Clinical and Health Affairs
Advances in Cataract Surgery and Intraocular Lenses
By Sherman W. Reeves, M.D., M.P.H.
Abstract
Cataract surgery is one of the most common medical procedures among persons age 65 and older. Once an inpatient procedure, it has become a simple outpatient surgery with extremely low rates of morbidity. With the development of implantable intraocular lenses (IOLs), patients undergoing cataract surgery gained the benefit of corrected pre-existing refractive error, astigmatism, and presbyopia. This article reviews developments in cataract surgery and describes current IOL technology.
Age-related cataract is the leading cause of visual impairment in the United States and the world.1,2 Although cataract surgery was historically considered a last-ditch effort to prevent blindness, today’s cataract surgery with intraocular lens (IOL) implantation is a quick, painless, outpatient procedure associated with extremely low rates of surgical morbidity and rapid recovery of visual function.3 Recent advances in surgical techniques and IOL technology have allowed for highly accurate correction of pre-existing refractive error, astigmatism, and, to some extent, presbyopia, often lessening or even eliminating the patient’s dependence on spectacles postoperatively. These developments have not only improved the safety of the procedure and the outcomes for the patient, but they have also begun to blur the distinction between traditional cataract surgery and refractive surgery.
Cataract Development
The word cataract comes from the Latin word cataracta (“waterfall”), which, to the ancients, was an apt description of the clouding of the lens that occurs with an advanced cortical cataract. Although the Romans considered this change to be a suffusion occurring between the pupil and the lens, we now know that oxidative damage to and hydration changes in the lens itself are responsible for the clouding that occurs with cataracts.4 Such changes are most commonly brought about during the natural aging of the eye. In addition, trauma, drugs such as steroids and phenothiazines, and congenital insults can also result in lens opacities (Figures 1 and 2).
In its early stages, a cataract may cause only a minimal decline in visual clarity or quality, or only intermittent symptoms such as glare in bright or low lighting. As the opacity develops and the cataract grows increasingly dense, however, the eye’s refractive error may begin to change, often drifting more myopic and causing the patient’s vision to suffer noticeably. At that point, surgical removal may be the only option for restoring visual function. Although investigators continue to search for medical treatments for cataract, no study to date has shown any clear advantage of nutritional or medical treatment in preventing or reversing age-related clouding of the lens.5
A History of Cataract Surgery
Cataract surgery was first described in India around 500 BC. From that time until the mid-1700s, cataracts were surgically addressed by couching, a procedure in which the surgeon would insert a needle in the eye through the pars plana or cornea and push the lens into the vitreous cavity posteriorly, thus clearing the visual axis of the opacity. Although the resulting aphakic vision allowed the patient to see only blurry shapes, the procedure at least could restore ambulatory vision to individuals whose densely opacified crystalline lenses had left them blind. Not surprising, complication and infection rates were high.
In 1747, French surgeon Jacques Daviel attempted but failed to couch a lens. Undeterred, he used a knife and scissors to cut open the patient’s cornea along the inferior limbus, incised the lens capsule, and expressed the nucleus from the eye. His publication of a paper about the procedure the same year ushered in the era of extracting the lens rather than simply displacing it. Despite high complication rates, lens extraction techniques would continue to advance slowly over the next two centuries, substantially improving with the introduction of local anesthesia, aseptic technique, and specialized instrumentation.
By the middle of the 20th century, lens extraction was still the standard procedure for treating cataracts. Although it involved making large incisions extending almost 180 degrees around the patient’s cornea, it generally produced good results. However, the procedure was still hampered by a 5% rate of complications that could result in blindness. Further, because the eye was left without a lens, patients were required to wear unattractive high-powered hyperopic spectacles (“Coke-bottle” glasses) or contact lenses in order to see clearly after the operation.6,7 The cataract glasses rendered a good retinal image when viewing through the center of the lens, but patients’ peripheral vision was highly distorted. And while distortion was far less with contact lenses, older individuals often had trouble inserting and removing the lenses as their dexterity declined. Even through the latter part of the 20th century, cataract extraction was typically delayed until the patient had severe vision loss from the lens opacity and considered life with cataract glasses or aphakic contacts the lesser of two evils.
