Glossary
A B C D E F G H I J K L M N O P Q R S T U V W X Y Z
A
Ablation (L. ablatio = removed) – Removal of micron thickness layers of the cornea during LVC surgery.
Aberration – a vision defect that occurs when light rays are improperly refracted in the eye. An aberration may occur because of a error in the structure of the eye, such as the cornea or natural lens.
Ablation Profiler – Measuring device used to assess the quality of the ablation profiles on a special polymer plastic.
Accommodation – here: The ability of the eye to focus.
Acuity (L. acuitas = sharpness) – Clearness, as in visual acuity. The most common measure of visual acuity is the Snellen Acuity- completely normal acuity being 20/20 or 6/6 (metric version).
Amblyopia (Gr. amblys = dull + Gr. ops =eye) – Poor vision without any visible abnormality of the eye. Synonym = lazy eye.
Anterior Chamber – The front section of the eye’s interior where aqueous humour flows in and out of providing nourishment to the eye and surrounding tissues.
AK – Astigmatic Keratotomy.
ALK – Automated Lamellar Keratectomy.
Ametropia (Gr. ametros = disproportionate + ops = eye + -ia ) – Any imperfection in refractive state of the eye – i.e. hyperopia, myopia, or astigmatism.
Argon Fluoride – ArF the “excited dimer” gas mixture from which the excimer laser got its name.
Argon Laser – Laser light produced from argon gas. The main wavelengths are 488.0 nm blue and 514.5 nm green light, but nine separate wavelengths in the blue-green visible light spectrum are produced.
Astigmatism (a = negative + Gr. Stigma = point) – A refractive abnormality where light is not focused to a point on the retina. This is often caused by the cornea or front window of the eye being elliptical (shaped more like an egg) rather than spherical (shaped like an orange), resulting in blurred vision. It can accompany either myopia or hyperopia or be a stand-alone disorder. Refractive correction includes a lens, which has focusing power in one axis and no power in the opposite one. The amount of astigmatism you have will appear in the second number of most glasses prescriptions e.g. -4.00 -1.00 x 30°, or -5.00 +1.00 x 120. In both of the above cases you would have 1.00 diopter of astigmatism.
B
BCVA – Best Corrected Visual Acuity: This is a measure of best corrected acuity or in other words the best acuity while wearing glasses or contact lenses – the best your eye can see.
Binocular (L. bini = two + oculus = eye) – Both eyes.
Binocular Vision – The ability to use both eyes at once.
C
Calibration – The process of measuring and setting the laser to the needed energy for an accurate tissue removal. Some machines sculpt a PMMA plastic button to be measured by optical instruments, and others employ a 1 micron thick foil as a method of calibration.
CE Mark – Regulatory approval system for all medical devices to be sold in the European Union (EU) – implemented in July 1998.
Central Island – A complication of LASIK where the laser beam fails to remove a portion of cornea – usually in the center of the area to be removed. If one views the concave area of the ablation like a lake one can imagine an island sticking up in the center – the visual symptoms would be monocular double vision or distortion – just as one would expect it if there were an irregularity in the surface of one’s glasses.
Cornea (L. corneus = horny) – The transparent front window of the eye. The cornea is the first part of the eye that bends (or focuses) the light and provides most of the focusing power of the eye. The cornea can be considered to have 5 layers:
- corneal epithelium
- Bowman’s layer
- corneal stroma
- Descemet’s layer
- the corneal endothelium
Corneal Curvature – The shape of the front surface of the eye.
Corneal Endothelium (Gr. endon = within) – The inner layer of cells on the inside surface of the cornea – like the inside pane of a double pane window.
Corneal Epithelium (Gr. epi = on) – The outer surface layer of the cornea, like the epidermis or outer layer of the skin.
