CONTACT LENS I
OPT 414
LECTURE NOTES
LECTURER: DR. BAZUAYE K.N
08033742563
1
CONTACT LENS
INTRODUCTION
1. Development
INTRODUCTION
2. Definition
3. Determinants
1. Development
PEOPLE/
COUNTRY
Leonardo da
Vinci Italy
YEAR
ACHIEVEMENT
LENS MATERIAL
1508
First to develop the concept that
A water – filled lens
Eugene Fick
1887
First to design The first powered
contact lens
Feinbloem
1936
First to utilize
Rohm & Haas
company
Muller and
Obrig
Theodore Obrig
1936
GLASS that affected lid
movement which czapods
& dallas introduce
moulding tecknique to
improve sceral leses
Synthetic plastic for
contact lens
PMMA
Norman Bier
1943
Kelvin Tuohy
1948
Muller –Welt
(America)
William
Sohnges, Frank
Dickson, John
Neil
Hofsthelter &
Graham
1950
Introduced transparent
SCERA
methlmethacrylate material
Developed techniques for making PMMA
SCLERA
sclera lens with it
Introduced plastic
polymethylmethacrylate
SCLERA
(PMMA)
Minimum clearance Perforated
PMMA
SCLERA
scleral lens to reduce central
corneal clowding
Conceived the idea of PMMA
PMMA
Cornea
material for the cornea
Muller –welt plastic fluid less
PMMA
Sclera
lens
Using tuohy’s Idea, developed a PMMA
Cornea
microlens several diopters flatter
than the corneal apex. Diameter
9.50mm, thickness 0.20mm
Reported in a journal these development of Leonardo da Vinci and others.
1936
1940
1951
1953
2
PART OF THE
EYE WORN
Can be placed
directly on the
eye
SCLERA
SCLERA
Norman Bier
1955
Wesley and
jessen
Wichterle & lim
1956
1960
Contour lens of same diameter
PMMA
Cornea
with microlens but 0.50mm in
the central optical portion with
flatter peripheral curves
Deloped a sphericon lens with
PMMA
Cornea
diameter 8.50mm
Publish the first reference to development of soft hydrogel material that uses
water as vehicle to supply oxygen to the cornea
1961
1970
hydrogel material was introduced
A decade after soft contact lens was introduced , rigid contact lenses came back
and was launched as rigid gas permeable lens (RGP materials) that uses silicon as
vehicle to supply oxygen to the cornea
Bauch and lomb
(America)
1971
Griffin (Canada)
1971
Soflens :
hydroxymethylmethacrylate
(HEMA) chains cross linked with
ethylene glycol dimethacrylate
(EGDMA)
Naturalens lathe cut made
hydroxymethylmethacryla
te (HEMA)
HYDROGEL
Cornea
Cornea
The mechanical support of the scleral lenses was the sclera itself. They may be perforated to allow better
circulation of tears which may enhance lens tolerance.
Scleral lenses do occasionally allow a patient to wear a CL in cases of severe keratoconus, severe lid problems
and marked astigmatism
2. Definition of contact lens
Contact Lenses are
Medical devices which are made exclusively of plastics and molded to conform to the complex curves of the
cornea, corneoscleral junction (limbus) and sclera for the correction of ametropia (myopia, hyperopia and
astigmatism), cosmetic (aniridia, coloboma, corneal scars) actresses changing colour of their eyes.
3. Determinants of use of contact lenses
 Indications and contraindications for CL use
Indications
 Visual : Anisometropia, high myopia, aphakia, irregular corneae,scarring, keratoconus, grafts
 Occupational: Theatre, film and other stage performers Armed forces Professional sports
 Cosmetics
 To avoid spectacles
 Change eye colour
 Prosthetic lenses (eg, masking lenses to conceal corneal scar) or shells
 Medical: Therapeutic Bondage
 Psychological: Where the patient cannot accept wearing spectacles
3



Others
Sports
Physical inability to wear spectacles (allergy to frame materials, nasal problems)
Contraindications
 Visual: Low refractive errors (eg, +1.00 – 0.75; -0.25/-0.50) , Correction required only for near
vision, Acuity with lenses may be worse than with spectacles. Prism required horizontally or
3prism vertically
 Occupational: Where legal constraints abounds
 Cosmetics: Where spectacles are better with alarge angle squint , Where spectacles hide facial
disfigurement, Where a patient has previously been reconciled to a long standing scarred eye.
 Medical: Active infection or pathology, Recurrent corneal erosion, Severe sinus or catarrhal
problems ,Allergies, Vernal catarrhal, Diabetes (fragile epithelium) Anatomical (eg, mis-shapen
lids)
 Psychological: Cannot accept the idea of lens on the eye., Cannot tolerate any level of
discomfort, Unable to cope with insertion and removal, Total perfectionist
 Sensitivity: Cornea too sensitive, Lid or lid margins
 Dryness: Poor volume or quality of tears , Dry environment, Drug-induced (eg, antihistamine)
Word-induced (eg, VDUs)
 Advantages and disadvantages of CLs
 compared with spectacles
1Wide field of view
2 Better for refractive anisometropia
3.Retinal image size almost normal with refractice ametropia (eg, with aphakia, high minus)
4 No unwanted prismatic effects with eye movements
5 Less convergence required by hyperopes for near vision
6 Avoid surface reflections
7 Minimal oblique or other aberrations
8 Cosmetically superior
9 More practical for sports
10 Avoid weather problems (rain, snow, fogging up)
11 Therapeutic uses
12 Provide good acuity for irregular corneae (keratoconus, trauma and subsequent to refractive surgery)
13Vocational uses.
Disadvantages
1 Time required for fitting and adaptation
2 Handling skills required by patient
3 Hygiene procedures and lens disinfection necessary
4 Wearing time may be limited
5 For binocular problems, only vertical prism possible
6 Greater convergence required by myopes for near vision
7 Lenses can be lost or broken
8 Problems with foreign bodies
9 Peripheral flare (especially at night)
4
10 Deteriorate with use and age
11 retinal image size disparity in axial anisometropia
 Hard Contact Lenses Over Soft Contact Lens
Advantages Of Hard Contact Lenses Over Soft Contact LenS.
1Absolute control over the lens parameters
2 The lenses are easy to check
3 Limited modifications
4 Vision is superior – greater rigidity of the optical system
5 They are usually easier to care for.
6 Almost any desired shape can be incorporated into the hard CLs.
Disadvantages
1. Limited wearing tolerance- many will not adapt to the movement of images as the lens move upward and
downwards with each blink.
2. Shape of the eyelids and cornea. With tight lids, the hard lens may not stay as easily upon the eye and be
pushed off the central cornea. Certain corneal shapes do not allow adequate centration of the hard CL.
3. If there is any limbal or corneal irregularities, a soft CL is preferable.
 Soft Contact Lenses Over Hard Contact Lenses
Two basic types of plastics: The hydrophilic lens comprises the vast majority of soft CLs and the silicon rubber
(elastomere) accounts for the rest.
The plastic material used for manufacturing soft CLs is hydroxymethylmethacrylate (HEMA)
Advantages
1. They are generally more comfortable than their cousins, the hard CLs.
2. It is possible to fit many of the patients who cannot be fitted with hard CLs for reasons of sensitivity or
shape of lids or eyes.
Disadvantages
1. Vision may not be so crisp with soft as it is with hard CLs
2. Increased care is a must for continuation of successful wear
3. The lenses are less durable with a greater tendency to tear and deposits which almost never occur with
hard CLs area problem
4. Certain soft CLs are available only in one country. Foreign travel can be a problem.
5. Limited design and parameters are the other problems for commercially available soft CLs.
6. For the most complicated toric and bifocal CLs, exact reproduction is not a reality at present and
replacement may be quite difficult at times.
7. Although initial purchase of soft CLs may be similar in price to or even less than hard CLs they are
more expensive to maintain owing to the decreased lifespan of the lenses and the largest volume of CL
solutions used.
 USUAGE
 DAILY WEAR IF USED DAILY
 EXTENDED WEAR if worn all the time even when bathing
5
CONTACT LENS MATERIALS & THEIR PROPERTIES
The ideal contact lens material would:
Provide sufficient oxygen for normal metabolism
Be physiologically inert
Be very wettable on the eye
Resist lens spoilage especially deposit formation
Maintain stable dimensions
Be durable when handled by wearers
Be transparent with minimal light loss
Be optically regular so its optics are predictable
Have physical properties which allow the creation and retention of high quality surface
Require minimal maintenance by wearers
Be easy to fabricate lenses from.
Good wettability is necessary for long term lens tolerance
Scratch resistance is essential to the maintanace of good optical surface properties
Rigidity (rigid lenses) is a key determinant of the minimum lens thickness necessary to resist lens
warpage on the eye, particularly if the cornea is astigmatic.
Material must be stable if lens parameters are to remain as manufactured.
For comfort, good vision and minimal adverse responses, the lens must resist deposits.
GENERAL 1. Physical
PROPERTIES
2.Chemical
3. Optical
1. PHYSICAL PROPERTIES
 Oxygen permeability
One of the most important properties of a contact lens material is its permeability to oxygen (DK). This
property is an inherent material property (like specific gravity or refractive index). Oxygen permeability
is a material property and not a lens property It is not a function of lens thickness, shape or back vertex
power. The units of 10-11(cm2/s)(mlxmmHg) are often omitted for convenience. In this nomenclature, D
is diffusion coefficient- a measure of how fast dissolved molecules of O2 move within the material and
6
K is a constant representing the solubility coefficient or the number of O2 molecules dissolved in the
material
The DK value is a physical property of a CL material and describes its intrinsic ability to transport O2. It
is defined as the rate of oxygen flow under specific conditions through unit area of CL materialof unit
thickness when subjected to unit pressure differences. The DK varies with temperature. The higher the
temperature the greater the DK.