As early as 1795, attempts were made to place a glass lens inside the eye for correction of postcataract extraction aphakia. These early IOLs quickly dislocated into the vitreous cavity, and the procedure was soon abandoned. Sir Harold Ridley, a British ophthalmologist, is credited with the development of the modern IOL. Ridley observed that plexiglass fragments from shattered Spitfire cockpit windows could remain inert in the eyes of Royal Air Force pilots for years after the penetrating injury. He fashioned a polymethylmethacrylate lens and first implanted it after a cataract procedure in 1949. Intraocular lens development proceeded slowly over the next several decades because of high complication rates with early IOLs. Thus, most surgeons remained skeptical of IOLs through the 1970s and continued to leave eyes aphakic after surgery.8
The development of less-invasive surgical techniques, including lens extraction by ultrasound emulsification (phacoemulsification) and improved instrumentation in the late 1970s and 1980s, breathed new life into IOL development. Phacoemulsification could be conducted through incisions of 3 mm or less, greatly reducing perioperative morbidity and hastening the time to visual recovery. Concomitant advances in instrumentation allowed for a more complete removal of proinflammatory lens cortical material and for IOLs to be more safely placed.
By the early 1990s, cataract surgery had become almost exclusively an outpatient procedure with posterior chamber IOL placement as the standard of care. Today, more than a million cataract surgeries are performed annually in the United States. It is the most common surgical procedure performed in the Medicare population at an annual cost to the federal government of approximately $3.4 billion.9,10
Refractive Cataract Surgery
With modern phacoemulsification technique and IOLs, cataract patients can enjoy not only the clearing of lenticular opacity but also correction of much of the refractive error inherent to the eye at the time of surgery. In a recent study of 100 patients without concurrent ocular comorbidity undergoing routine cataract surgery with IOL placement, 96% were within 1 D of their intended postoperative target and 93% had distance visual acuity of 20/30 or better without correction.11 Thus, the patient can expect high-level visual function after the procedure.
♦ Astigmatism Correction
Since Ridley’s first intraocular lens implant, IOLs have been able to correct the spherical portion of refractive error, decreasing (or increasing, if so desired) the overall myopia or hyperopia of the eye. However, astigmatic error from the patient’s cornea often remained, necessitating spectacle correction postoperatively. Surgical techniques and specialized IOLs have been developed to decrease corneal astigmatism at the time of cataract surgery.
One approach to correcting corneal astigmatism is making limbal relaxing incisions (LRIs). By making partial thickness incisions on the steep axis of the cornea, the axis is flattened and the overall astigmatism of the cornea is reduced. LRIs are popular among surgeons because they require relatively simple instrumentation and surgical skill. Still, because the surgeon’s hand is responsible for the correction, the effect of LRIs is somewhat variable. As such, this approach is considered best for correcting a small degree of corneal astigmatism or debulking higher corneal astigmatic errors.
During the past several years, astigmatism often has been corrected with implantation of astigmatism-correcting toric IOLs. These lenses differ from traditional spherical IOLs because toricity is incorporated into the lens optic. Hash marks on the lens signify the axis of toricity and guide the surgeon in aligning the lens along the steep axis of the cornea. Small postoperative misalignment or lens rotation, however, can greatly reduce the correcting effect of these lenses. Although the first toric IOL was approved for use in the United States in 1998, problems with postoperative rotational stability limited its acceptance by surgeons. The release of the Acrysof Toric IOL in 2006 marked a significant advance in toric IOL technology. The relatively sticky acrylic material and flexible stabilization arms of the new lens improved its rotational stability over the slippery silicone of the older design. In an FDA trial, the average rotation of the lens was less than 4 degrees, with 97% of lenses rotating 15 degrees or less after 6 months.12,13 In addition, 97% of patients who were bilaterally implanted in the FDA trial no longer needed spectacles for distance vision versus 50% of control patients who were bilaterally implanted with monofocal spherical IOLs.13
The advent of the excimer laser offered an additional option for correction of corneal astigmatic and spherical error after cataract surgery. The excimer, applied in either the laser-assisted in-situ keratomileusis (LASIK) procedure or by photorefractive keratectomy, offers the most precise avenue for correction of refractive error including astigmatism at the corneal plane. It also allows for the fine-tuning of the refractive result after IOL implantation. However, this additional procedure is expensive and must be done several months after the cataract surgery wounds have fully healed.
