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contact lens material

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contact lens

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contact lens material

  1. 1. RAJESHWORI NGAKHUSHI Bachelor of Optometry
  2. 2. Content Introduction Lens design Lens materials Verification Standards and ordering
  3. 3. Introduction A thin glass shell bounded by concentric and parallel spherical segments ( Fick ) A contact lens, or simply contact, is a thin lens placed directly on the surface of the eye. considered medical devices and can be worn to correct vision, or for cosmetic or therapeutic reasons
  4. 4. Lens design Design of contact lens is an important issue – it optimizes the ocular response for the individual and purpose is to achieve comfort, safety and vision Design of the RGP lens can be more complex than the soft lens Design matters - Most with physiologically poorer materials and Least with better materials
  5. 5. Soft lens design
  6. 6. 94N27-9S.PPT LENS PARAMETERSLENS PARAMETERS tEA tER Ø0 Ø1 ØT Øa0 Ø0 = Back Optic Zone Diameter (BOZD) Øa0 = Front Optic Zone Diameter (FOZD) Ø1 = Back Peripheral Zone Diameter (BPZD) ØT = Total Diameter (TD) tER = Radial Edge Thickness tEA = Axial edge Thickness
  7. 7. Soft lens design factors • Geometric centre thickness • Lens diameter (total diameter, TD) • Back optic zone radius (BOZR) • Back surface design • Front optic zone radius (FOZR) • Front surface design
  8. 8. Radial edge thickness  Edge design Material physical/mechanical properties Material physiological properties  Peripheral junctional thicknesses if transitions exist
  9. 9. Soft contact lens design 1. DIAMETER :All soft lenses are fitted 1-2mm larger than the horizontal visible iris diameter(HVID) 2. THICKNESS: 1. Along with central, mid peripheral and edge thickness the overall lens thickness profile is also important. 2. Local thickness is the only relevant thickness when calculating local O2 availability since there is little tear mixing under a soft lens
  10. 10. 3. CURVATURE: the back and front optic zone Radii are important to Rx determination other radii define the physical design of the lens which also affects lens behaviour .Corneal curvature -----flatter by 3- 5D 4. DESIGN: After defining centre thickness , front and back radii in the optical zone, the remainder of the lens design is defined by the radii of peripheral curves , their widths , their numbers and the junctional thickness. Design—high prescription------aspheric design, multi curve design
  11. 11. RELATIONSHIP WITH THE EYES: the parameter of a contact lens should closely match the dimensions of the ocular surface  eg- corneal topography HVID
  12. 12. 1.Material properties Material properties are very significant in soft lens design Material properties of a soft lens have a significant effect on fitting behaviour, comfort, durability, etc Water contents of 25 - 79% means material properties vary greatly Significance of material properties often leads lens designers to develop material-specific lens series.
  13. 13. 2.Center thickness consideration 1. Dk/t consideration- cornea’s O2 requirements must be met 2. Pervaporation prevention: a high water material with thin lens design, pervaporation corneal dessication may result 3. Fitting considerations: too thin lens - excessive flexing no dispersal of metabolic wastes due to conformity overall lens performance is not good. Lens wrinkiling causes ----corneal wrinkling and staining
  14. 14. Minus lens series Lenses of lower minus power (<2.00D) are made thicker and with a larger FOZD to improve handling For -3.00 to -6.00D,the lens series have constant centre thickness
  15. 15. Plus lens series Geometrical lens thickness cannot be decreased since it is a function of BVP. Reduction of FOZD is limited by vision issues – not be tolerated by most wearers except with small pupils
  16. 16. 3.Water content and thickness thickness lens Below 0.10mm Thin lens Below 0.07mm Ultrathin lens Below 0.05mm Superthin or hyperthin lens
  17. 17. Diagrams Representing the O2 Performance of Low/High Water and Thick/Thin Lenses
  18. 18. TRANSMISIBILITY (Dk/t) Dk H2O content∝ O2 and CO2 transmissibilities 1/t∝ corneal respiration is best served by a thin high water lens. Higher the H2O content, higher Dk/t Greater the thickness, lesser the Dk/t tc for minus lenses overestimates Dk/t tc in plus lenses underestimates Dk/t
  19. 19. To prevent corneal oedema Holden & Mertz(1984) derived a criteria of critical oxygen transmissibility and EOP values Equivalent oxygen percentage Type of lens wear O2 transmissibility Dk/t 9.9% Daily wear 24 17.9% EW 87 12.1% Compromised lens wear 34.3
  20. 20.  To achieve zero daytime edema thickness are physiologically desirable they are impractical H2O content Daily wear Extended wear Compromise EW 38% 0.033mm 0.009mm 0.023mm 75% 0.166mm 0.117mm
  21. 21. Pervaporation If the lens is too thin, corneal dehydration may result due to bulk flow of water through the lens and instability of water flow at the lens surface Produces epithelial desiccation staining - pervaporation staining High water content lenses loose more water than less water content due to temperature difference, pH and tonicity
  22. 22. HIGH WATER CONTENT LENSES  Lose more water than low water lenses (% of total) on eye  Lose water even when worn in a high humidity environment Experience on-eye lens shrinkage which affects TD and BOZR.
