ANDREW GASSON CONTACT LENSES

6 De Walden Street, London W1G 8RL, 020 7224 5959

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A GLOSSARY OF CONTACT LENS TERMS

[ A- D ]  [ E-K ]  [ L-R ]  [ S-Z ]

 

* Denotes separate entry

ABERRATION CONTROLLED LENS: Contact lens to improve visual performance by the control of spherical aberration, usually by means of an *aspheric front surface.  Sometimes beneficial for early presbyopes.

 

AESTHESIOMETER: Instrument for measuring the sensitivity of the cornea or lid margins.  The most common device is the Cochet-Bonnet aesthesiometer which uses a nylon filament of constant diameter but variable length.

 

ALIGNMENT FITTING: Fitting technique where the *BOZR is selected to be parallel to the corneal surface.

 

ALTERNATING VISION: Achieved with a bifocal contact lens which has distinct zones for distance and near.  The lens must *translate for the different portions to line up with the pupil area for the appropriate visual task.  The reading segment may be either fused or solid and only hard lenses have only proved successful.

 

ANOXIA: The complete absence of oxygen.

 

APICAL CLEARANCE (apical pooling): A contact lens fitting, usually steep, in which there is a pool of tears between the back surface of a hard lens and the anterior surface of the corneal apex. Generally observed with *fluorescein.

 

APICAL TOUCH: A contact lens fitting, usually flat, in which the back surface of the lens rests on the apex of the cornea. Generally observed with *fluorescein.

 

ASEPTICIZATION: See pasteurization.

 

ASPHERIC (aspherical) LENS: Lens design where one or both surfaces are of non-spherical construction. Aspherics usually take the form of a parabola, ellipse or hyperbola, and are defined by *eccentricity.

 

AUTOCLAVE: Instrument for heat sterilization of contact lenses under pressure, usually at a temperature of 121C for a minimum of 15 minutes.

 

AXIAL EDGE LIFT (AEL): Distance between a point on the back surface of a lens at a specified diameter and the continuation of the back central optic zone, measured parallel to the lens axis.

BACK OPTIC ZONE DIAMETER (BOZD): Diameter of the central, optic zone of a contact lens. Previously known as the optic diameter.

 

BACK OPTIC ZONE RADIUS (BOZR): Radius of curvature of the central, optic zone of a hard contact lens; previously known as back central optic radius (BCOR).

 

BACK PERIPHERAL RADIUS (BPR): Radius of curvature of a peripheral curve of a contact lens.

 

BACK SURFACE TORIC: Lens design where part or all of the back surface is of toric construction. The front surface may be either spherical, or toroidal in which case it is a *bitoric.

 

BANDAGE LENS: Soft contact lens used to protect the cornea, reduce pain and assist healing in conditions such as bullous keratopathy, ulcers and burns.

 

BASE CURVE: Term used to specify the back optic radius of a soft contact lens. For hard lenses, see back optic zone radius.

 

BI-ASPHERIC: Lens design having a back surface consisting of two different aspheric curves.

 

BICURVE: Lens design consisting of the central radius and one peripheral curve.

 

BIOCOMPATIBILITY: The ability of a material to interface with a natural substance without provoking a biological response.

 

BIOMIMESIS: Where the principles of the complex structure and chemistry of nature are emulated by much simpler scientific means which nevertheless achieve the same results.

 

BITORIC: Lens design with both front and back surfaces of toric construction.

 

BLENDING: The smoothing of a lens *transition with a curve intermediate between the two radii. Blending may be light, medium or heavy.

 

BREAK-UP TIME (BUT): The time in seconds for the break up of the precorneal tears film in a non-blinking eye. Generally observed with fluorescein and the slit lamp. A normal eye has a BUT of 15 seconds or greater. An important diagnostic test in assessing dry eyes (see non-invasive break-up time).

 

BULLOUS KERATOPATHY: A degeneration of the cornea, often following trauma, resulting in vesicles or bullae which cause severe pain on bursting. Frequently assisted by the use of a *bandage lens.

 

BURTON LAMP: A source of ultraviolet (blue) light of approximate wavelength 400nm used to excite *fluorescein and for observation.

 

CARRIER: That part of a lenticulated lens surrounding the front optic zone.

 

CAST MOULDING: Method of soft lens manufacture employing heat and closed moulds.

