Phakic intraocular lenses (IOLs) are a new technology for the correction of high refractive errors. They include both anterior and posterior chamber varieties.

The main anterior chamber IOLs under investigation are:

The lens is supported in an angle of the anterior chamber and lies in front of the iris. Complications include: Glaucoma, pupillary distortion, and corneal damage

 

The two main posterior chamber IOLs under investigation are:

The lens is put between the back of the iris and the front of lens. Complications associated with it are: Mainly cataract formation. These lenses may be difficult to remove because of adhesion formation.

 

Refractive Lensectomy

Refractive lensectomy, or clear lens extraction, is the removal of the natural crystalline lens for the treatment of high refractive errors. A monofocal or multifocal lens implant is inserted based on the desired refractive outcome. Refractive lensectomy has been used for the correction of myopia, hyperopia, and presbyopia.

Refractive lensectomy is essentially the same surgical procedure as cataract extraction and has the same complications, including:

Retinal detachment remains a significant concern when performing refractive lensectomy on patients with high myopia. In a group of 41 eyes with retinal detachment after clear lens extraction, only 9 eyes achieved final visual acuity of 20/60 or better. Retinal detachment rates after refractive lensectomy vary from 1.9% to 8.1%, depending on the study and time to follow-up.

Patients with high myopia have a higher incidence of retinal detachment than the general population. These patients account for 42% of rhegmatogenous retinal detachments, despite being only 10% of the population. Patients with myopia who undergo lens extraction and Nd:YAG capsulotomy may further increase their risk of retinal detachment.

Barraquer et al. found a clear association between Nd:YAG laser posterior capsulotomy and retinal detachment (11% with YAG capsulotomy vs. 5.5% without YAG capsulotomy) in eyes undergoing refractive lensectomy. Clinically significant posterior capsule opacification requiring Nd:YAG capsulotomy after refractive lensectomy ranges from 8% to 61%, depending on the study and time of follow-up.

Refractive lensectomy is a viable option for refractive correction at high extremes of ametropia, but caution should be exercised in cases of high axial myopia. Refractive lensectomy is a good option for patients who have corneas too thin or irregular for corneal refractive surgery. Furthermore, patients with evidence of early nuclear sclerosis may be better candidates for refractive lensectomy if cataract extraction would be anticipated in the next several years.

Refractive Surgery for Myopia, Myopic Astigmatism, and Mixed Astigmatism Refractive surgical options for the treatment of myopia and myopic astigmatism include laser surgeries, incisional surgeries, intrastromal ring segments, phakic intraocular lenses, and refractive lensectomy. Bioptics, or a planned combination of more than one refractive surgical modality, is also gaining popularity. For mixed astigmatism, several techniques are being used, including astigmatic keratotomy, photorefractive keratectomy, and laser in situ keratomileusis.

Laser Surgery

Photorefractive Keratectomy

Photorefractive keratectomy (PRK) was developed in the late 1980s as the first laser vision correction procedure. In October 1995, PRK became the first FDA-approved laser treatment for the correction of myopia and myopic astigmatism.

In PRK, a surgeon uses a 193-nm argon fluoride excimer laser to resculpt the surface of the cornea to correct refractive errors. In this procedure, the epithelium is removed by one of several techniques, including:

Following the epithelial removal, the laser reticule is centered over the entrance pupil and the laser ablation is performed on Bowman's membrane. The cornea is irrigated with a balanced salt solution, and a bandage contact lens is left in place for 3-7 days, until the epithelium regenerates. Most surgeons treat one eye at a time because functional visual acuity does not return until the epithelium has healed.

Depending on the type of laser used, PRK is approved for the treatment of myopia up to -13.0 D and astigmatism up to -4.5 D. PRK is more predictable in patients with a lower degree of myopia (<6.0 D). Patients with a higher degree of myopia who are treated with PRK tend to have more regression of their refractive effect , and more significant haze.

To minimize haze formation following PRK, surgeons prescribe the use of topical steroids for several months. In larger treatments, the use of antimetabolites to prevent haze formation may be beneficial. Preliminary rabbit and human studies suggest that a single intraoperative application of topical mitomycin C (0.2 mg/mL) may reduce corneal haze associated with PRK. However, the long-term safety of antimetabolite use in refractive surgery has not been established.

