VISION CORRECTION PROCEDURES
• Refractive errors are an extremely common cause for blurred vision.
• Refractive errors include myopia (nearsightedness), hyperopia (farsightedness), astigmatism, and presbyopia, which is the age- related loss of near vision.
• Medical management includes the use of spectacles, contact lenses, or both.
• Both corneal and intraocular surgical options exist to successfully and permanently correct refractive errors.
• Corneal refractive surgery options include excimer laser, such as laser in situ keratomileusis (LASIK) and photorefractive keratectomy (PRK), and corneal relaxing incisions for the management of astigmatism.
• In eyes that are not candidates for corneal refractive surgery, intraocular surgery with phakic intraocular lens implants or refractive lens exchange can be considered.
• Increasing advancements now offer, and continue to expand, the various surgical options available to correct myopia, hyperopia, astigmatism, and presbyopia.
• Laser in situ keratomileusis (LASIK) and photorefractive keratectomy (PRK) use an excimer laser to reshape the anterior surface of the cornea and permanently correct the eye’s refractive error. LASIK and PRK can correct myopia, hyperopia, and astigmatism and help reduce presbyopia.
• In LASIK, a corneal flap is created and lifted before the excimer laser is applied to correct the refractive error. In contrast, the PRK procedure requires no lamellar flap. In PRK, the laser treatment is applied directly to the anterior stromal surface after the surface epithelium is removed.
• Radial keratotomy (RK) was the first modern form of corneal refractive surgery to correct myopia. In RK, spokelike corneal incisions were created to flatten the central cornea and reduce myopia. Although RK surgery is effective, the results were often unpredictable and unstable and led to overcorrection, with progressive hyperopia.
• Astigmatic keratotomy (AK) incisions are a modification of RK surgery and are used to reduce corneal astigmatism. The two forms of corneal relaxing incisions, AK and limbal relaxing incisions (LRI) —or, more accurately, peripheral corneal relaxing incisions (PCRIs) —are differentiated by their location on the cornea.
• A refractive lens exchange (RLE) is a surgical option that can correct refractive errors by removing and replacing the crystalline lens. In essence, RLE is cataract surgery performed before a visually significant cataract forms; a visually significant cataract is the clouding or opacification of the natural lens. RLE is a viable option for high myopes and hyperopes, where laser vision correction is not an option, and to correct presbyopia.
• Presbyopia correction is a very dynamic field in refractive surgery. Surgical correction options include surgical monovision, which is popularly used with presbyopic contact lens wearers, where one eye is corrected for distance vision and the other eye for near vision. Monovision can be produced through excimer laser, RLE, or cataract surgery. Other options include various intraocular lenses (IOLs) and corneal inlays that can provide presbyopia correction.
• Surgically implanted lenses, or phakic intraocular lens implants, can be used to treat myopia only in the United States. Phakic IOLs are an alternative to laser vision correction and can successfully correct myopia in patients for whom keratorefractive surgery is not an option.
• No surgery is without its potential for complications. The more devastating complications of corneal refractive surgery include flap- related complications in LASIK, corneal weakening and ectasias, and vision-limiting haze in PRK. Endophthalmitis is a sight- threatening infection that can occur with any intraocular surgery such as RLE, phakic intraocular lenses, and cataract surgery.
The term refractive error describes any condition where light is poorly focused within the eye, resulting in blurred vision. This is the most common eye problem encountered in the United States and includes such conditions as nearsightedness (myopia), farsightedness (hyperopia), astigmatism, and age-related loss of near vision (presbyopia). A person who is able to see without the aid of spectacles or contact lenses has minimal to no refractive error. A wide variety of techniques are available for correcting refractive errors and restoring visual function. The most common methods employ corrective eyewear, such as eyeglasses and contact lenses. In addition to these noninvasive modalities, several surgical procedures can also be used to treat these conditions. These surgical techniques range from minimally invasive procedures, such as laser vision can be divided to corneal and intraocular procedures.
Whichever method is selected, the primary goal of every vision- correcting procedure should be to choose the technique that is most appropriate for each patient. The chosen method should not only correct the patient’s visual deficit but also satisfy the patient’s goals for visual function. This chapter focuses on the various surgical options that are available for vision correction. To set the groundwork for this discussion, we begin with some background information on ocular anatomy, refractive errors, and the clinical assessment of visual function.
