CL 2-7: Astigmatic Correction with GP Contact Lenses

Question 1

What are some of the ways we can Correct Astigmatism in GP CLs?

1. Tear Lens Differential can induce what?
2. FRONT SURFACE Curvature differential can induce what?
3. BACK SURFACE Curvature differential can induce what?

Answer 1

*All 3 can induce a Different Power in EACH MERIDIAN

Question 2

Spherical Lens Problems

1. On Highly Astigmatic Corneas what can happen? (6)

Answer 2

1. Lens Rocking
2. Decentration-Induced Flare
3. Corneal Desiccation
4. Flexure-Induced VA Fluctuation
5. Excessive Areas of Pressure on the Cornea
6. Corneal Distortion

Question 3

Causes of Flexure

1. What 5 things can cause Flexure?

Answer 3

1. Thin Lens
2. High Dk Material
3. HIGHLY TORIC CORNEA
4. Tight Lid
5. Extended Wear

Question 4

Steeper Lenses Tend to FLEX More than Flatter Lenses

1. Remember, we use a STEEPER BCR with INCREASING what?
2. This is necessary to better match each Meridian as they DIVERGE from one another with INCREASING what?
3. Keep in mind, that this MAY LEAD to WHAT?

Answer 4

1. corneal Cylinder
2. Corneal cylinder
3. to MORE FLEXURE

Question 5

Base Curve vs. K Cyl

1. If NO Corneal Cylinder, FIT with what?
2. More Corneal Cylinder, Tend to FIT how?

Answer 5

1. FTK

2. STK

Question 6

Spherical Lens Designs

1. Guidelines to Minimize Flexure when using Spherical GPs on Toric Corneas:
   a. What Dk Material?
   b. Request Higher what?
   c. What should you do to help RULE OUT FLEXURE?

Answer 6

1. a. Low Dk Material (

Question 7

Corneal Warpage

1. Relative Flattening of the Cornea Underlying what?
2. Can occur with any type of CL, but MORE Common with which TYPE?
3. Signs
   a. Topographic Changes (what ones?)

b. Myopia: Increased/Decreased?
c. Corneal Cyl = Refractive Cyl? (or no)
d. Keratometry Mires are what?

Answer 7

1. underlying the Resting Area of the CL
2. with GPs
3. a. Corneal Flattening, Irregular Astigmatism
   b. Decreased Myopia
   c. DOES NOT EQUAL
   d. Distorted

Question 8

Soft Toric Lens Designs (1)

1. these are Options for Patients in what 3 situations w/GPs?

Answer 8

1. GPs not tolerable, not desired, or a Front Toric GP is Indicated
   * SiHy Material now available in custom parameters

Question 9

Soft Toric Lens Designs (2)

1. Advantages: What are they? (3)
2. Improvements in last few years? (4)

Answer 9

1. a. Readily available in diagnostic sets
   b. Often have Wide axis and power ranges
   c. Custom options available
2. a. Newer edge designs improve comfort
   b. better lens reproducibility
   c. Enhanced Stability of Toric Lenses
   d. Greater Oxygen Transmissibility

Question 10

Soft Toric Lens Designs (3)

1. Disadvantages
   a. Small amts of rotation can do what?
   b. Not as useful if what is present?
   c. Higher Risk of Infection in what?

Answer 10

1. a. can significantly affect visual performance (esp. In high amts of Astigmatism)
   b. if Corneal Distortion or Irregular Astigmatism is Present (Better visual result w/a GP)
   c. in SCL vs. GPCL

Question 11

3 Types of Toric GPCL

Answer 11

1. Back Surface Toric
2. Bitoric
3. Front Surface Toric

Question 12

Back Surface Toric

1. Front Surface is Spherical and has what?
2. Back Surface is Toric (How many curvatures)

Answer 12

1. 1 Curvature

2. 2 Curvatures

Question 13

Bitoric

1. Front surface is what?
2. Back surface is what?

Answer 13

1. Toric (2 curvatures)

2. Toric (2 Curvatures)

Question 14

Recording Bitoric GP Rx

1. How is it done?

Answer 14

1. Most Plus/Least Minus Power comes first.
   * No axis recorded…

Ex: +3.00/+1.00 D

Question 15

Front Surface Toric

1. Front Surface is what?
2. Back Surface is what?

Answer 15

1. Toric (2 curves)

2. Spherical (1)

Question 16

When to use which

1. Toricity on the BACK SURFACE of a GP CL CORRECTS for what?
2. Toricity on the FRONT SURFACE corrects for anything that’s not corrected by what?
3. We can apply toricity on what?

