Chapter 9 - Contact lenses I (1) Flashcards
What are the different types of contact lenses?
1- Rigid corneal lenses (RCL)
2- Soft contact lenses
3- Speciality lenses- either made up of rigid or soft contact lenses
What are the properties of rigid corneal lenses?
1- Approx 9mm in diameter
2- Reusable (1-3 years)
3- Clean after each wear
What are the properties of soft contact lenses (SCL)?
1- Approx 14 mm in diameter
2- Disposable or reusable- can last a day or can be used for 2 weeks, 6-12 months, etc
What are the properties of specialty lenses?
1- You can have hybrid lenses- can be a combination of RCL and SCL
2- You can have ortho-keratology lenses- ortho keratology clinical procedure used in contact lenses to try and correct someone’s prescription.
How do ortho-keratology lenses work?
This involves putting a lens in and wearing it overnight.
By wearing that lens overnight, the front of the eye reshapes in order to fit that lens.
So when they then take the lens out, the shape of the cornea changes and by it changing, it provides a prescription correction.
What is the disadvantage of the way ortho-keratology lenses work?
These lenses don’t change the shape permanently and will go back to its normal shape over the next 48 hours.
So the patient has to wear these lenses every night in order for this to be effective.
Rigid gas permeable lenses (RGP) is the same as…
Rigid corneal lenses (RCL)
What is modalities?
Modalities means how often a contact lens is replaced.
This is how often soft contact lenses can be replaced:
1- Daily disposables
2- 2 weekly
3- Monthly disposables
4- Annually
How often can RCL be replaced?
1- Replacement every 1-3 years
What is wear modality?
How the person wears the lens.
For soft contact lenses, you can have three types of wear modality:
1- Daily wear modality- wear them during the day but not at night
2- Extended wear lenses- can be worn for up to 7 days and 6 nights
3- Continuous wear lenses- can be worn for up to 30 days and 29 nights.
For RCL, the wear modality is…
Typically worn as daily wear modality
What are the types of contact lens prescriptions?
1- Spherical
2- Toric
3- Multifocal
The materials in soft lenses can be split into two types:
1- Hydrogel (Hy)- a soft polymer which contains water and transports oxygen
- Very comfortable
2- Silicone hydrogel- where the polymer material has silicone associated with it
- Allows for more oxygen than Hy
- Allows for people to sleep in lenses
- More expensive
The materials in rigid lenses can be split into two types:
1- PMMA (polymethyl methacrylate)- No oxygen permeability which is not healthy for the eye
- Provides excellent vision
2- Gas permeable- much better oxygen permeability and much healthier.
What are the baseline measurements that you have to measure for a contact lens fitting?
1- Spectacle prescription
2- Measure curvature of the cornea
3- Measure diameter of cornea
What is the air gap?
The gap between the glasses and the eye.
When we get rid of the air gap, the power of the lens…
doesn’t have to be the same to focus the light into the right place on the back of the eye.
If there is a lack of air gap, what do you have to change in order to get that same focusing effect?
We must change the number of the lens that’s on the eye
What is this?
A table which is used to convert spectacle Rx into an ocular Rx which is needed for contacts.
If the spectacle Rx is below +4.00DS, then you keep it the same. So +3.00DS will have the ocular Rx of +3.00DS.
If the corresponding value is like -6.12, round it to the nearest 0.25 which will be -6.00
What is astigmatism?
Astigmatism is the imperfection in the curvature of the ocular cornea and/ or the crystalline lens.
How would you correct astigmatism?
You use two different prescriptions in two different meridians.
What is regular astigmatism?
Where the two meridians are 90 degrees apart
E.g, One meridian is 10 degrees which means the other meridian will be 100 degrees.
What is irregular astigmatism?
Where the meridians are not 90 degrees apart. This could be due to eye disease, trauma, etc.