Although the human eye is a complex structure, from a conceptual standpoint, it can be thought to function much like a simple camera. In general, light enters the eye and needs to be focused on the center of the retina, or the fovea, to generate images. Light first enters through the cornea, a convex transparent window that performs approximately 66% to 75% of the focusing for the eye. After passing through the cornea, the light encounters the iris and pupil. The pupil is an aperture centered within the iris, a muscular diaphragm that controls the diameter of the pupil and thus the amount of light that continues into the eye.
The crystalline lens sits behind the pupil and provides the remaining 25% to 34% of the eye’s focusing ability (Figure 1). The lens is suspended by a network of hundreds of supporting cables called zonular fibers. These zonules insert into the ciliary body, a muscular ring that is a peripheral extension of the iris. The refractive power of the lens is somewhat adjustable and can be increased to move the focal point of the eye from distance to close range, a process known as accommodation (Figure 2). Accommodation results from contraction of the ciliary muscle, which results in a decreased diameter of the ciliary ring, similar to a lens aperture of a camera. This loosens the zonules, which simultaneously reduces zonular tension on the lens, thereby causing the thickness and anterior curvature to increase, along with its amplified refractive power.
FIGURE 1 The refraction, or bending, of light in the eye occurs through the cornea and the crystalline lens. Refractive errors result when the light is not perfectly focused onto the retina. (Illustration courtesy A.D.A.M, Inc.)
FIGURE 2 The lens is suspended in place by the zonular fibers. These zonules insert into the ciliary body. Accommodation, or the ability to focus at close range, occurs as the result of the contraction of the zonules and a change in shape to the crystalline lens.
After being focused by the lens, light passes through the transparent vitreous humor until it reaches the retina, which lines the inside of the back of the eye. The retina functions like the film in a camera, converting the focused image into an electrical signal that is transmitted to the brain via the optic nerve.
Emmetropia is the condition where the eye has essentially no refractive error and requires no correction for distance vision.
Refractive errors result when the cornea and lens inadequately focus incoming light, resulting in blurred images projected onto the retina. The unit of measure for refractive error is the diopter (D), which for a thin lens (in air) is defined as the reciprocal of the lens focal length.
Most refractive errors refer to the patient’s visual status when viewing objects in the distance. For example, a lens that focuses light over a distance of 0.5 m has a refractive power of +2.0 D.
In myopia, or nearsightedness, the focusing powers of the cornea are too strong or the axial length of the eye is too long, or both. The resulting image comes into focus anterior to the retina and is out of focus by the time it reaches the back of the eye. As a result, myopic eyes can see better at close range than at a distance. To correct a myopic eye, its refractive power must be decreased by using a lens with negative refractive power, which weakens the focusing of light and redirects it toward the retina.
Hyperopes, or farsighted persons, are the opposite of myopes. The cornea is flatter and focuses too weakly or the axial length is too short (or both) in the hyperopic eye. The images from objects viewed at a distance are not yet in focus by the time they reach the retina. To see clearly, a hyperopic eye must accommodate to increase its lenticular power to bring distant objects into sharp focus. Because this requires contraction of the ciliary muscle, the farsighted eye is never at rest and must work even harder to see near objects clearly. Because of this, hyperopic refractive corrections must add positive focusing power to the eye (Figure 3).
FIGURE 3 Emmetropia, myopia, and hyperopia. In emmetropia, the secondary focal point (F2) is at the retina. In myopia, the secondary focal point (F2) is in the vitreous. In hyperopia, the secondary focal point (F2) is behind the eye. (Reprinted with permission from Mimura T, Azar DT: Part 3: Refractive Surgery. In Yanoff M, Duker JS (eds): Ophthalmology, 3rd ed. St. Louis, Mosby, 2008.)
In astigmatism, the eye has different refractive powers along different meridians; light entering in the vertical direction gets focused differently than light in the horizontal direction.
