Contact lenses

OPHTHALMOLOGY PRESENTATION

1. CONTACT LENSES Dr.Sidesh Hendavitharana (Registrar-ophthalmology)

2. The concept for the development of contact lens came from Leonardo De Vinci(1508 AD) In 1946 ,corneal contact lens,made of polymethyl methacrylate(PMMA) were popularized In 1961 ,introduction of soft contact lenses were made by Wichterle In same period,development of rigid gas permeable contact lenses were made. History

3. Contact lens classified as, Depending upon the anatomical position occupied Depending upon the nature of material used for manufacturing Depending upon the mode of wear Depending upon the water content(hydrogel lenses) Contact lens terminology

4. Sceral contact lenses ,which cover the cornea and conjunctiva overlying the sclera Semiscleral contact lenses ,which cover the limbus to lie partially on the conjunctiva Corneal contact lenses ,which confirm the cornea Depending upon the anatomical position occupied

5. Rigid (hard)non gas permeable contact lenses eg:those made of PMMA Rigid gas permeable contact lenses , eg:cellulose acetate butyrate(CAB) and silicone lenses Soft contact lenses eg:those made of HEMA Depending upon the nature of material used for manufacturing

6. Daily wear contact lenses Extended wear contact lenses Disposable contact lenses Depending upon the mode of wear

7. Low water content(0-40%) Medium water content(40-55%) High water content(>55%) Depending upon the water content(hydrogel lenses)

8. Wettability It is the adherance of a liquid to the surface of a solid despite the cohesive forces holding the liquid together,lower the wetting angle better the wettability Complete wetting-wetting angle 0 Partial wetting-wetting angle 70 Non wetting-wetting angle 150 Terms used in relation to lens material properties

9. Water content Contact lens has pores that are formed by cross-linking of monomers. These pores absorb water,forming the water content of the lens Water content↑→oxygen transmisability ↑ If water content increases by 20%,oxygen permeability is doubled .

10. Permeability , Property of the polymeric material to transmit gaseous substances It is express as a coefficient of variable DK, D-diffusion coefficient K-solubility coefficient Transmissibility , Refer to the oxygen permeability for a contact lens of given thickness Expressed as DK/L Oxygen permeability and transmissibility

11. Light transmission-measure of optical property Refractive index Heat resistance Dimensional stability Other properties are,

12. Biocompatibility-should not contain material potentially harmful to the ocular tissues Optical properties- transparent,ref.index approximate to that of tears Gas permeability-increases wearing time and tolerance Moulding -amenable to easy shaping Sterility-easily srerilizable Stability-curvature and thickness of contact lens should be stable for an efficient optical purpose Properties of ideal material for contact lens are,

13. Broadly contact lens material classified into two groups Focons - term focon is used to hydrophobic rigid lens materials Filcon -term used for the hydrophillic non rigid lens material Contact lens materials

14. Rigid (hard)non gas permeable contact lenses Manufactured from plastic or thermoplastic material Commercially available plastic contact lens material was PMMA-derivative of acrylic acid Depending upon the nature of material used for manufacturing

15. The PMMA has a high optical quality and stability and is light in weight It has excellent moulding and machining qualities and so is easy to manufacture pure PMMA is non toxic and does not excite allergic reactions Advantages of PMMA

16. PMMA is practically impermeable to oxygen,with DK value essentially zero,thus restricting the tolerance Being relatively hard,it can cause corneal abrasions Being hydrophobic in nature,it resists wetting,but a stable tear film can be formed over it. Disadvantages of PMMA

17. Silicone acrylate Commonly manufactured from the copolymer of PMMA and silicone-containing vinyl monomer Cross-linking of silicone and PMMA copolymers have resulted in siloxanyl methacrylate lenses. It has wide range of DK value(15-55)and oxygen permeability Rigid gas permeable lens materials

18. Fluoropolymers Copolymer containing fluorine molecule These lenses have been accepted for extended wear because of their high oxygen permeability Three subtypes of fluorine-containing contact lens materials Pure fluoropolymer Fluorosiloxanylmethacrylate Perfluoroalkyl itaconate-siloxane .

19. Made from hydrogels Hydrogel contains hydrophillic group such as hydroxyl group Hydrogels are obtained by polymerization and copolymerization of hydrophillic monomers with a cross linking agents,ethelene glycol dimethacrylate (EGDM) Soft contact lens materials

20. HEMA lenses HEMA-VP(Vinyl pyrrolidone ) MMA-PVP(polyvinyl pyrrolidone ) Glycidyl methacrylate Commmonly used hydrogel lens materials are

21. FDA group 1-low water content(<50%),non ionic polymers FDA group 2-high water content(>50%),non ionic polymers FDA group 3-low water content(<50%),ionic polymers FDA group 4-high water content(>50%),ionic polymers Contact lens materials FDA classification

22. Neutralization of cornea With conact lens in situ,2 lenses present Glass lens Fluid lens-formed by a thin tear film Ref.index of tear-1.336,cornea-1.376 The tear film would neutralize1.336 or about 9/10 th of the power of ant.surface of cornea. Ametropia is corrected by glass lens. Optics of contact lens

23. When a contact lens is in place,four media of different refractive indices must be taken into account. These are air(R.I.-1),contact lens material(PMMA,R.I.-1.49),tears(1.33),and cornea(1.37) Most significant refractive power lies in the interface between air and lens Front surface optically replaces the front surface of cornea. The curvatures of interfaces have equal importance in the refractive properties of the system. Correction of ametropia by contact lenses

24. Afocal lenses -the correcting properties of optical system imparted by different curvatures of surfaces of fluid lens Powered lenses -correction of ametropia is fundamentally due to difference in curvature of anterior and posterior surfaces of contact lens Correction of ametropia by contact lens with two basic principles.

