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CONTACT LENSES DR PRATIBHA THILAK CLASSIFICATION ANATOMICAL POSITION MATERIAL MODE OF WEAR WATER CONTENT CLASSIFICATION OF CONTACT LENSES FDA contact lens classification Group 1 – Low water contenta(<50%),nonionic polymers Group 2- high water content(>50%),nonionic polymers Group 3- low water content(<50%),ionic polymers Group 4- high water content(>50%),ionic polymers Refractive correction Spherical contact lenses Toric contact lenses Bifocal contact lenses(annular,segmental,diffractive,aspheric) Uv blocking With or without UV blocker ANATOMICAL POSITION SCLERAL SEMI SCLERAL CORNEAL CONTACT MATERIAL RIGID NON GAS PERMEABLE RIGID GAS PERMEABLE SOFT MODE OF WEAR DAILY WEAR EXTENDED WEAR DISPOSABLE WATER CONTENT LOW 0-40% MEDIUM 40-55% HIGH >55% CONTACT LENS DESIGN SINGLE CUT LENTICULAR CUT IDEAL MATERIAL BIOCOMPATIBLE TRANSPARENT,RI= TEARS GAS PERMEABILITY TOLERANCE MOULDING STERILITY STABILITY SURFACE CHEMISTRY WETTABILITY Complete= o * Partial=70* Non wetting=150* WATER CONTENT Wc = Wc= oxygen tansmissibilty thickness OXYGEN PERMEABILITY Coefficient of variable DK D=diffusion coefficient K=solubility OXYGEN TRANSMISSIBILTY DK/L L=thickness Light transmission Refractive index Heat resistance Dimensional stability flexure RIGID NON GAS PERMEABLE PMMA Derivative of acrylic acid advantages disadvantages High optical quality Impermeable to oxygen Excellent moulding Hard Non toxic hydrophobic RIGID GAS PERMEABLE Cellulose acetate butyrate Silicone acrylate Silicone Styrene fluoropolymers HYDROPHILIC SOFT LENS HYDROGELS HEMA HEMA VP MMA PVD INDICATIONS OPTICAL –refractive errors THERAPEUTIC-non healing ulcers,bullous kp,recurrent corneal erosions,filamentary keratitis,aniridia,coloboma,albinism PREVENTIVE-symblepharon.exposure keratitis,trichiasis DIAGNOSTIC-gonioscopy,fundoscopy OPERATIVE-goniotomy,vitrectomy COSMETIC-corneal scars CONTACT LENS vs SPECTACLES Irregular astigmatism Normal field of vision Aberrations with spectacle use is eliminated Binocular vision retained Rain and fog do not condense Clinical contraindications Active ocular disorders-inflammatory diseases of lids,conjunctiva, cornea, anterior uveal tract Corneal anaesthesia due to any cause Caution-glaucoma and retinal detachment Allergic predisposition,contact dermatitis Pregnancy(fitting) Lack of manual dexterity Psychological factors High altitudes Optics of Contact lenses Optics Contact lens placed in contact with cornea with a thin fluid film in between It eliminates cornea as ref. surface Afocal contact lens: ant & post curvatures of CL same as cornea No optical power Surface irregularities of cornea are taken care of Fluid lens Curvature of posterior surface of CL derives the power of CL Glass lens Post surface of CL same curvature as cornea CL power derived by curvature of ant surface of CL Combined lens Curvature of both surfaces contribute Both glass lens & fluid lens give dioptric power. CLINICALLY IMPORTANT FEATURES Field of vision larger field avoid peripheral distortion Image size Power of CL Vertex Distance The power of a lens is the reciprocal of the focal length, the relative or effective power of a corrective lens changes with the placement of the lens or the distance between the lens and the eye. This relationship is expressed by the formula D= 1 /f Where D = power in diopters, f = focal length in meters. Example. In a +10.00 diopter lens, the focal length is 10 cm (0.1 m): D = 1/0.1 D= 10 - The closer a lens comes to the corneal surface and nodal point of the eye, the greater the plus power required and the less the minus power needed to correct the refractive error. -The