♦ Presbyopia Correction
Although the majority of cataract patients have been presbyopic for years, the removal of the crystalline lens during cataract surgery and implantation of a distance-vision targeted monofocal IOL results in complete dependence on reading glasses for clear near vision. As such, the ophthalmic community has sought to develop IOLs that provide an increased range of clear vision and relieve the symptoms of pseudophakic presbyopia. Within the past decade, manufacturers have been working on multifocal IOLs and accommodative IOLs to correct presbyopia.
Adding multiple optical zones to an intraocular lens is one way to correct both distance and near vision with a single lens implant. Although this technology has been effective for correcting both myopia and presbyopia, all multifocal lenses present more than one image to the retina. Thus, multifocal IOLs may deliver an increased range of vision, but they also are invariably associated with some degree of unwanted visual phenomena such as glare and halos. The challenge for lens designers has been to increase quality across a range of focal lengths while decreasing unwanted side effects.
Several multifocal presbyopia IOLs are now available in the United States. The newest of these, the Tecnis Multifocal, which received FDA approval in January of 2009, uses multizone diffractive optics to create multifocality, splitting the incoming light evenly into a near and a distant focal point. Of the patients who took part in the FDA trial of the Tecnis Multifocal IOL, 94% achieved simultaneous near vision of 20/32 (J2) and distance vision of 20/25 with best-distance correction in place.14 Clinical studies in Europe, where the Tecnis Multifocal has been in use for several years, suggest that near acuity is enhanced with the compromise of some intermediate vision but that patient satisfaction was high and that most did not need spectacles after bilateral implantation with the lens.15
Accommodating IOLs offer another strategy for presbyopia correction after cataract surgery. These lenses attempt to provide enhanced near-focus capabilities by changing shape or position in the eye when a near object is viewed. Currently, only one accommodating IOL, the Crystalens, is FDA-approved for use in the United States. The Crystalens has a unique hinged haptic design that allows the lens to vault forward during contraction of the ciliary muscle. Initially approved for use in 2004, early versions of the lens performed well at distant and intermediate ranges, adding approximately 1 to 1.5 D of increased near power over that of monofocal IOLs. However, only 57% of subjects in the original FDA trial reported being able to read a newspaper without spectacles, a less-favorable result than that produced by current-generation multifocal IOLs.16 A recent change to the design of the lens in 2008 incorporated an additional power zone in the middle to help improve near vision performance. The hinged haptics of the IOL, however, add some degree of instability to the lens after surgery. As the capsular bag contracts around the lens, its position in the eye can occasionally change enough to cause clinically significant shifts in refractive error. YAG laser capsulotomy can often relax these capsular contraction forces, and excimer laser vision correction is sometimes needed to eliminate the induced refractive error.
Summary
Cataract surgery is one of the oldest surgeries recorded in human history. Thanks to technological advances during the last 50 years, it is now a highly successful procedure with a remarkably low rate of complications. Recent advances in IOL and surgical technology have decreased or even eliminated the need for spectacle correction postoperatively. Although the primary goal of cataract extraction is to clear the visual axis of opacity and restore visual function, the correction of refractive errors has been an additional benefit to patients who wish to no longer depend on spectacles for distance and near vision. MM
Sherman Reeves is a cornea, cataract, and refractive surgeon with Minnesota Eye Consultants in Minneapolis and an adjunct assistant professor of ophthalmology at the University of Minnesota.
References
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