Intraocular lens
An intraocular lens is a lens implanted in the eye usually as part of a treatment for cataracts or for correcting other vision problems such as near-sightedness and far-sightedness ; a form of refractive surgery. If the natural lens is left in the eye, the IOL is known as phakic, otherwise it is a pseudophakic lens. Both kinds of IOLs are designed to provide the same light-focusing function as the natural crystalline lens. This can be an alternative to LASIK, but LASIK is not an alternative to an IOL for treatment of cataracts.
IOLs usually consist of a small plastic lens with plastic side struts, called haptics, to hold the lens in place in the capsular bag inside the eye. IOLs were originally made of a rigid material, although this has largely been superseded by the use of flexible materials, such as silicone. Most IOLs fitted today are fixed monofocal lenses matched to distance vision. However, other types are available, such as a multifocal intraocular lens that provides multiple-focused vision at far and reading distance, and adaptive IOLs that provide limited visual accommodation. Multifocal IOLs can also be trifocal IOLs or extended depth of focus lenses.
As of 2021, nearly 28 million cataract procedures took place annually worldwide. That is about 75,000 procedures per day globally. The procedure can be done under local or topical anesthesia with the patient awake throughout the operation. The use of a flexible IOL enables the lens to be rolled for insertion into the capsular bag through a very small incision, thus avoiding the need for stitches. This procedure usually takes less than 30 minutes in the hands of an experienced ophthalmologist, and the recovery period is about two to three weeks. After surgery, patients should avoid strenuous exercise or anything else that significantly increases blood pressure. They should visit their ophthalmologists regularly for three weeks to monitor the implants.
IOL implantation carries several risks associated with eye surgeries, such as infection, loosening of the lens, lens rotation, inflammation, nighttime halos and retinal detachment. Though IOLs enable many patients to have reduced dependence on glasses, most patients still rely on glasses for certain activities, such as reading. These reading glasses may be avoided in some cases if multifocal IOLs, trifocal IOLs or EDOF lenses are used.
Medical uses
Intraocular lenses have been used since 1999 for correcting larger errors in near-sighted, far-sighted, and astigmatic eyes. This type of IOL is also called phakic intraocular lens, as it is implanted without removing the patient's natural crystalline lens.Phakic IOLs appear to be lower risk than excimer laser surgery in those with significant near-sightedness.
More commonly, IOLs are implanted via Clear Lens Extraction And Replacement surgery, also known as refractive lens exchange or clear lens extraction. 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, last approximately 30 minutes, and 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 visit their ophthalmologists regularly for several weeks to monitor the IOL implants.
CLEAR has a 90% success rate. CLEAR can be performed only on patients ages 40 and older. This is to ensure that eye growth, which disrupts IOL lenses, will not occur post-surgery.
Once implanted, IOLs have three major benefits. First, they are an alternative to the excimer laser procedure, a form of eye surgery that does not work for people with serious vision problems. Effective IOL implants also eliminate the need for glasses or contact lenses post-surgery for most patients. Cataracts will not appear or return, as the lens has been removed. The disadvantage is that the eye's ability to change focus has generally been reduced or eliminated, depending on the kind of lens implanted.
Some of the risks that were found in the early 2000s during a three-year study of the Artisan lenses were:
- a yearly loss of 1.8% of the endothelial cells,
- 0.6% risk of retinal detachment,
- 0.6% risk of cataract, and
- 0.4% risk of corneal swelling.
- 0.03–0.05% eye infection risk, which in worst case can lead to blindness.
- glaucoma,
- astigmatism,
- remaining near- or far-sightedness,
- rotation of the lens inside the eye one or two days after surgery.
When standard IOLs are implanted with a CLEAR procedure, in substitution of the patient's crystalline, astigmatism is typically not corrected, as astigmatism is mainly attributable to a deformation of the cornea. Toric IOLs may be used during the CLEAR procedure to correct astigmatism.
The surgeon can ascertain 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 lenses in front of the eye, asking the patient, "Which is better, 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, 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.
Indications for refractive lens exchange
Severe myopia or hyperopia with coexisting presbyopia are the primary indicators for refractive lens exchange, as RLE leads to complete loss of accommodation. Underlying regular astigmatism can also be managed by RLE, even beyond the scope of corneal incisional techniques, by toric lens implants. Marginal indications for RLE are presbyopia without ametropia, using a multifocal lens implant, presbyopia with underlying astigmatism, and prepresbyopoa hyperopia of from +5 to +10 D not amenable to keratorefractive surgery or phakic IOL due to a shallow anterior chamber.Complications
Complications of RLE are similar to those after cataract surgery, but with the difference that RLE is often used in very short or very long eyes and patients' ages tend to be significantly lower, so consideration must be given to longer-term effects.Type of surgery
Implants can be used with or without removal of the natural crystalline lens:- Phakia is the presence of the natural crystalline lenses. Phakic IOL refers to an intraocular lens implanted without removal of the original crystalline lens, and this is performed solely to correct refractive error.
- Aphakia is the absence of the natural crystalline lens. The aphakic state is usually due to surgery to remove a cataractous lens, but post-surgical aphakia is rare nowadays because of the ubiquity of intraocular lenses. Rarely, aphakia can be post-traumatic or congenital in nature. Aphakic IOL refers to a lens implanted secondarily in an eye already aphakic from previous surgery or trauma.
- Pseudophakia is the substitution of the natural crystalline lens with an IOL, as is often done after cataract extraction or less often to correct major refractive error. Pseudophakic IOL refers to a lens implanted during surgery, immediately after removal of the patient's crystalline lens.
Location of implant
- Posterior chamber IOL. This is by far the most common type of implanted lens after cataract surgery, as this is the natural and optimum position for a lens.
- Anterior chamber IOL. A less-common type of intraocular lens, which is sometimes used if a PCIOL is not an option for a patient or if the situation requires a phakic IOL.
Pseudophakic IOLs