Glossary of laser eye surgery terms

You might have encountered a few specialised words related to laser eye treatment whilst reading this website. Therefore, to help you we’ve put together some of the terms commonly used when talking about laser eye surgery, and in the articles on Laser Eye Facts.

Ablation: Refers to the act of removing material - for instance, from the surface of the cornea.
Accommodation: The ability of the eye to change focus from distant to closer objects. The loss of accommodation is called presbyopia, which can be treated with laser eye surgery.
ACS: ACS (Anterior Ciliary Sclerotomy) is an experimental eye surgery technique. ACS can be used to treat presbyopia by making incisions in the sclera.
Amblyopia: (Also known as lazy eye) is a condition in which, although the eye is still healthy, a limitation of visual acuity occurs as a loss of vision and depth perception. Amblyopia normally affects only one eye but can sometimes cause reduce vision in both eyes. The lazy eye condition is not treated with laser correction, but can usually be corrected with drops, glasses and vision therapy.
Ametropia: Can refer to any condition of imprecise focus, such as short sightedness, long sightedness or astigmatism, that can be easily addressed with laser eye treatment.
Argon Laser: A type of eye laser filled with argon gas, extensively used to correct glaucoma, retinal, and diabetic eye diseases such as diabetic retinopathy (leaking blood vessels).
AK: AK (Astigmatic Keratomy) is a variant of radial keratotomy used in the treatment of astigmatism. AK involves the making of incisions into the surface of the cornea.
Astigmatism: A common eye disorder occurring when the shape of the cornea ceases to be spherical causing the asymmetrical focus of the light rays.
ALK: ALK (Automated Lamellar Keratoplasty) is a surgery procedure for correcting eye disorders such as short sightedness and long sightedness. ALK does not involve the use of eye laser in which a layer of cornea is cut, and sometimes an additional layer is removed.
Axis: The direction of astigmatism, normally measured in degrees ranging from 1 to 180.
Bilateral: A laser eye surgery term used in relation to eye disorders when they pertain to both eyes.
Cataract: Occurs when light does not pass smoothly through the eye lens, causing blurred vision and a clouding of the eye. In patients with cataracts the crystalline lens needs to be surgically removed and replaced by an artificial lens.
Central Islands: An infrequent complication of the laser eye surgery procedure in which the central treatment zone becomes raised, interfering with vision. Central islands can occur when the eye laser does not manage to remove enough tissue from the corneal surface.
Conductive Keratoplasty: Often abbreviated to CK, this technique used to correct farsightedness and presbyopia. Conductive keratoplasty uses radio frequency energy that shrinks the collagen in the corneal area. This eye treatment is also known as radio frequency keratoplasty.
Contrast Sensitivity: A measure of visual ability, specifically the ability to see black and white distinguish details under varying degrees of contrast. Contrast sensitivity is measured to quantify the quality of vision. Sometimes the level of contrast sensitivity can decrease temporarily after undergoing laser eye treatment, but recovers to normal level in a few weeks time.
Cornea: The transparent tissue at the front of the eye covering both the iris and the pupil. The cornea is a powerful refracting surface that has also great focusing power. The cornea surface, which can be reshaped during laser eye surgery, is transparent and shiny and has more nerve endings than any other part of the human body.
Corneal Haze: Corneal haze is a complication of laser eye surgery in which the cornea develops cloudiness, or 'hazy vision'. Corneal haze can normally be treated with topical steroids.
Corneal Ring: Small plastic insert placed in the cornea to correct low levels of myopia, or short sightedness. Corneal rings, also known as intrastomal corneal rings or Intacs, can be easily removed to return the cornea to the shape it had before surgery. The treatment constitutes a soft alternative to laser eye surgery.
Corneal Topography: Computer-assisted technique for measuring the curvature of the surface of the cornea. Patients undergoing laser eye surgery must have corneal topography, as good candidates ought to have healthy eyes and an eye prescription that has been unvaried for at least two years.
