Various Soft Contact Lens- designs and their indications

Introduction to soft contact lenses - various designs and their indications

1. Various soft contact lenses – Designs and their indications Presenter : Urusha Maharjan 18 th batch, IOM

2. Contents Introduction to soft contact lenses Design factors Material properties Centre thickness Water content and thickness Other design considerations Physiological considerations Fitting characteristics Back and front surface designs Aspheric soft lenses Lens design limitations Various types of soft contact lenses and their indications

3. Introduction to soft contact lenses These lenses are very thin and pliable and conform to the front surface of the eye . Also known as hydrophilic lenses, Hydrogel lenses, etc. These rely on their water content to transmit oxygen through the lens to the cornea. As a result, higher water content lenses allowed more oxygen to the cornea

4. Lens parameters : ISO terminology r = Back Optic Zone Radius (BOZR) r 1 = Back Peripheral Radius, First (BPR1) r 2 = Back Peripheral Radius, Second (BPR2) r a0 = Front Optic Zone Radius (FOZR) r a1 = Front Peripheral Radius, First (FPR1) t c = Geometric Centre Thickness t pj1 = Peripheral Junction Thickness, First t pj2 = Peripheral Junction Thickness, Second t ER = Radial Edge Thickness t EA = Axial edge Thickness

5. Terminology cont. Ø = Back Optic Zone Diameter (BOZD) Ø a0 = Front Optic Zone Diameter (FOZD) Ø 1 = Back Peripheral Zone Diameter (BPZD) Ø T = Total Diameter (TD) Miscellaneous (not shown): Fv´ = Back Vertex Power (BVP) Fv= Front Vertex Power (FVP)

6. Design factors For the ease of understanding, soft lens design factors is divided as: Material properties Centre thickness Water content and thickness Back surface designs Front surface designs Edge designs Aspheric soft lenses Other design considerations

7. A. Material properties More significant in soft lens than in RGPs Water contents of 25 - 79% means material properties vary greatly Hydrogel contact lens materials are made with a stable, solid polymer component that can absorb or bind with water . The polymers consist of small building blocks called monomers .

8. By polymerizing different combinations of monomers, the physical and chemical properties of the lens material can be created, such as water content, refractive index, hardness, mechanical strength, and Dk . The following monomers are those that are most commonly used to create hydrogel contact lenses:

9. HEMA 2-Hydroxyethyl methacrylate (HEMA) is the monomer that was used to create the first commercial hydrogel contact lens, and it continues to be the monomer most often utilized. By itself , it will allow a water content of about 38%. When it is combined with other monomers ( such as N-vinylpyrrolidone or methacrylic acid), the water content can be increased from 55 % to 70 %. 2-HEMA is a extremely stable material, and variations in temperature, pH, or tonicity have relatively little effect on its water content. It also offers good wettability.

10. Ethylene glycol dimethacrylate Ethylene glycol dimethacrylate(EGDMA) is used primarily as a cross-linking agent. Its primary function is to increase the dimensional stability of the material . Increased use of EGDMA will tend to make the material stiffer, lower in water content, and less stretchable .

11. Methacrylic acid Methacrylic acid (MAA) is used to increase the water content of the lens material. It is extremely hydrophilic because of the presence of a free carboxylic acid group that bonds water. It therefore also tends to impart ionic (charged) properties to a material.

12. Methyl methacrylate Methyl methacrylate (MMA) is sometimes used to lower water content or to increase the material hardness or strength of a material . It offers excellent optical clarity and is completely inert and very stable, but does not offer any permeability to oxygen.

13. N- vinyl pyrrolidone (NVP) N-vinyl pyrrolidone (NVP) is very hydrophilic and is used to increase water content. I t offers excellent wettability, and its high water uptake allows for increased Dk . It generally imparts an ionic property to the material.

14. Glyceryl methacrylate Glyceryl methacrylate (GMA) offers good wettability and helps to increase deposit resistance because it creates smaller pore sizes. Because it also lowers the water content of the material , it imparts a lower Dk.

15. Polyvinyl alcohol Polyvinyl alcohol (PVA) is very hydrophilic, thereby increasing the water content and the Dk of the lens material. It is highly biocompatible and extremely resistant to deposits. It also imparts increased hardness and strength, along with excellent optical clarity. It is completely inert and very stable.

