thumb|A phakic IOL

An intraocular lens (IOL) 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 (myopia) and far-sightedness (hyperopia); 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 (or false 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 IOLs were originally made of a rigid material (PMMA), although this has largely been superseded by the use of flexible materials, such as silicone.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 (EDOF) lenses.

As of 2021, nearly 28 million cataract procedures took place annually worldwide. That is about 75,000 procedures per day globally.

Phakic IOLs appear to be lower risk than excimer laser surgery (LASIK) in those with significant near-sightedness.

More commonly, IOLs are implanted via Clear Lens Extraction And Replacement (CLEAR) surgery, also known as refractive lens exchange (RLE)

Multifocal

Multifocal IOLs attempt to provide simultaneous viewing of distance vision and near vision. Trifocal IOLs can provide intermediate vision in addition. Many multifocal IOL designs attempt to achieve this simultaneous viewing focus using a concentric ring design, which alternates distance and near focal points. However, concentric ring multifocal lenses are prone to glare and mildly compromised focus at all ranges of vision.

People who have a multifocal IOL after their cataract is removed may be less likely to need additional glasses compared with people who have standard monofocal lenses.

The most common adverse visual effects from multifocal IOLs include glare, halos (rings around lights), and a loss of contrast sensitivity in low-light conditions.

Adjustable lens

An adjustable IOL is unlike any other lens as its prescription power can be adjusted after surgery once all healing is complete. All other IOLs require surgeons to use pre-surgery measurements to determine a patient's post-surgery lens power. The drawback of this is that pre-surgery measurements are taken while a patient still has cataracts, and they cannot account for minuscule shifts that occur during healing. An adjustable IOL allows surgeons to implant it and then, once healing is complete, use an ultraviolet light delivery device to fine tune it until it suits the patient. An early example being the (RxSight) Light Adjustable Lens (LAL).

The eyes and lenses must not be exposed to random ultraviolet light before and during the adjustment process, and protective glasses must be worn from the operation until the lens is locked. When the eye has healed, which is usually 2 to 4 weeks after IOL implantation, refraction adjustment is done. A prescription is formulated for the patient and customization of the IOL's refractive power is done by exposing either the centre or the periphery of the lens to a metered dose of UV light, with the help of a contact lens on the cornea. This exposed part will swell slightly, adjusting the lens surface curvature. Meridional exposure can be done in much the same way to correct for astigmatism. Several exposures can be made for fine tuning, spaced over several days. Once the lens has been optimised a final exposure of the whole lens is made to lock the changes, after which the lens will no longer be adjustable and can be used outdoors.

Accommodating

Some newer lens designs attempt to allow the eye to regain some ability to change focus from distance to near (accommodation). However, many accommodating IOLs used today only achieve very limited improvements in near vision which reduce over time. PCO is a common side-effect of many cataract surgeries and is easily treatable with a one-time laser capsulotomy procedure.

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 nerves and musculature 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.

An accommodating lens made by Eyeonics, Astigmatism can also be treated with limbal relaxing incisions or an excimer laser procedure. About 40% of Americans have significant astigmatism and thus may be candidates for a toric IOL.

Extended depth-of-focus

Extended depth-of-focus (EDOF) is an intraocular lens technology for treating presbyopia. Where multifocal IOLs have two or more focal points, EDOF lenses form a single elongated focal point to enhance depth of focus. The intention is to reduce glare, halos, and other photic phenomena which occur with multifocal IOLs. A possible drawback is decrease in image quality due to aberrations.

  • Angle-supported PIOLs, placed in the anterior chamber. They are notorious for their negative impact on the corneal endothelial lining, which is vital for maintaining a healthy clear cornea.
  • Iris-fixated PIOLs, attached by claws to the mid-peripheral iris by a technique called enclavation. It is believed to have a lesser effect on corneal endothelium. The main complication with this type is their tendency to cause endothelial cell reduction.
  • Sulcus-supported PIOLs, placed in the posterior chamber in front of the natural crystalline lens. This type of PIOLs is gaining more and more popularity. They have special vaulting so as not to be in contact with the normal lens. The main complication with older versions was a small possibility of cataract formation.

In 2006, a centrally perforated ICL (i.e., the Hole-ICL) was created to improve aqueous humour circulation.

Blue-light filtering IOLs

Blue-light filtering IOLs filter the UV and high-energy blue light present in natural and artificial light, both of which can cause vision problems; however, too much filtering of blue light can increase depression, especially in the winter months (SAD). 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 in order to attempt to protect the retina after cataract extraction of the natural crystalline lens.

A Cochrane Review found little evidence of important differences between blue‐light filtering and non‐blue‐light filtering lenses for protecting the macula (back of the eye) after cataract surgery. This may have been due to studies being too small and too short‐term to provide reliable results.

Posterior capsule opacification

thumb|A posterior capsular opacity (PCO) around a posterior chamber IOL (as seen on retroillumination in a [[slit lamp)]]

Posterior capsule opacification (PCO), often referred to as "after cataract", is the most common complication of cataract surgery.

In a large percentage of patients (20-40%), posterior chamber intraocular lenses may form PCOs a few months after implantation. They are easily treatable, and typically only require a one-time capsulotomy procedure (using a Nd:YAG laser) to clarify.

Materials

thumb|Acrylic MICS-IOL in holder

The materials that have been used to manufacture intraocular lens implants include poly(methyl methacrylate) (PMMA), silicone, hydrophobic acrylate, hydrophilic acrylate and collamer. PMMA was the first material to be used successfully in intraocular lenses.

Advances in technology have brought about the use of silicone and acrylic polymers, both of which are soft foldable inert materials. This allows the lens to be folded and inserted into the eye through a smaller incision.

Selection of refractive power

The appropriate refractive power of the IOL is selected, much like a spectacle lens prescription, to provide the desired refractive outcome. Pre-operative measurements, including corneal curvature, axial length, and white-to-white measurements are used to estimate the required power of the IOL. These methods include several formulae, including Hagis, When writing his memoirs in the 1790s, Giacomo Casanova likely heard of Casaamata's attempt, because Casanova falsely claimed that oculist Felice Tadini told him the idea of intraocular lenses in Warsaw in 1766, but Casanova's claim is false, because Tadini was in Constantinople in 1766. That lens was manufactured by the Rayner company of Brighton, East Sussex, England from Perspex CQ polymethylmethacrylate (PMMA) made by ICI (Imperial Chemical Industries). Ridley had 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, and deduced that the transparent material was inert and useful for implantation in the eye.

Makers of IOLs

  • Alcon (US)
  • Eyekon (US)
  • Bausch & Lomb (US)
  • Johnson & Johnson (US)
  • Rayner (UK)
  • Morcher (Germany)
  • ZEISS (Germany)

See also

References

Further reading