Soft contact lens
History of contact lenses (CL)
- In 1508, Leonardo da Vinci sketched the first
forms of new refracted surface on the cornea.
- He used the example of a very large glass
bowel filled with water; immersion of the
eyes in water theoretically corrected vision
Figure1. A hollow glass semi-spheroid filled
- In 1636, Descartes suggested applying a tube
full of water directly to the eye to correct a
Figure2. Optical apparatus described by Descartes
In 1887, Adolf Fick was
apparently the first to
successfully fit contact
lenses, which were made
from brown glass
- In 1887, Dr. Fick a physician in Zurich. He
described the first contact lens with
refractive power known to have been worn.
- The contact lens was made by A.Muller a
manufacturer of artificial eyes.
- Early contact lenses were crude disk of ground
or blown glass and made spherical although
the cornea is not.
- Theses lenses because the developer had
little knowledge of the metabolic need and
physiology of the cornea.
Did not conform to the shape of the cornea.
Causes corneal abrasions.
Wearing time short and frequently painful.
- This led to the development of a larger lenses
that rested on the sclera (1888 to 1938) and
provided a clearance between the lens and the
- However, the edge of the first plastic lenses
formed a seal with the sclera, trapping a pool
of bathing medium under the lens and
precluding vital metabolic exchange and poor
The proposed therapeutic uses for contact lenses
is to protect and reshape the optical properties of
irregular cornea in disease such as keratoconuse
- It become clear that corneal shape is critical
importance in CL design, and the cornea obtains
the bulk of its oxygen supply from the air and
that the medium of exchange is the tears.
- As result scleral lenses were abandoned and
hard contact lenses were redesigned.
- The newer lenses were smaller, thinner and
more flexible. They ride on the surface of the
tear film, and each blink of the lids provides a
flow of oxygenated tear that supplies the
cornea the oxygen.
- Even these early suggestions the history of CL
did not begin until the 19th century.
- In 1964 Dr. Wichterle in Czechoslovakia
introduced soft hydrophilic plastic contact
- These lenses had the advantage of comfort and
permeability by water and oxygen.
- The main potential in these lens lay in the
ability of mass production methods that would
bring down the manufacturing costs.
- In 1966 Baush & Lomb introduced the spin-
cast soft CL on an experimental basis
- In 1967, in the USA the first lathe-cut lens was
Contact lens materials
- Contact lenses can be classified according
to their material as hard or soft contact
lenses. Or according to hydrophobic
(non-loving water) materials or
hydrophilic (loving water) materials.
In general these materials should be
Hard contact lenses (HCL)
- It is organic plastic compounds stable at room
temperature and water content 0.5%.
- It is easily to work with and has excellent
optical clearly, durability, stability, lack of
toxicity, resistance to deposit formation.
-The refractive index 1.48-1.50.
- Disadvantage lack of oxygen permeability.
- It is used to produce hard contact lens.
Cellulose acetate butyrate (CAB)
- The first rigid gas permeable lenses (RGP)
were made from CAB.
- CAB has slightly better oxygen permeability
- It is strong, durable.
- Disadvantage prone to warpage.
- It is a polymer of dimethyl-silicone, permeable
to oxygen and glucose.
- It refractive index 1.43.
- There are two types of Silicone contact lenses:-
a- Silicone rubber lenses their stiffness and
rigidity are intermediate between typical rigid
gas permeable RGP and they hydrogels.
b- Silicone resin lenses are in hard state the
stiffness to typical rigid gas permeable
Soft contact lenses (SCL)
- Soft or hydrophilic contact lenses are
characterized by the ability to absorb water,
elasticity and flexibility.
- HEMA is hydrophilic because it contains a
free Hydroxyl group that bounds with water.
- The water content range from 38% to 60%.
- Its refractive index 1.43.
- Example Bausch&Lomb (Soflens)
The passage of oxygen molecules and certain
other ions and molecules through a contact
lens is very important in maintaining normal
- The passage of oxygen is one of the most
important aspects of a contact lens material,
and much attention is directed to this topic by
contact lens practitioners and researchers.
The cornea obtain most of oxygen from the tear
film. The tear film supply the cornea with
oxygen from the atmosphere when the eyes are
-Thecornea has no blood vessels, the oxygen
supply necessary for normal metabolism.