16. The U.S. Food and Drug Administration (FDA) classifies soft contact lenses into four groups for the U.S. Market. These 'water-loving' soft contact lens materials are categorized as "Conventional Hydrophilic Material Groups ("-filcon "): Group Water Content Percentage Ionic/Non-Ionic I Low Water Content (<50%) Nonionic II High Water Content (>50%) Nonionic III Low Water Content (<50%) Ionic IV High Water Content (>50%) Ionic

17. Group 1- Low Water (<50% H 2 O) Nonionic Hydrogel Polymers Material % Water Oxygen Transmissibility (Dk/t) Brands Helfilcon A/B 45 12 Continental Toric , Flexlens , Flexlens Toric, Flexlens Aphakic, Optima Toric , All X-Cel lenses Hioxifilcon B 49 15 Alden HP, Alden HP Toric, Aqua ease, Essential Soft Toric Multifocal , Flexlens , Quattro, Satureyes, Satureyes Toric and Multifocal, All X-Cel Lenses Polymacon 38.6 8.5-24.3 Allvue, Biomedics 38 , Clearview , CustomEyes 38 , EpconSOFT , EsstechPS , Esstech PSD, Esstech SV, Frequency 38 , HD, HD-T, HDX, HDX-T, Horizon 38, Hydron Mini, Hydron Zero 4 SofBlue , Hydron Zero 6 SofBlue , Hydron Versa Scribe, Lifestyle MV2, Ideal Soft, Lifestyle Xtra , Lifestyle 4Vue, Lifestyle Toric Bifocal, LL38, Metrosoft ll Multifocal, Metrosoft Toric, Natural Touch, Occasions, Optima 38/SP/FW , PS-45 Multifocal, Simulvue 38, Sof -form II, Soflens , SofLens38 , Soflens Multi-Focal, Softics , SoftView , Unilens 38, Westhin Toric Tefilcon 38 8.9 Cibasoft , Illusions, Torisoft , Softint , STD, LL Bifocal Tetrafilcon A 43 9 Cooper Clear, Cooper Toric , Preference , Preference Toric, Vantage, Vantage Accents, Vantage Thin, Vantage Thin Accents, Classic

18. Group 2 - High Water (>50% H₂O) Nonionic Hydrogel Polymers Material % Water Oxygen Transmissibility (Dk/t) Brands Alphafilcon A 66 16 Soflens Toric for Astigmatism, Medalist 66, Soflens 66 Hefilcon B 45 10 Optima Toric Hilafilcon A 70 35 Hilafilcon B 59 22 Soflens Daily Disposable , Soflens Daily Disposables for Astigmatism Hioxifilcon A 59 28 ExtremeH₂O 59% Thin/Extra, Biocurve Gold Sphere and Toric, Aura ADM Hioxifilcon D 54 21 ExtremeH₂O 54%, Clarity H₂0, C- Vue Advanced Custom Toric Nelfilcon A 69 26 Focus Dailies , Focus Dailies Toric/Progressive, Dailies AquaComfort Plus, FreshLook One-Day , Synergy, Triton Omafilcon A 58-60 28-36.7 Proclear 1-Day, Proclear EP, Proclear 1 day Multifocal, Proclear Multifocal Toric, Biomedics XC, Aveo Omafilcon B 62 21.3-52.3 Proclear Sphere , Proclear toric, Proclear toric XR, Proclear multifocal, Proclear multifocal XR, Proclear multifocal toric

19. Group 3 - Low Water (<50% H₂O) Ionic Hydrogel Polymers Material % Water Oxygen Transmissibility (Dk/t) Brands Bufilcon A 45 16 Hydrocurve II 45, Soft Mate B Deltafilcon A 43 10 Amsoft , Amsoft Thin, Comfort Flex , Custom Flex, Metrosoft , Soft Form Toric Phemfilcon 38 9 Durasoft 2

20. Group 4 - High Water (>50% H₂O) Ionic Hydrogel Polymers Material % Water Oxygen Transmissibility (Dk/t) Brands Bufilcon A 55 16 Hydrocurve I , Hydrocurve 3 Toric, Softmate II Perfilcon A 71 34 Permalens Etafilcon A 58 23.8-28 Acuvue , Acuvue Bifocal, Acuvue 2, Acuvue 2 Colors, 1-Day Acuvue, 1-Day Acuvue Moist, 1-Day Acuvue Moist for Astigmatism, 1-Day Acuvue Moist Multifocal, 1-Day Acuvue Define, Colornova , Discon , Waldo, Natural Vue , Surevue Phemfilcon A 55 16 Durasoft 3 , Freshlook , Wildeyes Methafilcon A 55 17.9-37.6 Biocurve Advanced Aspheric, Biocurve 1-Day, Biocurve Toric & Sphere, C- Vue 1-Day ASV, C- Vue 55, Edge III 55, Elite AC, Elite Daily, Elite AC Toric, Expressions Colors, Flexlens , Frequency 55 Sphere /Multifocal, HD2, HDX2, Horizon 55 Bi-Con, Hubble, Kontur, LL55, New Horizons, Revolution, Sauflon 55, Sof -form 55, Sunsoft Eclipse, Sunsoft Toric, Vertex Sphere, Vertex Toric Vilfilcon A 55 16 Focus 1-2 Week Softcolors , Focus Monthly Softcolors , Focus Toric, Focus Progressives, Soft 55, Soft 55 EW