  23. 23. Advantages of high water content lenses • Better comfort because of material softness. • Faster adaptation. • Longer wearing time. • Extended wear. • Easier to handle because of greater thickness. • Better vision because of greater thickness. • Better for intermittent wear.
  24. 24. Disadvantages of high water content lenses • Shorter life span and Greater fragility. • More deposits, especially white spots. • More discolouration. • Reproducibility less reliable. • Greater variation with environment. • Fitting requires longer settling time. • Greater variability in vision. • More solutions problems. • Lens dehydration and Corneal desiccation.
  25. 25. Advantages of low water content lenses • Greater tensile strength. • Less breakage. • Longer life span. • Better reproducibility. • Easier to manufacture. • Can be made thinner. • Less dehydration on the eye. • Less discolouration with age. • Fewer solutions problems.
  26. 26. Disadvantages of low water content lenses • A greater tendency to cause corneal oedema. • A long-term tendency with thicker lenses (e.g. with high powers) to cause vascularization
  27. 27. 4.Other Design Considerations Centration Quality of vision, comfort and mechanical effects of a lens on the eye, depend to some extent on centration. Movement - minimal amount of movement is required for all soft contact lenses to remove debris under the lens.
  28. 28. Design factors Back surface designs Front surface designs Edge designs Aspheric soft lenses Lens design - limitations
  29. 29. Back Surface Designs  Single curve - simplest design but not commonly used. Bicurve - second curve often 0.8 - 1.0 mm flatter than BOZR and about 0.5 - 0.8 mm wide. Blended multiple spherical curves (multicurve) – fexible lenses don’t need a multicurve design Aspheric – shapes cornea better
  30. 30. BACK PERIPHERAL CURVES Presence or absence of back peripheral curves is insignificant physiologically Changes in back peripheral curves,especially radical edge lift, affect lens movement substantially
  31. 31. Front Surface Design it tends to be ignored important to lens fit and on- eye behaviour also influence the comfort of the lens - especially true in cases of higher Rxs because of their greater thicknesses
  32. 32. Bicurve - with a peripheral curve chosen to produce a thin edge. Multiple blended peripheral spherical curves. Continuous aspheric front surface curves are not commonly used.
  33. 33. Front surface may also include bifocal or multifocal components such as: Continuous aspheric surface  Concentric bifocal  Flat-top segment
  34. 34. Edge Design and Thickness Edge is already under both lids & has relatively little effect on comfort Edge thickness is governed by durability considerations rather than comfort or physiology concerns. Too thick- discomfort Too thin- tearing of the edge
  35. 35. Aspheric Soft Lenses  ‘aspheric’ means a conicoid  A mathematically regular nonspherical surface usually take the form of a parabola, ellipse or hyperbola and are defined by eccentricity. Circle, e =0 Ellipse, e = 0.5 Parabola, e = 1 As eccentricity increases , the rate of peripheral flattening or steepening increases exponentially
  36. 36. Contd. e - Defines mathematically the departure of an aspheric curve from a circle. Used to describe both a lens form and the curvature of the cornea. P value - Defines the rate of flattening with eccentricity:p = 1 — e2. closest mathematical approximation to the topography of the human cornea is an ellipse. Mean eccentricity = 0.45; p = 0.8.
