Contact lens designs-1.pptx

About contact lens materials and design

1. School of Medical and Allied Sciences Course Code : BOPT5001 Course Name: Contact lens Design Contact lens Design Name of the Faculty:Mr.Labishetty Sai Charan Program Name: B. Optometry

2. C on t e n t  Introduction  Lens design  Lens materials  Verification  Standards and  ordering

3. In t roduct i on  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. 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.  Soft lens design

6. LE NS P ARAM E TER S t E A t ER Ø Ø 1 Ø T Ø a 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. 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.  Radial edge thickness Edge design  Material physical/mechanical properties  Material physiological properties Peripheral junctional thicknesses if transitions exist

9. Soft contact lens design DIAMETER :All soft lenses are fitted 1-2mm larger than the horizontal visible iris diameter(HVID) THICKNESS: Along with central , mid peripheral and edge thickness the overall lens thickness profile is also important. Local thickness is the only relevant thickness when calculating local O 2 availability since there is little tear mixing under a soft lens

10. 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 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.  RELATIONSHIP WITH THE EYES: the parameter of a contact lens should closely match the dimensions of the ocular surface  eg- corneal topography HVID

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. 2.Center thickness consideration Dk/t consideration- cornea’s O2 requirements must be met Pervaporation prevention: a high water material with thin lens design, pervaporation corneal dessication may result 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. 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. 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. 3.Water content and thickness l e ns Thin lens Ultrathin lens thickness Below 0.10mm Below 0.07mm Below 0.05mm Superthin or hyperthin lens

17. Diagrams Representing the O2 Performance of Low/High Water and Thick/Thin Lenses

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.  To prevent corneal oedema Holden & Mertz(1984) derived a criteria of critical oxygen transmissibility and EOP values Equivalent oxygen p e r c e n t a ge Type of lens wear O2 t r a n sm i ssi b i li ty Dk/t 9.9% Daily wear 24 17.9% EW 87 12.1% C o m p r o mised lens wear 34.3

20.  To achieve zero daytime edema thickness are physiologically desirable they are impractical Ext e n d e d wear C om p ro m i s e EW 0.009 m m 0.023 m m H2O Daily wear content 38% 0.033mm 75% 0.166mm 0.117 m m

21. P e rvapora t ion  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. 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. 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. 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. Advantages of low water content l en s es 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. 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. 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. Design factors  Back surface designs  Front surface designs  Edge designs  Aspheric soft lenses  Lens design - limitations

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. 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. 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.  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. Front surface may also include bifocal or multifocal components such as:  Continuous aspheric surface Concentric bifocal Flat-top segment

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. 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. C on t d.  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. 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. ASPHERIC DISADVANTAGES More expensive to manufacture Not as readily available Perceived to be more complex May decentre and move more than spherical design

39. Manufacturing process may limit Method Lathing Anhydrous Molding- We Molding & Lathing Spin-casting & Lathing stabilized Spin-casting lens design: Limitations 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. 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.  The desirable properties of an RGP lens are : Optimal design Material : High Dk Wettability Deposit resistance Stability Ease of manufacture: manufacturing difficulties with a particular material can be a barrier to its usage.

42. DESIRED FITTING Moderate edge width and clearance Central and mid-peripheral alignment Smooth movement Centration

43. DESIRED PERFORMANCE Comfortable Clear vision Adequate wearing time Minimal ocular response Normal facial appearance

44. KEY DESIGN FEATURES Back surface design Back optic zone diameter Front surface design Lens thickness Edge configuration Lens diameter

45. Tricurve corneal lens  Ø t - total diameter  Ø1 - first back peripheral zone diameter;  Ø0 - back optic zone diameter;  r o - back optic zone radius  r 1 - back peripheral radius  r 2 - second back peripheral radius

46. Continous non spherical design  Singl e con t inuou s curv e - approxi m a t es cornea’s shape  Aspheric designs  Regular non spherical curves whose centers of curvature appear to be off the axis of symmetry

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. Back surface design – clinical considerations

49. Back optic zone radius Aspheric  Better alignment  Difficult to manufacture  Difficult to verify  more decentration Spheric  Better vision  Better centration

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. 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. 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. 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. 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. Edge width and tear reservoir

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. Edge shapes of lenses: (a) posterior; (b) central; (c) anterior; (d) blunt; (e) sharp

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. Edge shape vs comfort

60. IDEAL FITTING  Centre - aligned  Mid-periphery - align/min. clearance  Pheripheral curve - 0.3-0.5 mm wide  AEL - 75-100μm clearance

61. LENS THICKNESS Determined by: Rigidity Permeability Back vertex power CONSIDERATIONS  ‘On-eye’ lens flexure Correction of corneal astigmatism Dk/ t

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. Suggested minimum thicknesses for different materials (BVP-3.00D) Material tc (mm) te (mm) PMMA 0.10 0.12 CAB 0.16 0.12 Silicon acrylate 0.15 0.13 Fluorosilicon 0.14 0.15 acrylate

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. Lenticulation affects: Centre thickness - In plus lenses only. Lens mass - true for all lenses. O2 transmission - true for all lens types  comfort

66. influence comfort, movement and centration

67. Junction angle & thickness Affects Comfort Lens movement Centration Lens bulk

68. Lens diameter Determined by:  Corneal diameter HVID of patient  Inter-palpebral aperture Lens power (minus/plus)

69. Lens diameter Affects: Centre of gravity Stability Option to have larger BOZD/FOZD Comfort 3 & 9 staining

70. Centre of Gravity

71. OTHER DESIGN ISSUES  Tints Handling Aid to colour defectives  Lens Markings For ‘piggyback’ fits