9. Other Refractive Surgery and Orthokeratology Flashcards
briefly define PRK and LASIK. what does it stand for, each procedure, and which has an advantage over the other.
techniques to modify refractive status by removing corneal tissue.
photorefractive keratectomy: epithelium and some underlying stroma removed.
laser assisted insitu keratomileusis: flap of epithelium is removed, stroma ablated. quicker healing, less uncomfortable.
what can PRK and LASIK be used for?
correction for myopia, hypermetropia, astigmatism.
for PRK and LASIK, briefly describe the process and important zones when correcting myopia (write notes). typical ablation size.
in myopia, loss of tissue greatest in corneal centre. tissue is ablated for particular diameter. outside of an “optical” zone, there may be transition zone so there is smoothing between treated and untreated parts of the cornea.
ablation size varies, but typically of the order of 6mm.
list some effects expected of PRK and LASIK?
induce high level of positive spherical aberration.
problems with large pupils (e.g. glare at night)
likely to be increased aberrations associated with peripheral vision.
(+bifocal effect resulting from different refractive effects occurring simultaneously through central and peripheral zones)
what aberration is associated with hypermetropia treatment through surgery?
treatment for hypermetropia results in negative spherical aberration.
What is “SuperVision”? discuss characteristics and theory of how it is achieved. name a system used to produce optimum correction.
It is theoretically possible to design the ablation to eliminate the aberrations of the eye, thus producing diffraction limited vision - “SuperVision”. the aberrations of the eye must be measured before surgery, and the ablation designed accordingly. there are different systems available to produce this optimum correction (e.g. Bausch and Lomb’s Zyoptix system.
concerning “SuperVision”, why is it not yet possible to eliminate aberrations?
Issues in healing process, such as changes to posterior cornea following treatment.
because of this, it is not possible to eliminate aberrations at present, but it is possible to reduce from what would otherwise be obtained.
regarding the optics for myopic surgery, what are some assumptions in the calculations?
assumes spherical surfaces, no allowance for effectivity, and no transition zone.
name the important variables when calculating optics for myopic surgery. draw diagram. (p.7)
say the refractive correction required is X dioptres and the surface has power K.
the surface required after surgery has power K + X.
untreated surface has radius of curvature r1 = (n-1)/K
the treated surface has radius of curvature r2 = (n-1)/(K+X)
how to calculate the amount of tissue to be removed from cornea in PRK and LASIK surgery? draw diagram and equation. (p.7)
if the ablation zone has diameter D, the amount of tissue t to be removed from the center of the cornea can be determined by finding the difference between the sags for the two surfaces.
describe tools and process of SMILE, difference from LASIK.
SMILE uses femtosecond laser to separate a thin lenticule (or disc) of corneal tissue from within the cornea. the disc is removed through a very small incision. there is no corneal flap and no tissue is vaporized. less disruptive than LASIK.
describe corneal inlays.
in recent years, there have been corneal inlays to correct refractive errors. A lens has been designed to fit on the surface of the cornea after removal of the epithelium. the epithelium then grows back over the top. inlay supposably easy to remove.
