Test 1 Review Flashcards
What is the formula for mm to diopter conversion?
D = 337.5 / mm
What is the average HVID?
11.7mm
What is the average VVID?
10.6mm
What is average CCT?
0.52mm
What is average limbal corneal thickness?
0.67mm
What is the average central radius of the cornea?
7.8mm
What is the average sagittal depth of the cornea?
2.6mm
What is the average index of refraction of the cornea?
n = 1.376
What percentage of the cornea is made up of water?
78%
How thick is the corneal epithelium?
50 microns
What ensures the transparency of the stroma?
The lamellar arrangement
Which lenses are made via lathing or molding?
GP
Which lenses are made via cast-molding or spin casting?
SCL
What are some advantages to lathing GP lenses?
Almost unlimited designs
Good for custom lenses
What is the GP manufacturing method of choice?
Lathing
What are some disadvantages to lathing GP lenses?
Slow production
High cost per lens
Labor intensive
What is plasma treatment?
A “super cleaning” using room temperature, ionized oxygen and UV energy to break organic bonds
Which lenses can be plasma treated?
GP
How long does plasma treatment last?
Few days, to months
What is the purpose of plasma treatment?
To increase wettability
What is Dk?
Oxygen permeability
What is the “D” in Dk
Diffusion coefficient
What is “k” in Dk?
Solubility of the gas in the material
What is Dk/t?
Dk value / thickness
What is the necessary Dk/t for DW lenses under open eye conditions?
24
What is the necessary Dk/t for EW lenses under open eye conditions?
87
What is the necessary Dk/t for DW lenses under closed eye conditions?
35
What is the necessary Dk/t for EW lenses under closed eye conditions?
125
What is the required EOP for DW lenses?
9.9%
What is the required EOP for EW lenses?
17.9%
What removes lipids from CLs?
Isopropyl alcohol
Surfactants
What removes proteins from CLs?
Proteolytic enzymes
Surfactants
Proteins are attracted to which type of CL?
Ionic lenses
Lipids are attracted to which type of CL?
Non-ionic lenses
What is the usual percentage of Hydrogen Peroxide in peroxide based cleaners?
3%
How long do lenses need to be left in peroxide to kill acanthomoeba?
3-6 hours
Do multipurpose solutions sterilize lenses?
No - they disinfect and preserve
What is the main difference between the Andrasko and IER staining studies?
Andrasko was measured after 2 hours of exposure
IER was 3 months worth of data
Which type of soft CL is prone to lipid deposits?
Si-Hy
Si-hy lenses are prone to lipid deposits, and what else?
Mucin balls
What damage do mucin balls do to the cornea?
Epithelial dimples, causing NaFl pooling, but not staining
What is PMMA, and some of it’s properties?
Hard lens materia
No Oxygen transmission
Old material
What are the two old, traditional Hard lens materials?
PMMA
CAB
What are the 4 common hard lens materials?
PMMA
CAB
SA
FSA
What is the “standard” soft lens material?
HEMA
What is EGDMA and its properties??
A crosslinking agent for soft lenses that stiffens material and lowers the water content
What does GMA do?
Increases wettability and deposit resistance in SCLs
Why does GMA increase deposit resistance?
Smaller pore size
What does MMA do?
Decreases water content
Increases strength of lens
No Oxygen transmission
What does PVA do?
Increases water content
Increases resistance to deposits
Increases Dk
Increases Strength
What are the characteristics of MAA and NVP?
Increase water content
Very hydrophilic
Ionic
What is the water content of hydrogel lenses?
80%
What is the water content of Si-hy lenses?
24%
Higher water content usually means ___Dk value.
Lower
What is the “stem” for the hydrogel family?
Filcon
What group are Si-Hy lenses in?
Group V
What group are hydrogel lenses in?
Group IV
What is the “stem” of non hydrogel lenses?
Focon
What is the average lens diameter of a soft lens?
12-14.5
What is the average diameter of a GP lens?
9-10mm
How do you calculate the optic zone diameter of a GP lens?
OAD - (2 x peripheral curve widths)
If you have a high plus lens, and you make the diameter bigger, what happens to the center of gravity?
It pushes forward
If you have a high plus lens, and you make the diameter smaller, what happens to the center of gravity?
It moves backward
If you have a high minus lens, and you increase the diameter, what happens to the center of gravity?
It moves backward
Why is a minus carrier used?
To reduce center thickness in a high plus lens
Why is a plus carrier used?