Crystalline Lens: The transparent, hard tissue composed of fibres located behind the iris, which focuses light rays. The crystalline lens is made of four layers: capsule, subcapsular epithelium, cortex and nucleus.
Decentration: Complication during laser eye treatment in which the laser is not lined up on the eye undergoing surgery, and tissue is removed off-center. This can cause blurred vision or, if the amount of the decentration is higher, even double vision. Both side effects can be easily cured via surgical techniques.
Dioptre: Unit of measurement of the focusing power of a lens. One dioptre of focusing ability focuses parallel rays of light at one meter.
Ectasia: See kerato-ectasia.
Emmetropia: The normal condition of the eye, i.e. a state of vision where the light rays focus on the retina. This occurs when the axial length of the eye and the cornea refractive power balance out. Therefore, an eye in a state of emmetropia does not require any correction.
Epikeratophakia: Discontinued eye surgery technique used to correct nearsightedness in which donor corneal tissue previously frozen and reshaped is sewn onto the surface of the cornea.
Epithelium: The thin, jellylike outer surface layer of the cornea. In some laser eye treatments this tissue is removed prior to laser corneal reshaping, and then grows back in a few days.
Excimer Laser: An argon-fluorine gas laser that produces an ultraviolet beam used in laser eye treatment to remove corneal tissue, in order to correct short sightedness, long sightedness and astigmatism. Also known as an erciplex laser.
FDA: Abbreviation for Food and Drug Administration, a US regulatory medication body that monitors new medical devices and drugs.
Flap and Zap: Nickname referring to LASIK (laser in-situ keratomileusis) surgery, a procedure where a flap is made in the cornea and excimer laser energy is applied to tissue inside the cornea. In flap and zap, the surgeon uses an automated blade to cut the surface of the eye instead of scraping away the outer layer.
Glaucoma: Disease characterised by abnormally high pressure within the eye, i.e. increased intraocular pressure (IOP), due to malfunctioning eye drainage structures.
Hexagonal Keratotomy: A type of eye surgery used to correct long sightedness or hyperopia, where cuts are made into the cornea within a 6mm diameter zone in the shape of a hexagon.
Holmium Laser: Surgical laser used to correct long sightedness.
Hyperopia: Also known as long sightedness, a focusing error in which the light rays are focused behind the retina. Hyperopia occurs when the cornea and the crystalline lens have too little focusing power for the length of the eye.
ICR: Soft alternative to laser eye surgery in which a plastic insert is placed in the cornea to correct myopia. ICR, also known as corneal rings, intrastomal corneal rings, or Intacs, can be easily removed to return the cornea to its pre-surgical shape.
Informed Consent: The legal process whereby a patient receives and acknowledges the risks, benefits, and alternatives to a medical procedure prior to laser eye surgery.
Intrastromal: Plastic lenses placed in front or behind the iris to correct eye disorders such as short sightedness, long sightedness, or astigmatism.
Intraocular Implants: Term used in relation to laser eye treatment, literally means ‘within the cornea’ as opposed to ‘on the surface of the cornea’.
Intrastromal Corneal Ring: Small plastic ring used to correct mild-to-moderate short sightedness. The ring is placed inside the edge of the cornea to flatten the anterior corneal curvature without intruding on the visual axis.
Intrastromal Lens: A clear hydro gel lens placed within the cornea used to correct focusing abnormalities.
Iris: The visible coloured tissue inside the eye. The iris surrounds the pupil and is embedded with tiny muscles that can change the pupil’s size to allow different amounts of light to penetrate the eye, just like the aperture of a camera does.
Irregular Astigmatism: As opposed to regular astigmatism, irregular astigmatism refers to the irregular curvature of the cornea. A person suffers from irregular astigmatism when the curvature of his/her eyes’ cornea varies in different parts of the same meridian.
Kerato: Prefix used in laser eye treatment terminology that means ‘of (or pertaining to) the cornea’. Kerato derives from the Greek word for ‘cornea’.
