Most candidates for intraocular refractive surgery have extreme myopia,hyperopia, and/or significant presbyopia. For such patients, laser ablation of corneal tissue may not be the best option. The risk of corneal instability, dry eye, and bothersome visual side effects make excimer therapy suboptimal for high refractive errors. Although cornea-based procedures have dominated the refractive surgery market, newer intraocular techniques are presenting options for select patients seeking independence from glasses and contact lenses.
All intraocular procedures utilize implantation of an artificial lens to ideally focus light. The practice of lens implantation was first developed for cataract surgery. Lens implantation has multiple potential advantages over corneal refractive procedures, including:
- accuracy for extreme refractive errors,
- fewer visual side effects, and
- preservation of normal corneal architecture.
Intraocular lenses are now used in conjunction with the patient’s own natural lens (phakic intraocular lens) or as a replacement for it.
Phakic Intraocular Lens
The first intraocular lens (Verisyse™) for use without removal of the crystalline lens gained FDA approval in September 2004. The implant corrects 5 to 20 diopters of myopia by affixing the rigid lens to the anterior iris. Results have been excellent with 85% of participants seeing 20/40 or better (uncorrected) at 6 months postoperatively. Patients retain the ability to accommodate naturally, and some have demonstrated better corrected vision than was possible preoperatively.
Another phakic intraocular lens, the Visian ICL™ (Implantable Collamer Lens), is designed for placement behind the iris. It is inserted with a folding technique that minimizes incision size and potential astigmatism. FDA approval data for myopia between 3 and 20 diopters showed that at 3 years postoperatively 59% had 20/20 or better, and 95% had 20/40 (uncorrected) acuities. Less than 1% lost greater than or equal to 2 lines of best corrected visual acuity.
While phakic intraocular lens technologies have brought surgical vision correction to patients with extreme refractive errors, some important considerations deserve mention. The Verisyse™ requires a peripheral corneal incision of at least 5 millimeters, which is closed with sutures. The incision for the Visian ICL™ is approximately 3 millimeters and does not typically require sutures. Both phakic intraocular lenses necessitate deep anterior chambers and peripheral iridectomies to equalize pressure on both sides of the iris. The complications that can occur with phakic intraocular lenses are similar to those of other intraocular procedures (see below).
Refractive Lens Exchange
With the recent success of smaller incisions, high-frequency ultrasound, and superior lens implants, cataract surgery itself has become a refractive procedure. Some surgeons are now using the sameskills to offer refractive correction through lens removal with implant placement prior to cataract formation. Refractive lens exchange has been especially useful for higher levels of hyperopia. Multiple options exist for the intraocular lens implant, which is selected after careful patient interview, examination, and lens removal.
The vast majority of implants today are monofocal, which allow a clear image to be formed on the retina from a given distance. Most patients choose to have an implant that will correct for distance but will necessitate reading glasses for closer work. Occasionally, monovision or planned myopia is used so that near and intermediate distances are clear without glasses. Monovision may be an option in motivated patients who are likely not to be bothered by a mild decrease in depth perception.
Certain implants contain concentric rings of varying powers along the visual axis, thereby providing excellent near and distance acuities. Newer designs (ReSTOR® and ReZoom™) may create less halo side effect and contrast sensitivity loss, which have limited the use of multifocal lenses in the past.
The Crystalens™ is a silicone monofocal implant with a flexible structure that is believed to allow a forward accommodative motion. FDA approval data at one year postoperatively demonstrated 92% of eyes were 20/25 or better for uncorrected distance, and 89% could read 20/40 at near and intermediate without magnifying spectacles. The implant has shown superb stability for all ranges of vision in patients implanted at least 4 years earlier. Studies are ongoing to verify its intraocular movement with near effort.
Risks of Intraocular Procedures
The complications of intraocular refractive procedures may be uncommon, but their consequences can be devastating. Hemorrhage, infection, retinal detachment, and persistent edema of the cornea or retina are all possible with lens implantation procedures. Phakic intraocular lens procedures carry the additional risk of cataract formation, although this occurred in less than 5% of patients in the respective FDA trials.