Your eyes and refractive error

How your eyes work: 
To see clearly, the cornea and the lens must bend - or refract - light rays so they focus on the retina - a layer of light-sensing cells that line the back of the eye. The retina converts the light rays into impulses that are sent to the brain, where they are recognized as images. If the light rays don't focus on the retina, the image you see is blurry. This is called a refractive error. Glasses, contacts and refractive surgery attempt to reduce these errors by making light rays focus on the retina. 

Refractive errors are caused by an imperfectly shaped eyeball, cornea or lens, and are of three basic types:

Hyperopia: The optical power of the eye is too weak, and the light rays from distant object are focused to a point behind the retina. Picture

Astigmatism: The anterior refracting surface of the eye is slightly elliptical rather than spherical. This causes light rays from a distant object to come into focus at two discrete points in the eye.

Presbyopia: The ability to focus at near is deteriorated according to ages. If you are more than 40, you need to wear reading glasses.

Myopia: The optical power of the eye is too strong, and the light rays from distant object are focused to a point in front of the retina.

Excimer Laser: Is based on the combination of 2 gases, a noble gas and halogen. These gases are stable in their normal low-energy state. When a high-voltage electrical discharge is delivered in a laser cavity containing these gases, however, the gasses combine to form a high-energy excited-gas state compound. The term “excimer” is derived from a contraction of “excited dimer”. On the dissociation of this high-energy compound, a photon of energy is released that corresponds to the bond energy of the noble gas-halogen molecule. This wavelength of light is amplified in the laser system and results in the production of a high-energy discrete pulse of laser energy. Current excimer laser systems use Argon and fluoride gases, and emit energy at a wavelength of 193 nanometer.

The 193 nm excimer laser energy is well absorbed by the proteins, glycosaminoglycans and nucleic acids that make up the cornea. Because a 193 nm photon is of higher energy than the molecular bond strength of these compounds, absorption of the laser energy results in a breaking of the bonds. The excimer laser tissue removal process is termed “photoablation”.

LASIK procedure: Lasik is an outpatient surgical procedure. The only anesthetic is an eye drop that numbs the surface of the eye. The surgery takes 10 to15 minutes for each eye.

The procedure starts with administration of a topical antibiotic drop and a topical anesthetic to numb the eye. The surgeon then places a wire-lid speculum to stretch back the eyelids to expose the full surface of the eye. The surgeon then uses a special marker on the corneal surface to permit proper realignment of the corneal flap at the end of the procedure.

In order to stabilize the eye for flap creation, a pneumatic suction ring is placed on the eye and the intraocular pressure is raised to about 65 mm of mercury. With the eyeball now stabilized, the surgeon uses a microkeratome to create the corneal flap on the outermost layer of the eye. The corneal flap is hinged along the superior border of the cornea. The flap is then peeled back to expose the cornea, which is then subjected to the excimer laser. The laser reshapes the underlying corneal tissue, and the surgeon replaces the flap, which quickly adheres to the eyeball. There are no stitches. A shield — either clear plastic or perforated metal — is placed over the eye to protect the flap.