Intraocular Lenses for Cataract Surgery 2

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. Several studies have shown that the Alcon Acrysof and the Allergan silicone lenses are associated with less frequent clouding of the posterior capsule. Although this is not a serious problem, it is a slight advantage.

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.

The toric IOL - an IOL that corrects astigmatism:

People wear eyeglasses if their eyes are not in good focus naturally. Such eyes are said to have "refractive error". Besides myopia (nearsightedness) and hyperopia (farsightedness), the third major category of refractive error is called astigmatism. This blur results from a cornea that is not perfectly spherical in shape. Unlike eyeglasses, standard IOLs cannot correct astigmatism. Thus, the more astigmatism a patient has, the more dependent they will be upon eyeglasses, regardless of whether or not they have had cataract/IOL surgery.

In 1999, the first IOL to correct astigmatism gained FDA approval, and became available for use in the United States. This category of lens implant is called a toric IOL and is manufactured by the American company, Staar Surgical. Unlike near or farsightedness, astigmatism is the only category of blur which has a specific orientation to it, called the "axis". A lens which corrects astigmatism - whether eyeglasses or an IOL - also has an axis. It must be aligned so that its axis perfectly matches the axis of the patient's astigmatism. If the corrective lens becomes rotated out of proper alignment, the vision becomes blurred.

Like other posterior chamber IOLs, the toric IOL is implanted into the capsular bag. It has a different shape, however, in order to prevent it from later rotating out of proper alignment. Unlike the conventional three-piece posterior chamber IOL design, this particular shape, called a plate haptic IOL design, has four corners. Since the capsular bag is round, the squared corners prevent rotation once the capsular bag eventually contracts around it during the first postoperative week.

Immediately after cataract surgery, the capsular bag is still loose and baggy. Thus, approximately 5-10% of these IOLs may rotate out of acceptable alignment during these first few days postoperatively. For these patients, a tiny instrument inserted into the eye can rotate the lens back into proper and permanent alignment. Although this may need to be done in the operating room, it is usually a fairly minor manipulation compared to the original cataract surgery.

As with other types of refractive error such as myopia (nearsightedness) and hyperopia (farsightedness), different patients possess varying amounts of astigmatism. The toric IOL may not correct all of the astigmatism, particularly with severe amounts. Eyeglasses can correct whatever residual amount remains. However, the less the remaining amount is, the better the patient is able to see without glasses. In February, 1999, Dr. Chang became the first Bay Area ophthalmologist to implant the toric IOL.

The ARRAY multifocal IOL:

Standard IOLs are called "monofocal" lenses, because they set the focus at a single distance. Since it is a single lens, the monofocal IOL cannot provide the ability to see both in the distance, and up close without glasses. Over time, several attempts have been made to engineer an artificial lens that can provide both focal distances. Such a lens is called a "multifocal" IOL, because it provides more than one focal distance without glasses. Most of these initial design attempts did not prove viable.

Finally, in 1997, the FDA approved the ARRAY multifocal foldable IOL, manufactured by Allergan (Irvine, California). In December, 1997, Dr. Chang became the first Northern California ophthalmologist to implant the ARRAY multifocal IOL. The three-piece design and material of this IOL is identical to that of the most commonly implanted silicone IOL (also manufactured by Allergan). It therefore maintains all of the advantages of conventional foldable IOLs - allowing implantation through an extremely small incision, and providing the same optical quality and safety of this proven material. However, the optic is designed very differently in order to produce a dual focus.

The multifocal IOL optic is also completely different from the design of bifocal eyeglasses where a horizontal line separates the distance half and the reading half. Instead, the center of the multifocal IOL optic is used for distance focus. The periphery of the optic provides near focus. Since most of the lens is set for distance, it performs just as well as a monofocal IOL in providing good distance vision either with or without glasses.

With a monofocal IOL set for far distance, the close vision is quite blurred until the patient dons reading glasses or bifocals. The multifocal IOL, however, is able to provide some focus for near without glasses. This reading ability is further enhanced when patients have a multifocal IOL implanted in both eyes. Because the reading area is in the periphery of the optic, patients must have sufficiently large enough pupils to capitalize on the near focus capability of this system.

Because the multifocal IOL is distant dominant, most patients will still find it easier to read with glasses. Patients should therefore not expect this system to eliminate the need for eyeglasses. However, their ability to see at near without glasses will be better than if they had received a standard monofocal IOL in each eye set for far distance. This typically provides the convenience of being able to read many things (e.g. letters, receipts, price tags, and menus) without glasses. In situations with dimmer illumination, small print, or prolonged reading, reading glasses will allow patients with the multifocal IOL to read just as well as those with standard monofocal IOLs using reading glasses.

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