Photoablation Flashcards
(31 cards)
excimer laser - wavelengths and photochemically
193-nm argon-fluoride (ArF)
excimer laser - in which methods
PRK, LASEK, epi-LASIK
LASIK - short definition
In LASIK, the excimer laser ablation is performed under a lamellar flap that is created with either a mechanical microkeratome or a femtosecond laser
What does excimer on molecular level
A single 193-nm photon has sufficient energy to directly break carbon–carbon and carbon–nitrogen bonds that form the peptide backbone of the corneal collagen molecules. Excimer laser radiation ruptures the collagen polymer into small fragments
PRK - good for
irregular or thin corneas; epithelial basement membrane dystrophy, previous corneal surgery, such as PKP and RK; and treatment of some LASIK flap complications, such as incomplete or buttonholed flaps
Surface Ablation vs LASIK
eliminates the potential for stromal flap–related complications and may have a decreased incidence of postoperative dry eye as compared to LASIK. Corneal haze, the major risk of PRK, decreased markedly with the use of adjunctive mitomycin C; subsequently, the use of PRK for higher levels of myopia has increased
When Wavefront-guided is inappropriate
after cataract surgery, particularly with multifocal intraocular lenses
Topography-guided systems
use corneal topography data to create ablation profiles that treat existing corneal shape irregularities and optimize corneal curvature. In the treatment of corneas with irregular surfaces, such as those with small or decentered optical zones from prior excimer ablations, LASIK flap complications, or post-RK corneal irregularities
Is keloid contraindication for surface ablation or LASIK?
No
Postoperative dry eye due to corneal denervation is more common with
LASIK than with surface ablation
Corneas steeper than 48.00 D are more likely to
have thin flaps or frank buttonholes (central perforation of the flap) with procedures using mechanical microkeratomes
Corneas flatter than 40.00 D are more likely to
have smaller-diameter flaps and are at increased risk for creation of a free cap due to transection of the hinge with mechanical microkeratomes. These problems may be reduced by using a smaller or larger suction ring
estimate the postoperative keratometry
flattening of 0.80 D for every diopter of myopia treated and a steepening of 1.00 D for every diopter of hyperopia treated
which layers are exposed in 1) surface ablation and 2) LASIK
Bowman layer for surface ablation and the midstroma for LASIK
Manifest or cycloplegic refraction is more accurate determining cylinder axis and amount?
manifest refraction
What is blend zone?
The blend zone is an area of peripheral asphericity designed to reduce the possible undesirable effects of an abrupt transition from the optical zone to the untreated cornea. A prolate blend zone reduces the risk of glare and halo after excimer laser photoablation
Epithelial debridement techniques for surface ablation. The epithelium can be removed with
sharp blade, blunt spatula, rotary corneal brush, application of 20% absolute alcohol to the corneal surface for 10–45 seconds, mechanical microkeratome with an epi-LASIK blade, transepithelial ablation from the excimer laser
LASEK
goal is to preserve the patient’s epithelium. loosens the epithelium with 20% alcohol for 20 seconds and folds back an intact sheet of epithelium
Epi-LASIK
epithelial flap is fashioned with a microkeratome fitted with a blunt epikeratome and a thin applanation plate that mechanically separates the epithelium
Suction ring dimention
The thicker the vertical dimension of the suction ring and the smaller the diameter of the ring opening, the less the cornea will protrude, and hence a smaller-diameter flap will be produced.
Advantages of femtosecond
more customizable flap, size and thickness of flap less dependent on corneal contour, centration easier to control, epithelial defects on flap are rare, less risk of free cap and buttonhole, more reliable flap thickness, hemorrhage from limbal vessels less likely, ability to re-treat immediately if incomplete femtosecond laser ablation
Disadvantages of femtosecond
longer suction time, more flap manipulation, opaque bubble layer interfere with excimer ablation, bubbles in the AC may interfere with tracking and registration, increased overall treatment time, difficulty lifting flap after 6 months, increased risk of transient light sensitivity, increased cost, delayed photosensitivity or good acuity plus photosensitivity, which may require prolonged topical corticosteroid therapy
How quick heals the epithelium after surface ablation?
4-7 days
Why corticosteroids after surface ablation?
modulate postoperative wound healing, reduce anterior stromal haze, and decrease regression of the refractive effect remains controversial. Patients who received mitomycin C at the time of surgery have a reduced risk of haze formation and thus may have a shorter duration of corticosteroid use.