21. SiHy - Silicone Hydrogel Polymers Material FDA Group % Water Oxygen Transmissibility (Dk/t) Brands Balafilcon A 3 36 91-130 PureVision , PureVision Toric , PureVision Multi-Focal, PureVision2, PureVision2 for Astigmatism, PureVision2 Multi-Focal for Presbyopia Comfilcon A 1 48 116-160 Biofinity , Biofinity toric, Biofinity XR, Biofinity XR toric, Biofinity Energys , Biofinity multifocal Delefilcon A 33-80 (water gradient) 140 Dailies Total1 Enfilcon A 1 46 100 Avaira, Avaira Toric Galyfilcon A 1 47 86 [17] Acuvue Advance with Hydraclear , Acuvue Advance for Astigmatism Lotrafilcon A 1 24 140 Air Optix Night & Day Aqua Lotrafilcon B 1 33 110 O 2 Optix, Air Optix for Astigmatism , Air Optix Aqua, Air Optix Aqua Multifocal Samfilcon A 46 [18] 114-163 [19] Bausch & Lomb Ultra, Bausch & Lomb Ultra for Astigmatism, Bausch & Lomb Ultra for Presbyopia Senofilcon A 1 38 103-147 Acuvue Oasys , Acuvue Oasys for Astigmatism, Acuvue Oasys for Presbyopia, Acuvue Oasys 1-Day, Acuvue Oasys 1-Day for Astigmatism Sifilcon A 1 32 82 O 2 Optix Custom

22. Silicone hydrogels vs. conventional hydrogels The most striking difference between conventional and silicone hydrogels is that unlike conventional hydrogels, the high oxygen transmissibility of silicone hydrogels is not dependent on the water content of the lens material . Conventional hydrogels are primarily composed of hydrophilic monomers (HEMA) that attract and bind water into the polymer. Oxygen dissolved in the water is transported through the lens to the cornea and the higher the water content, the higher the oxygen permeability of the material In addition, siloxane groups are hydrophobic in nature which means silicone hydrogels need to be surface treated to increase wettability and to reduce the level of lipid deposits that would occur if the surface was left untreated

23. Compatibility with lens care solutions is different among conventional hydrogels and silicone hydrogels. (because their basic characteristics like wettability, water content, Dk, etc. differ ) The vast differences between silicone hydrogels and conventional hydrogels in chemical composition and on-eye behavior suggest that perhaps a new FDA lens group should be instituted that better describes the unique characteristics of silicone hydrogels.

24. Few commonly used materials Pure toric : Material Polymacon Water content 38% and Dk 9 2. Axis toric Material Polymacon Water content 38% 3. Soflens 66 toric (B&L) Material alphafilcon A Water content 66% and Dk 32

25. 4. Soflens daily disposable (B&L) Hilafilcon B Water content 59% and Dk 22 5. J&J acuvue moist Etafilcon A Water content 58% and Dk 28 6. Pure vision toric Balafilcon A Water content 36% and Dk 91

26. 7. Air optix for astigmatism (Ciba vision) Lotrafilcon B Water content 33% and Dk 110 8. Ciba focus dailies (daily disposable toric) Nelfilcon A Water content 69% and Dk 26 9. Optima toric (B&L) Hefilcon B Water content 45% and Dk <12

27. 10. Soflens daily disposable (B&L) Hilafilcon A Water content 70% and Dk 35 11. Acuvue (J&J) Acuvue, Acuvue Bifocal, Acuvue 2, Acuvue 2 Colors, 1-Day Acuvue, 1-Day Acuvue Moist, Acuvue clear Etafilcon A Water content 58% and Dk 28 12. Soflens comfort (B&L) Hilafilcon B Water content 59% and Dk 22

