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Reshaping cornea for improved vision

LASIK is a 15 to 20 minute procedure that corrects nearsightedness, farsightedness and astigmatism by reshaping the cornea, the transparent membrane that covers the front of the eye. When successful, this procedure allows a person eliminate or reduce the need for glasses or contact lenses. When successful, this procedure allows a person to enjoy life without glasses or contact lenses. LASIK may well be the fastest growing surgical procedure. It is estimated that 500,000 to 750,000 Americans will have this procedure each year.

Anatomy and Physiology

There are two eyes. The eyes lie in bony casings within the skull called orbits. Each eye is globular in shape and about one inch in diameter. Light entering the front of the eye passes through the cornea, anterior (front) chamber, iris, lens and vitreous and finally is focused on the retina, the film of the eye. (Figure 1,2)

Figure 1 Cross-sectional anatomy of the eye. See text for descriptions.

Figure 2 Normal eye. An object is focused on the retina.
  • The cornea is a curved, highly transparent tissue that separates air from clear fluid in the anterior chamber of the eye, which lies between the cornea and lens. The cornea is approximately 0.5 mm. thick.
  • The lens is a firm gel-like transparent tissue that is almost eight millimeters (one-third inch) in diameter and biconvex in shape, that is, thicker in the center than at the edge. A thin transparent capsule surrounds the lens.
  • The iris is in front of the lens and consists of a circular pigmented muscle that gives the eye its color. The iris acts like the diaphragm of a camera and adjusts the amount of light that enters the eye through the hole in its center called the pupil. Light then passes through the vitreous, a clear gel-like material that fills the center of the eye, onto the retina.
  • The retina is the film of the eye. It is a true extension of the brain and is composed of special nerve cells sensitive to light.
  • The optic nerve is formed from these nerve cells and carries the light image entering the eye to the brain


Three abnormalities of vision may be corrected by LASIK surgery. The degree of abnormality from 20/20 vision is measured in diopters, a unit of measurement of the refractive (light bending) power of a lens

  • Nearsightedness occurs when the cornea has a steeper curve than normal or the length of the eyeball from front to back is too long. In either situation distant light entering the eye focuses in front of the retina. The degree of nearsightedness is measured in negative (-) diopters
Figure 3 Nearsighted eye. A distant object is focused in front of the retina causing a blurred image.
  • Farsightedness occurs when the cornea is flatter than normal or the length of the eyeball from front to back is too short. In either situation light from near objects focuses behind the retina. The degree of farsightedness is measured in positive (+) diopters
Figure 4 Farsighted eye. A near object is focused behind the retina causing a blurred image.
  • Astigmatism is caused by an irregularity of the curved surface of the cornea and produces a blurring of vision. The degree of farsightedness is measured in positive or negative (+/-) diopters of cylinder correction

History and Physical Examination

  • LASIK an elective or voluntary surgery and, therefore, it is important that the person requesting the surgery be fully realistic about the outcome. Expectations that are too great may result in an unhappy patient. The procedure will not correct the need for reading glasses in the older person (presbyopia)
  • No history of ocular disease that may cause failure
  • No history of general disease that may contraindicate surgery such as diabetes
  • Stable refraction. A good result will not occur If the degree of diopter change in the eye is unstable
  • Uncorrected and best corrected (with lenses) vision is tested
  • Vision is tested with the pupils dilated (wide open)
  • The following are evaluated
    1. shape of the orbit
    2. condition of the eyelids
    3. vitreous and retina
    4. tear function
  • Slit-lamp evaluation (the slit lamp is a special instrument for examining the cornea and lens)
    1. Determine if there is abnormal small vessels in the cornea or lens
    2. Determine the diameter of the cornea
    3. Evaluate the condition of the epithelium (outer layer of cells) of the cornea
    4. Evaluate the clarity of the lens
  • Determine the maximum diameter of the pupil

Contraindications to Surgery

  • Absolute general contraindications
    1. Uncontrolled diabetes mellitus
    2. Pregnancy
    3. Active collagen vascular disease such as lupus
    4. Active or severe vessel disease
    5. Certain medications such as Cordarone or Accutane
  • Absolute eye contraindications
    1. Herpes zoster of the eye
    2. Active corneal or other eye disease
    3. Cataract formation
    4. Severe abnormal curvature of the cornea
    5. Both eyes cannot be corrected better than 20/40
    6. Abnormally thin cornea
    7. Certain abnormalities of the retina
    8. Severe dry eye particularly with breakdown of the epithelial surface of the cornea
    9. Failure of the eyelids to protect the cornea
  • Relative contraindications
    1. Very irregular corneal curvature
    2. Corneal scar
    3. Recurrent erosion of the cornea
    4. Too steep a corneal curvature
    5. Too flat a corneal curvature
    6. Glaucoma
    7. The worst eye cannot be corrected with lenses better than 20/50
    8. Breast feeding
  • Psychological Contraindications
    1. Emotional instability
    2. Unrealistic expectations

Surgical Procedure

  • The procedure tends to be less successful when the correction is great or the pupils are large
  • The procedure is carried out with the patient awake and with local anesthetic drops placed in the eye. The anesthetic blocks pain from the cornea

An automated knife cuts through the outer one-fourth of the cornea except for a small hinge. The flap is flipped back on the hinge by the surgeon

  1. The computer guided excimer laser passes over the cut surface of cornea sculpting the surface point by point, each point being less than one-thousandth of a millimeter
  2. The surgeon replaces the corneal flap
  • The ophthalmic surgeon marks the cornea as a guide to positioning the epithelial flap
  • A suction apparatus is placed on the eye to hold the eye steady
  • Gentle pressure is applied to the eye to make it stiffer so that any cut in the cornea can be made more easily. Vision is dimmed by the pressure
  • A automated miniature knife (microkeratome) is then used to cut through the outer epithelium of the cornea except for a small uncut segment that acts as a hinge. The flap is approximately 9mm. in diameter and one-fourth the thickness of the cornea
  • The surgeon then folds back the flap that is formed on the uncut hinge
  • A computer guided excimer laser is then used to sculpt the corneal tissue beneath the flap area. The excimer laser is a ‘cool’ laser. It produces very little heat. Each burst of the laser removes less than one-thousandth of a millimeter of corneal tissue. The central 6-7 mm. of the cut area is sculpted
    1. For astigmatism, the laser smoothes the irregularities of the cornea
    2. For nearsightedness, the laser flattens the central portion of the cornea. This corrects vision up to –14 diopters
    3. For farsightedness, the laser sculpts out a doughnut shaped area at the periphery of the flap area. This can correct vision up to +6 diopters
  • The flap is then replaced


  • Glare and halos of light are frequently present shortly after surgery and usually disappears
  • Decreased visual acuity
  • Under correction or over correction in as high as 15% of procedures. A second procedure may be required to make the proper correction
  • Irregular astigmatism with further blurring of vision
  • Off center sculpting of the cornea
  • Recurring erosions of the cornea
  • Scarring of the cornea
  • Allergy to the anesthetic
  • Infection
  • Dryness of the cornea
  • Cutting through the full thickness of the cornea
  • Progressive myopia after surgery
  • Macular hemorrhage or swelling. (The macula is a yellowish point on the retina upon which focuses the most central beam of light)

Surgical After Care

  • After surgery the eye may be covered with a contact lens 24 hours after which the contact lens is no longer needed
  • The surgeon will usually see the patient within the next 48 hours for an examination in order to be sure the corneal flap is OK
  • If there are no complications, a typical follow up schedule is one week, one month, three months and one year
  • It is important to keep post-operative visits as scheduled since any necessary enhancements are best carried out by 3-4 months