Conceptually, it is easier to think of the astigmatic cornea or lens as shaped like a football rather than a basketball, with the meridian of steeper curvature having greater refractive power. The astigmatic eye produces a blurred image because essentially two focal points of images are being produced. This requires different corrections along each of these meridians to produce a single focused image on the retina (Figure 4).
FIGURE 4 Astigmatism occurs when the refractive power of the eye is not symmetrical. The focusing poser of one axis is stronger than the other axis. This effectively leads to multiple focal points of light, which results in blurred images. (Reprinted with permission from Haw WW, Manche EE: Vision correction procedures. In Bope ET, Rakel RE, Kellerman R (eds): Conn’s Current Therapy 2010. Philadelphia, Saunders, 2010, pp. 193–197.)
Presbyopia describes the normal age-related loss of near vision. To see near objects clearly, young distance-corrected eyes must accommodate to increase their refractive power. However, this ability progressively declines with age, usually reaching clinical significance in the 5th decade. Several factors have been implicated in this process, including loss of lens elasticity, decreased zonular tension, and altered ciliary muscle function. Although there is currently no way to reverse this natural consequence of aging, several vision-correction options are available to improve near vision in presbyopic persons.
Population-based studies indicate that approximately 25% to 40% of people in their 40s have myopia, and 10% to 20% have hyperopia.
Myopia is the most common refractive error and affects about 35% of whites and 13% to 30% of African Americans. Approximately three- quarters of the American population older than 40 years have refractive errors greater than 0.5 D. Astigmatism of more than 0.5 D is common in adults, and the prevalence increases to approximately 28% in persons in their forties. In general, a higher prevalence of hyperopia and less myopia is observed with increasing age from about 45 to 65 years. This levels off with older age and is eventually followed by an increase in myopia at older ages, which is thought to largely be from cataract formation. Regarding the correction of refractive errors, about 150 million Americans currently use some form of eyewear to correct refractive errors at a cost of approximately $150 billion annually, including 36 million who use contact lenses.
Diagnosis (Assessment of Vision)
There are many aspects of vision, including visual acuity, contrast sensitivity, color perception, and peripheral vision. The most common assessment of visual function is to test the central vision through visual acuity. Visual acuity testing determines a patient’s ability to read high-contrast symbols (usually black letters on a white background) of varying sizes at a standard testing distance. This reference distance approximates optical infinity and is typically 20 feet in the United States and 6 meters in Europe. A 20/20 letter on the standard eye chart devised by Snellen is approximately three-eighths of an inch tall at a distance of 20 feet (subtending a visual angle of 5 minutes of arc). Twenty-twenty vision is considered normal visual acuity.
Visual acuities less than 20/20 are represented by ratios whose denominator is greater than 20. For example, a visual acuity of 20/60 means that the smallest letter the eye can read is three times larger than a 20/20-size letter.
Refractive errors can result in uncorrected visual acuities that fall below 20/20. However, in the absence of other disease, the conditions of myopia, hyperopia, astigmatism, and presbyopia can be corrected with restoration of normal visual function. This can be achieved with spectacles, contact lenses, or the various surgical procedures discussed next.
Treatment (Surgical Correction of Refractive Errors)
Laser Vision Correction
Laser in situ keratomileusis (LASIK) and photorefractive keratectomy (PRK) use an excimer laser to reshape the anterior surface of the cornea and permanently correct the eye’s refractive error. The excimer laser was developed in the 1970s and emits ultraviolet light at a wavelength of 193 nm. This particular wavelength has been found to accurately and efficiently ablate corneal tissue without causing thermal damage to the surrounding collagen. In myopia, the excimer laser treatment flattens the central cornea to decrease its focusing power. Conversely, for hyperopia, laser pulses are applied to the periphery, indirectly steepening the central cornea and thereby increasing its refractive power. Astigmatism is corrected by combining central and peripheral treatments to differentially steepen the flattest corneal meridian and flatten the steepest meridian. Since the first excimer treatment in the late 1980s, LASIK and PRK have been used to treat refractive errors in millions of patients.
Laser in Situ Keratomileusis
In LASIK, a lamellar or partial-thickness corneal flap is first created in the cornea. The flap thickness typically varies from about 100 to 180 µm, as compared with a total corneal thickness that usually ranges from 500 to 600 µm. The LASIK flap is reflected at its hinge and the excimer laser ablation is applied directly to the corneal stroma. Once the ablation is complete, the flap is returned to its original position (Figure 5).
