Ablate
In surgery, to remove.

Ablation
The vaporization of tissue with the excimer laser.

Ablation zone
The area of tissue that is removed during laser surgery.

ALK
See Automated lamellar keratoplasty.

Anatomy of the Eye

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Aniseikonia
A difference in imaging size between the two eyes.

Anisometropia
A difference in refractive power of the two eyes in which the variance is at least one diopter.

Anterior chamber
The fluid-filled area between the cornea and the lens.

Aqueous humor
The fluid in the anterior chamber.

Astigmatism
A refractive error caused by an irregular shape of the cornea (much like a football). Astigmatism is measured in terms of diopters, cylinder meridian or axis. Uncorrected astigmatism may produce ghosting or double images.

Automated lamellar keratoplasty (ALK)
A procedure in which the surgeon first creates a flap in the uppermost layer of the cornea using a device called a microkeratome. Then the surgeon makes an optical cut after removing additional tissue with a second pass of the microkeratome.

Axis
In ophthalmology, a line that is the symmetrical center of a curved optical surface. Measure of astigmatism.

BCVA
See best corrected visual acuity.

Best corrected visual acuity (BCVA)
The best possible vision a person can achieve with corrective lenses measured in terms of Snellen lines on an eye chart.

Bowman’s membrane
The non-regenerative layer of tissue between the epithelium and the stroma (5-10 microns thick – .005 to .01 millimeters).

Broad beam laser
A medical instrument that produces a powerful beam of light that is focused at close range to remove corneal tissue. A broad beam laser uses a relatively large beam diameter (from 6.0 to 8.0 millimeters) which can be manipulated to ablate the cornea.

Center islands
A manageable complication of LASIK related to ablation. The incidence of center islands has been greatly reduced as more ophthalmic research has been devoted to its contributing factors.

Cornea
The outer part of the eye that provides 70% of the eye’s refractive power. The cornea is approximately 500 microns thick (.5 millimeter) and consists of 5 layers epithelium, Bowman’s membrane, stroma, Descemet’s membrane and endothelium.

Cylinder meridian
In ophthalmology, a line that is the symmetrical center of a curved optical surface. Measure of astigmatism.

Decentration
A complication caused by movement of the pupil that can be corrected with an enhancement procedure.

Descemet’s membrane
The layer of the cornea between the stroma and endothelium. Five microns thick (.005 millimeters), this membrane provides an adhesion layer for the endothelium.

Deturgescence
The balance of hydration in the eye.

Diopters
A measurement of refractive error. Hyperopia is measured in terms of positive diopters. Myopia is measured in terms of negative diopters. The most common refractive errors ranged between +6 to -6 diopters.

Dry eye
A Syndrome characterized by corneal dryness due to deficient tear production.

Ectasia
A progressive corneal thinning and bulging.

Emmetropes
People who have no refractive error.

Emmetropia
The ophthalmic term for a perfect refractive state – no nearsightedness, farsightedness or astigmatism.

Endothelium
The innermost layer of the cornea. The endothelium is one cell layer thick (5-10 microns or .005-.01 millimeters) and provides hydration balance to maintain the cornea’s transparency. The endothelium serves three main purposes it regulates the stroma’s water content, provides a barrier to ingress of several constituents of the aqueous humor, and actively transports glucose.

Enhancement
A secondary refractive procedure performed after the initial one in an attempt to achieve better visual acuity.

Epithelial ingrowth
A complication of LASIK wherein epithelial cells grow underneath the corneal flap.

Epithelium
The outermost layer of cells of the cornea. Six cells thick (20 microns), the epithelium is the eye’s first defense against infection.

Excimer laser
A “cold” laser used in refractive surgery to remove corneal tissue.

Farsightedness
See “Hyperopia”

Glare
A complication of refractive surgery in which the patient sees additional luster around lights. Glare is a subjective experience that often decreases with time.

Ghosting
A distortion of image due to irregular healing of the corneal surface.

Globe
The eyeball.

Halos
A complication of refractive surgery in which the patient sees additional rings around lights at night. Halos are subjective experiences that often decrease with time.

Haze
A relatively rare complication of LASIK caused by the deposition of ground substance in the cornea. An ophthalmologist can measure the haze response of a patient’s eye under a slit lamp, but patients’ experiences of haze vary. Outcomes for the patient include decreased night vision, halos or loss of best corrected visual acuity. Caused by the eye’s reaction to the laser, haze often decreases with time and is more common with PRK.

Hyperopes
People who are farsighted.

Hyperopia
The ophthalmic term for farsightedness. In the hyperopic eye, images are focused in back of the retina. The hyperopic eye is often described as being too flat or too short.

Intraocular pressure
The pressure the fluid contained within the eye exerts on the globe.

Irregular astigmatism
A refractive error caused by an irregular shape of the cornea in which the curve on one side of the meridian or axis is not symmetrical with the curve on the other side.

Keratectomy
The portion of the LASIK procedure in which the surgeon raises a thin layer of the cornea – creates a corneal flap – with an instrument called a microkeratome, to expose the layer of the cornea called the stroma.

