contact lens

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Contact Lens
Dr Abdul Hannan

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Classification
1. Anatomical Position
1. Scleral
2. Semi Scleral
3. Corneal
2. Nature of Material
1. Rigid Non gas permeable
2. RGP
3. Soft contact lens
3. Mode of wear

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Classification
 Mode of wear
 Daily wear
 Extended wear
 Disposible
 Depending on Water Content
 Low water content
 Medium water content
 High water content

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Contact Lens Design
 Single Cut design
 Lenticular cut lens

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Terms Related to CL
 Wettability
 Water Content
 Oxygen Permeability
 Oxygen Transmissibility
 Light Transmission
 Refractive Index
 Flexure
 Dimentional Stability

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CL Materials
 Ideal Material:
 Biocompatibility
 Optical Properties
 Gas permeability
 Tolerance
 Moulding
 Sterility
 Stability
 Surface chemistry

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RIGID non-gas-permeable PMMA
 Advantages
 Light weight
 Excellent Moulding
 Non-toxic
 Disadvantages
 Impermeable to O2
 Hard – can cause corneal abrasion
 Resists wetting

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RIGID gas permeable lenses
 Cellulose
 Good wetting
 Easy scratching
 Easy wraping
 Silicone
 High oxygen permeability
 Lack of wetting
 Styrene
 Surface durability and brittleness

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Present RGPs
 Silicone Acrylate
 Fluoropolymers

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Hydrophillic Soft Contact Lens
 Combination of monomers and crosslinking agents
 Characteristics:
 Amount of Water Content
 Hydration of soft lenses
 Degree of swelling
 Average pore size
 Oxygen permeability
 HEMA, HEMA-VP, MMA-PVD

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Indications
1. Optical
2. Therapeutics
3. Preventive
4. Diagnostics
5. Operative
6. Cosmetic
7. Occupational

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Optical
 Anisometropia
 Unilateral aphakia
 High Myopia
 Keratoconus
 Irregular astigmatism

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Optical
 Advantages over glasses:
1. Normal field of vision
2. Irregular astigmatism
3. Aberrations
4. Binocular vision
5. Rain n fog don’t condense
6. Cosmesis

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Therapeutics
 Corneal Disease
 Iris defect
 Amblyopia
 BCL after surgery

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Preventive
 Symblephron
 Restoration of chemical burns
 Exposure keratopathy
 Trichiasis

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Diagnostic
 Gonioscopy
 ERG
 Fundal Contact Lens
 Fundus Photography
 Triple mirror

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Operative
 Goniotomy
 Vitrectomy
 Endocular photocoagulation

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Contraindications
 Mental incompetence
 Lacrimal Disease
 Disease of eyelids
 Episcleritis or scleritis
 Uveitis
 Ocupational Hazard
 Facial Palsy
 Poor hygiene
 Allergy to lens material
 Dry eyes

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CL and TEAR FILM
 Affect of CL on Corneal Nutrition:
1. Retards Evaporation
2. Barrier to Oxygen
3. Blocks Waste disposal
4. Traumatize the delicate epithelium

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Grip of CL
 Cohesion of water molecules
 Negative pressure

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Supply of Nutrition
 Lid Pump Mechanism
 20% replacement of post CL tear film is required

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OPTICS of CL
 Spectacles and its effect on eye:
 Are 12 to 13 mm away from the corneal vertex
 Are separated from eye by air
 No change in refractive status of eye
 Vergence of eye is not altered but the rays is altered
 CL changes the refractive power of eye ball the substituting the
cornea with its own surface

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Neutralization of Cornea
 Refractive index of Cornea: 1.376
 Refractive index of tears: 1.336
 With CL insitu it neutralizes the power of cornea by 90%
 From 48.83 to 5.19D

24. +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

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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

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 Combined lens
 Curvature of both surfaces contribute
 Both glass lens & fluid lens give dioptric power.

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Physical factors of CL
1. Overall diameter of Hard lenses- 8mm, about 1.5-2mm
less than corneal diameter
• GP & soft lenses allow larger diameters
• Larger lenses are more stable
• Larger lenses – vaulting effect
2. Optical zone- central zone of 5-6mm
• Average of maximum & minimum pupil size

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3. Curvatures
Ant. Curvatures:
• C.A.C (Cental ant.Curvature)- ant surface of optical zone
• 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

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 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.
These form ski for contact lens movement.
CL can have bicurve, tricurve or even multi
curve contour design.

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4. Blend
Smooth area of transition of radius of curvature from one
curve to other
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

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 Power of CL determined by central ant & post surfaces at O.Z.
determined by the ammetropic correction required
 Tint- to reduce the glare for cosmesis
 Ref. error determined by retinoscopy
 Curvature determined by keratometry
 Fit can be assessed by fluorescein pattern- flat , steep, ideal