BCSC Refractive Surgery Flashcards

1
Q

What are the two main classes of refractive surgical procedures?

A

1) Corneal

2) Lenticular

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2
Q

What are 3 types of maps produced in corneal topography?

A

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

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3
Q

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

A

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

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4
Q

What is “forme fruste keratoconus?

A

Subclinical keratoconus

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5
Q

Are patients with keratoconus commonly considered for refractive surgery?

A

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

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6
Q

Can irregular astigmatism be corrected with spherocylindrical lenses?

A

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

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7
Q

What is one indication of irregular astigmatism during a refraction?

A

Inability to identify the axis meridian of the astigmatism

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8
Q

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

A

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

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9
Q

Where does corneal protrusion occur in a patient with Keratoconus?

A

At the point of maximal thinning.

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10
Q

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

A

2nd order

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11
Q

Does myopia produce positive or negative defocus?

A

Positive. (Hyperopia produces negative defocus)

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12
Q

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

A

Yes

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13
Q

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

A

No

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14
Q

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

A

anterior lamellae

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15
Q

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

A

posterior lamellae

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16
Q

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

A

In the meridian of the incision AND the orthogonal meridian

17
Q

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

A

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

18
Q

What are the 5 classes of corneal refractive procedures?

A

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

19
Q

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

A

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

20
Q

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

A

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

21
Q

What is keratophakia?

A

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

22
Q

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

A

6mm

23
Q

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

A

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

24
Q

What is the advantage of multizone ablations in PRK?

A

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

25
Q

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

A

55 to 60 degrees Celsius

26
Q

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

A

1) thermokeratoplasty – with laser

2) conductive keratoplasty – with radiofrequency diathermy probe