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Keratometry: Main Points so Far• Keratometry uses the anterior cornea as a mirror
• Distant object: h rAC
• Keratometer Equation:
• Virtual corneal image (h) inaccessible, small and
unstable (eye movements), so:– Use objective lens to focus reflected rays as a real image
– magnify the real image with an eyepiece lens (~ 5 mag)
– split the real image inside the keratometer into two using a half-field prism; adjust prism to “double” images
h
hbrAC
2
OBJECTIVE
h
½ hC
MIRE
F
CORNEA
½ h
h
< h
PRISM (P)IMAGE PLANE
P
Fig 13.17, Page 13.18
Moving prism toward image plane decreases image displacement (x)
Previously doubled images are no longer doubled (now overlap)
What new corneal radius would this prism position “suit”?
What happens if we move the prism?
x
Q1: Based on the previous figure, how could the keratometer prism be used to yield a measure of anterior cornea radius?
for s
horter c
orneal radii,.
..
for s
horter c
orneal radii,.
..
It co
uld provid
e a qualita...
33% 33%33%
1. for shorter corneal radii, the prism would be moved LEFT to double the images
2. for shorter corneal radii, the prism would be moved RIGHT to double the images
3. It could provide a qualitative comparison only between corneas based on separation or overlap of the images
Fig 13.22, Page 13.27
ILLUMINATED MIRE
HORIZONTAL & VERTICAL PRISMS
EYEPIECEOBSERVER
GRATICULE PLANE
APERTURE PLATE
CORNEAL MIRE
IMAGEPH
PV
OBJECTIVE LENS
Schematic View of the B & L Optical System
Two prisms means two deviated images
OBJ
Topcon Keratometer
What the Clinician Sees
V 90 / H 180
h'90
Corneal vertex
B & L: Oriented to Measure r90 and r180
h'180
Question: If most corneas are aspheric, what is one drawback with a keratometer?
Answer: only measuring radius at one location (annulus) on cornea; and it is NOT central radius
OBJ
h'60
Corneal vertex
B & L: Oriented to Measure r60 and r150
h'150
60
150 30
90
180 (0)
15
45
75105120
135
165
180
Q2: What does this appearance indicate?
Spheric
al corn
eal surfa
ce
Irregular A
stigm
atism
Obliq
ue Astigm
atism
With
-the-R
ule Astigm
atism
Against-
the-Rule Astig...
20% 20% 20%20%20%
1. Spherical corneal surface
2. Irregular Astigmatism
3. Oblique Astigmatism
4. With-the-Rule Astigmatism
5. Against-the-Rule Astigmatism
h'90
Corneal vertex
B & L: Oriented to Measure r90 and r180
h'180
Question: What does the above appearance indicate?
Answer: anterior corneal astigmatism. What type?
Against-the-rule
OBJ
Estimation of Total Corneal Power Most keratometers read out both anterior radius and total corneal power. How is this possible?
It is not!
Keratometer gives only anterior corneal radius - it cannot measure posterior radius total corneal power reading is an estimate
Estimate usually reasonable because the anterior cornea carries so much of the total corneal power (big n)
Page 13.23
OBJ
To see how we could estimate total corneal power from Keratometry (anterior radius alone) modify the Exact Eye to simulate what the keratometer is measuring
Basis of Corneal Power Estimate
Effectively creating a new schematic eye with an anterior cornea only that gives the same total corneal power as the Exact Eye
Basis of Corneal Power Estimate - Exact Eye
r1 = +7.7 mm
naqueous
1.336
ncornea
1.376
nair
1.000
r2 = +6.8 mmFe (cornea) +43.05 D
F1 = +48.83 D
F2 = 5.88 D
Page 13.23
Basis of Corneal Power Estimate - Modified Exact Eye
r1 = +7.7 mm
naqueous
1.336nair
1.000ncornea
1.376
r2 = +6.8 mmFe (cornea) +43.05 D
F1 = +48.83 D
F2 = 5.88 D
Based on Keratometry want anterior surface only
Basis of Corneal Power Estimate - Modified Exact Eye
r1 = +7.7 mm
naqueous
1.336nair
1.000
r2 = +6.8 mmFe (cornea) +43.05 D
F1 = +48.83 D
F2 = 5.88 D
Based on Keratometry want anterior surface only
Basis of Corneal Power Estimate - Modified Exact Eye
r1 = +7.7 mm
naqueous
1.336nair
1.000
Keep true anterior corneal radius - this is what keratometry measures
Want single surface cornea to give same
+43.05 D as the Exact Eye cornea
1r
nnFcornea
Only option is to change naqueous
Using n = 1.3315, the +7.7 mm Exact Eye
anterior corneal radius yields correct total
corneal power +43.05 D
Why is new n < 1.336?
Estimation of Total Corneal Power Calibration Refractive Index = 1.3315 works for real corneas if:
anterior : posterior corneal radii are in the same proportion as the SEEE cornea (7.7/6.8)
central thickness of the cornea is 0.5 mm
Usually a good estimate, but keratometer cannot verify either of these properties
Page 13.24
Calibration Refractive Indices - Real Keratometers
Zeiss, Rodenstock 1.332
B & L, Haag-Streit (Javal-Schiötz) 1.3375
American Optical 1.336
B&L and AO index based on corneal back vertex power estimate (using posterior cornea as reference plane)
Calibration Refractive Index - B & L Keratometer Different keratometer calibration refractive indices will give different total power estimates
Contact lens practice corneal power estimate used to estimate total corneal astigmatism.
Astigmatism rarely exceeds 10% of total corneal power(~ +43 D) 0.78 D discrepancy in total power estimate translates to 0.078 D discrepancy in corneal astigmatism
Intraocular implant design: formula uses total corneal power estimate from keratometry directly with 1.3375, the SEEE cornea’s in situ power is 0.78 D higher
OBJ
Page 13.24-25
Intraocular Implant Design
Relies heavily on axial length and keratometer readings:
KLAP 9.05.2
IOL power for
emmetropia
Constant based on IOL type
Axial length in mm
Average total corneal power based
on keratometry
OBJ: when applying formula, the basis (ncal) of the K value must be consistent
with the ‘A’ value (design constant)
Corneal Power Estimate - Routine Applications Estimating total corneal astigmatism.
Estimating total ocular astigmatism: intraocular astigmatism averages 0.5 D atr for most patients with moderate to high astigmatism, corneal astigmatism is a good predictor of total ocular astigmatism
Problem with estimates of total ocular astigmatism keratometry will not identify exceptions to the trend
Page 13.25