  37. 37. ASPHERIC ADVANTAGES • Better lens/cornea-peri-limbal fitting relationship • Fewer base curve steps required • Lens fit less sensitive to lens diameter changes • Increased lens movement • Bearing pressure more uniform
  38. 38. ASPHERIC DISADVANTAGES • More expensive to manufacture • Not as readily available • Perceived to be more complex • May decentre and move more than spherical design
  39. 39. Manufacturing process may limit lens design: Method Limitations Lathing Molding-Anhydrous Molding-Wet stabilized Spin-casting Molding & Lathing Spin-casting & Lathing Simple designs only Few, but anisotropic expansion on hydration changes lens shape Almost none Only simple back surface design Possible Lathing limitations Lathing limitations
  40. 40. Rigid gas permeable Lens Design Design is the cornerstone of any contact lens fitting. Ultimate goal of rigid lens design is to achieve ideal fit Essential for optimizing response
  41. 41. The desirable properties of an RGP lens are : 1. Optimal design 2. Material : High Dk Wettability Deposit resistance Stability Ease of manufacture: manufacturing difficulties with a particular material can be a barrier to its usage.
  42. 42. DESIRED FITTING Moderate edge width and clearance Central and mid-peripheral alignment Smooth movement Centration
  43. 43. DESIRED PERFORMANCE • Comfortable • Clear vision • Adequate wearing time • Minimal ocular response • Normal facial appearance
  44. 44. KEY DESIGN FEATURES • Back surface design • Back optic zone diameter • Front surface design • Lens thickness • Edge configuration • Lens diameter
  45. 45. Tricurve corneal lens Øt - total diameter Ø1 - first back peripheral zone diameter; Ø0 - back optic zone diameter;  ro -back optic zone radius r1 - back peripheral radius r 2 - second back peripheral radius
  46. 46. Continous non spherical design Single continuous curve - approximates cornea’s shape Aspheric designs Regular non spherical curves whose centers of curvature appear to be off the axis of symmetry
  47. 47. BACK SURFACE DESIGN Controls Lens/Cornea Interaction Affects both centration and movement DESIGN FREEDOM • Spherical or aspheric • Single or multiple curves • Fitting relationship
  48. 48. Back surface design – clinical considerations
  49. 49. Back optic zone radius Aspheric Spheric Better alignment Difficult to manufacture Difficult to verify  more decentration Better vision Better centration
  50. 50. Optic zone should be larger than the pupil size and should cover it during the movement Also dependent upon the overall diameter and the peripheral curve and power
  51. 51. Optimal Back Surface Design: • Alignment or a very slight tendency towards apical clearance over the central 7 – 8 mm. • Mid-peripheral alignment about 1 – 2 mm wide. • Edge clearance about 0.5 mm wide. • An obvious tear meniscus at the lens edge.
  52. 52. Back Surface Mid-Periphery Should align flattening cornea secondary and peripheral zones must have curves which are flatter than the BOZR Affects: • Tear flow • Stability of the fit • Corneal mid-peripheral shape • Centration
  53. 53. Back surface periphery affects Fluorescein pattern at the periphery of the lens eg. A flat and wide peripheral curve will result in excessive edge clearance producing a bright band of fluorescein Tear exchange is greater with a wide and flat peripheral curve Excessive edge clearance results in an unstable fit with excessive lens movement
  54. 54. Peripheral or edge curve Radius - 2.50 mm flatter than BOZR Width - 0.30 to 0.50 mm Affects: • Peripheral fluorescein appearance • Centration • Tear exchange • Lens fit • 3 & 9 staining
  55. 55. Edge width and tear reservoir
  56. 56. Edge configuration Position of apex – centrally located apex was more comfortable Should not exhibit any high point The topography of lens just inside the lens edge aka blend of junctions, influences the edge profile, thickness, junction angles.. Affects Comfort Durability Tear meniscus
  57. 57. Edge shapes of lenses: (a) posterior; (b) central; (c) anterior; (d) blunt; (e) sharp
  58. 58. Rounded edge – most comfortable Edge profile rough or square at the anterior side – least comfortable Posterior design – square  Comfort is determined by interaction of lens edge with the lid
  59. 59. Edge shape vs comfort
  60. 60. IDEAL FITTING Centre - aligned Mid-periphery - align/min. clearance Pheripheral curve - 0.3-0.5 mm wide AEL - 75-100μm clearance
  61. 61. LENS THICKNESS Determined by: •Rigidity • Permeability • Back vertex power CONSIDERATIONS ‘On-eye’ lens flexure  Correction of corneal astigmatism  Dk/t
  62. 62. Center thickness Each lens material has a critical thickness – minimum ct which can be made of a particular lens material so that the lens does not flex on the eye Ct – more in higher dk lenses
  63. 63. Suggested minimum thicknesses for different materials (BVP-3.00D) Material tc (mm) te (mm) PMMA CAB Silicon acrylate Fluorosilicon acrylate 0.10 0.16 0.15 0.14 0.12 0.12 0.13 0.15
