From Wikipedia, the free encyclopedia - View original article
An intraocular lens (IOL) is a lens implanted in the eye used to treat cataracts or myopia. The most common type of IOL is the pseudophakic IOL. These are implanted during cataract surgery, after the cloudy crystalline lens (otherwise known as a cataract) has been removed. The pseudophakic IOL replaces the original crystalline lens, and provides the light focusing function originally undertaken by the crystalline lens. The second type of IOL, more commonly known as a phakic intraocular lens (PIOL), is a lens which is placed over the existing natural lens, and is used in refractive surgery to change the eye's optical power as a treatment for myopia or nearsightedness. IOLs usually consist of a small plastic lens with plastic side struts, called haptics, to hold the lens in place within the capsular bag inside the eye. IOLs were traditionally made of an inflexible material (PMMA), although this has largely been superseded by the use of flexible materials. Most IOLs fitted today are fixed monofocal lenses matched to distance vision. However, other types are available, such as multifocal IOLs which provide the patient with multiple-focused vision at far and reading distance, and adaptive IOLs which provide the patient with limited visual accommodation.
Insertion of an intraocular lens for the treatment of cataracts is the most commonly performed eye surgical procedure. Surgeons annually implant more than 6 million lenses The procedure can be done under local anesthesia with the patient awake throughout the operation. The use of a flexible IOL enables the lens to be rolled for insertion into the capsule through a very small incision, thus avoiding the need for stitches, and this procedure usually takes less than 30 minutes in the hands of an experienced ophthalmologist. The recovery period is about 2–3 weeks. After surgery, patients should avoid strenuous exercise or anything else that significantly increases blood pressure. They should also visit their ophthalmologists regularly for several months so as to monitor the implants.
IOL implantation carries several risks associated with eye surgeries, such as infection, loosening of the lens, lens rotation, inflammation and night time halos, but a systematic review of studies has determined that the procedure is safer than conventional laser eye treatment. Though IOLs enable many patients to have reduced dependence on glasses, most patients still rely on glasses for certain activities, such as reading.
|This section does not cite any references or sources. (March 2010)|
Sir Harold Ridley was the first to successfully implant an intraocular lens on 29 November 1949, at St Thomas' Hospital at London. That first intraocular lens was manufactured by the Rayner company of Brighton, East Sussex, England from Perspex CQ Polymethylmethacrylate (PMMA) made by ICI (Imperial Chemical Industries). It is said the idea of implanting an intraocular lens came to him after an intern asked him why he was not replacing the lens he had removed during cataract surgery. The acrylic plastic material was chosen because Ridley noticed it was inert after seeing RAF (Royal Air Force) pilots of World War II with pieces of shattered canopies in their eyes (this acrylic resin is known by several trade names including Lucite and Plexiglas). The intraocular lens did not find widespread acceptance in cataract surgery until the 1970s, when further developments in lens design and surgical techniques had come about. By the 21st century, more than a million IOLs are implanted annually in the United States.
|This section does not cite any references or sources. (March 2010)|
|This section requires expansion with: reorganization, clarifications, better differentiation between the practices of different countries, and specific differences between PMMA, silicone, and silicone acrylate. (January 2008)|
The various materials which have been used to manufacture intraocular lens implants include polymethylmethacrylate (PMMA), silicone, hydrophobic acrylate, hydrophilic acrylate and collamer.Polymethylmethacrylate (PMMA) was the first material to be used successfully in intraocular lenses. British ophthalmologist Sir Harold Ridley observed that Royal Air Force pilots who sustained eye injuries during World War II involving PMMA windshield material did not show any rejection or foreign body reaction. Deducing that the transparent material was inert and useful for implantation in the eye, Ridley designed and implanted the first intraocular lens in a human eye.
Advances in technology have brought about the use of silicone and acrylic, both of which are soft foldable inert materials. This allows the lens to be folded and inserted into the eye through a smaller incision. Specifically, acrylic lenses are a better choice in people who have a history of uveitis, or are likely to have to undergo retinal surgery requiring vitrectomy with replacement by silicone oil, such as persons with proliferative diabetic retinopathy or who are at high risk of retinal detachment, such as persons with high myopia.
The trademarked "Natural Yellow" material is available in three hydrophilic IOLs. Dr. Patrick H. Benz of Benz Research and Development created the first IOL material to incorporate the same UV-A blocking and violet light filtering chromophore that's present in the human crystalline lens. This breakthrough material provides the exact chromophore the human retina has already specified for light protection.
Other IOLs include:
The root of these words comes from the Greek word phakos 'lens'.