formula for the change in vertex power of the lens is Δ= D²d Where Δ = change in power due to vertex distance; D = lens power; d = distance lens in meters. An aphakic spectacle correction of +13.00 diopters at 13 mm in from the eye. power of the contact lens be? Δ = 13² x 0.013 Δ = 169 X 0.013 Δ =2.197 diopters. The power of the required contact lens corrected for vertex distance is +13.00 +2.20= +15.20 D If a -10.00 lens is at 15 mm from the eye the power of the required contact lens is calculated as follows: Δ =D²d Δ = -10² X 0.015 Δ = 1.5 D The total required power is -10.00 + 1.50= -8.50 diopters - The power and position of the correcting lens must be such that the focal point of the lens is conjugate to the focal point of the eye. DESIGN DESCRIPTION AND PARAMETERS Overall diameter Linear measurement of greatest distance across physical boundaries of lens PMMA-7.5-8.8 mm RGP-9-9.8 mm Soft contact lenses-13-15mm 1. Optical zone diameter dimension of Central optical zone which is meant to focus rays on retina 3. Curvatures Ant. Curvatures: • C.A.C (Cental ant.Curvature)- ant surface of optical zone,determines power of cl • P.A.C. (Peripheral ant curvature): slope on the periphery of ant surface • I.A.C. (Intermediate anterior curvature) for high power plus & minus lenses in between CAC & PAC Posterior curvatures: CPC (central post curve )– Base curve to fit the front surface of cornea I.P.C.(Intermediate)- flatter than CPC P.P.C.(Peripheral)- flatter than IPC These are meant to serve as tear fluid reservoir. CL can have bicurve, tricurve or even multi curve contour design. 4. BlendSmooth area of transition of radius of curvature from one curve to other Light Medium heavy 5. Edge-Polished & blended union of ant & post surfaces • Too sharp- may dig into corneal epithelium • Too thick- may irritate the lids • Edge lift or Z factor- comfort & stability Edge lift-extent to which peripheral curvatures differ from base curve PPMA > RGP Greater need for tear renewal 6.Power of CL- determined by central ant & post surfaces at O.Z. determined by the ammetropic correction required Measured in terms of posterior vertex power in diopters 7.Tint- to reduce the glare for cosmesis Central thickness-measured at geometric centre Varies depending upon posterior vertex power of lens FITTING SPHERICAL RIGID GAS PERMEABLE CONTACT LENSES CONTACT LENS FITTING Patient screening Preliminary examination & measurements Trial lens fitting Lens dispensing After-care PATIENT SCREENING Factors to consider in patient selection Anatomical and physiological Psychological Pathological Personal and occupational needs Refractive HISTORY General health Ocular health Medication Ocular history Occupational, recreational, environmental factors Refraction-Retinoscopy -Subjective verification -BCVA -Spherocylinderical notation-Vertex distance Keratometry PRELIMINARY MEASURES Corneal radius of curvature Corneal diameter Lid characteristics Pupil size Spectacle refraction TRIAL LENSES Range of designs required: For low and high minus For low and high plus Diameters BOZR TRIAL LENS SELECTION Based on: Corneal topography Corneal size Prescription Pupil size Lid position Lid tonus TRIAL LENS SELECTION Corneal topography controls: BOZR BOZD Total lens diameter TRIAL LENS SELECTION Corneal size controls: Lens total diameter BOZD TRIAL LENS SELECTION Prescription controls: Total lens diameter TRIAL LENS SELECTION Pupil size controls: BOZD TRIAL LENS SELECTION Lid tonus controls: Lens total diameter TRIAL LENS SELECTION The trial fitting lens will be a guide to the lens design which is considered optimal for the patients eye Base curve Detemined from