week 2

Question 1

what happens if you find >.75D on steep or flat meridian when OR’ing a back surface toric lens?

Answer 1

may need a bitoric design instead

Question 2

If cyl is found with an axis oblique to the flat K axis on back surface toric lens OR?

Answer 2

either the lens is rotating excessively, or the pt’s corneal and internal astigmatism are not at the same axis

Question 3

when would you use a SPE or spherical power effects lens?

Answer 3

-toricity of BC=toricity of CLP
-eyes with little to no internal
astigmatism
-Corneal astigmatism >/= 2D
-Refastigmatism w/i 0.75 D of corneal cyl

Question 4

when would you use a CPE or cylindrical power effects lens?

Answer 4

Toricity of BC is NOT = to toricity of CLP
-eyes w/ significant (>0.75 D)
internal astigmatism
-Corneal astigmatism >/= 2 D
– Internal astigmatism > 0.75 D

Question 5

a SPE or spherical power effects lens has what kind of surfaces front and back? toric or spherical

Answer 5

both front and back toric surfaces

Question 6

How do you determine power of front and back surface of SPE or spherical power effects lens?

Answer 6

back surface toricity: same as toric back surface lenses (based on corneal toricity & fitting relationships)
-front surface toricity: formulas so resulting lens has the same amount of toriciy in BC and power

Question 7

If OR cyl < 0.75 D on SPE lenses, what do you do?

Answer 7

If OR cyl < 0.75 D, add the SE of OR to both meridians

Question 8

If OR cyl > 0.75 D on SPE lenses, what do you do?

Answer 8

If OR cyl > 0.75 D, try back surface toric or CPE toric

* If OR cylinder > 0.75 and is oblique to K axis, assess rotational stability

Question 9

a CPE or spherical power effects lens has what kind of surfaces front and back? toric or spherical
*what does each surface correct?

Answer 9

A CPE bitoric lens has toric front and back surfaces

*back surface toricity corrects corneal astigmatism, front surface toricity corrects any internal astigmatism

Question 10

Eyes with less than _ D of corneal toricity will not support a back surface toric

Answer 10

less than 2.00 D of corneal toricity will not support a back surface toric

Question 11

what are some disadvantages of front surface toric lenses vs spherical lenses

Answer 11

• have prism, so drop inferiorly, not as comfy
• more rotation, worse VA
• not as much oxygen transmission, thicker due to prism ballasting

Question 12

how do you select BC and power of front surface toric of front surface toric?

Answer 12

Cylindrical power of front surface = internal astigmatism
BC: same as spherical lens fitting
• Kave – 0.50 D for 9.2 diameter
• Kave – 1.00 D for 9.6 diameter

Question 13

prism ballasting in front surface toric lenses: where is it, how does it change for WTR and ATR?

Answer 13

prism ballasting is always down

• Larger WTR cylinders require larger amounts of prism
• ATR corrections do not need as much prism

Question 14

How do you determine CT for front surface toric?

Answer 14

First determine CT as for spherical lenses based on spherical lens power
– Increase CT by 0.10 mm for each prism diopter