Unit 5: Rgp Review Flashcards
Up to what amount of a stigmatism should we consider a spherical lens?
2.50D
How do we know if astigmatism is corneal?
Find RA (Rx cyl)-(K cyl). If it’s within half a diopter, it can probably be neutralized by the tear layer
When is a front surface toric lens used?
There is over 1.25 of Rx cylinder correction with a spherical or near spherical cornea.
When is a back surface toric lens used?
When corneal astigmatism is approximately 2/3 of the refractive astigmatism
(=0.666 of Rx power or lower)
What are two designs of bitoric lenses?
Spherical power effect
Cylindrical power effect
What is an SPE bitoric?
Spherical power effect
-optics of a sphere when combined with tear lens
What does an SPE bitoric correct?
Corrects only induced cylinder. Does not correct residual astigmatism.
How does an SPE bitoric work?
Plus cylinder is ground on the front surface, which cancels out the induced cylinder power created by the tear lens and the posterior curve.
What is a CPE bitoric?
Cylindrical power effect toric
When do we use a CPE bitoric?
When a spherical power effect bitoric results in significant cylindrical over refraction.
SPE or CPE: which one needs to be rotationally stable?
CPE, To prevent inducing further astigmatism
What is a saddle fit?
-on k in each meridian
-allows for better alignment with overall toric corneal shape
-tighter fit, more centered
-not used often
What is a low toric simulation?
-Fit flat Meridian on K and steep Meridian 0.75 flatter than steep k
-More common fitting philosophy
-Provides stability
How to solve a prescription for low toric simulation?
-Flat k is unchanged
-Vertex meridians
-Add 0.75 to the steepest Meridian
What is the name of the fit factor guide?
Mandell-Moore
Why would you choose GP multifocals over soft multifocals
-Better optical correction.
– corrects corneal astigmatism.
– resists deposits better than soft contacts.
– more stable with blink
– often more comfortable for dry eye patients
What are two kinds of simultaneous multifocals?
Aspheric
-gradual change in power from distance to near
Concentric
-Individual rings of distance and near
How does a translating GP work?
There is an area for near at the bottom of the lens, and when the patient looks down, the lower eyelid holds the lens in place so the pupil looks through the near zone
When to not prescribe translating GPs
When they don’t rest on or about the lower lid
When to prescribe Concentrics and aspherics
There is some degree of upper lid attachment
How to find residual astigmatism
Rx cyl-K cyl
How to find spherical overrefraction
Sph-trial lens
