Corneal Toricity & Keratometry/Topography

Regular vs. Irregular Astigmatism

• Regular: principle medirian 90* apart

• Irregular: principle medirian NOT 90* apart

Keratoconus

irregular cornea

What are With the Rule and Against the rule indicating?

• it indicate the axis that is 90* away from the cyl

With the rule

axis 0/180 - 030 and 150 - 180

Against the rule

axis 60 - 120

oblique astigmatism

axis 030 - 060 & 120 - 150

pros vs cons of keratometry

PROS:

• To assess the curvature, power and toricity of the cornea

• Assess the integrity of the corneal/tear surface

CONS:

• Measures very small central portion of cornea, ~ 2-3 mm

How is automated corneal topography better?

• Map of the entire surface curvature of cornea

• Map of the relative “steepness” or “flatness” of the cornea

K-map for spherical cornea

• red = steeper

• less steep as you move away from center → cooler colors 

how to read K-map

• it represents corneal map

• Steep corneal meridian - more RED

  • has more plus → requires minus rx in that position

• Flatter corneal meridian - cooler colors

  • has more minus → requires plus rx in that position

Is this ATR or WTR?

with the rule

Is this ATR or WTR?

against the rule

Keratoconus K-map characteristics

• small radius of red circle (steep)

• medium radius of orange circle (avg)

• large radius of green circle (flat

as radius decreases, the curve becomes steeper

as radius increases, the curve becoems flatter

diopter and radius (mm) conversion

index of refraction of cornea (n) - 1.3375

D=(n-1)/r

1.3375-1 = 0.3375 × 1000 = 337.5 (makes math easier for mm)

=> RADIUS = 337.5 / DIOPTER

Why is anterior corneal power is not useful for measuring refractive power?

it does not consider the posterior corneal curvature

→ we calculate the “central” corneal power 

Why is measuring the curvature of cornea important?

• fitting contacts

• determining amt of astigmatism