Intraocular Lenses for Cataract Surgery (General Info)
By David F. Chang MD
To take the place of the clouded human lens, a precisely engineered artificial lens called the intraocular lens, or “IOL”, is implanted into the eye at the time of cataract surgery. Unlike a contact lens, it is permanently fixated inside the eye. In this way, it cannot fall out, does not require cleaning, and does not change the appearance of the eye. It produces no sensation, and cannot be felt by the patient.
The IOL is permanent because it is made of a perfectly transparent material that will never cloud. Once in place, it will not move, and unlike an artificial joint or heart valve, there are no moving parts in the lens that could wear out over time. Because it is lightweight and flexible, it will not be affected by physical activities or by rubbing of the eye. Patients can ultimately resume all of their normal activities without any restrictions.
Replacement organs that come from another human body are called transplants. The clear, front part of the eye – the cornea – is the only ocular organ that can be transplanted. Just as we can make perfectly engineered optical lenses for cameras, microscopes, and telescopes, IOL’s are man-made. Unlike transplanted organs, there is no shortage of tissue and there is no chance of rejection. The term implant is used because the artificial lens is surgically and permanently placed inside the eye.
Function of the intraocular lens
Intraocular lenses replace three main functions of the natural human lens. First, like a window, both the cornea and the lens must be perfectly transparent in order for external light and images to be focused onto the retina at the back of the eyeball’s interior. Second, the lens acts as a structural partition between the vitreous gel behind it, and the clear fluid-filled chamber in front of it. Finally, it provides necessary focusing power.
The two ocular structures that focus incoming light onto the retina are the cornea and the lens. About two thirds of the total focusing power requirement is provided by the cornea. The lens provides the remaining one third. An eye that has undergone cataract surgery without implantation of an IOL is called aphakic (phakic is the Greek root denoting lens). Without a natural or artificial lens inside, everything is completely defocused, and the eye is functionally blind at all distances. The missing power must be replaced in order to restore useful vision. Besides the intraocular lens, there are two other options that have been used, historically, to accomplish this.
Spectacles: Special glasses, called aphakic spectacles or “cataract glasses”, can be used to restore focus after lens removal. Prior to the 1980’s, this was the most common way to correct vision after cataract surgery. Prior to the 1970’s, it was the only way. This is still the primary method used in some third world countries where artificial lenses and other modern medical technology are not available.
Because so much power must be placed in these glasses, they do not appear or work like conventional eyeglasses. The lenses are extremely thick and heavy, appearing like “coke-bottle” glasses. The exaggerated lens thickness causes the view to be magnified by approximately 25%. Although this is fine for reading, this creates a very distorted and unnatural type of vision during walking. These spectacle lenses compromise depth and peripheral vision, and have optical “blind spots” where images pop in and pop out.
Because of their weight, they are uncomfortable, and the unusual optics makes them extremely difficult to adapt to. Cosmetically, the thickness and power of the lenses magnifies the patient’s eyes creating a “bug-eyed” appearance. Finally, such lenses cannot be used for just one eye, because that one eye would see everything magnified by 25% compared to the other eye. This would create a type of double vision that the brain could not fuse together. Aphakic spectacles were therefore not an option when only one eye needed cataract surgery.
Contact lenses: The advantage of contact lenses following cataract removal without an IOL is that they can carry large amounts of optical power without significant magnification (only approximately 5%). The unnatural distortion of aphakic spectacles is avoided. Because the magnification is minimal, this method of correction can be used following cataract surgery in only one eye.
Especially as the eye gets older, it is preferable and safer to remove contact lenses daily. Extended wear contact lenses should not be worn for longer than a week at the most. Contact lenses wear may be more problematic in an older patient for several reasons. With age, the eye surface tends to more dry and prone to discomfort. In addition, the patient must have good eyesight and good manual dexterity to be able to clean, handle, insert, and remove the lens.
The fact that the aphakic eye (no IOL) is functionally blind without the lens makes it more difficult when the contact lens falls out, is lost, or when the patient arises in the middle of the night without it. Unlike a nearsighted patient who can focus on the contact lens when it is held in their hand, an aphakic patient is completely blurred at near. This can make handling the contact lens quite challenging.
