Unit 6: Contact Lenses Flashcards
Orthokeratology
Gas permeable lenses worn while sleeping to reshape the cornea
Hard contact lens materials
-PMMA (Polymethylmethacrylate)
-GP (Gas permeable)
Soft contact lens materials
-hydrogel
-Silicone hydrogel
Characteristics of PMMA lenses
-worn by less than onepercent of contact lens wearers
-Very durable but not oxygen permeable
-Small diameter to allow cornea to breathe
Characteristics of GP lenses
-Less than 10% of contact lens wearers
-Vision is better than with soft lenses
-Last longer
-Less comfortable than soft lenses
-Can’t be used for part time where
Characteristics of HEMA lenses
-Worn by 90% of contact lens wearers
-Can be worn occasionally
-More comfortable and stable
-Less durable and more expensive
-Vision not as good
-Prone to deposits
Characteristics of silicone hydrogel lenses
-All soft contact characteristics
-More expensive
-Easier to handle
-Reduced lens wettability
Advantages of hybrid lenses
-Improved comfort over hard lenses
-Lower cost than scleral lenses
-Easier to remove than scleral
-More stable than soft toric lenses
Disadvantages of hybrid lenses
-More expensive than soft or hard lenses
-More chair time to fit
-Limited ability to customize
-Insertion is similar to scleral
Contact lens modalities
-Single use
-Two week
-Daily or extended wear
-Monthly
-Daily or extended wear
-Quarterly or yearly
-Daily wear only
Types of ballast for toric lenses
-prism ballast
-Peri ballast
-Thin zone
What is monovision
-Two different lenses, distance power in one by, near power in the other one
-Good for high astigmatism
-Compromises depth perception
What are lenticular lenses
-Used for aphakic patients
-Sharply defined transition between carrier and prescription zone
Eight contact lens fitting parameters
-Base curve
-Diameter
-Center thickness
-Edge design
-Oxygen transmissibility
-Water content
-Sagittal depth
-Wetting angle
Base curve
-Curvature of the posterior surface
-the HIGHER the number, the FLATTER the lens
Average diameter smallest to largest
-PMMA (7.5-8.8mm)
-GP (8.0-9.6mm)
-Soft (13.0-15.0mm)
-Hybrid (14.5mm)
-Scleral (15.0-25.0mm)
How far away from the limbus should a soft contact lens be
2mm
HVID
Horizontal visible iris diameter
Center thickness
-Distance from the front surface of the lens to the back surface
-Hi plus we’ll have a thicker centre
Dk
The oxygen permeability of the lens
D=The diffusion coefficient for oxygen movement in the material
k=The solubility constant
Dk/t
The oxygen transmissibility of the lens
t=Centre thickness
Three categories of water contacts and their percentages
High- 60%
Medium- 40-60%
Low- <40%
What happens to lens characteristics as the water content increases
-More oxygen
-Lower stiffness
-Higher evaporation, decreasing comfort
-More fragile
Sagittal depth
The distance from the centre of the lens to surface it’s sitting on
Wetting angle
-The angle that the edge of a bead of water makes with the surface of the contact lens
-The smaller the angle the better the wettability
Difference between cleaning and disinfecting
Cleaning removes debris and deposits
Disinfecting kills bacteria and pathogens
Four methods of contact lens care systems
-Chemical
-Hydrogen peroxide
-Thermal
-Ultraviolet
Parameters to determine if a patient is a good contact lens candidate
-Motivation
-Occupation
-Lifestyle
-Ocular history
-General health
-Medications
-Age/ maturity
Questions to ask a current contact lens wearer
-How long have they been wearing CL?
-How long since last CL check?
-How often replace lenses?
-How old is current pair?
-What is typical wear time?
-How often do they sleep with lenses on?
-What solution do they use?
Vertex distance
The distance of the lands from the surface of the eyeball
-Closer adds minus power
-Further adds plus power
(So CL must have more plus power)
Range of average K values
38.00D-46.00D
Higher than 48D is suspicious for keratoconus
Average corneal diameter
11.8mm
Average soft contact lens diameter
14.5 mm
Monovision
Two different contact lenses, distance power on dominant eye, near power on non-dominant eye
Modified monovision
Single vision distance power on dominant eye, multifocal lens on non-dominant eye. 
Some CL insertion methods
-Directly on cornea
-On sclera then slide onto cornea
-Slide under upper eyelid for a small aperture

Some CL removal methods
-Pinch off
-J stroke
-Scissors versus blink
Possible lens complications
-Chipped cracked or torn lens
-Protein deposits
-Scratched GP lens
-Mucus film
-Lens discolouration
GPC
Giant papillary conjunctivitis
CLARE
Contact lens acute red eye
SPK
Superficial punctate keratitis