Oxygen transmissibility
Oxygen transmissibility is referred to as DK/t, with units of 10(cm2/s)(mlO2/mlxmmHg)/cm. Here t is
the lens thickness or material sample thickness and D & K are as defined above. In vivo,there are three
main indirect methods which infer the oxygen transmissibility
Corneal washing after overnight wear
Equivalent oxygen percentage or EOP
Corneal oxygen demand following lens removal
The DK/t for a particular lens under specified conditions defines the ability of the lens to allow O2 to
move from the anterior to posterior surface. The value of t is generally an average lens thickness for
powers between + 3.00D. DK/t is however not a physical property of a CL material, but is a specific
characteristic related to the sample thickness.
Equivalent oxygen percentage
The EOP refers to the level of O2 at the surface of the cornea under a CL. For the uncovered cornea
exposed to the atmosphere, the amount of O2 available is 20.9%. With the eye close the cornea receives
8%, whereas to avoid oedema the EOP should be 9.9% for DW lenses (DK/t=24) & 17.9% for EW
(DK/t=87). EOP therefore states that, the oxygen conc. of a gas mixture (the balance is nitrogen & H2O
vapour) which produces a corneal response is equivalent to that resulting from wearing the CL.
Low DK/t can result in corneal changes:
Epithelial microcysts
Thought to be spherules of disorganized cellular growth, necrotic tissue & cellular debris which
accumulate between epithelial cells and which probably contain metabolic by-products. As DK/t of
lenses increases, the incidence of microcysts decreases in both DW % EW
Polymegathism
An increase in the range of endothelial cell sizes believed to be a result of hypoxia.
Corneal pH
An acidic shift results from CO2 retention
Oedema
Oedema-induced reduction in the efficacy of the endothelial pump results in fluid retention and swelling
of the cornea.
Endothelial blebs
 Wettability
Sessile drop (water-in-air)
A drop of water is placed on the test surface. The angle between the tangent to the drops surface at the
point of contact and the horizontal test surface (thelal) is measured.
A zero angle signifies a completely wettable surface
A low angle signifies a somewhat wettable surface
A large angle (esp >90) signifies poorly wettable surface
When the bubble is expanded by adding more water, the advancing angle is determined. By withdrawing
some water, the drop decreases in size and the receding angle can be measured. Receding angles are
usually smaller (indicating better wetting), because the angle involves surfaces previously wetted.
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What do we require from CL material?
Biocompatibility
All lens materials must be biocompatible since they are in intimate contact with a physically and
physiologically sensitive organ for extended periods of time. In particular, the material should contain
virtually no leachable untreated chemical components which may affect the cornea and/or conjunctiva
Ease of manufacture
From a manufacturer’s point of view, ease of manufacture is essential. A CL material should:
Be homogenous
Have consistent mechanical properties
Be stress free and dimensionally stable
Be durable and resist heating
Be easy to polish/retain surface finish
Have predictable hydration characteristics
2. Chemical properties
Most contact lens materials are polymers or macro moleclecs. Polymers are long chain high molecular wts
made of many smaller repeating units called monomers, linked together by covalent bonds. The numbers of
monomers in a particular chain is known as the degree of polymerization which may be 100 or 10000 for most
classes of plastics. The molocular wts should be between 10,000 and 1million.
When the monomers are of one type the polymer is called homoploymer. If the polymer is of two or more type
it is called co- polymer. A homo or co- polymer can also be linear . eg linear homopolymer, liner random
copolymer or linear alternating or regular co-polymer and graft co-polymer. or branched
3. Optical Properties
Material must be transparent with little light transmission loss
Material must be optically homogenous i.e, its refractive index should not be subject to regional
variation unless such variation is intentional and well controlled.
1. Physical
SPECIFIC
PROPERTIES
2. Chemical
3. Optical
 RIGD CONTACT LENS Polymethylmethacrylate (PMMA)
The properties of PMMA lenses are easily shaped on lathe, non allergenic, stable, very transparent,
somewhat hydrophobic and DK=0
CH
l
H2C-C – CH
8
l
C=O
l
O
l
CH2
Methylmethacrylate
It is thermoplastic, which may be heat-set to take any desired shape. PMMA may also be manufactured to have
thermosetting properties, which means that there are increased cross-links, providing greater rigidity and less
susceptibility to becoming scratched.
 SOFT CONTACT LENSES
Flexibility and hydrophilicity mark the very important advantages of the widely used hydrogel soft contact
lenses. A large varietyof plastics is now employed in the many types of soft contact lenses now available.
The first soft contact lenses were composed of 2-hydroxyethylmethacrylate (HEMA)
CH
l
-CH3-C-CH3
l
COOCH3CH2OH
Methlacrylate 2-hydroxylethyl
When polymerized, it becomes polyhydroxylethylmethacrylate (pHEMA). With the addition of a small amount
of cross-linking. It is still the most commonly used soft lens plastic at present. Its water content can be increased
above the usual limit of 38% by the addition of other monomers.
Polyvinylpyrrolidone (PVP), Methacrylic acid (MA),methylmethacrylate (MMA), Diacetone acrylamide
(DAA), Glyceryl methacrylate (GMA), Polyvinyl alcohol (PVA).
pHEMA, PVP = vifilicon A High water content (WC)
pHEMA, DAA, MA = bufilicon A low/high WC
pHEMA, PVP, MA = Perfilicon A high WC
Forming a copolymer
The degree of cross-linking can have a profound effect upon the physical characteristic of the plastic. A
monomer commonly used for this, is ethylene glycol dimethacrylate (EGDMA). Other monomers may be added
to HEMA to
Form a copolymer. Poly vinyl pyrrolidone in combination with pHEMA forms a copolymer (vifilicon A) used
in the manufacture of higher water hydrogels.
Water content is one variable that can be easily controlled by the choice of copolymers. Gas permeability
increases with WC. Other factors in the overall performance of a SCL plastic are relative stiffness or softness,
fragility, and the attraction of the plastic for foreign chemical debris from the tear film.
The formation of copolymer(s)
-HEMA-Vinyl
-HEMA-Vinyl
Pyrrolidone
pyrrolidone
l
/
I
/
EGDMA
9
/
/
-vinyl
Pyrrolidone
(long chain)
-HEMA-Vinyl
pyrrolidone
Soft contact lens materials
Physical compatibility
Mechanical properties of the lens in combination with its fit must allow for lens movement
The lens material must be flexible enough, especially in thicker lenses, to allow the lens to conform somewhat
to the anterior eye’s topography. This ensures comfort and satisfactory physiological performance.
Oxygen permeability of soft CLs is influenced by:
Water content
The higher the water content, the greater the DK. This is believed to be the result of oxygen dissolving in H2O
especially if it is unbound (free)
Chemistry of polymer
Mechanical properties of the lens in combination with its fit must allow for lens movement.
The lens material must be flexible enough, especially in thicker lenses, to allow the lens to conform somewhat
to the anterior eye’s topography. This ensures comfort and satisfactory physiological performance.
Oxygen permeability of soft CLs is influenced by:
Water content
The higher the water content, the greater the DK. This is believed to be the result of oxygen dissolving in H2O,
especially if it is unbound (free)
Chemistry of polymer
The packing density of a material’s molecules influences the ease with which O2 may pass through the material.
If large, open, but rigid side molecules are present, the packing density is limited and the permeability is
enhanced.
Method(s) of water retention
The greater mobility of free water enhances DK
Temperature
Higher temperatures increase agitation of molecules, resulting in an increase in potential intermolecular space
and easier passage of oxygen through the material. DKs are often quoted at eye temperature (34oc)
pH
as the pH of the lens environment decreases (becomes more acidic) so too does the water content, and as it
increases(becomes more alkaline or basic) the water content increases.
Tonicity
The tonicity of the surrounding medium (tears or lens care pdts) can affect the water content. Hypertonic
solutions decrease the water content, hypotonic solutions increase it.
Water content Influences
Oxygen permeability
Refractive index
Rigidity (handling)
Durability
Minimum thickness to prevent pervaporation
Environmental susceptibility including spoilage. Higher water content lenses, especially if ionic, are more easily
spoiled and/or influenced by their environment.
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Lens care choice
Soft lens materials
DK@34oC
Water content
DK
<40%
5-8
40-55%
7-19
>55%
18-28
Low water content (advantages)
Less susceptible to environmental influences more stable parameter
More rigid, easier to handle
Virtually any lens care product can be used
Ease of manufacture
Greater reproducibility
More wettable
High water content (disadvantages)
Greater fragility
More deposit prone
More susceptible to environmental influences
Lower refractive index
More difficult to manufacture
Less stable parameters
Lower reproducibility
Cannot be made too thin
 Hard gas-permeable lenses
Corneal lenses represent the majority of lenses fitted today. In the past PMMA was the usual material. It is
easily shaped on a lathe, non-allergenic, stable, very transparent and somewhat hydrophobic and very slightly
gas permeable
A newer plastic – cellulose acetate butyrate (CAB) the first generation of gas permeable plastics were
introduced in 1970’s
Possibly more wettable than PMMA but were very unstable with strong tendency to warp. Not popular these
days.
Introduction of Silicon-PMMA copolymer plastics lenses that are very gas permeable, stable, non toxic and
easily shaped.
Disadvantage- increased brittleness and excessive flexibility.
The introduction of fluorosoilicon acrylate brought another phase of unremarkable success in RGP with fluorine
as the vehicle for oxygen transfer.
Generally, the base curve, diameter, power and type of plastic with its colour are given. Thickness is sometimes
a necessary part of the prescription.
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Gas-permeable lens materials are given the suffix-focon and classified according to the chemical groups shown
in the table 1 below
Chemical group classification of hard lens materials(By courtesy of Association of CL manufacturers)
Group 1a- Essentially pure polymethylmethacrylate (99%) DK=0
Group 1b-Copolymers of PMMA with not more than 10% max of other monomers that may alter hardness,
wettability and stability DK=0
Group 2a- Essentially pure cellulose acetate butyrate (90%) DK=2-8
Group 2b- Copolymers of mixtures of CAB and other monomers.