In the past, removing a cataract meant that the patient would either have to wear cataract glasses or a contact lens. A patient was therefore trading their preoperative blur, for new problems postoperatively – either the visual distortion of the aphakic spectacles, or the inconvenience of the contact lens. Understanding this, ophthalmologists would often wait until the preoperative disability was so major that even these postoperative compromises would amount to an improvement. This factor, and the previously higher complication rate, meant that many patients were urged to delay cataract surgery in the past. Thus arose the excuse that their cataract wasn’t “ripe” yet.
Intraocular lens (IOL): The above discussion reveals why the intraocular lens is one of the most important medical advances in the history of ophthalmology. Because there is no unwanted magnification, the IOL provides the most natural vision of the three alternatives following cataract surgery. It is permanent, requires no care or handling, and comes in a range of powers that can be selected for the individual patient. Nowadays, it is standard for all cataract surgeries.
Anatomy of an intraocular lens
The central viewing zone is called the optic. This is a clear, round disc measuring 5.5 to 6.5 mm in diameter (about 1/4 inch). The optical power is contained in the optic, and there are generally about 40 powers available for surgeons to choose from. The unit of optical power is called a “diopter”.
On opposite sides of the optic are two flexible struts, called haptics. These act like tension loaded springs to automatically center the lens within the eye compartment where they are implanted. Haptics have varying shapes depending on the type of IOL. The most common lens used is the three-piece posterior chamber IOL. It is called a three-piece IOL because the round optic is fused with two plastic haptics that are shaped like curved wires.
Evolution of the intraocular lens
The pioneer of the intraocular lens was a British ophthalmologist, Sir Harold Ridley. In caring for British aviators with eye injuries during World War II, Dr. Ridley made a critical and interesting observation. He noted that pieces of plastic from shattered airplane canopies that became lodged inside the eyeball were surprisingly well tolerated with little inflammation. He also recognized the tremendous advantages that an artificial lens would provide for cataract patients, and courageously performed the first intraocular lens implant in 1949 in London.
Although the lens, like modern IOLs, was made of PMMA plastic, the prototype design was not ideal and the surgical techniques were limited by today’s standards. As a result, the early results were not satisfactory. Although a true visionary, Dr. Ridley’s ideas were ridiculed and strongly opposed by the medical establishment and the idea of lens implantation became largely abandoned.
Almost 20 years passed before a pair of Dutch ophthalmologists resurrected the mission of developing an artificial lens. At that time, the entire lens – capsule and all – was removed during cataract surgery. For fixation, these early artificial lenses were either clipped or sewn onto the iris. However, the iris is not rigid, and this created problems with excessive movement of the IOL that intermittently bumped and damaged the cornea.
The next generations of IOLs were designed to rest in front of the iris, and were called anterior chamber IOLs. The junction where the dome-like cornea joins the iris peripherally is called the “angle”. For fixation, the two ends of these anterior chamber IOLs were wedged into the angle. Improper sizing and incompatible designs of these initial prototypes again resulted in poor tolerance by the eye or excessive IOL movement that could damage the cornea.
The modern intraocular lens design was the brainchild of an American ophthalmologist, Dr. Steven Shearing. His revolutionary, three piece model was designed to be placed just behind the iris, which is where the natural human lens is located. What was needed was some structure to support the artificial lens, and it took a major advance in cataract surgical technique to provide this.
Position and fixation of the IOL
The natural human lens is about the size and shape of a piece of M&M candy. The lens is encased in a thin, transparent wrapper called the capsule. Tiny microscopic support ligaments, called zonules, insert into the capsule 360 degrees around the edge of the lens circumference. This suspends the lens just behind the colored iris. The front of the lens capsule is called the anterior capsule, and the back is called the posterior capsule.
Until the early 1980’s the primary method used to remove the clouded natural lens was to make a large incision and to insert a freezing probe that would adhere to the lens. This was then used to pull out the entire lens – capsule and all. Although far more difficult, techniques have since evolved that allow the lens to be dissected apart under an operating microscope while the lens is still inside the eye!
