Unit 1

Question 1

With the rule astigmatism

Answer 1

-Greatest refractive power is w/in 30 degrees of the verticle meridian
-Minus cyl axis around the horizontal meridian
-Most common type
-Steeper in the verticle
-Most common in children

Question 1

Against the rule

Answer 1

-Greatest refractive power is within 30 degrees of the horizontal meridian
-Minus cyl axis around vertical meridian
-Most common in adults
-Steeper in horizontal

Question 2

Oblique astigmatism

Answer 2

-Greatest refractive power is w/in 30 degrees of the oblique meridians
-Between 30 and 60 or between 120 and 150

Question 3

Irregular astigmatism

Answer 3

-The 2 principal meridians are NOT perpendicular to each-other
-Associated with trauma, disease or degeneration (kerataconus, pellucid marginal degeneration, scaring)

Question 4

What is keratometer

Answer 4

An instrument used to measure the front surface of the cornea
-Curvature power, degree and type of corneal astigmatism
-DOES NOT measure any lenticular astigmatsim
-Average is +43D

Question 5

What does keratometer measure

Answer 5

-Measures central 3mm of cornea
-36-52 diopters
-Addition of a -1.00D auxiliary lens extends range to 30D to get a flatter reading
-Addition of a +1.25 auxiliary lens extends range to 61 D to get a steeper reading

Question 6

How do you calculate corneal astigmatism

Answer 6

1. Find the difference in the Ks by subtracting (ignore signs)
   2 Place a minus sign in front of your answer
   3, The meridian of your SMALLER number becomes the axis
   EX.. -1.00D x 180

Question 7

How to transpose RX

Answer 7

1. Add the first 2 numbers together algebraically
2. Change the sign of the cylinder
3. Change the cylinder axis by 90 degrees so the new number falls between 1 and 180

Question 8

What lenses have Tear lens / lacrimal lens??

Answer 8

GP lenses have a layer of tears that form between the posterior surface of the CL and the anterior surface of the cornea

Question 9

Does a soft CL have tear lens?

Answer 9

NO, it is plano power

Question 10

Rule of thumb for base curves

Answer 10

0.1mm BC radius is equivalent to 0.50D

ex. if there is a 0.2mm change that means we change the power by 1.00

Question 11

TRUE OR FALSE
if a patient has astigmatism <2.50D then a spherical GP lens might work

Answer 11

TRUE // the lower amount of astigmatism can be neutralized by the tear film

Question 12

can we ‘ignore’ the cly value during sam/fap?

Answer 12

Yes

Question 13

What is fluroscein used for ?

Answer 13

-Help visualize corneal damage - dry eyes, corneal abrasions, infections etc..
-Helps to visualize different characteristics of a GP lens fit

Question 14

Fluroscien pattern for WTR cornea

Answer 14

Bow tie shape
-Steeper in the verticle / flatter in horizontal

Question 15

Fluroscein pattern in ATR cornea

Answer 15

Figure 8 shape
-Steeper in horizontal flatter in verticle

Question 16

What is residual astigmatism?

Answer 16

The amount of astigmatic refractive error that remains uncorrected with a CL applied to the cornea
“left over” astigmatism

Question 17

Residual astigmatism formula

Answer 17

Residual astigmatism = spectacle astigmatism - corneal astigmatism

Question 18

Types of residual astigmatism?

Answer 18

-Physiologic
-Induced

Question 19

Physiologic RA

Answer 19

From curvature refractive index differences between the corneal and crystalline lens

Question 20

Induced RA

Answer 20

Caused from lens application
EX.. warpage, flexure, decentration

Question 21

Can RA very depending on what type of CL being used?

Answer 21

Yes it depends on whether it is spherical GP or spherical soft CL

Question 22

To calculate RA what 2 pieces of info do you need?

Answer 22

1. The spectacle astigmatism (cyl in the gls)
2. The corneal astigmatism (calculated from the K values)

Question 23

When solving for RA and it asks for minus cyl what do we do ?

Answer 23

We need to transpose it so we switch the power of the cyl and change the axis so it is 90 degrees different from current axis

Question 24

T OR F
Do we need to vertex even when the spec rx is given?

Answer 24

YES
We need to find the spectacle rx that has been vertexed to the corneal plane
ALWAYS VERTEX

Question 25

Why is over refraction needed ?

Answer 25

To determine if a change in the final lens power is necessary
Goal is to maximize the pxs VA

Question 26

Why may we have a significant power in our over refraction?

Answer 26

-If tear lens is not fitting on alignment, unwanted power is created
-Fitting a spherical GP lens on a toric cornea

Question 27

What data do we need to solve OR problems”?

Answer 27

-The spec correction-vertex if needed
-Pxs K readings
-The power of the diagnostic GP lens (often -3.00

Question 28

Things to calculate/solve for during OR

Answer 28

-Residual astigmatism
-The spherical portion of the OR
-Impact of the tear lens from the CL base curve

Question 29

Spherical OR formula

Answer 29

Sphere power @ the corneal plane - diagnostic sphere power of GP lens

Question 30

Steps to solve OR problems

Answer 30

1. Find the RA (cyl power and axis of the RA = cyl power and axis of the OR)
2. Find the spherical power of the OR
   (sphere power @corneal plane - diagnostic sphere power of GP)
3. Apply the SAM FAP rule to your spherical OR power from step 2 if the CL is NOT an on-k fit
4. The final answer to the OR problem is the sphere power from step 3 combined with the cyl power and axis from step 1