Contact Lenses

Liezel Fourie B.Optom (RAU)

Professional Services Manager

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Contact Lenses

Contents

1. Principals of contact lens correction…………………………………………………..2

2. The advantages of contact lens wear…………………………………………………3

3. Contact lens materials…………………………………………………………………..5

a. Hard (rigid) contact lenses……………………………………………………...5

b. Soft (hydrophilic) contact lenses……………………………………………….6

4. Contact lens terms……………………………………………………………………..10

5. The contact lens consultation…………………………………………………………13

6. Types of soft contact lenses…………………………………………………………..16

a. Spherical contact lenses………..……………………………………………..16

b. Toric contact lenses……………………………………………………………16

c. Conventional vs. disposable contact lenses………………………………...18

d. Daily wear, extended wear and flexi-wear…………………………………..19

e. Daily disposable contact lenses………………………………………………20

f. Bifocal and multi-focal (progressive) contact lenses……………………….21

g. Cosmetic contact lenses………………………………………………………24

h. Prosthetic contact lenses……………………………………………………...25

7. Contact lens prescription vs. spectacle prescription……………………………….25

8. References……………………………………………………………………………...27

Principles of Contact Lens Correction

Contact lenses are optical devices designed to correct visual problems. A contact

lens floats on the tear film, which makes it adhere to the eye. The eyelids also help

to keep the contact lens in place. With every blink the eyelids slide over the surface

of the contact lens, and cause it to move slightly. This movement is very important as

it allows the tears to flush away any debris that may be trapped underneath the lens.

The fresh tears also help to lubricate the cornea (the clear surface of the eye that lies

directly underneath the contact lens).

Figure 1: Anatomy of the eye Figure 2: Position of a soft and a hard

(rigid) contact lens on the eye

Contact lenses can be used to correct visual problems like short-sightedness,

farsightedness, astigmatism and presbyopia (age-related loss of the ability to focus

on near objects). In these conditions light entering the eye is not focused properly on

the retina (the sensory layer at the back of the eye) (Figure 3). This results in a

blurred image being sent to the brain. A contact lens is an optical instrument which

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bends light (similar to spectacle lenses). When placed on the eye, the contact lens

focuses the light entering the eye onto the retina, resulting in a clear image being

sent to the brain (Figure 4).

Figure 3: The short-sighted eye Figure 4: Visual correction with a contact lens

Contact lenses are medical devices and need to be prescribed by either an

optometrist or an eye specialist.

The Advantages of Contact Lens Wear

Visual

Because there are no spectacle frames to block the peripheral vision (side

vision), contact lens wearers have a wider field of view than spectacle wearers.

A contact lens moves with the eye which allows the eye to look through the

optical centre of the lens at all times. This eliminates certain optical distortions

which might be present when looking through the sides of spectacle lenses.

Spectacles can make objects appear smaller or bigger than what they actually

are. This can become a problem when there’s a big difference between the two

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eyes (anisometropia). Most people’s eyes differ slightly, but when there’s a

difference of around 2.00 D or more, it can lead to certain problems. When

wearing spectacles objects will appear much bigger through the one lens than

through the other, which may lead to disorientation, confusion and even double

vision. Because there is no space between contact lenses and the surface of the

eye, contact lenses don’t minify or magnify objects as much as spectacles. The

size of the images falling on the two retinas will be more similar.

With contact lenses you don’t have the reflections normally associated with

spectacles. These reflections interfere with vision, especially when driving at

night.

In general, the higher a patient’s prescription, the more he will benefit, in terms of

vision, from wearing contact lenses in stead of spectacles.

Cosmetic

Vision correction that is not visible like with spectacles.

Can be used to enhance or change the natural eye colour.

Can be used to make a damaged/disfigured eye appear more natural.

Convenience

A number of problems associated with spectacle wear can be eliminated with contact

lenses:

Getting wet due to splashing water or rain

Fogging up on cold days, in steamy environments or when perspiring

Sliding down the nose, or causing pressure sores on the nose or ears

The frame material reacting with certain skin types

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Contact lenses are much more practical to wear during strenuous activities such

as sports.

