RGP- Materials & History

Question 1

How did they first make CLs?

Answer 1

Blown Glass and ground glass served as scleral lenses

Question 2

In 1870, John and Issac Hyatt in Albany, NY, were printers who developed the first commercially plastic. What was this plastic?

Answer 2

Celluloid (same stuff in photo films, combs, buttons, records, billiard balls)

Question 3

I don’t know how in depth they want us to know it, obviously Pat taught this section so we have that going for us, but know a general progression of CLs

Answer 3

1931 Acrylics, PMMA
1938 Cellulose Acetate Butyrate (CAB)
1938 Polystyrene
1943 Fluoropolymers
1943 Silicone
   -Definitely know the materials PMMA and silicone

Question 4

What was one of the first signs when wearing PMMA CLs

Answer 4

CCC- central corneal clouding from edema

Question 5

The endothelium responds to low O2. What is the term where the cells change in size?

Answer 5

Polymegathism

Question 6

What is the term where cells change in chape

Answer 6

Pleomorphism

Question 7

What is a general physiologic explanation of how PMMA caused edema?

Answer 7

Hypoxia leads the epithelium to release toxins into underlying tissues. This in turn changes the normal state and draws in water, hence edema.

Question 8

What tissue layer of cornea layer needs the most oxygen?

Answer 8

They said the epithelium demands a lot. The stroma and endothelium require very little O2.

Question 9

What material helps with durability in silicone lenses?

Answer 9

Acrylate

Question 10

Pure silicone lenses have a DK of 340, but there is only one found on the market and is used for pediatric aphakia.

Answer 10

Free card

Question 11

How do they make RGP lenses?

Answer 11

Simply, it takes about 5 days for polymer to harden as it is heated and cooled. It hardens into a rod, from which they cut off ‘buttons’ which are then created into a single lens.

Question 12

Describe some general GP lens properties

Answer 12

Oxygen permeability, surface characteristics (wettability, reactivity), dimensional stability, flexural/fracture resistance, hardness, specific gravity, index of refraction, machining capabilities

Question 13

What is the oxygen transmissibility? Need to know this!

Answer 13

Dk/t

Question 14

What is Dk? t?

Answer 14

Material’s permeability to oxygen. D-diffusion, k- solubility constant for oxygen being adsorbed, t- thickness

Question 15

What is the Dk value of PMMA?

Answer 15

Zero! Good Dk values should be much higher, hopefully up to 100 or more

Question 16

What is diffusion?

Answer 16

Inherent ability of material to allow oxygen to diffuse through holes or voids in the plastic, 7th grade science class review

Question 17

Cool fact, they were able to develop pseudo-blood cells out of a plastic like material that could deliver oxygen to tissue in the body

Answer 17

Did you know that? Now you know…..

Bill, Bill, Bill, Bill

Question 18

We material do we use in clinic that has a Dk value between 30-160?

Answer 18

Fluorosilicone acrylate

Question 19

How do they measure oxygen transmissibility for CLs?

Answer 19

In vitro Polarographic Technique- gold standard for measuring O2 transmissibility

Question 20

They were able to get a rabbit eye (in vivo) and find out how ‘thirsty’ the cornea is for oxygen. How did they do this?

Answer 20

They put a cylinder on the eye (did not previously have a lens) and measured the amount of O2 soaked up by the cornea. They also put a test lens on a rabbit eye, after the lens was removed they measured the rate of oxygen withdrawal and found it significantly faster.

Question 21

When do our corneas normally swell?

Answer 21

At night, our corneas swell up to 3% with no Contact lenses.

Question 22

With night & day contacts, how much on average do corneas swell?

Answer 22

Surprisingly very similar to have no contact lenses on. Only raised by less than 1%. Acuvue 2 lenses made student’s corneas swell up to about 13%.

Question 23

What are two easy to see signs of potential CL abusers?

Answer 23

Limbal Hyperemia and Limbal Redness

Question 24

What is oxygen flux (for our purposes)?

Answer 24

Actual amount of oxygen that diffuses through a contact lens and becomes available to the cornea. It is driven by oxygen availability. 3-7% for a closed lid (through lid vasculature), about 21% for an open lid.

Question 25

There is less of a driving force of oxygen through a contact lens if you’re at higher altitudes (less oxygen in the air).

Answer 25

Free card.

This is also called oxygen tension, 155 mmHg for an open eye at sea level vs 55 mmHg for a closed eye at sea level

Question 26

There was a number given for general RGP lens thickness when discussing GP lens wetting, do you remember this thickness?

Answer 26

100 microns

Question 27

What can happen if there’s not enough mucin in the tear film?

Answer 27

Filamentary Keratitis

Question 28

How do you check ‘wetting’ of a lens?

Answer 28

Put wetting drops on the lens to see if it is soaked up or runs off.

Question 29

Like some other questions, this was written in the slides but wasn’t highlighted in class. What are others ways to measure material wetting properties?

Answer 29

Sissile Drop, Captive Bubble, Wilhelmy Plate

Question 30

What is Plasma Treatment Surface Modification?

Answer 30

Electron ejected from oxygen to bombard the contact lens surface. The reaction of gas and the surface silicon molecules cleans the lens and alters the hydrophobic silicon surface. It makes it MORE HYDROPHILIC.

Question 31

What is HydraPEG?

Answer 31

Surface treatment for CLs, permanently bonded and makes a mucin-like hydrophilic shell to improve surface wettability and deposit resistance.

Question 32

How do you get rid of scratches on RGP lenses?

Answer 32

You can polish them, careful not to change the power

Question 33

Specific gravities of CLs range from 1.15-1.27

Answer 33

Free Card

Question 34

How far does the eyelid approximately travel per year?

Answer 34

46.5 miles, here to Portland

Question 35

How often do we blink on average? In a year?

Answer 35

12.55 times, over 4 million

Question 36

Distance traveled per blink?

Answer 36

8.5 mm, I assume this is an average palpebral fissure.