Keratometry Flashcards
Why is Keratometry important?
- To determine the site of ocular astigmatism
- To fit contact lenses
- When ocular refraction is difficult
- To determine if ametropia is axial or refractive
- To examine corneal health
Where is the power in 42.00 M 45/ 43.75 M 135?
= -1.75 x 45
This gives
What is the aim of keratometry?
To measure the power (and astigmatism) of the anterior corneal surface, the most powerful refractive element in the eye
What is the power of most corneas?
How much dos the cornea contribute to the eye’s total refractive power?
40-45D
60-75%
How are keratometry findings recorded?
Power and orientation of the principal meridia
ie: 42.00 @ 45 or 42.00 M 45
What is an incorrect way to record a keratometry recording?
42.00 x 45
How would you correct 42.00 M 45/ 43.75 M 135
By adding a -1.75 lens to the 135 meridian to bring the total power down to 42.00
If you do keratometry what is being measured?
Which type of astigmatism is measured?
Anterior corneal surface
Corneal astigmatism only
Where is the power in with the rule astigmatism?
Vertical meridian
Where is the power in against the rule astigmatism?
Horizontal meridian
Which astigmatism is most common for people younger than 45?
WTR
Which astigmatism is most common for people older than 45?
ATR
What is oblique astigmatism?
Astigmatism that does not meet the definition of WTR or ATR
Ie: Meridians at 45 and 135
What is irregular astigmatism?
Astigmatism where the principle meridia are not 90 apart
Ie: Axis 180 and axis 100
What are causes of irregular astigmatism?
Keratoconus
Refractive surgery
What can irregular astigmatism be corrected with?
What can it not be correct with?
Rigid contact lenses
Can’t be corrected by spectacles
How do you calculate non corneal astigmatism?
Ocular (total astigmatism)=
Corneal astig. + NC astig
Which type of astigmatism does a refraction give you?
Total ocular astigmatism
What is Javal’s Rule?
OA= 1.25CA + 0.5atr
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?
53D- you would expect a high corneal power
34D- you would expect a low corneal power
What do reflected mires show us?
The state of the anterior surface of the cornea they should be both clear and regular
What does the anterior surface of the cornea act as?
Convex mirror
If the patient has corneal astigmatism what will their reflected mires be shaped as?
Oval
Which type of patient has blurred and irregular mires?
Contact lens overwear-er
How do the mires of a dry eye patient look?
They are clear immediately after blinking then go out of focus until the patient reblinks
What is used to measure the radius of curvature of the anterior corneal surface?
Which equation is used?
Keratometers
F= (n’-n)/r
Do different keratometers give different eye powers? Why?
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
Which is most important to evaluate when fitting contact lenses?
Radius of curvature
Which is the most important to evaluate when calculating corneal astigmatism?
Power
Where does keratometry measure the radius of curvature?
It does not measure at the corneal apex but measures an annulus around the apex
What is the shape of the corneal surface?
Aspheric- flattening ellipse with great variation between individuals
What are corneal topographers used to measure?
- Fitting CL, RGP
- Keratoconus
- Ortho-K
- Pre/Post refractive surgery
- IOL calculations pre cataract surgery
What does a corneal topographer give?
More thorough estimation of corneal shape, corneal topographers take measurements from a large number of points on the corneal surface.
Calculate NC in:
Refraction= -3.00-1.00x180
Keratometry= -1.00 x 90 AM 42.00
OA= 1.00 WTR
CA= 1.00 ATR
OA= NC + CA
1WTR= NC + 1ATR
NC= 2.00D WTR
Write:
42.00 M 25/ 43.37 M 115 in terms of negative correcting cylinder?
Is this WTR or ATR or oblique?
-1.37 x 25 AM 42.00
Oblique
Write:
-2.00 x 25 AM 43.37 in terms of
X @ 90/ Y @ 180
43.37 @ 25/ 45.37 @ 115
Is:
-2.25 x 170 AM 38.87 with the rule or against the rule?
With the rule
