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OT > Keratometry > Flashcards

Keratometry Flashcards

1
Q

Why is Keratometry important?

A
  1. To determine the site of ocular astigmatism
  2. To fit contact lenses
  3. When ocular refraction is difficult
  4. To determine if ametropia is axial or refractive
  5. To examine corneal health
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2
Q

Where is the power in 42.00 M 45/ 43.75 M 135?

A

= -1.75 x 45
This gives

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3
Q

What is the aim of keratometry?

A

To measure the power (and astigmatism) of the anterior corneal surface, the most powerful refractive element in the eye

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4
Q

What is the power of most corneas?

How much dos the cornea contribute to the eye’s total refractive power?

A

40-45D

60-75%

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5
Q

How are keratometry findings recorded?

A

Power and orientation of the principal meridia

ie: 42.00 @ 45 or 42.00 M 45

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6
Q

What is an incorrect way to record a keratometry recording?

A

42.00 x 45

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7
Q

How would you correct 42.00 M 45/ 43.75 M 135

A

By adding a -1.75 lens to the 135 meridian to bring the total power down to 42.00

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8
Q

If you do keratometry what is being measured?

Which type of astigmatism is measured?

A

Anterior corneal surface

Corneal astigmatism only

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9
Q

Where is the power in with the rule astigmatism?

A

Vertical meridian

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10
Q

Where is the power in against the rule astigmatism?

A

Horizontal meridian

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11
Q

Which astigmatism is most common for people younger than 45?

A

WTR

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12
Q

Which astigmatism is most common for people older than 45?

A

ATR

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13
Q

What is oblique astigmatism?

A

Astigmatism that does not meet the definition of WTR or ATR

Ie: Meridians at 45 and 135

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14
Q

What is irregular astigmatism?

A

Astigmatism where the principle meridia are not 90 apart

Ie: Axis 180 and axis 100

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15
Q

What are causes of irregular astigmatism?

A

Keratoconus
Refractive surgery

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16
Q

What can irregular astigmatism be corrected with?
What can it not be correct with?

A

Rigid contact lenses

Can’t be corrected by spectacles

17
Q

How do you calculate non corneal astigmatism?

A

Ocular (total astigmatism)=
Corneal astig. + NC astig

18
Q

Which type of astigmatism does a refraction give you?

A

Total ocular astigmatism

19
Q

What is Javal’s Rule?

A

OA= 1.25CA + 0.5atr

20
Q

If the Rx= -9.00sph and the astigmatism is refractive what would the corneal power be?

If the Rx=+12.00sph and the astigmatism is refractive what would the corneal power be?

A

53D- you would expect a high corneal power

34D- you would expect a low corneal power

21
Q

What do reflected mires show us?

A

The state of the anterior surface of the cornea they should be both clear and regular

22
Q

What does the anterior surface of the cornea act as?

A

Convex mirror

23
Q

If the patient has corneal astigmatism what will their reflected mires be shaped as?

A

Oval

24
Q

Which type of patient has blurred and irregular mires?

A

Contact lens overwear-er

25
Q

How do the mires of a dry eye patient look?

A

They are clear immediately after blinking then go out of focus until the patient reblinks

26
Q

What is used to measure the radius of curvature of the anterior corneal surface?

Which equation is used?

A

Keratometers

F= (n’-n)/r

27
Q

Do different keratometers give different eye powers? Why?

A

Yes, ie: Zeiss uses n’= 1.332 while Haag uses n’= 1.3375.

The two instruments would give different eye powers since a different n’ is used

28
Q

Which is most important to evaluate when fitting contact lenses?

A

Radius of curvature

29
Q

Which is the most important to evaluate when calculating corneal astigmatism?

A

Power

30
Q

Where does keratometry measure the radius of curvature?

A

It does not measure at the corneal apex but measures an annulus around the apex

31
Q

What is the shape of the corneal surface?

A

Aspheric- flattening ellipse with great variation between individuals

32
Q

What are corneal topographers used to measure?

A
  1. Fitting CL, RGP
  2. Keratoconus
  3. Ortho-K
  4. Pre/Post refractive surgery
  5. IOL calculations pre cataract surgery
33
Q

What does a corneal topographer give?

A

More thorough estimation of corneal shape, corneal topographers take measurements from a large number of points on the corneal surface.

34
Q

Calculate NC in:

Refraction= -3.00-1.00x180
Keratometry= -1.00 x 90 AM 42.00

A

OA= 1.00 WTR
CA= 1.00 ATR

OA= NC + CA
1WTR= NC + 1ATR
NC= 2.00D WTR

35
Q

Write:
42.00 M 25/ 43.37 M 115 in terms of negative correcting cylinder?

Is this WTR or ATR or oblique?

A

-1.37 x 25 AM 42.00

Oblique

36
Q

Write:
-2.00 x 25 AM 43.37 in terms of
X @ 90/ Y @ 180

A

43.37 @ 25/ 45.37 @ 115

37
Q

Is:
-2.25 x 170 AM 38.87 with the rule or against the rule?

A

With the rule

OT (13 decks)

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