When the natural lens of the eye opacifies, an ophthalmologist surgically extracts the opacified lens or cataract and inserts a clear, artificial lens, called an intraocular lens implant (IOL). Intraocular lens implants take the place of and function like the natural lenses to focus light to the back of the eye. IOLs have been available since the mid-1960s, but the United States Food and Drug Administration (FDA) did not approve of artificial lens implants until 1981. The implants consist of three key components — a round or ovoid optic and two arc-shaped holding arms, called haptics. Intraocular lens implants are made of various materials, including acrylic, silicone, and polymethylmethacrylate.
Manufacturers vary the design of intraocular lenses for implantation in different anatomic areas of the eye. The older implants, called anterior chamber implants, sit anterior to the iris, the colored part of the eye. In the vast majority of cataract surgeries within the past two decades, surgeons typically elect to insert posterior chamber implants. Placed behind the iris, these lenses sit in the elastic capsule that once enclosed the cataractous lens. When the capsular bag is torn or unstable, surgeons position the posterior chamber lens so that the holding arms fit into the angle anterior to the capsular bag called the sulcus.
Most standard intraocular lens implants are monofocal, meaning that they provide clear vision for one distance. Without glasses, patients may see well in the far distance, intermediate distance, or close range. Patients with monofocal implants who see well for distance invariably need glasses for reading. Advanced accommodating and multifocal implants provide the potential of seeing well at more than one distance, without glasses or contacts, but most health insurance plans, including Medicare, do not cover the additional cost for the premium IOLs required for simultaneously clear distance and near vision.
Astigmatism is a focusing error of the eye that produces distortion of the image. It is characterized by an oval-shaped curve to the transparent window on the front of the eye, the cornea. Traditional intraocular lens implants do not correct astigmatism. The FDA has approved two styles of toric IOLs, which correct from 1.5 to 3.00 diopters of astigmatism. A risk of using a toric implant is poor vision resulting from rotation of the implant within the eye, which necessitates additional surgery to reposition the lens.
Traditional intraocular lens implants are spherical, meaning their front surface is uniformly curved. Aspheric IOLs are somewhat flattened in the periphery, a modification that enhances contrast sensitivity. Contrast sensitivity enables a patient to perform better visually in dim lighting, particularly pertinent at dusk. Other implants absorb ultraviolet and blue light, which have been implicated in causing macular degeneration, a degradative process affecting the center part of the retina with age. The cataract surgeon will typically counsel each patient in the advantages and disadvantages of the implant designs and recommend an implant for each patient.
