Optical Parameters of the Cornea Flashcards
The average power of the total cornea for young adults is approximately A. 40D B. 43D C. 47D D. 49D E. 51D
B. 43D
Keratometry is used to measure the
A. Anterior corneal surface power
B. The anterior corneal radius of curvature
C. The posterior corneal radius of curvature
D. The total corneal thickness
E. The corneal diameter
B. The anterior corneal radius of curvature
The calibration index used by the keratometer is
A. Higher than the typical corneal refractive index
B. Used to calculate total refractive power of the reduced cornea
C. Equal to the index of the Gullstrand #1 model aqueous
D. Equal to the index of the Gullstrand #1 model vitreous
E. Equal to the index of the Gullstrand #1 model cornea
B. Used to calculate total refractive power of the reduced cornea
Which of the following is not part of the principles of keratometry?
A. Measurement of the PS #I image of the mires on the cornea
B. A Scheiner’s disk that creates 2 clear but split images
C. A doubling prism that slides along the optical axis to align the displaced images
D. An internal calibration index of 1.332-1.3375 depending on the brand of keratometer
E. A central corneal measurement zone of 6-8mm in diameter
E. A central corneal measurement zone of 6-8mm in diameter
Which is the most common shape for the normal cornea?
a. Aspheric with high positive asphericity values, +2.00 or more
b. Aspheric with high negative asphericity values, -2.00 or more
c. Aspheric with low positive asphericity values, +0.50 or more
d. Aspheric with low negative asphericity values, -0.50 or less
e. Spherical with an asphericity value of 0
d. Aspheric with low negative asphericity values, -0.50 or less
Which layer of the cornea has the greatest thickness?
a. Epithelium.
b. Bowman’s layer.
c. Stroma.
d. Endothelim
e. The tear film.
c. Stroma.
Given the size of the mires is 5cm and the first Purkinje- Sanson image of the mires is 2.79mm. If u, the distance from the mires to the first PS#I image is 7cm what is the radius that the keratometer will read?
a. 5.85mm
b. 6.78mm
c. 7.81mm
d. 8.28mm
e. 9.58mm
c. 7.81mm
. If the radius of the front surface of the cornea is measured and read off the keratometer, as 7.6mm what is the actual power of the front surface of the cornea?
a. 52.47D
b. 41.47D
c. 48.87D.
d. 44.40D.
e. 49.47D.
e. 49.47D.
For valid keratometric readings, the cornea must be spherical (or sphero-cylindrical) over a region of diameter of about
a. 1 mm.
b. 3 mm.
c. 5 mm.
d. 7 mm.
e. 12 mm.
b. 3 mm.
What is the approximate power of the posterior cornea in vivo?
a. +48.8D
b. +43.0D
c. +6.00D
d. –6.00D
e. -43.0D
d. –6.00D
If a luminous object is 5 meters in front of an eye, where is the 1st PS image located? Assume that the anterior corneal radius of curvature is 6mm and the corneal index of refraction is 1.376.
a. 1mm or less behind the cornea (in the eye)
b. 2mm behind the cornea (in the eye)
c. 3mm behind the cornea (in the eye)
d. 4mm behind the cornea (in the eye)
e. 5mm behind the cornea (in the eye)
c. 3mm behind the cornea (in the eye)
If the K reading is 40D and the calibration index is 1.3375, what is the radius of curvature of the cornea?
a. 6.84 mm
b. 7.44 mm
c. 7.84 mm
d. 8.44 mm
e. 9.44 mm
d. 8.44 mm
Which of the following is not true about keratoscopy
A. It is quick and simple to use
B. It gives precise measurements of the corneal diameter
C. It creates a reflection of ring shaped mires on the entire cornea
D. It is used to observe corneal distortion by viewing the images of the mires
E. It employs a concentric ring system called Placido’s disk
B. It gives precise measurements of the corneal diameter
All of the following are true of Pachymetry except which one?
A. It is used as a preoperative test for corneal surgeries
B. It measures corneal thickness
C. Thick corneas may overestimate IOP however rigidity may also play a role
D. Pachymetry will show lower readings in a swollen cornea
E. Thin corneas with high IOP readings may be at most risk for glaucoma
D. Pachymetry will show lower readings in a swollen cornea