keratometry Flatter k reading Astigmatism-steeper k astigmatism 0.5-1.0-BC 0.25 D> steeper K 1-2 D0.5 D steeper k > 2 D1/3 toricity added to K TRIAL LENS FITTING ASSESSMENT POSITION OF LENS &LID POSITIONS EVALUATION: Base curve determination TRIAL FITTING: DESIRED RGP LENS FITTING EFFECTS OF DIAMETER same BOZR S2 > S 1 > S3 S1 S2 D1 ‘Original Fit’ D2 ‘Steeper’ same BOZR flatter BOZR S S4 3 D4 ‘Same’ S1 D1 DS D3 ‘Flatter’ 4 4 EFFECTS OF SAGITTAL HEIGHT Constant Diameter S2 > S1 > S3 S2 S1 D1 D1 Steeper BOZR ‘Original BOZR’ S3 D1 Flatter BOZR Finalization of power Once a lens of satisfactory fit has been found,the power to be ordered has to be finalized. POST FIT MANAGEMENT Ordering rigid lenses Examination of ordered lens Evaluation of ordered lens Ordering rigid lenses Specify known variables Base curve radius Optic zone diameter First back peripheral zone First peripheral curve radius Second peripheral curve radius Overall diameter power AFTER - CARE Investigate complaints Perform general ocular examination: Over-refraction Slit-lamp microscopy Other tests particular to the patient Assess lens fit Review lens care regimen Schedule next after-care visit FITTING SPHERICAL SOFT CONTACT LENSES SOFT LENS FITTING PHILOSOPHY Corneal Apex Limbus Peri-Limbal Region INITIAL PATIENT WORKUP: History General ocular examination Refraction Keratometry Corneal Diameter SELECTING THE INTIAL TRIAL LENS:- 1- Selection of lens diameter :- Obtain patient's horizontal visible iris diameter (HVID) measurement - CL diameter = HVID + (1 to 3mm, average = 2mm) - Increase or decrease lens diameter in 0.50mm step if necessary during evaluation process. - Most soft CLs are available from diameter of 13.50mm to 15mm. 2- Selection of Base curve (BC) - SCL are usually fitted flatter than the flattest K - The flattest K minus 3.00 diopter BC = flattest K - 3.00D - Convert the diameter value to millimeters using a converting table. - Increase or decrease BC in 0.30mm steps if necessary. Measure Ks Select trial lens from manufacturer's fitting guide for lenses Add 0.7 mm or more to flattest K for less flexible lens materials (thicker, low water) Add 0.3 - 0.6 mm for standard and flexible materials (thinner, high water) 3- Selection of lens power - Refraction prescription must be converted to minus cylinder from: 1- If cylinder in refraction is (less than or equal to) less or equal ≤ 0.50D, power = spherical component 2 - If cylinder in refraction is 0.750D to 1.00D, the contact lens power = spherical equivalent (spherical component + 1/2 Cyl) EVALUATION OF TRIAL LENS FIT:1.Base curve Evaluation:- A well-fitted lens will show five basic qualities; good centration, adequate movement, stable vision, crisp retinoscopic reflex, clear undistorted keratometry mires, and clear endpoint over-refraction SOFT LENSES CENTRATION DECENTRED SOFT LENS 2.Lens diameter evaluation:- IDEAL-should extend 1.0-1.5 mm over the cornea. 3.Lens power evaluation:- CONTACT LENS FITTING APPROACHES FOR SOFT CL’S FITTING: Corneal Diameter Technique Corneal Curvature Technique Median Fit Technique Saggital Depth Tecnique Corneal diameter technique:- Corneal Curvature Technique:K-reading Less < 41.00D Soft CL Base Curve Flat ( >9.00mm) Between 41.00– 45.00 D Medium (8.00-.00mm) Larger > 45.00D Steep (< 8.00mm) Median Fit Technique:- Sagittal Depth Technique:- Correction The following steps should be taken to correct a loose lens: - Either changing the base curve by decreasing it by 0.2 to 0.3 mm OR - Increasing the diameter of the lens by 0.5mm up to 15mm. The following steps should be taken to correct a light lens: - Either changing the base curve by increasing it by 