Modern contact lenses are classified as either hard or soft. Hard contact lenses include PMMA and RGP lenses. PMMA lenses were the first rigid lens material patented in 1934 and have good optical properties but extremely low oxygen permeability. RGP lenses allow oxygen to pass through and are more durable than soft lenses, though initially less comfortable. Soft contact lenses are made of polymers that absorb water, such as poly HEMA, which increases their oxygen permeability and makes them more comfortable and pliable than hard lenses.

1. Contact Lenses (1) Continued
DR. EL-BASHIR EL-SAEED EL-BASHIR
FACULTY OF OPTOMETRY-MAKKA
23 JANUARY 2020

2. Introduction
Modern contact lenses are classified into:
Hard and soft contact lenses.
1. Hard contact lenses: are made of
PMMA (polymethylmethacrylte).
RGP (rigid gas permeable)
Of two types:
- Corneal contact lenses: Has an overall diameter
less than the cornea itself.
- Scleral contact lens: Override the cornea and rest
on sclera.

3. Rigid (hard) lenses
• Polymethylmethacrylate (PMMA):
• Is backbone of all rigid lens materials.
• It was the first rigid lens material which was
patented in 1934.
• It is thermoplastic material (soften with heat).
• It is trade name is Perspex.

4. Properties of PMMA
i. Excellent biocompatibility (inert)
ii. Light
iii. Good optical properties (has outstanding
optical properties).
iv. Do not scratch so easily.
v. Good manufacturing properties (can highly
be abolished).

5. Major drawback
• Even through PMMA is an excellent contact
lens materials as far as physical properties are
concerned, it has a major drawback, that it
has extremely low oxygen permeability.
• This hindered with corneal physiology and has
ultimately made PMMA material an obsolete
for use.
• It produced corneal exhaustion on prolonged
use.

6. PMMA (polymethylmethacrylate)
 1934.
 Good optics.
 Easy to manufacture.
 Very stable.
 Easily wettable.
 Almost zero oxygen (O2).

7. Rigid Gas Permeable Lenses(RGP)
Are made of rigid plastic material and contain no
water. RGP lenses permit oxygen to pass directly
through the lens to the eye. Because they transmit
oxygen, through the material these lenses are
referral to as gas permeable. They are more durable
and resistant to deposits, and generally provide a
crisper vision. They tend to be less expensive over
the life of the lens as they last longer than soft
contact lenses. They are easier to handle and less
likely to tear. However, they are not as comfortable
initially as soft contacts and it may take a few weeks
to get used to wearing RGP lenses.

8. 2. Soft contact lenses :
Are made with a stable, solid polymer component
that can absorb or bind water.
poly HEMA (Hydroxyethylmethacrylate) is used to
manufacture soft hydrogel lens. The lenses are
made in their dry state and then hydrated in saline
solution where they absorb water. The water so
absorbed gives the lens its softness and makes
them comfortable and pliable. Water content also
increases its oxygen permeability. On an average
water content varies from 38-80%. Higher water
content makes a lens less durable.