Refractive Corneal Surgery

1. See Also
   1. Refractive Error
2. Epidemiology
   1. Incidence: 600,000 procedures per year in U.S. (peaked at 1.4 Million in 2006)
   2. Age: 30-40 years old most typical
3. Indications
   1. Refractive Error Correction
4. Contraindications
   1. See Laser In-Situ Keratomileusis (LASIK)
5. Background: Refractive Surgery reshapes Cornea
   1. Myopia (Near-sightedness)
      1. Eye globe is too long, and light rays focus in front of Retina
      2. Surgery flattens Cornea to focus light further back
   2. Hyperopia (Far-sightedness)
      1. Eye globe is too short, and light rays focus behind Retina
      2. Surgery steepens Cornea to focus light closer
   3. Astigmatism
      1. Cornea is non-spherical with uneven surface
      2. Surgery evens out Cornea, directs light at Retina
6. Background
   1. See Cornea for anatomy
   2. Cornea undergoes computer-aided reshaping with multiple different techniques
      1. For Myopia, Corneal curvature decreased by removing central Corneal tissue
      2. For Hyperopia, Corneal curvature increased by removing peripheral Corneal tissue (less predictable)
      3. For Astigmatism, Corneal surface is smoothed
   3. Refractive Surgery mechanism
      1. Pulsatile laser (excimer laser) is used to a pre-programmed section of Corneal Stroma
7. Techniques
   1. Laser In-Situ Keratomileusis (LASIK)
      1. Contact Lens sized flap is made into the Corneal Epithelium (via femtosecond/cutting laser or microtome)
      2. Cornea epithelial flap is folded over, allowing laser access to the underlying Corneal Stroma
      3. Corneal Stroma is removed via pulsatile (excimer) laser
      4. Corneal flap is folded back into place and spontaneously re-adheres
      5. Variant methods use Lenticule (lens-shaped) extraction: FLEx, SMILE
   2. Photorefractive Keratectomy (PRK)
      1. Epithelial surface is scraped away allowing access to Corneal Stroma (but causing a large Corneal Abrasion)
      2. As with LASIK, Corneal Stroma is removed via pulsatile (excimer) laser
      3. Replaced by LASIK for most patients
   3. LASEK
      1. Variant of PRK, in which the epithelial layer is removed with Alcohol
   4. Radial Keratotomy (RK)
      1. Replaced by LASIK
   5. Laser Thermal Keratoplasty (LTK)
      1. YAG laser mediated radial spot placement
      2. Heats Cornea and shrinks stromal Collagen
      3. Modifies anterior Corneal curvature
      4. Corrects presbyopic patient with <4 D Refraction
   6. Intrastromal corneal ring (ICR or Intacs)
      1. Polymethylmethacrylate (PMMA) ring implanted
      2. Flattens central Cornea
      3. Corrects low Myopia (<3 D)
      4. Surgery easily adjustable
      5. No risk of central Corneal scarring
   7. Phakic Intraocular Lens
      1. Plastic intraocular lens
      2. Placed anterior to natural crystalline lens
      3. Corrects high Myopia and Hyperopia
8. References
   1. Bower (2001) Am Fam Physician 64(7):1183-90 [PubMed]
   2. Sanders (1998) J Cataract Refract Surg 24:607 [PubMed]
   3. Schanzlin (1997) Ophthalmology 104:1067 [PubMed]
   4. Wilkinson (2017) Am Fam Physician 95(10): 637-44 [PubMed]
   5. (1999) Med Lett Drugs Ther 41(1068):121-3 [PubMed]