Occupational Advantages

Preferred by photographers and people who work with optical instruments like

microscopes, telescopes etc. because there is no frame that can get in the way.

People who perspire a lot when working don’t have the problems of spectacle

lenses fogging up.

Sports enthusiasts don’t have to worry about spectacles getting in the way.

Safety

In contact sports a broken spectacle frame can cause serious injury to the eye.

Contact lenses might be the safer option to wear.

Please note that contact lenses are no substitute for safety glasses.

Contact Lens Materials

Contact lenses are classified into two main types according to the type of material

from which they are made:

Hard (rigid) lenses

Soft (hydrophilic) lenses

Hard (Rigid) Contact Lenses

Hard lenses are made from a rigid plastic and will flex only a small amount. If

excessive force is applied to the lens, it will break. A hard lens retains its shape

when placed onto the cornea. It does not conform to the shape of the cornea. For

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this reason, vision through a hard lens is crisp and sharp, and it can even correct

astigmatism.

Hard lenses can further be divided into two groups:

Non-gas-permeable lenses (PMMA)

Rigid gas permeable lenses (RGP)

RGP lenses allow oxygen to pass through the lens material, PMMA doesn’t.

Hard lenses are much smaller than soft lenses. Soft lenses are slightly bigger than

the cornea - hard lenses are smaller. This can cause haloes at night when the

patient’s pupils dilate. Hard lenses move more in the eye than soft lenses and can

fall out more easily. In general, hard lenses are less comfortable and it takes a

patient longer to adapt.

Soft (Hydrophilic) Contact Lenses

The word hydrophilic means “water-loving” (hydro=water, philos=love). Soft contact

lenses are made from plastics that absorb water like a sponge. This makes the lens

material soft and pliable. The first soft contact lenses have been approved by the

FDA in 1974.

Soft lenses are generally very comfortable and easy to get used to. This makes it

ideal for occasional wear. Because they are slightly bigger, they don’t easily fall out

of the eye, which makes it the lens of choice for sports activities.

Unlike hard lenses, soft contact lenses conform to the shape of the cornea. (Soft

lenses ‘drape’ over the cornea.) Because of this they do not correct a lot of

astigmatism unless a toric lens is used.

Because soft lenses are so absorbent, they are more prone to the adherence of lens

deposits (little particles accumulating on the surface of the lens). Soft lenses are also

more susceptible to the growth of micro-organisms like bacteria and fungi. Soft

lenses are less durable than hard lenses.

Soft contact lenses can further be divided into two groups:

Hydrogel lenses

Silicone hydrogel lenses

The main difference between the two groups is the amount of oxygen it transports

through to the cornea.

Why is oxygen so important? The cornea needs oxygen to stay healthy. It derives it’s

oxygen directly from the air around it. A contact lens forms a

barrier between the cornea and it’s oxygen supply. A lack of

oxygen can lead a variety of adverse effects on the cornea.

In a nutshell, it lowers the cornea’s defence system, leaving

it more susceptible to infection. Most contact lens complications can be traced back

to a lack of oxygen.

Traditionally all soft contact lenses were made from a hydrogel material. As

mentioned earlier, soft contact lenses absorb water like a sponge. Hydrogel lenses

transport oxygen through the water molecules in the lens. Newer developments in

contact lens technology saw the birth of silicone hydrogel lenses. Silicone is very

porous. Little channels run through the material (almost like Swiss cheese). These

channels allow oxygen to physically pass through the material, without being bound

to water. Therefore silicone hydrogel contact lenses allows much higher

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amounts of oxygen to pass through to the eye than traditional hydrogel lenses.

Figure 5: Microscopic structure of Silicone Hydrogel

One of the disadvantages of silicone hydrogel lenses is that the lens material is

slightly “stiffer” than hydrogel lenses. When changing a patient form a hydrogel lens

to a silicone hydrogel lens, initially the patient might be more aware of the new

lenses in his eyes. This is temporary in most patients, and lasts only a few days.

