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2-Spring18-ContactLenses > Block 5 > Flashcards

Block 5 Flashcards

1
Q

Which meridian is with-the-rule astigmatism steeper?

A

Vertically (90 degree)

Wider than it is tall

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2
Q

What should the keratometer read if calibrated correctly?

A

7.5/45

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3
Q

Which is steeper? 45.0 D or 42.0 D?

A

45 is steeper

Higher the diopter is steeper

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4
Q

Which is steeper? 7.5mm or 8.04mm

A

7.50mm is steeper

The lower the mm is steeper

Meters and diopters are inversely related

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5
Q

How many millimeters is 0.5 D?

A

0.1mm

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6
Q

How many millimeters is 5 D?

A

1mm

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7
Q

How many millimeters is 1 D?

A

0.2mm

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8
Q

If 42.00 D = 8.04mm, how many millimeters is 42.50 D?

A
  1. 94mm
  2. 5 - 42 = 0.5 D = 0.1mm
  3. 04 - 0.1mm = 7.94mm
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9
Q

If 45 D = 7.50mm, then how many mm are in 44.50 D?

A
  1. 60mm

7. 50mm + 0.10mm = 7.60

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10
Q

What is the index of refraction of the cornea?

A

1.3375

If calculating with radius of curvature, use 337.5

N-1(1000) = 337.5

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11
Q

At what power do you have to start worrying about vertex distance?

A

After +/- 4 Diopters

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12
Q

What do the hotter colors on the computer keratometry mean?

A

Those are the steeper areas

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13
Q

What are the Sim K values on the computer keratometry reading?

A

They are the equivalent to the manual keratometry reading

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14
Q

How do you expand the range of the ketatometer for a very high reading? (Above 52 diopters)

A

Place a +1.25 D trail lens over the objective opening to increase the range by 9 diopters

OR multiply the K reading by 1.185 for an exact reading

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15
Q

How do you expand the range of the keratometer for a very low reading? (Less than 36D)

A

Place a -1.00 D trail lens over the objective opening to shift it down by 6 D

OR

Multiply K reading by 0.840

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16
Q

What are the advantages of the keratometer?

A

Ease of use

Low cost

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17
Q

What are the disadvantages of the keratometer?

A

Only central 3 mm
Apex is not directly measured, ONLY the geometric center - which is not always the steepest K reading (contacts rest at the corneal apex)
Examiner error possible
The keratometer is change may not correspond with the refractive change

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18
Q

What is keratometry good for?

A

Initial selection of lens parameters
Initial base curve selection
Prediction of residual cylinder

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19
Q

What is the most important assessment in GP contact lens fitting?

A

Fluorescein pattern evaluation

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20
Q

What is the most important assessment for soft lens fit?

A

Contraction and lens lag

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21
Q

What does it mean if there is fluorescein pooling in a ring around the center and none at the edge?

A

Fluorescein needs 20 microns to show up, so it places like the middle where there is no fluorescein it means it is sitting tightly against the cornea there

No fluoro at the edges means CL is too tight

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22
Q

What are the regions of the cornea?

A

Optic cap - steepest area at the center - 3-4mm

Paracentral/mid peripheral region - flattens as you go out - 4-8mm

Peripheral region - 8-11mm

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23
Q

What should you do if you can’t get the keratometer in focus?

A

Move the head reset placement

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24
Q

What do you need to make sure you do in strabismic patients?

A

Definitely occlude one eye

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25
Q

Back surface is what shape?

A

Concave

MINUS power

-45

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26
Q

What shape is the front surface?

A

Convex

PLUS power

+45

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27
Q

The back surface is the ____ curve?

A

Fitting

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28
Q

The front surface is the _____ curve

A

Power

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29
Q

If you require a power of -5 D, what will the front and back surface powers be?

A

Back surface will be -45 and front surface will be +40 (flatter)

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30
Q

If you need a power of +5 D what should the front and back surfaces be?

A

Back is -45, front is +50

Steeper

31
Q

To accurately measure the total corneal power, what do you need to measure? 3 things

A

Anterior corneal curvature
Posterior corneal curvature
Central corneal thickness

32
Q

Which is the one surface that the keratometric equation takes into consideration?

A

Anterior corneal curvature

This is why it is only an approximate calculation of optical power of the cornea

33
Q

The curvature (in diopters) is a reciprocal of what?

A

Radius of curvature (in meters)

34
Q

A large radius of curvature means?

A

Small curvature

35
Q

What is normal astigmatism?

A

2 principal meridians are perpendicular

36
Q

Keratometry can not measure?

A

Asphericity

37
Q

What is toricity?

A

A difference in the horizontal meridian vs the vertical meridian

Aka astigmatism

38
Q

What is asphericity?

A

Steeper in the middle vs flatter in the outer or vice versa

Not a perfect sphere

39
Q

Steeper in the vertical meridian is what kind of astigmatism?