FIGURE 5 LASIK procedure. 1, The normal cornea. 2 and 3, A microkeratome (or laser) is used to create a partial-thickness corneal flap attached at a hinge. 4, The corneal flap is lifted. 5, The excimer laser ablates the corneal stroma. 6, The corneal stroma is reshaped. 7 and 8, The corneal flap is repositioned. (Reprinted with permission from Haw WW, Manche EE: Vision correction procedures. In Bope ET, Rakel RE, Kellerman R (eds): Conn’s Current Therapy 2010. Philadelphia, Saunders, 2010, pp. 193–197.)
The LASIK flap is created by either a microkeratome, a device containing a motorized oscillating blade connected to a suction ring, or via a femtosecond laser. The femtosecond laser uses ultrashort microscopic pulses of infrared light to define the lamellar flap. The femtosecond laser has allowed greater accuracy with the flap thickness. Also, there are significantly less flap-related complications, such as free (without flap hinge), partial, or buttonhole (doughnut- shaped) flaps.
Recovery of vision usually takes a few days; some patients note gradual improvement over a few weeks. Postoperative medications usually include topical antibiotic and steroid eye drops for approximately 5 to 7 days.
Complications with laser vision correction in general are extremely low. Most intraoperative complications associated with LASIK involve the flap. If during the surgery the patient moves the eye (fixation loss), the laser can focus on the wrong part of the eye (decentration of the laser treatment), which can degrade the postoperative vision creating a chromatic aberration called coma.
LASIK carries the postoperative risks of flap displacement or induction of flap striae. Most of these cases require a lifting and repositioning of the flap. However, more severe or refractory cases can require further intervention, such as suturing.
On rare occasions, epithelial cells from the corneal surface can migrate underneath the LASIK flap and proliferate in the interface. Usually, the peripheral nests of epithelial cells are small and insignificant visually, thus requiring no intervention. Larger collections of cells can compromise vision or cause corneal necrosis and scarring, so they need to be removed. Epithelial ingrowth requires lifting the flap and manually débriding the cells. This treatment might need to be supplemented with alcohol or suturing, or both, to prevent recurrence.
Corneal weakening and subsequent distortion are other potential sequelae of laser vision correction. Certain preoperative corneal shapes suggest a predisposition toward weakening, especially those with steeper curvature in the inferior region as compared with the superior region. Preoperative corneal thicknesses less than 500 µm or post-LASIK residual stromal bed thicknesses less than 250 µm can also be associated with corneal weakening.
As with any surgery, LASIK and PRK are also associated with a risk of infection. The incidence varies, but published rates have been about 0.03%. Staphylococcal and streptococcal species are most common, but atypical organisms such as mycobacteria and fungi have also been reported. Sterile inflammation can also occur in the flap interface and is known as diffuse lamellar keratitis. Diffuse lamellar keratitis has been associated with bacterial endotoxin, cleaning solutions, corneal abrasions, and excessive femtosecond laser energy levels. Treatment primarily relies on topical steroid eyedrops, but it may also include oral steroids and irrigation of the flap interface.
PRK also uses the excimer laser to reshape the cornea, but this procedure requires no lamellar flap. In this method, the laser treatment is applied directly to the anterior stromal surface after the surface epithelium is removed. Several techniques are available to remove the corneal epithelium. Mechanical débridement with a spatula or rotating brush is very common. Advanced surface ablation has replaced other techniques used to manipulate the surface epithelium, including a devitalizing 20% alcohol solution, followed by gentle débridement with a blunt spatula or an epikeratome (similar to a LASIK microkeratome) to separate a flap of epithelium at the level of the basement membrane.
Laser-assisted subepithelial keratomileusis (LASEK) was another modification of PRK but has now fallen largely out of use. In LASEK, a flap consisting of only epithelium is created and replaced onto the cornea, which some surgeons believed provided greater postoperative comfort and expedited re-epithelialization of the corneal surface. Each of these methods has certain advantages and disadvantages, but all do an effective job of preparing the corneal surface for laser ablation.