To reduce edge thickness of high minus lenses
Describe Lipcof grade 1
1 small conj fold, smaller than the tear meniscus
Mild dry eye
Describe Lipcof grade 2
Conj fold up to the height of the tear meniscus
Moderate dry eye
Describe Lipcof grade 3
Fold of greater height than the tear meniscus, or multiple folds
Severe dry eye
What is normal for Phenol Red Thread test?
Greater than 20mm in 15 seconds
When a GP is placed on a toric cornea, how much astigmatism is left?
10%
A flat fitting GP gives what lacrimal lens?
minus
A steep fitting GP gives what lacrimal lens?
Plus
If your SCL is decentered, what should you do to make it fit better?
Use a steeper BCR
How long does a SCL need to “settle” in the eye?
5 minutes
What is the average HVID?
11.7mm
What is the average VVID?
10.6
What is the approximate water content of the cornea?
78%
Your patient wears +2.00D SCLs. The Va is decreased. In over the lens keratometry, the mires are irregular. What is the most likely reason?
Lenses are inside out
Why do we use n=1.3375 for the cornea in calculations?
We consider the cornea to be a single surface for calculations
According to Holden and Mertz, a dialy wear contact lens should have at least what Dk/t value?
24
According to Holden and Mertz, an extended wear contact lenst should have at least what Dk/t value?
87
What do sessile drop and captive bubble measure?
Wetting angle
Which is a way to in-vivo test CL wettability?
Break-up time
What is the role of an enzymatic cleaner?
Removes proteinaceous layer from a CL surface
What material is added to a CL material to improve oxygen permeability?
Silicone
What is a component added to a GP lens to improve wettability?
Fluroine
Which study is critical for MPS products made by Bausch and Lomb?
Andrasko
What is the key difference between the methodologies of the Andrasko and IER studies?
Duration of time preceding assessment
What sequesters contaminants through formation of micelles?
Surfactant
What has a main role of inhibiting microbial growth?
Preservative
Which MPS additive contains mercury and is known to be toxic?
Thimerosal
The killing and/or the removal of some or all resident pathogenic organisms is called___.
Disinfection
What is the typical GP lens diameter?
9-10mm
A GP lens fitted steeper than the flat K will result in what power lacrimal lens?
Positive
What is the purpose of prism in a CL?
Stabilize cyl power
What are Bennet’s classification of CL Dk values?
Low: 25-50
High: 51-99
Hyper: 100+
What was the first SCL material?
HEMA
What is the water content of a HEMA polymer?
38%
What is the water content of a Si-Hy lens?
24%
What does PVA do as a CL material?
Increases water content
Increases resistance to deposits
Increases Dk
Increases hardness and strength
What does MMA do as a CL material?
Lowers water content
Increases hardness and strength
Has NO oxygen permeability
What are the two core components of modern Silcone hydrogel lenses?
Si-O
MMA
In hydrogel lenses, water content and Dk are usually ___ related.
Directly
Most silicone hydrogel lenses are ___ water content, ___Dk.
Low water
High Dk
CAB, FSA and PMMA are all materials for which type of CL?
GP
What is the maximum acceptable corneal swelling induced by a CL?
4%
What is the approximate atmospheric percentage present at the anterior cornea in an open eye condition?
21%
What is the approximate atmospheric pressure present at the anterior cornea in an open eye situation?
747mmHg
What is the approximate partial oxygen pressure present at the anterior cornea in an open eye situation?
155mmHg
What is the approximate partial oxygen pressure present at the posterior cornea in an open eye situation?
55mmHg
What is the approximate partial oxygen pressure present at the anterior cornea in a closed eye condition?
55mmHg
What is the primary source of oxygen for the corneal epithelium in a closed eye condition?
Palpebral Conjunctiva
A modern GP lens from FSA is most commonly made how?
Lathing
The power of a lens stated on the packaging is mostly likely stated as what?
Back vertex power
A GP lens change of 0.1mm equal how many diopters?
0.50
Which curve is the steepest in a tetra-curve GP lens?
BCR
What is the typical center thickness of GP lenses?
0.2mm
Decreasing OAD in a minus power GP lens will do what to the center of gravity?
Move it forward
Evaluating the amount of fluorescein under the edge of a GP lens is called what?
Edge clearance
Decreasing OAD in a high minus power GP lens will do what?
Reduce edge thickness
What kind of lag is expected with SCLs?
1mm
According to Young, which test is most predictive in assessing a hydrogel lens fit?
Lower lid push up test
When a hydrogel lens undergoes dehydration on the eye, what happens to the fit?
The lens tightens and moves less
How much corneal astigmatism does a spherical soft lens compensate?
0%