25. Distance in relation to the vertex plane determines the power and size of retinal image contact lens is placed at the vertex plane while spectacle lens is placed 13mm in front of vertex plane. The power of contact lens required to correct the ametropia can be calculated from the spectacle correction Power and magnification by contact lens

26. The size of retinal image depends on the site of lens Eg:in aphakia,magnification is 22% with spectacle lens and 7% for contact lens which is within tolerable limits for binocular single vision in most individuals. .

27. Tear lens is formed between the posterior surface of the contact lens and anterior surface of cornea Two variable factors concerning the fluid lens Thickness Surface curvatures Influence of tear lens(fluid lens)

28. Contact lenses increase the accomodative and convergence requirments of myopic eyes and decrease those of hyperopic eyes Effect of contact lens on accomodation and convergence demand

29. To understand the contact lens specifications ,designs and parameters, following standard nomenclatures have been recommended Diameters of lens Curves of lens Blends Edge of lens Power of lens Central thickness of lens Tint Design prescription and parameters of a contact lens

30. Overall diameter or chord diameter of the lens Expressed in mm Optic zone diameter ,dimension of the central optic zone of lens which is meant to focus rays on retina Diameters of the lens

31. Base curve or central posterior curve Is a curve on the back surface of the lens to fit the front surface of the cornea Peripheral curve Serve as a reservoir of tears Central ant.curve-ant.surface of optical zone of lens Peripheral ant.curve Intermediate ant.curve -fabricated only in the high power minus and plus lens Curves of the lens

32. Is a smooth area of transition the radius of curvature from one curve to another Classified as, Light-transition between two posterior curves distinctly visible Medium-the transition between two posterior curves just visible Heavy-transition between two posterior curves not visible Blends

33. Optical indications Anisometropia Unilateral aphakia High myopia Keratoconus Irregular astigmatism Indications for contact lens use

34. Corneal diseases Eg : non healing corneal ulcers, bullous keratopathy,filamentary keratitis and recurrent corneal erosion syndrome Diseases of iris Eg : aniridia,coloboma and albinism to avoid glare Bandage contact lens are used following keratoplasty and in microcorneal perforation Therapeutic indications

35. Prevention of symblepharon and restoration of fornices in chemical burns Exposure keratitis trichiasis Preventive indications

36. Use during ERG Examination of fundus in the presence of irregular corneal astigmatism Goldman 3-mirror examination Diagnostic indications

37. Used during Goniotomy operation for congenital glaucoma Vitrectomy Endocular photocoagulation Operative indication

38. Unsightly corneal scar Cosmetic scleral lenses in phthisis bulbi Cosmetic indications

39. Devided into blinding and nonblinding complications Blinding Infective keratitis Corneal scarring Corneal warping Complication of contact lens wear

40. Related to desication Dry eye syndrome Related to microtrauma Punctate epithelial erosions Corneal abrasion Superior limbic keratoconjuctivitis Related to hypoxia Corneal oedema Epithelial microcyst Acute overwear syndrome(rupture of cysts) Corneal vascularization Related to hypersensitivity Giant papillary conjunctivitis Allergic conjunctivitis Sterile corneal infiltrate Nonblinding complications

41. Initial patient work-up History Exclude medical C/I for contact lens wear Eg : allergies Evaluate emotional status and reasons for wanting contact lenses General occular examination Conjunctival and limbal injection or corneal infiltration Assesment of tear film Blink characteristics Measurement of corneal diameter, pupillary diameter and palpebral width Refraction including retinoscopy and subjective verification Keratometry -measure corneal curvature in two principle meridia Contact lens fitting procedure

42. Diameter of lens-overall diameter of 9mm+/-0.5mm Base curve radius Derived from keratometry If presence of astigmatism, 0.5-1.0D-0.25D steeper than K is chosen Eg:44/45D-base curve 44.25 1.0-2.0D-0.5Dsteeper than K More than 2.0D ,1/3 rd toricity is added Selecting lens from the trial set

43. Position of lens Lens may ride high,low or horizontal decentring of the lens Base curve determination -flat/steep or ideal base curve Finalization of overall diameter Finalization of the power Evaluation of the trial lens fit

44. The various solutions available can be grouped as, Wetting agents Cleaning agents Storage (soaking)agents Rewetting agents Contact lens solutions and care of contact lenses

45. Wetting solution is an agent that coats the contact lens uniformly with the film which minimizes the friction They are sterile , isotonic,non-irritable,stable and it should be buffered to pH of tears Eg:polyvinyl alcohol,polysorbate 80, polyetheline oxide Wetting agents

46. It has two functions Detergent which remove the lacrimal film and mucous deposits Bactericidal agent Two types of cleaners Sufactant cleaners -K sorbate,sorbic acid Enzymatic cleaners -papain, incorperation of enzyme lipase Cleaning solutions

47. Serves as an antiseptic storage medium It maintains the lenses in a hydrated state Storage(soaking)solutions

48. Used to rewet the corneal surface or lens device Commonly used contact lens solutions are, Benzalkonium chloride Chlorobutanol Thiomersal Chlorhexidine Hydrogen peroxide and povidone -iodine systems EDTA Polyvinyl alcohol Rewetting agents

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50. Thank you