Refractive & Optics Flashcards
Munnerlyn formula
Ablation Depth (µm) ≈ Degree of Myopia (D) × (Optical Zone diameter)2 (mm)/3
Minimum residual corneal stromal bed thickness has not been definitively established, but should not be thinner than __ µm
Minimum residual corneal stromal bed thickness has not been definitively established, but should not be thinner than 250 µm
Post-op keratometry readings lower than about __ D and higher than about __ D increase the risk of ___ ___ and poor-quality vision after refractive surgery.
Post-op keratometry readings lower than about 34.00 D and higher than about 50.00 D increase the risk of spherical aberration and poor-quality vision after refractive surgery.
Differentiate DLK from microbial keratitis (3)
DLK is confined to the flap interface, is characterized by diffuse inflammation, and is seen usually within the first 24 hours
Microbial keratitis extends beyond the interface and the flap edge, is characterized by focal inflammation around the infection, and its onset occurs 2-3 days postoperatively and even later in some cases of infection with atypical microorganisms
Compare and contrast AKs and LRIs (2 differences, 2 similarities)
AK incisions are made in midperiphery (7-9 mm optical zone) while LRIs are made at the limbus, and AK incisions are made at greater depth.
Both procedures correct astigmatism without inducing a hyperopic shift (ie, the coupling ratio is 1.0). Both procedures have increased effect in older patients.
Second-order aberrations (3)
Myopia (positive defocus), hyperopia (negative defocus), regular astigmatism
Third-order aberrations (2)
Coma, trefoil
Fourth-order aberration
Spherical aberration
Patients should discontinue use of soft contact lenses for __ days to __ weeks, soft toric contact lenses for __ weeks or longer, and rigid contact lenses for __–__ weeks prior to refractive surgery evaluation.
Patients should discontinue use of soft contact lenses for 3 days to 2 weeks, soft toric contact lenses for 2 weeks or longer, and rigid contact lenses for 2–3 weeks prior to refractive surgery evaluation.
Which has greater effect, LASIK or PRK, and why?
LASIK because ablation occurs in weaker posterior stroma
___ keratometry and loss of ___ are associated with an increased incidence of buttonhole flaps
Steep keratometry and loss of suction are associated with an increased incidence of buttonhole flaps
Differentiate DLK from PISK (2)
DLK has an earlier onset and is responsive to steroids, whereas interface fluid from elevated IOP (PISK) typically occurs later and is unresponsive to steroids.
Stages and treatment of DLK
Stages 1 and 2: scattered interface opacities in the periphery and centrally, respectively. Topical corticosteroids.
Stages 3 and 4 are characterized by central opacities that have coalesced, and corneal scarring or melting, respectively. Lift flap and irrigate, topical or oral corticosteroids.
It is important to check the ___ before cutting a corneal flap with a microkeratome to ensure adequate suction has been achieved. Inadequate suction can result in irregular, ___ flaps.
It is important to check the IOP before cutting a corneal flap with a microkeratome to ensure adequate suction has been achieved. Inadequate suction can result in irregular, partial flaps.
Risk factors for epithelial ingrowth following LASIK (2)
Epithelial defect along flap edge
Secondary flap lifts
Mechanism of femtosecond laser
Photodisruption: tissue changed into plasma that leads to microscopic cavities
A pupil size smaller than __ mm can interfere with near vision.
A pupil size smaller than 2 mm can interfere with near vision.
A post-LASIK ___ shift, especially with onset of ___-the-rule astigmatism, can be a sign of post-LASIK ectasia.
A post-LASIK myopic shift, especially with onset of with-the-rule astigmatism, can be a sign of post-LASIK ectasia.
Complications of glare, halos, and regression increase when the optical zone ___. For this reason, the optical zone should be ___.
Complications of glare, halos, and regression increase when the optical zone decreases. For this reason, the optical zone should be 6 mm or larger.
Why are IOL calculations difficult to accurately obtain in someone who has had laser refractive surgery? (2)
(1) Standard IOL calculations make assumptions about the relationship between the anterior and posterior curvatures of the cornea, which is altered post ablation.
(2) Effective central optic zone after refractive surgery is small (imagine a flatter central cornea after post-myopic LASIK), and most keratometers measure curvature several mm away from the center of the cornea, which results in inaccurate estimation of the true central corneal power.
Refractive surgery for myopia results in a ___ surprise post cataract surgery; refractive surgery for hyperopia results in a ___ surprise.
Refractive surgery for myopia results in a hyperopic surprise post cataract surgery; refractive surgery for hyperopia results in a myopic surprise.
For an arcuate keratotomy procedure, cylindrical correction can be increased by ___ the length or depth of the incision, using multiple incisions or ___ the optical zone.
For an arcuate keratotomy procedure, cylindrical correction can be increased by increasing the length or depth of the incision, using multiple incisions or reducing the optical zone.
___-order aberrations are highly dependent on pupil size and ___ as the size of the pupil increases.
Higher-order aberrations are highly dependent on pupil size and increase as the size of the pupil increases. (More peripheral rays enter pupil)
Lower-order aberrations, such as hyperopia, myopia, and regular cylinder, are not significantly affected by pupil size.
Spherical aberration results in ___ and decreased ___ ___, but does not typically result in a significant loss of ___ ___.
Spherical aberration results in halos and decreased contrast sensitivity, but does not typically result in a significant loss of best-corrected acuity.
