RAJESHWORI NGAKHUSHIBachelor of Optometry

ContentIntroductionLens designLens materialsVerification Standards and ordering

IntroductionA thin glass shell bounded by concentric and parallel spherical segments ( Fick )A contact lens, or simply contact, is a thin lens placed directly on the surface of the eye. considered medical devices and can be worn to correct vision, or for cosmetic or therapeutic reasons

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Lens designDesign of contact lens is an important issue it optimizes the ocular response for the individual and purpose is to achieve comfort, safety and visionDesign of the RGP lens can be more complex than the soft lensDesign matters - Most with physiologically poorer materials and Least with better materials

Soft lens design

0 = Back Optic Zone Diameter (BOZD)a0 = Front Optic Zone Diameter (FOZD)1 = Back Peripheral Zone Diameter (BPZD)T = Total Diameter (TD)tER = Radial Edge ThicknesstEA = Axial edge Thickness

Soft lens design factors Geometric centre thickness Lens diameter (total diameter, TD) Back optic zone radius (BOZR) Back surface design Front optic zone radius (FOZR) Front surface design

Radial edge thickness Edge designMaterial physical/mechanical propertiesMaterial physiological properties Peripheral junctional thicknesses if transitions exist

Soft contact lens designDIAMETER :All soft lenses are fitted 1-2mm larger than the horizontal visible iris diameter(HVID)

THICKNESS: Along with central, mid peripheral and edge thickness the overall lens thickness profile is also important. Local thickness is the only relevant thickness when calculating local O2 availability since there is little tear mixing under a soft lens

CURVATURE: the back and front optic zone Radii are important to Rx determination other radii define the physical design of the lens which also affects lens behaviour .Corneal curvature -----flatter by 3-5DDESIGN: After defining centre thickness , front and back radii in the optical zone, the remainder of the lens design is defined by the radii of peripheral curves , their widths , their numbers and the junctional thickness.Designhigh prescription------aspheric design, multi curve design

RELATIONSHIP WITH THE EYES: the parameter of a contact lens should closely match the dimensions of the ocular surface eg- corneal topography

HVID

1.Material propertiesMaterial properties are very significant in soft

lens design Material properties of a soft lens have a significant effect on fitting behaviour, comfort, durability, etcWater contents of 25 - 79% means material

properties vary greatlySignificance of material properties often leads

lens designers to develop material-specific lens series.

2.Center thickness consideration

Dk/t consideration- corneas O2 requirements must be metPervaporation prevention: a high water material with thin lens design, pervaporation corneal dessication may resultFitting considerations: too thin lens - excessive flexing no dispersal of metabolic wastes due to conformity overall lens performance is not good.Lens wrinkiling causes ----corneal wrinkling and staining

Minus lens seriesLenses of lower minus power (

Plus lens seriesGeometrical lens thickness cannot be decreased since it is a function of BVP. Reduction of FOZD is limited by vision issues not be tolerated by most wearers except with small pupils

3.Water content and thickness

thicknesslensBelow 0.10mmThin lensBelow 0.07mm Ultrathin lensBelow 0.05mmSuperthin or hyperthin lens

Diagrams Representing the O2 Performance ofLow/High Water and Thick/Thin Lenses

TRANSMISIBILITY (Dk/t)Dk H2O contentO2 and CO2 transmissibilities 1/tcorneal respiration is best served by a thin high water lens.Higher the H2O content, higher Dk/t Greater the thickness, lesser the Dk/ttc for minus lenses overestimates Dk/ttc in plus lenses underestimates Dk/t

To prevent corneal oedema Holden & Mertz(1984) derived a criteria of critical oxygen transmissibility and EOP values

Equivalent oxygen percentageType of lens wearO2 transmissibility Dk/t9.9%Daily wear2417.9%EW 8712.1%Compromised lens wear34.3

To achieve zero daytime edema

thickness are physiologically desirable they are impractical

H2O contentDaily wearExtended wearCompromise EW38%0.033mm0.009mm0.023mm75%0.166mm0.117mm

Pervaporation If the lens is too thin, corneal dehydration may result due to bulk flow of water through the lens and instability of water flow at the lens surfaceProduces epithelial desiccation staining - pervaporation stainingHigh water content lenses loose more water than less water content due to temperature difference, pH and tonicity

HIGH WATER CONTENT LENSES Lose more water than low water lenses (% of total) on eye Lose water even when worn in a high humidity environmentExperience on-eye lens shrinkage which affects TD and BOZR.