28. 13. Purecon company Q3 Etafilcon A Water content 58% and Dk 22 14. Soflens 38 Polymacon Water content 38.6% and Dk 9 15. Purecon Supersoft Water content 45% and Dk 9 pHEMA 16. Purecon UV filter , same

29. 17. Purecon O2X extended wear Filcon II 2 Water content 70% and Dk 43 18. Purecon prolong wear PGMA/HEMA (Hiofilcon B) Water content 49% and D k 15 19. Bandage lens Water content 58% and Dk 21 Material filcon II

30. 20. B&L yearly lens : B-series, U- series, HO series, Optima 38, Plano- T (bandage cl) Water content 38.6% HEMA 21. Ciba vision Freshlook yearly Water content 55% and Dk 16 Phemfilcon A Colored/ non colored 22. Silklens Aquasoft visiblu Material HEMA Water content 42%

31. 23. Silklens UV soft pHEMA with UV blocker Water content 55% 24. Silklens soft extended wear pHEMA /vinyl pyrrolidone Water content 75% and Dk 35 25. P urecon natural eyes Yearly disposable and colored Water content 38% and Dk 9

32. 26. Aryan colored lens (Migwang company ) Water content 42% and Dk 10 pHEMA 9 dazzling colors available 27. Optima natural look (B&L) pHEMA WC 38.6% 28. Freshlook color blends Phemfilcon Wc 55%

33. 29. Soflens star color II (B&L) Water content 38.6% HEMA material Colors available are hazel, grey, topaz blue, blue, violet, green 30. Acuvue II colored lenses (J&J) Etafilcon A Water content 58% and Dk 25.5 Colors available are: ocean blue, sapphire blue, green, warm honey, deep blue, chestnut brown, hazel green, pearl grey, aqua marine, emerald green

34. 31. Neo cosmo lens (Neo vision) Water content 45% Blue, green, brown, grey, purple, honey, hazel, violet 32. Prosthetic contact lenses: Water content 70% E.g., prosthetic Purecon, Silklens, flexilens

35. B. Center thickness Center thickness consideration : Dk/t considerations : must be met under the conditions of intended lens usage Pervaporation prevention : high water material in a thin lens Fitting considerations : if the lens is too thin, there may be excessive flexing and it may not remain in cornea If the lens is too thin but remains on the cornea, it may move a little or not at all because of conformity.

36. Design considerations : Minus lens series While a large FOZD is desirable, other limitations apply and may need to be considered, especially in high plus and minus Rxs. Lenses of lower minus power (<2.00D) are often made thicker and/or with a larger FOZD to improve handling While higher powered lenses may have FOZD decreased to reduce mid-peripheral thickness (see above), such a decrease is limited by vision issues. The latter may be governed by pupil size under conditions of reduced illumination . Central thickness for negatively powered soft lenses range from 0.035 to 0.15mm

37. Design Considerations: Plus Lens Series While zero/ near zero will give the thinnest geometric center thickness, these figures are not practical Center thickness reduction by FOZD reductions is limited by vision issues. (will not be tolerated by most wearers except those with small pupils) Only BVP determines the tc so no degrees of freedom remain Central thickness for positively powered lenses may be more than 0.35mm

38. C. Water content and thickness The number of arrows to the right of the lens profile is intended to convey a qualitative measure of the O2availability under the lens. However , too literal an interpretation of this diagram may be misleading .

39. Transmissibility (Dk/t ) The oxygen permeability is proportional to the water content , i.e. Dk ∝ H 2 O content. The thicker a lens is, the lower are its O 2 & CO 2 transmissibilities,i.e . Transmissibility (O2&CO2) ∝ 1/t and = DK (O2&CO2) /t Therefore, corneal respiration is best served by a thin , high water lens, however this sort of lens is also subject to pervaporation

40. Pervaporation If the lens is too thin, corneal dehydration may result due to bulk flow of water through lens and instability of water flow at the lens surface. This dehydration: Is subject to individual variation Is worse with higher water content lens Results in corneal dehydration from water loss to air via lens Produces epithelial desiccation staining (pervaporation staining)

41. Water lost to the atmosphere from the anterior lens surface is replaced with water from the back o f the lens . This in turn taps the supply in the tears . When this source is exhausted, water is extracted from the cornea and pervaporation-induced desiccation staining of the superficial epithelium can be demonstrated objectively

42. High water content lenses: Lose more water than low water lenses Lose water even when worn in a high humidity environment Probably due to the higher ‘on-eye’ temperature and possibly other eye environment factors such as tonicity and pH Experiences ‘on-eye’ lens shrinkage which affects TD and BOZR and hence affect fit This suggests that ‘on-eye’ shrinkage must be taken into consideration when selecting trial lens or ordering final lens parameters.