During sleep, the eyelids block oxygen from
the atmosphere, and most of the oxygen in the
tears diffuses from the blood vessels of the
limbus and the palpebral conjunctiva. This
reduces the amount of oxygen in the tear film
to approximately one third.
Palpebral conjunctival blood vessels
- All contact lenses act as a barrier between the
cornea and its oxygen supply.
- The oxygen is able to reach the cornea in two
In the form of oxygen dissolved in the tears
being pumped behind the lens when the lens
moves upon blinking, and
By diffusing directly through the lens
Fig. The pumping mechanisum
- Tear exchange not only provides oxygen and
other nutrients to the cornea, but also removes
waste products (such as carbon dioxide and
lactic acid) and dead epithelial cells.
The contact lens
Fig. The diffusion mechanism
- Tear pumping is the major source of corneal
oxygenation with PMMA lenses, since these
lenses have almost no oxygen permeability.
- The tear pump alone is insufficient to provide
adequate amounts of oxygen to the cornea.
- The PMMA lenses cause unacceptable levels
of corneal hypoxia (lack of oxygen; even in
the presence of an active tear pump.
- Diffusion significant amounts of oxygen
directly pass through the lens that is necessary
to provide an adequate oxygen level for
normal cornea metabolism.
MESUREMENT OF OXYGEN TRANSMISSION
- Direct diffusion is the major source of oxygen
transmission with soft lenses,
- it is very important to measure this parameter.
- Permeability is the degree to which a
substance is able to pass through a membrane
- Diffusion is the process by which molecules
pass through a material (such as a contact
lens); the direction of movement is always
from the area of higher concentration to the
area of lower concentration.
- Permeability is natural function of the
molecular composition of the
- Permeability is affected by
concentration, temperature, pressure,
and barrier effects.
- The permeability of a material is
expressed as a permeability
coefficient, denoted Dk.
- The diffusion coefficient (D) is the speed with
which gas molecules travel (diffuse) through
the material (Figure 15).
- The solubility coefficient (k) defines how
much gas can be dissolved in a unit volume of
the material at a specified pressure (Figure
- In order for oxygen to pass through a
contact lens material, the molecules
must first dissolve into the material
and then travel through it.
- Permeability is the product of the
diffusion coefficient ( D) and the
solubility coefficient (k).
- The Dk value is specified in standard
units. The actual testing conditions
may vary, but the results must be
converted to the standard Dk units.
- A typical Dk value, expressed in its
standard units: Dk = 8.9 x 10‾¹¹
(cm²/sec)(mlO2 / mL x mm Hg) @
- The temperature of the testing
conditions should always be noted
because Dk increases with increasing
- Because increasing temperature
increases the energy of the gas
molecules, causing them to travel at a
faster rate through the material.
- The oxygen permeability coefficient
(the Dk value) of a contact lens
material is an inherent characteristic
of the material, regardless of its
thickness. As a rule, Dk is a constant
for a given lens material.
- The Dk value of a material is not how much
oxygen will actually pass through a given
- The actual rate at which oxygen will pass
through a specific contact lens of a given
thickness is called its oxygen transmissibility,
- To calculate the oxygen
transmissibility of a given contact
lens, the Dk value for the material is
divided by the lens thickness, denoted
- Lens thickness is expressed in
centimeters, so care must be taken to
convert lens thickness (which is
typically expressed millimeters) to the
- The lens thickness chosen to
calculate Dk/L is usually the center
thickness of a -3.00 D lens, as this is
typically the midrange power of the
minus lens range for many
manufacturers; +3.00 lenses are
typically used as the midrange of plus
- The lens thickness chosen to calculate
Dk/L is usually the center thickness of a
-3.00 D lens, as this is typically the
midrange power of the minus lens range
for many manufacturers; +3.00 lenses are
typically used as the midrange of plus
- It is important to remember that most
published Dk/L values represent only
-3.00 D lenses.
- It is significant that as lens thickness
increases, the oxygen transmissibility
decreases. This means that plus lenses
(which are thickest at the center of the
lens) will have lower calculated oxygen
transmissibilities than minus lenses
(which are thinnest at the center of the
lens) of the same material.
- Dk is a function of water content in hydrogel
lenses. As a general rule, this is a linear
function with Dk increasing at the same rate as
- Although lenses with higher water content
typically have higher Dk values, they often
must be made thicker than lower water
content lenses for several reasons:
They dry out, or dehydrate, more rapidly in
thin designs, leading to corneal drying which
is observed as corneal desiccation staining.
