2. 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

3. x 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?
MIRE
PRISM (P)
IMAGE PLANE
CORNEA
½ h
h h P C F
½ h
< h x
OBJECTIVE
Fig 13.17, Page 13.18

4. Q1: Based on the previous figure, how could the keratometer prism be used to yield a measure of anterior cornea radius?
for shorter corneal radii, the prism would be moved LEFT to double the images
for shorter corneal radii, the prism would be moved RIGHT to double the images
It could provide a qualitative comparison only between corneas based on separation or overlap of the images

5. Schematic View of the B & L Optical System
ILLUMINATED MIRE
HORIZONTAL & VERTICAL PRISMS
OBJECTIVE LENS
EYEPIECE
OBSERVER PV
Two prisms means two deviated images
GRATICULE PLANE
CORNEAL MIRE IMAGE PH
APERTURE PLATE
OBJ
Fig 13.22, Page 13.27

6. Topcon Keratometer What the Clinician Sees
V 90 / H 180

7. B & L: Oriented to Measure r90 and r180
Corneal vertex
h'90
h'180
OBJ
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

8. B & L: Oriented to Measure r60 and r150
Corneal vertex
h'150
h'60

9. Q2: What does this appearance indicate?
Spherical corneal surface
Irregular Astigmatism
Oblique Astigmatism
With-the-Rule Astigmatism
Against-the-Rule Astigmatism

10. B & L: Oriented to Measure r90 and r180
Corneal vertex
h'90
h'180
OBJ
Question: What does the above appearance indicate?
Answer: anterior corneal astigmatism. What type?
Against-the-rule

11. Estimation of Total Corneal Power
Page 13.23
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)
OBJ

12. Basis of Corneal Power Estimate
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
Effectively  creating a new schematic eye with an anterior cornea only that gives the same total corneal power as the Exact Eye

13. Basis of Corneal Power Estimate - Exact Eye
Page 13.23
Fe (cornea) D
r2 = +6.8 mm
F1 = D
F2 = 5.88 D
naqueous 1.336
ncornea 1.376
nair 1.000
r1 = +7.7 mm

14. Basis of Corneal Power Estimate - Modified Exact Eye
Based on Keratometry  want anterior surface only
Fe (cornea) D
F1 = D
F2 = 5.88 D
ncornea 1.376
r2 = +6.8 mm
naqueous 1.336
nair 1.000
r1 = +7.7 mm

15. Basis of Corneal Power Estimate - Modified Exact Eye
Based on Keratometry  want anterior surface only
Fe (cornea) D
F1 = D
F2 = 5.88 D
r2 = +6.8 mm
naqueous 1.336
nair 1.000
r1 = +7.7 mm

16. Basis of Corneal Power Estimate - Modified Exact Eye
Keep true anterior corneal radius - this is what keratometry measures
Why is new n < 1.336?
Want single surface cornea to give same D as the Exact Eye cornea
naqueous 1.336
nair 1.000
Using n = , the +7.7 mm Exact Eye anterior corneal radius yields correct total corneal power D
Only option is to change naqueous
r1 = +7.7 mm

17. Estimation of Total Corneal Power
Page 13.24
Estimation of Total Corneal Power
Calibration Refractive Index = 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

18. Calibration Refractive Indices - Real Keratometers
Zeiss, Rodenstock
B & L, Haag-Streit (Javal-Schiötz)
American Optical
B&L and AO index based on corneal back vertex power estimate (using posterior cornea as reference plane)

19. Calibration Refractive Index - B & L Keratometer
Page
Calibration Refractive Index - B & L Keratometer
Different keratometer calibration refractive indices will give different total power estimates
OBJ
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  D discrepancy in corneal astigmatism
Intraocular implant design: formula uses total corneal power estimate from keratometry directly  with , the SEEE cornea’s in situ power is 0.78 D higher

20. Intraocular Implant Design
Relies heavily on axial length and keratometer readings:
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)

21. Corneal Power Estimate - Routine Applications
Page 13.25
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