Group 3- Copolymers of one or more siloxanylmethacrylates, plus other water active monomers and crosslinking agents DK=6
Group 4- Hard lens materials formed from polysiloxanes
Group 5- Copolymers of one or more alkylmethacrylate and/or siloxanylmethacrylates, plus other water active
monomers, cross-linking agents, & at least 5% by wt of a fluoroalkylmethacrylate or other fluorine containing
monomers DK=20
Hard gas-permeable lenses are therefore available in a wide range of materials & DKs. Oxygen considerations,
however, must take into account:
The barrier effect, which reduces the DK on the eye to approximately 55% of that measured in air in the gas/gas
situation
Centre & average lens thickness
For physiological reasons, lenses should be as thin as possible,although in practical terms making them too thin
is counte-productive since they are very likely to distort throughout the power range & also become too brittle.
In most cases, a realistic minimum centre thickness is 0.14mm, even for high minus powers.
A. Cellulose acetate butyrate (CAB)
Cellulose acetate butyrate (CAB) was one of the first non-PMMA materials introduced in 1977. By modern
standards, its DK(b/w 4 & 8) is low, and it is now fitted infrequently. Itsmain difficulty when manufactured by
traditional lathing methods is dimensional instability due to lack of cross-links. However, when manufactured
by moulding, this problem was largely overcome, and lenses such as conflex and Persecon E have given very
good clinical results.
Advantages
Good wettability
Relatively inert
Does not attract protein
Low breakage rate
Very low incidence of CLIPC
Disadvantages
Low DK
Moulding necessary for dimensional stability
Scratches easily
Corneal adhesion in some cases.
Lens flexure and distortion on toric conrneae with tight lids
Silicon acrylates (siloxanes)
Silicon acrylates are coplopymers in varying propoertions of acrylate(PMMA), which provides lens rigidity &
silicon which controls the degree of oxygen permeability. Oxygen permeability results from the existence of the
siloxane bonds which permit oxygen to migrate freely along them. Also included are cross-linking agents to
improve the strength of the material and wetting agents to improve the strength of the material and wetting
agents such as methacrylic acid to improve the naturally hydrophobic properties of silicon. They give superior
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oxygen and physiological performance compared with CAB and most have stood the test of time in terms of
dimensional stability and optical and mechanical results. They are routinely tilted for daily wear and to a limited
degree have been used for extended wear.
Table 2 Silicon acrylates
Lens
DK@35oC
Polycon II
12
Boston II
15
Parapem O
15
Boston IV
26.7
DK 43
43
Parapem EW
56
Advantages of silicon acrylates
Wide range of materials available
Low to medium DKs available
Good dimensional stability
Good vision with limited lens flexure
Good scratch resistance
Disadvantages
Attract protein from tears
Some materials are brittle with a breakage problem
Some incidence of CLIPC
High incidence of 3 & 9oclock staining
Fluorosilicon acrylates
Fluorosilicon acrylates are composed of fluoromonomers and siloxy acrylate monomers. The addition of
fluorine atoms to replace some of the hydrogen present in methacrylate monomers improves surface wettability,
tear film stability and deposit resistance as well as increasing oxygen permeability. The solubility of oxygen in
fluoro-materials is enhanced & so higher DKs can be achieved (table 3)
Table 3 Fluorosilicon acrylates
Lens
DK@35oC
Fluoroperm 30 (Fluorocon 30)
30
Boston ES
31
Fluoperm 60 (Fluorocon 60)
65
Equalens
71
Fluoroperm 90
95
Quantum
92
Boston 7
73
Equalens 125
125
Aquila
143
Fluoroperm 151
151
Optacryl F
160
Quantum 2
210
Alternatively, moderate DKs can be achieved with a lower siloxy acrylate content and hence provide improved
wettability. The silicon content ranges from 5-7% with Boston 7 to 16-18% with Fluoroperm 90
Advantages
Very high DKs possible
Suitable for flexible extended wear
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Better wettability
Fewer deposit problems
Lower incidence of CLIPC
Easy to modify
Disadvantages
Brittle if too thin
Dimensional stability depends on material and manufacture
Corneal adhesion in some cases
Fluoropolymers
Fluoropolymers contain no silicon and have a fluorine content of up to 50% by weight, several times that
incorporated into fluorosilicon acrylates. They have so far found limited application despite their high Dk, good
surface wettability, good deposit resistance and lack of brittleness. These advantages have been outweighed by
problems associated with lens flexure, high specific gravity and cost.
COMPARATIVE SUMMARY OF PROPERTIES
1
Properties
Physical
-Arrangement: of
polymers,
-Porousity and
strength
PMMA
HEMA
- linear polymer of
MMA,
- high. Amorpous
and brittle
- H2O absorbtion of its
own wt when soaked
in water.
-Permeability to gases
- Refractive index n
- absorb 0.5 –
1.5%
- Contact angle
(wettability)
- Low 60 -75%
relatively
hydrophobic
- Good for
machining &
moulding .
selective tinting
- linear polymer of HEMA Similar to PMMA except
for cross linkages of with compounds.
- low and stronger but weaker with increasing H2O .
Allows HEMA to bind to H2O and further reduction
by cross linkages with EDEMA, PMMA, OR PMMA
+ glycerol or vinyl pyrolidinic acid.
- Generally, H2O content of HEMA varies from 5 –
90% But in CL practice we are concerned with H2O
content of 30 80%)
- High. DK Increases with water content
- Dehydrated state after manufacture N= 1.52
Hydrated state. N varies with H2O hydration in its
structure.1.43 - 1.53 or (30 -38% dehydrated)
Lower, highly hydrophilic
- Ease of manufacture
&dimentional stability
2
Chemical properties
- Chemical Reations
- Poor
- N =1.49
-
RGP
Both
Weaker with increasing hydration
Both
- Unaffected
by Cpds except Affected by
solvents like
ketones such as
acetones , ester,
aromatic
- Absorb
soluble substances like preservatives for CL soln, due
to its high water contents.
Reactants in ophthalmic drugs used for eyes which
can cause allergic and toxic rxns.
Deposits or unwanted surface coating. Greater
affinity tha PMMA.
14
- Biocompactibilty
- Temperature
3
Optical properties
hydrocarbons
- Non toxic and
inert
- Softens at 1250 C
Clear, excellent
and stable
- Non toxic and inert
- Softens at 1200 C as a thermo setting plastic
Inferior when compared to PMMA
OPTICAL PRINCIPLES
TO SPECTACLES
Both
OF CONTACT LENSES COMPARED
POWER OF
A LENS
1. SURFACE
2. EFFECTIVE
3. VERTEX
RIGID CONTACT LENS OPTICS
1. SURFACE POWER
This is the power of a curved surface of a lens in diopters or radius of curvature in mm.
The power of a spherical CL is the combination of the front and back surfaces. The total refractive power of
2 adjacent surfaces of differing indices of refraction is given by the formula
F = (n2 – n1)/r
F = power (in dioptres)
n1 – index of refraction for 1st medium
n2= index of refraction for 2nd medium
r= radius of curvature.
Power of a lens using surface powers, F = F1 + F2
For a thin ophthalmic lens, where r1 & r2 are large in comparism to the thickness , the formular,
F = F1 + F2 holds .
F1 = n – n1 / r1
F1
= 1.49
,
, F2 = n2 - 1 / r2 , n = 1.49 , n1 = n2 = 1, r1 = 7.5mm, r2 = 7.5mm
– 1 / 0.0075 = 65.332 , F2 = 1 – 1.49 / 0.0075 = -65.332
F = 65.332 + (-65.332) = 0 ( A plano lens to be fitted on the cornea)
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If r2 = 7.00mm, (ie steeper than r1) F2 = - 70.00D.
F = 65.33 + (-70.00) = -4.77D
Most times in contact lens practice, we talk of curvatures in terms of keractometric values.
The keratometer is calibrated for index of refraction as 1.3375 with the cornea in mind. When used to determine
the surface power or radius of curvature of a glass or plastic lens with a different index of refraction such as
PMMA material, we multiply the result with a ratio or conversion factor.
1.49 -1/1.3375 -1 = 1.452, for instance if the keratometer is used to determine the surface power of the plano
lens above reads 45.00 for a glass or plastic lens with n = 1.49, the true value will be 45.00 x 1.452 = 65.34D
(the two values are close as you can see about 0.01D)
NOTE: The keratometric estimated reading of the corneal curvature is 89.76% but the accuracy of the actual
measurement is 89.36%.
Surface power can also be obtained using the instrument
2. VERTEX OR NEUTRALIZING POWER
The power of a lens can also be determined using the reciprocal of the focal length. I / f. This method
has a limitation in that 2 lenses of the same power with different radii will have different focal lengths.
A better formular is the vertex or neutralizing power. This is the reciprocal of the focal length measured
from the vertex place on the surface of the lens
Contact lens have short radii of curvature and is regarded as a thick lens.
The total power when t is not equal to zero is less than when t =0.
That is for a thick lens F = F1 + F2 - ( t/n) F1. F2
If the thickness for the last lens parameters = 0.30mm, F = -3.75 instead of – 4.77 for thin lens formular.
Using the last parameter for the thin lens here
Front vertex power, Ffvp = F2/1 – ( t/n) F2 + F1
FFV = -3.26
Back vertex power Fbvp = F1/1 – ( t/n) F1 + F2
Fbvp = -3.80D
The back vertex power is more commonly used in the clinic and as such lenses are clearly specified in that
form.
3. EFFECTIVE POWER
If the power of the required spectacle lens is known, the power of a CL required to correct the same eye can
be determined by means of the effective power formula:
Fcl = Fsp/1 - dFsp
Where Fsp = the power of the spectacle lens, Fcl = the power of the contact lens, vertex distance (in metres).
Contact lens power begins to differ by as much as 0.25D from spectacle lens power when it is about
+4.50D.
Eg a) Fsp = -5.00 , vertex distance = 14mm, Fcl = -5.00/ 1- 0.014(-5.00) = -4.67D
b) Fsp = + 5.00 , vertex distance = 14mm, Fc = + 5.00/1 - 0.014(5.00) = 5.39D
16
THE TEAR LENS
When a contact lens is placed on the cornea, the lens will often alter the curvature of the air-tear interface
but is not expected to alter the refraction taking place at the tear-cornea interface. There is always a tear lens
between the contact lens and the cornea even if the base curve of the contact lens equals that of the cornea.
So thin that with a thickness less than 0,02mm without any consequential effect on the contact lens – eye
optical system.
Uair=1.000
u=1.490
ucornea=1.376
utear=1.336
FTL.