A

With the rule

40
Q

Steeper in the horizontal meridian is what kind of astigmatism?

A

Against the rule

41
Q

What is the difference between axial vs tangential maps?

A

In axial maps all the circles are lined up on the same optical axis
(Most people prefer these maps)

In the tangential maps the radius is just measured from off the tangent to the surface (the radii may not be in a line)

42
Q

What are elevation maps?

A

All the notches in or out have the same curvature

There is a best fit sphere placed on top, the cornea below it is a depression (show up as blue on map) and the cornea above it is an elevation (show up as red on the map)

43
Q

What is the difference in the axial curvature maps vs the elevation maps?

A

The axial curvature maps look like the normal astigmatism maps (bow tie pattern)

The elevation maps show the elevation or depression at the edges, so there are red spots in the superior and inferior with yellowish color in the middle

44
Q

What does bright green mean on elevation maps in a contact lens fitting?

A

Green means the more clearance between the lens and the cornea

Black means less clearance

45
Q

What does the Scheimpflug do for centration that the OCT does not?

A

The Scheimpflug uses the corneal apex as a standard reference

The OCT relies on the examiner’s identification of the pupil center

46
Q

Which machine takes several pictures across the cornea and then puts them together like pizza slices

A

Pentacam

But this one takes more time

47
Q

This is the opposite of the pentacam.. it is quicker because it only takes one picture?

A

Corneal topography

48
Q

What is Cassini’s elevation based topography?

A

It is asymmetrical with 7 trianglular plates

It is more sensitive to small differences

It can discern feature changes along both the radial and tangential directions

49
Q

What is the usual radius of curvature for a contact lens?

A

7-9mm

50
Q

What is the purpose of having bicurve or tricurve contact lenses?

A

Helps in fitting the lens

There are no optical properties

Most CLs are bi or tri curves

51
Q

Where is the power located in CLs?

A

Optical zone

It must be larger than scotopic pupil size

52
Q

What is the base curve of the contact lens?

A

The curvature of the central portion of the posterior surface of the lens

53
Q

The cornea is _____ and the base curve is ______

A

Cornea is steeper

Base curve is flatter

54
Q

What is the chord diameter?

A

The width of the contact lens

Usually 13-15mm

Some riding gas permeable (RGP) are smaller

55
Q

Are the peripheral curves flatter or steeper than the base curve?

A

Usually flatter to blend and enhance comfort

56
Q

Why is large thickness detrimental to contact lenses?

A

Thicker the lens means less oxygen permeability

57
Q

What is the Sagittal depth of the contact lens?

A

The distance between the center of the lens to the plane connecting the edges

The sagittal depth decreases as the base curve increases(lens is flatter)

58
Q

With the same diameter, an increased lens curvature means?

A

More sagitta

59
Q

Black in a fluorescein picture means?

A

Contact, near zero clearance

60
Q

What is the tear lens?

A

An optical lens formed by the entrapped tear film layer between the posterior surface of a contact lens and anterior surface of the cornea

61
Q

As a rule of thumb for tear lens power, for every 0.05mm radius of curvature difference, what is the diopter power?

A

0.25 D

The steeper the base curve, the more plus lens power

The flatter the base curve, the less tear lens power

62
Q

The steep part of the cornea that usually includes the geometric center

A

Apical zone

Usually 3-4mm

63
Q

This is the steepest point of the cornea

A

Corneal apex

64
Q

What is the usual degree between the pupillary axis and the light of sight?

A

3-5 degrees

65
Q

What is the vertex distance?

A

The distance between the back surface of the lens and the front surface of the cornea

66
Q

What is the formula for the downstream optical power?

A

Power/(1-transposition (in m) * power)

67
Q

How do plus and minus lenses affect the retinal image?

A

Plus lens magnifies

Minus lens minifies

So with CLs, less magnification for a plus and less minification for a minus lens

68
Q

When switching myopes from glasses to contacts what may happen?

A

Highly myopic patients may have better VAs in contacts b/c of the minification that happens with glasses

Hyperopes have the opposite effect - less VA with contacts

This is important for aniseikonia and aphakia***

69
Q

In myopia, the need to converge is ___ with a CL

A

More

70
Q

In hyperopia, the need to converge is ___ with CL

A

Less

71
Q

In myopia there is ___ accommodative demand with CLs

A

More

A myope when viewing a near object accommodates more with CLs than white spectacles

72
Q

In hyperopia ____ accommodative demand is needed with CLs

A

Less

A hyperope when viewing a near object will accommodate less with CLs

73
Q

Who is credited to first thinking about contact lenses?

A

Leonardo DaVinci

74
Q

What material supposed to have the best oxygen permeability

A

Silicone hydrogel

2-Spring18-ContactLenses (1 decks)

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