cataract operation involves removing
the natural cloudy lens from the eye.
Without a lens within it the eye is
widely out of focus and remains legally
blind. The best remedy for this is
to insert an artificial lens into
the eye. This is now a standard part
of cataract surgery, though it has
not always been so.
This artificial lens
is called "The Intra Ocular Lens Implant".
Surgeons often refer to it as the IOL (Intra
Ocular Lens). If a lens implant is not used
the eye must be brought into focus with
either a spectacle lens or contact lens.
Each of these has disadvantages. A spectacle
lens will be thick, heavy and restrict the
field of vision. A contact lens has to be
looked after and taken on and off the eye.
An intra ocular lens once inserted can be
forgotten about by the patient and restores
the eye to the most nearly natural state.
section provides more information about
the Intra Ocular Lens Implant. It expands
on the brief explanation given on the previous
page. It includes information about:
history of intra ocular lens implants.
Types of intra ocular lens implants .
Intra ocular lens implant power.
Innovations & possible future developments
with intra ocular lens implants.
cataract operation involves removing the
natural lens from the eye. In order to restore
focused vision to the eye an artificial
plastic lens is placed into the eye after
the cataract has been removed. This is now
a standard part of cataract surgery. During
the second world war some aircrew suffered
injuries to the eyes from shattered pilot
and turret canopies. These structures were
made of perspex (polymethyl methacrylate
or PMMA) and in some cases tiny fragments
penetrated and remained within the eye.
It was then observed that this material
caused very little reaction within the eye
and remained stable over many years. It
was deduced that perspex would be a good
material from which to make intra ocular
lens implants. It remained the principle
material used until the mid 1990's and is
only now being superseded by more modern
British ophthalmologist Harold Ridley is
credited as the first eye surgeon to place
an artificial lens into the eye following
removal of a cataract. This was around 1949
- 1950. At the time he was considered to
be a bit of a maverick by his colleagues.
In the face of opposition from the rest
of the medical profession he initially abandoned
the technique though continued to pursue
the concept with lens manufacturers. In
2000, aged 93 and just before his death,
he was knighted for his pioneering work.
It was not until the late 1970's and early
1980's that intra ocular lenses gained widespread
popularity and acceptance amongst eye surgeons
within the UK. The idea of inserting a lens
into the eye had to wait until surgical
techniques had developed to the point where
it was safe and possible to do so. There
have been two particularly important factors
the development of gel like substances
(called viscoelastics) which can be
placed within the eye during surgery
to create space and protect the delicate
internal structures of the eye. These
substances allow the lens implant to
be safely manipulated into position
within the eye.
second factor was the development of
surgical techniques which preserve a
platform within the eye onto or into
which a lens implant can be securely
positioned. The best location for an
intra ocular lens implant is behind
the pupil, i.e. as near as possible
to the position of the natural lens
of the eye. The move away from intra
capsular cataract surgery to the extracapsular
technique in the 1980's made it possible
to support a lens implant behind the
pupil. Phacoemulsification cataract
surgery popularised in the 1990's now
means that the lens implant can be placed
within the pocket (the so called capsular
bag) of the natural lens.
ocular lens implants may be classified according
- their particular
material from which they are made
- whether they are
rigid or foldable
- where they are
positioned within the eye
intra ocular lens implants have two parts.
The central part is the focusing or true
lens component of the device. This is shaped
like a thin round disc and is usually between
5 and 7 mms in diameter. This part of the
lens is called the “optic”.
From the edge of this optic extend limbs,
called “haptics”. These support
the lens in position within the eye and
ideally keep the optic centred on the line
of vision. Most lens designs have two haptics
extending from the optic in opposite directions.
There are a few lenses on the market with
more than two haptics. The
lens as a whole may be made from a single
piece of plastic. It is then referred to
as a one piece lens. Alternatively the optic
and the supporting haptics may be manufactured
separately and then fused together. If the
lens has two supporting haptic limbs bonded
to the central optic it is referred to as
a three piece lens. Different
lens designs may have differently shaped
haptics. These may be simple 'C' or 'J'
shaped loops or have more complex sinuous
forms. In one design, known as the plate
haptic lens, the central lens optic is supported
by two paddle shaped limbs and the whole
lens has a rectangular form.
ocular lens Material
ocular lenses are made from clinical grade
plastics. These materials need to be chemically
stable and inert within the eye. They must
not cause any irritation or chemically induced
damage to the tissues of the eye. In other
words they must be safe and reliable. They
must last a lifetime. The
first plastic used to make an intra ocular
lens was perspex (polymethyl methacrylate);
also known as PMMA. It is a rigid material
and the same substance from which traditional
hard contact lenses are made. Many millions
of such lenses have been implanted. One
company has devised a means of bonding a
substance called 'heparin' to the surface
of lenses made from this material. This
has the effect of making them 'non stick'
and ideal for use in eyes with a history
of inflammation or in patients with diabetes.
