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. Content
Introduction
Lens design
Lens materials
Verification
Standards and
ordering

3. Introduction
•  A thin glass shell bounded by concentric a
n
d
parallel spherical segments ( Fick )
•  A contact lens, or simply contact, is a thin
lensplaced directly on the surface of the eye.
• considered medical devices and can be worn t
o
correct vision, or for cosmetic or therapeutic
reasons

4. Lens design
• Design of contact lens is an important issue
–i
t
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
thanthe soft lens
• Design matters - Most with physiologically
poorermaterials and Least with better materials

5. Soft lensdesign

6. LENS 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

8. 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

9. Radial edgethickness
 Edge design
Material physical/mechanicalproperties
Material physiological properties
 Peripheral junctional thicknesses if transitions
exist

10. 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

11. 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

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

13. 1.Material properties
Material properties are very significant insoft
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.

14. 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

15. Minus lens series
• Lenses of lower minus power (<2.00D) are
made thicker and with a larger FOZD to improve
handling
•  F o r -3.00 to -6.00D,the lens series have
constant centre thickness

16. 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

17. 3.Water content and thickness
lens
Thin lens
Ultrathin lens
thickness
Below 0.10mm
Below 0.07mm
Below 0.05mm Superthin or hyperthin lens

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

20. TRANSMISIBILITY (Dk/t)
 D k ∝H2O content
 O 2 and CO2 transmissibilities ∝1/t
corneal respiration is best served by a thinhigh
water lens.
Higher the H2O content, higher Dk/t
Greater the thickness, lesser theDk/t
 t c for minus lenses overestimates Dk/t
 t c in plus lenses underestimates Dk/t

21.  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

22.  To achieve zero daytime edema
thickness are physiologically desirable they are
impractical
Extended
wear
Compromise
EW
0.009mm 0.023mm
H2O Daily wear
content
38% 0.033mm
75% 0.166mm 0.117mm

23. Pervaporation
 I f 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 thanless
water content due to temperature difference, pH and
tonicity

24. HIGH WATER CONTENT LENSES
 Lose more water than low water lenses (% o
f
total) on eye
 Lose water even when worn in a high humidity
environment
Experience on-eye lens shrinkage which affects
TD and BOZR.

25. 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.

26. 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.

27. 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.

28. 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

29. 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.

30. Design factors
Back surfacedesigns
Front surfacedesigns
Edge designs
Aspheric softlenses
Lens design -limitations

31. Back Surface Designs
 Single curve - simplest design but not commonly
used.
Bicurve - second curve often 0.8 - 1.0 mmflatter
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 corneabetter

32. BACK PERIPHERAL CURVES
Presence or absence of back peripheral curves i
s
insignificant physiologically
Changes in back peripheralcurves,especially
radical edge lift, affect lens movement
substantially

33. Front Surface Design
 i t tends to be ignored
important to lens fit ando
n
-
eye behaviour
also influence the comfortof
the lens - especially true in
cases of higher Rxs because of
their greater thicknesses

34. Bicurve - with a peripheral curve chosen to
produce a thin edge.
Multiple blended peripheral sphericalcurves.
Continuous aspheric front surface curves are n
o
t
commonly used.

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

36. Edge Design and Thickness
Edge is already under both lids & has relativelylittle
effect on comfort
Edge thickness is governed by durability
considerations rather than comfort or physiology
concerns.
 To o thick-discomfort
 To o thin- tearing of theedge

37. Aspheric Soft Lenses
 ‘aspheric’ means a conicoid
 A mathematically regular nonspherical surface
usually take the form of a parabola, ellipseo
r
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

38. 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.

39. 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

40. ASPHERIC DISADVANTAGES
• More expensive to manufacture
• Not as readily available
• Perceived to be more complex
• May decentre and move more than spherical
design

41. Manufacturing process may limit
• Method
• LathingAnhydrous
Molding-
• WeMolding & 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

42. Rigid gas permeable Lens Design
Design is the cornerstone of any contactlens
fitting.
Ultimate goal of rigid lens design is to achieve
ideal fit
Essential for optimizing response

43.  T h e 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.

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

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

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

47. Tricurve corneal lens
 Ø t - totaldiameter
 Ø 1 - first back peripheral
zone diameter;
 Ø 0 - back optic zone
diameter;
 ro- back optic zone radius
 r 1 - back peripheral radius
 r 2- second back peripheral
radius

48. 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

49. BACK SURFACE DESIGN
Controls Lens/CorneaInteraction
Affects both centration and movement
DESIGN FREEDOM
• Spherical or aspheric
• Single or multiple curves
• Fitting relationship

50. Back surface design – clinical
considerations

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

52. 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

53. Optimal Back Surface Design:
• Alignment or a very slight tendency towardsapical
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.

54. Back Surface Mid-Periphery
Should align flatteningcornea
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

55. Back surface periphery affects
Fluorescein pattern at the periphery of the lens
 e g . A flat and wide peripheral curve will result in
excessive edge clearance producing a bright band of
fluorescein
Tear exchangeis greater with a wide and flat
peripheral curve
Excessive edge clearance results in an unstablef
i
t
with excessive lens movement

57. Peripheral or edge curve
Radius - 2.50 mm flatter thanBOZR
Width - 0.30 to 0.50mm
Affects:
• Peripheral fluorescein appearance
• Centration
• Tear exchange
• Lens fit
• 3 & 9 staining

58. Edge width and tear reservoir

59. Edge configuration
Position of apex – centrally located apex was more
comfortable
Should not exhibit any high point
 T h e topography of lens just inside the lens edge a
k
a
blend of junctions, influences the edge profile,
thickness, junction angles..
Affects
Comfort
Durability
Tear meniscus

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

61. Rounded edge – mostcomfortable
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

62. Edge shape vs comfort

63. IDEAL FITTING
Centre -aligned
Mid-periphery - align/min. clearance
Pheripheral curve - 0.3-0.5 mmwide
 A E L - 75-100μmclearance

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

65. 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
 C t – more in higher dk lenses

66. 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

67. More stable and comfortable – center of gravity i
s
posteriorly located
 C a n be madestable by the diameter of the lens,
mass by lenticular design or adding minus
carrier lenses

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

69. influence comfort, movement and centration

70. Junction angle & thickness
Affects
• Comfort
• Lens movement
• Centration
• Lens bulk

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

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

73. Centre of Gravity

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