Warning the patient before hand about this possibility, and explaining the

advantages of changing from a low-oxygen hydrogel lens to a high oxygen silicone

hydrogel lens, will help him understand and accept this adaptation period.

Figure 6: Examples of Hydrogel lenses Figure 7: Examples of Silicone Hydrogel lenses

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Which type of contact lens is best?

As you can see, each type of contact lens has it’s advantages and disadvantages.

There is no such thing as the “best type” of contact lens. There is no single contact

lens on the market that will completely satisfy all the needs of each and every

contact lens patient. The optometrist will decide which type of contact lens is best for

each patient, based upon their specific visual, ocular and lifestyle needs.

Even though most people can wear contact lenses, there are patients who are not

suitable candidates for contact lens wear. These include patients with a history of

repeated eye infections, or patients with extremely dry eyes.

As with other medical devices, contact lenses are not without a degree of risk of

adverse effects. Some of these include infection and allergic reactions. These

complications are infrequent. It is very important that the patient follow the

optometrist’s instructions on wearing and caring for the lenses in order to minimise

the risk of complications.

Contact Lenses

Hard Contact Lenses

Soft Contact Lenses

Non-gas-permeable (PMMA)

Rigid Gas-permeable (RGP)

Hydrogel Silicone Hydrogel

Figure 8: Classification of contact lens materials

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Contact Lens Terms

Power

The prescription of the contact lens

Diameter (Dia)

The size of the lens when measured from side to side

Figure 9: The Diameter of a contact lens

Centre Thickness

The thickness, measured in millimetres, in the centre of a -3.00 D lens. Lenses of the

same type, but with different powers, will have different thicknesses in the centre. A

plus lens, for example, is thicker in the centre than at the edge, while a minus lens is

thinner in the centre than at the edge. For this reason, the centre thickness for a

specific lens type, as specified by the suppliers, is always that of a -3.00 D lens.

Example, if we say the centre thickness of Focus® DAILIES® is 0.1 mm, it means a

-3.00 D Focus DAILIES contact lens is 0.1 mm thick in the centre. A +3.00 D or a

-10.00 D Focus DAILIES lens will have a different centre thickness. -3.00 D is just a

standard used in the contact lens industry.

Water content

Different types of soft contact lenses absorb different amounts of water. The water

content is the amount of water contained by a contact lens. A hydrogel lens with a

water content of 55% consists of 45% hydrogel material and 55%

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water. In general lenses with a water content higher than 50% are classified as high

water lenses. If the water content is lower than 50% the lens is being classified as a

low water lens. Because hydrogel lenses transfer oxygen through the water in the

lens, the higher the water content, the more oxygen gets transmitted through the

lens material. Silicone hydrogel lenses does not need water to transmit oxygen

through the lens. It simply passes through the porous structure of the lens. Therefore

in silicone hydrogel lenses water content is not an indicator of oxygen

transmissibility. Based upon various factors, the optometrist will decide what water

content will be most suitable for a specific patient.

Dk/t (Oxygen Transmissibility)

The Dk/t of a lens is its ability to transmit oxygen. The higher the Dk/t of a lens, the

more oxygen pass through the lens to the eye. Silicone hydrogel lenses has much

higher Dk/t’s than traditional hydrogel lenses. E.g. a NIGHT & DAY® lens with a Dk/t

of 175 transmits much more oxygen than a Focus® Visitint® lens with a Dk/t of 20.

Again, when we specify the Dk/t of a certain lens type, we refer to the Dk/t in the

centre of a -3.00 D lens. If we say NIGHT & DAY has a Dk/t of 175, it means the Dk/t

in the centre of a -3.00 D NIGHT & DAY lens is 175. At the edge it will be less,

because the edge is thicker. The Dk/t will also be lower in thicker, higher powered

lenses.