= F1 + F2
, =
n – n1 / r1
FTL = 1.3375-1/0.0078
+
n2 - 1 / r2 ,if r1 = 7.8mm, r2 = 8.0mm
+ 1- 1.3375/0.0080 = 1.00D
If r1 = 8.2mm, r2 = 8.0mm , FTL = 1.3375-1/0.0082
+ 1- 1.3375/0.0080 = - 1.00D
Rule of thumb
If a contact lens BC is specified as steeper or flatter than the corneal curvature (i.e. the tearlayer curvature),
the power of the lens must be specified 0.50D more minus for each 0.1mm of lens steepening, or 0.5D more
plus for each 0.1mm of lens flattening.
Spherical tear layer
If the tear layer is spherical, and if a contact lens is firm enough so it will retain its curvature while on the
eye, the curvature of the tear layer will (i) not change if the radius of curvature of the back surface is the
same as that of the tear layer. (ii) steepen if the radius of curvature of the back surface of the lens is steeper
than that of the tear layer and (iii) flatten if the radius of curvature of the back surface of the lens is flatter
than that of the tear layer. It is obvious that when the curvature of the tear layer is steepened,the eye is made
more myopic (SAM) and that when the curvature is flattened, the eye is made more hyperopic (FAP).
Fitting steeper or flatter than “K” and Accomodation and the contact lens wearer.
A practitioner may decide for one reason or the other to fit a contact lens with a base curve steeper of flatter
than the flattest meridian of the cornea. When this is done, the curvature of the tear layer is changed, a
steeper lens making the eye more myopic and a flatter lens making the eye more hyperopic.
FSP = FCL + FTL + FOR
For toric tear lens/toric cornea, a rigid CL with spherical base curve eliminates the astigmatism.
17
FSP = -2.00@90 / -1.00@180 , K : 45.00@90/ 44.00@180 , BC: 44.50@90/ 44.50@180,
FTL = -0.50@90/+0.50@180, Compensation = +0.50@90/-0.50@180
New FCL = -1.50@90/-1.50 @180
Contact lens doess not eliminate all the cornea astigmatism but only eliminates the astigmatism measured
by the keratometrer .this is because of the difference in index of refraction between the cornea and the tear
lens. That means it will only eliminate all spectacle astigmatism refered to the the cornea plane when both
have the same axix and magnitude , otherwise there will be residual astigmatism.
Residual astigmatism
SOFT LENS OPTICS
The soft contact lens s flexible and conforms to the cornea surface in situ. This torisity and cornea
astigmatism remains
Steeping or flatting the base curve does not change the the cornea curvature nor affect the refractive error
18
CONTACT LENS AND NEAR TEST or BINOCULAR VISION RELATIVE TO SPECTACLES
1. ACCOMMODATION
NEAR TESTS
2. PHORIA &
FUSIONAL VERGENCES
3. OTHER EFFECTS
eg Prismatic effect, Retinal image size effect,
and the Field of view effect.
1. ACCOMODATION AND THE CONTACT LENS WEARER
When a spectacle lens wearer switches to contact lens, the demand on accommodation and convergence are
altered in a number of ways. The total effects of these alterations may be negligible or great enough to
constitute a major problem. In any event, the practitioner should be aware of their effects, able to predict
their occurrence, and if necessary, to take measures to minimize them.
Accommodative effort
Pascal (1947) pointed out that the dioptre is an unsuitable unit to apply to accommodation for spectacle
wearers. This is because the spectacle-wearing hyperope must put more accommodative effort into each
dioptre of accommodation than an emmetrope. When contact lenses rather than glasses are fitted the
hyperope loses a disadvantage and then myope loses an advantage.
The accommodative effort required by an emmetrope, first while wearing glasses and then while wearing
contact lenses, can be determined by considering that the refraction of the eye takes place at the corneal
plane rather than at the spectacle plane. The myopic patient with CLs has to accommodate more strongly for
close vision than the myope wearing glasses. This difference is significant in fitting the myopic prepresbyope (age 35 or more) who desires CLs. Such a patient should be warned before the fitting that reading
glasses may be necessary to be used with the CLs for near vision. The change in accommodation is due to
the divergence difference at the cornea between the contact lens and the spectacle lens.
19
example
20
2. Accommodative and fusional convergence with CLs
The myope will use more accommodative effort with contact lenses than with glasses, and therefore will use
correspondingly more accommodative convergence. If the wearer happens to be exophoric at near, the
increased amount of accommodative convergence will result in a decreased exophoria and thus,will reduce
the need for positive fusional vergence (PFV)
Accommodative convergence and Magnification/minification in contact lens and spectacles.
However, if the wearer is esophoric at near, the resulting increase in the esophoria will require the use of
more negative fusional vergence (NFV) than is required with glasses, which may result in complaints of
eye strain. The increase in NFV may bring about adaptation problems, which the practitioner may blame on
the ‘fit’ of the lenses. The hyperope wearing contact lenses and using less accommodation will use less
accommodative convergence. The patient will be expected to be more exophoric or less esophoric at near
than while wearing spectacles.
3. Other effects:
(A) Prismatic effect at near with spectacle and contact lens
(B) Magnification/minification in cls and spectacles
Although it is useful to be aware of the possibilities of controlling image size in various combinations of
contact lenses spectacle. It is a known fact that certain amount of anisometropia can result in aniseikonia,
21
hence the knowledge of magnification to aid in the spectacle/contact lens combination in anisometropia is
dispensable. When you suspect that there are significant image size differences between the two eyes it may
be useful to resort to manipulating the spectacle and contact lens powers. Magnification of any lens system
in general is given by the formula:
M = 1/1-dp
Where:
M = magnification
d = distance of
correcting lens in meters
P = power of the
CONTACT LENS – EYE correcting lens
INTERACTION
(A) Field of view
CONTACT LENS – EYE INTERACTION
1. ANATOMY
2. PHYSIOLOGY
3. EXAMINATION
During CL wear, there is a good interaction between CLs (regardless of the type) and the anterior segment,
hence the anatomy and physiology are considered. The anterior segment is composed of the eyelids (and
margins), conjunctiva, sclera, cornea, precorneal tear film, limbus and iris. A knowledge of the anatomy and
physiology of the eye is required to detection of changes induced by disease of CLs wear this will also help to
facilitate the diagnosis of such changes.
1 & 2 ANATOMY AND PHYSIOLOGY
CORNEA
Corneal tissue is transparent and avascular, consisting of 3 layers and 2 membranes. It is protects the ocular
structure contributes about 70 % of the refractive power of the eye.
Dimensions of the conea
 Corneal shape is elliptical because the encroachment of the opaque limbus into the cornea’s superior and
inferior borders.
 Horizontal visible iris diameter (HVID) is 10.6mm on average. These may be about 0.1mm less in
females.
 Corneal area is 1.3cmsq or 1/14 of the total area of the globe
22

Sagittal depth of the cornea is 2.6mm with variations largely dependent on the radius of corneal
curvature (S= (d/2)2 D/2000(u-1)

23
0.52mm (520um) in the centre and 0.67mm (670um) at the limbus (Maurice, 1969).
 The corneal is not symmetrical and corneal curvature flatten towards the periphery .
Corneal shape:
The cornea is a meniscus lens
The average front apical radius is 7.8mm (corneal power = 43.27D with an instrument calibrated for a refractive
index of 1.3375)
24
The average back optical radius is 6.5mm gives the posterior cornea a power of -6.15D (assumptions naq = 1.336
and npost Cor=1.376. rpost cor=6.5mm)
The actual refractive index of the cornea (ncornea) is 1.376
The cornea is not optically homogenous. However, the actual refractive indices of the individual layer are not
known accurately.
Most values are for the homogenized corneal material.
Little light is reflected from internal layer interfaces, suggesting minimal refractive index differences. However,
Nground subst = 1.354 & ncollagen = 1.47
Corneal composition
The corneal is composed of
 78% water
 15% collagen
 5% other proteins
 1% glycosaminoglycans (GAGs)
 1% salts
These are wet weight figures
The epithelium accounts for approx 10% of cornea’s wet weight.
Transverse section of the cornea (Warurick, 1976)
The cornea consists of 5 layers.
 Epithelium
 Bowman’s membrane
 Stroma (substantia propria)
 Descemet’s membrane.
 Endothelium
Examination and assessment of the cornea is best done using the slit lamp biomicroscope. Observation of a
tansverse section of the cornea requires a narrow slit beam, together with high magnification.
(a) Epithelium
The outer surface is smooth and regular to provide a tear layer substrate.
It is uniform in thickness and benefits from an orderly replacement scheme after desquamation.
Any slight irregularity is compensated for by the overlying tear film.
Layers of the epithelium (Hogan et al, 1971)
The epithelium is made up of 3 types of cells
 Squamous cells
 Wing cells
 Columnar (or basal) cells (deepest)
The outer surface is smooth and regular, allowing it to act as a tear layer substrate. It is of uniform thickness
and benefits from an orderly replacement process following desquamation.
The epithelium is avascular and is usually transparent.
(b) Bowman’s membrane
It is an acellular tissue with a thickness of 8-14um. It is thicker in the peripheral 1/3 and thins as it round off
at the limbus.
It is usually differentiated or modified anterior stroma and is sometimes called the anterior limiting lamina.
It is composed mainly of collagen fibrils and some ground substances. The randomly dispersed collagen
fibrils are some 24-27nm thick and are therefore smaller than the stroma fibrils (32-36nm). They are also
less densely packed than the stroma fibrils. Damage to this membrane results in laying down of fibrous scar
tissue leading to permanent opacity.
25
(c) Stroma
The stroma is approximately 0.5mm thick centrally (90% of corneal thickness). It is composed almost entirely
of collagenous lamellae whose turnover time is 12 months or more. It contains 2-3% keratocytes (also known as
corneal fibroblasts) and about 1% ground substance (giycosaminoglycans). It is very hydrophilic. It is
responsible for
Exact sparing of fibrils
Water imbibitions pressure of the cornea. This is due largely to their hydrophilicity.
Keratocytes
Interspersed between collagenous lamellae
Thin, flat cells 10µm in diameter with long processes
5-50µm of intercellular space.
