BCSC Refractive Surgery

Question 1

What are the two main classes of refractive surgical procedures?

Answer 1

1) Corneal

2) Lenticular

Question 2

What are 3 types of maps produced in corneal topography?

Answer 2

1) Axial power and curvature
2) Instantaneous power and curvature
3) Mean curvature

Question 3

What parameter is used to described the asphericity of a cornea?

Answer 3

Q-value (Q=0 for spherical cornea, Q < 0 for prolate cornea, Q > 0 for oblate cornea)

Question 4

What is “forme fruste keratoconus?

Answer 4

Subclinical keratoconus

Question 5

Are patients with keratoconus commonly considered for refractive surgery?

Answer 5

No, unpredictable and often negative responses to procedures usually preclude patients with keratoconus from refractive surgery.

Question 6

Can irregular astigmatism be corrected with spherocylindrical lenses?

Answer 6

No (the principal axes, highest and lowest power, are not orthogonal to each other)

Question 7

What is one indication of irregular astigmatism during a refraction?

Answer 7

Inability to identify the axis meridian of the astigmatism

Question 8

Where does corneal protrusion occur in a patient with Pellucid Marginal Degeneration?

Answer 8

The thinning occurs at the superior or inferior margin of the cornea, and the thinning occurs above the thinning in both cases.

Question 9

Where does corneal protrusion occur in a patient with Keratoconus?

Answer 9

At the point of maximal thinning.

Question 10

Defocus (positive or negative) and astigmatism are considered aberrations of what order?

Answer 10

2nd order

Question 11

Does myopia produce positive or negative defocus?

Answer 11

Positive. (Hyperopia produces negative defocus)

Question 12

Do the lamellae of collagen fibrils comprising the cornea slide easily over one another?

Answer 12

Yes

Question 13

Does the corneal stroma distribute tensile stress equally throughout its thickness?

Answer 13

No

Question 14

Which lamellae of the corneal stroma take up the strain in the healthy or edematous state?

Answer 14

anterior lamellae

Question 15

Which lamellae of the corneal stroma take up the strain in the dehydrated state?

Answer 15

posterior lamellae

Question 16

Where does flattening occur after a radial incision in the cornea?

Answer 16

In the meridian of the incision AND the orthogonal meridian

Question 17

Where does flattening occur after a tangential (arcuate or linear) incision in the cornea?

Answer 17

In the meridian in the incision. Steepening in the orthogonal meridian (coupling) often occurs, but may not equal the amount of flattening.

Question 18

What are the 5 classes of corneal refractive procedures?

Answer 18

1) Lamellar
2) Keratotomy
3) Keratectomy
4) Collagen shrinkage
5) Penetrating keratoplasty

Question 19

What are the 5 major methods by which corneal biomechanics are altered in corneal refractive procedures?

Answer 19

1) Incisional effect
2) Tissue addition or subtraction
3) Alloplastic material addition
4) Laser effect
5) Collagen shrinkage

Question 20

What depth of cornea must be maintained to retain structural integrity?

Answer 20

approximately 50%% of stromal depth (estimated at 250 to 300 microns)

Question 21

What is keratophakia?

Answer 21

The addition of a tissue lenticule or synthetic inlay to the corneal stroma.

Question 22

What is the minimum size of the optical zone in PRK needed to avoid glare, haloes, and regression?

Answer 22

6mm

Question 23

How does the ablation depth required for PRK change with the size of the ablation zone?

Answer 23

Ablation depth grows as square of the optical zone diameter (as given by Munnerlyn’s formula)

Question 24

What is the advantage of multizone ablations in PRK?

Answer 24

Multizone ablation can reduce the overall ablation depth by providing the full correction centrally and less than the full correction

Question 25

What temperature needs to be achieved to cause corneal collagen to shrink?

Answer 25

55 to 60 degrees Celsius

Question 26

What are the two primary methods of heating corneal collagen to produce shrinkage?

Answer 26

1) thermokeratoplasty – with laser

2) conductive keratoplasty – with radiofrequency diathermy probe