PMMA lenses are conventional hard contact lenses made from a transparent rigid plastic material called polymethyl methacrylate (PMMA).
PMMA lenses have excellent optical properties, but they do not allow for oxygen transfer to the cornea. The only way for this vital element to reach the cornea is for tears to wash underneath the lens with each blink. In order for this blink-induced, tear-pumping action to occur, PMMA lenses have to be made relatively small in size.
PMMA lenses have been replaced by gas permeable lenses and are rarely prescribed today.
Gas permeable (GP) contact lenses are rigid lenses made of durable plastic that transmits oxygen. Most GP contact lenses incorporate silicone, which makes them more flexible than PMMA. Silicone is oxygen permeable, so oxygen can pass directly through GP contact lenses to keep the cornea healthy.
They are designed for patients with keratoconus and irregular astigmatism and can be used to correct any type of refractive error. GP contacts are the best choice for individuals for whom soft contacts don’t produce the desired visual acuity.
It usually takes some time for eyes to adjust to gas permeable contact lenses when users first start wearing them.