43. D. Other design considerations Centration : Proper centration required for quality of vision, comfort and mechanical effects of a lens on the eye. More adverse effects in higher prescriptions If significantly decentered, corneal exposure becomes a possibility and hence more problematic wearer’s comfort Decentration can also lead to localized elevated bearing pressures, conjunctival indentation and even an altered fit. The amount of Decentration may also increase with lens age due to increased lens traction caused by progressive lens spoilage.

44. Movement : A minimum amount of movement is required for all soft contact lenses for the removal of the debris. Tear mixing (oxygen exchange) which has been demonstrated to be minimal , is not a pressing reason for lens movement per se to be pursued. While too little or no movement is prejudicial to lens tolerance and wearer success in the medium to long term, excess movement usually results in immediate lens awareness/discomfort . As long as the excess movement persists, this awareness / discomfort seldom abates.

45. Intended Lens Usage is Relevant because : •If extended wear (EW) is envisaged, the cornea’s minimum requirements during eye closure must be met. • Lesser open-eye requirements apply to daily wear (DW). If an EW lens is practical for daily wear , then it represents a better alternative than a minimalist approach . •EW requirements always > DW because of, among other things, the significantly reduced O 2 availability in the closed eye

46. Soft contact lenses can be worn according to a number of different wearing modalities. DAILY WEAR Lenses worn on a daily wear basis are worn during waking hours , usually for periods of 8-16hours . On removal they are either cleaned and disinfected in preparation for the next wearing period ,or, in the case of single-use daily disposable lenses, discarded . FLEXIBLE WEAR Lenses worn on a flexible wear basis are typically worn on a daily wear basis, with occasional, infrequent overnight use. When removed , they should either be cleaned and disinfected or discarded .

47. EXTENDED WEAR W orn constantly for up to 7 consecutive days and nights. When removed, they should either be cleaned and disinfected (reusable extended wear) or discarded (disposable extended wear). However , modern silicone hydrogel lenses are worn for four consecutive 1-week periods of extended wear and then discarded, making them reusable extended-wear lenses . CONTINUOUS WEAR Used to be worn even upto 3 mths historically but discarded in SCLs However, in silicone hydrogels, it means lenses are worn on a 24 –hour periods for 30 consecutive days and nights

48. Edema Cycle Overnight edema Corneal edema results from prolonged eye closure and the chronic reduction of O2 availability under closed lids during sleep. This is largely due to water imbibition and the reduced capacity of the endothelial pump to maintain corneal deturgescence. Incomplete deswelling Once a certain level of overnight swelling is exceeded , the cornea is incapable of thinning itself sufficiently during the day to return its thickness to baseline levels while wearing the contact lens . The incompletely thinned cornea then exhibits an apparent residual daytime level of corneal edema.

49. Overnight Lens Wear Corneal edema Edema depends largely on O2availability. Overnight, during eye closure/sleep, O2 availability is at its lowest. The O2 transmissibility of the lens indicates the magnitude of the negative contribution the lens makes to the condition the cornea is exposed to during eye closure. On eye opening. Upon eye opening, the partial pressure of O2 nearly triples. The level of hypoxia is reduced and corneal function endeavors to return to normal levels. The endothelial pump increases its fluid removal rate and the cornea thins.

50. Holden & Mertz (1984) found an equivalent oxygen percentage(EOP) of 9.9% or a Dk/t of 24.1 was required for DW . EW has a much greater requirement ( 17.9% O2or a Dk/t of 87 ). These figures (DW and EW) are difficult to meet with present materials and manufacturing technology . In view of these difficulties, a compromise based on the level of overnight swelling which can be reversed on awakening while lens wear continued, was also presented . A Dk/t of 34.3 , or an EOP of 12.1% , was suggested .

51. Lens Thicknesses to Prevent Edema Daily wear: •38% water lenses: 0.033 mm. •75% water lenses: 0.166 mm . Extended Wear: •38% water lenses: 0.009 mm. •75% water lenses: 0.046 mm . Compromise figures for Extended Wear : •38% water lenses: 0.023 mm. •75% water lenses: 0.117 mm .

52. Hydrogel Lenses On average, all current soft lenses cause more than 8 % O/N edema which the cornea is unable to eliminate completely during the open-eye phase . The residual is a measure of the level of chronic hypoxia produced by these lenses . Conservatively, EW should not be recommended or prescribed if it can be avoided. If not, a 1-2 nights per week schedule at most should be advised, along with a recommendation for regular aftercare visits .