High water lenses are generally more fragile
m thin designs.
- The thicker designs of high water lenses often
result in Dk/L values that are similar to thinner
lenses with lower water content. Table 6
There are 2 main types of water content
materials in soft CLs
- Low water content materials (Bausch & Lomb,
water content 38.6%, trade-name lens Optima
- High water content materials (Bausch &
Lomb, water content 70%, trade-name lens
B&L 70 Minus)
Contact Lens Optics
- The general principle of correction of
refractive errors with contact lenses is
substituting a new refractive surface (contact
lens) for the old surface (cornea).
- The new surface is uniform, with a different
index of refraction and anterior radius
curvature. This substitutes for the cornea
which may be irregular.
The power of the eye is dependent upon
- The radius (r) of curvature of the cornea and
- The index of refraction (n)
- The length of the eye.
The dioptric value of each surface can be
calculated with the formula
D = n2 – n / r
D = dioptric power;
n = index of refraction of first medium;
n2= index of refraction of second medium.
The power of the typical anterior corneal surface
D = 1.376 – 1.000 / 0.007
D = 0.376 / 0.007
D = + 48.83 D
The power of the typical posterior corneal surface
D = 1.336 – 1.376 / 0.0068
D = 0.040 / 0.0068
The total corneal power in round numbers is
+ 48.83 - 5.88 = + 42.90 D
- The typical anterior human lens surface power
D = 1.41– 1.336 / 0.010
D = 0.074 / 0.0010
D = + 7.40 D
- The typical posterior human lens surface power
D = 1.376 – 1.000 / 0.0006
D = - 0.0074 / 0.0006
D = + 12.33 D
- The total corneal power in round numbers is
+ 7.40 + 12.33 = + 19.70 D
The total power of the eye is arrived at by use of
the formula for combination of lenses.
If the total power of the eyes s 58.00 diopters,
then the total length is
F = 1/D
f = 1/58.00
f = 0,017 = 17 mm
Where f = focal length; D = dioptric power.
Index of Refraction
If the radius (r ) of the refractive surface and length
of the eye are constant, then the variations of the
index of refraction (n) (Table 2.4) will change the
power of the refracting surface. Using the
D = n2 – n / r
assigning a value to r of r = 7.50 mm, then if n is
given, the amount of change dioptric power can
For example, if n2= 1.33, then
D = 1.33 – 1.00/0.0075
D = 0.33/0.0075
D = 44.0
- AS the index of refraction (n) increases, the
refractive power increases (Table 2.5).
Radius of Curvature
The power of the refractive surface is dependent
upon the radius of curvature ( r) and the index of
If n is kept constant and r is changed, the power
will also change
If D = n2 – n / r,
n = air
n2 = PMMA contact lens (n = 1.49)
then the effect of changes in r can be
If r = 7.50
D = 1.49 – l-00/0-0075
D = 0.49/0.0075
D = 65.333 diopters
The smaller the radius or the steeper the
refractive surface, the greater the refractive
power of the lens (Table 2.b).
The power of a lens is the reciprocal of the focal
length, the relative or effective power of a
corrective lens changes with the placement of the
lens or the distance between the lens and the eye.
This relationship is expressed by the formula
D= 1 /f
Where D = power in diopters, f = focal length in
Example. In a +10.00 diopter lens, the focal
length is 10 cm (0.1 m):
D = 1/0.1 D= 10
- The closer a lens comes to the corneal surface
and nodal point of the eye, the greater the plus
power required and the less the minus power
needed to correct the refractive error.
- These vertex power changes can be arrived at
in four ways: The formula for the change in
vertex power of the lens is
Where Δ = change in power due to vertex
distance; D = lens power; d = distance lens in
Example 1. An aphakic spectacle correction of
+13.00 diopters at 13 mm in from the eye.
What power should the contact lens be?
Δ = 13² x 0.013
Δ = 169 X 0.013
Δ =2.197 diopters
The power of the required contact lens corrected
for vertex distance is
+13.00 +2.20= +15.20 D
Example 2. If a -10.00 lens is at 15 lens the power
of the required contact is calculated as follows:
Δ = -10² X 0.015
Δ = 1.5 D
The total required power is
-10.00 + 1.50= -8.50 diopters
- The power and position of the correcting lens must
be such that the focal point of the lens is conjugate
to the focal point of the eye.
1- External examination.