Advantages of high water content lenses

Better comfort because of material softness. Faster adaptation. Longer wearing time. Extended wear. Easier to handle because of greater thickness. Better vision because of greater thickness. Better for intermittent wear.

Disadvantages of high water content lenses Shorter life span and Greater fragility. More deposits, especially white spots. More discolouration. Reproducibility less reliable. Greater variation with environment. Fitting requires longer settling time. Greater variability in vision. More solutions problems. Lens dehydration and Corneal desiccation.

Advantages of low water content lenses

Greater tensile strength. Less breakage. Longer life span. Better reproducibility. Easier to manufacture. Can be made thinner. Less dehydration on the eye. Less discolouration with age. Fewer solutions problems.

Disadvantages of low water content lenses A greater tendency to cause corneal oedema. A long-term tendency with thicker lenses (e.g. with high powers) to cause vascularization

4.Other Design ConsiderationsCentration Quality of vision, comfort and mechanical effects of a lens on the eye, depend to some extent on centration.Movement - minimal amount of movement is required for all soft contact lenses to remove debris under the lens.

Design factorsBack surface designsFront surface designsEdge designsAspheric soft lensesLens design - limitations

Back Surface Designs

Single curve - simplest design but not commonly used.Bicurve - second curve often 0.8 - 1.0 mm flatter than BOZR and about 0.5 - 0.8 mm wide.Blended multiple spherical curves (multicurve) fexible lenses dont need a multicurve designAspheric shapes cornea better

BACK PERIPHERAL CURVESPresence or absence of back peripheral curves is insignificant physiologically

Changes in back peripheral curves,especially radical edge lift, affect lens movement substantially

Front Surface Designit tends to be ignoredimportant to lens fit and on-eye behaviouralso influence the comfort of the lens - especially true in cases of higher Rxs because of their greater thicknesses

Bicurve - with a peripheral curve chosen to produce a thin edge.

Multiple blended peripheral spherical curves.

Continuous aspheric front surface curves are not commonly used.

Front surface may also include bifocal or multifocal components such as:Continuous aspheric surface Concentric bifocal Flat-top segment

Edge Design and ThicknessEdge is already under both lids & has relatively little effect on comfortEdge thickness is governed by durability considerations rather than comfort or physiology concerns.Too thick- discomfortToo thin- tearing of the edge

Aspheric Soft Lenses aspheric means a conicoid A mathematically regular nonspherical surfaceusually take the form of a parabola, ellipse or

hyperbola and are defined by eccentricity.

Circle, e =0Ellipse, e = 0.5Parabola, e = 1

As eccentricity increases , the rate of peripheral flattening or steepening increases exponentially

Contd.e - Defines mathematically the departure of an aspheric curve from a circle. Used to describe both a lens form and the curvature of the cornea.P value - Defines the rate of flattening with eccentricity:p = 1 e2.closest mathematical approximation to the topography of the human cornea is an ellipse. Mean eccentricity = 0.45; p = 0.8.

