contact lens

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Contact Lens

Dr Abdul Hannan

+Classification

1. Anatomical Position1. Scleral

2. Semi Scleral

3. Corneal

2. Nature of Material1. Rigid Non gas permeable

2. RGP

3. Soft contact lens

3. Mode of wear

+Classification

Mode of wear Daily wear Extended wear Disposible

Depending on Water Content Low water content Medium water content High water content

+Contact Lens Design

Single Cut design

Lenticular cut lens

+Terms Related to CL

Wettability

Water Content

Oxygen Permeability

Oxygen Transmissibility

Light Transmission

Refractive Index

Flexure

Dimentional Stability

+CL Materials

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

+RIGID non-gas-permeable PMMA

Advantages Light weight Excellent Moulding Non-toxic

Disadvantages Impermeable to O2

Hard – can cause corneal abrasion Resists wetting

+RIGID gas permeable lenses

Cellulose Good wetting Easy scratching Easy wraping

Silicone High oxygen permeability Lack of wetting

Styrene Surface durability and brittleness

+Present RGPs

Silicone Acrylate

Fluoropolymers

+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

+Indications

1. Optical

2. Therapeutics

3. Preventive

4. Diagnostics

5. Operative

6. Cosmetic

7. Occupational

+Optical

Anisometropia

Unilateral aphakia

High Myopia

Keratoconus

Irregular astigmatism

+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

+Therapeutics

Corneal Disease

Iris defect

Amblyopia

BCL after surgery

+Preventive

Symblephron

Restoration of chemical burns

Exposure keratopathy

Trichiasis

+Diagnostic

Gonioscopy

ERG

Fundal Contact Lens

Fundus Photography

Triple mirror

+Operative

Goniotomy

Vitrectomy

Endocular photocoagulation

+Contraindications

Mental incompetence

Lacrimal Disease

Disease of eyelids

Episcleritis or scleritis

Uveitis

Ocupational Hazard

Facial Palsy

Poor hygiene

Allergy to lens material

Dry eyes

+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

+Grip of CL

Cohesion of water molecules

Negative pressure

+Supply of Nutrition

Lid Pump Mechanism

20% replacement of post CL tear film is required

+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

+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

+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