Introduction to soft contact lenses - various designs and their indications

1. Various soft contact
lenses – Designs
and their indications
Presenter : Urusha Maharjan
18th 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
r0= Back Optic Zone Radius (BOZR)
r1= Back Peripheral Radius, First (BPR1)
r2= Back Peripheral Radius, Second (BPR2)
ra0= Front Optic Zone Radius (FOZR)
ra1= Front Peripheral Radius, First (FPR1)
tc= Geometric Centre Thickness
tpj1= Peripheral Junction Thickness, First
tpj2= Peripheral Junction Thickness, Second
tER= Radial Edge Thickness
tEA= Axial edge Thickness

6. Terminology cont.
Ø0 = Back Optic Zone Diameter (BOZD)
Øa0 = Front Optic Zone Diameter (FOZD)
Ø1 = Back Peripheral Zone Diameter (BPZD)
ØT = Total Diameter (TD)
Miscellaneous (not shown):
Fv´ = Back Vertex Power (BVP)
Fv= Front Vertex Power (FVP)

8. Design factors
For the ease of understanding, soft lens design factors is divided
as:
A. Material properties
B. Centre thickness
C. Water content and thickness
D. Back surface designs
E. Front surface designs
F. Edge designs
G. Aspheric soft lenses
H. Other design considerations

9. 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.

10. 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:

11. 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.

12. 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.

13. Methacrylic acid
Methacrylic acid (MAA) is usedto 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.

14. Methyl methacrylate
Methyl methacrylate (MMA) is sometimes usedto
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.

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

16. 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.

17. 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.

19. 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

20. Group 1- Low Water (<50% H2O) 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

21. Group 2 - High Water (>50% H₂O) Nonionic Hydrogel
Polymers
Material % Water
Oxygen
Transmissib
ility (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

22. 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

23. 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

24. 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
O2Optix, 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 O2Optix Custom

25. 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

26. 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.

27. Few commonly used materials
1. 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

28. 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

29. 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

30. 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

31. 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

32. 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 Dk 15
19. Bandage lens
Water content 58% and Dk 21
Material filcon II

33. 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%

34. 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. Purecon natural eyes
Yearly disposable and colored
Water content 38% and Dk 9

35. 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%

36. 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

37. 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

38. 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.

39. 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

40. 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

41. 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.

42. Transmissibility (Dk/t)
The oxygen permeability is proportional to
the water content, i.e. Dk ∝H2O content.
The thicker a lens is, the lower are its O2& CO2
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

43. 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)

44. Water lost to the atmosphere from the anterior lens surface is
replaced with water from the back of 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

45. 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.

46. 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.

47. 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.

48. 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 O2 availability in the closed eye

49. 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.

50. EXTENDED WEAR
• Worn 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

51. 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.

52. 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.

53. 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.

54. 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.

55. 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.

56. 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/tlocal because of
the lack of tear mixing under a soft lens.
 Hence local thickness is the ONLY relevant lens dimension.

57. 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.

58. 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.

59. 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.

60. • 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.

61. • 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

62. 2. Fitting characteristics: FOZR
• The upper lid covers more of a soft lens than in RGP lens.
• This influences:
Lens resting position(static) :
oThe 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.
oTo a lesser extent the lower lid does the same.
oThe 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.

63. Movement induced by a blink
oIf 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.
oBy 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.

64. 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.

65. • 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

66. VI. Design factors
1. 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

67. 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

68. 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

69. 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

70. 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

72. 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

73. Advantages of aspheric:
1. Aspherics attempt to optimize the lens/ anterior eye
relationship
2. Aspherics reduce local bearing pressure due to peripheral
curve/ transition zone discontinuities
3. 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

74. 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

76. A. Comfort
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

77. 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

78. 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

79. 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

80. 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

81. 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

82. 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.

83. 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

84. 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

85. 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)

86. 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.

87. 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

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

89. 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.

90. 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

91. 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)

92. 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.

93. 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).

94. Chromagen lenses
• ChromaGen contact lenses are a range of soft lenses with
precision – tinted pupils of varying hue and saturation which
when used singly or 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.

95. 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

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