ASPHERIC ADVANTAGES Better lens/cornea-peri-limbal fitting relationship Fewer base curve steps required Lens fit less sensitive to lens diameter changes Increased lens movement Bearing pressure more uniform

ASPHERIC DISADVANTAGES More expensive to manufacture Not as readily available Perceived to be more complex May decentre and move more than spherical design

Manufacturing process may limit lens design:

MethodLimitationsLathing

Molding-Anhydrous

Molding-Wet stabilized

Spin-casting

Molding & Lathing

Spin-casting & LathingSimple designs only

Few, but anisotropic expansionon hydration changes lens shape

Almost none

Only simple back surface designPossible

Lathing limitations

Lathing limitations

Rigid gas permeable Lens DesignDesign is the cornerstone of any contact lens

fitting. Ultimate goal of rigid lens design is to achieve ideal fit Essential for optimizing response

The desirable properties of an RGP lens are :

Optimal design Material : High DkWettabilityDeposit resistanceStabilityEase of manufacture: manufacturing difficulties with a particular material can be a barrier to its usage.

DESIRED FITTINGModerate edge width and clearanceCentral and mid-peripheral alignment Smooth movement Centration

DESIRED PERFORMANCE Comfortable Clear vision Adequate wearing time Minimal ocular response Normal facial appearance

KEY DESIGN FEATURES Back surface design Back optic zone diameter Front surface design Lens thickness Edge configuration Lens diameter

Tricurve corneal lens t - total diameter1 - first back peripheral zone diameter; 0 - back optic zone diameter; ro - back optic zone radiusr 1 - back peripheral radiusr 2 - second back peripheral radius

Continous non spherical designSingle continuous curve - approximates corneas shapeAspheric designsRegular non spherical curves whose centers of curvature appear to be off the axis of symmetry

BACK SURFACE DESIGNControls Lens/Cornea Interaction Affects both centration and movement

DESIGN FREEDOM

Spherical or aspheric Single or multiple curves Fitting relationship

Back surface design clinical considerations

Back optic zone radiusAspheric Spheric Better alignment

Difficult to manufacture

Difficult to verify

more decentration

Better vision

Better centration

Optic zone should be larger than the pupil size and should cover it during the movementAlso dependent upon the overall diameter and the peripheral curve and power

Optimal Back Surface Design: Alignment or a very slight tendency towards apical clearance over the central 7 8 mm. Mid-peripheral alignment about 1 2 mm wide. Edge clearance about 0.5 mm wide. An obvious tear meniscus at the lens edge.

Back Surface Mid-Periphery

Should align flattening corneasecondary and peripheral zones must have curves which are flatter than the BOZRAffects:

Tear flow Stability of the fit Corneal mid-peripheral shape Centration

Back surface periphery affectsFluorescein pattern at the periphery of the lens eg. A flat and wide peripheral curve will result in excessive edge clearance producing a bright band of fluoresceinTear exchange is greater with a wide and flat peripheral curveExcessive edge clearance results in an unstable fit with excessive lens movement

Peripheral or edge curveRadius - 2.50 mm flatter than BOZRWidth - 0.30 to 0.50 mm

Affects:

Peripheral fluorescein appearance Centration Tear exchange Lens fit 3 & 9 staining

Edge width and tear reservoir

Edge configurationPosition of apex centrally located apex was more comfortableShould not exhibit any high pointThe topography of lens just inside the lens edge aka blend of junctions, influences the edge profile, thickness, junction angles..AffectsComfortDurabilityTear meniscus

Edge shapes of lenses: (a) posterior; (b) central; (c) anterior;(d) blunt; (e) sharp

Rounded edge most comfortableEdge profile rough or square at the anterior side least comfortable Posterior design square Comfort is determined by interaction of lens edge with the lid

Edge shape vs comfort

IDEAL FITTINGCentre - alignedMid-periphery - align/min. clearancePheripheral curve - 0.3-0.5 mm wideAEL - 75-100m clearance

LENS THICKNESSDetermined by:Rigidity Permeability Back vertex powerCONSIDERATIONSOn-eye lens flexure Correction of corneal astigmatism Dk/t

Center thicknessEach lens material has a critical thickness minimum ct which can be made of a particular lens material so that the lens does not flex on the eyeCt more in higher dk lenses

Suggested minimum thicknesses for differentmaterials (BVP-3.00D)

Materialtc (mm)te (mm)PMMA CAB Silicon acrylate Fluorosilicon acrylate0.10 0.160.150.140.120.120.130.15