Corneal Haze – An opacification, or cloudiness, of the normally clear cornea. Any build up of inflammatory infiltrates (white blood cells), extra moisture, scar tissue, or foreign substances (like drugs) can cause a clouding of the cornea. Most types of haze will disappear with time or drug treatment but sometimes a permanent haze or scars can form.
Corneal Topographer – An instrument or system, which measures features of the corneal surface of living human eyes in a non-invasive manner.
Corneal Topography – A process of mapping the surface details of the cornea with a unique camera/computer combination. It is used to determine corneal slope and astigmatism for LASIK refractive correction. It can also be used post-operatively to measure the results of LASIK treatment. The computer data show the efficacy of ablation and the quality of smoothness of the final refracted surface.
Corneal Topographical Map – A corneal topographic map shows the surface profile of the cornea.
CLVC – Customized Laser Vision Correction
D
Decentration – In perfect centering the center of the LASIK corneal ablation exactly coincides with the center of the visual axis and/or pupil. This is like looking through the very center of your spectacle lens. If you look through the periphery of your lens you might end up seeing partly through the lens and partly through the edge of the lens – this is de-centering. It can cause various symptoms including edge glare or even monocular double vision. Other factors such as the normal size of the pupil (if it is dark outside your pupil will enlarge), or the size of the LASIK ablation zone will affect the severity or presence of symptoms.
Depth Perception – The ability to distinguish objects in a visual field.
Diopter/dioptre (Gr. dioptra = optical instrument for measuring angles) – The diopter is the unit of measurement describing the extent of refractive error and/or for optical lenses. A negative diopter value signifies an eye with myopia, a positive diopter value signifies an eye with hyperopia. A one diopter lens will focus parallel rays of light 1 meter from the lens, a two diopter lens will do so 0.50 meter from it. A plus (+) 1.00 diopter lens is convex and will converge the light rays so they focus as a visible image 1 meter past the lens. A minus (-) 1.00 diopter lens is concave and will diverge or spread light. The minus lens will not actually focus as a visible image on an optics table. Its image is known as a virtual image and if the diverging rays were followed to their point of origin they would focus 1 meter in front of the minus lens.
Diplopia (Gr. diplous = double + Gr. ope = sight) – Double vision or seeing double usually with both eyes open as in binocular diplopia, but can be with only one eye as in monocular diplopia.
E
Elevation – The distance between the corneal surface and a defined reference surface, measured in a defined direction from a specified position.
Emmetropia (Gr. emmetros = in proper measure + ope = sight + -ia) – Perfect distance vision without the need of glasses.
Endo (Gr. endon = within) – A prefix meaning within or inside.
Endophthalmitis (Gr. endon = within + ophthalmia = eye + itis = inflammation) – An inflammation within the eye. Inflammations may be caused by organisms such as bacteria or may be sterile as in immune disorders. Endophthamistis is usually used to indicate an infectious disease, which occasionally occurs as a complication of surgery.
Enhancement – Extra laser treatments made to refine or improve the original visual result. (s. “regression” for discussion of normal, slow, and vigorous healing) It is important to appreciate that, although refractive predictability is excellent at lower planned corrections, the predictability is reduced at higher corrections. Higher corrections and wider optical zones require deeper sculpting and consequently under-correction and over-correction are more common. An enhancement treatment by contrast is usually a small correction and also has a highly accurate outcome.
Excimer Laser – Laser energy produced by several rare gas-halide mixtures. The term excimer comes from the concept of an energized molecule with two identical components or excited dimer (contracted to one word exci-mer). In LASIK the term has for practical purposes become synonymous with the argon-fluoride (ArF) gas version. The wavelength of an ArF excimer laser is in the far ultraviolet range at 193 nm.
Eye Tracker – device which realigns the Laser to any changes in the position of the eye thereby ensuring proper centering of ablation.
F
Flap & Zap – A term meaning LASIK.