Joined together by macular accludens or hemidesmosomes
(d) Descemet’s membrane
10-12µm thick
Structureless
Slightly elastic
Secreted by the endothelium
Very regularly arranged stratified layer
(e) Endothelium
A single layer of cells in direct contact with the aqueous layer .18-20µm in diameter and 5µm thick. Its
pump mechanism maintains the cornea’s fluid balance, which is in turn responsible for transparency. No
mitosis occur, but enlargement and spreading of existing cells take place. Age-induced endothelial cell
degeneration/loss combined with an inability to produce lost cells, results in a decrease in uniform
(pleomorphism), thickness and cell density of the endothelium. These age-related changes manifest
themselves as an increase in the range of cell size. This is termed polymegattism. The details of the
endothelium can be appreciated by specular reflection with high magnification of 25-40x
Endothelial ultrastructure
Organelles are engaged in active transport (an active pump) which is necessary for hydration control and
protein synthesis for secretory purposes
There are large numbers of mitochondria which are more numerous around the nucleus.
Vasculature band nerve supply
Peripheral corneal vasculature
The peripheral cornea and sclera adjacent to schlemm’s canal are supplied by conjunctival, episcleral
and sclera terminals of the circumcorneal vessels.
These vessels play a minor role in corneal nutrition.
The rest of the cornea is normally avascular.
Corneal innervations
The cornea has one of the richest sensory nerve supplies in the body.
It is supplied by the ophthalmic division of the trigeminal nerve (N5)
Nerve
The nerve fibres become more visible when the cornea is oedematous.
Medullar changes of corneal nerves
These are about 30 nerves entering the cornea which appears as axon bundles near the limbus.
The cornea nerves lose their medullar coats (myelin sheath) before their first division, a division which
is usually dichotomous.
26
The sensitivity of the cornea is greatest centrally and in the horizontal meridian. It reduces towards the
vertical meridian and the periphery. Conjunctival sensitivity increases from a minimum at the limbus
towards a maximum at the fornix and lid margins. Sensitivity reduces with age and contact lens wear.
Conjunctiva
.The conjunctiva is a mucous membrane consisting of loose,vascular connective tissue. It is transparent.
It is continous :
The lining of the globe beyond the cornea
The upper and lower fornices
The innermost lower portion of the upper and lower eyelids
The skin at the lid margin.
The cornea epithelium at the limbus .
The nasal mucosa at the lacrimal puncta.
Regional Divisions of the Conjunctiva
Palpebral:
Conjunctiva lining the inner eyelids and extending to the margins.
Fornices:
Loose conjuntival tissue that is continous with the palpebral and bulbar conjuuctiva.
Bulbar:
Conjunctival tissue overlying the sclera and limbus.
Plica semilunaris:
Fold of conjunctival at the inner canthus, appearing as a biconcave crescent partly covered by the
caruncle.
Caruncle:
Conjunctival tissue nasal to the plica semilunaris and defining the medial canthus.
Conjunctival glands
Goblet cells are :
Unicellular and sero-mucous secreting
Found in the epithelial layer.
Absent near the limbus and lid margin.
More numerous in the bulbar region.
Glands of wolfring are:
Contributors to the aqueous layer of tear film
Similar in structure in to the lacrimal gland
Located near the fornices in the conjunctival stroma
Approximately 20 in the upper and the lower conjunctival
More numerous laterally.
Conjunctival Arteries
The conjunctiva is supplied by
Palpebral branches of the nasal lacrimal arteries of the lids
Larger branches from peripheral and marginal arteries arcades
Anterior ciliary arteries.
The limbus is:
A transition zone of the epithelia and connective tissues of the cornea,the conjunctiva and the sclera
combined.
An anatomical surgical procedures,biomicroscopy and contact lens fitting.
27
Limbal function
Provides limited nourishment to the peripheral cornea via the limbal vasculature.
Site of the aqueous humour drainage system via the schlemm’s canal. This system is important for the
maintenance of IOP.
Limbal vasculature
Begins with arteries from the superficial marginal plexus which originates from the anterior ciliary
arteries.
There are two types of limbal vessels:
Terminal arteries which form the peripheral cornea arcades.
Recurrent arteries which form the peripheral arcades which pass through the palisades of Vogt to supply
the prelimbal conjunctiva. The blood then returns via the venous system.
Limbal vasculature
Limbal nerves arise from the intrascleral and conjunctival nerves.
These originate from the long ciliary nerves.
Innervations extends into the trabecular meshwork and the cornea.
Note that:
Th deep cornea has few nerves
The cornea adjacent top descemet’s membrane has no nerves.
LACRIMAL GLAND
Each lacrimal gland is located in a depression called the lacrimal fossa(fossa glandulae lacrimalis) in the
supero-temporal orbit just behind the orbital margin. This lacrimal fossa is one of two in each orbit,the
other is a more obvious but smaller oval depression in the anterior nasal orbital wall in the lacrimal sac
is located.
The lacrimal gland is divided by the levator palpebral superioris muscle(LPS) into:
Larger upper orbital portion
Smaller lower palpebral portion
Lacrimal gland: innervations
Lacrimal gland innervations comes from the lacrimal branch of the ophthalmic division of the
trigerminal nerve(CN5)-afferent, facial (CN7),-efferent and the superior cervical ganglion and carotid
plexus(sympathetic). It is believed that the sphenopalatine ganglion provides parasympathetic
innervation to the lacrimal gland via very fine fibres(the rami lacrimalis) passing directly to the lacrimal
gland.
The lacrimal gland has up to 12 ducts
2-5 from the upper (orbital)portion
6-8 from the lower (palpebral) portion
The ducts open into the superior palpebral conjunctiva.
Accessory lacrimal glands
Glands of Krause
Supply the aqueous phase of basal tears
Glands of Wolfring
Supply the aqueous phase of basal tears
Glands of Zeis (lids)
Sebaceous glands
Associated with lash follicles
Partially supply the lipid layer of tears
28
Meibomian glands(lids)
Sebaceous glands
Main supply of the lipid layer of tears
25 in the upper lid and 20 in the lower lid. The glands in the lower lids are shorter
Oil on the lid margins prevents tear overflow
Goblet cells
Situated in the epithelium of the conjunctiva
Provide the mucoid layer of tears.
TEAR FILM
Tear Distribution
The tears are distributed by the following mechanisms
Normal and voluntary eyelid action with each blink resurfacing the pre-corneal tear film
Normal and voluntary movements of the globe
The tears from a lacrimal ‘river’ on the lid margin and a lacrimal ‘lake’ at the inner canthi
Tear flow
Tear flow is aided by:
Capillary action
Gravity
Blinking
Distribution of tear volumes
Precorneal 1um
Tear meniscus 3ul
Cul-de-sac 4ul
Tear film stability
The mucin layer is spread by the action of the lids. The wettability of the epithelium is enhanced by the
renewed mucin layer.
Tear thinning leaves an oil & mucin admixture which does not wet the epithelium leading to break-up of
the tear film.
Mechanics of tear film spreading
Upward lid movement draws the aqueous component over the cornea and conjunctiva.
This is followed by the lipid layer spreading over the surface. This increases tear film thickness and
stability.
Tear flow: lid closure
Movement of the medial canthus
Contraction of the orbicularis oculi muscle causes lid closure with a scissor like action towards the nose.
Lid closure propels the tears towards the medial canthus.
Lacrimal pump
The upper part of the lacrimal sac is distended by a negative pressure which draws tears into the
drainage system. On eye opening this action is reversed.
Capillary action and gravity play a part in tear drainage.
The turnover rate of tears is approximately 16% per mm
Tear flow directions
The compressive force of blinking on the tears combined with the scissorlike action of lid closure, gives
a directional flow towards the medial canthus.
Tear Drainage
29
Tears are drawn into the upper and lower canaliculi via the upper and lower lacrimal puncta located on
the lacrimal papillae.
The upper and lower canaliculi join and then almost immediately enter the lacrimal sac.
The tears pass through the naso lacrimal duct and finally into the nose.
Exit to the nasolacrimal duct is guarded by the valve of Hasner.
The valve of Hasner prevents reflux of nasal contents into the nasolacrimal duct or lacrimal sac.
The normal action of the lids can also activate the valve of Hasner. Lid closure closes the valve of
Hasner.
The Eyelids.
The eyelids are a 4 layered structure.
Cutaneous layer (the skin)
The skin of the eyelids is atypical being loose and elastic having very few hair and containing no
subcutaneous fat. It has 3 layers and is rich in blood vessels, lymphatics and nerves.
Muscular layer
This layer consists mainly of the oval, concentric orbicularis oculi muscle involving the lids and orbital
margins.
It also contains the smooth Muller’s muscle which helps the levator palpebrae superioris(LPS) keep the
eye open when awake.
Fibrous tissue layer
This layer is made up of tarsal plate, orbital septum and Meibomian glands.
The semi rigid tarsal plates are made of fibrous and elastic tissue arising from the medial and lateral
palpebral ligaments.
The orbital septum is a thin layer of elastic connective tissue. It is fused with the sheaths of the LPS.
The Meibomian glands are longer in the upper lids and are embedded in the tarsal plate. Each gland
opens into the lid margin.
Mucosal layer
This mucosal layer is largely is largely the palpebral conjunctiva.
Meibomian Gland Array.
20 (lower lid) and 25 (upper lid) vertical glands situated parallel to each other.
These glands are located in the tarsal plates. The orifice of each gland is located along the inner side of
the lid margins.
Meibomian glands are well developed sebaceous glands.
Blood vessels of the eyelids.
There are two blood sources of arterial bloodsupply to the eyelids
Facial system
Orbital system
The palpebral vascular structure provides oxygen to the cornea via palpebral conjunctival vessels when
the lids are closed.
Physiology
Cornea physiology is primarily concerned with
The sources of energy which fuel the cornea metabolic activity
Corneal transparency and its maintenance
Corneal permeability
Water
30
The endothelium has greater water permeability than the epithelium
The epithelium has low permeability to lactic acid which moves through the stroma to the aqueous
humour. This lactate induced osmotic gradient towards the posterior interface produces an influx of
water.