Keratoconus: Eye disease that normally affects both eyes and commences at puberty. In a patient with keratoconus the cornea is cone-shaped and the central part of the cornea becomes thinner and irregularly shaped. This results in a distorted image that cannot be corrected with the use of glasses. Therefore, patients with keratoconus are often prescribed rigid contact lenses.
Kerato Ectasia: A bulging of the cornea, resulting from too much thinning, that can also be caused by laser eye surgery.
Keratome: Instrument used in LASIK surgery to create a flap in the eye.
Keratomileusis: Discarded surgical procedure used to correct short sightedness, which involved the removal of a piece of cornea. The cornea was then reshaped and reattached to the eye.
Krypton Laser: Laser used in the treatment of the retinal eye disorder. The active medium of krypton laser is krypton ionized by electric current and its beam is in the yellow-red visible light spectrum.
Lamellar Keratoplasty: Eye surgery procedure used in the correction of both short sightedness and long sightedness. In lamellar keratoplasty a layer of cornea and sometimes an additional layer are cut and then subsequently removed using a precision cutting device.
Laser: A device that creates a beam of light that is perfectly synchronised and of the same wavelength. In laser eye treatment, a beam of laser light is pulsed onto the surface of the eye in order to reshape its surface and correct eye disorders such as short sightedness, long sightedness and astigmatism.
Laser In-situ Keratomileusis: See definition of LASIK.
Laser Thermal Keratoplasty: Also known as LTK, an experimental laser eye surgery procedure in which a laser is used to heat portions of the cornea in order to correct long sightedness.
LASIK: Also known as laser in-situ keratomileusis or flap and zap, a laser eye treatment procedure in which a flap is made in the cornea and excimer laser energy is applied to tissue inside the cornea.
Lazy Eye: Also known as amblyopia, a condition in which, although the eye is still healthy, a limitation of visual acuity occurs as a loss of vision and depth perception. Lazy eye normally affects only one eye but can sometimes cause reduce vision in both eyes. It is not treated with laser eye surgery, but can usually be corrected with drops, glasses and vision therapy.
Lens: A device that focuses light rays.
Long Sightedness: Also known as hyperopia, a focusing error in which the light rays are focused behind the retina. Long sightedness occurs when the cornea and the crystalline lens have too little focusing power for the length of the eye. This eye disorder can be easily treated with laser eye surgery.
Micron: A minute unit of measurement that is one millionth of a meter, or a thousandth of a millimetre; equivalent to about 39 millionths of an inch, which is also known as ‘micrometer’.
Monovision: A way to treat presbyopia by adjusting the vision in one eye for distance and the other eye for near vision. Monovision is an alternative to reading glasses and can be accomplished with contact lenses or refractive surgery.
Myopia: Also known as short sightedness, a focusing error in which the light rays are focused in front of the retina. Myopia occurs when the cornea and the crystalline lens together have too much focusing power for the length of the eye. Laser eye surgery is a common treatment for myopia.
Optic Nerve: The nerve behind the eye that transmits visual information from the eye to the brain.
Optical Zone: Term used in relation to laser eye surgery and in particular to radial keratotomy, where it defines the arca of the central cornea in which incisions are not placed. Also called ‘cranial nerve’.
Orthokeratology: Technique for temporarily decreasing the curvature of the cornea using rigid gas-permeable contact lenses that are normally worn only at night. Orthokeratology involves the reshaping of the cornea and therefore represents an alternative to laser eye treatment. Also known as ‘overnight vision correction’.
Overcorrection: Complication or possible side effect of laser eye surgery where too much tissue is removed from the cornea. Overcorrection is quite rare and the blurred vision caused by it can be temporary rather than permanent.
Pachymeter: Medical device used to measure the thickness of the cornea, usually by ultrasound, prior to a laser eye treatment procedure.
Phakic: Referring to the natural crystalline lens of the eye.
Phakic Intraocular Implants: Lenses placed into the eye in addition to the natural crystalline lens.
PRK: Also known as photoastigmatic refractive keratectomy, an excimer laser surgical procedure to correct short sightedness, in which tissue is removed from the surface of the cornea to remove scars and other irregularities.
