Keratometry

Question 1

Javan’s rule

Answer 1

Predicts the total refractive astigmatism with keratometry findings

ARx = 1.25 AC + (-0.50 D x 090)
Where ARx = total refractive astigmatism, and AC= Corneal Astigmatism

(Corneal Astigmatism = (Flat - Steep) x Flat)

Question 2

Corneal Astigmatism

Answer 2

Amount of difference between principle meridians on the cornea

Flat reading (D) - Steep reading (D) x Flat Meridian

Question 3

Oblique Astigmatism

Answer 3

Prescription axis 30-60 or 120- 150

Question 4

Against the rule astigmatism

Answer 4

Sleeper horizontal meridian

Prescription axis 060 - 120 degrees

Question 5

With the rule astigmatism

Answer 5

Steeper vertical meridian

Prescription axis 0-030, 150-180

Question 6

Steps in Manual keratometry

Answer 6

1. Focus the eye piece
2. Align patient
3. Focus the system
4. Rotate the barrel to align the crosses/minuses
5. Use the horizontal and vertical focusing knobs to make crosses/minuses continuous
6. Interpret your findings

Question 7

cornea mirror properties

Answer 7

convex mirror

Question 8

Elevation knob

Answer 8

Raises and lowers the barrel of the keratometer.

The device needs to be unlocked prior to adjusting this knob.

If done while locked the barrel tends to fall abruptly once unlocked.

Question 9

Chin rest elevation knob

Answer 9

Raises and lowers the chin rest without altering the height of the barrel

Used when positioning the patient in the keratometry

Question 10

Locking knob

Answer 10

Locks the barrel in place to prevent any vertical or horizontal adjustments from being made but does not prevent focusing or chin rest adjustments

Question 11

Chin rest

Answer 11

Keeps the patient at the proper height as adjusted by the chin rest elevation knob

Question 12

Forehead rest

Answer 12

keeps the patient at the proper distance and helps maintain focus when patient is fully against the headrest

Question 13

Head rest adjustment knob

Answer 13

Adjusts the tilt of the entire forehead rest forwards or backwards.

Most patients should be situated vertically without angle.

If performing keratometry and one reaches the limit of adjusting the focusing knob before achieving clarity this should be checked

Question 14

Occluder

Answer 14

Used to cover the fellow eye not being evaluated to aid in fixation.
The side facing towards the
examiner is often white to help aid in initial focusing of the Eyepiece

The occluder can be rotated clockwise
or counterclockwise to occlude the next eye to avoid scraping/bumping into the patient’s nose. Do not attempt
to pull the occluder towards yourself. It WILL break!

Question 15

Alignment knob

Answer 15

A sighting mechanism to help align the barrel vertically.

Question 16

cornea refractive properties

Answer 16

convex refractive surface

Question 17

photokeratoscope

Answer 17

placido disc image, detects subtle shifts in corneal curvature/corneal topography

Question 18

videokeratograph

Answer 18

provides color-coded map of virtually entire corneal surface - can detect corneal irregularities that may be causing changes in VA

Question 19

keratometry readings

Answer 19

measured to the nearest 1/8 diopter and truncated to the hundredth’s place

Question 20

converting dioptric power to radius of curvature

Answer 20

D = 337.5(mm)/Rmm

Question 21

Extending range via trial lenses more than 52.00 D

Answer 21

add +1.25D

extends upper range to 61.00 D by adding 9.00 D to K reading

Question 22

Extending range via trial lenses less than 36.00 D

Answer 22

add - 1.00D

extends lower range to 30.00 D by subtracting -6.00 D from your k-reading