Oxygen
Oxygen is needed to maintain corneal integrity and derived mostly from the palpebral conjunctiva and
the limbal vasculature (especially in closed eye circumstances)
Carbon dioxide
The cornea is highly permeable to carbondioxide. The DKCO2 is about 7x DKO2. This is necessary to
resist pH and metabolic changes in the cornea.
Oxygen.
Oxygen is the most important metabolite
The cornea derives its oxygen supply from several sources
Atmospheric (main supply) via the tear film
Capillaries of the limbal region
Aqueous humour via the corneal endothelium
Capillaries of the palpebral conjunctiva
Carbon dioxide efflux
carbon dioxide from the cornea and aqueous humour pass out through the tears during open eye
conditions.
During closed eye conditions,carbon dioxide exita through the aqueous humour.
The amount of carbon dioxide that diffuses freely from thr cornea for every 5uL O2/cm2 cornea/hour
consumed is 2uL Co2/cm2 cornea/hour.
Contact lenses are barrier to oxygen and CO2 transmission.
Contact lenses act as a barrier to oxygen influx and carbondioxide efflux. Oxygen tension at various
levels of the cornea in open and closed eye circumstances, for central and superior corneal locations,
without a contact lens are:
Open eye, central cornea: 20.9%
Open eye, suoerior cornea: 10.4%
Closed eye, central cornea 7.7%
Closed eye, superior cornea 6.6%
With contact lens
Open eye, central cornea: 15%
Open eye, superior cornea: 10%
Closed eye, central cornea: 5%
Closed eye, superior cornea: 0%
METABOLISM- Corneal energy by carbohydrate metabolism.
Sources of glucose: corneal metabolism.
The aqueous humour contains 3.0umol/ml of glucose.
Epithelial glucose is derived from mainly (90%) from the aqueous humour of the anterior chamber.
The limbal vasculature and tears supply less than 10% of the needs of the epithelium.
Glucose consumption
38-90ug/hr of glucose is consumed
40-66% of this is used by the epithelium
Glucose metabolic pathway
Produces lactate (aerobic) + 2ATP
31
Tricarboxylic Acid cycle
Aerobic (along with epithelial cell mitochondria produces carbon dioxide, water and 36ATP
Hexose monophosphate shunt
Aerobic under the hexose monophosphate shunt produces NADPH, carbondioxide, water and ATP.
TEARS
Tear functions
Optical: form and maintain a smooth optical surface over the cornea.
Physiologic: maintain a moist environment for the epithelium of the cornea, conjunctiva and lids.
Bacteriocidal/Bacteriostatic: antibacterial props are impacted by the prescence of tear lysozyme,
lactoferrin, B-lysin and immunological cells. The leucocyte pathway is invoked in the case of injury.
Metabolic: transport of nutrients and metabolic pdts to and from the cornea via tears.
Protective: elutes and dilutes noxious stimuli, foreign bodies from the eyes anterior surface.
Mucin layer
Converts the hydrophobic corneal epithelium to a hydrophilic surface (extremely hydrophilic)
Greatly enhances epithelial wettability.
Maintain stability of tear film
Secreted by Goblet cells of conjunctiva
Aqueous layer
The only layer involved in true tear flow
Vehicle for most of tears component
Transfer medium for oxygen and carbon dioxide.
Produced by the lacrimal gland and accessory lacrimal glands of Wolfring and Krause.
Lipid layer
Main function is anti-evaporative
Prevents tear fluid overflow
Anchored at the orifices of Meibomian gland
Some produced by the Zeis glands
Contains some dissolved lipid and mucins
Tear proteins
98.2% water, n=1.336: some glucose (mainly fromaqueous humour. pO2= 155mmhg (closed eye)
Tear dimension
Volume 6.5-8uL. Flow rate: 0.6uL: Turn over rate 16%/min
32
4. EXAMINATION
 INSTRUMENTATION/TECHNIQUES
Slit-lamp biomicroscopy
keratometry
Burton lamp
Placido disc
Klein keratoscope
Slit-lamp biomicroscope
The biomicroscope derives its name from the fact that it enables the practitioner to observe under magnification
the living tissues of the eye. The biomicroscope consists of an illumination system, and the necessary
mechanical apparatus for their support and coordination.
Types
Haag-Streit and Zeiss biomicroscopes.
Methods of Illumination
Direct methods
The slit beam and microscope are focused at the same point to give;
Diffused illumination; Direct focal illumination
Diffuse illuminatiom illumination is used for a general look at the ocular tissues under low magnification.
In direct illumination, either a wide or narrow slit can be used. Under direct focal illumination, three types of
beam can be modified (narrow slit)
Parallelepiped (medium width)
Optic section (1mm)
Conical beam (wider slit with reduced height)
In direct parallelepiped, the beam and microscope are focused at point of interest.
Slit width: medium (2mm)
Mirror: click stop
Angle: 30-45o
Mag: Begin with low (20X)
Using the parallelepiped, the corneal stroma (which makes up 90% of corneal thickness) will appear to be
optically empty in the absence of any opacities. The normal corneal stroma contains sensory nerve fibres which
are fine and silky in appearance, slitting into branches at acute angles. By reducing the width of the
parallelepiped to a much narrower slit (say 1mm), optic section is formed.
Optic Section
This is used to examine the integrity of the cornea, define the depth of foreign body or scar, observe the
naturally occurring structures such as nerves of the cornea. When the width of the optic is increased to medium
and the height reduced to about half, a conical beam is formed.
Conical beam
Observe small number of cells and flares in the anterior chamber.
INDIRECT ILLUMINATION
33
The beam is focused on an opaque or translucent structure located to one side of the area to be observed by the
microscope and this area is observed somewhat in shadow by virtue of scattered light.
Sclerotic scatter (limbal)
If a slit of medium is directed towards the limbus from a wide angle, the scattering of light by the cornea at the
limbus will cause the appearance of a halo all the way around the cornea at the limbus. The light transverses the
cornea by total internal reflection. Useful in the detection of corneal abrasion, embedded foreign bodies,
oedema and other conditions that may occur in connection with contact lens wear.
Specular reflection
Using a slit of medium width, if the source and the microscope are placed at equal angles from the normal to the
corneal surface, the image of the source will be observed. This method is particularly useful in observing both
the corneal epithelium and endothelium. Also, examination of this bright area, looking for any debris or
oiliness, gives a quantitative assessment of the tear film. Changing focus to the rear of the beam section brings
an area of the endothelium into view. In summary, specular reflection is used to assess the integrity of the
corneal endothelium; mosaic palor is observed slightly to the right of the reflection; also detect central corneal
clouding (CCC) due to contact lens over wear.
Slit width: medium
Mirror: click stop
Angle: 45o
Mag: High
NB: The endothelium looks like a patch of beaten gold to the side of the much brighter specular zone.
RETRO-ILLUMINATION
This is also referred to as trans-illumination. Retro-illumination involves focusing the beam on a surface beyond
(or behind) the area to be observed. For example, the cornea can be observed in retro-illumination by
illuminating the iris and focusing the microscope on the cornea, observing the cornea in the light reflected back
from the iris. Used for assessing the density and boundary of corneal lesions.
Direct retro-illumination
To differentiate between lesion types spanning over transparent tissues.
Slit width: 2-3mm
Mirror: off clickstop
Angle: 45o
Mag: Low to Moderate
Indirect retro-illumination
Check the holes in the iris as light bounces off the fundus
Slit width: 1-2mm
Mirror: off clickstop
Angle: 45o
Mag: Low to moderate.
Both the cornea and conjunctival are then examined for evidence of epithelial staining using a broad scanning
beam and the cobalt blue filter. A yellow filter (Kodak Wratten 12) in the observation system gives enhanced
contrast and assists identification. Other stains used are rose Bengal and less commonly, alcian blue.
Handheld Burton lamp
This can also be used for broad examination of the lid, fornices, conjunctivae, cornea apart from its use in
fluorescein evaluation of hard contact lens fit.
KERATOMETERS AND AUTOKERATOMETERS.
Keratometers measure the curvature of the central cornea over an area of approximately 3-6mmto determine
The radii of curvature
34
The directions of the principal meridians
The degree of corneal astigmatism
The presence of any corneal distortion
The variable doubling keratometer is one of two types. The mires have a fixed separation. The separation of the
mires images is found by varying the doubling power. The Bausch and Lomb (B & L) keratometer has two
variable doubling devices and two sets of fixed mires. Both principal meridians can therefore be measured
simultaneously and it is called a one position instrument.
Eye piece focus: wind the eye piece fully counter clockwise (most plus direction). View the graticule through
the eye piece and slowly turn it back clockwise until the graticule cross-hairs are in sharp focus. Then stop.
Patient’s comfort: Adjust the instrument height and chin rest height so that your patient is comfortable with
head against the fore head rest. Check the patient’s outer canthus is more or less level with the eye-level mark
on the head rest.
Instrument alignment: Move the instrument vertically so that the height is level with the subject.
Patient’s fixation: Patient looks down the objective and may see
(a) A reflection of his own eye or (b) a small fixation light. You may have to adjust the instrument for him
to fixate properly. If no fixation target is visible, the patient should look down the objective, at the centre
of the aperture. Some patients prefer to have their other eye occluded.
Mire Alignment: Use the vertical and horizontal drums to adjust the mires centrally in your view through
the eye piece. Move the keratometer in and out of focus.
Axis location: rotate the body of the keratometer until the axis markers on the mires are correctly aligned. If
you make an incorrect adjustment at this stage you will be able to correct it later.
Doubling adjustment: adjust the doubling until the mires just touch.
Other meridian adjustment: with one piece instruments, adjust and align the 2nd meridian in exactly the
same way as you adjusted the first.
Record your results: you should now record all the information from the keratometer as follows:
FLAT K @ meridian
44.00D @ 180
STEEPER K @ meridian
45.00D @ 90
(Flat K – steep K) X Flat
-1.00DC X 180 AM 44.00D
This is lens power required to neutralize corneal astigmatism. This astigmatic power, however, may differ in
amount, axis and even sign from that which the patient finally accepts as the subjective refraction (i.e.
corneal astigmatism = refractive astigmatism)
NOTE:
Obtain readings quickly to minimize accommodation
Take three readings of each eye. Measured values should be within 0.50D and 5o of each other.
