الفهرس 
Laser In Situ Keratomileusis (LASIK)Only 14 pages are availabe for public view
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Abstract
The cornea, conjunctiva, and the limbus comprise the tissue at the ocular surface. All are covered by a nonkeratinized stratified squamous epithelium which support the tear film and serve as a barrier to fluid loss and pathogen entrance.
LASIK is done under topical anesthesia and require proper preoperative instrument setup then performing the keratectomy, laser ablation, then reposition of the flap. Postoperative management includes; topical antibiotics, steroids, and preservative-free lubricating DROPs, follow up and patient cautions as not to rub the cornea.
Incisional wound healing starts with formation of a fibrin plug in the wound cleft which is replaced in a few days by invaginating epithelial cells. Keratocytes lining the plug undergo myofibroblastic transformation which is followed by withdrawal of the epithelial cells from the wound cleft, constriction of the wound and appearance of extracellular matrix typical of wound fibrosis. Later the active myofibroblasts are replaced by quiescent keratocytes. After LASIK some of the stromal nerves within the flap shows degenerative changes, but the architecture of the nerves under the flap remain reasonably normal. Circumferentially located regenerative nerve fronds are formed at the flap margin. The epithelial innervation is restored several months after LASIK.
LASIK affects the tear film leading to dry eye symptoms, fluctuating vision that improve after blinking or instillation of artificial tear substitutes, mild to severe punctuate epithelial erosion and rose Bengal staining confined to the flap in about 4% of patients with decreased tear secretion and decreased tear film stability. Three conditions that can occur associated with dry eye after LASIK; one is neurotrophic epitheliopathy as a result of surgical amputation of the corneal nerves and laser ablation of the nerve fibers innervating the central corneal surface, second is a decrease in conjunctival and corneal sensitivity, and third is changes in the tear lipid layer by reducing corneal sensitivity and rate of blinking leading to tear film lipid layer dysfunction, which increase tear evaporation during the inter blink interval leading to dry eye.
Epithelial hyperplasia can lead to regression of refractive effect in the early postoperative months after LASIK. The epithelium showed a tendency toward hyperplasia starting from the first day reaching the maximum thickness between the first and third months after LASIK.
Epithelial defects are the most common intraoperative complication in LASIK, it occurs when the shearing force from microkeratome pass overwhelms the adhesion between the epithelium and its basement membrane. The consequence of such defects lead to delayed achievement of desired visual acuity, diffuse lamellar keratitis, recurrent erosion syndrome, epithelial ingrowth and microbial keratitis.
Ectopic epithelium growing within the LASIK lamellar interface can produce significant morbidity, its incidence vary from 1% to 20%. It occurs when the microkeratome blade cuts through the peripheral epithelium, there will always be some epithelial tissues that adhere to the blade which may be deposited within the interface. If it contains viable cells, it may proliferate and produce a nest of cells within the interface, this leads to foreign body sensation and decrease of the best corrected visual acuity if it extends into the pupil.
Flap striae and wrinkles are fine lines in a lattice pattern that may represent folds in the epithelium or Bowman’s layer after LASIK which are related to disparity between the curvature of posterior surface of the flap and the bed following ablation, thus the incidence of striae appears to increase as the level of correction increase. It may be asymptomatic, cause monocular diplopia, multiplopia, glare, halos, star burst, and reduction of the visual acuity if the wrinkles are in the visual axis.
Thin and even buttonhole flaps are one of the more common complications during the learning curve. Buttonhole in the flap occur when the blade travels too superficially and breaches the central epithelial-Bowman’s complex. Thin flap occurs when the keratome cut within or anterior to Bowman’s layer. It manifest with a shiny reflex on the stromal surface and can be recognized by a flap thickness of less than 60 nanometer.