Lec 6 Torics Flashcards
Why are toric RGP fitted
Improve fitting on astigmatic cornea with mod to high cyl
Correct significant residual astig that occurs when fitting a spherical RGP
What’s residual astigmatism
Uncorrected astigmatism found by over refraction when a spherical RGP is fitted on cornea
SPEC CYL - CORNEAL CYL
Why is flexible important in RGP
You can have more or less residual astigmatism than predicted
Can be measured by keratometer over CL as a cyl result
How much astig warrants a bitoric or back surface toric
2D of regular astig
How to fit a back toric (?)
Fit a BC near K for each meridian but go for one where vertical is slightly flatter so there’s a better tear fluid pump for movement
Why are Front surface toric less prescribed
Because soft torics are successful so they can fit over the cornea well and get rid of remaining astigmatism
What’s main disadvantage of front surface toric
Prism is used to stabilise the lens but thick increases thickness (decreases 02) and lens awareness
What’s good for low corneal cyl that’s equal to spectacle cyl
Spherical RGP as it fits well and TLP corrects for it
What’s good for low corneal cyl that matches a significant non matching refractive cyl
Front toric as it fits well and then gives extra correction for cyl from lens
What’s good for medium corneal cyl that matches spectacle cyl
Spherical RGP as fits Ok and TLP corrects
Toric periphery as same as spherical RGP but better fit
SPE Bitoric as fits well and not affected by rotation so is same as spherical RGP
What’s good for medium corneal cyl with a non matching refractive cyl
Front surface toric as fits ok and gives extra correction from lens
Bitoric as fits well and gives extra correction for lens
What’s good for high corneal cyl that mace he’s the spectacle cyl
SPE Bitoric as fits well and has spherical effect so doesn’t matter if rotates and corrects all corneal cyl
What’s good for high corneal cyl with a non Matching refractive cyl
CPE Bitoric as fits well and gives extra correction for lens
Back surface toric as fits well and ….
What is a TSP
Spherical back optic and toric periphery (Toric Secondary and Peripheral curves)
Used if normal spherical RGP is good vision but uneven edge lift on a toric cornea
1.5-2.5DC
What does TSP 0.8 mean?
Difference between the peripheral curves is 0.8
Eg SC curve be 9.0 in one meridian and the flatter meridian will be 9.8
What is the bi-sym and quad-sym lens
Bi- Lower mid periphery of lens steepens independent of upper mid periphery = top and bot diff while middle used as blending zone
Quad - 4 independent curves
What’s induced astigmatism
An over correction that’s pretty much a Minus cyl at the axis of flatter meridian
Proportional to CL toric it’s and refractive index of material (hi RI =hi cyl)
Why do we care about how much induced astigmatism there is
Because if BC toricity plus the induced over correction equals the refractive astigmatism then it causes the front surface to become spherical so u get back surface toric
Advantage of back surface toric
Easy to manufacture as one side is just sphere so better lens quality
Allows front surface modifications e.g plus power or polishing
Costs less as only one curve is larger in front
What’s a Bitoric RGP
Lenses with 2 central BC in back 90deg apart and 2 front surface curves which produce a BVP for each meridian
Advantages of SPE Bitoric
Works for refractive astigmatism that matches corneal cyl
Rotation doesn’t produced IA as tears and front surface neutralises itself (as curves rotate together)
Good as trial lenses to easily over refract to get final rx to check if there’s any residual astig