More stable and comfortable center of gravity is posteriorly locatedCan be made stable by the diameter of the lens, mass by lenticular design or adding minus carrier lenses

Lenticulation affects: Centre thickness - In plus lenses only. Lens mass - true for all lenses. O2 transmission - true for all lens typescomfort

influence comfort, movement and centration

Junction angle & thickness

Affects Comfort Lens movement Centration Lens bulk

Lens diameterDetermined by:Corneal diameter HVID of patientInter-palpebral aperture Lens power (minus/plus)

Lens diameterAffects: Centre of gravity Stability Option to have larger BOZD/FOZD Comfort 3 & 9 staining

Centre of Gravity

OTHER DESIGN ISSUESTints

Handling Aid to colour defectives

Lens Markings

For piggyback fits

Contact lens material

IntroductionGlass was used exclusively for some yrs PMMA began to replace glass in 1940s toughness, optical properties and physiological inactivity

IDEAL CONTACT LENS MATERIAL Meets corneas oxygen requirements Physiologically inert Excellent in vivo wetting Resists spoilation Dimensionally stable Durable Optically transparent Requires minimal patient care Easily machineable

IMPORTANT MATERIALPROPERTIES Oxygen permeability Wettability Scratch resistance Rigidity (RGPs) Flexibility (SCLs) Durability Deposit resistance

OPTICAL PROPERTIES Refractive index Spectral transmission Dispersion Scatter

Rigid contact lensMaterial used is PMMA Stable materials Resists warpage , wets well and clean easilyLack of permeability to oxygen tear exchange phenomenonBackbone of all rigid lens materialsTrial lens

Properties:- excellent biocompatibility good optical properties scratch resistance good manufacturing properties Fairly wettable when clean. Easy to care for. Rigid. 0.2 - 0.5% water when hydrated fully. Almost zero oxygen permeability.Produces spectacle blur

Gas permeable lensesEssentially rigid lensesMaterial used are:

Cellulose acetate butyrateSilicone acrylateFluoropolymers( teflon)styrene

Cellulose acetateCellulose is combined with acetic and butyric acids ( 13% acetyl, 37% butyryl and 1-2%free hydroxyl groups)Low oxygen permeability dk range of 4-8Lack of dimensional stability i.e. Warpage, scratching and coatingNo longer available

Silicone acrylateSilicone and oxygen are combined to make into siloxane Combined with PMMA to produce a gas permeable lens Most successful rigid gas permeable material introduced in 1970Dk value range 12 to 60 are achievable

Negative charge Tended to become coated with proteinaceous materials from the tearsScratches easily may cause flexure problems if made thinnerPure silicone o2 permeability is high but poor wettabilityPolycon II 14.2

Fluoro-Siloxane Acrylates (FSAs)

Fluorine monomer added to SA materialLower surface charge Withstand high heat and chemical attack O2 permeability is like silicone but more wettableDks 40 to 100+ (med-high)Surface easily scratched Greater lens flexure

Perfluoroethersconsists of: Fluorine, Oxygen, Carbon and HydrogenDk 90+ (high)Neutral surface chargeGreater flexibility on eyeLow refractive index High specific gravity

SOFT CONTACT LENSMATERIALS

PHEMAIncorporation of hydroxyl group into PMMA gives 2-hydroxy ethylmethacrylate and makes it more hydrophilicclose relative of poly(methyl methacrylate)Water content is approximately 38%

Other variants to improve PHEMA are:PVP Poly Vinyl Pyrrolidone MA Methacrylic AcidMMA Methyl Meth AcrylateGMA Glyceryl Meth AcrylateDAA Di Acetone AcrylamidePVA Poly Vinyl Alcohol

Convenient to consider the polymers that have been used as contact lens materials under four heading:

1.Thermoplastics capable of being shaped or moulded under heat or pressureEg: PMMAPolyethylene and polyvinyl chlorideCopolymer of tetrafluoroethylenePoly(4-methyl pent-1-ene)Cellulose acetate butyrate(CAB)-

