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KMK Optics > CL > Flashcards

CL Flashcards

1
Q

The “BC” of a GP lens is actually determined based on

A

the front surface of the tear film

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2
Q

SAMFAP. For every 0.1mm of change in the BC, the power of the GP lens needs to change by

A

-0.50

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3
Q

Formula to calculate residual astigmatisms

A

RA= Rx - Ks
*People can tolerate up to -0.75DC of residual astigmatism. If there is more than this, the pt should be fit with a toric GP.

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4
Q

Javal’s Rule

A

Add -0.50 x 090 DC to the corneal astigmatism to account for lenticular cyl

= (1.25 x k cyl) + (-0.50x090)

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5
Q

Optical zone diameter

  • What happens if you increase or decrease it
  • for every 0.4mm change in OZD, how much should you adjust the power?
A

Increase OZD = Increased sag = tighter fit = need to flatten the BC

Decrease OZD= decrease sag = looser fit= need to steepen the BC

For every 0.4mm change in OZD, change the power by 0.25D.

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6
Q

What parameter is selected to minimize flare? (most commonly occurs when the edge of the CL is close to the edge of the pupil)

A

Overall diameter. Larger diameter will maximize comfort.

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7
Q

What parameter allows alignment between the CL edge and the peripheral cornea.

A

Peripheral curves. GP CLs may have 1, 2, or 3 peripheral curves in addition to the BC.

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8
Q

The peripheral curve has what 3 main functions

A
  1. Prevent the edge of the CL from bearing on the cornea.
  2. Promotes tear exchange under the CL to maintain adequate K metabolism.
  3. Support a tear meniscus at the edge of the CL to promote CL centration.
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9
Q

The ideal edge thickness to promote lid attachment is the edge thickness of a ___D GP CL

A

-3.00D

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10
Q

How to alter edge thickness with lenticular carriers

A

GP with more than +1.50 will have flat edges and poor LA. Add a minus carrier to promote LA.

GP with more than -5.00D will have excessive edge thickness causing the lens to ride high due to too much LA. Add a plus carrier to decrease LA.

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11
Q

What is edge lift and what steps should you change it in

A

Distance between the peripheral edge of the GP and cornea.

Make 1mm changes.

Increase edge lift–> flatten
Decrease edge lift–> steepens

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12
Q

Center thickness (CT)

  • What role does this play in GPs
  • What steps should you change it in
A

Influences oxygen transmissibility of the CL, flexure, and center of gravity.

Change it in 0.03mm steps.

A thinner CT has more oxygen transmissibility, better contraption due to less mass, but more flexure.

A thicker CT has less oxygen transmissibility and less flexure, but tends to drop on the eye due to increased mass.

In general, high DK lenses require a thicker CT in order to minimize flexure.

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13
Q

In general, high DK lenses require a ____ CT in order to minimize flexure.

A

Thicker

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14
Q

GP CLs with more anterior center of gravity will tend to drop on the eye. 4 examples

A

Flat BC
Small diameter
Plus lenses
Thicker CT

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15
Q

A lens that is moved closer to the eye becomes effectively more

A

MINUS.

Add more plus into the Rx

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16
Q

Why would you not fit each meridian exactly in a GP bitoric lens? What are the other options?

A

On K fit is often too tight, resulting in poor tear exchange.

Saddle fit: Equal alignment between both principal meridians.

LTS: Flat K is 0.25 FTK and steep K is 0.75 FTK to mimic a -1.00 WTR K

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17
Q

When can you use a back surface toric lens?

A

If 1.5x k cyl = refractive cyl

Usually seen in people with ATR astig

18
Q

Primary disadvantage of fitting aspheric GP lenses

A

Decreased tear exchange due to the decrease in contact lens movement on the eye. Especially Back surface aspheric lenses.

Decentration may also induce unwanted astigmatism.

19
Q

2 types of multifocal GP designs

A
  1. Simultaneous
    - Aspheric MF
    - Concentric BF
  2. Translating
20
Q

What characteristics will increase flexure

A 
Thin center thickness 
High K cyl 
Large diameter 
High DK material 
Steep BC
21
Q

Why does a soft lens have total flexure?

A

Because it completely forms to the shape of the K, resulting in a plano lacrimal lens.

RA= refractive astig.

22
Q

Diff between GP flexure and warpage

A

GP flexure is only on the eye

Warpage is on AND off the eye. Usually due to aggressive digital cleaning.

23
Q

steep CLs or high DK lenses have increased flexure, how can you counteract this?

A

Increase CT by 0.03mm

24
Q

For every 0.4mm change in the GP diameter, how much do you need to change the BC?

A

By 0.25D, same with power.

25
Q

Low, high, and hyper DK values

A

Low: 25-50
High: 50-99
Hyper: 100+

26
Q

Transmissivity =

A

DK/ thickness (cm)

DK is gas permeability

27
Q

Patients usually can be fit with a sphere soft lens if they have how much astigmatism?

A

0.75DC or less

28
Q

Prism ballasting
Periballasting
Dynamic stabilization

A

Prism ballasting: BD incorporated into the bottom of the lens.
Periballasting: BD prism incorporated at the bottom of the lens OUTSIDE the optic zone.
Dynamic stabilization: Thicker horizontal meridian.

29
Q

For every 10 degrees rotation in a soft toric lens, how much cyl is induced

A

1/3

30
Q

Average K values

A

41-45D

31
Q

Good fit characteristics of a soft CL

A

Centration
0.25-1mm movement in primary gaze with blink
1mm lag
Lens extends 1.5mm beyond limbus

32
Q

How to find diameter of Soft CL

A

HVID + 3mm

33
Q

Water and DK relationship based on conventional or silicone hydrogels

A

Conventional: As water increases, DK increases.
Silicone hydrogel: As water increases, DK decreases.

DK= permeability

34
Q

DK

A

Permeability

35
Q

The FDA classifies soft CLs in to 5 groups

A

1: Low water, non-ionic
2. High water, non-ionic
3. Low water, ionic
4. High water, ionic
5. Silicone hydrogels

36
Q

Deposits are most likely to occur in soft CLs with

A

High water content, ionic (Group 4)

37
Q

Rigidity of a CL is dependent on the

A

Thickness and modulus

38
Q

Modulus =

A

1/stretchability

or rigidity

39
Q

Describe the modulus and lens if it is low water

A

Low water = high modulus = more rigid = more durable = can be made thinner = easier handling

40
Q

Some drawbacks to high modulus SiHy lenses

A

Giant papillary conjunctivitis, SEALs, mucin balls, edge fluting.

41
Q

How does accommodation, magnification, and vergence change when switching a hyperope from glasses –> CLS

A

Accommodation decreases
Magnification decreases
Less vergence required (bc in specs, there was BO prism moving the image closer, requiring more convergence)

42
Q

How does accommodation, magnification, and vergence change when switching a myope from glasses –> CLS

A

Increased accommodation
More magnification
More convergence (Because in glasses, BI helped move the image away so the pt wouldn’t have to converge as much)

Pre-presbyopic myope may notice near blur sooner in CLs than specs due to increased accommodation demand.

KMK Optics (4 decks)

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