Visibility (handling) tint

Contact lenses often have a slight blue, green or grey tint on them to make them

more visible when in the contact lens case. It might also make them easier to find

when dropped. This tint is very light, and doesn’t affect the natural eye colour.

Base Curve (BC)

The base curve refers to the back curvature of the contact lens which fits onto the

eye. The curvature of people’s eyes differ. For a contact lens to fit properly, the back

surface of the lens must be similar in curvature to the front surface of the eye. This is

important to insure good comfort, vision and health.

Figure 10: Base Curves

The higher the base curve, the flatter the lens. A Focus® Visitint® lens with an 8.9

base curve will be flatter, and fit looser on the eye than an 8.6 Focus Visitint lens.

Figure 11: The effects of base curve on contact lens fit

A soft lens needs to move slightly on the eye with every blink. This movement is

important to flush debris from underneath the lens. A lens that fits too tight can lead

to serious complications in the long run. A lens that fits too loose will move

excessively on the eye, causing fluctuating vision and discomfort.

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The Contact Lens Consultation

When a patient wants to try contact lenses for the first time he needs a few more

tests than a spectacle patient. The optometrist needs to check whether the patient is

a suitable candidate for contact lens wear and that the lenses fit properly. The

patient then needs to be instructed in handling and caring for contact lenses. The

optometrist might also require the patient to return for follow-up exams to check if

everything is fine.

Every practitioner has his own way of conducting contact lens examinations. Some

types of contact lenses, like hard lenses, also requires more consultations before a

satisfactory fit is achieved. Here is a rough outline of the contact lens fitting

procedure:

Pre-fitting Examination

Before contact lenses are fitted, the following procedures are being performed:

Case history

A detailed case history is taken to assess among others a patient’s symptoms,

ocular history, general health, medication that might be used, previous

experience with contact lenses, visual and lifestyle requirements. The case

history is important in deciding which type of contact lens will be most suitable for

the patient. A patient who wants to wear contact lenses occasionally for sports

activities, might enjoy the freedom of daily disposable lenses. A doctor who is on

call might benefit from extended wear lenses, which can be worn while sleeping.

Refraction (eye test)

To determine the patient’s prescription

Keratometry

The optometrist takes a measurement of the

curvature of the patient’s corneas. These

measurements are called K-readings and are taken

with an instrument called a keratometer. This gives

an indication of the required base curve.

Figure 12: The Keratometer

Slit-lamp examination

The slit-lamp is a microscope used to examine the cornea. The optometrist uses

the slit-lamp, to assess the health of the cornea and to

evaluate the tear layer. Problems with the tear layer include

not enough tears being produced, or tears evaporating too

quickly. This leads to dryness which can cause problems

with contact lens wear. Figure 13: Slit-lamp examination

Based on these findings, the practitioner will select a lens type to try.

Contact Lens Fitting

The optometrist inserts the selected trial lenses into the patient’s eyes and allow the

patient to walk around with them for a while (usually 30 minutes to 3 hours, based on

fitter preference). On return the optometrist will enquire about the comfort and vision

the patient experienced with the trial lenses. He will measure the vision

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and test the patient’s eyes over the contact lenses (over-refraction) to determine

whether the prescription needs to be adjusted or not. Using the slit-lamp he will

evaluate the fitting of the lens. If he is not happy with the lens fit, he will select a

different lens type and repeat the trial fitting. Soft contact lenses are usually quick

and easy to fit. A hard lens fitting is a bit more tricky and often needs a series of

consultations before an acceptable fit is achieved.

Contact Lens Instruction

If the comfort, vision and fitting of the lenses are

acceptable, the patient will be taught how to handle

and care for the lenses. Some practitioners prefer to

do the contact lens instruction themselves, while

others leave it up to their front staff assistants. The contact lens instruction is very

important. It supplies the patient with valuable information on how to take care of

their lenses. Patients who haven’t been taught how to handle their lenses can hurt

themselves when trying to insert and remove the contact lenses. They can also

damage the lens. If patients don’t clean and disinfect their lenses correctly, it can

lead to serious eye infections and even corneal ulcers, which can lead to a

permanent loss of vision.