Phakic IOLs are implanted without removal of the patient's original crystalline lens, and this is performed solely to correct refractive error in the presence of a clear crystalline lens.
Aphakic IOLs generally refer to lenses implanted secondarily in an eye already aphakic from previous surgery or trauma some time ago.
Pseudophakic IOLs refer to lenses implanted during cataract surgery, as a sequential step after removal of the cataractous lens of the patient.
Many aphakic and pseudophakic IOLs such as anterior chamber IOLs or 3 piece posterior chamber IOLs can be used interchangeably. The exception are one piece IOLs, which must be placed within the capsular bag at the time of cataract surgery and hence cannot be used as secondary implants.
In the United States, a new category of intraocular lenses was opened with the approval by the Food and Drug Administration in 2003 of multifocal and accommodating lenses. These come at an additional cost to the recipient beyond what Medicare will pay and each has advantages and disadvantages.
New FDA-approved multifocal intraocular lens implants allow most post operative cataract patients the advantage of glass-free vision. These new multifocal lenses are not a covered expense under most insurance plans (In the United States, Medicare decided to stop covering them in May 2005) and can cost the patient upwards of $2800 per eye. Latest advances include IOLs with square-edge design, non-glare edge design and yellow dye added to the IOL.
Multifocal IOLs – provide for simultaneous viewing of both distance vision and near vision. Some patients report glare and halos at night time with these lenses.
Accommodating IOLs – allow for both distance vision and midrange near vision. These IOLs are typically not as strong for closer vision as the multifocal IOLs.
To incorporate the strengths of each type of IOL, some eye surgeons recommend using a multifocal IOL in one eye to emphasize close reading vision and an accommodating IOL in the other eye for further midrange vision. This is called "mix and match." Distance vision is not compromised with this approach, while near vision is optimized.
One of the major disadvantages of conventional IOLs is that they are primarily focused for distance vision. Though patients who undergo a standard IOL implantation no longer experience clouding from cataracts, they are unable to accommodate, or change focus from near to far, far to near, and to distances in between. Accommodating IOLs interact with ciliary muscles and zonules, using hinges at both ends to “latch on” and move forward and backward inside the eye using the same mechanism as normal accommodation. These IOLs have a 4.5-mm square-edged optic and a long hinged plate design with polyimide loops at the end of the haptics. The hinges are made of an advanced silicone called BioSil that was thoroughly tested to make sure it was capable of unlimited flexing in the eye. There are many advantages to accommodating IOLs. For instance, light comes from and is focused on a single focal point, reducing halos, glares, and other visual aberrations. Accommodating IOLs provide excellent vision at all distances (far, intermediate, and near), project no unwanted retinal images, and produce no loss of contrast sensitivity or central system adaptation. Accommodating IOLs have the potential to eliminate or reduce the dependence on glasses after cataract surgery. For some, accommodating IOLs may be a better alternative to refractive lens exchange (RLE) and monovision.
The FDA approved Eyeonics Inc.’s accommodating IOL, Crystalens AT-45, in November 2003. Bausch & Lomb acquired Crystalens in 2008 and introduced a newer model called Crystalens HD in 2008. Crystalens is the only FDA-approved accommodating IOL currently on the market and it is approved in the United States and Europe.
Studies and peer reviews
In a September 2004 FDA trial involving 325 patients:
Generally, patients over 50 with cataract problems and no serious eye diseases are good candidates for the procedure. The patient must have functional ciliary muscles or zonules for haptics positioning. In addition, the pupils must dilate adequately, as the IOL will induce glares in low-light environments if the pupils dilate too large. Accommodating IOLs are beneficial not only for patients with cataracts, but also those who wish to reduce their dependency on glasses and contacts due to myopia, hyperopia and presbyopia.
Post-operative care is similar to that of normal IOLs. However, patients must include ophthalmologic exercises such as puzzles and word games as a part of their daily regimen in order to tone up their ciliary muscles and attain the maximum benefit from the accommodating lenses. These exercises should be done consistently for 3–6 months and the patient's performance monitored by their eye care professional.
Other promising multifocal/accommodating IOLs currently in clinical trials include Accommodative 1CU (HumanOptics, Erlangen, Germany), Smartlens (Medennium, Irvine, CA), and dual optic accommodating lenses such as Sarfarazi (Bausch and Lomb, Rochester, NY) and Synchrony (Abbott Laboratories, Abbott Park, IL).
Synchrony IOL is anticipated to receive FDA approval in 2012.
|This section does not cite any references or sources. (March 2010)|
Intraocular lenses have been used since 1999 for correcting larger errors in myopic (near-sighted), hyperopic (far-sighted), and astigmatic eyes. This type of IOL is also called phakic intraocular lens (PIOL), and the crystalline lens is not removed.