 

COMBINATION LENS: Sometimes called a 'piggy-back' lens and used for keratoconus or corneal grafts, where a hard lens is fitted on top of a soft lens for better comfort.  The reverse combination of a thin soft lens over a hard lens is used to improve lens stability.

 

COMPRESSION MOULDING: Method of hard lens manufacture employing granules of polymer, heat and pressure.

 

CONDITIONING SOLUTION: A storage solution used to enhance the biocompatibility of lens surfaces.

 

CONOID: Design of fenestrated PMMA lens introduced in the 1960s fitted 0.3mm steeper than 'K'. Noted cause of corneal moulding.

 

CONSTANT AXIAL EDGE LIFT (CAEL): A lens design in which the axial edge lift of the peripheral curves is calculated to remain constant for all BOZRs in a series.

 

CONTACT ANGLE: Angle formed by a tangent to a sessile drop of fluid at the point where the drop meets a surface. A more wettable material has a smaller angle of contact. The angle is zero for a completely hydrophilic material.

 

CONTACT LENS-INDUCED PAPILLARY CONJUNCTIVITIS (CLIPC); giant papillary conjunctivitis: Condition of the palpebral conjunctiva characterized by the presence of large papillae. Suggested causes with contact lens wearers are allergic, mechanical and chemical.

 

CONTINUOUS WEAR: The use of contact lenses without removal for periods in excess of 1 week. See *extended wear and *flexible wear.

 

CONVENTIONAL LENS: Soft lens intended for repeated, non-disposable use.

 

CORNEAL EXHAUSTION: Loss of tolerance to contact lenses from long-term hypoxia resulting in chronic oedema.

 

CORNEAL LENS: A hard gas-permeable lens fitted within the area of the cornea. Typical overall sizes are 8.50-10.00mm.

 

CORNEAL MOULDING: Change in corneal curvature caused by the presence of a contact lens. Predominantly associated with PMMA-induced oedema but also found with hard gas-permeable lenses, especially aspherics, and occasionally with soft lenses.

 

DIAGNOSTIC LENS:  A special complex diagnostic contact lens is used to assess the performance of the design on the eye.  Although it may be of any type it is nearly always hard. The re-use of such lenses is permitted only under certain stringent  conditions.

 

DIMPLING: The formation of trapped air bubbles beneath a contact lens. Usually associated with hard lenses but can also occur with soft lenses.

 

DISINFECTION: The process of reducing the number of viable micro-organisms to a level which is harmful neither to ocular health nor to the quality of contact lenses and accessories.

 

DISPOSABLE LENS: Soft lens designed for frequent replacement, usually on a daily, weekly or monthly basis.

 

Dk and Dk/t:  See oxygen permeability and oxygen transmissibility.

 

ECCENTRICITY: Defines mathematically the departure of an aspheric curve from a circle. Used to describe both a lens form or the curvature of the cornea which has a typical eccentricity of 0.5.

 

EDGE LIFT: See Axial edge lift and radial edge lift.

 

EXPIRY DATE: The date, designated by the manufacturer, beyond which a product should not be first used.

 

EXTENDED WEAR: The regular use of contact lenses without removal, overnight or during sleep, for periods of up to 1 week. See flexible wear and continuous wear.

 

FENESTRATED LENS FOR OPTIC MEASUREMENT (FLOM): A diagnostic hard lens with typical overall size of 13.50-14.50mm, used to assess the optic fitting and power of a scleral lens.

 

FENESTRATION: A ventilation hole drilled in a contact lens. Provides additional oxygen to the cornea and may assist the dispersal of air bubbles or dimples.

 

FLARE: Peripheral blur, usually experienced by hard lens wearers as a reflections or halation around the edge of the contact lens.  Caused by decentration or too small a *BOZD and therefore worse with large pupils.

FLEXIBLE WEAR: The intermittent use of contact lenses overnight or during sleep.

 

FLEXURE: The bending of a soft contact lens fitted flatter or steeper than 'K' to conform to the corneal curvature. Usually applied to soft lenses where negative power is induced, but also applicable to steep-fitting hard lenses where visual distortion may occur.