A round opening is made in the anterior capsule, and the interior contents are removed in several delicate steps. All that remains is the transparent capsular bag. Since the support zonules still hold this empty capsular bag in place, it provides an optimal location to place the permanent artificial lens implant. Like a shrink-wrap, the capsular bag soon contracts slightly around the implanted IOL. This immobilizes the IOL so that it cannot jiggle or rotate with head movements. The spring-like haptics assure that the optic is centered within the capsular bag. In this way, there is no need to sew or clip the artificial lens into place, and it occupies the same position as the original natural lens.
These implants are called posterior chamber IOLs, because they are placed behind the iris. They require that the lens capsule be present to support the IOL. Dr. Shearing’s model was the first such lens designed. Lenses placed in front of the iris are called anterior chamber IOLs. Because the iris supports them, they do not require the presence of the posterior lens capsule. This is why anterior chamber IOLs were used in the earlier period when cataract surgeons only possessed techniques to remove the entire lens – capsule and all.
Although the posterior chamber is the preferred location of the IOL, modern anterior chamber IOL designs have improved a great deal. These lenses also perform very well and have a long and proven track record. Cataract surgeons may select an anterior chamber IOL at the time of surgery if the posterior capsule support is insufficient. This is not common, but can occur.
Other patients may not have received an IOL at the time of their original cataract surgery many years ago. Even if the natural lens capsule was already removed, an anterior chamber IOL can still be implanted many years later. Whether the IOL is placed in the posterior or anterior chamber (i.e. behind or in front of the iris,) there is no difference in optical quality, vision, or comfort. Thus, modern anterior chamber IOLs still provide a viable and successful second option.
Types of IOL materials
IOL’s can be manufactured from different materials. The type of material determines whether or not the IOL can fold. All of the lens materials discussed provide excellent optical quality and have gained FDA approval through large, rigorous studies and a proven track record of safety.
Non-foldable IOL’s
The original lens implanted over 50 years ago by Dr. Ridley was made of a clear plastic called polymethylmethacrylate, or “PMMA”. This was the same type of firm plastic used in rigid contact lenses. PMMA lenses were the only lenses available until 1990, and thus have enjoyed a long track record of proven safety. This type of material is still used today for posterior chamber IOL’s. Because PMMA is rigid, these lenses do not fold, and require a larger incision to be implanted.
Some eyes require an unusually strong prescription to focus. In these unusual cases, the appropriate power may not be available in a foldable lens, and a PMMA non-foldable lens is used. Some special situations require an IOL to be sutured into the eye. These lenses are also made of PMMA. All anterior chamber IOL’s are currently made of PMMA, as well. Because PMMA lenses are the least expensive to manufacture, they are still the most commonly implanted lenses in many areas of the world.
Foldable IOL’s
Foldable IOL’s are comprised of materials flexible enough to allow the lens to be rolled, compressed, or folded in half. This property allows these IOL’s to be introduced through a much smaller incision than their non-foldable PMMA counterparts. Although more expensive to manufacture, these are now the most commonly implanted lenses in North America for this reason. They have the basic same design as the PMMA three-piece posterior chamber IOLs. The main difference is that the optic can be folded during insertion.
Foldable IOLs can be inserted into the eye using one of two techniques. One is to fold the IOL in half using a special forceps. An alternative is to use a special device that rolls the IOL and injects it slowly into the capsular bag. Once in the eye, foldable lenses open up and return to their original configuration without leaving any crease or mark in the optic.
There are many advantages to using the smallest incision possible for cataract surgery. A small incision increases safety both during the operation, and during the early postoperative period. Because a small incision heals faster, it allows for a more rapid recovery. By not weakening the eye, patients do not have to avoid physical exertion or bending their head over. The vision will improve faster compared to if a large incision had been used. Because the small incision will stabilize and heal faster, new eyeglasses can be prescribed much sooner. Finally, the larger the incision, the more it can potentially alter the natural spherical shape of the cornea. Thus, small incisions are much less likely to induce unwanted changes in this desired spherical shape called astigmatism.