Depending on the study and the amount of myopic correction, PRK has been successful in achieving uncorrected visual acuity of 20/40 or better in 67%-98% of patients, with 48%-81% of patients achieving 20/20 uncorrected visual acuity. Long-term refractive outcomes of PRK and LASIK are similar.

Laser Surgery

Laser In Situ Keratomileusis
Since the introduction of laser in situ keratomileusis (LASIK) in 1990, there have been many reports describing its safety and efficacy. Uncorrected visual acuity has been reported at 20/40 or better in 46.4%-100% of eyes, depending on the study and degree of myopia. Higher refractive errors have less predictable results, resulting in more under- and overcorrections. Depending on the laser used, LASIK is approved by the FDA for treatments of myopia up to -15.0 D and astigmatism up to -5 D. There have been reports, however, of LASIK being used to treat myopic corrections of -25.0 D or more.

In LASIK, the microkeratome (laser or mechanical types) suction ring increases intraocular pressure to greater than 65-70 mm Hg. This is confirmed by

The microkeratome is used to make a corneal flap of 130-200 µm. Depending on the type of microkeratome used, either a superior- or nasal-hinged flap can be made. The corneal flap is reflected back toward the hinge, and the stromal bed is dried. The laser reticule is centered on the entrance pupil and the excimer laser ablation is performed. Balanced salt solution is irrigated under the flap, which is then stretched back into place and dried.

The flap is inspected for lack of striae and symmetry of the peripheral gutters. If the flap or stromal bed is irregular, laser treatment should not be performed. The flap should be left to heal in place, and a new flap can be cut in 6 months.

Postoperatively, topical antibiotics and steroids are used for 1-3 weeks. It may take up to 1 month per diopter of correction to achieve refractive stability. An enhancement should not be considered before 3 months, and in most cases it is prudent to wait 6 months.

Advantages of LASIK over PRK

Exclusion criteria are the same as for PRK but also include situations that make flap creation difficult, including:

Poor exposure (anterior buckles or deep-set eyes) may interfere with the microkeratome pass. Very steep or flat corneas increase the risk of buttonhole formation or free caps, respectively. Anterior basement membrane dystrophy increases the risk of epithelial defects and subsequent lamellar inflammation.

In addition, LASIK is not recommended for patients at risk for ectasia, including those with thin corneas, pellucid marginal degeneration, or suspected keratoconus. The current standard of care is that 250 µm of corneal tissue should be left in the stromal bed to minimize the risks of ectasia. However, there have been reports of iatrogenic ectasia even when the residual stromal bed was of sufficient thickness. Therefore, some surgeons recommend leaving up to 250 µm in the stromal bed.

Laser Surgery

Laser-assisted Subepithelial Keratectomy (LASEK)
In recent years, LASIK has become the preferred choice for vision correction because results demonstrate reduced postoperative discomfort and immediate improved postoperative visual acuity. However, as reports of LASIK complications surface, many surgeons and patients are indicating a preference for PRK. Nevertheless, significant postoperative pain, slower visual recovery, and haze remain deterrents to patient and surgeon acceptance of PRK.

Laser epithelial keratomileusis (LASEK) is a recent modification of PRK conceived by Massimo Camellin, MD. LASEK may reduce the incidence of postoperative pain, speed visual recovery, and reduce regression and haze when compared to PRK.

In this procedure, a trephine is used to make an epithelial groove. A reservoir is filled with an alcohol solution and left on the eye for 30-60 seconds. Then a microhoe is used to retract a hinged epithelial flap. Laser treatment is applied directly to Bowman's layer, and the epithelium is replaced and covered by a bandage contact lens. If the epithelium is torn or lost, the procedure is converted to a PRK by removing the residual epithelium.

In LASEK, the epithelial covering of the stroma may reduce haze formation and improve postoperative pain as compared to PRK. The advantages of LASEK compared to LASIK include:

It may also be preferred in patients who had previous glaucoma filtering surgery.

The few published studies to date show encouraging results of this new refractive procedure. Scerrati et al. compared their results from treating two groups of 15 patients with either LASIK or LASEK. The results in the LASEK group were superior to those in the LASIK group when comparing postoperative corneal topography, best spectacle-corrected visual acuity, and contrast sensitivity. Lee et al. studied 27 patients with low to moderate myopia in which one eye was treated with LASEK and the other with conventional PRK. At 3-months' follow-up, no between-eye differences in epithelial healing time, uncorrected visual acuity, or refractive error was found. The LASEK eyes, however, had lower pain scores and corneal haze than the PRK eyes.