STUDIES

# Keratometry

Keratometry is the measure of the curvature of the cornea.

# Corneal Curvature

The cornea has the most refractive power. Even for then the lens. The average thickness of the cornea is 0.5 mm(100 um). Corneal curvature is measured in millimeters/diopter.

## With the rule vs against the rule.

**With-the-rule** astigmatism means that the strongest axis is the vertical axis (90 degrees) in plus cylinder which is the same as 180 degrees minus cylinder.

**Against-the-rule** astigmatism means that the strongest axis is the horizontal axis (180 degrees) in plus cylinder which is the same as 90 degrees minus cylinder.

Example:

Is the following prescription with-the-rule or against-the-rule.

+1.00 +2.00 @90

Answer: With-the-rule, because the cylinder is positive and at 90 degrees.

Is the following prescription with-the-rule or against-the-rule.

+1.00 +2.00 @180

Answer: Against-the-rule, because the cylinder is positive and at 90 degrees

**Oblique astigmatism **– the steepest curve lies in between 120 and 150 degrees and 30 and 60 degrees.

**Irregular astigmatism** is when the axes of astigmatism are not 90 from each other. In other words they are not perpendicular.

**Regular astigmatism** is he opposite of irregular astigmatism. Regular astigmatism means that the axes of astigmatism are 90 degrees apart. With-the-rule, against-the-rule, and oblique astigmatism are all regular astigmatisms.

# Keratometry

The following are three useful measurement which require keratometry

- Keratoconus – an eye disease that affects the curvature of the eye
- Preoperative cataract surgery
- Contact lens fitting

Steps in using the keratometer

- Focus the eye piece
- Position the patient
- Occlude the eye not being tested
- Focus the mires, and center the cross-hairs in the lower right hand circle
- Turn the drum so that the horizontal plus signs are aligned exactly tip-to-tip
- Turn the dials to superimpose the horizontal plus and vertical minus signs

### Calibration

The keratometer is calibrated with a sphere metal ball with a known radius. If the keratometer needs calibration it needs to be sent to he the manufacturer for repair.

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