- The external examination can be done with a
penlight combined with hand magnifier or a slit
lamp which is better choice.
- CL is contraindicated if there is any active
pathology of the eye, e.g. inflammation, injury of
the cornea, conjunctiva or lid.
1- Procedure for penlight:
- Seat the patient comfortably in room with good
- Direct the penlight illumination at the area to be
examined while you look through the magnifier.
- Examine the eyelid skin, lid margin (blepharitis
marginalis), conjunctiva, cornea (scars), sclera,
anterior chamber and iris.
- You may need to avert the lids to examine the
conjunctiva properly for follicles or papillae.
- Note any inflammation or injury of the area
2- Procedure for slit lamp:
- Seat the patient comfortably at the slit lamp by
adjusting the patient seat, the slit lamp height
and chine rest or both.
- Examine all the tissue mentioned above.
Particularly those directly related to contact lens
- Diffuse illumination used to examine the
conjunctiva and the lids.
Direct illumination used to examine the cornea
- The Patient tear quality and quantity should be
tested, because wearing CL on dry cornea can
cause poor tear circulation, corneal edema,
blurry vision and burning sensation.
Therefore, there are two tests commonly used
- Tear quality→ Tear breakup time (TBUT)
- Tear quantity→ Schirmer test
◊ Tear breakup time (TBUT)
- Blinking helps in distributing tear over the
cornea, immediately after a blink, evaporation
begins and tear film begin to thin. Therefore,
the tear breakup time is often used as an index
for an abnormal tear formation.
- Tear breakup time is the interval time between
a complete blink and the first randomly
distributed dry spot.
- If an eye is kept open without blinking for
15-34 seconds the tear will show dry spot
- When fluoresecin applied these dry areas
appear black when examined with ultraviolet
- Any dry areas occur in less than 10 seconds is
considered a negative factor in patient
selection for CL fitting.
◊ Schirmer test
- It used to evaluate the rate of tear flow. It
provides information on hypo and hyper
secretion of tears.
- A special filter paper (5x35mm) is used; this
paper has an indentation at the uppers 5mm of it
- After 5 minutes the paper is removed and the
length moistened by tear is measured with a
- Normal tear secretion moistens 10-15mm of the
strip, yet older patient have less reading.
- The corneal diameter has effect on the
specification of the CL prescribed especially
lens diameter. Because the corneal diameter is
assumed to be equal the diameter of the iris.
- The actual measurement is made with a P.D.
ruler. The pupil diameter can be approximated
by using the iris as reference scale.
- For older children and adult the iris is usually
about 12mm in diameter.
- The palpebral Aperture height is important
factor in determining corneal contact lens
- the palpebral Aperture height is measured by
instructing the patient to relax and fixating
straight ahead, and measurement should be
made of the maximum vertical distance when
the lids are separated.
- This measurement is difficult to obtain since
the lid aperture is under voluntary control, so
patient tend to squint when ruler is placed near
- The refractive error of the patient must be
measured and final prescription is written in
minus cylinder form for ordering the contact
- Three reading of keratometer measurement for
the patient is obtained for maximum accuracy,
and then the median value of the three is
Contact fitting procedure lens
1- Selection of lens diameter (Dia)
- Obtain patient's horizontal visible iris diameter
- CL diameter = HVID + (1 to 3mm, average =
- Increase or decrease lens diameter in 0.50mm step
if necessary during evaluation process.
- Lens must completely cover cornea.
- Most soft CLs are available from diameter of
13.50mm to 15mm. However, large diameter e.g.
15mm tends to tighten on the cornea and may
result in complication e.g. ulcers or
2- Selection of Base curve (BC)
- SCL are usually fitted flatter than the flattest K
- The flattest K minus 3.00 diopter
BC = flattest K - 3.00D
- Convert the diameter value to millimeters
using a converting table.
- Increase or decrease BC in 0.30mm steps if
- Clinical experience shows that majority of
patient can be fitted with an average or median
BC. This is usually is the 8.50-9.00mm range.
As a guide and an alternative method of BC
selection. The following table can be used:
K-reading Soft CL Base Curve
Less < 41.00D Flat ( >9.00mm)
Between 41.00– 45.00 D Medium (8.00-.00mm)
Larger > 45.00D Steep (< 8.00mm)
3- Selection of lens power
- Refraction prescription must be converted to
minus cylinder from:
1- If cylinder in refraction is (less than or equal to)
less or equal ≤ 0.50D, power = spherical
2 - If cylinder in refraction is 0.750D to 1.00D, the
contact lens power = spherical equivalent
(spherical component + 1/2 Cyl)
- If overall spherical component in 1 and 2 is
greater than ± 4.00D, compensate for vertex
distance using either method 1 or 2.
