astigmatism_torics_flashcards
What does ‘With the rule astigmatism’ mean?
The flattest K is at 180 degrees.
What does ‘Against the rule astigmatism’ mean?
The flattest K is at 90 degrees.
When is BVS used for astigmatism correction?
For astigmatism up to 0.75D: BVS (Spherical + ½ Cylinder).
When is exact Rx used for astigmatism correction?
For astigmatism 0.75D and above, exact prescription (Rx) is used.
What does the CAAS acronym stand for when assessing toric lens positioning?
CAAS stands for: Clockwise, Add; Anti-clockwise, Subtract.
How is a Toric RGP lens stabilised using prism ballast?
The lower lens edge is thickened, and the lid pushes the thickest part of the lens downwards during blinking.
What is a disadvantage of prism ballast in Toric RGP lenses?
The added thickness reduces oxygen transmissibility, increasing the risk of neovascularisation.
How is a Toric RGP lens stabilised using dynamic stabilisation?
The upper and lower lens edges are thinned, allowing both lids to stabilise the lens without inducing a prismatic effect.
Why is dynamic stabilisation preferred for against-the-rule astigmatism?
Dynamic stabilisation thins the areas of the lens that would otherwise be thickened by the cylinder in against-the-rule astigmatism.
How is truncation used to stabilise a Toric RGP lens?
Part of the lower lens is sectioned off to lie flat on the lid margin. Often combined with prism ballast.
When is a front surface toric RGP lens used?
Used when there is over 0.75DC with a spherical or near spherical cornea (lenticular astigmatism).
How is a front surface toric lens designed to prevent rotation?
A 1.5Δ base-down prism acts as a weighted ballast to prevent the lens from rotating.
What is the diagnostic marker on a front surface toric RGP lens?
A dot is placed at 6 o’clock on the lens for diagnostic purposes.
When is a back surface toric RGP lens used?
Used when there is a highly toric cornea (3.00D or more) with a spherical correction.
What is the purpose of a back surface toric RGP lens?
The back surface of the lens matches the curvature of the corneal surface to align and create a plano tear lens.
