New siloxane hydrogel contact lenses
have enabled considerable leaps in the oxygen transmission
capabilities of soft contact lenses. However, the numbers
cited for oxygen transmissibility are calculated from the
central thickness of a -3.00D lens, which gives the highest
number. This does not take into consideration the transmissibility
across the rest of the lens, which is dependent on its design.
Silicone hydrogel lenses, and for comparison, a widely
used ordinary hydrogel lens, were sectioned and the thickness
measured over the diameter of the lenses using a high resolution
microscopic measurement technique previously described1.
The thicknesses were used to profile the oxygen transmissibility
perpendicular to the posteriour surface of the lenses across
a section of the lens, and this is presented graphically.
The mean of these values, based on harmonic mean lens thickness2
and the manufacturers' cited Dk, were 139.1, 74.8, and 22.2.
x 10-9 barrers/cm for the two lenses tested and an ordinary
soft lens control respectively. The maximum transmissibility
at the thinnest part of the lens was 191.8, 101.5, and 32.7,
x 10-9 barrers/cm respectively.
Oxygen transmission with the new siloxane hydrogel lenses
have been increased by a maximum factor of approximately
six times over ordinary hydrogel lenses for both the central
and mean values, making it possible to satisfy even the
most stringent estimates of the corneal oxygen requirements
for contact lenses during sleep.