Refractive Flashcards
In LASIK, how does the myopic correction change the post-operative K’s? Hyperopic?
- 0.8 X every Diopter corrected (e.g. 2 diopter corrected make the post-op K’s 1.6 flatter2. 1 X every diopter corrected for hyperopia
In general, what is the accepted post-op K’s in LASIK?
34-50 (to flat can risk free cap, to steep can risk K buttons)
Multifocal lens Restore and Tecnis are both what type of lens? (refracting or diffracting)?
Diffracting
The natural cornea is prolate or oblate?
prolate (steeper at center and flatter at periphery)
For monovision, which eye is usually set for near? What is the target refraction often?
The non-dominant is set for near and the target is often -1.5 to -1.75. “mini monovision” is -0.75
The RSB (residual stromal bed) should not be less than what number?
250 microns
What are the two refractive errors that are common in patients having undergone RK?
- diurnal fluctuation2. Hyperopic shift (ave about 1.0 D)
LRI’s tend to induce no change in spherical equivalence and a net coupling of 1.0. However, straight keratotomies tend to induce what?
a hyperopic shift (they have a positive coupling factor)
Keratophakia
A high plus powered lens is placed intrastromally to correct high hyperopia or aphakia. Now obsolete.
Epikeratoplasty
homoplastic (living transplant) placed onto bowmans layer of the host. No reshaping of the cornea. Basically a contact lens placed into the cornea. Obsolete.
Spherical aberration is what type of higher order abberation?
4th order
Which type of higher order abberation is more visually significant?
spherical abberation
What is the difference between tracking and registration in laser treatment?
tracking follows the pupil, registration the iris
For the same amount of correction, does a hyperope or a myope take longer to have stable vision following refractive surgery?
hyperopes
What is the most common cause of interstial keratitis following LASIK?
Staph A and other gram positives
INTACS is ideal for what type of patient
low myopic keratoconus patient
What is a distinguishing exam finding that can often distinguish interstitial keratits from DLK in a post refractive patient?
DLK is usually contained under the flap and IK can spreak past the flap border.
When does Pressure-induced stromal keratoapthy usually manifest post-LASIK?
About 10-14 days
What is the treatment for pressure induced stromal keratopathy?
Rapid taper off corticosteroid
What is the optimal temperature to induce collagen shrinkage in cornea when performing conductive keratoplasty?
65 degrees celcius +/- 10 degrees
Conductive keratoplasty can be used for what type of refractive error?
low hyperopes
How is corneal cross linking performed?
- Epithelium is denuded2. Riboflavin administered X 30 minutues3. UVA light administered X 30 minutes with continued riboflavin instillation
What does bioptics mean in refractive surgery?
combining refractive surgery and intraocular surgery sequentially
ND:YAG uses photodisruption like what other laser used in refractive surgery?
femtosecond
When calculating power for two piggy backs, how much power should be added to the combined power so as to compensate for posterior shift of the more posterior IOL?
+1.50 to +2.50
Does Restore or Tecnis multifocals have harder time with reading in dark?
Restore because the diffraction component is only 3.6mm
In regards to higher order abberrations, what is rms value is considered normal?
Less than 0.3 um
What is the normal q value for asphericity of the human cornea?
-0.26, negative value means it is a prolate cornea, i.e. Steeper centrally
A myopic patient that had overcorrection from PRK/LASIK can be helped to induce myopia by doing what?
Going off steroids