Follow-up Examinations

A newly fitted contact lens patient needs to return for follow-up examinations. This is

just to make sure the patient adapted well to contact lens wear, and that everything

is still fine with his eyes and the contact lenses. Follow-up examinations can be

performed after a week, and then again after a month, based on fitter preference.

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Types Of Soft Contact Lenses

Spherical Contact Lenses

These are lenses with a spherical prescription e.g. -3.00 D. There is no cylinder and

axis. Most patients with low levels of astigmatism (usually under -1.00 D) can

successfully be fitted with spherical contact lenses. The reason for this is that

spherical contact lenses will often compensate for a certain amount of astigmatism.

This is phenomenon is called ‘masking’. Sometimes the optometrist may add a bit of

minus to the spherical prescription to help compensate for the astigmatism.

E.g. If a patient’s spectacle prescription in his right eye is -2.00/-0.50X180, the

optometrist might fit him with a -2.25 D AIR OPTIX™ lens.

Figure 14: Examples of spherical contact lenses

Toric Contact Lenses

Patients with higher amounts of astigmatism will benefit from wearing toric contact

lenses. A toric contact lens has a sphere, cylinder and axis. Spherical contact lenses

constantly rotate on the eye, due to the forces of the eyelids during blink. With toric

contact lenses we want the axis to stay in the correct position in front of the eye.

There are different methods of preventing a lens to turn on the eye. The two most

often used are prism ballast and double thin zones.

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Prism Ballast

The contact lens is designed so that it is thicker at the bottom than at the top. This

creates a ‘wedge’ effect. During blink, the forces of the eyelids cause the lens to

rotate in such a way that the thinnest part of the lens rests underneath the upper

eyelid and the thickest part stay at the bottom.

Figure 15: Prism Ballast Lens Design Figure 16: Example of a Toric Contact Lens with

a Prism Ballast Design

(Double) Thin Zones

In this method of stabilisation the contact lens is shaped in such a way that it’s got

thinner areas at both the top and bottom sections of the lens. This creates a double-

wedge effect where the lens rotates in such a way that the thin parts of the lens rest

underneath the top and bottom eyelids.

Figure 17 : Double Thin Zones Figure 18 : Example of a toric contact

lens with a Double Thin Zone design

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Despite all of these measures, a toric lens may still rotate on the eye. Toric contact

lenses have fine lines engraved into the lens material at certain positions. These

lines are called fitting marks or rotation marks and their purpose is to help the

optometrist determine the orientation of the lens. By looking at the lens through the

slit-lamp, he can determine whether or not the lens has rotated on the eye, and by

how many degrees. He might then decide to alter the axis of the contact lens

prescription to compensate for this rotation, or try a different type of toric lens.

The fitting marks of different brands of toric lenses are located in different positions

on the lens.

Figure 19 : Examples of Fitting Marks

Conventional vs. Disposable Contact Lenses

Conventional lenses can be worn for 6-12 months before being replaced.

Disposable lenses are replaced at more regular intervals. The recommended

replacement modality (how often it should be replaced) for a specific lens is

specified by the manufacturer. This includes monthly, 2-weekly and daily

replacement.

Figure 20: Conventional Contact Lenses Figure 21: Disposable Lenses

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Daily Wear, Extended Wear and Flexi-wear

If a lens is indicated for daily wear, it should be worn only during waking hours.

Under no circumstances should a patient sleep with them. A lens prescribed for

extended wear can be worn while sleeping.

Patients should only sleep with their lenses with the consent of their optometrist. Not

everyone is suitable for extended wear. Some people might develop problems. A

lens like NIGHT & DAY® has got FDA approval for up to 30 nights extended wear,

but only under prescription and supervision of an optometrist.

