RGP- Materials & History Flashcards
How did they first make CLs?
Blown Glass and ground glass served as scleral lenses
In 1870, John and Issac Hyatt in Albany, NY, were printers who developed the first commercially plastic. What was this plastic?
Celluloid (same stuff in photo films, combs, buttons, records, billiard balls)
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
1931 Acrylics, PMMA 1938 Cellulose Acetate Butyrate (CAB) 1938 Polystyrene 1943 Fluoropolymers 1943 Silicone -Definitely know the materials PMMA and silicone
What was one of the first signs when wearing PMMA CLs
CCC- central corneal clouding from edema
The endothelium responds to low O2. What is the term where the cells change in size?
Polymegathism
What is the term where cells change in chape
Pleomorphism
What is a general physiologic explanation of how PMMA caused edema?
Hypoxia leads the epithelium to release toxins into underlying tissues. This in turn changes the normal state and draws in water, hence edema.
What tissue layer of cornea layer needs the most oxygen?
They said the epithelium demands a lot. The stroma and endothelium require very little O2.
What material helps with durability in silicone lenses?
Acrylate
Pure silicone lenses have a DK of 340, but there is only one found on the market and is used for pediatric aphakia.
Free card
How do they make RGP lenses?
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.
Describe some general GP lens properties
Oxygen permeability, surface characteristics (wettability, reactivity), dimensional stability, flexural/fracture resistance, hardness, specific gravity, index of refraction, machining capabilities
What is the oxygen transmissibility? Need to know this!
Dk/t
What is Dk? t?
Material’s permeability to oxygen. D-diffusion, k- solubility constant for oxygen being adsorbed, t- thickness
What is the Dk value of PMMA?
Zero! Good Dk values should be much higher, hopefully up to 100 or more
What is diffusion?
Inherent ability of material to allow oxygen to diffuse through holes or voids in the plastic, 7th grade science class review
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
Did you know that? Now you know…..
Bill, Bill, Bill, Bill
We material do we use in clinic that has a Dk value between 30-160?
Fluorosilicone acrylate
How do they measure oxygen transmissibility for CLs?
In vitro Polarographic Technique- gold standard for measuring O2 transmissibility
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?
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.
When do our corneas normally swell?
At night, our corneas swell up to 3% with no Contact lenses.
With night & day contacts, how much on average do corneas swell?
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%.
What are two easy to see signs of potential CL abusers?
Limbal Hyperemia and Limbal Redness
What is oxygen flux (for our purposes)?
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.
There is less of a driving force of oxygen through a contact lens if you’re at higher altitudes (less oxygen in the air).
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
There was a number given for general RGP lens thickness when discussing GP lens wetting, do you remember this thickness?
100 microns
What can happen if there’s not enough mucin in the tear film?
Filamentary Keratitis
How do you check ‘wetting’ of a lens?
Put wetting drops on the lens to see if it is soaked up or runs off.
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?
Sissile Drop, Captive Bubble, Wilhelmy Plate
What is Plasma Treatment Surface Modification?
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.
What is HydraPEG?
Surface treatment for CLs, permanently bonded and makes a mucin-like hydrophilic shell to improve surface wettability and deposit resistance.
How do you get rid of scratches on RGP lenses?
You can polish them, careful not to change the power
Specific gravities of CLs range from 1.15-1.27
Free Card
How far does the eyelid approximately travel per year?
46.5 miles, here to Portland
How often do we blink on average? In a year?
12.55 times, over 4 million
Distance traveled per blink?
8.5 mm, I assume this is an average palpebral fissure.