Internal astigmatism average -0.50D X 90 (usually crystalline lens) for the population.
Extending the range
Radii steeper than the range of the instrument (e.g. keratoconus) can be obtained by placing a +1.25D trial
lens (add 8D to the steeper radius) in front of the keratometer objective. At the flatter end, the range can be
similarly extended with a -1.00D lens (subtract 6D from the flatter radius). Prior calibration is necessary
using steel balls of known radii. In the calibration kit, we have the lenscometer, lens holder and steel balls of
different radii of curvature [40.50D, 42.50D & 44.75D]. calibration can be by direct adjustment of the
affected drum by loosening and adjusting for the appropriate value when steel ball is in place or by always
adjusting the affected drum with the correction factors.
Corneal topographers
Keratometer
35
The keratometer can also be used to explore the paracentral and peripheral areas of the cornea by means of a
graduated fixation.
Autokeratometer
Autokeratometers determine the radii and principal meridians along the visual axis. They can also measure
peripheral radii at predetermined positions away from the corneal apex (e.g. at 23o and 30o with the Nikon
NRK 8000)
Placido disc
The Placido disc is a flat, circular disc with alternate black and white concentric rings. The width when
reflected froman average cornea gives a quantitative assessment of the regularity of the cornea itself. The
eye is viewed through a convex lens in the centre of the disc.
Klein keratoscope
The Klein keratoscope is an internally illuminated version of the Placido disc. Corneal topography
TEAR FILM STABILITY
The time taken for the tear film to break up following blink cessation
Tear BUT
Sodium fluorescein instilled unto the eye
Tear film monitored under blue light
Record occurrence of first dry spot
Repeat measurements required due to:
Defects in anterior segment
Surfactants in paper strip
Abnormal eyes may not form a complete film
<10s is abnormal
15-45s is considered normal.
Non-invasive tear breakup time (NIBUT)
NIBUT is BUT test which does not require staining.
Results are more consistent and reliable than those using introduced fluids. Recently, Tear thinning time
(T-T-T) was introduced as a form of NIBUT since no fluid is introduced.
The patient is comfortably seated with chin rest, forhead on head rest of the Bausch and Lomb
keratometer.
With the instrument well adjusted, the mires are focused properly.
The patient is instructed to blink once and keep the eye open and with the mires well adjusted, the stop
watch starts counting while the doctor observes the focusing mire through the objective. The time taken
fpr the focusing mire to double with the watch stopped is the T-T-T (which is same as NIBUT)
Schirmer’s Test
Thin strip of filter paper is bent into an L shape and inserted into the lower fornix.
Wet length after a fixed time period (5mins) is measured.
Short wet length means a possible dry eye.
Test is subject to many artefacts
Cheap and readily available
Phenol red thread test (PRT)
The PRT test was introduced by Hamano et al in 1983. It is
Used to assess the basal tear volume
More comfortable than Schirmer’s test.
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 Preliminary considerations
It is important to discuss the various aspects of CLs at first examination and assess potential suitability in
relation to patient expectations, spectacle refraction, ‘k’ readings and slit lamp examination. The discussion,
reinforced by introductory patient leaflets, should cover many other related aspects of lens wear and fitting.
General health, including allergies, hay fever and systemic drugs
Ocular health, previous infections or surgery
Vision, nature of Rx, amblyopia
Previous contact lens history, success or failure.
Reasons for contact lens wear
Types of lens currently available
Preconceived ideas and misconceptions
Outline of fitting procedures
What is required of the patient in terms of after care examinations, hygiene and proper use of solutions
Fees for both initial fitting and future after care
External ocular measurements
Certain ocular measurements help us best determine the size and viewing areas of rigid lenses. Measuring
the size of the cornea helps us select the proper overall diameter (OAD) of a contact lens. Because the
cornea is a transparent tissue and the colored iris behind it is about the same size, we measure the size of the
iris that is visible to us with a millimeter rule. We call the measurement the visible iris diameter (VID) and
use it to represent the size of the cornea. The palpebral aperture helps us determine the proper lens size. The
size of the pupil under ambient and dim illumination is necessary so as to ensure that the optical zone
diameter ( where the usable optical power is located) is larger than the pupil in darl conditions. Otherwise,
glare and unclear vision may result in low light conditions.
Prescription
To determine the correct contact lens R, a spectacle refraction is necessary. This may be done with a
handheld instrument called a retinoscope, an automated refractor or a manual refractor. Using the
retinoscope yields an objective refraction because the contact lens practitioner may obtain a prescription
without the patients assistance. Automated refractors are objective as well. A subjective refraction occurs
when the practitioner uses the phoropter or trial lens set, asking the patient assist. Most practitioners use the
objective refraction as a starting point and refine the prescription with the subjective refraction.
This prescription is determined at the spectacle plane (where eye glasses would be as the face), but we need
the prescription at the corneal plane (where contact lens would be), so modifications may be necessary. The
difference in distance from the spectacle plane to the corneal plane is called the vertex distance and the
subsequent prescription change is the vertex distance compensation. Most practitioners use a conversion
chart for this computation.

TRIAL CONTACT LENS FITTING
After placing the contact lens on the eye further refinement may be necessary. Spectacle Rx to contact lens
Rx.
Depending on the Rx, some adjustments may be required to compensate for vertex distance. Vertex distance
takes into account the fact that the CL works ‘on’ the eye. Slight adjustments must be made to the Rx to
compensate for the change in the distance for spectacle powers over 4.00D. Remember Fcl = Fsp/1-vd Fsp.
There is always another exception. For patients fitted with RGP lenses, an adjustment to the Rx may be
neede to compensate for the lacrimal lens which is created. This is where the concept of ‘SAM’ and ‘FAP’
come in.
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The curvature
A contact lens must be curved to fit the curvature of the eye. A corneal CL is composed of curved surfaces
that are either spherical or non-spherical (aspheric). The back (concave) surface of a spherical CL can have
one or more curves. The central most radius of curvature is called the base curve (BC) & is specified in
millimeters. Most BC’s are spherical, which means they are shaped like the surface of a round ball.
However, if the patient has a high degree of astigmatism (or differences In the curvatures on the front
surface of the eye), a toric BC is required. If you placed the rounded- ball (spherical) BC on such a highly
toric cornea may require a toric shaped CL ( two different curvatures)
When fitting the back surface of the CL to the front surface of the eye. It is critical that the lens “ride” on the
layer of tears properly as the patient blinks . without a good fit , the lens will move unnecessarily about the
eye, not position its centrally, be comfortable to the patient and ineffective in correcting vision working the
curves.
Determining the Rx for an RGP lens takes many factors into account, including the lacrimal lens. We begin
with the curvature of the cornea Remember, we obtain this by using the keratometer. The keratometer
measures the major meridians of the corneal surface and is recorded in dioptres as K readings. These units
are converted to millimeters to specify the back curve of the CL that will fit that eye “k” is the flattest
meridian on the cornea and the starting point. The contact lens maybe fit.
On K --- The base curve matches that of the cornea sleeper than K --- the base curve is fitted steeper (more
curved) than the cornea.
Flatter than K --- The base curve is fitted flatter (less curved) than the cornea.
There are two considerations when selecting the fitting philosophy of the lens: (1) lens movement and
positioning, evaluated by observing how the lens positions and moves on the eye as well as observing the
pattern the tears make under the lens and (2) change in the refractive power of the lens, or adjustments made
to Rx in order to compensate for the tears or lacrimal lens.
Fitting on K—when an RGP CL is fitted on k, no lacrimal lens power is created by the lens fit, therefore no
compensation need to be made to the prescription
Fitting steeper than k--- A plus(+) lacrimal lens is created by the steeper than k fit, therefore minus (-) power
must be added to the contact lens Rx to compensate for the tear layer to obtain the appropriate lens
power --- SAM
Fitting flatter than k--- A minus (-) lacrimal lens is created by a flatter than k fit, therefore plus (+) power
(or less minus) must be added to the CL Rx t0 compensate for the tear layer to obtain the appropriate
lens power
Worked examples in class
BVP = Dx + TL + OR
Fluorescein patterns
Fitting
The central & peripheral fit are independent variables and should always be assessed separately
The fitting should be evaluated with the lens both centered on the apex of the cornea & in a decentered
position.
Lens movement during and after blink should be noted.
Lens position after a blink is important
Lens movement on eye excursions is also significant.
Assessment with fluorescein
Appearance (e.g. with modern multicurve designs)
The ideal fluorescein pattern should show three fitting areas
Alignment or the merest hint of apical clearance over the central 7.00mm
38
Mid-peripheral alignment over about 1.50mm
Edge clearance about 0.5mm wide
Flat fitting
The fitting patterngives a dense central area of dark blue touch surrounded by fluorescein to the edge of the
lens.
The area of touch is small with an indistinct as opposed to a sharply demarcated border.
Fluorescein encroaches beneath the periphery of the central portion where alignment would be expected
with a correct fit.
The lens is unstable and decentres
The entire periphery of the lens shows clearance as a wide area of fluorescein.
Blinking gives excessive and rapid lens movement which may be uncomfortable.
To correct a flat fit
Use a steeper BOZR to improve centration
Increase the TD to stabilize the lens
Increase the BOZD to give a larger sag and steepen the fit.
Use tighter peripheral curves to reduce dynamic forces and the effect of the lids.
Use a thinner lens to reduce mobility.
Steep fitting
Appearance
The fluorescein pattern gives central pooling
An air bubble is sometimes present with excessive central clearance
Heavy bearing is seen at the transition as an area of dark blue touch beyond the central pooling
The smaller the area of central pooling, the greater the degree of steepness.
The periphery gives only a thin annulus of fluorescein around the lens edge.
There is little lens movement on blinking.
To correct a steep fit
Use a flatter BOZR
Decrease the TD to increase lens mobility.