 

FLUORESCEIN (sodium fluorescein): A dye which stains live tissue, used in 1% solution or by applicator strips: (1) to reveal lesions in the corneal or conjunctival epithelium; (2) to assess hard lens fitting characteristics; and (3) to evaluate tears film (see *break-up time).  The dye is orange under white light but fluoresces bright green when excited by ultraviolet light.  Molecular weight 330.

 

FLUOROSILICON ACRYLATES: Hard lens copolymers composed of fluoromonomers and siloxy acrylate monomers. Sometimes loosely called fluorocarbons.

 

FOOD AND DRUGS ADMINISTRATION (FDA): The regulatory authority in the USA that licenses the manufacture and supply of contact lenses, solutions and ancillary products.

 

FREQUENT REPLACEMENT: The regular replacement of soft lenses at predetermined intervals, usually at 1, 3 or 6 months. See disposable lenses.

 

FRONT SURFACE TORIC: Lens with a spherical back surface and toroidal front surface, used for the correction of *residual astigmatism. Stabilization is necessary to control axis orientation.

 

GHOST VESSELS: Vessels in the cornea caused by vascularization and which have emptied of blood after the removal of the stimulus (e.g. hypoxic or chemical).

 

GIANT PAPILLARY CONJUNCTIVITIS (GPC): also called *contact lens-induced papillary conjunctivitis. Condition of the palpebral conjunctiva characterized by the presence of large papillae. Suggested contact lens causes are allergic, mechanical and chemical.

 

HARD GAS-PERMEABLES: Hard lenses made from materials such as *silicon acrylates and *fluorosilicon acrylates which permit the flow through their structure of gases, particularly oxygen and carbon dioxide.

 

HYDRATION: The uptake of water by a hydrogel material.

 

HYDROGEL: A material made from a hydrogel polymer which absorbs and binds water into its molecular structure. Describes those lenses which have a percentage water content, although not all hydrogels are necessarily soft.

 

HYDROPHILIC (water loving): Frequently used as a synonym for soft lenses but more properly applied to define the surface characteristic of a material in relation to its wetting angle.

 

HYDROPHOBIC (water hating): A surface characteristic of a material which causes the surface to repel water.

 

HYPERCAPNIA: The accumulation of carbon dioxide within the ocular tissues.

 

HYPOXIA: Reduced supply of oxygen to the ocular tissues.

 

IMPRESSION LENS: *Scleral lens fitted by taking a mould of the eye.

 

INDUCED ASTIGMATISM: Astigmatism created optically when a toric lens is fitted on the cornea because of the difference in the refractive index between tears and contact lens material.

 

INFILTRATES: Inflammatory cells within the cornea occurring as a response to viral or other infection, toxic or chemical stimulus. Typically seen as greyish disciform patches near the limbus.

 

ISOTONIC SOLUTION: A solution having the same tonicity as 0.9% sodium chloride.

 

KERATOMETER: See ophthalmometer.

 

LID-ATTACHMENT: Hard lens fitting technique to ensure that the lens periphery is held in a superior 'hitch-up' position by the upper lid and moves with it on blinking. Reduces lid sensation and can avoid 3 and 9 o'clock staining.

 

MEDICAL DEVICES DIRECTIVE (MDD)

The licensing authority in the UK for contact lenses, solutions and associated procedures.

 

MICROCYSTS: Small vesicles in the corneal epithelium containing fluid and cellular debris. Occur as a typical response to corneal stress, particularly extended wear.

 

MICRON; micrometre: Unit of length (1/1000th of a mm or 10-6m) used, for example, in defining *tear layer thickness.

 

MONOVISION: Technique for correcting presbyopia in which reading addition is incorporated into the contact lens for the non-dominant eye.

 

MUCIN BALLS: Translucent or opalescent dimples containing cellular debris trapped behind a contact lens.  Most often associated with *silicone hydrogels and sometimes known as lipid plugs or pre-corneal deposits.

 

MULTICURVE: Lens design consisting of the central radius and multiple peripheral curves.

 

MULTI-PURPOSE SOLUTION: Solution for lens disinfection that combines more than one function, such as cleaning, soaking and wetting.

 

NEOVASCULARIZATION: Growth of blood vessels within the corneal stroma towards the pupil area (see vascularization).

 

NEUTRALIZATION: The process by which active ingredients in contact lens care products are rendered inactive and non-toxic to ocular tissues. Usually applied to systems containing hydrogen peroxide.

