Intro to contact Lenses Flashcards
What are the different types of contact lenses?
Rigid Gas Permeable Soft Hybrids Silicone Rubber Lenses (SRCL)
Are rigid gas permeable lenses comfortable?
No they are definitely not as comfortable as soft lenses
Why are Rigid Gas permeable lenses still used if they are uncomfortable?
Myopic control –> Orthokeratology refers to the use of gas-permeable contact lenses that temporarily reshape the cornea to reduce refractive errors such as myopia.
What were the first contact lenses made from and why was this bad?
The first contact lenses were made from PMMA (Polymethylmethacrylate) - essentially glass. (They were smaller than the cornea) This was bad as glass doesn’t let air through and so hypoxia would occur thus corneal swelling and neovascularisation.
How does the size of an RGP lens relate to comfortability and why?
The bigger the RGP lens, the more comfortable it is. Small lenses move around a lot causing discomfort whereas, large lenses hug the eye staying in place.
What are the rough sizes RGP lenses come in?
Smaller than the corneal diameter Semi-scleral (so just bigger than the cornea) Scleral so border clearly exists in sclera
What are the two materials soft lenses are made out of?
Hydrogel (Hy) and Silicone hydrogel (SiH)
Why is Hydrogel a good material for soft contact lenses?
It is Hydrophilic. It has different water contents - it is the water that lets the oxygen through. The more water content you have the more oxygen can get through.
What is a disadvantage to Hydrogel as a material for soft contact lenses?
Although a high water content means more oxygen it also means more deposits are attracted to the lens. This makes it uncomfortable for the patient.
What is Silicone Hydrogel (SiH)?
A mixture of silicone rubber (it is a hydrophobic material) and Hydrophilic materials.
What is an Advantage of using Silicone hydrogel over Hydrogel as a material for soft contact lenses?
The silicone rubber in Silicone hydrogel has a high oxygen permeability (even though its hydrophobic) this paired with the hydrogel components means an increased oxygen levels reach the cornea as a result Silicone hydrogel lenses can be worn for longer!
What is the typical size of Soft contact lenses?
Larger than the corneal diameter
What is an advantage of RGP lenses?
They provide very good vision And cannot take up bacteria (thus are healthier for the eye (surface may contain bacteria but they can’t take it up)).
What is the structure of Hybrid lenses?
There is Rigid Centre (RGP) and Soft skirt attached.
What is the advantage of a hybrid lens?
The rigid centre (RGP) provides good vision whilst the soft skirt provides comfort.
What is a disadvantage of Hybrid Lenses?
They are more expensive
What does it mean to be therapeutic?
Relating to the healing of disease.
What is an advantage of Silicone Rubber Lenses (SRCL)?
They have an EXTREMELY high oxygen Permeability.
When are Silicone Rubber Lenses used?
Therapeutically only e.g. for those with Aphakia, corneal perforation, dry eyes, babies/children.
Why are Silicone Rubber Lenses only used therapeutically?
Because they are very expensive.
What is Aphakia?
Aphakia is the absence of the crystalline lens of the eye
What are indications and motivations in regards to contact lenses?
Indications are what we the optometrist think the patient needs. Motivations is what the patient wants (i.e. cosmetic reasons).
What are common motivations for contact lenses?
Cosmetic reasons - people don’t like the look of glasses Convenience Refractive/Pathological condition Expectations and Preconceived Ideas about Contact Lenses
What are some indications for contact lenses?
Refractive (Prescription) Pathological condition in which they may need them Cosmetic (coloured contact lenses) Sports (Contact sports or ones with fast head movement) Occupational reasons (e.g. sportsperson, armed forces, entertainment industry).
For myopes is visual acuity improved or decreased when comparing contacts to glasses?
Contact lenses provide a higher visual acuity for myopes than glasses
What is the benefit of contact lenses for myopes?
Improved acuity when compared to glasses Greater Field of View They can provide myopic control
What are the benefits of contact lenses for Hyperopes?
Greater Field of view
Why may acuity be decreased (when compared to glasses) for hyperopes when contact lenses are worn?
There is a possibility that the decreased acuity is due to decreased magnification and fluctuating accommodation.
Why is acuity for astigmatic patients reduced (compared to that in use of glasses) when wearing contact lenses?
Contact lenses move around very slightly but they still move. This changes the axis of the Cyl.
In what cases of astigmatism may contact lenses be more successful at providing acuity equal to or similar to that provided by glasses?
If the patient is Myopic and if the DC< DS (cyl is less than sphere).