53. Tear Mixing It has been shown by experiment and clinical observation that there is little useful tear mixing under a soft contact lens. This means that oxygenated tears do not find their way to areas of lower oxygenation, hence there is little compensation for any local hypoxia . Corneal swelling at any point is related to Dk/t local because of the lack of tear mixing under a soft lens. Hence local thickness is the ONLY relevant lens dimension .

54. Reasons for poor tear mixing under soft lenses include: Conformance results in a thin, approximately parallel , tear film with little aqueous phase being present . This results in a more viscous tear film, because it consists mainly of lipids and mucin . The lens profile, which is affected by lens design as well as BVP, influences local lid-induced pressures under both static (rest) and dynamic(blink ) conditions .

55. Material rigidity is directly related to its water content. Higher water lenses conform more closely than low water lenses . Lens movement, especially with a thin tear film containing little water, has little effect on the physiological conditions under a soft lens.

56. V Fitting Characteristics 1. Fitting characteristics: BOZR Less significant than in RGPs SCL fitting is based on ‘three-point touch’ approach. The three points are peri-limbal conjunctiva, corneal apex and peri- limbal conjunctiva. More flexible material results in greater conformity and a thinner post lens tear film.

57. Larger changes are required for clinically significant alterations to ‘on-eye’ behavior . The BOZR of soft lenses can be custom made from approximately 7.00 to 9.50 mm but is generally available in a limited range from 8.30 to 9.20mm Typically a soft lens BOZR needs a change of 0.3 mm whereas RGP is sensitive to change of 0.05-0.1mm For most conventional polyHEMA based materials, a suitable BOZR will be 0.7-1.0 mm flatter than the flattest K-reading Soft lenses are more environmentally susceptible and the effect less predictable. E.g. pH, tear tonicity, temperature, etc.

58. Initial fitting relationship is lost due to conformity, osmotic equilibration, lid pressure and elastic forces induced. Visco- elastic forces induced on decentering aid lens self-centering. When a lens is displaced from its central position, it rests on the flatter peri-limbal and scleral zones. Its apparent diameter also increases due to flattening After the blink, the lens relaxes but the relaxation process lags behind the retreat of the lids

59. 2. Fitting characteristics: FOZR The upper lid covers more of a soft lens than in RGP lens. This influences: Lens resting position(static) : The ramp- like edge shape of a soft lens will result in a force, approximately parallel to the eye surface, which attempts to squeeze the lens from under the lid. To a lesser extent the lower lid does the same. The balance of these forces, in addition to forces within (viscoelastic) and around the lens (surface tension, negative squeeze pressures under etc..) will decide the resting position of the lens in situ.

60. Movement induced by a blink If a lid margin positions well over soft lens, its effect on lens movement will be less than one which covers only a little of the lens. By keeping the lens as thin as possible, the front surface profile as low as possible and the surface as continuous (stepless) as possible, the lid/lens interaction is reduced further.

61. 3. Altering lens fit(sagittal height) The diameter can be left unaltered and the sag changed by altering the BOZR The diameter can be altered and the BOZR changed in such a manner that the sag is left unchanged.

62. Both the sag and diameter can be changed By increasing the sag height independent of diameter, the fit of the lens becomes effectively steeper and predictably tighter By decreasing the sag height independent of diameter, the fit of the lens becomes effectively flatter and predictably looser

63. VI. Design factors Back Surface designs Single curve(monocurve) : simplest but not commonly used Bicurve : often 0.8- 1.0 mm flatter than BOZR about 0.5-0.8 mm wide A common design because it performs satisfactorily clinically and is easy to design and make Blended multiple spherical curves (multicurve) : relatively uncommon Aspheric : relatively uncommon Surface shape may be a continuous single conic section or a progressive change complex of conics

64. 2 . Front surface designs : It tends to be ignored since the front surface is not in contact with the cornea But it is important for lens fit and on eye behavior and hence the comfort of the lens. Somewhat dependent on manufacturing processes

65. Front optic zone diameter (FOZD) Usually the FOZD defines the effective optic zone diameter of a soft lens, especially when used in combination with simple back surface designs. In this diagram, the front peripheral design defines the FOZD and therefore the OZD of the lens

66. It is commonly bicurve with a peripheral curve chosen to produce a thin edge. Multiple blended peripheral spherical curves: not required Continuous aspheric front curves are not commonly used Front surface design may also include bifocal or multifocal components such as : Continuous aspheric surface Concentric bifocal Flat-top segment

