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. 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
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.
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
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 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.
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
For -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
20. 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
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
H2O
content
Daily wear Extended
wear
Compromise
EW
38% 0.033mm 0.009mm 0.023mm
75% 0.166mm 0.117mm
23. 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
24. 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.
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.
31. 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
32. 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
33. 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
34. 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.
35. Front surface may also include bifocal or
multifocal components such as:
Continuous aspheric surface
Concentric bifocal
Flat-top segment
36. 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
37. 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
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
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
42. 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
43. 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.
46. KEY DESIGN FEATURES
• Back surface design
• Back optic zone diameter
• Front surface design
• Lens thickness
• Edge configuration
• Lens diameter
47. 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
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/Cornea Interaction
Affects both centration and movement
DESIGN FREEDOM
• Spherical or aspheric
• Single or multiple curves
• Fitting relationship
51. Back optic zone radius
Aspheric Spheric
Better alignment
Difficult to manufacture
Difficult to verify
more decentration
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 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.
54. 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
55. 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
56.
57. 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
59. 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
61. 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
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
Ct – more in higher dk lenses
66. 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
67. 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
68. Lenticulation affects:
Centre thickness - In plus lenses only.
Lens mass - true for all lenses.
O2 transmission - true for all lens types
comfort
76. Introduction
Glass was used exclusively for some yrs
PMMA began to replace glass in 1940s –
toughness, optical properties and physiological
inactivity
77. 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
80. 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
81. 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’
82. Gas permeable lenses
Essentially rigid lenses
Material used are:
Cellulose acetate butyrate
Silicone acrylate
Fluoropolymers( teflon)
styrene
83. 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
84. 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
85. 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
86. 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
87. Perfluoroethersconsists of: Fluorine, Oxygen, Carbon and
Hydrogen
Dk 90+ (high)
Neutral surface charge
Greater flexibility ‘on eye’
Low refractive index
High specific gravity
89. 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%
91. 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)-
92. 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)
93. 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).
94. 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).
95. 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.
96.
97. 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
98. 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.
99. 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.
102. 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
103. 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
104. 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
106. 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
107. Lens must be centered
concave side down on
the focimeter stop -
BVP
with lens convex side
down - FVP
focimeter
109. Measuring magnifier
An adjustable eye piece through which an engraved
scale is viewed
Held with the concave surface towards the scale ( 20
mm long )
110. V gauge
Made of metal or plastic
V shaped channel cut into the material
Channel may vary in width from 6 to 12.50mm
112. 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
115. 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
116. 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
118. 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
120. 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
121.
122. 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)
123. 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 )
124. 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
126. 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
