Corneal Measurements and Keratometry Flashcards
(15 cards)
1619: Christoph Scheiner
First described the principle of comparing the curvature of reflections coming off the cornea to the curvature of reflections coming off small convex mirrors placed next to the cornea.
1728: Francois Pourfour du Petit
Invented a device to measure the anterior and posterior chambers of the eye and cornea.
1794: Jesse Ramsden
Developed an instrument to measure the cornea to prove that the cornea was the accommodative aspect of the eye.
1881: Javal and Schoitz
Modified the 1851 design and created adjustable size mires and fixed doubling.
1932: Bausch and Lomb
Created a keratometer that has improved focus, circular mires, and the ability to look at two meridians at once.
What is the process of doubling in regards to keratometry?
Doubling is the process in which a prism is introduced into the measuring system and the prism is moved until the images touch each other.
What is the standard index of refraction for a keratometer?
1.3375 because it is the combined index of the aqueous and the cornea. This is the basis for the conversion table for the radius of curvature to diopters of power.
What is the minimum radius of curvature of the keratometer?
36D which requires a minimum chord length of 3.8mm
What is the maximum radius of curvature of the keratometer?
52D which requires a minimum chord length of 2.6mm
What are the main steps of using a keratometer?
- Adjust the eyepiece
- Align the patient
- Level the keratometer
- Align with the center of the cornea
- Locate the cylinder axis
- Measure the horizontal meridian
- Measure the vertical meridian
- The difference between the two meridians is the amount of corneal astigmatism
What are limitations of a keratometer?
- Corneas can be irregular and measurements are sometimes not exact.
- Only looks at the center 3-4mm of the cornea
- Not very accurate on steep or flat corneas
- Diopters of power are based on an inaccurate index of refraction
What is an Axial map (corneal topography)?
It is one of the quickest ways to get an overview of corneal power because it averages the corneal power to create a smooth map. Averages are not as accurate, and can become more innaccurate due to the cornea becoming more aspheric in the periphery. Good for corneal and soft contact base curve selection.
What is an Tangential Map (corneal topography)?
The most accurate of the power maps because it measures power and curvature at individual points. This is ideal if the fitter needs to identify minute changes in the corneal surface.
What is the size of the fixed mire on the keratometer?
64mm
What is the distance between the mire and the cornea?
75mm