67. 3. Edge design : less consideration is given to details of edge design than for RGPs because of the size of the lenses: The edge is already under both lids Edge has relatively little effect on comfort Too thick an edge may produce discomfort Too thin an edge may lead to tearing of the edge. This is considered to be more important

68. 4. Aspheric soft lenses Aspheric means a conicoid A mathematically regular non- spherical surface/shape Based on conic sections Conic sections relevant to soft contact lenses are : Circle Ellipse Parabola

69. Advantages of aspheric : Aspherics attempt to optimize the lens/ anterior eye relationship Aspherics reduce local bearing pressure due to peripheral curve/ transition zone discontinuities Aspherics require fewer back curve steps to cover the range of fits required because: Fit of aspherics is less sensitive to changes in back curvature due to peripheral flattening Conformance produced by a blink deforms an aspheric soft lens less, hence greater change is required in the base curve to produce clinically significant alteration in lens fit Compared to a spherical lens, alterations in lens diameter result in smaller height changes. This is because of the peripheral flattening

70. Disadvantages of aspherics Lens shape may not be optimal optically – aberrations may be enhanced Adverse visual effects of a decentered lens are greater than with a spherical lens Hence, visual acuity may not be optimum More difficult to manufacture More expensive Not as readily available Perceived to be more complex If greater Decentration and movement exhibited then more diligence at after–care may be required

71. Indications for various soft contact lenses

72. A. Comfor t Because of low tensile modulus, high degree of elasticity and total diameter extending beyond limbus Hence ideal for patients with sensitive eyes Who have previously failed rigid lenses due to discomfort Photophobia and lacrimation often encountered with rigid lenses is generally absent with SCLs

73. B. Rapid adaptation Comfort of soft lenses makes these the lens of choice in case where slow adaptation with rigid lenses is difficult due to constraints of work or study While abnormal head posture and unnatural facial expressions may be seen with rigid lens wearers in the early stages of adaptation, soft lens wearers generally look quite natural from the beginning

74. C. Occasional wear Many patients only require social or part-time wear The ease of adaptation of soft lenses makes alternating between spectacles and contact lenses straight forward Patients with less than 1 D of ametropia are often poorly motivated to wear lenses and the comfort of soft lenses is preferable to adaptation required for rigid lenses D. Low refractive errors

75. E. Uniocular wear Patients who only require a lens in one eye for unilateral ametropia frequently find it easier to adapt to a soft lens than a rigid one This does not apply to unilateral lens wear for ocular pathology such as injury or Keratoconus, where optimum visual acuity would require that the eye be fitted with a rigid lens

76. F. Sports The excellent stability and initial comfort of soft lenses has made them the lens of first choice for many sporting activities, particularly contact sports. Well- fitting soft lenses are not easily ejected from the eye by rubbing or lid tension and are not normally dislodged onto the sclera

77. Contact lenses worn for sports must satisfy the following criteria. • They must not move when accidental foreign pressure is applied . • The optics must remain centered. • Corneal metabolism must be adequately maintained during the period required for the sport. Large soft lenses are manufactured specifically for sports purposes such as the Cantor & Nissel Sportlens, TD 14.90mm . This has an aspheric front surface and large optical zone with aberration - controlled optics designed to reduce flare under adverse lighting conditions

78. G. Occupational The size of a soft lens and the way in which it conforms to the shape of the eye means that there are few problems with dust and foreign bodies. For patients who work in windy or dusty atmospheres, soft lenses are generally the lens of choice. However, this is still controversial. E.g., sportsmen, pilots, actors, etc.

79. H. Flare with rigid lenses Patients with large pupils who wear rigid lenses may suffer from ‘flare’ in reduced illumination. The large diameter of soft lenses usually eliminates this

80. I. Persistent 3 and 9 o’clock staining Patients with poor quantity of tears, low/high- riding lenses, incomplete blinking, wide palpebral apertures or high negative prescriptions may predispose to ‘3 and 9o’clock’ staining with RGP lenses If all attempts to optimize the rigid lens fit and blinking have proved unsuccessful, then a soft lens may resolve the problem

81. J. Pediatric Wear The comfort associated with soft lenses make them ideal for pediatric use in cases of, for example, aphakia High myopia High hyperopia (accommodative esotropia) Anisometropia Nystagmus Photophobia (Aniridia, iris coloboma, oculo- albinism, Amblyopia (occlusive contact lens)

82. K. Therapeutic wear Soft lenses are used as bandage lenses in many pathological cases in order to relieve pain, promote stable epithelialization, maintain epithelial hydration and provide mechanical protection Occasionally used as drug delivery systems E.g., exposure keratitis, trichiasis ( mechanical effects ), bullous keratopathy( symptom relief ), keratoplasty, recurrent epithelial erosions, aqueous leak ( adjunct to healing ), etc.