Fc = Fs / 1- d Fs
Where Fc = power of CL, Fs = power of spectacle
d = distance between spectacle lens and CL in
Ref; -5.00 +1.00 x 90 (plus cyl form)
-4.00 -1.00 x 180 (minus cyl form)
vertex distance = 13mm » 13 / 1000 = 0.013m
Fc = 4.5 / 1- (0.013 x 4.5)
Fc = 4.5 / 1- 0.0585
Fc= 4.5 / 0.9415 = 4.249 ≈ 4.25D
Add 1/2 of cyl to sphere,
Ref; -5.00 + 1.00 x 90 (plus cyl form)
- 4.00 - 1.00 x 180 (minus cyl form)
contact lens power = - 4 + - 0.50 = - 4.50D
The contact lens power from the table = - 4.25D
This is greater than ± 4.00D, so compensate for
vertex distance e.g. 13mm (by calculation or
Types of Soft contact lenses
Soft contact lenses divided into four categories
1- Daily ware
- These lenses are worn on daily basis for 12 to
14 hours and removed before bedtime for
cleaning and disinfection.
- Methods of disinfection are heating, chemical
and oxidation (hydrogen peroxide).
2- Flexible/Extended ware
- Usually corresponds to patients who wear
lenses overnight only on an occasional basis
such as weekends, and should be cleaned and
disinfection upon removal.
- Most practitioners now recommend lens wear
without overnight removal 3 to 7days.
- Methods of disinfection involve chemical and
- These lenses also available in planned
replacement, tinted and toric forms.
3- Planned replacement
- These lenses allow the patient to replace lenses
weekly or biweekly depend on their
- For example, disposable lenses are worn for a
one- week extended wear period and
discarded, therefore, a care regimen is not
These are usually cosmetic Soft contact lens
- The tear film is vital. Not only does it provide
oxygen exchange as the lens is moved, but it
also passes lysozyme, an antibacterial enzyme
that inhibits bacterial proliferation.
- Patients with a tear deficiency are more prone
to infections and often cannot be fit
comfortably with lenses.
- Many complications with soft contact lens wear
occur after lenses are successfully fit, when
patients care and handle their lenses.
- Problems arise due to nature of hydrogel lens
materials which vulnerable to contamination by
bacteria and fungi.
- Routine soft contact lens care including
disinfection and cleaning.
- There are three methods of disinfection used with
SCL: thermal, chemical, and oxidative, each of
these methods has advantages and disadvantages
which will aid the practitioner in selecting the
care regimen best suited for each patient and
- This technique is not expensive and most
effective system in the short term.
- The thermal disinfection technique is
contraindicated with lenses containing greater
than 55% water.
- Thermal care regimen consists of saline,
surfactant cleaner, enzymatic cleaner and
rewetting or lubricating drops.
After soft contact lens removal:-
1- The CL should be cleaned with surfactant
2- Stored in a case filled with saline.
3- Enzymatic cleaner should be used weekly.
The advantages of thermal disinfection
- Quick (20 min) require very few steps
- Preservative-free solution for patients sensitive to
- Effective against all form of bacteria such as
pseudomonas and AIDS virus.
The disadvantage of thermal disinfection
- The heat bakes on the deposits so lens not cleaned
- The lens life shortened
- Not interchangeable with other care systems
-Complications such as giant papillary conjunctivitis
(GPC), or red eye occurs due to deposited lens.
- This method consist of a disinfecting solution
that contain preservatives, surfactant cleaner,
enzymatic cleaner and rewetting or lubricating
drops, and many of these solutions may be used
for rinsing and to store of the lenses.
- For example, ReNu Multi-purpose solution can be
used as a cleaner, saline and with the enzymatic
tablets; however disinfection must still be
performed following enzymatic cleaning.
The advantages of chemical systems
- It can be used for all type of SCL.
- Little effect on lens life.
- It remove 90% of a measured amount of
- The solutions number and steps are less make it
simple and convenient for patient.