 

NON-INVASIVE BREAK-UP TIME (NIBUT): Methods used to measure the stability of the tears film without a staining agent, employing a cold diffuse light source or grid pattern for observation (see Break-up time).

 

OPHTHALMOMETER (keratometer): Instrument used for measuring the curvature of the anterior surface of the cornea. Can also be used to measure the radius of curvature of a contact lens.

 

OPTIC ZONE DIAMETER: Diameter of a specified optic zone, measured to the surrounding junction.  If the latter is not circular, the major and minor diameters define the size. N.B. The term may be qualified, for example, 'back central optic zone diameter'.

 

ORTHOKERATOLOGY: The reduction, modification or elimination of a visual defect by the programmed application of contact lenses. The technique for reducing myopia consists of changing the shape of the cornea by fitting a series of hard lenses progressively flatter than 'K'.  Also called Corneal Refractive Therapy (CRT) and Overnight Vision Correction (OVC).

 

OVER-REFRACTION: Refraction carried out with a contact lens on the eye.

OVERWEAR SYNDROME (acute epithelial necrosis; 3am syndrome): Extreme, painful response to gross corneal oedema as a result of excessive contact lens wear. Usually found with PMMA and typically occurring in the middle of the night.

 

OXYGEN PERMEABILITY (Dk): The rate of oxygen flow under specified conditions through the unit area of contact lens material of unit thickness when subjected to unit pressure difference.

 

OXYGEN TRANSMISSIBILITY (Dk/t): The value for oxygen permeability divided by the thickness of the measured sample under specified conditions.

 

PACHOMETER (pachymeter): Instrument for measuring the thickness of the cornea using optical alignment.

 

PASTEURIZATION: Method of lens disinfection employing heat which reduces micro-organisms to a safe level. Falls short of the absolute efficacy of *sterilization.

 

PERMEABILITY: See oxygen permeability.

 

PHOTO-REFRACTIVE KERATECTOMY: Surgical technique employing a laser beam to sculpt the central corneal surface. Reduces myopia, hypermetropia or astigmatism.

 

PLACIDO DISC: Instrument to assess qualitatively the regularity of the cornea or a contact lens surface using concentric circles and a magnifying lens. An illuminated version is known as a Klein keratoscope and when combined with a camera as a photokeratoscope.

 

PMMA (polymethyl methacrylate): The plastics material from which nearly all rigid lenses were made prior to the introduction of *hard gas-permeables. Also known as perspex or plexiglas.

 

POLYMEGATHISM: Irregularity in the size of cells of the corneal endothelium. Observed in extended wear and in long-standing PMMA wearers.

 

PREFORMED LENS: Scleral lens fitted from trial sets without taking a mould of the eye.

 

PRESERVATIVE: Agent intended to prevent the growth of micro-organisms in a care product.

 

PRISM BALLAST: The use in a contact lens of base down prism as a weighting or stabilizing device to assist with correct orientation on the eye in toric or bifocal fitting. The amount of prism is usually between one and two dioptres. 3D is about the maximum available.

 

PROFILE MATCHING: Technique for assessing the *BOZR of a contact lens by matching its profile shape with a curve of known radius.  This can be by optical projection or physically matching calibrated domes.

RADIAL EDGE LIFT (REL; 'Z' factor): Distance between a point on the back surface of a lens at a specified diameter and the continuation of the back central optic zone, measured along a radius of curvature of the latter.

 

RADIAL KERATOTOMY (RK): Surgical technique to reduce myopia by flattening the cornea with radial incisions.

 

RADIUSCOPE: Instrument used to measure the radius of curvature of a contact lens by means of Drysdale's method.

 

RESIDUAL ASTIGMATISM: Uncorrected astigmatism found by refraction when a spherical contact lens is placed on the cornea. Derives from the crystalline lens and is usually against-the-rule.

 

REVERSE GEOMETRY LENS: A lens where the second radius is steeper than the base curve.  If more than one intermediate curve is steeper than the *BCOR, the lens may be termed double reverse geometry.  Such lenses are used mainly for *orthokeratology but also for other fitting applications such as corneal grafts and post refractive surgery.

 

ROSE BENGAL : A deep red dye used to stain devitalized (dead) epithelial cells in the cornea and bulbar conjunctiva.