When a patient wants to try extended wear, the optometrist needs to do frequent

check-ups in the beginning to make sure he is a suitable candidate. Once the

optometrist gave the patient trials, it is recommended that he returns for check-ups

the next morning, after he slept with his lenses for the first time, a week later, and

then again a month later. If the optometrist is satisfied that his eyes are in a good

condition, and no problems came up, the patient can continue with extended wear. It

is also recommended that the optometrist do a quick follow-up exam every 6 months,

when the patient orders his next supply of contact lenses.

If the optometrist feels that the patient is not a suitable candidate for extended wear,

they should not sleep with their lenses. Sometimes the optometrist might prescribe

extended wear, but for shorter periods of time. He might recommend that the patient

removes his lenses once a week, clean them, and sleep without them for one night.

Figure 22: Example of a 30 night extended wear lens

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Flexi-wear (flexible wear) is the term used when a patient does not routinely sleep

with his lenses, but may occasionally sleep or nap with his lenses.

Figure 23: Example of a flexi-wear lens (up to 6 nights overnight wear)

Daily disposable contact lenses

Daily disposable (one-day disposable) contact lenses are lenses intended for one

use only. The patient inserts a fresh pair in the morning, and throws it away at the

end of the day. There are a lot of advantages to wearing daily disposable lenses:

Allergy sufferers

Great for allergy sufferers. Because the lenses are thrown away after each wear,

there is no need for cleaning and disinfecting. Most contact lens cleaning

solutions contain preservatives to which patients can be allergic. Daily

disposables cuts out the need for cleaning solutions.

Health

Studies have shown that patients wearing daily disposable contact lenses get

fewer contact lens complications compared to patients wearing 2-weekly, monthly

and conventional lenses.

Convenience

Because there’s no need for cleaning, daily disposables are often preferred by

patients with busy schedules. People who travel frequently like it, because they

don’t have to carry bottles of solution around in their suitcases. Daily disposables

are also convenient for people with active lifestyles, e.g. hikers.

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Even though daily disposable lenses are a bit more costly than your other

replacement modalities, it’s becoming more and more popular, due to it’s

convenience.

Figure 24: Examples of daily disposable contact lenses

Bifocal and Multi-focal (Progressive) Contact Lenses

A presbyope (a person over the age of 40, wearing bifocals, multi-focals or reading

glasses) have two different prescriptions for far and near. These patients are often

interested in contact lenses. There are several options available to them:

Contact lenses together with reading glasses

Presbyopes can wear contact lenses for far, and just where reading glasses over

them for reading.

The next 3 methods all make use of the principal of simultaneous vision. During

simultaneous vision a patient looks through both the distant and the near sections

of their contact lenses at the same time. Generally, if you look far through your near

prescription, things will be out of focus, and vice versa. During simultaneous vision

the brain subconsciously suppresses the image that is out of focus.

The advantage of simultaneous vision is that the patient can see both far and near

with their contact lenses, without the help of spectacles. Disadvantages

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are that it takes longer to adapt, and there is also usually some degree of

compromise to the vision. The patient often has to return for several follow-up

consultations before satisfactory vision is obtained.

Monovision

With monovision the optometrist fits a patient with a contact lens for far in the one

eye, and one for near in the other eye. When the patient looks at an

object with both eyes open, one eye will see the object in focus while the other

eye will see a blurred image. Because this blurred image is suppressed by the

brain, the patient won’t notice it.

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Distance Rx

Near Rx

Distance

Rx Near Rx

Right Lens Left Lens

Figure 25: Monovision

Bifocal Contact Lenses

With bifocal contact lenses the distance and the near prescriptions are alternated

in concentric circles from the centre to the edge of the lens. Again, the brain

suppresses the blurred image and makes use of the clear image.

