dk/t.

The study measured 20 spherical disposable lens brands (power range: -15.00 to +8.00 D) and 8 toric lens brands (power range: -8.00 to +4.00 D sphere, with a -1.00 D/-1.25 D x 180). An electro-mechanical gauge (Heidenhain, Germany) was used to measure lens thickness (t) at different positions on the lens. Oxygen permeability (Dk) of each lens material was calculated from the equation Dk =1.67 e (0.0397 x water content) of Morgan and Efron [Contact Lens Anterior Eye 21 (1998) 3] who used ISO/ANSI standard methodologies. The local Dk/t value was calculated from the point of maximum thickness on the lens and compared to the criterion of 12 Fatt units set by Benjamin [Int. Contact Lens Clin. 23 (1996) 188], to provide corneal oxygenation greater than or comparable to that available in the normal closed eye.

All lens types with a water content over 65% had a local Dk/t of 12 Fatt units or more for most or all lens powers: CIBAVision FocusDailies, Bausch & Lomb Soflens66, CooperVision Actifresh400, and CIBAVision PrecisionUV. In addition, some lenses in the 55-62% water content range had some lens powers with a local Dk/t in excess of 12: CooperVision Proclear, Johnson & Johnson Acuvue 1 and CIBAVision Focus 2 week. None of the disposable toric lenses had a minimum Dk/t of 12 or more. The silicone-hydrogels, Bausch & Lomb PureVision and CIBAVision Focus Night&Day, comfortably exceeded Dk/t of 50 for all lens powers.

Some current disposable soft lenses have local Dk/t values below physiological requirements. Contact lens manufacturers and international standards organisations should consider introducing the labelling of local Dk/t values on lens packaging so that clinicians can make informed prescribing decisions.