Tvrda gas-propusna (RGP) kontaktna sočiva Koncepcija

Slide 46
Debljina sočiva - Razmatranja
• Savitljivost “na oku”
• Korekcija kornealnog astigmatizma
• Dk/t
Rigid Gas Permeable Contact Lenses Design
Lens Thickness - Considerations:
‘On-eye’ lens flexure.
A lens needs to be thick enough to resist flexure while being thin enough to deliver the
potential physiological benefits inherent in higher Dk materials.
Some materials show a direct relationship between Dk and flexibility, i.e. the higher the
Dk the greater the flexibility. This often means in practice that any gains from increasing
the Dk are offset by the need for a thicker lens having an acceptable level of flexure, i.e.
Dk/t does not increase by as much as is suggested by the increase in the Dk of the
material used. However, Cornish et al ., (1991) found that over the Dk range 0 – 140, Dk
and flexibility were independent properties. Therefore, at least for the materials tested,
the oxygen transmissibility can be increased without the need to increase lens thickness
to maintain the level of lens rigidity desired.
Correction of astigmatism.
The dominant use of RGPs in many practices is for the correction of corneal astigmatism
with a spherical lens. If the lens flexes on an astigmatic cornea, then less than the 90% of
astigmatism (which is normally corrected by such lenses) will be corrected.
Dk/t.
Since Dk/t is a measure of the oxygen transmission through the lens, t is an important
determinant of oxygen availability especially for low to moderate Dk materials.
Debljina sočiva – Razmatranja:
Savitljivost “na oku”.
Sočivo treba da bude dovoljno debelo da se odupire savijanju na oku, ali i dovoljno tanka
da daju potencijalne fiziološke pogodnosti koje su nerazdvojive sa materijalima sa
visokim Dk.