Corneal Refraction & CL Optics Flashcards
Briefly explain how modern excimer lasers work
Argon-monofluoride complex decays emitting excess photons as UV radiation which break covalent bonds within corneal stromal tissues creating microscopic ablations
controlled pulses (100-500Hz), longer the stromal bed is exposed the greater amount of tissue ablated
What are som pros/cons of radial keratometry?
improves uncorrected vision and simple
light scattered at incision interfaces (night glare), unpredictable healing with more susception to corneal perforations with refractive regression
Describe LASIK flap and zap technique
plate applied onto cornea with scleral suction ring
mechanical oscillating microkeratome cuts flap 160um thick of epithelium, Bowman’s, anterior stroma
stromal tissue bed ablated/reshaped with excimer laser then flap is replaced/positioned
What are the main pros/cons of LASIK (flap & zap)?
improves high uncorrected vision with(out) astigmatism, quick procedure/recovery, pain-free
if post-op residual stromal bed tissue thickness is too thin (<250um), forward bulging (corneal ectasia) can occur ~ iatrogenic keratectasia (keratoconus)
reduced post op. corneal sensitivity (dry eye), regression, flap complications, inflammatory cells trapped under flap (DLK - intra-stromal haze)
Explain the procedure for advanced LASIK (INTRALase)
Femtosecond (10^-15) laser fires extremely high speed pulses (IR@1053nm) to cause stromal collagen ‘photo-disruption’ (covalent bonds in stromal tissue break creating ablations/CO2/H2O bubbles)
repeated pulses create overlapping pattern of numeours ablations/bubbles (corneal flap) for surgeon to peel back with tweezers
increased LASIK flap creation precision with less complications
Explain the procedure for LASEK and why it’s typically reserved for thin corneas
Laser Assisted Sub-Epithelial Keratectomy
only epithelium disrupted: ethanol applied onto cornea (~30secs) to lossen epithelial cells’ bond to Bowman’s
Epithelium (40um thick) is retained/temporarily moved aside with flat spatula
excimer laser applied to underlying Bowman’s/anterior stromal tissue to ablate/correct any refractive error
epithelium is re-spread and protective, bandage ExW CL is fitted for 2-7 days for epithelium healing
What are the pros/cons of LASEK?
quick, effective over RK, improves uncorrected vision, usually re-treats LASIK px
much longer healing (up to 7 days) where px can experience significant pain, px can develop idiopathic, corneal sub-epithelial haze (reduced VA) or long-term dry eye
Describe the relationship between the RGP CL and Fluid Lens system
we assume the CL/FL/cornea are each separated by infinitely thin films of air and the RGP/tear FL powers combine to correct the eye’s ametropia
overall FL power depends on the RGP BOZR/FL’ front surface power
What limitations and benefits are there when correcting corneal astigmatism with spherical RGP CLs?
Tear Fluid Lens underneath CL has n1.336 closely matching corneal n1.376
on-eye lens comfort compromised due to undue corneal staining (lens rocking) if cyl >2.00DC so here a back surface toric RGP CL should be fitted
