dr. sami ata dassan medics index member 7122010 1

8/8/2019 Dr. Sami Ata Dassan Medics Index Member 7122010 1

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Medics Index Member

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Light then passes though the gel-like vitreous andfalls upon the retina, the light-sensitive tissue atthe back of the eyeball (functioning much like thefilm in a camera) that converts the light intoelectrical signals. The retina contains cells, calledrods and cones, which serve the task of detectingthe intensity and the frequency of the incominglight. The rods and cones then send nerveimpulses through the optic nerve to the brain,where translation of the impulses into vision takesplace.

THE CORNEA

The cornea is approximately 500 microns thick (.5millimeter) and is responsible for 80% of thefocusing (refracting) of the light entering the eye.The cornea consists of five cell layers, whichmoving from the front of the eye inward are theepithelium, Bowmans Membrane, Stroma,

Descemets Membrane, and Endothelium.

The Epithelium is the outermost layer of the corneaand is the eyes first barrier to infectiousorganisms.

Riding on the very outside of the corneal epithelium isa very thin film of water and other chemicals (salt,antibiotics, etc.) Known as the tear film.

The tear film lubricates the cornea and keeps it moist.A problem with the tear film causes the sensationof dry eye.


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Coursing through the epithelium are a tremendousnumber of nerve cells with bare ends. If thesenerve endings become exposed to the air by theslightest defect in the epithelial surface, a cornealabrasion is created which is usually accompaniedby exquisite pain.

REFRACTIVE ERRORS

In order for our eyes to be able to see, light rays mustbe bent or refracted so they can precisely focusupon the retina. A refractive error means that the

optics of the eye do not refract the light properly,so that the image formed on the retina is blurred.While refractive errors are called eye disorders,they are not diseases. There are three primaryrefractive errors which are addressed by refractivesurgery: myopia, hyperopia, and astigmatism.

Myopia (nearsightedness) is where the distance visionis blurred at all times while near vision is oftenexcellent within a certain range. In the myopic eye,the image ultimately comes to a focus at a point infront of, rather than directly on, the retina. Myopiais due to an excessively long eyeball and/ or acornea that is too steeply curved, creating anexcessive amount of focusing. It is this cornealcurvature which is altered in the various refractivesurgery techniques to be discussed.

Conversely, hyperopia, (farsightedness) is therefractive condition where near objects may appearblurred while distant objects typically appear clear.

The third refractive error deal with through refractivesurgery is astigmatism.


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Refractive errors are measured in Diopters. Myopia ismeasured in terms of minus -diopters, hyperopiain + diopters.

REFRACTIVE SURGERY MODALITIES

The intent of refractive surgery is to change thenatural curvature of the cornea in order to alter theeyes focusing power to make a myopic corneaflatter or a hyperopic cornea steeper.

In the early 80s, eye surgeons became aware of theExcimer laser, then being used in the computerchip industry. While most surgical laser beamsaffect tissue by producing heat, the Excimer laseruses a charged mixture of argon and fluorine gasesto produce a cool beam of ultraviolet light. Thebeam breaks the molecular bonds between cellsand vaporizes tissue, one microscopic layer at atime. The Excimer laser was formally approved foruse in PRK in 1995, although many eye surgeonswere flying their patients to Mexico or Canada priorto that to circumvent the FDA prohibition.

In PRK, the Excimer laser is used to reshape thecornea in an effort to effect a change in therefractive characteristics of the eye and therebycorrect or lessen myopia, hyperopia, and/orastigmatism. Before the laser is applied, the

epithelial layer of the cornea is removed by eithermechanical means (simply scraped away) orchemical (application of alcohol solution). Thelaser is then used to photoablate (vaporize) severalmicrons of tissue from the central and mid cornea.Usually from 3% to 15% of the central cornealtissue is utilized for corneal reshaping for myopiccorrections from -1.00 to -7.00.


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The epithelium usually regrows over the treated areawithin several days. To reduce the amount ofmyopia in the eye, the cornea is flattened byremoving more tissue from the center of the corneathan from the midzone cornea. The resultantcentral corneal flattening moves the focus pointfarther back toward its desired spot on the retina.To reduce hyperopia, more tissue is removed fromthe midzone cornea, thereby steepening the centralcornea.

The LASIK procedure is similar to PRK

(photorefractive keratectomy) but does not treat oralter the very front surface of the cornea(epithelium). In the LASIK procedure, a liquidanesthetic is dropped into the patients eye,numbing it for surgery. The surgeon then props theeyelids open and marks the cornea with watersoluble ink to guide in the later repositioning of theflap. A suction ring is placed on the eye to secure

the eye and maintain pressure within the eye whilethe cornea is drawn outward.

Simultaneously, a microkeratome (similar to acarpenters plane, but automated) is placed in thetrack of the suction ring . The blade of themicrokeratome then moves across the cornea,creating a flap of corneal tissue some 20-25% ofthe total corneal thickness. This layer(down intothe corneal stroma) is not cut away completely, butremains attached at one side and is then openedlike a door on a hinge to reveal the stroma beneath.


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dr. sami ata dassan medics index member 7122010 1

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