  64. 64. More stable and comfortable – center of gravity is posteriorly located Can be made stable by the diameter of the lens, mass by lenticular design or adding minus carrier lenses
  65. 65. Lenticulation affects:  Centre thickness - In plus lenses only.  Lens mass - true for all lenses.  O2 transmission - true for all lens types comfort
  66. 66. influence comfort, movement and centration
  67. 67. Junction angle & thickness Affects • Comfort • Lens movement • Centration • Lens bulk
  68. 68. Lens diameter Determined by: Corneal diameter  HVID of patient Inter-palpebral aperture  Lens power (minus/plus)
  69. 69. Lens diameter Affects: • Centre of gravity • Stability • Option to have larger BOZD/FOZD • Comfort • 3 & 9 staining
  70. 70. Centre of Gravity
  71. 71. OTHER DESIGN ISSUES Tints Handling Aid to colour defectives Lens Markings For ‘piggyback’ fits
  72. 72. Contact lens material
  73. 73. Introduction Glass was used exclusively for some yrs PMMA began to replace glass in 1940s – toughness, optical properties and physiological inactivity
  74. 74. IDEAL CONTACT LENS MATERIAL • Meets cornea’s oxygen requirements • Physiologically inert • Excellent in vivo wetting • Resists spoilation • Dimensionally stable • Durable • Optically transparent • Requires minimal patient care • Easily machineable
  75. 75. IMPORTANT MATERIAL PROPERTIES • Oxygen permeability • Wettability • Scratch resistance • Rigidity (RGPs) • Flexibility (SCLs) • Durability • Deposit resistance
  76. 76. OPTICAL PROPERTIES • Refractive index • Spectral transmission • Dispersion • Scatter
  77. 77. Rigid contact lens Material used is PMMA Stable materials Resists warpage , wets well and clean easily Lack of permeability to oxygen – tear exchange phenomenon Backbone of all rigid lens materials Trial lens
  78. 78. Properties:-  excellent biocompatibility  good optical properties  scratch resistance  good manufacturing properties Fairly wettable when clean.  Easy to care for.  Rigid.  0.2 - 0.5% water when hydrated fully.  Almost zero oxygen permeability. Produces ‘spectacle blur’
  79. 79. Gas permeable lenses Essentially rigid lenses Material used are: Cellulose acetate butyrate Silicone acrylate Fluoropolymers( teflon) styrene
  80. 80. Cellulose acetate Cellulose is combined with acetic and butyric acids ( 13% acetyl, 37% butyryl and 1-2%free hydroxyl groups) Low oxygen permeability dk range of 4-8 Lack of dimensional stability i.e. Warpage, scratching and coating No longer available
  81. 81. Silicone acrylate Silicone and oxygen are combined to make into siloxane Combined with PMMA to produce a gas permeable lens Most successful rigid gas permeable material introduced in 1970 Dk value range 12 to 60 are achievable
  82. 82. Negative charge Tended to become coated with proteinaceous materials from the tears Scratches easily may cause flexure problems if made thinner Pure silicone – o2 permeability is high but poor wettability Polycon II 14.2
  83. 83. Fluoro-Siloxane Acrylates (FSAs) Fluorine monomer added to SA material Lower surface charge  Withstand high heat and chemical attack O2 permeability is like silicone but more wettable Dks 40 to 100+ (med-high) Surface easily scratched  Greater lens flexure
  84. 84. Perfluoroethersconsists of: Fluorine, Oxygen, Carbon and Hydrogen Dk 90+ (high) Neutral surface charge Greater flexibility ‘on eye’ Low refractive index  High specific gravity
  85. 85. SOFT CONTACT LENS MATERIALS
  86. 86. PHEMA Incorporation of hydroxyl group into PMMA gives 2- hydroxy ethylmethacrylate and makes it more hydrophilic close relative of poly(methyl methacrylate) Water content is approximately 38%
  87. 87. Other variants to improve PHEMA are: PVP Poly Vinyl Pyrrolidone MA Methacrylic Acid MMA Methyl Meth Acrylate GMA Glyceryl Meth Acrylate DAA Di Acetone Acrylamide PVA Poly Vinyl Alcohol
  88. 88. Convenient to consider the polymers that have been used as contact lens materials under four heading: 1.Thermoplastics – capable of being shaped or moulded under heat or pressure Eg: PMMA Polyethylene and polyvinyl chloride Copolymer of tetrafluoroethylene Poly(4-methyl pent-1-ene) Cellulose acetate butyrate(CAB)-
  89. 89. Synthetic elastomers Not only fexible but show rubber like behaviour Intermediate characteristics b/w thermoplastic and hydrogel materials. Oxygen permeabilities 100x-1000x more than PMMA Hydrophobic – surface treatment Ethylene propylene terpolymer(EPT) Silicone rubber or poly(dimethyl siloxane)
  90. 90. Hybrid RGPs have a rigid GP central optical zone, surrounded by a peripheral fitting zone made of a soft contact lens material. second generation silicone hydrogel hybrid contact lens called Duette. The lens features a highly oxygen-permeable GP center (Dk 130), surrounded by a soft silicone hydrogel "skirt" for comfort (Dk 84; 32 percent water).