Decrease the BOZD to give a smaller sag
Use flatter peripheral curves to increase the dynamic forces on the lens
Rule of thumb for spherical lenses
An increase in the BOZD of 0.50mm requires the BOZR to be flattened by 0.50mm to maintain the same
fluorescence pattern
RIGID LENS SELECTION
Trial sets usually have steps of 10mm although Rx lenses can normally be ordered in 0.05mm steps
The preferred fitting for most corneal lenses is alignment or very slightly flatter
Trial lens selection is based on keratometry
The initial lens usually has a BOZR nearest to flattest ‘K’
The radius must be considered in relation to BOZD. The radius is usually flatter ( e.g.’K’ + 0.10mm) to
achieve an alignment fitting
Additional factors such as lid tension, BVP, and center of gravity must also be considered
(II) Total diameter (TD)—Overall diameter (OAD)
TD is chosen on the basis of corneal size to be approximately 2.0mm smaller the HVD.
39
TD depends on the pupil size, especially in low illumination
The choice can be regarded as small (<9.20mm), medium (9.20-9.70mm) or large (>9.80mm)
Changing to a larger diameter generally stabilizes the fitting, although it does not always have a significant
effect on the fluorescence pattern
The TD should be evaluated in relation to Back vertex power (BVP) High powers requires larger diameters
for lens stability
The final choice of diameter also depends on the method of fitting (e.g, whether lid attachment or
inerpalpebral)
(iii) Back optic zone diameter (BOZD)
Often predetermined by the laboratory
Depends on the pupil size, especially in low illumination
The pupil position must be assessed for aphakes
BOZD is chosen to be least 1.50mm larger than the pupil size
The choice may be regarded as small (<7.30mm), medium (7.30-7.90mm) or large (>7.90mm)
A larger BOZD for a particular radius gives a greater sag and therefore a steeper fitting
A smaller BOZD is often chosen with a toric cornea to reduce the area of mismatch
A larger BOZD is often chosen to permit a flatter BOZR which gives less flexure on a toric cornea
The BOZD should be considered in relation to BVP and lenticulation. High minus lenses frequently require
very large BOZDs to avoid flare
(iv) Peripheral curves
The first peripheral curves should be at least 0.70mm flatter than the BOZR and rather more for lenses at the
flatter end of the scale
A flat peripheral curve gives less corneal irritation but greater lid sensation
(v) Back vertex power (BVP) and over-refraction (OR)
Trial lenses should be as near as possible to anticipated BVP
If a lens is fitted steeper than ‘k’ a positive lens is created requiring more negative power in the overIf a lens is fitted flatter than ‘K’ a negative near lens created requiring more positive power in the over
refraction
Different BVPs are likely for the same degree of with and against the rule astigmatism
The final BVP should correlate with the spectacle Rx after taking into account the vertex distance
Worked examples in class
SOFT LENS DESIGN AND FITTING
The two main fitting philosophies into which soft lenses may be divided are therefore corneal and semisclera, although there is now considerable overlap, especially with single diameter lenses between these
two approaches. Emphasis will be placed on semi-sclera lenses are significantly larger with better
stability of both vision and fitting in order to provide good physiological response, they are now
manufactured from medium to high water content materials with high DK values
Indications
Most straight forward cases
Large cornea
Large palpebral apertures
Sensitive lid margin
Hyperopes and high powers, if high water content
Moderate degrees of astigmatism (0.75-1.25D)
Contradictions
Very small cornea
40
Small palpebral apertures and tight lids if handling is difficult
Cornea prone to oedema if low water content
Where cosmetic appearance is not important
Fitting radius
Radius selection is based on keratometry
Most radii are fitted between 8.30 and 9.20mm
High water content lenses are fitted between 0.30 and 1.00mm flatter than ‘K’
Low water content lenses are fitted flatter between 0.70 and 1.30mm flatter than ‘K’
Fitting steps are usually 0.30mm or 0.40mm
Most semi-sclera lenses are bicurve construction, with a relatively flat but narrow peripheral curve.
Total diameter
Lenses are fitted significantly larger than HVID, to give deliberate apical touch with further support beyond
the limbus where they overlap onto the sclera.
The TD isselected to be 2.00 – 3.00mm larger than the HVID
The majority of semi sclera lenses are fitted with total diameters of14.20-14.80mm, the possible range is
from 13.50-16.00mm
Power
Mainly because of flexure effects, the power of a correctly fitting lens often shows approximately 0.250.50D less minus than the spectacle Rx, after allowing for any vertex distance considerations. With more
rigid low water content lenses, this difference can be as great as 0.75D
Fitting appearance and lens movement
It is essential that a correctly fitting lens should be sufficiently large to span the limbus and not interfere
with the blood vessels in this region. Soft lens fittings are assessed in relation to lens movement with
Blinking
Upward and lateral gaze
The push up test
This simple test is used to assess the speed of recovery when a soft lens is displaced vertically upwards by
the practitioner with the eye in the primary position. A rapid recovery movement suggests a satisfactory
fitting, whereas a slow recovery may be indicative of tightness.
Rule of thumb
A change in the radius of 0.30mm= a change of diameter of 0.50mm
Examples:
8.10 :13.50 = 8.40:14.00
8.70: 14.50 = 9.00: 15.00
To improve a loose fitting
Select a steeper radius
Select a larger diameter
Use a more rigid or lower water content material
Use a different lens thickness
Worked examples in class
To improve a tight fitting
Select a flatter radius
Select a smaller total diameter
Use a less rigid or higher water content material
Use a different lens thickness
Worked examples in class
41
CARE SYSTEMS
Components of solutions
Buffers
Buffers (e.g., sodium phosphate, borate or tromethamine) are included where there is need to keep the pH
within narrow limits necessary for contact lens wear(pH 6-8). The antimicrobial buffer system (ABS)
patented by CIBA vision combines three borate buffers (boric acid, sodium borate andsodium perborate)
in bottled saline. These constituents yield 0.006% hydrogen peroxide which acts as an antimicrobial
agent.
Preservatives
Preservatives restrict the growth of microorganisms and maintain the sterility of the solution remaining in
the bottle and in the CL case. Typical examples are given below
Benzalkonium chloride (BAK)
Frequently used with PMMA lenses. It destabilizes the tear film in concs over 0.004%, so wetting solutions
or rewetting drops do not exceed this conc. The majority of the soaking and cleaning solutions can use
0.004-0.01% because they do not come in contact with the eye.
Chlorhexidine digluconate (CHX)
Usually included with other preservative systems in conc of 0.006% for hard lens solutions. If it is used
alone, the kill time is slow and lenses must be stored for a minimumof 10hrs.soft lens solutions employ a
reduced conc of 0.002-0.005%
Thiomersal
A mercurial derivative used in concs of 0.001-0.002% with both hard and soft lens solutions, more often
found with the latter
Thiomersal is effective against fungi, but toxic reactions are fairly common. It is slow acting as a
preservative andsois usually incorporated with chlorhexidine or EDTA (Ethylenediamine Tetra acetic
acid)
Dymed
Dymed is the proprietary name for polyhexanide (polyaminopropyl biguanide). It is one of the new
generation of high molecular weight preservatives used in multifunction soft lens solutions that come
under theumbrella title of polyquats. Dymed is found in concs of 0.00005-0.0001%. all polyquats bind
to negatively charged phospholipids found in the bacterial plasma membrane. Their action results in
cellular lysis rather than disrupting bacterial cell walls like antimicrobials (e.g., ReNu, ReNu Multiplus,
complete).
Polyquad
Polyquad (pollyquaternium) is a new generation polyquat used as a high molecular weight preservative. The
molecular weight is 14 times that of chlorhhexidine, nearly 4 times that of Dymed and comes in a conc
of 0.001% (e.g., Opti-free, Opti-1)
Phenylmercuric nitrate and chlorbutol
Phenylmercuric nitrate is found in some multipurpose hard lens solutions in a conc of 0.004%. Chlorbutol
binds to CAB and now uncommon due to its volatile nature. Used in concs of 0.4% (e.g., Liquifilm
tears)
Quaternary ammonia
Used in soft lens soaking solutions in concs of 0.013-0.03%
Sorbic acid
Used in surfactant cleaners with a concof 0.1% (e.g. Pliagel)
Tonicity (invariably sodium chloride)
To adjust the salt conc and ensure compatibility with the tears
42
Viscosity agents (e.g Hydroxy Ethyl cellulose)
To improve the wetting time and comfort of the solution
Wetting agents (eg. Polyvinyl alcohol, polysorbate 80)
To help the solution spread across the lens surface. Also found in soft lens disinfection solutions (eg.
Polyvinyl pyrrolidone)
Chelating agents [eg. Ethylenediamine tetra acetic acid (EDTA)] known as sodium edentate. Found in concs
0.01-0.2%. Use to enhance the action of preservatives, exceptions are mercurial derivatives.
Surfactants (e.g. Poloxamine. Miranol)
The most common surfactants used in multipurpose soft lens solutions for their cleaning action are
poloxamine and tyloxapol.
Surfactants and solutions for hard gas permeable lenses.
These are supplemented in recent products with non-ionic surfactants e.g. Lubricare (Quattro) and protein
removal agents, e.g. Hydranate (ReNu MultiPlus)
SOLUTIONS FOR HARD GAS PERMEABLE LENSES
The surface of a hard gas permeable lens is prone to deposits and interaction with some formulations
compared to more inert PMMA.
Wetting solutions.
Wetting solutions are used on insertion to act as a cushion between the lens and the cornea. They also
enhance the spread of tears across the lens surface, although the effect only lasts for a maximum of 15
minutes and sometimes for as little as 5 seconds.
Formulation: tonicity, viscosity agent, wetting agent, preservatives, chelating agent.
Soaking solutions:
Soaking solutions keep lenses hydrated during overnight storage in a sterile, bacteriocidal environment.
They facilitate good surface wetting and assist the removal of deposits. Hydration is important to
maintain correct BOZR both for modern hard lens materials and for PMMA over about -10.00D.
Formulation: tonicity agent, wetting agent, detergent, preservatives, chelating agent.
Chelating agent
Cleaning solutions remove surface debris including lipids and mucus (e.g. LC65) and enhance the
disinfecting action of the soaking solution. With the advent of fluorosilicon acrylates, greasing of the
lens surface has become a more frequent problem. Manufacturers have therefore developed
43