Foldable IOL materials
There are three classes of foldable lens materials – silicone, hydrophobic acrylic, and hydrogel.
The first foldable IOL to become FDA approved was the Allergan silicone lens in 1990. Soon, other manufacturers received FDA approval for their silicone IOLs. This material has enjoyed a superb track record of safety and performance. Unlike the liquid silicone that can leak out from breast implants, IOLs are made of solid silicone polymers that are very biocompatible and completely unrelated to the liquid material. There have been no complications attributable to the material itself. Although the three-piece design is the most popular silicone IOL, there is a variation called the plate haptic design.
The first hydrophobic acrylic IOL, Alcon’s Acrysof, became available in the U.S. in 1995. The acrylic material has been very popular. It and silicone currently comprise the two most commonly used IOL materials in the United States. The newest class of foldable IOL materials to become FDA approved is the hydrophilic acrylic, or hydrogel class. Their market share is currently much smaller than that of the other two materials.
All of these foldable posterior chamber lenses are of excellent optical quality, are safe, FDA approved, and produce outstanding outcomes. While cataract surgeons may have personal preferences, no clear superiority of one foldable material over another has been demonstrated.
What can be seen without glasses following cataract/IOL surgery
The following discussion applies to healthy eyes with no other ocular diseases, problems, or history of surgery. After the age of 50, most patients no longer have perfect natural focus for distance. Eyeglasses will usually improve the focus for far distance objects, such as road signs. Depending upon the activity, this additional improvement may or may not be enough to compel the patient to acquire or wear glasses.
Because of presbyopia – the loss of accomodation [link to presbyopia] – everyone by this age must do something to shift their focus from distance to near. [See FAQ – “Does having an IOL mean that I won’t need glasses anymore?”]. People with contact lenses or otherwise excellent distance vision don reading glasses. People wearing glasses for distance either switch to separate reading glasses or use bifocals. Some nearsighted patients are able to simply remove their distance spectacles and read without eyeglasses.
Although healthy eyes can be focused at any distance with the aid of appropriate eyeglasses, there will be some distance at which the eye will be in natural focus without glasses after cataract surgery. However, no eye over the age of 50, with or without an IOL, can shift the focus between far and near without glasses. Thus, optimal distance focus without glasses may not be everyone’s preference. If the overriding priority is to be able to read up close without glasses, a patient may prefer to remain nearsighted after cataract/IOL surgery. For others, a slight amount of myopia (nearsightedness) may represent a good compromise between being either very blurred for distance or very blurred for near without glasses.
IOL power selection
All optical lenses – whether eyeglasses, contact lenses, or IOLs – are manufactured in a large range of different powers. The process of determining one’s eyeglass prescription – a refraction – is really a trial and error exercise. After the examiner narrows down the choices to several lens powers, the patient selects which one best focuses their vision by repeatedly comparing different pairs of lenses … “Which is better – one or two?” In this way, the patient ultimately ends up receiving that one contact lens or one spectacle lens that best corrects their vision for the distance. Vision without glasses is called “uncorrected” vision.
Each IOL model is also manufactured in a large range of powers. Of the 40 to 50 available IOL powers to choose from, there is only one that will be the single perfect lens for far distance focus. Three or four others will be very close, and the rest will result in significant blur for distance without glasses. Regardless of what the uncorrected vision is after surgery, eyeglasses can always be prescribed to provide excellent distance vision for an otherwise healthy eye.
With the patient’s input, the surgeon must decide what general distance (far/near/or intermediate focus) to target for the patient’s uncorrected vision after IOL surgery. The patient’s lifestyle, the prescription of the other eye, and the patient’s prior eyeglass prescription are factors to consider. A specific lens power for the IOL is then selected with the goal of achieving this target.