Synthetic elastomers

Not only fexible but show rubber like behaviourIntermediate characteristics b/w thermoplastic and hydrogel materials.Oxygen permeabilities 100x-1000x more than PMMAHydrophobic surface treatmentEthylene propylene terpolymer(EPT)Silicone rubber or poly(dimethyl siloxane)

Hybrid RGPs have a rigid GP central optical zone, surrounded by a peripheral fitting zone made of a soft contact lens material.second generation silicone hydrogel hybrid contact lens called Duette. The lens features a highly oxygen-permeable GP center (Dk 130), surrounded by a soft silicone hydrogel "skirt" for comfort (Dk 84; 32 percent water).

HydrogelsCalled as soft, elastic, water containing gelsWitcherle and coworkers first developed hydrogels polymers (PHEMA)Made from HEMA, lightly cross linked with ethylene glycol dimethacrylate. (spin cast)Monomers commonly used in hydrogel contact lens materials include N-vinyl pyrrolidone (NVP), Methacrylic acid (MA) and Poly-2-hydroxyethyl me-thacrylate (polyHEMA).

Some Examples of Hydrogel materials, by Water Content

Group 1Low Water ContentNonionicGroup 2High Water ContentNonionicGroup 3Low Water ContentIonicGroup 4High Water ContentIonicCrofilconDimefilcon AGenfilcon AHefilcon A & BHioxifilcon BIotrafilcon AIsofilconMafilconPolymaconTefilconTetrafilcon Agenerally show lower levels of protein depositAlphafilcon AAltrafilconOfilcon AOmafilcon AScafilcon ASurfilcon AVasurfilcon AXylofilcon AHeat andsorbic acid should be avoided for disinfection because of the riskof lens discolouration.Balafilcon ABufilcon ADeltafilcon ADroxifilcon AEtafilcon AOcufilcon APhemfilcon ABufilcon AEtafilcon AFocofilcon AMethafilcon A, BOcufilcon BOcufilcon COcufilcon DOcufilcon EPerfilcon APhemfilcon ATetrafilcon BVifilcon Ashow the highest level of protein deposition ,heat and sorbic acid should be avoided for lens disinfection.

VerificationContact lens verification undergoes two stages, laboratory and clinicalLaboratoryDuring the final phase of manufacture, an overall parameter check is performed to ensure the lenses do not differ significantly from the parameters ordered by the practitioner.

ClinicsVerification of lenses upon receipt, rather than during the dispensing visit, is advisable

Why Verify Contact Lens Parameters?

Ensure correct lens is dispensedAssess changes in contact lens with wearTo ensure that proper over-refraction and trial fitting examination has been conductedTo correlate with the manufacturers parameter to actual lens parameterPrior to initial dispensing of CLs, the clinician should verify that all parameters of the lenses are as ordered and that they meet established (e.g., ANSI) standards.

Rigid and soft lenses have similar parameters which require verification by the practitioner. Radii of curvatureLinear parameters Edge profile Power Lens quality

Rigid and soft contact lenses should be hydrated in a soaking solution for 12 - 24 hours before verification procedures are conducted.

INSTRUMENTS

Radiuscope Keratometer (modified) Toposcope Moir fringe deflectometer Radius checking device Topographical mapping system Electrical conductivity method Microspherometer

Drysdales Principle

based on the theory that when a curved reflecting surface is positioned so that the real image created by

the instrument is located at its centre of curvature an image will be formed in the same plane as the aerial object.real image/aerial object is formed at the first focal plane and an aerial image is formed at the second focal planedistance between the real image at the lens surface and the aerial image

keratometerused to measure the BOZR of a contact lens by using special attachments.Used with special contact lens holder which utilizes the front surface silvered mirror and a lens support