Figure 26: Bifocal Contact Lens

Progressive/Multi-focal Contact Lenses

Progressive or multi-focal contact lenses also make use of the principal of

simultaneous vision. These lenses are classified into two groups: centre distance

(far) progressive designs and centre near progressive designs.

o Centre Distance (Far)

The centre of the lens contains the distance prescription. As you move from the

centre of the lens towards the edge, the prescription gradually changes from the

distance prescription to the near prescription.

o Centre Near

Distance Rx

Near Rx

Distance Rx

Near Rx

The prescription changes from the near prescription in the centre of the lens, to

the distance prescription towards the edge.

Figure 27: Centre Distance Progressive Lens Figure 28: Centre Near Progressive Lens

Figure 29: Two examples of Progressive Contact Lenses

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Cosmetic Contact Lenses

Cosmetic contact lenses are contact lenses that will change or enhance a patient’s

natural eye colour. Coloured contact lenses can be divided into two groups: Colour

enhancers and Opaque Lenses.

Colour enhancers have a transparent tint. These lenses will change the colour

of light coloured eyes, but not dark eyes.

Figure 30: Examples of colour enhancers

Opaque lenses have a more dense tint that will change both light and dark eyes.

Figure 31: Examples of opaque coloured contact lenses

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Cosmetic contact lenses are available in prescription (to correct vision problems) as

well as PLANO (where no vision correction is needed). Even if a patient doesn’t

need vision correction, it is important for her to still have an eye test. The reason for

this is that the optometrist needs to check whether she is suitable for contact lens

wear, and what type of lens will be best for her. He needs to do a trial fitting to make

sure the lens fits properly and the patient needs to be instructed in how to insert,

remove and clean the lenses. Lenses that don’t fit properly or aren’t taken care of

correctly, can lead to serious complications and even cause permanent damage to a

patient’s eyes.

Prosthetic Contact Lenses

A prosthetic contact lens is a tinted lens, designed to enhance the appearance of a

damaged or injured eye, e.g. to hide a scar or an irregular pupil. Sometimes

standard cosmetic lenses can be used, but often a more specialised prosthetic lens

is called for. These lenses can be hand painted. Sometimes an iris with a black pupil

is painted onto the contact lens to hide a scar. Often the eye in question has lost its

ability to see.

Contact Lens Prescription vs. Spectacle Prescription

It is important to note that the contact lens prescription is not always the same as the

spectacle prescription. This might be due to several reasons:

Over-refraction

When the optometrist test the patients eyes over the trial contact lenses, it might

reveal that the contact lens prescription needs to be adjusted by ± 0.25 D.

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Astigmatic patients

Patients with low amounts of astigmatism are often fitted with spherical contact

lenses in stead of torics. Sometimes the optometrist will make the sphere a bit more

minus to help compensate for the astigmatism.

When a patient is fitted with toric lenses, it does sometimes happen that the lens

turns (rotates) on the eye. In order to compensate for that rotation the optometrist

might order a contact lens with a different axis to the spectacle axis.

Prescription Availability

Most soft disposable contact lenses are available in 0.25 D steps up until a certain

prescription. The higher plus and minus prescriptions (which are less common) are

only available in 0.50 D steps. In a case where the patient’s prescription is not

available, the optometrist will select the closest available prescription.

Vertex Distance

The vertex distance is the distance between the back surface of the spectacle lens,

to the patient’s cornea. With lower prescriptions this has no influence on the contact

lenses prescription. With higher prescriptions the patient’s contact lens prescription

will start to differ from his spectacle prescription (due to the fact that it’s closer to the

eye). With prescriptions higher than -4.00 the contact lens prescription is lower than

the spectacle prescription. With prescriptions higher than +4.00 the contact lens

prescription is higher than the spectacle prescription. The optometrist uses a Vertex

distance conversion chart to convert the spectacle prescription to a contact lens

prescription.

Figure 32: Vertex Distance Figure 33: Vertex distance conversion table

References

1. The CIBA Vision® Learning Program, CIBA Vision 1991

2. Basics of Contact Lenses, Anne Austin Thompson, O.D. CIBA Vision 1993

3. Design and Fitting of Soft Contact Lenses, Anne Austin Thompson, O.D. CIBA

Vision 1995

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