The surgeon uses a computer program to determine the appropriate IOL power in advance of the surgery. The calculations are based upon those dimensions of the eyeball, which determine the unique optical properties of that individual eye. These painless measurements are taken preoperatively. The most important are 1) the amount of corneal curvature, which correlates with the cornea’s optical power and 2) the distance from the cornea to the retina. Since this distance cannot be determined with a ruler, ultrasound (medical sonar) or similar technology is used to measure this distance in tenths of millimeter accuracy.
Let us assume that the surgeon is seeking to optimize distance focus without glasses postoperatively. The IOL power selected is an estimate derived from these calculations. However, without the benefit of trial and error (as is employed in prescribing glasses or contacts), perfect distance focus is not assured. Since one cannot try out different IOL powers, the hope is to pick one of the two to three IOL powers that will get the patient into the right “ballpark”. Eyeglasses can then be worn as an option to fine-tune and achieve best distance focus.
Every individual’s situation may be different. Although there is a wide range of targeted results, a very common outcome following IOL surgery is that the patient can see reasonably well indoors and around the house without glasses. They will utilize reading glasses to read comfortably. They will pick and choose when to don distance glasses to enhance their far focus. This might be for driving, for example. Many patients will continue to choose bifocals out of habit or for convenience. In essence, the patient will have the same focusing options that all other patients over the age of 50 have. As with any other patient, contact lenses or refractive surgery are available options as well.
Secondary intraocular lens implantation
For a variety of reasons, some patients may not have received an IOL at the time of their cataract operation. The most common reason is that they underwent cataract surgery many years ago, before IOLs had become the standard. Although very uncommon, it is possible that complications at the time of the original surgery made it inadvisable to implant a lens at that time. Finally, the natural lens may occasionally need to be removed at the time of retinal surgery with the plan to implant an IOL at a later time. A subsequent re-operation to implant the artificial lens is called a secondary IOL procedure.
An eye that has no IOL following cataract surgery is called “aphakic”. Most aphakic patients must wear a special post-cataract contact lens in order to see. Thick cataract glasses require that both eyes be aphakic, and are generally a less satisfactory option. As described above, both of these options for visual correction have inconveniences and potential drawbacks. However, many people function perfectly well with contact lenses, which will usually provide the same quality of vision as would be possible with an IOL. Aside from the care and handling of the contact lens, the main disadvantage is that the eye is functionally blind when the contact isn’t worn.
Aphakic patients have the option of a secondary IOL implantation at any time. However, they must submit to the small but irreducible risks of a second operation. The risk varies depending on the condition of the eye following the original operation. In the absence of other eye problems, the success rate with a secondary IOL operation should approach 95%.
Someone who has been successfully wearing an aphakic contact lens may eventually elect to have a secondary IOL implantation for a number of reasons. Handling the lens may become difficult because of worsening eyesight or reduced manual dexterity from arthritis. Dry eyes or corneal problems may cause irritation and reduce the amount of time the contact lens can be worn each day. Frequent lens loss or ripping becomes increasingly inconvenient and expensive.
The type of IOL used for a secondary lens implantation depends upon the anatomy of the eye following the original cataract surgery. In some eyes, enough of the lens capsule remains to support a posterior chamber IOL. If not, an anterior chamber IOL can be properly positioned in front of the iris. In some eyes, prior injuries or surgical complications may have resulted in internal iris scarring or removal of iris tissue. If this precludes support of an anterior chamber IOL, posterior chamber IOLs can be permanently sutured into place. These decisions are made by the ophthalmologist at the time of surgery.
FAQ about IOLs
How long will the IOLs last?
The IOL is permanent and, unlike an artificial joint or heart valve, there are no moving parts in the lens that could wear out over time. Artificial lenses are even placed in children following congenital cataract surgery because they will last a lifetime.
Can the IOL be removed and replaced?
Although it is rarely necessary, the IOL can be removed and replaced. Although the need to remove the lens is very unusual, the most common reason would be that the power is incorrect, despite all of the preliminary calculations. Another reason would be if the IOL shifted out of position inside the eye. This is very rare. Because the artificial lens is designed to be permanent, it is not a simple task to remove the IOL.
Does having an IOL mean that I won’t need glasses anymore?