The disadvantage of chemical systems
- The use of preservatives such as thimerosal and
chlorhexidine that are toxic to some patient so it
is more likely that the lens will have to be
3- Oxidative disinfection
- This technique consists of a 3% hydrogen
peroxide solution, neutralizing (solution, tablet,
and disc), saline, surfactant cleaner, enzymatic
cleaner and rewetting or lubricating drops.
- Hydrogen peroxide
- Effective against bacteria
- It can be used in a disinfection cycle of 10min
with 10min of neutralization.
- Yet longer exposure time is recommended to be
effective against fungi.
Disadvantages this system
- Large number of solutions and steps
- Storing the lens in hydrogen peroxide for
lengthily periods may affect the base curve
radius of the lens, especially with high-water
content lens materials.
- The acidity of hydrogen peroxide could cause
mild to moderate punctuate keratitis. To prevent
this there are many methods to neutralizing it.
Advantages of this system
- Safe, effective, and preservatives-free.
Examples of Oxidative disinfection
1- Allergan Optical has two systems Oxysept and
UltraCare both are preservatives-free.
A- The Oxysept is two step processes
1- The CL is placed in the case containing
hydrogen peroxide after proper time interval of
2- Place neutralizing tablet in the case to
neutralize hydrogen peroxide acidity.
B- The UltraCare is one step process
- The CL is placed in the case containing
hydrogen peroxide and place UltraCare
neutralizing tablet at the same time.
- The UltraCare neutralizing tablet is coated with
a viscosity agent that prevents activation of
tablet for 20-30mins this allow disinfection with
hydrogen peroxide to occur prior to
3. Ciba Vision has one system called AODisc.
- The CL is placed in the case containing
hydrogen peroxide after proper time interval of
- The platinum disc attached to the lens cage
begins neutralizing hydrogen peroxide
immediately upon contact when the lens cage is
placed in the case.
- The disc should be replaced after 3 months of
1- Saline solution
- It is necessary part of hydrogel CL care,
because the hydrogel CL must stay hydrated.
- Saline is non toxic to eye and sterile used to
rinse the lens from foreign body as well as to
dissolve enzyme tablets.
- Distilled water not suitable since it not sterile
and easily contaminated.
- Saline solution is not capable of disinfecting the
lens when used a lone
- It available in preserved (with thimersol or
sorbic acid) and unpreserved (e.g. aerosol
2- Surfactant cleaners
- It prevents buildup of lens deposition thus it
should be used after every lens removal.
- It acts as a soap to remove debris, unbound
proteins, lipid deposits and some microbial
- The lens placed in the palm of the hand with few
drops of the cleaner, the lens rubbed gently
back and forth for 20 to 30 seconds, and then
the lens rinsed and soaked in disinfection
Alcon introduced three generations of cleaners for
- Opticlean (preservative was Thimerosal )
- Opticlean II (preservative was Polyquad )
- Opti-Free Dialy Cleaner (preservative was
- Introduce Mira Flow contain among other
cleaning ingredients, isopropyl alcohol.
- Isopropyl alcohol eliminates the need for a
preservative because of its broad-spectrum
- It excellent cleaner especially for patients with
the tendency toward lipid deposits, but the lens
should be rinsed to avoid the risk of parameter
3- Enzymatic cleaner
- It is used once a week to break down peptide
bonds, allowing protein to be rubbed off
- The proper care sequence when enzyming
hydrogel lenses are cleaning, rinsing, enzymatic
cleaning, rinsing, and disinfecting.
4- Lens lubricants/ rewetting
- It is optional, but may be beneficial in cases of
dry eyes, foreign body sensation, irritations and
for morning and evening use in extending wear.
- Lens lubricants used directly in the eye with and
without the lenses.
- It is not suitable to use ophthalmic medication as
lubricants because this could cause
discoloration and cause toxic reaction.
- Soft lens should be fitted with what is known as
1- The lens should parallel the superior and
inferior sclera as well is the corneal apex.
2- When the lens rests only on the superior and
inferior sclera and jump the corneal apex, the
lens is too steep (Fig. 61).
- If the lens rests on the corneal apex and the
edges stand off from the sclera, the lens is too
flat (Fig. 6-3).
- All soft lenses, regardless of power, size, or
manufacturer, should be fitted to obtain this
three-point touch (Fig. 6-4).
- A well-fitted lens will show five basic qualities;
good centration, adequate movement, stable
vision, crisp retinoscopic reflex, clear
undistorted keratometry mires, and clear
1- Good centration.