  91. 91. Hydrogels Called as soft, elastic, water containing gels Witcherle and coworkers– first developed hydrogels polymers (PHEMA) Made from HEMA, lightly cross linked with ethylene glycol dimethacrylate. (spin cast) Monomers commonly used in hydrogel contact lens materials include N-vinyl pyrrolidone (NVP), Methacrylic acid (MA) and Poly-2-hydroxyethyl me-thacrylate (polyHEMA).
  92. 92. Some Examples of Hydrogel materials, by Water Content Group 1 Low Water Content Nonionic Group 2 High Water Content Nonionic Group 3 Low Water Content Ionic Group 4 High Water Content Ionic Crofilcon Dimefilcon A Genfilcon A Hefilcon A & B Hioxifilcon B Iotrafilcon A Isofilcon Mafilcon Polymacon Tefilcon Tetrafilcon A generally show lower levels of protein deposit Alphafilcon A Altrafilcon Ofilcon A Omafilcon A Scafilcon A Surfilcon A Vasurfilcon A Xylofilcon A Heat and sorbic acid should be avoided for disinfection because of the risk of lens discolouration. Balafilcon A Bufilcon A Deltafilcon A Droxifilcon A Etafilcon A Ocufilcon A Phemfilcon A Bufilcon A Etafilcon A Focofilcon A Methafilcon A, B Ocufilcon B Ocufilcon C Ocufilcon D Ocufilcon E Perfilcon A Phemfilcon A Tetrafilcon B Vifilcon A show the highest level of protein deposition , heat and sorbic acid should be avoided for lens disinfection.
  93. 93. Verification Contact lens verification undergoes two stages, laboratory and clinical Laboratory During the final phase of manufacture, an overall parameter check is performed to ensure the lenses do not differ significantly from the parameters ordered by the practitioner. Clinics Verification of lenses upon receipt, rather than during the dispensing visit, is advisable
  94. 94. Why Verify Contact Lens Parameters? Ensure correct lens is dispensed Assess changes in contact lens with wear To ensure that proper over-refraction and trial fitting examination has been conducted To correlate with the manufacturer’s parameter to actual lens parameter Prior to initial dispensing of CLs, the clinician should verify that all parameters of the lenses are as ordered and that they meet established (e.g., ANSI) standards.
  95. 95.  Rigid and soft lenses have similar parameters which require verification by the practitioner.  Radii of curvature  Linear parameters  Edge profile  Power  Lens quality  Rigid and soft contact lenses should be hydrated in a soaking solution for 12 - 24 hours before verification procedures are conducted.