Not exactly. While we are young, our internal eye muscles can change and control the shape of the natural human lens. This alters the lens power, and allows us to shift our focal distance, from far to near. This process of moving the focus closer is called accomodation. It is so fast and automatic (like an auto-focus camera) that we aren’t even aware that it is occurring. Whether the eye has good distance focus with or without glasses, it is this accomodation of our natural lens that then enables us to focus closer up to read.
Presbyopia is the natural and unavoidable process by which everyone slowly loses the ability to accomodate over time. This is due to a loss of lens flexibility with age. As we progress from our forties through to our sixties, everyone gradually loses the ability to focus in the direction from far to near. This function is instead replaced with reading glasses (for the person who sees distance well without glasses) or bifocals and trifocals (for the person who needs eyeglasses to see distance).
The conventional artificial lens is a single, fixed focus lens. It cannot give distance focus one moment, and near focus the next. Following cataract surgery, patients will therefore choose from the same options available to every other person over the age of 50 without cataracts or implants. They can wear separate glasses for help at distance (if needed); separate glasses for reading and near focus; or combine the two functions into bifocals. They can wear contact lenses if they choose to. No one is “required” to wear eyeglasses. Rather, depending upon the activity and at what distance their eye is in natural focus already, patients can pick and choose when they want to wear glasses to enhance their vision further. [link to IOL section on vision without glasses after cataract/IOL surgery]
Since there is no opportunity for trial and error in the IOL selection process, there is no guarantee that the targeted focal distance will be attained following surgery. Fortunately, eyeglasses can be used to provide clear focus, just as they do for any other patient whose eyes do not have perfect focus naturally.
Does the multifocal IOL take the place of eyeglasses?
The multifocal IOL is a special situation with its own pros and cons. Although a number of these patients do not seem to need spectacles, the majority will use still wear spectacles part time – particularly for reading. Some still require spectacles most of the time. However, compared to patients with conventional monofocal IOLs, those with multifocal IOLs usually perform significantly better at near without glasses .
Does the IOL replace the need for sunglasses?
Sunglasses provide two benefits. Their darker tint reduces the brightness of our surroundings by decreasing the amount of light that reaches the eye. The major health benefit is that they contain a transparent UV coating which blocks out the invisible ultraviolet rays of the sun. Ultraviolet rays are what cause sunburn, and are present even on overcast days. Because of the potential for cumulative damage to the retina over time, it is advisable to block out ultraviolet light. All modern IOL’s are permanently coated so as to provide this UV protection at all times. Since IOLs have no dark tinting, patients may still choose to wear sunglasses for comfort, just as they did before their cataract surgery.
Where are IOLs manufactured?
The commonly used IOLs in North America are produced in the United States, where the major manufacturers are located. The quality control is very strict and of the highest standard, in order to obtain FDA approval. In the non-industrialized world, inexpensive IOLs may be produced locally, which are not subject to the rigid standards of the United States FDA.
If I previously had LASIK, R.K., or other refractive surgery, can I still have an IOL?
Yes. However, prior refractive surgery significantly impairs the IOL power selection process [link to IOL power selection]. Because of the significant surgical alteration in the corneal shape, the corneal measurements used to calculate the necessary IOL power become very inaccurate. One must often rely upon prior records from exams immediately before and after the refractive surgery. Even so, it is still very difficult to estimate the required IOL power. The further off this calculation is, the stronger the postoperative eyeglasses for distance focus will need to be.
With so many different kinds of IOLs, which one is best for me?
Your cataract surgeon is in the best position to make this decision. The patient’s input regarding preference for near or distance focus is important. However, there are many other factors to consider. These include individual risk factors and the health of the other structures inside the eye.
All of the IOL materials and designs that have been FDA approved have passed a rigorous and long-term test of optical quality, efficacy, and safety. Current IOL manufacturing is of the highest possible standard with strict quality control. The overall results are excellent with all of the IOLs currently in use in North America today. The intraocular lens is one of the most important technological advances to ever evolve – not only from ophthalmology, but also from all of medicine. Cataract patients are the winners!