Far-Sightedness – A refractive abnormality of the eye which requires a plus (positive or convex) lens for correction. The term originated because people who are far or distance sighted can see at a distance more clearly than they can see objects being are closer. The medical term is hyperopia or hypermetropia, (also known as long-sightedness).
FDA – Food and Drug Administration. The FDA is the United States government agency responsible for the evaluation and approval of pharmaceuticals and medical devices.
FDA PMA – Pre Market Approval: FDA’s certificate, required before a medical device can be commercially marketed in the US.
Fovea – is the retina center where cones are most densely populated. This is the point of highest acuity in the eye.
G
Gaussian Curvature – The product of the two principal normal curvature values at a surface location.
GMP – Good Manufacturing Practice
H
Hypermetropia / Hyperopia (Gr. hyper = above + ops = eye + -ia) – A refractive abnormality of the eye requiring a plus (positive or convex) lens for correction. Synonyms: far-sighted, hypermetropia. Far or distance sighted people can see at a distance more clearly than they can see objects which are closer, (also known as long or far-sightedness).
I
IDE – Investigational Device Exemption; issued by the FDA to permit clinical trials of a medical device in the U.S.A.
Inflammation (L. inflammatio; inflammare = to set on fire) – A tissue’s reaction to trauma, often with pain, heat, redness, swelling, and/or loss of function. Inflammation may be caused by a mechanical trauma, infections by bacteria or viruses as well as by immune reactions, and other causes.
In Situ – In place.
Iris – The colored ring of tissue suspended behind the cornea and immediately in front of the lens (the colored part of the eye). The iris is partly responsible for regulating the amount of light permitted to enter the eye.
ISO-9001 – The International Organization for Standardization issued the ISO-9000 series of standards in 1987 to harmonize internationally comprehensive quality management concepts and guidance, together with models for external assurance requirements. The ISO-9001 contains the standards for quality assurance in design, development, production, installation and servicing.
-itis – An ending term meaning inflammation.
K
Katana – The word Katana means sword in the Japanese language. The Katana is a samurai precision sword known for its quality, effectiveness, precision, flexibility, great speed, solidity and extreme durability. To manufacture a Katana affords a meticulous process and has to be made with great care. Katana is considered as “the soul of the samurai”.
Kerato (Gr. keras = horn, cornea) – A prefix indicating relationship to the cornea or window of the eye. Kerato also can indicate a relationship to horny tissue.
Keratitis (Gr. keras = horn, cornea + itis = inflammation) – An inflammation of the cornea. Inflammation may be caused by trauma as in an abrasion, PRK, or LASIK; or be caused by infections by bacteria or viruses; or be caused by immune disorders.
Keratoconjuntivitis (Gr. keras = horn, cornea + conjuntiva + itis = inflammation) – An inflammation (infectious or auto-immune) of the cornea and conjunctiva.
Keratoconus (Gr. keras = horn, cornea + knous = cone) – An abnormality of the eye where the cornea becomes deformed in the shape of a cone. This condition is a contraindication to (or strong reason against having) refractive surgery such as LASIK or PRK.
Keratometric Diopters – Curvature, in inverse millimeters (mm-1), multiplied by the keratometric constant, 337.5.
Keratomileusis (Gr. keras = horn, mileusis = chiselling) – A refractive surgical technique where a partial thickness circular flap of cornea is removed, frozen, lathed to a new shape and replaced upon the cornea. The lathe can shape either a convex or concave lens. It can now also be done with excimer lasers.
Keratectomy (Gr. keras = horn, cornea + ektome = excision) – Surgical excision (removal) of any portion of the cornea. In a penetrating keratectomy (or PK) a button-like full thickness segment of the cornea is removed and replaced with a donor cornea from another person.
Keratotomy (Gr. keras = horn, cornea + temnein = to cut) – Surgical incision (cut) of the cornea as in radial keratotomy i.e. a radially or clock-hour aligned incision in the cornea.
L
Laser – Light Amplification by Stimulated Emission of Radiation: Any of several devices that emit highly amplified and coherent radiation of one or more discreet frequencies.