Keratoconous
A rare inherited condition of the cornea in which the cornea is steepened to the point of being cone-shaped.

Keratomileusis
The carving of the cornea formerly done with a lathe and blade, now done with an excimer laser.

Keratoplasty
The replacement (transplantation) of the cornea. Keratoplasty can be lamellar (replacement of superficial layers) or penetrating (replacement of the full thickness of the cornea).

Keratotomy
A surgical incision (cut) of the cornea.

Lamellar keratoplasty
The replacement of superficial layers of the cornea with the layers of another donor cornea.

Laser
An acronym for light amplification by stimulated emission of radiation. A medical instrument that produces a powerful beam of light and can produce intense heat or cool vaporization when focused at close range. Lasers are often used in surgery to remove tissue.

LASIK
The acronym for laser assisted in situ keratomileusis. The name refers the use of a laser to reshape the cornea without invading the adjacent cell layers. For more information, see Step-by-Step of the procedure.

Leucomas
Scars that are dense and white.

Meibomian secretions
Oily secretions from the eyelid glands that supply the outer portion of tear film, prevent rapid tear evaporation and tear overflow.

Micron
One thousandth of a millimeter. The symbol for a micron is.

Microkeratome
The instrument a surgeon uses to create the corneal flap in the uppermost layer of the cornea during the LASIK procedure.

Monovision
The purposeful adjustment of one eye for near vision and the other for distance vision accomplished with either corrective lenses or surgery.

Myopes
People who are nearsighted.

Myopia
The medical term for nearsightedness. Eye is too steep, too long, image is focused in front of the retina.

Nearsightedness
See “Myopia”

Nebulas
Scars that are translucent.

Nomogram
A surgeon’s adjustment to the laser’s computer calculation to further refine his or her own results.

Off label use
The permissible use of an approved drug or instrument in a way that has not been specifically sanctioned.

Optic nerve
The millions of optical nerve fibers connecting to the eye and terminating in the brain where images are created and processed.

Overcorrection
The result achieved when the change to refractive error exceeds the attempted correction.

Pachymetry
The process of measuring corneal thickness, usually using an ultrasonic probe.

Photorefractive keratotomy
A procedure involving the removal of the surface layer of the cornea (epithelium) by gentle scraping and use of a computer-controlled excimer laser to reshape the stroma. Acronym is PRK.

Plano
Characterized by no refractive error.

Practice of medicine
A regulatory body’s allowance of practitioners to make decisions to best serve their patients.

Presbyopia
The natural deterioration of near vision caused by loss of flexibility in the eye’s lens as one ages.

PRK
The acronym for photorefractive keratotomy. A procedure involving the removal of the surface layer of the cornea (epithelium) by gentle scraping and use of a computer-controlled excimer laser to reshape the stroma.

Ptosis
Droopy eyelid.

Radial keratotomy
A surgical procedure designed to correct myopia (nearsightedness) by flattening the cornea with incisions. The procedure is called a radial keratotomy because the incisions resemble the spokes in a wheel. Acronym is RK.

Refraction
The bending of light wave as they pass from one medium to another.

Refractive surgery
Any surgical procedure that attempts to decrease the patient’s refractive error. Typically the surgeon alters the shape of the cornea in order to change the angle at which an image is projected onto the retina.

Regression
A backwards shift from the initial visual outcome.

Regular astigmatism
A refractive error caused by an irregular shape of the cornea (usually a football shape) in which the curvature is symmetrical across one or more meridians or axes.

Retina
Light processing membrane; converts light into electrical impulses that are transmitted to the optic nerve.

RK
Acronym for radial keratotomy. A surgical procedure designed to correct myopia (nearsightedness) by flattening the cornea with incisions. The procedure is called a radial keratotomy because the incisions resemble the spokes in a wheel.

Slit lamp
Table-top microscope for examining the eye.

Snellen chart
An eye chart used to test a patient’s vision.

Snellen line
A line of same-sized letters on an eye chart that is used to test a patient’s vision.

Spot scanning laser
A medical instrument that produces a powerful beam of light that is focused at close range to remove corneal tissue. Spot scanning lasers use radar technology to track the eye’s movement.

Stroma
Thickest part of the cornea (450-600 microns – approximately .5 millimeters). Between Bowman’s membrane and Decemet’s membrane.

Symmetry of refractive error
The refractive error in both eyes are close to the same value.

Tear film
A very thin film of water and other chemicals riding on top of the epithelium that lubricates the front of the eye.

Topical proparacain hydrochloride
Anaesthetic eye drops.

UCVA
See uncorrected visual acuity.

Uncorrected visual acuity (UCVA)
A person’s vision without corrective lenses measured in terms of Snellen lines on an eye chart. Acronym is UCVA.

Undercorrection
The result achieved when desired change in refractive error is not fully achieved.

Vitreous humor
The gel-like fluid in the main cavity of the eye behind lens and pupil.

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