Basic Contact Lens Flashcards
USANC stands for what?
United States adopted names council
What is the purpose for USANC (United States adopted names council)?
Selects simple, informative, and unique nonproprietary names for drugs by establishing logical nomenclature classifications based on pharmacological and/or chemical relationships
Implies the contact lens is in harmony with the eye and has little or no adverse effects
Biocompatibility
Ease of manufacture includes what? (10 things)
Material must be easy to manufacture
Be inert
Be stress-free, dimensionally stable and durable
Material must be optically homogeneous - unless such variation is intentional (ex: Bifocal and DT1)
Material must be biocompatible: no leachable unreacted chemical components
Be easy to polish/retain surface finish
Have predictable hydration characteristics
Exhibit low friction in SItu
Not induce inflammatory or immunological responses
UV absorbers may cause NaFl pattern detail to be less visible
Filter that will enhance the NaFl pattern
Written 12 yellow filter
Does having lower water content make the lens more rigid or less rigid? Is this a good or bad thing?
More rigid, easier to handle
Advantage
Low water content advantages
Less susceptible to environment influences More rigid, easier to handle Higher refractive index = thinner Any lens care product can be used Ease of manufacture Greater reproducibility More wettable
If a lens has low water content, what care products may be used on it?
Any can be used
If a lens has a higher refractive index, is it thicker or thinner, and does it have low or high water content?
Thinner
Low water content
Low water content disadvantages
Low Dk (have to be made thinner > oxygen) Less flexible (comfort concerns?) Thin lenses are difficult to handle
High water content advantages
Higher Dk
More flexible
Faster restoration of shape following deformation
High water content disadvantages
Fragile More deposit prone - ionic, pore size More susceptible to the environment Lower refractive index Less stable parameters, lower reproducibility More difficult to manufacture
If a lens has a lower refractive index, does it have a lower or higher water content? Is this a good thing or a bad thing?
Higher water content
Disadvantage
Ionic materials have what charge on the surface?
Net negative charge on the surface
Non-ionic materials have what charge on the surface?
Still have charged sites within polymer…however, the charges are internal to the matrix = no net surface charge
Advantages to having ionic materials
More wettable
Disadvantages to having ionic materials
Deposit more readily
More susceptible to pH changes
Advantages to having non-ionic materials
Less deposit prone
Do not bind charged particles
Less susceptible to pH changes
Disadvantages to having non-ionic materials
Less wettable
Material properties (7)
Oxygen permeability Wettability Flexibility (SCL’s) Rigidity (GP’s) Scratch resistance Durability (stability, wear and tear) Deposit resistance
ANSI Standard for ophthalmics - contact lenses - standard terminology, tolerances, measurements and physicochemical properties
ANSI Z80.20-2016
ANSI Z80.20-2016
Applies to contact lenses worn over the front surface of the eye in contact with the preocular tear film. The standard covers rigid intracorneal and haptic (scleral) contact lenses, as well as soft paralimbal contact lenses
What is the role of oxygen in the cornea, and where does it mainly come from?
Oxygen to maintain corneal integrity and mostly from the atmosphere
Some oxygen is derived from the blood supply (peripheral) and aqueous humor (especially for closed-eye)
Atmospheric oxygen
~21% or 155 mmHg
Closed eye oxygen
~1/3 or 55 mmHg
What increases oxygen demand in the eye?
Eye temperature and
Tear pH
Percentage of overnight corneal swelling due to closed eye lack of oxygen
4%
Minimum oxygen goal for contact lens wear
Swelling no greater than normal without a CL - Eye open or closed
Low Dk/T corneal changes involves low or high oxygen transmissibility
Low oxygen transmissibility
Low oxygen transmissibility can result in these corneal changes (low Dk/T)
Microcysts Polymegethism (endothelium) Corneal pH Edema (stroma) Blebs
Disorganized cellular growth, necrotic tissue and cellular debris that accumulates btw cells
Epithelial microcysts
What may microcysts do?
Migrate and rupture, causing discomfort and create a portal for infection and/or ulceration
With an increased Dk/l, do microcysts increase or decrease?
Decrease
What do you use to check for polymegethism?
Speculation microscopy
Polymegethism
Decreased pH
Hypoxia
EW is worse
Hydrogel vs SiHy’s or GP’s
Is polymegethism reversible?
Some people think so, but typically not
Presentation of polymegethism on speculation microscopy
Larger, fewer cells
Chronic condition in endothelial architecture
What is the cornea’s interaction with carbon dioxide?
The cornea is highly permeable to carbon dioxide
Dk of CO2
About 7 times Dk of oxygen
Why is it necessary for CO2 Dk to be so much higher than O2 Dk?
To resist pH and metabolic changes in the cornea
An acidic shift due to CO2 buildup as a consequence of epithelial hypoxia and an increase in lactic acid
pH Shift
