keratometry

Question 1

keratometer measures what?

Answer 1

refractive power in the two principle meridians of the cornea. only gives info regarding astigmatism of cornea NOT about spherical ametropia.

Question 2

the keratometer measures what directly? it then converts it to? what are the normal values of the radius of curvature of the cornea?

Answer 2

radius of curvature of the center corneal thickness and then converts it to diopters.
normal radius values are around 3.5mm to 3.75mm.

Question 3

the cornea acts as what type of surface?

Answer 3

convex refracting surface or a convex mirror.

Question 4

wollaston or biprism uses what? B&L?

Answer 4

two position keratometers (2 real mires)

uses two prisms oriented horizontally and vertically (one position keratometer, 1 real mire).

Question 5

keratometer object is what? image?

Answer 5

object is the fixed circle

images are called mires (they are virtual and erect, smaller than object, these images are formed by prisms)

Question 6

a steeper cornea has what kind of power with respect to radius?
a flatter cornea has what kind of power with respect to radius?

Answer 6

higher power therefore shorter radius

lower power therefore longer radius

Question 7

avg cornea has a power of? range of powers measured on the B&L keratometer?

Answer 7

43-44D

36-52D

Question 8

if the powers in each meridian (90 degrees apart from each other) are the same what is expected of the cornea? if they are not the same?

Answer 8

if the same the cornea is spherical

if they differ, the there is some corneal astigmatism

Question 9

what is the general order in which you focus your image (mires)?

Answer 9

1. get image in focus
2. once image is in focus try to align, two minuses are together and 2 positives are together and cross is in the middle
3. once alignment is done, merge the two pluses to make one plus, merge the two minuses to make one minus once this is done these are your two principle meridians.

Question 10

when recording which measurement comes first?

Answer 10

horizontal meridian power @ axis, vertical meridian power @ axis difference between the two powers (which is your corneal astigmatism) WR or AR, MCAR (mires clear and regular)

Question 11

what are the four types of astigmatism?

Answer 11

1. WR: more power in vertical meridian
2. AR: more power in horizontal meridian
3. oblique: not within 30 degrees of 90 or 180
4. irregular: two principle meridians are NOT 90 degrees apart.

Question 12

what are the general forms of Rx’s for WR, AR and oblique?

Answer 12

• cylx180 +/- 30 (WR)
• cylx90 +/- 30 (AR)
• cyl x30 to 60 and x120-150 (oblique)

Question 13

javals rule used for what?

Answer 13

predicting total (refractive) astigmatism
total astig= 1.25(corneal astig) + internal astig
internal astig is a constant= 0.50 AR

Question 14

problems with javals rule?

Answer 14

internal astig is variable and increases with age, may be sources of internal astig other than the lens, constant of 1.25 is an aprox (doesnt consider ametropes)

Question 15

common sense rules with javals rule?

Answer 15

if there is a sig corneal astigmatism, it will show on refraction
if there is a sig refractive astig, it probably comes from cornea
K’s and refraction are HIGHLY correlated.