To determine and generate oxygen transmissibility (Dk/t) profile maps across various contact lenses in different powers.
Lens thickness measurement and Dk/t measurement and calculation of material-specific Dk were performed on five silicone hydrogel lenses (PureVision, Acuvue Oasys, Acuvue Advance, Focus Night and Day and O2Optix) in two lens powers (-3.00DS and +3.00DS), where available. A purpose built soft lens profilometer was used to measure the radial thickness profiles of 5 lenses for each type and power in 4 meridians. The 20 meridians from the same lens type were all averaged to obtain a single mean radial thickness profile for each lens type and power. Dk measurements were taken using the Dk1000 coulometric oxygen apparatus (JDF Company, GA, USA). From the mean harmonic thickness for the optical zone (8 mm diameter) and peripheral area (8 to 14.2mm), the mean Dk/t was calculated.
Transmissibility profiles for each lens were calculated using the measured radial thicknesses and oxygen permeabilities. Colour coded plane view and 3-D maps were generated, in which elevation corresponds with Dk/t to illustrate differences between lens types and designs, as well as across the power range. For the –3.00DS Acuvue OASYS lens Dk/t dropped from 153 at the optical zone to 85 in the periphery. However, FND lens maintained a relative constant Dk/t of 174 to 151 respectively. PureVision lenses showed the greatest difference in peripheral Dk/t, 68 for the -3.00DS and 114 for the +3.00DS lens. In contrast, the peripheral Dk/t for Acuvue Advance is consistent (48) for both lens powers.
Contact lenses made from the same material in different shapes supply different amounts of oxygen to the cornea. Dk/t based on centre thickness of a -3.00D lens is unrepresentative of physiological performance. A more relevant measure is to calculate the area-weighted mean transmissibility. Practitioners can assess lens oxygen supply by referring to average Dk/t across the optical or the peripheral zone.