LASIK – Laser Assisted In-Situ Keratomileusis: In this LVC procedure, the surgeon creates a flap by cutting across the front edge of the cornea with a microkeratome, folding it back to reveal the corneal “bed” or stroma. The stroma is ablated with a laser, reshaping the underlying cornea. The flap is then replaced. The amount and shape of the removed tissue is determined by the preoperative refractive error i.e. myopia, hyperopia or astigmatism.
Lens – A part of the eye that provides some focusing power. The lens is able to change shape allowing the eye to focus at different distances.
Limbus (L. limbus = border) – The visible borderline between the clear window (cornea) and the white globe (sclera) of the eye. The conjunctival layer, which covers the globe and also joins at the limbus.
Long-sightedness – A common term for hyperopia or hypermetropia. (Also known as far-sightedness)
LVC – Laser Vision Correction, an alternative to the conservative methods of correction of refractive errors mentioned above which eliminates or reduces the need for eyeglasses or contact lenses. The idea is to reshape the surface of the cornea, by removing tiny portions of tissue, in order to change the refractive power of the eye.
M
Macula – The portion of the eye that allows us to see fine details clearly.
Masks (Fr. masque = to cover or conceal) – Masks are used in LASIK surgery to modify the removal of surface tissue by the laser. LASIK surgery involves the removal or fashioning of concave (myopic) or convex (hyperopic) lenses into the inner corneal bed. Most refractive errors also include some element of astigmatic irregularity. Most excimer laser machines employ different kinds of masks to customize, refine and smooth the corneal surface. Many machines have an internal constricting diaphragm (like a camera f-stop mechanism) as an internal mask for use in treating Myopia. One manufacturer uses an extra material applied unto the cornea (an ablatable or destroyable mask) for treating astigmatism. Another manufacturer uses a different shaped metal mask for treatment of hyperopia or myopia. Variable rotation of the masks is used to deal with astigmatism.
Microkeratome – A surgical device that is affixed to the eye by use of a vacuum ring. When secured, a very sharp blade shaves a small amount of the cornea at a predetermined depth. It is used to create the “flap” in the LASIK procedure.
Micrometer (abbr.: µm = 10 to the -6 power meter or 10-6 m) – The term micrometer has replaced the term ‘micron’ which was used until 1967.
Micron – A micrometer, or 10 to the -6 power meter or 10-6 m. The term micron was used prior to 1967.
Millimicron – is one nanometer or 10 to the -9 power meter, or 10-9 m. The term was used before 1967.
Miosis – Constriction of the pupil.
Myopia (Gr. myein = to shut + ops =eye + ia) – A refractive abnormality of the eye requiring a minus (negative or concave) lens for correction (also known as short- or near-sightedness ). People who are near sighted can see objects up close or at near distance without glasses.
Mydriasis – Dilation of the pupil.
N
Nanometer (abbr.: nm = 10 to the -9 power meters or 10-9 m) – The term nanometer has replaced the term millimicron which was used prior to 1967.
Nd:YAG Laser – A common near infra-red neodymium laser source used in many industries, including the medical industry.
Near Point of Accommodation – The closest point in front of the eyes that an object may be clearly focused.
Near Point of Convergence – The maximum extent the two eyes can be turned inward.
Near-Sightedness – A refractive abnormality of the eye requiring a minus (negative or concave) lens for correction. The term originated because people who are near-sighted can see up close more clearly than they can see objects in the distance. The medical term is myopia.
O
Ophthalmic (Gr. ophthalmos = eye ) – Anything to do with the eye.
Ophthalmologist (Gr. ophthalmos =eye) – A medical doctor having specialized in medical and surgical care of the eyes, and also in the prevention of eye disease and injury. An ophthalmologist uses its medical education, training and experience to diagnose, treat and manage the visual systems.
