What Part of the Cornea Does Keratometry Assess?
- The a 3mm zone in the central cornea
- Corneal topography is able to assess the entire cornea
Uses of a Keratometer
- Determining degree of corneal astigmatism present
- Obtaining location of astigmatic meridians (WTC or ATR)
- Radii of curvature for contact lens fitting
- Assessing CL fit
- Detection of corneal pathology
Determining Degree of Astigmatism from K Readings
- For each 0.05mm of difference this equates to approx 0.25DC of corneal astigmatism
- Any residual astigmatism in the prescription is lenticular
Flat and Steep K Readings
- Flatter cornea’s have larger numerical values and are less dioptriclally powerful
- A steeper cornea has a smaller numerical value and are more dioptric value
With the Rule or Against the Rule Astigmatism
- In WTR the flatter K is located along the horizontal meridian
- In ATR the flatter K is located along the vertical meridian
- Oblique astigmatism doesn’t fall into either of these categories
Where is WTR and ATR Astigmatism More Common
- WTR is more common in young and myopic eyes
- ATR is more common in older eyes with cataract
How to Obtain SCL BOZR from K Readings
- Take flattest K and add 0.7 or multiply by 1.1
How to Obtain RGP BOZR from K Readings
- Use formula and round answer to nearest 0.05mm
- FlatK- ([FlatK-SteepK]/3)
- It gives a slightly flatter fit than the flattest K
Keratometer in Assessing SCL Fit
- Bausch and Lomb Keratometer only
- A good fitting lens will show clear mires before and after a blink
- A tight fitting lens will show distorted mires which clear up after a blink
- A flat fitting lens will show clear mires which distort right after blink
Keratometry in Detection of Pathology
Keratoconus will show as:
- Minification of mires due to high myopia
- Oval mires due to significant astigmatism
- Wavy and irregular mires seen in advanced keratoconus
Principle of Keratometry
Based on that principle that:
- Cornea acts as a mirror which produces images of the mires
- The size of these images is dependent upon the radius of curvature of the corneal surface
- Image size can then be used to determine K values
Bausch and Lomb Keratometer Design
- One position keratometer
- Works on principle of constant object size and varying image size
Javal- Schiotz Keratometer Design
- Two position keratometer
- Works on principle of varying image size and constant image size
Bausch and Lomb Keratometer Pros and Cons
Advantages
- Measures both meridians at same time
- Quick and easy to use
- Can be used for other purposes e.g. SCL fitting and keratoconus assessment
Disadvantages
- Inaccurate in presence of irregular astigmatism
- Less accurate due to shortened working distance
Javal-Schiotz Keratometer Pros and Cons
Advantages
- Can be used to measure irregular astigmatism
- More accurate due to longer working distance
Disadvantages
- Need to move to second position
