The Intra Ocular Lens Implant

This section provides more information about these devices including information on:

Once placed within the eye the Intra Ocular Lens implant can be forgotten about by the patient. It requires no looking after.

The Intra Ocular Lens Implant Power

The 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 those eyes that require them.

The History Of Intra Ocular Lens Implants

During the second world war some aircrew suffered injuries to the eyes from scattered cockpit 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 therefore 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 It remained the principle material used until the mid 1990s and is only now being superseded by more modern plastics.

The British ophthalmologist Harold Ridley is credited as the first eye surgeon to have placed an artificial lens into the eye following removal of a cataract. This was around 1949/1950. At the time he was considered 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 1970s and early 1980s that Intra Ocular Lenses gained widespread popularity and acceptance amongst eye surgeons within the UK. Improvements in lens design have depended heavily on advances in the techniques of cataract surgery.

The idea of inserting a lens into the eye has had to wait for surgical technology to catch up. There have been two particularly important factors in this.

Firstly 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 make it possible to safely manipulate a lens implant into the eye. In the early 1980s, before these substances were widely available, a rather bulky lens implant style was introduced for clipping to the iris. In many cases this lens caused damage to the unprotected inner surface of the cornea and subsequent clouding of the cornea. This lens design was therefore abandoned.

The second factor has been 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 position for an intra ocular lens implant is for it to be as near as possible to the location of the natural lens of the eye. This means placing the lens implant behind the pupil and iris. Phacoemulsification surgery popularised in the 1990s now means that the lens implant can be placed within the natural pocket (the so called capsular bag) of the natural lens.

Types Of Intra Ocular Lens Implants

Intra Ocular Lens Implants may be classified according to:

their particular design
the material from which they are made
whether they are rigid or foldable
where they are positioned within the eye

Lens Implant Designs

All 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.

Intra ocular lens Material

Intra 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 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.

Rigid v Foldable Lens Implants

Lens 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 suture will be required to leave the eye in a sealed and secure state at the end of the operation. Phacoemulsification has made it possible to remove the natural lens through an incision of about 3mms. If a rigid lens implant is used the incision will need to be enlarged to enable the lens to be posted into the eye. It is therefore the lens that will determine the size of the incision. There are though individual cases where a rigid lens implant may still be the lens type of choice.

Newer silicone and acrylic materials are pliable. Lenses can be made from them that can be folded. This means that the entire Lens Exchange 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 require a suture. This makes the surgery quicker, simpler and the focus of the eye settles more rapidly post-operatively.

Position within the eye

In Lens Exchange surgery it is best to place the Lens Implant into the same position as the natural lens formerly occupied. This is behind the pupil. This position is anatomically known as the posterior chamber. Such Lens Implants are therefore called Posterior Chamber Intra Ocular Lens Implants (P/C IOLs).

One of the advantages of phacoemulsification, the procedure used in Lens Exchange, is that part of the lens capsule is preserved (see: Lens Exchange - the operation). This capsule is a thin membrane like structure that surrounds the natural lens of the eye. With this surgical technique the substance of the natural lens is removed from within the capsule. The capsule remains as an empty pocket. Eye surgeons call this the capsular bag. This can be used to support a posterior chamber lens implant. A lens implant may be slipped into the capsular pocket. In this position is said to be in the bag.

Sometimes a surgeon may wish to use a posterior chamber lens implant when there is no capsule to support such a lens. Specially designed posterior chamber lens implants can be held in place behind the iris with sutures. These sutures may be anchored through the wall of the eye or through the iris. Non absorbable sutures must be used to ensure that the lens is permanently held in position.

Alternatively the lens implant may be placed on (rather than within) the capsular bag rather like resting on a mini trampoline.

Without the support of the capsule 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.

It is possible to support a lens implant elsewhere within the eye, e.g. between the cornea and the iris or clipped to the iris.

The fluid filled space between the cornea and the iris is called the anterior chamber. Lens implants positioned here are therefore known as Anterior Chamber Intra Ocular Lens implants (A/C IOLs). The focusing part of the lens (the optic) sits in front of the pupil. The lens as a whole is held in position by s-shaped legs (haptics) that rest in the periphery of the anterior chamber where the root of the iris and the dome of the cornea meet. A/C IOLs are available in a range of overall sizes and have flexible supporting legs. This permits a lens to be chosen that will fit snugly and comfortably within an individual eye. Too loose or too tight a fit might damage the delicate tissues within the eye. A/C IOLs need to hold their form and are therefore made from PMMA which is a rigid material. When implanting such a lens implant it is usually necessary to make a small hole in the periphery of the iris to ensure that the lens does not interfere with the circulation of fluid within the eye.

Innovations & Possible Future Developments

Recent and possible future advances in intra ocular lens implant design includes:

Implant edge design
Multifocal implants
Accommodating implants
Implants that correct the optical aberrations of the eye
Adjustable implants

Implant Edge Design

There 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.

Multifocal Lens Implants

In 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 Lens Exchange surgery the eye has a fixed focus. Traditional lens implants have only a single focus power which cannot be adjusted by the eye. If after 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 and loss of contract sensitivity. 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.

Accommodating Implants

The ultimate dream, or the holy grail, of cataract and Lens Exchange surgery is to produce a lens implant with an adjustable focus. If this were possible not only would the refractive error be corrected by Lens Exchange but the eye would be returned to its youthful freedom from reading spectacles. Some lenses have now been designed which may be able to do this. However 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 increased.

Implants that correct the optical aberrations of the eye

The 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.

Devices 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. This technology could be adopted to 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.

Adjustable Implants

An 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 an 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 shape.