Optician – A professional who makes, verifies and delivers glasses (lenses and frames), and contact lenses using a prescription from an optometrist or ophthalmologist. Opticians’ analysz and interpret prescriptions, determine which products meet the customer’s needs best, and prepare and deliver the finished product.
Optometrist (Gr. optos = seen + metron = measure) – A primary eye care provider who examines, diagnoses, treats, and manages diseases and disorders of the eye and associated structures. They prescribe eyeglasses and contact lenses, low vision aids, vision therapy to treat eye diseases.
Orthoptist – An allied health professional who diagnoses and treats patients with eye alignment and eye movement disorders as well as binocular vision disorders. Orthoptists are often employed as refractive co-ordinators in laser vision clinics.
Over-correction – A complication of LASIK where the expected amount of correction is more than the one desired. (s. “regression”). Over-correction happens occasionally in a person where healing occurs less vigorously than expected. Over-correction can also occur in the uncommon event that the laser is improperly calibrated.
P
Pachymetry (Gr. pachy = thick, thickness + L. meter = instrument used in measurement) – Measuring of the thickness of the cornea with ultrasound machine. The purpose of measuring is to determine the thickness of the cornea prior to PRK or LASIK treatment, so as not to exceed the maximum treatment depth. In RK the radial cuts are usually set at 90% of the depth (thickness). PRK and LASIK treatment depths are dependent on the ablation diameter.
Phoropter – An optical instrument containing many lenses which is used to determine the power of glasses or refractive error.
Photoablation (Gr. phos, photos = light + L. ablatio = remove) – The “cold” process of tissue removal which occurs with laser radiation in the 200 nm wavelength range. This far-UV wavelength possess light photons so powerful that the molecular bonds of the target tissue both break down and have sufficient extra kinetic energy to fly off the surface; hence ablation. Microscopic pictures show incredibly precise cuts with no evidence of tissue burning in adjacent tissue.
Photocoagulation (Gr. photos = light + L. coagulatio = clot) – The process of tissue destruction accomplished by visible light radiation. Tissue is broken down by the light and “clots” as if it were cooked.
Photorefractive Keratectomy – See PRK
Photovaporization (Gr. photos = light + L. vapors = steam or gas) – The process of tissue destruction as occurs with infrared light radiation such as with a yag laser. The target tissue is ionized causing a plasma formation followed by a shock wave.
Pinguecula (L. pinguis = fat) – A yellowish spot seen on the white of the eye at the junction of the clear cornea and white sclera of the eye. These lesions are usually caused by UV radiation. The white surface of the eye cannot “tan” and therefore cannot protect itself from sunburn.
Posterior Chamber – The back section of the eye’s interior.
Pre Market Approval (PMA) – Refer to FDA PMA above.
Presbyopia (Gr. presbyo = old + opia = eye) – The “Old eye” is a condition in which the ability to accommodate for near vision falls off because of the loss of elasticity of the crystalline lens of the eye and because of the weakness of the ciliary muscle limiting the ability of the eye to change its point of focus from distance to near. The individual is no longer able to read clearly and requires reading glasses or bi-focals.
PRK – Photo Refractive Keratectomy: Sculpting of an astigmatic, a myopic or a hyperopic lens for refractive reasons on the front surface of the eye with the use of a “cold” laser light.
PTK – Photo Therapeutic Keratectomy: “Cold” laser removal of surface tissue of the cornea such as scar tissue for medical or optical treatment reasons.
Ptygeria (Gr. ptergion = wing) – A growth of scar tissue on the cornea. Like with pinguecula an ultraviolet burn to the surface layer or epithelium causes these lesions.
Pupil – It appears as a small, black circle in the center of the iris and changes its diameter in response to ambient lighting.
Pupillary response – The constriction or dilation of the pupil as stimulated by light.
R
Radial – Pertaining to the radius or line from a circle center to the circle itself.