0.2 to 0.3 mm OR - Decreasing the diameter of the lens by 0.5mm. Extended Wear Lenses: Fitting of Soft EWL’s:- Base curves of 8-9 mm and Overall diameter of 13.5 -14.5 mm are commonly used Fitting of Rigid EWL’s:- selected RGP-EW lenses should have a DK value of 90 SPECIAL CONTACT LENS FITTING: CONTACT LENS FITTING IN ASTIGMATISM LENS TYPES: RGP LENSES-Front surface toric Back surface toric Bitoric Peripheral toric SOFT LENSES-Spherical -Toric FRONT SURFACE TORICS Spherical back surface Base down prism Cylindrical front surface Circular design Truncated design SPHERICAL BACK SURFACE Start with: Optical zone about 7.70 mm Total diameter about 9.20 mm Tricurve BASE DOWN PRISMS Meridional orientation Requires 1 - 1.75 prism dioptres Provide thickness/weight differential Nasal offset by 10-15 degrees RIGHT EYE UPPER LID CORNEA LENS LOWER LID PRISM BASE AT 280o BASE DOWN PRISM The tendency for the lens to rotate with a blink is counteracted by the thickness differential of the lens The heavier prism base helps to maintain meridional orientation of the cylindrical correction BASE DOWN PRISM There is a continuous variation in the thickness across the lens Apex is the thinnest portion and the base the thickest CYLINDRICAL FRONT SURFACE The astigmatic correction is provided by a plus cylinder on the lens front surface CIRCULAR DESIGN Optical zone is centred Base down prism Easier manufacture and duplication TRUNCATED DESIGN Inferior part of lens is truncated Resets against lower lid for stability Prism ballast Optical zone is decentred superiorly TRUNCATED LENS TRUNCATED DESIGN Possible contour Taper from front to back Flat base Taper from back to front a b c TRUNCATED DESIGN Shaped to match lower lid contour Removes some of the prism ballast effect (more in minus powers than in plus) TRUNCATED DESIGN TRUNCATION DOUBLE TRUNCATION A superior truncation can be added to increase lens stability if a single truncation is not sufficient DOUBLE TRUNCATION DOUBLE TRUNCATION UPPER LID LENS CORNEA TRUNCATIONS LOWER LID BACK SURFACE TORIC The back surface toric design is chosen to optimize the lens-to-cornea bearing relationship that would be unsatisfactory with a spherical lens FITTING REQUIREMENTS Corneal cylinder of 2.00D or greater Physical compatibility with the cornea Stable meridional orientation LENS DESIGN Back surface is toric Front surface is spherical BITORIC A bitoric lens is required when a back surface toric/spherical front surface lens results in an unacceptable amount of residual astigmatism LENS DESIGN Toric back surface for physical fit Toric front surface for astigmatism correction Rotational stability PERIPHERAL TORIC A toric periphery with a spherical back optic zone is designed to improve the fit on the cornea of moderate toricity PERIPHERAL TORIC A toric peripheral curve(s) design allows even bearing and/or clearance at the periphery, resulting in improved centration and comfort LENS DESIGN Spherical back optic zone Toric back peripheral curves Spherical front optic zone and peripheral curves oval shaped optic zone 10.60 8.6 0 9.0 0 11.00 Flatter secondary and peripheral curves Steeper secondary and peripheral curves PERIPHERAL TORIC FEATURES Improved lens centration Reduced uneven bearing Prevents peripheral bubble formation CONTACT LENS FITTING IN APHAKIA: RGP LENSES-Single cut -Large lenticular cut SOFT LENSES-Particularly EWL CONTACT LENS FITTING IN KERATOCONUS: Rigid CL’s fitted with 3 point touch technique: Sopper’s technique Piggyback lenses CONTACT LENS FITTING IN PRESBYOPIA: Monovision CL’s Modified Monovision CL’s Binocular Bifocal CL’s- Annular Bifocal - Segmental - Aspheric - Diffractive