 

SCHIRMER TEST: Diagnostic test to assess quantitatively the volume of tear flow using strip of absorptive filter paper placed in the outer temporal part of the lower fornix. Normal tears flow wets at least 15mm in 5 minutes. A frequently quoted but unreliable procedure.

 

SCLERAL LENS: A contact lens which fits over both the cornea and bulbar conjunctiva. Typical overall sizes are 22-24mm.

 

SEMI-SCLERAL (mini-scleral): A soft lens which extends beyond the limbus onto the bulbar conjunctiva. Typical overall sizes are 14.00-15.00mm.

 

SILICON ACRYLATES (siloxanes): Hard lens copolymers with varying proportions of acrylate and silicon.

 

SILICONE HYDROGELS:  Soft lens materials made from a combination of silicone rubber and *hydrogel monomers.  They were developed for *extended wear because of their extremely high *oxygen permeability but have several applications for daily wear.

 

SIMULTANEOUS VISION: Achieved with a bifocal contact lens which positions both distance and reading portions in front of the pupil at the same time.  Lenses may be either hard or soft.

 

SOAKING SOLUTION: A solution designed to keep a contact lens in its functional condition when not in the eye.

 

SPECTACLE BLUR:  Blurred vision with spectacles after wearing contact lenses because of oedema and corneal moulding.  Mainly caused by PMMA but also encountered with hard gas-permeable and soft lenses.

 

SPIN CASTING: Method of soft lens manufacture employing liquid polymer spun to the required shape in rotating open moulds.

 

STABILIZATION: Used to ensure the correct orientation of a contact lens. Important in the fitting of torics and bifocals (see truncation and prism ballast).

 

STAINING: Usually refers to the uptake of *fluorescein by lesions in the corneal epithelium but applies to any of the commonly used tissue stains (see rose bengal).

 

STERILIZATION: The killing of all micro-organisms (see disinfection).

 

STRIAE: Vertical stress lines observed as folds in the corneal stroma, caused by hypoxia. Vogt striae of different origin are observed in keratoconus.

 

SURFACTANT: Agent that modifies the surface energy of a contact lens solution.

 

TEAR LAYER THICKNESS: The thickness of the layer of tears between the back surface of a contact lens and the front surface of the cornea. Usually expressed in microns.

 

THREE AND NINE O'CLOCK STAINING: Staining of the nasal and temporal areas of the peripheral cornea. Frequently associated with conjunctival injection in the horizontal meridian. Influenced by dry eyes, poor blinking, lens design and lens material.

 

TORIC LENS: Lens with all or part of at least one surface of toroidal construction (see back surface toric, front surface toric, bi-toric, toric periphery).

 

TORIC PERIPHERY: Lens with one or more peripheral curves of toric construction.

 

TRANSLATION: The movement of a bifocal contact lens to bring either the distance or reading portion in front of the pupil as the eye changes fixation.  

 

TRANSITION: The junction between two adjacent curves on the surface of a contact lens; usually applied to the central radius (BOZR) and first peripheral radius. Transitions may be sharp or blended.

 

TRANSMISSIBILITY: See oxygen transmissibility.

 

TRIAL LENS:  A contact used to assess fitting, following which it is either disposed of or dispensed to the patient.

 

TRICURVE: Lens design consisting of the central radius and two peripheral curves.

 

TRUNCATION: The shaping of a lens, normally with a straight edge, to assist with correct orientation on the eye in toric or bifocal fitting. Truncation may be single, usually at the base, or double, at top and bottom of the lens.

 

VASCULARIZATION: Superficial extension of blood vessels from the limbal arcades into the cornea.

 

WATER CONTENT: Volume of water (0.9% saline) absorbed by a hydrophilic lens, expressed as a percentage of the total weight of the fully hydrated lens.

 

WATER UPTAKE: Volume of water (0.9% saline) absorbed by a hydrophilic lens, expressed as a percentage of the weight of the lens prior to hydration.

 

WETTABILITY: A property of the contact lens surface as defined by the contact angle and measured under specified conditions.

 

WETTING SOLUTION: A solution used with hard lenses to improve the wettability of the lens surface.

 

XEROGEL: Hydrogel lens prior to hydration.

This glossary is taken from The Contact Lens Manual, 3rd edition by Andrew Gasson and Judith Morris.

lenses@andrewgasson.co.uk

 

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