- The lens will center itself well easily after
insertion in the eye. After the patient blinks, it
will not show more rim of lens on one side of
the cornea than on the other side.
- Lens decentration requires refitting with either a
steeper base curve or a larger diameter (Fig. 6-
2- Adequate movement
- The slit lamp is very useful for evaluation of
- Fitting should be evaluated while the patient
looks straight ahead, upward, and laterally. The
patient should be asked to blink under slit lamp
- Evaluation should then be made clinically as to
whether the movement is excessive, negligible,
- A standard-thickness lens may show movement
of 0.5 to 1 mm on upward gaze after a blink,
and it should show no greater movement on
- If the lens is even with tears and does not move,
the person should be switched to a lens with a
flatter base curve (Fig. 6-6, A).
- If the lens moves excessively, a lens with a
steeper base curve (Fig. 6-7) series or one with
a larger diameter should be substituted (Fig. 6-
6, C and D)
3- Stable vision.
- When he patient blinks, the vision should remain
equally clear before and during the blink and
visual acuity should be as sharp as possible (Fig.
6-8). If trial-set lenses re used for fit evaluation,
an over-refraction should be performed.
- If visual acuity is not adequately sharp after
changing the lenses or holding over low-plus or
low minus lenses. It is useful to have the patient
view an astigmatic clock. If some of the clock
lines are significantly blurred, residual
astigmatism is present and vision cannot be
improved soft lenses.
- Variable vision initially may be caused by a
lens that is either too loose or too tight. If
the fit is found to be adequate and the
patient still complains of fluctuating vision,
such factors as dryness of the eye or from
the environment, lack of blinking, or excess
mucus secretions must be considered as
- Normally, blinking may be reduced with
driving and reading. The patient should be
warned of soft lens variable vision. It is
easily reduced by a series of blinks or
4- Crisp retinoscopic reflex.
- As confirmatory evidence of a good fit, the
retinoscope, streak is flashed in all meridians
while the patient blinks. When the patient is
adequately fitted, the retinoscopic reflex will be
sharp and crisp as if no lens were in place, both
before and after blinking (Fig. 6-9, A).
- If the lens is steep, there will be a spreading of
the streak centrally in the rest position, which
will clear after a blink because of ironing out of
the apical jump (Fig. 6-9, C).
- If the lens is flat, it may ride low; a position that
can be detected by the retinoscopic shadow may
be blurry immediately after a blink (Fig. 6-9,
5- Clear, undistorted keratometry mires.
The mires that are reflected from the keratometer
while the person is wearing the soft lens will
often indicate if the fit is adequate. With the
correct fit, the mires of the keratometer should
not be distorted either before or after a blink
(Fig. 6-8, A).
- If the mires are blurred, the patient should blink
several times; if the mires are still distorted, the
lens should be changed (Fig. 6-10, B and C).
Characteristic of steep fitting
There are characteristics of steep fitting for soft
- Little or no movement either on blinking or as the
eye change fixation.
- Tight fit is quit comfortable, sometimes more so
than a correct fit, because a complete immobile
lens produces the minimum of lid sensation.
- Usually good centration
- The slit lamp may show irritation of the
conjunctival or limbal vessels and, with very
light lenses an annular ring of conjunctival
compression may be seen this often visible when
- Vision unstable and poor because momentary
pressure on the eye during blinking occurred with
steep fitting jump the corneal apex.
- Subjective refraction is difficult with no clearly
defined end point, and more negative power than
predicated may be required because of a positive
- Retinoscopy and keratometer mires both show
irregular distortions these mires improve with
Characteristic of loss fitting
There are characteristics of loss fitting for soft
- Easily to diagnose because of poor centration,
greater lens mobility on blinking and excessive
lag on lateral eye movements.
- Very uncomfortable especially on looking
upwards, lower lid sensation experienced if the
- Vision and over refraction are variable, but
nevertheless may still give satisfactory results.
- The retinoscopy reflex may be clear centrally but
with peripheral distorted.
- The keratometry mires change according to lens
The following steps should be taken to correct a
- Either changing the base curve by decreasing it by
0.2 to 0.3 mm
- Increasing the diameter of the lens by 0.5mm up to
The following steps should be taken to correct a light
- Either changing the base curve by increasing it by
0.2 to 0.3 mm
- Decreasing the diameter of the lens by 0.5mm.