BCSC Refractive Surgery Flashcards
What are the two main classes of refractive surgical procedures?
1) Corneal
2) Lenticular
What are 3 types of maps produced in corneal topography?
1) Axial power and curvature
2) Instantaneous power and curvature
3) Mean curvature
What parameter is used to described the asphericity of a cornea?
Q-value (Q=0 for spherical cornea, Q < 0 for prolate cornea, Q > 0 for oblate cornea)
What is “forme fruste keratoconus?
Subclinical keratoconus
Are patients with keratoconus commonly considered for refractive surgery?
No, unpredictable and often negative responses to procedures usually preclude patients with keratoconus from refractive surgery.
Can irregular astigmatism be corrected with spherocylindrical lenses?
No (the principal axes, highest and lowest power, are not orthogonal to each other)
What is one indication of irregular astigmatism during a refraction?
Inability to identify the axis meridian of the astigmatism
Where does corneal protrusion occur in a patient with Pellucid Marginal Degeneration?
The thinning occurs at the superior or inferior margin of the cornea, and the thinning occurs above the thinning in both cases.
Where does corneal protrusion occur in a patient with Keratoconus?
At the point of maximal thinning.
Defocus (positive or negative) and astigmatism are considered aberrations of what order?
2nd order
Does myopia produce positive or negative defocus?
Positive. (Hyperopia produces negative defocus)
Do the lamellae of collagen fibrils comprising the cornea slide easily over one another?
Yes
Does the corneal stroma distribute tensile stress equally throughout its thickness?
No
Which lamellae of the corneal stroma take up the strain in the healthy or edematous state?
anterior lamellae
Which lamellae of the corneal stroma take up the strain in the dehydrated state?
posterior lamellae
Where does flattening occur after a radial incision in the cornea?
In the meridian of the incision AND the orthogonal meridian
Where does flattening occur after a tangential (arcuate or linear) incision in the cornea?
In the meridian in the incision. Steepening in the orthogonal meridian (coupling) often occurs, but may not equal the amount of flattening.
What are the 5 classes of corneal refractive procedures?
1) Lamellar
2) Keratotomy
3) Keratectomy
4) Collagen shrinkage
5) Penetrating keratoplasty
What are the 5 major methods by which corneal biomechanics are altered in corneal refractive procedures?
1) Incisional effect
2) Tissue addition or subtraction
3) Alloplastic material addition
4) Laser effect
5) Collagen shrinkage
What depth of cornea must be maintained to retain structural integrity?
approximately 50%% of stromal depth (estimated at 250 to 300 microns)
What is keratophakia?
The addition of a tissue lenticule or synthetic inlay to the corneal stroma.
What is the minimum size of the optical zone in PRK needed to avoid glare, haloes, and regression?
6mm
How does the ablation depth required for PRK change with the size of the ablation zone?
Ablation depth grows as square of the optical zone diameter (as given by Munnerlyn’s formula)
What is the advantage of multizone ablations in PRK?
Multizone ablation can reduce the overall ablation depth by providing the full correction centrally and less than the full correction
What temperature needs to be achieved to cause corneal collagen to shrink?
55 to 60 degrees Celsius
What are the two primary methods of heating corneal collagen to produce shrinkage?
1) thermokeratoplasty – with laser
2) conductive keratoplasty – with radiofrequency diathermy probe