CONTACT LENS FITTING IN MYOPIA: High myopia >-8.00 d Relatively flat ant. Surface Peripheral lenticular bevel THERAPEUTIC CONTACT LENS Bandage lenses Provide mechanical support Facilitate wound healing Proper surface hydration Reduce discomfort from corneal surface disorders Drug delivery system Types and choice Hydrogel therapuetic CL High water content-desmatocoele ABK/PBK Coexistent corneal or anterior segment inflammation Moderate water content Low water content Epithelial defect COSMETIC SOFT CONTACT LENS Indications Disfigured corneas Albinism/aniridia/iris colobomas/photophobia/diplopia Occlusion therapy Pure cosmesis COMPLICATIONS OF CONTACT LENS WEAR CORNEAL CONJUNCTIVAL CONTACT LENS RELATED CORNEAL COMPLICATIONS Epithelial edema Epithelial microcysts SPKs 3 and 9 o’clock staining Sterile corneal infiltrates Neovascularisation Microbial keratiti Warping Endothelial changes CONJUNCTIVAL COMPLICATIONS Allergic conjunctivitis GPC SLKC CONTACT LENS RELATED COMPLICATIONS Physical damage Discolouration Lens loss Deposits CONTACT LENS SOLUTION For hydrophobic rigid CLs For hydrogel soft CLs For both Wetting/cleaning/storage/rewetting/multifunctional Vehicle/buffering agent/preservatives Low concentration of preservatives Benzalkonium chloride Chlorobutanol Thiomersal Chlorhexidine Ethylene diamine Tetra acetic acid Sorbic acid Potassium sorbate Wetting solutions Minimise friction between lens and palpebral conjunctiva and cornea Buffer or cushioning agent for brief period to be replaced by lacrimal fluid characteristics Wet thoroughly,spread entirely Form a film Non-irritating,non sensitizing Not leave a residue Cleaning antiseptic and self preserving Proper degree of viscosity Allow wear agents Polyvinyl alcohol Polysorbate 80 Polyethylene oxide Cellulose like derivatives PVP CLEANING SOLUTIONS Detergent and bactericidal action Surfactant and enzymatic cleaners Surfactant-non-ionic Enzymatic- papain,lipase tablets SOAKING SOLUTIONS Bactericidal Hydrated state maintainance REWETTING AGENTS Rewet corneal surface or lens while it is on cornea Agents in lens solutions Benzalkonium chloride Chlorbutanol Thiomersal Chlorhexidine Hydrogen peroxide and povidone iodine EDTA Polyvinyl alcohol CONTACT LENS CARE Personal hygiene Removal of lens Storage lens Routine cleaning Enzyme cleaning Disinfection Special lens care Personal hygiene Hands washed,dried,nails clipped Hand cream not to be used Removal of lens Lens cleaned before storage Storage of lens Hard lens-dry state,flat case Lens cases to be cleaned Soft lens-wet state,hydrating kit Routine cleaning Hands washed.dried,lens kept concave side up Solution added and rubbed for 15- 30 sec Lens held between finger and thumb Held in running water Soft lens Not washed with water Normal saline Enzyme cleaning Once a week Papain For protein coating Enzyme tablet+distilled water+lens for 4 hours Lens rinsed clear of enzyme Repeated enzyme cleaninglens decentres more DISINFECTION Destroys vegetative microorganisms Moist heat and chemical methods Thermal disinfection Saline based solution of thiomersal + EDTA boiling For 80 *C for 10-15 mins Suspending lens case over steam column Above procedure in >100 * C with 5 lbs of steam pressure Chemical disinfection Cold disinfecting system Thiomersal-alkyl triethanol ammonium chloride Thiomersal sorbic acid or potassium sorbate Hydrogen peroxide system Chlorine tablets with water Iodine based solution Special lens care technique In womencontamination with cosmetics Insert lens before cosmetics In men contamination with hair oil In childrencarelessness Special cleaning Laboratory cleaning to remove grease paint nail polish etc