More commonly, aphakic IOLs (that is, not PIOLs) are implanted via Clear Lens Extraction and Replacement (CLEAR) surgery. During CLEAR, the crystalline lens is extracted and an IOL replaces it in a process that is very similar to cataract surgery: both involve lens replacement, local anesthesia, both last approximately 30 minutes, and both require making a small incision in the eye for lens insertion. People recover from CLEAR surgery 1–7 days after the operation. During this time, they should avoid strenuous exercise or anything else that significantly raises blood pressure. They should also visit their ophthalmologists regularly for several months so as to monitor the IOL implants. CLEAR has a 90% success rate (risks include wound leakage, infection, inflammation, and astigmatism). CLEAR can only be performed on patients ages 40 and older. This is to ensure that eye growth, which disrupts IOL lenses, will not occur post-surgery.
Once implanted, IOL lenses have three major benefits. First, they are an alternative to LASIK, a form of eye surgery that does not work for people with serious vision problems. Effective IOL implants also entirely eliminate the need for glasses or contact lenses post-surgery for most patients. The cataract will not return, as the lens has been removed. The disadvantage is that the eye's ability to change focus (accommodate) has generally been reduced or eliminated, depending on the kind of lens implanted.
Two types of PIOLs have been approved by FDA. The VisianICL (Visian Implantable Collamer Lens), (FDA approved in 2004), produced by Staar Surgical Company and Artisan Myopia lens (FDA approved in 2004), produced by Ophtec USA Inc. The VisianICL is made of collamer, a biocompatible material, and the Artisan is a plastic lens.
Some of the risks that FDA have been found so far during a three-year study of the Artisan are:
Other risks include:
One of the causes of the risks above is that the lens can rotate inside the eye, if the PIOL is too short if the eye was incorrectly measured, or because the sulcus has a slightly oval shape (the height is slightly smaller than the width). Toric IOLs must be powered and aligned inside the eye on a meridian that corrects the patient's preexisting astigmatism. Again, these lenses can rotate inside the eye postoperatively, or be placed incorrectly by the operating surgeon. Either way, the patient's preexisting astigmatism may not be corrected completely or may even increase. When IOLs are implanted in the aphakic patient—either after clear lensectomy for the correction of refractive error or after cataract extraction to restore eyesight—astigmatism in these patients is mainly attributable to the cornea. The surgeon can ascertains the astigmatic, or steepest, meridian in a number of ways, including manifest refraction or corneal topography. Manifest refraction is the familiar test where the eye doctor rotates various lenses in front of the eye, asking the patient, "Which is better (or clearer), this one or this one?" Corneal topography is considered a more quantitative test, and for purposes of aligning a toric IOL, most surgeons use a measurement called simulated keratometry (SimK), which is calculated by the internal programming of the corneal topography machine, to determine the astigmatic meridian on the surface of the cornea. The astigmatic meridian can also be identified using corneal wavefront technology or paraxial curvature matching.
Recently, Alpins et al. described a vector analysis algorithm, which they call CorT, to quantify corneal topographic astigmatism. The authors compared CorT with manual keratometry, SimK, corneal wavefront, and paraxial curvature matching. CorT proved to be significantly more reliable (less variable with smaller standard deviation) than the other approaches. The magnitude of ocular residual astigmatism (ORA) using CorT was the least and its magnitude was closest to refractive cylinder than all other parameters examined.
CorT can provide a reliable meridian (and magnitude) for the alignment (and power) of a toric IOL. The current standard practice amounts to estimating the meridian from multiple differing inputs (e.g., SimK, manual keratometry, IOL master keratometry, and Lenstar), which often differ in the meridian identified as “steepest.” CorT provides the most reliable orientation of the steep meridian and the most effective magnitude of the corneal astigmatism.
The variability of SimK values has been a point of frustration in the field in regard to the alignment of toric IOLs. Alpins expects that CorT will replace the SimK value, which has been the standard measure since the inception of Placido ring topography technology. This is described in greater detail in the corneal topography section.
As with IOLs inserted after cataract extraction, phakic IOLs (PIOLs) can be either spheric or toric. The difference is that they are placed in an eye that retains the natural human crystalline lens. As with aphakic eyes, toric PIOLs have to be aligned with the meridian of astigmatism; toric IOL misalignment or rotation can lead to residual or even greater astigmatism postoperatively.
Depending on their placement site in the eye, PIOLs can be divided into: