CL 2-7: Astigmatic Correction with GP Contact Lenses Flashcards Preview

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Flashcards in CL 2-7: Astigmatic Correction with GP Contact Lenses Deck (41):
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?

*All 3 can induce a Different Power in EACH MERIDIAN

2

Spherical Lens Problems

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

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

3

Causes of Flexure

1. What 5 things can cause Flexure?

1. Thin Lens

2. High Dk Material

3. HIGHLY TORIC CORNEA

4. Tight Lid

5. Extended Wear

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?

1. corneal Cylinder

2. Corneal cylinder

3. to MORE FLEXURE

5

Base Curve vs. K Cyl

1. If NO Corneal Cylinder, FIT with what?

2. More Corneal Cylinder, Tend to FIT how?

1. FTK

2. STK

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?

1. a. Low Dk Material (

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?

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

8

Soft Toric Lens Designs (1)

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

1. GPs not tolerable, not desired, or a Front Toric GP is Indicated

*SiHy Material now available in custom parameters

9

Soft Toric Lens Designs (2)

1. Advantages: What are they? (3)

2. Improvements in last few years? (4)

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

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?

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

11

3 Types of Toric GPCL

1. Back Surface Toric

2. Bitoric

3. Front Surface Toric

12

Back Surface Toric

1. Front Surface is Spherical and has what?

2. Back Surface is Toric (How many curvatures)

1. 1 Curvature

2. 2 Curvatures

13

Bitoric

1. Front surface is what?

2. Back surface is what?

1. Toric (2 curvatures)

2. Toric (2 Curvatures)

14

Recording Bitoric GP Rx

1. How is it done?

1. Most Plus/Least Minus Power comes first.

*No axis recorded...

Ex: +3.00/+1.00 D

15

Front Surface Toric

1. Front Surface is what?

2. Back Surface is what?

1. Toric (2 curves)

2. Spherical (1)

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?

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

17

Effect of Applying Toricity to Concave Surface of a GP

1. Applying Toricity to Back Surface CREATES what?

1. Better Corneal Alignment to BOTH MERIDIANS

18

Astigmatism can be of 2 types?

1. Corneal

2. Internal (Lenticular)

19

CRA is what?

Eqn?

1. Leftover Cyl that remains uncorrected by the CL Power and the Tear Lens Power

*CRA = Spectacle Cyl - Corneal Cyl

20

Spherical Design

1. Good Candidates

a. Corneal Cylinder is what?

b. Spectacle Cyl is Approximately Equal to what?


i. CRA is what?

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

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?

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

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?

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

23

Back Surface Toric

1. Induces Cyl Correction EQUAL to what?

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

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?

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)

25

Back Surface Toric in the Lensometer

1. Verifies as having what?

1. Having 2 POWERS

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?

1. a. to STABILIZE LENS ROTATION

b. about 1.5x's the Amt of Corneal Cyl

c. and Spec cyl are Similar

27

Back Surface Toric (2)

1. Closer Alignment of the Lens and Cornea vs. Sphere GP Lenses : Does what 4 things

1. Improved Centration

2. Less Flexure

3. Less Lens Rocking

4. Less Flare

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?

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

29

Meridian of Least Resistance

1. GP will do what?

a. WTR Cornea?

b. ATR Cornea?

1. "Rock" around the meridian of LEAST RESISTANCE

a. UP and DOWN

b. SIDE to SIDE

30

What is a Bitoric Lens

1. Front and back surface?

2. Back Surface toricity?

3. Front Surface Toricity

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

31

Why A Bitoric?

1. Back surface toric INDUCES CYL where?

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

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?

1. Applying toricity to Back Surface Creates Better Corneal Alignment to Both Meridians

a. the EFFECT of Induced Cyl by Index of Refraction Difference

33

Corneal Cyl > Spectacle Cyl

1. Need toricity on the front surface to negate the effect of what?

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

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...

1. Align Meridians on back surface to corneal toricity but this does not provide enough correction.

*Add to this with front surface cylinder.

35

Front Surface Toric Design

1. Good Candidates

a. Corneal Cyl

b. Spec Cyl is what?

c. Residual Astigmatism is ?

1. a. Less than or EQUAL to 1.00 D

b. > Corneal Cyl

c. Greater than or Equal to 1.00 D

36

Front Surface Torics

1. All toricity incorporated on the front surface.

a. Used when TORICITY IS what?

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

37

Front Surface Toric

1. Stabilization Methods (2)

1. Prism Ballast and Truncation

2. Prism Ballast Only

38

Stabilizing Front Surface Toric GP

1. Prism Ballast and/or Truncation

a. Truncation = ?

1. a. Flat and Even Against the Lower Lid

39

Truncation in Minus Lenses

1. Truncation decreases the effectivity of Prism Ballast in Minus Lenses

a. high Minus Powers Require what?

1. MORE PRISM

*1.25 to 1.50 Prism

40

Truncation in Plus Lenses

1. What does it do?

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

41

Front Surface Toric

1. Problems

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.