Toposcope Moire fringes were used - measuring radii and diameters of corneal lenses. target consist of a series of straight linesshape and orientation of the fringes formed were a function of the relationship between the two sets of linesStraight parallel fringes indicate a spherical surface, curved fringes indicate an elliptical surface. Any warpage or dimples in the surface was indicated by irregularly shaped fringes

Measures spherical, toric and aspheric contact lensesQuality of lens surface can also be assessed

Power verificationverified with a lensometerA smaller lens stop however, is recommended to reduce the amount of light passing through the lens focusing the light source more through the central area of the lens.May be recorded as being a greater positive or smaller negative value than it actual valueTrue back vertex focal length is actually greater - Back surface of lens is not in the plane of the stop

Lens must be centered concave side down on the focimeter stop - BVP

with lens convex side down - FVP

focimeter

Verification proceduresDiameters and linear parameters

Measuring magnifierV gauzeCast, dividers and transparent ruleMicrometer & spheres

Measuring magnifierAn adjustable eye piece through which an engraved scale is viewedHeld with the concave surface towards the scale ( 20 mm long )

V gaugeMade of metal or plastic V shaped channel cut into the materialChannel may vary in width from 6 to 12.50mm

Cast, dividers & transparent ruleMicrometer & spheres:

Thickness verificationDial thickness gauzeCentre or edge thickness may be determined with a suitable thickness gauze which usually incorporates a dial gauge calibrated to 0.01mmCentre thickness is measured at a common geometric and optical centre

LENS THICKNESS VERIFICATION:

Edge profile verification: Instruments/techniques:

Edge molding Projection magnifier Ehrmann profilometer Palm test Radiuscope (modified)

LENS AND SURFACE QUALITY ASSESSMENT: for RGP AND SCL

Instruments:Magnifying 10x loupeProjection magnifier Contact lens optical quality analyzer (CLOQA) Dark field microscope Moire fringe deflectometer

Differences between verification soft and RGP lensesHydrogel contact lenses are flexibleIf exposed to atmosphere, they dehydrate and alter their contour. Verification in air is inaccurate due to-Shrinkage of Hydrogel on dehydrationAccumulation of surface moistureSo, artifact liquid cells are used to measure parameters of soft lensesBut RGP lenses can be measured in air

Contact lens standards

American National Standard Institute (A.N.S.I.) 1999 Contact Lens Tolerances

Power Tolerance O to 5.00D+/- 0.12DSPHERE POWER5.12 to 10.00D+/- 0.18D10.12 to 15.00D+/- 0.25D15.12 to 20.00D+/- 0.50D

POWERTOLERANCECYLINDER POWER0 to 2.00D+/- 0.252.12 to 4.00D+/- 0.37Over 4.00D+/- 0.50

For toric lens cylindrical axis is specified in relation to the base apex meridian

Power Tolerance Cylinder axis0.50 to 1.50D+/- 8Above 1.50D+/- 5

Parameter Tolerance Bifocal refractiveAdd power+/-0.25DSeg height+/-0.10mm

The terminology for a standard tricurve lens in ISO 8320-1986symbols is:

Example:7.90:7.80/8.70:8.60/10.75:9.20 tc 0.15 BVP -3.00D Tint light blue7.90 = back optic zone radius (BOZR) r07.80 = back optic zone diameter (BOZD) 008.70 = first back peripheral radius r28.60 = first back peripheral zone diameter 0210.75 = second back peripheral radius r29.20 = total diameter 0T0.15 = geometric centre thickness tc-3.00 = back vertex power (BVP)

The ISO 11539 standard for the classification of contact lenses, describes the use of a six part code to describe a material typePrefix stem series suffix group suffix Dk range modification codePrefix and series suffix administrated by United States Adopted Names (USAN) council and such are only relevant for materials with FDA approval.Two types of stem filcon stem - materials which contain >10% water by mass (hydrogels )

Focon stem materials which contain

Contact lens ordering

Soft contact lensIn addition to the trade name of the lens being orderedAn order for soft contact lens should include the following parameters:

Base curve radius Overall diameter And power

Thank you!!!

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