83. L. Cosmetic and prosthetic lenses Colored lenses are generally soft lenses, whether for prosthetic purposes in cases of scarred or unsightly corneas, or to cosmetically enhance or change eye color Note: patients who wear colored contact lenses should be informed about the possible reduction in visual function, in dim illumination (such as driving). for safety reasons, pilots should not use cosmetic contact lenses when flying

84. M. Presbyopic contact lens option Single vision contact lens wear and reading glasses Monovision, enhanced monovision and modified mono vision Bifocal and multifocal contact lenses - Alternating vision bifocal contact lenses - Simultaneous vision contact lenses : Aspheric Concentric/annular Diffractive

85. N. Soft toric Lower cylindrical components and regular astigmatism (generally from >0.75 Dc to 3.00Dc) If the astigmatism is lenticular or partly non-corneal, it will be easier to correct with a toric SCL as opposed to an RGP toric or spherical RGP For some patients who are unable to adapt to RGP lenses because of discomfort, toric SCLs should be used.

86. O. Orthoptic use A contact lens may act as an occluder in cases of intractable diplopia . Complete occlusion can only be achieved by having an opaque iris and pupil but an opaque pupil in an otherwise clear lens may be sufficient P. Tinted lenses Soft lenses can be made any color, density or light absorption while RGP tints are limited in density

87. Q . Climbing and cold weather sports and activities Socks (1983) found no significant problems when lenses were worn for cold weather activities and contact lenses had the advantage over spectacles; they did not mist up, become brittle or break as spectacles may do when cold. Contact lenses protect the eye from wind- driven ice and snow and have been successfully worn upto 26,000 feet on mount Everest (Clarke 1975)

88. R . Contact lens- spectacles combination as low vision aid Extra magnification is provided in the form of Galilean telescope device, in which a minus contact lens forms the eyepiece and a conventional plus spectacle lens as the objective. High minus rigid contact lenses may produce physical and optical problems, including edge thickness and prismatic effects secondary to lens movement and position. A custom, high minus, soft contact lens by Flexlens has dramatically lessened these problems.

89. S. As an aid to defective color vision and specific learning difficulties (dyslexia) THE X-CHROMLENS The X-Chromlens is a red contact lens, with peak transmission of 595 nm and worn in one eye only. It attempts to overcome certain red- green color deficiencies by allowing a comparison of the different contrasts perceived by the two eyes. The eyes have a different perception of hues, altering their saturation or brightness, or imparting a luster that the wearer learns to relate to a particular color name. Clear soft lenses with a red-tinted pupil area are cosmetically More acceptable and comfortable than smaller rigid lenses and almost as effective in improving Colour perception (Wood& Wood 1991 ).

90. Chromagen lenses ChromaGen contact lenses are a range of soft lenses with precision – tinted pupils of varying hue and saturation which when used singly o r sometimes in combination, enhance color perception in color defectives ChromaGen system is also recommended in cases of specific learning difficulties to ease reading problems. However, a wide range of precisely chosen colors must be available.

91. The fitting set contains 25lenses and so- called diagnostic Haploscopic filters, in conjunction with an optional computerized test Tints used are violet, purple, orange, yellow, green, amber and magenta Three intensities of tint and three diameters are available ranging from 5 to 7 mm A spectacle lens filter is held in front of the non- dominant eye and the patient decides which color enhances a rainbow test screen the most. There may be two or three colors that enhance the color range seen and make certain colors fluoresce The appropriate soft contact lens with this optimum tint is inserted into the non-dominant eye

92. Disadvantages of soft contact lenses Variable vision as a result of blinking and changes in tear film Breakage and tearing Deposition more in SCLs Lens care expensive and time consuming. Soft lenses cannot be verified as easily as rigid lenses Chemical contamination more

93. References MANUAL OF CONTACT LENS PRESCRIBING AND FITTING- edited by Milton M. Hom IACLE Modules CET articles Soft Lens Fitting- Elseveir Contact lens manual 4 th edition Speedwell and Philips contact lens Contact lens optics and design 3 rd edition Thank you !!