  96. 96. INSTRUMENTS  Radiuscope  Keratometer (modified)  Toposcope  Moiré fringe deflectometer  Radius checking device  Topographical mapping system  Electrical conductivity method  Microspherometer
  97. 97. Drysdale’s Principle
  98. 98. based on the theory that when a curved reflecting surface is positioned so that the real image created by the instrument is located at its centre of curvature an image will be formed in the same plane as the aerial object. real image/aerial object is formed at the first focal plane and an aerial image is formed at the second focal plane distance between the real image at the lens surface and the aerial image
  99. 99. keratometer used to measure the BOZR of a contact lens by using special attachments. Used with special contact lens holder which utilizes the front surface silvered mirror and a lens support
  100. 100. Toposcope Moire’ fringes were used - measuring radii and diameters of corneal lenses.  target consist of a series of straight lines  shape and orientation of the fringes formed were a function of the relationship between the two sets of lines  Straight parallel fringes indicate a spherical surface, curved fringes indicate an elliptical surface. Any warpage or dimples in the surface was indicated by irregularly shaped fringes
  101. 101. Measures spherical, toric and aspheric contact lenses Quality of lens surface can also be assessed
  102. 102. Power verification verified with a lensometer A smaller lens stop however, is recommended to reduce the amount of light passing through the lens focusing the light source more through the central area of the lens. May be recorded as being a greater positive or smaller negative value than it actual value True back vertex focal length is actually greater - Back surface of lens is not in the plane of the stop
  103. 103. Lens must be centered concave side down on the focimeter stop - BVP with lens convex side down - FVP focimeter
  104. 104. Verification procedures Diameters and linear parameters Measuring magnifier V gauze Cast, dividers and transparent rule Micrometer & spheres
  105. 105. Measuring magnifier An adjustable eye piece through which an engraved scale is viewed Held with the concave surface towards the scale ( 20 mm long )
  106. 106. V gauge Made of metal or plastic V shaped channel cut into the material Channel may vary in width from 6 to 12.50mm
  107. 107. Cast, dividers & transparent rule Micrometer & spheres:
  108. 108. Thickness verification Dial thickness gauze Centre or edge thickness may be determined with a suitable thickness gauze which usually incorporates a dial gauge calibrated to 0.01mm Centre thickness is measured at a common geometric and optical centre
  109. 109. LENS THICKNESS VERIFICATION:
  110. 110. Edge profile verification: Instruments/techniques: • Edge molding • Projection magnifier • Ehrmann profilometer • Palm test • Radiuscope (modified)
  111. 111. LENS AND SURFACE QUALITY ASSESSMENT: for RGP AND SCL Instruments: Magnifying 10x loupe Projection magnifier  Contact lens optical quality analyzer (CLOQA)  Dark field microscope  Moire fringe deflectometer
  112. 112. Differences between verification soft and RGP lenses Hydrogel contact lenses are flexible If exposed to atmosphere, they dehydrate and alter their contour. Verification in air is inaccurate due to- Shrinkage of Hydrogel on dehydration Accumulation of surface moisture So, artifact liquid cells are used to measure parameters of soft lenses But RGP lenses can be measured in air
  113. 113. Contact lens standards
  114. 114. American National Standard Institute (A.N.S.I.) 1999 Contact Lens Tolerances Power Tolerance O to 5.00D +/- 0.12D SPHERE POWER 5.12 to 10.00D +/- 0.18D 10.12 to 15.00D +/- 0.25D 15.12 to 20.00D +/- 0.50D
  115. 115. POWER TOLERANCE CYLINDER POWER 0 to 2.00D +/- 0.25 2.12 to 4.00D +/- 0.37 Over 4.00D +/- 0.50
  116. 116. Power Tolerance Cylinder axis 0.50 to 1.50D +/- 8° Above 1.50D +/- 5° Parameter Tolerance Bifocal refractive Add power +/-0.25D Seg height +/-0.10mm For toric lens cylindrical axis is specified in relation to the base apex meridian
  117. 117. The terminology for a standard tricurve lens in ISO 8320-1986 symbols is: Example:7.90:7.80/8.70:8.60/10.75:9.20 tc 0.15 BVP -3.00D Tint light blue 7.90 = back optic zone radius (BOZR) r0 7.80 = back optic zone diameter (BOZD) 00 8.70 = first back peripheral radius r2 8.60 = first back peripheral zone diameter 02 10.75 = second back peripheral radius r2 9.20 = total diameter 0T 0.15 = geometric centre thickness tc -3.00 = back vertex power (BVP)
  118. 118. The ISO 11539 standard for the classification of contact lenses, describes the use of a six part code to describe a material type Prefix – stem – series suffix – group suffix – Dk range – modification code Prefix and series suffix – administrated by United States Adopted Names (USAN) council and such are only relevant for materials with FDA approval. Two types of stem – filcon stem - materials which contain >10% water by mass (hydrogels )
  119. 119. Focon stem – materials which contain <10% water by mass ( non hydrogel ) Group suffix Filcon ( hydrogel) Focon ( non hydrogel) I II III IV Low water content non ionic <50% EWC <1% ionic monomer High water content, non-ionic >50%EWC, <1%ionic monomer Low water content, ionic <50%EWC, >1% ionic monomer Low water content, non ionic>50%,>1% ionic monomer No silicon + no fluorine CAB Silicone + no fluorine Silicon acrylate Silicon + fluorine Flurosilicon acrylate No silicon + flurine Flurocarbon
  120. 120. Contact lens ordering
  121. 121. Soft contact lens In addition to the trade name of the lens being ordered An order for soft contact lens should include the following parameters: Base curve radius Overall diameter And power
  122. 122. Thank you!!!

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