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Part 1 > Contact Lenses > Flashcards

Contact Lenses Flashcards

1
Q

What kind of lenses are CL considered?

A

Thick or Thin lenses

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2
Q

What is the equation for the back vertex power for a thick CL?

A

Fv=F2 + F1/(1-(t/n)*F1)

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3
Q

What is the equation for the thin CL?

A

Fv=F1+F2

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4
Q

If CT is given for a CL is it considered thick or thin?

A

Thick

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5
Q

The effective power of the lens changes where it is located in front of the eye is how many mm?

A

12-14 mm

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6
Q

CLRx equation?

A

CLRx = Tear Lens + CLP + OR

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7
Q

What is the back surface of the lacrimal lens equal to?

A

Equal to the radius of the curvature of the cornea (Keratometry; calibrated for n=1.3375)

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8
Q

What is the front surface of the lacrimal lens radius of curvature?

A

BC of the CL

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9
Q

True or False. the K readings are equal (but opposite in sign) to the back surface power of the lacrimal lens in air.

A

TRUE.

The BC of GP CL given by K reading is EQUAL to the POWER of the FRONT surface of the lacrimal lens in air

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10
Q

The K reading used to specify BC of GP CL is not the actual surface power of the CL why?

A

bc Keratometer uses n=1.3375 vs the n of the CL

The BC of GP CL expressed as a K reading is actually the power of the FRONT surface of the tear film

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11
Q

What happens when the BC CL is steeper than the curvature of the CL?

A

F1 > F2 of tear lens = (+) PLUS POWER

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12
Q

What happens when the BC CL is flatter than the curvature of the CL?

A

F1 < F2 of tear lens = (-) MINUS POWER

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13
Q

Equation for amount of astig not corrected by GP CL?

A

Ar = Arx - Ac
Residual astig = Astig on Manifest - Corneal astig

NOTE: in RGP, Arx is simply amount NOT attributable to the cornea assuming no flexture

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14
Q

How much WTR and ATR is generally tolerable for patients?

A

<1.00D WTR

< or = 0.75D ATR

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15
Q

Javal’s Rule equation?

A

Total refractive astig=1.25*(corneal astig) + 0.50 (WTR) OR -0.50 (ATR)

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16
Q

What is the typical useable area of optics in CL?

A

Avg 7.6 to 8.2 mm

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17
Q

What happens when you increase the OZD of a CL?

A

The sag increases, thereby INCREASING the STEEPNESS of the FIT

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18
Q

What is the purpose of the OAD/

A

The uncurved distance of the CL from edge to edge

  1. Selected to minimize flare (edge CL close to edge pupil)
  2. Avoid bottom lid
  3. Facilitate lid attachment
  4. Maximize comfort
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19
Q

For every 0.4 mm change in OZD what happens to BC?

A

It is adjusted by 0.25D

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20
Q

What is the average OAD and how is it adjusted?

A

9.4 - 9.6 mm

Adjusted in 0.4 mm steps

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21
Q

Which each progressive peripheral curve in a GP lens what happens?

A

It flattens towards the edge to:

  1. ensure comfort
  2. Promote tear exchange under CL
  3. Support tear meniscus at edge of CL lens centration
22
Q

The edge thickness to promote lid attach is the edge thickness of which lens power?

A

-3.00DS GP CL

23
Q

GP lenses that are more plus than -1.50DS have a “plus shape” and tend to drop why?

A

They are too thin at the edge and will drop inferiorly due to poor lid attachment

24
Q

What may be added to more plus GP lenses to promote lid attachment?

A

Plano/Minus carrier lenticular may be added to thicken and promote lid attachment

25
Q

How do you adjust for excessively thick edges in GP lenses that are >-5.00D?

A

You can add a PLUS lenticular OR CN beveling can be added to DEC thickness

26
Q

What is the distance between the edge of a GP lens at the periphery and cornea called?

A

Edge Lift

27
Q

How can you adjust the peripheral curve in GP lenses with edge lift?

A
  1. Changing the peripheral curve by 1.0 mm steps
  2. Steepen peripheral curve radii
  3. REDUCE Width of peripheral curve to DEC edge lift
28
Q

Excessive edge lift will cause what?

A
  1. Excessive pooling
  2. DEC centration
  3. Desiccation at 3/9 o clock
29
Q

What will you see in cases of inadequate edge lift?

A
  1. Minimal pooling in periphery
  2. Debris trapped under lens
  3. Poor movement of lens
  4. Adherence
  5. Inadequate tear exchange
  6. Vascularization limbal keratitis
30
Q

How do you treat inadequate edge lift?

A

Flatten the peripheral curve radius!

AND/OR

INCREASE the Width of the peripheral curve

31
Q

How much can CT be changed by?

A

0.03 mm steps

32
Q

what is the CT equation for RGP lenses?

A

CT=0.023*CLP + 0.19

33
Q

Benefits of a thinner CT RGP lens?

A

Better centration

34
Q

Cons of a thinner CT RGP lens?

A

More flexure*

35
Q

What happens with a thicker CT RGP lens?

A

Less flexure but drop inferiorly more often

36
Q

What is required in an RGP that has a larger Dk in order to minimize flexure?

A

CT needs to be thickened

37
Q

What affects center of gravity?

A
  1. Power
  2. Diameter
  3. CT
  4. BC
38
Q

A more POSTERIOR center of gravity will affect your RGP how?

A

Better centration of the CL

39
Q

What will cause an RGP to have a more anterior center of gravity and tend to drop more inferiorly on the eye?

A
  1. Thinner CT
  2. Smaller OAD
  3. Plus power
  4. Flatter BC
40
Q

When would a spherical RGP design be warranted?

A

Cyl in OR: <= 0.75

K Toricity: <=2.50D

Mostly corneal astig but not too much

41
Q

When would a BC Toric RGP design be warranted?

A

Cyl in OR: >= 0.75

K Toricity: >=1.50

Mixed Corneal and Lenticular astig

42
Q

When would a SPE Bitoric RGP design be warranted?

A

Cyl in OR: <0.75

K Toricity: >= 1.50

Mostly corneal astig

43
Q

When would a CPE Bitoric RGP design be warranted?

A

Cyl in OR: >= 0.75

K Toricity: >= 1.50

Mixed Corneal and Lenticular astig

44
Q

When would a F1 toric RGP design be warranted?

A

Cyl in OR: >= 1.00

K Toricity: <= 1.00

Lenticular astigmatism

45
Q

What lens design is it when (3/2)*BC difference = CLP difference?

A

Base Curve Toric

NOTE: when BC=/=CLP it is CPE

46
Q

When is a bitoric GP CL design utilized?

A

When corneal cyl is >= 2.50DC and WTR

NOTE: both front and back surfaces are toric

47
Q

The BC of an RGP CL measured w keratometry is the power for what?

A

The front surface of the tear film

48
Q

Which surface of a bitoric gives MORE astigmatism correction?

A

The back surface

NOTE: the toric front surface is just to compensate for any additional astigmatism correction from the back

49
Q

What are the 2 bitoric GP CL fitting design philosophies?

A
  1. Saddle Fit

2. Low Toric Stimulation

50
Q

Which bitoric GP CL fitting philosophy does this description fit:

  • Equal alignment in both principle meridians btwn the CL & cornea
  • Both principle meridians of the GP CL are fit 0.25D flatter than principle
  • For ATR and/or oblique astigmatism
A

Saddle Fit

Part 1 (31 decks)

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