Answer 16

1. CORNEAL CYLINDER
2. by the Base Curve-Corneal Curve Differential (or the Back surface Toricity)
3. on the Front, Back, or Both Surfaces, Depending on the Above Factors

Question 17

Effect of Applying Toricity to Concave Surface of a GP

1. Applying Toricity to Back Surface CREATES what?

Answer 17

1. Better Corneal Alignment to BOTH MERIDIANS

Question 18

Astigmatism can be of 2 types?

Answer 18

1. Corneal

2. Internal (Lenticular)

Question 19

CRA is what?

Eqn?

Answer 19

1. Leftover Cyl that remains uncorrected by the CL Power and the Tear Lens Power
   * CRA = Spectacle Cyl - Corneal Cyl

Question 20

Spherical Design

1. Good Candidates
   a. Corneal Cylinder is what?
   b. Spectacle Cyl is Approximately Equal to what?
   i. CRA is what?

Answer 20

1. a. Less than or Equal to 2.00-3.00 D
   b. and at the same axis as the Corneal Cyl
   i. Less than or Equal to 0.75 DC

Question 21

Back Surface Toric Lens

1. What is it?
   a. Fits SHAPE of what?
2. When the correction of cyl thru the creation of a tear lens causes fitting issues we can change the shape of what?

Answer 21

1. TORIC BACK SURFACE
   a. (Back surface toricity): Fits shape of highly toric cornea
2. of the Back Surface to Match the Contour of the Cornea
   a. *This Base Curve differential Corrects Corneal Cylinder

Question 22

Back Surface Toric

1. Good Candidates
   a. Corneal Cyl is what?
   b. SPECTACLE CYL is EQUAL to what?
   c. The Contact Lens itself INDUCES what?
   d. This is because Astigmatism will be INDUCED by what?

Answer 22

1. a. greater than or equal to 2.00-3.00 (3.00 WTR or 2.00 D ATR)
   b. about 1.5 times the CORNEAL CYL, and at the SAME AXIS
   c. CYL Correction equal to 0.5 times the CL Toricity
   d. by the Difference b/w the Refractive Index of the CL and the Refractive Index of the Tearfilm

Question 23

Back Surface Toric

1. Induces Cyl Correction EQUAL to what?

Answer 23

1. to HALF the AMT of TORICITY of the CL Base Curves (AXIS FLAT CORNEAL MERIDIAN)

Question 24

Back Surface Toric

1. Induced Cyl
   a. Due to what?
   b. HIGHER nlens –> ?
   c. In General, and for the sake of this course, use what?

Answer 24

1. a. Difference in Index of Refraction (nlens/ntears)
   b. More induced Cylinder
   c. 0.5 x induced Cyl (1.5 x’s total cyl correction)

Question 25

Back Surface Toric in the Lensometer 1. Verifies as having what?

Answer 25

1. Having 2 POWERS

Question 26

Back Surface Toric (1) 1. Indicated in the following 3 conditions: a. Enough CORNEAL TORICITY to do what? b. Refractive Cylinder is about what? c. AXES of Kcyl and what?

Answer 26

1. a. to STABILIZE LENS ROTATION b. about 1.5x's the Amt of Corneal Cyl c. and Spec cyl are Similar

Question 27

Back Surface Toric (2) 1. Closer Alignment of the Lens and Cornea vs. Sphere GP Lenses : Does what 4 things

Answer 27

1. Improved Centration 2. Less Flexure 3. Less Lens Rocking 4. Less Flare

Question 28

Bitoric Design 1. Good Candidates a. Corneal CYL is what? i. Especially Necessary with what? AND b. Spectacle Cyl is what? c. The Spectacle Cyl is > greater than what?