In the UK PMMA lens implants are now going
out of use and have been replaced by lens
types made from more modern plastics. These
materials include silicone and a variety
of acrylics. These substances are pliable
and lenses can be manufactured from them
that can be folded.
v Foldable Lens Implants
implants made from PMMA (described above)
are rigid. The 'optic', the main focusing
part of the lens, is inflexible. It cannot
be bent or folded. This part of the lens
is usually between 5 and 7 mms in diameter.
The incision in the eye must therefore be
at least this big in order to allow the
lens to be put into the eye. If the incision
is made in the form of a slit like tunnel
it may be self sealing and not require a
suture. However the larger the incision
the more likely it is that a suture will
be required in order to leave the eye in
a sealed and secure state at the end of
the operation. Newer
silicone and acrylic materials are pliable.
Lenses can be made from them that can be
folded. This means that the entire cataract
procedure can be performed through a small
incision of around 3mms or less. The lens
implant is folded and introduced into the
eye either with special forceps or an injection
device. Once within the eye the lens is
released and gently unfolds to its full
size. It is then manoeuvred into position.
Only a very slight enlargement of the incision
is necessary to permit the insertion of
a foldable lens implant. In the vast majority
of cases the incision does not then require
a suture. This makes the surgery quicker,
simpler and the focus of the eye settles
more rapidly post-operatively.
within the eye
intra ocular lens may be placed in the space
between the cornea and the iris, be attached
to the iris or be placed behind the iris
and pupil. Nowadays lens implants are usually
placed behind the iris and pupil. The
fluid filled space between the cornea and
the iris is called the anterior chamber.
Lens implants designed to be positioned
here are therefore known as anterior chamber
intra ocular lens implants (A/C IOL’s).
These implants are supported in position
by S-shaped haptics (supporting limbs) that
rest in the periphery of the anterior chamber
where the root of the iris and the dome
of the cornea meet. This is called the anterior
chamber angle and this type of lens implant
is therefore said to be 'angle supported'.
When using an anterior chamber lens implant
it is usually necessary to make a small
hole in the periphery of the iris. This
is called a peripheral iridotomy (or PI).
This is because the optic of the lens, which
sits immediately in front of the pupil,
may obstruct the flow of aqueous fluid through
the pupil. This could cause a rise in the
fluid pressure within the eye (glaucoma).
The iridotomy prevents this from happening.
An intra ocular
lens implant may be clipped or hooked to
the iris. This type of lens was abandoned
in the early 1980’s due to an unacceptably
high rate of complications. A newer design
has more recently been devised but in the
UK it is at present rarely used in the context
of cataract surgery. With
current techniques of cataract surgery the
intra ocular lens implant is usually placed
behind the iris. The “optic”
(the true focusing part of the lens implant)
sits just behind the pupil. This is very
close to the position of the natural lens
of the eye (which has been removed). The
small space behind the iris is called the
posterior chamber. Lenses designed for implantation
into this space are therefore called posterior
chamber intra ocular lens implants. Eye
surgeons refer to them as P/C IOL's.
of the advantages of phacoemulsification
(and extra capsular cataract surgery) is
that part of the lens capsule is preserved.
This capsule is a thin membrane like structure
that surrounds the natural lens of the eye.
With these particular surgical techniques
the opaque natural lens (the cataract) is
removed from within the capsule. The capsule
remains as an empty pocket. It can then
be used to support and fixate a posterior
chamber lens implant. Without this support
the lens implant would fall into the vitreous
gel that fills the main cavity of the eye
and would end up at the back of the eye.
posterior chamber lens implant can either
be placed onto the remaining capsule or
actually slipped into the capsular pocket.
Placing the implant onto the capsule is
rather like resting the lens onto a mini
trampoline. The supporting legs (haptics)
of the lens implant rest in the cul-de-sac
between the back of the iris and the ciliary
body. This recess is called the 'ciliary
sulcus'. Posterior chamber lens implants
resting on the surface of the capsule are
therefore said to be sulcus fixated, i.e.
held in position by haptics resting in the
ciliary sulcus. Alternatively the lens implant
may be placed inside the capsular pocket.
Eye surgeons call this the 'capsular bag'.
A lens held in place in this position is
said to be 'in the bag' or capsular bag
fixated. This is the probably the best place
for a lens implant as it is the exact position
of the natural lens of the eye.
Intra Ocular Lens Implant Power
optical power, or focus power, of the lens
implant is measured in dioptres (D). This
is the same measurement as that used for
spectacle and contact lenses. The higher
the number the stronger or more powerful
is the focus of the lens. For a posterior
chamber lens implant, the usual style of
implant used these days, the average lens
implant power is around +22D or +23D. For
short sighted (myopic) eyes the power will
tend to be less than this and for long sighted
(hyperopic) eyes it will be higher. The
exact lens power chosen for each operation
will depend on the desired post-operative
focus of the eye. Most lens implants are
available in a standard power range of +10D
to +30D. This will meet the requirements
of most eyes. Powers outside this range
are available in some designs for the occasional
eyes that require them. Innovations &
possible future developments with intra
ocular lens implants
possible future advances in intra ocular
lens implant design includes:
- Implant edge design.
