CL 2-3: GP Fitting 1 Flashcards Preview

Contact Lens II: Part I > CL 2-3: GP Fitting 1 > Flashcards

Flashcards in CL 2-3: GP Fitting 1 Deck (25):
1

Empirical Fitting: what 3 things are needed?

1. Call in order for starting lens based on the following 3 parameters (Ks, Rx, and Lid Position)

2

1. What's the benefit of Empirical Fitting?

2. What's the Benefit of Diagnostic Fitting?

1. Usually has a Benefit of Having the FIRST Experience with GP lenses being a visually POSITIVE ONE

2. of obtaining Optimum fit thru the application of lenses

3

Choosing a Diameter

1. OAD of a GP needs to be LARGE ENOUGH to allow for a sufficient what?

2. OZD typically Encompasses what % of OAD?

1. Optic Zone while providing good lag with the Blink

2. 65-80% of the OAD.

4

What 3 Factors influence the Choice of OAD size?

1. Lid Position (Primary Factor)

2. Pupil Size

3. Corneal Curvature

5

OAD Factors: Lid Position (PRIMARY FACTOR!)

1. Upper Lid relative to Superior Limbus:
a. Above

b. At

c. Below

1. Fit smaller lens (Goal: Interpalpebral Fit)

2. Fit smaller Lens (Goal: Interpalpebral Fit)

3. Fit Larger Lens (Goal: Lid Attached Fit)

6

OAD Factors: Pupil Size

1. Measure when?

2. OZD needs to be bigger than what?
a. Why?

3. OZD makes up what % of OAD?

4. A Lens w/a LARGER OAD usually has a LARGER WHAT?

1. light and dark

2. than pupil size in Dim illumination in order to MINIMIZE FLARE at NIGHT
(that's why)

3. 65-80% of OAD

4. OZD

7

OAD Factors: Corneal Curvature

1. To maintain Optimum Centration/Stability:

a. Select LARGER OAD for what BCR?

b. Smaller OAD for what BCR?

1. a. for FLATTER BCR

b. for STEEPER BCR

8

OAD in GPs for Normal Corneas

1. Small

2. Average

3. Large

4. They're Available in increments of what?

1. About 8.8-9.0 mm

2. 9.2-9.4 mm

3. 9.6-9.8 mm

4. 0.1 mm increments

9

BC (BCR)

1. 3 terms are used to describe how BC of a GP CL compares to Corneal Curvature: What are they?

1. FTK; On K; STK

10

Choosing a BCR for a 9.2 mm OAD

Corneal Cyl

1. 0-0.50 D

2. 0.75 - 1.25 D

3. 1.50 D

4. 1.75-2.00 D

5. 2.25-2.75 D

6. greater than or equal to 3.00 D

1. 0.50-0.75 D FTK

2. 0.25-0.50 D FTK

3. On K

4. 0.25 D STK

5. 0.50 D STK

6. Bitoric design recommended

11

Corneal Vs. Spectacle Cyl

1. Spectacle (Manifest) Cylinder: What is it?
a. What components could it have?

2. Corneal Cyl: What is it?
a. It may be different than what?

1. it's the Actual Cyl in a Pt's Refraction
a. can have a Corneal Component, an Internal (Lenticular) Component, or Both

2. Difference in Curvature b/w the 2 Major Corneal Meridians
a. May be different than the Spectacle Cyl

12

Javal's Rule (you know the eqn): What does it Provide?

1. Provides an ESTIMATED PREDICTION of the Manifest Cylinder Based on the Corneal Toricity

13

1. The chart says to fit 0.50-0.75 FTK

and your Cyl difference is at the higher end (0.50 K cyl): What do you fit?

1. the higher the K Cyl, the steeper the BC chosen; So, Fit 0.50 D FTK since K Cyl is on the high end of the range listed

14

1. K Values are generally stated in what?

2. BCR is stated in what?

1. DIopters

2. mm or Diopters

15

Sagittal Depth

1. The Sag of a lens is Dependent on what 2 Factors?

2. how to explain Sagittal Depths?

1. BCR, and OAD

2. S1: Original Fit

S2: Effectively Steeper (Larger Sag)

S3: Effectively Flatter (smaller Sag)

S2 > S1 > S3

16

Sagittal Depth

1. In order to keep the same fitting relationship, you can compensate for a change in diameter by adjusting what as Necessary?

1. the BCR

(so if you're using a fitting set that has 8.6 mm lenses, you may use a steeper base curve than the chart says)

17

General Rule to KEEP SAME FITTING RELATIONSHIP:

1. FLATTEN BCR by what?

2. STEEPEN BCR by what?

1. by 0.25 D for every 0.5 mm INCREASE in OAD

2. 0.25 D for every 0.5 mm DECREASE in OAD

18

1. If you INCREASE the OAD by 0.5 mm, what are you doing?

1. you're effectively STEEPENING the Fit by 0.25 D w/o having to change the BASE CURVE! (and vice versa)

19

Power Determination

1. Diagnostically?

2. Empirically?

1. Over-Refraction

2. Calculations

20

1. Prescription Delivered by SCL = ?

2. Prescription delivered by GP (Rx at the Corneal Plane) =

1. Power of SCL

2. Power of GP + Power of Tear Lens

21

GP Fit FLATTER than K

1. What does this induce (for the TEAR LENS)?

2. Tear Lens (D) = ?

1. Induces a NEGATIVE Tear Lens

2. GP BCR (D) - K Value (D) (difference b/w Curvature of the Lens and the Curvature of the Cornea)

22

GP fit ON K

1. What kind of Tear lens is Induced?

1. A ZERO TEAR LENS!

(Tear lens = Plano)

23

GP Fit STEEPER than K

1. What does it induce for the Tear Lens?

**Eqn: Rx Delivered by GP = ?

1. Induces a POSITIVE TEAR LENS!

** = Power of GP + Power of Tear Lens

24

Compensating for Tear LEns

1. SAM & FAP

2. What is CALCULATED RESIDUAL ASTIGMATISM?

1. Steeper add MINUS

and

Flatter ADD PLUS

2. Whatever is predicted to be left UNCORRECTED.

25

1. What is Residual Astigmatism?

a. So if a Spherical GP is put on the eye, the RA is what?

b. How do you Calculate it? (CRA) = ?

2. What can Calculated Residual Astigmatism (CRA) be used to help us determine?

a. When would we use a Spherical GP?

1. Refractive Astigmatism that is still Uncorrected when a CL is placed on an eye.

a. It's about equal to the Difference b/w the K Cyl and the Manifest Cyl

b. Rx Cyl - K Cyl

2. if we can use a spherical GP or not. Too much cyl left uncorrected by a spherical GP, then we need to use a Toric GP of some kind.

a. when CRA is LESS THAN or EQUAL TO 0.75 D

b. Toric GP when CRA is > 0.75 D