Answer 28

1. a. greater than or equal to 2.00-3.00 (3.00 DC WTR/ 2.00 DC ATR) b. is Approximately Equal to (and at the same axis as) the Corneal Cyl, Or c. than the Corneal Cy, or Corneal Cyl > Spectacle Cyl

Question 29

Meridian of Least Resistance 1. GP will do what? a. WTR Cornea? b. ATR Cornea?

Answer 29

1. "Rock" around the meridian of LEAST RESISTANCE a. UP and DOWN b. SIDE to SIDE

Question 30

What is a Bitoric Lens 1. Front and back surface? 2. Back Surface toricity? 3. Front Surface Toricity

Answer 30

1. Toric front AND Back surface 2. Fits shape of Highly Toric Cornea 3. Contains Cyl; Can use Plus Cyl to Neutralize the Back Surface Cyl created when the lens is on the eye (if it's undesirable) and Allows Lens to fully correct the Refractive error that's not corrected thru the back surface toricity

Question 31

Why A Bitoric? 1. Back surface toric INDUCES CYL where?

Answer 31

1. At the Same Axis the Corneal Cyl Equal to 1/2 the Amt of Toricity of the CL Base Curves

Question 32

Spec Cyl is about the same as Corneal Cyl 1. Why not just use a Back Surface Toric? a. BUT you NEED toricity on the Front surface to NEGATE what?

Answer 32

1. Applying toricity to Back Surface Creates Better Corneal Alignment to Both Meridians a. the EFFECT of Induced Cyl by Index of Refraction Difference

Question 33

Corneal Cyl > Spectacle Cyl 1. Need toricity on the front surface to negate the effect of what?

Answer 33

1. of Toricity on the Back Surface a. Align Meridians on Back surface to corneal toricity, but this induces too much correction b. Negate this with Front Surface Cylinder

Question 34

Spectacle Cyl > Corneal Cyl, But Spec Cyl DOES NOT EQUAL 1.5 x's K Cyl 1. Toricity from Back Surface is not sufficient to Correct All Cylinder...

Answer 34

1. Align Meridians on back surface to corneal toricity but this does not provide enough correction. * Add to this with front surface cylinder.

Question 35

Front Surface Toric Design 1. Good Candidates a. Corneal Cyl b. Spec Cyl is what? c. Residual Astigmatism is ?

Answer 35

1. a. Less than or EQUAL to 1.00 D b. > Corneal Cyl c. Greater than or Equal to 1.00 D

Question 36

Front Surface Torics 1. All toricity incorporated on the front surface. a. Used when TORICITY IS what?

Answer 36

1. a. is NOT Corneal in Nature * Achieving rotational stability can be challenging; * Rotation is measured same way as Soft Toric * Laboratory can Dot the Base of the Lens * Usually reserved for Patients who are unable to Wear Soft Toric Lenses

Question 37

Front Surface Toric 1. Stabilization Methods (2)

Answer 37

1. Prism Ballast and Truncation | 2. Prism Ballast Only

Question 38

Stabilizing Front Surface Toric GP 1. Prism Ballast and/or Truncation a. Truncation = ?

Answer 38

1. a. Flat and Even Against the Lower Lid

Question 39

Truncation in Minus Lenses 1. Truncation decreases the effectivity of Prism Ballast in Minus Lenses a. high Minus Powers Require what?

Answer 39

1. MORE PRISM | * 1.25 to 1.50 Prism

Question 40

Truncation in Plus Lenses 1. What does it do?

Answer 40

1. Increases the Effectivity of Prism Ballast in Plus Lenses | * Smaller Amts of Prism are needed for Plus Power (as well as low minus Power) Lenses

Question 41

Front Surface Toric 1. Problems

Answer 41

1. Vision blurred from rotation 2. Discomfort from prism ballast and/or truncation 3. Inferior Decentration causing flare or corneal desiccation 4. Can't modify front surface 5. Asthenopia can result from vertical imbalance (if unilateral) 6. Edema can develop if a LOW-Dk Lens material is used.