- Multifocal implants.
- Implants that
correct the optical aberrations of the
- Adjustable implants.
has been much discussion of late as to the
best shape for the edge of a posterior chamber
intra ocular lens implant. The current consensus
is that the back edge of the optic (the
main focusing part) of the lens should be
square and that the front edge should be
rounded. It is thought that a square back
edge will help prevent the development of
posterior capsule opacity. The lens implant
rests on the posterior capsule. Immediately
after surgery this membrane like structure
is clear. In some eyes as time goes by it
becomes hazy or opaque and vision worsens.
This opacification is partly due to the
proliferation of cells on the surface of
the capsule. If the lens implant has a square
back edge which abuts into the surface of
the capsule this may prevent cells from
proliferating across the capsule beneath
the lens. This would then help to keep the
capsule clear in the line of vision. A rounded
front edge is thought to reduce internal
reflections and stray light within the eye.
Many lens implant manufacturers have now
adopted these edge features.
youth the eye has the ability to naturally
adjust its focus. As well as having clearly
focused distance vision the eye is able
to re-focus for near vision. This ability,
called accommodation, is gradually lost
with ageing. This is why most people need
reading or near vision spectacles by mid
40's. This adjustment of focus is produced
by a small muscle inside the eye which alters
the shape of the natural lens of the eye.
As the years pass the natural lens becomes
less pliable until by age 60 all focus adjustment
has been lost. Similarly after a conventional
cataract operation the eye has a fixed focus.
Traditional lens implants have only a single
focus power which cannot be adjusted by
the eye. If after cataract surgery the eye
is in focus for distance vision spectacles
will be required for near vision. Theoretically
one solution to this is to use a lens implant
with both distance and near focus; rather
like bifocal spectacles. Multifocal lens
implants are now clinically available but
in most situations the patient must be prepared
to accept some degree of blur. If the distance
portion is in focus on the retina there
will be a ghost near focus and vice versa.
These lenses will only enable the patient
to be free of spectacles if there is no
significant astigmatism and the distance
and near lens implant powers are appropriate
for the individual eye.
explained above in the section on multifocal
implants after a conventional cataract operation
the eye has no ability to adjust its own
focus. The ultimate dream, or the holy grail,
of cataract surgery is to produce a lens
implant with an adjustable focus. If this
were possible then not only would sight
be restored from removal of the cataract
but the eye would also be returned to its
youthful freedom from reading spectacles.
Some lenses have now been designed which
may be able to do this. They are though
currently experimental and, at the time
of writing, their long term success in clinical
practice has not yet been proven. The theory
is that on attempting to focus for near
vision the little focusing muscle within
the eye (called the ciliary muscle) causes
the lens implant to move forwards slightly.
If the lens sits a little further forward
within the eye its power is in effect slightly
that correct the optical aberrations of
main focusing part of a conventional lens
implant is optically very simple. It is
usually biconvex, i.e. both the front and
back surfaces have a raised curvature. This
curvature is part of the surface of a sphere,
i.e. it is like the curvature on a soccer
ball. A lens of this design will correct
the main focus error of the eye. However
it does not correct the other optical aberrations
or imperfection of the eye (e.g. spherical
aberration). These aberrations cause vision
to be slightly degraded especially when
the pupil is large. The natural lens of
the eye is shaped in such a way that some
of these aberrations are neutralised. In
other words a traditional style of lens
implant is not as good as a clear natural
lens at achieving the best possible focus
and clarity of vision.
developed for use in laser refractive surgery
are able to measure the optical aberrations
of the eye. These devices are called 'wavefront
aberrometers'. They shine a low power laser
beam into the eye. This is reflected from
the back of the eye. From an analysis of
how much the beam is defocused by its passage
through the eye it is possible to calculate
the main aberrations of the eye. The laser
treatment can then be given to correct not
just the short or long-sightedness but may
also re-shape the cornea to correct the
optical aberrations of the eye. This technology
could be applied to cataract surgery and
the design of lens implants. If the aberrations
of an individual eye were known the curvatures
on either the front or back surface of the
lens implant could be manufactured so as
to correct these aberrations, as well as
the main focus error of the eye. In other
words the lens implant could be optically
“made to measure” for each individual
eye. Alternatively knowledge of the average
aberrations of the eye at a given age could
be used to produce an 'off the peg' aberration
correcting lens implant. Such a lens is
now commercially available.
alternative way to perfect the focus of
a lens implant to the individual requirements
of an eye is to adjust the focus of the
lens after it has been placed into the eye,
i.e. to do so post-operatively. A lens material
has now been produced whose shape can be
adjusted using a ultra violet light. A lens
of approximately the correct dimensions
is implanted and the eye is allowed to recover
from the operation. The focus and aberrations
of the eye are then determined. The lens
is exposed to a specific wavelength of light
in such a way that its shape is altered
slightly to perfect the focus of the eye.
This can be done more than once if necessary.
When the best focus has been achieved the
lens is exposed to a different light which
fixes or sets the lens and keeps the desired