Radial Keratotomy – A surgical procedure designed to correct myopia (short-sightedness) by flattening the cornea; commonly referred to as RK.
Refraction – The bending of light at an interface change. In ophthalmology it is the test performed to determine the refractive error of the eye.
Refractive Error – The degree to which the natural refraction of the eye is imprecise. This natural error is the result of the focal point of the image not falling precisely on the curve-linear plane of the retina. Refractive error is measured in Diopters (D) “-” denoting short-sightedness and “+” denoting long-sightedness.
Regression (L. regressio = a return) – A return to the original refractive state. In PRK the surface layer or epithelium is removed prior to or as part of the procedure and then the deeper corneal stroma is reshaped with the cold laser treatment. During the healing process both the surface epithelium and the deeper corneal stroma can reshape and reform at different rates. Computer-generated PRK treatments assume a normal “Bell” curve amount of healing for both layers and thus normally “overcorrect” the refractive error in order to finally heal with no refractive error and no need for glasses. A small percentage of patients will heal faster and more vigorously than expected and will thus regress or “heal” their planned over-correction back past no refractive error to their original type of error. This regression phenomenon occurs with both myopic and hyperopic corrections. It occurs more commonly with smaller ablation diameters and with abrupt transition zones at the edge of the laser treatment areas. Some surgeons have suggested that the epithelium may overgrow as if nature abhors a vacuum and overreacts to fill in the space. Steroid medications can usually be used to regulate and control regression.
Retina – Light sensitive nerve layer which converts light images into electrical signals for transmission to the brain. The retina can be compared to the film of a camera. See Peter’s diagram.
RK – Radial Keratotomy. (s. Radial Keratotomy)
S
Sclera – The tough, white, outer layer (coat) of the eyeball that, along with the cornea, protects the entire eyeball.
Scotoma – An area of partial or complete loss of vision surrounded by an area of normal vision.
Short-sightedness – A common term for myopia (also known as near-sightedness)
SLO – Scanning Laser Ophthalmoscope: This is a laser based device used for imaging, diagnosing and screening of optic nerve density and optic disc topography to aid in the diagnosis of diseases and conditions of the eye including retinal disease, glaucoma, macular degeneration, retinal tumors and diabetes and also potentially for diagnosing refractive error (wave front).
Stereopsis – Ability to perceive three-dimensional depth.
Stroma – Thick, middle layer of cells in the cornea.
T
TGA – Therapeutic Goods Administration: Australia’s regulatory authority for evaluation and approval of pharmaceuticals and medical devices.
Topography (see corneal topography) – Accurate and detailed description or drawing of places or items and their surface details. It is used to determine the corneal profile in order to program the LASIK computer for refractive correction as well as for postoperative corneal analysis.
U
UCVA – Uncorrected Visual Acuity: This is the best vision measurement taken without the use of glasses or contact lenses.
Under-correction – A complication of LASIK where the expected amount of correction is less than the one desired. (s. “regression”). Under-correction occurs in the occasional person where healing occurs more vigorously than expected. Under-correction can also occur in the uncommon event that the laser is improperly calibrated.
V
Visual Acuity – The clearness of vision or the ability to distinguish details and shapes or objects.
Vitreous Humor/Body – The transparent, colorless mass of gel that lies behind the lens and in front of the retina, filling the center of the eyeball.
W
Wave Length – The distance between the top of one wave and the top of the next wave. The argon fluoride excimer wavelength is 193 nm. This wavelength is in the far ultraviolet end of the electromagnetic spectrum.
Z
Zernike Polynomials – A series of mathematical formulas developed by astronomer Fritz Zernike to describe low- and high-order aberrations of an optical system. The complexity of these equations is beyond the ability of most refractive surgeons (and patients) to grasp. All current commercial wavefront analyzers provide a graphical display of aberrations in terms of their Zernike coefficients. No wavefront system currently available is capable of treating above the sixth radial order Zernike term.