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

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

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

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

Topcon Keratometer What the Clinician Sees V 90 / H 180

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

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

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

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

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

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

Basis of Corneal Power Estimate - Exact Eye Page 13.23 Fe (cornea) +43.05 D r2 = +6.8 mm F1 = +48.83 D F2 = 5.88 D naqueous 1.336 ncornea 1.376 nair 1.000 r1 = +7.7 mm

Basis of Corneal Power Estimate - Modified Exact Eye Based on Keratometry  want anterior surface only Fe (cornea) +43.05 D F1 = +48.83 D F2 = 5.88 D ncornea 1.376 r2 = +6.8 mm naqueous 1.336 nair 1.000 r1 = +7.7 mm

Basis of Corneal Power Estimate - Modified Exact Eye Based on Keratometry  want anterior surface only Fe (cornea) +43.05 D F1 = +48.83 D F2 = 5.88 D r2 = +6.8 mm naqueous 1.336 nair 1.000 r1 = +7.7 mm

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 +43.05 D as the Exact Eye cornea naqueous 1.336 nair 1.000 Using n = 1.3315, the +7.7 mm Exact Eye anterior corneal radius yields correct total corneal power +43.05 D Only option is to change naqueous r1 = +7.7 mm

Estimation of Total Corneal Power Page 13.24 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

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 Page 13.24-25 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  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

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)

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