Presbyopia: The gradual loss of the ability to adjust the focus of the eye, and is considered a normal part of the aging process. Patients with presbyopia experience a loss of the ability to adjust focus from distant to nearby objects. Also known as age-related long sightedness.
Progressive Hyperopia: A complication of radial keratotomy that involves a progressive flattening of the cornea. In patients with progressive hyperopia, the flattening of the cornea normally occurs months or years after the procedure.
Pupil: The black opening located in the centre of the iris of the eye though which lights penetrates. The pupil gets larger or smaller depending on the amount of light entering the eye. The size of the pupil can be an issue when undergoing laser eye treatment because of the risk of night vision problems.
Radial Keratotomy: Eye surgery procedure for correcting short sightedness that has been long replaced by more effective laser eye treatment techniques. Radial keratotomy involves the making of small incisions in the surface of the cornea of the eye.
Radio Frequency Keratoplasty: Technique used to correct long sightedness, astigmatism and presbyopia using high frequency radio waves to reshape the cornea. Also known as conductive keratoplasty.
Refraction: Ophthalmology term used to define the measuring of the focus of the eye. The refraction test is usually performed by an eye doctor that places multiple test lenses in front of the patient’s eye. Also known as an eye test.
Refractive Error: An inaccuracy in the focusing ability of the eye which occurs when the image cannot be focused onto the retina. Refractive errors that can be treated with laser eye surgery include short sightedness, long sightedness and astigmatism.
Refractive Surgery: The surgical correction, including laser eye surgery procedures, of refractive errors such as myopia, hyperopia and astigmatism.
Regression: Complication of laser eye treatment experienced by patients that had more than 6.0 diopters of refractive errors before undergoing surgery. In cases of regression an initially favourable result changes moderately in the direction of the original condition.
Retina: The multi-layered, delicate and light sensitive tissue located in the back of the eye. The retina captures light rays and converts them into electrical impulses that are turned into images when they reach the brain via the optic nerve.
Sclera: The white part of the eye, i.e. the opaque tissue that surrounds the eyeball and protects the eye. The sclera contains collagen and elastic fibres and is attached to the optic nerve. There are four layers composing the sclera: the episclera, stroma, lamina fusca, and endothelium.
Scleral Expansion Band: A device that is implanted into the white portion of the eye in order to correct presbyopia. The scleral expansion band segments are usually made of polymethylmethacrylate.
Sclerostomy: See the definition of anterior ciliary sclerostomy.
Thermal-Keratoplasty: Procedure commonly used to correct long sightedness and astigmatism. Thermal keratoplasty involves the application of heat into the cornea and can also be performed via a laser eye treatment known as laser thermal keratoplasty.
Topography: Computer-assisted technique used to measure the surface contours of the cornea and its curvature. During a topography test sophisticated software elaborates thousands of measurements within seconds.
Transepithelial Keratectomy: Variation of photorefractive keratectomy, a laser eye surgery procedure. In transepithelial keratectomy, a laser is used to remove the epithelium, the thin layer of cells on the surface of the cornea.
Undercorrection: A rare complication of eye laser treatment. Undercorrection might occur when too little tissue is removed from the cornea during a laser eye surgery procedure.
Visual Acuity: Clearness of vision, i.e. the ability of the eye to resolve visual detail. Visual acuity depends on the sharpness of the retinal focus. Below the visual acuity threshold of the eye, which varies greatly from person to person, objects get undetected.
Virtual Cortex: The area of the brain processing the visual information from the eye.
Vitreous Humor: Clear gel-like substance that fills the space between the crystalline lens and the retina of the eye. Vitreous humor (spelt in the UK as vitreous humour) is produced by the retinal cells.
Wavefront Analysis: Technique for measuring complex imperfections in vision by using light bounced off the back of the eye.
Wavelength: The distance between two identical, successive parts of a wave of light. Wavelength is responsible for the ‘colour’ of the light.
YAG: Also known as yttrium-aluminum-garnet, a laser used in ophthalmology to cut transparent membranes.

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Glossary of laser eye surgery terms