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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

  1. Keratoconus – an eye disease that affects the curvature of the eye
  2. Preoperative cataract surgery
  3. Contact lens fitting

Steps in using the keratometer

  1. Focus the eye piece
  2. Position the patient
  3. Occlude the eye not being tested
  4. Focus the mires, and center the cross-hairs in the lower right hand circle
  5. Turn the drum so that the horizontal plus signs are aligned exactly tip-to-tip
  6. 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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