Once the laser treatment is complete, a soft contact lens is placed on the cornea. The corneal epithelium typically heals in 4 to 7 days, after which the contact lens is removed. Once the contact lens is removed, recovery of vision can take a few more weeks, although some patients experience improvement in vision that continues over several months. Antibiotic eye drops are used until the contact lens is removed, and steroid eye drops may be tapered over several months.
Regarding complications, PRK has fewer intraoperative risks than LASIK because no lamellar flap is being created. Postoperatively, there is a higher risk of subepithelial haze. Fibroblastic transformation of keratinocytes can cause the deposition of disorganized collagen that decreases the smoothness and clarity of the post-PRK cornea. This development is usually associated with higher myopic corrections and may be reversed by increased application of topical steroid eyedrops. Short-duration intraoperative use of low-concentration topical mitomycin-C (Mitosol)1 (0.02%) appears to decrease the risk of haze formation. As discussed with LASIK, postoperative infectious or sterile inflammation is a rare but potential risk with PRK as well.
Both PRK and LASIK damage corneal nerves, which appears to have a secondary effect on the ocular hydration status. Postoperative dry eye is greater in LASIK than PRK owing to its greater depth of penetration into the cornea. The increased dryness appears to be temporary, and most patients return to baseline by 6 to 9 months, but a subset of patients experience chronic dry eyes following the procedure. A careful preoperative assessment of dry eye risk factors is recommended, and those at risk may be steered toward PRK or toward no surgery at all.
Modern keratorefractive surgery can attribute some of its origins to a flurry of research produced by Sato of Japan in the 1940s that led to radial keratotomy (RK) and astigmatic keratotomy (AK) surgeries.
Russian ophthalmologist Fyodorov is credited with advancing keratorefractive surgery in the 1970s through RK surgery. He created up to 16 spoke-like cuts radiating from the central cornea at 90% to 95% corneal depth to balloon the peripheral cornea. This would, in turn, flatten the central cornea and reduce myopia. The combination of RK and AK surgeries were very popular procedures that effectively reduced myopia and astigmatism throughout the 1970s and 1980s.
Although RK surgery was effective, the results were often unpredictable and unstable and led to overcorrection, with progressive hyperopia. RK surgery was quickly replaced by laser vision correction with the introduction of PRK in the late 1980s (Figure 6).
FIGURE 6 Radial keratotomy was the first modern form of refractive surgery for the correction of myopia. Up to 16 spokelike cuts radiating from the central cornea at 90% to 95% of corneal depth can be created to flatten the central cornea and balloon the peripheral cornea to correct myopia.
Astigmatic keratotomy incisions are a modification of RK surgery and are used to reduce corneal astigmatism. The two forms of corneal relaxing incisions, astigmatic keratotomy (AK) and limbal relaxing incisions (LRI), or more accurately peripheral corneal relaxing incisions (PCRI), are differentiated by their location on the cornea (Figure 7).
FIGURE 7 Incisions can be placed on the cornea to reduce astigmatism. The more central astigmatic keratotomy (AK) incisions have a greater effect than their peripheral limbal relaxing incision (LRI) counterparts. (Reprinted with permission from Yeu E, Rubenstein JB: Management of astigmatism during lens-based surgery. In American Academy of Ophthalmologists:
Focal Points, Clinical Modules for Ophthalmologists, February, 2008.)
Several nomograms exist for AK and PCRIs that consider the age of the patient and the amount and meridian of the steep axis of astigmatism in order to calculate the length of the incisions. In both procedures, incisions are made to a 90% to 95% depth in the cornea to flatten the steep meridian. The basic principles of astigmatism correction hold true for both types of keratotomy surgeries: a greater effect is achieved with longer incisions, with smaller optical zones, with deeper incisions, and in older patients. Hence, the more central AK incisions have a greater effect and can correct upwards of 6 to 7 diopters of astigmatism.
PCRIs have a weaker effect because of their more peripheral location and generally can correct 2 to 3 diopters of astigmatism. Because the incisions are made closer to the limbus, they heal faster, and thus the refractive effect stabilizes more quickly. Given their peripheral location, the ratio of flattening in meridian of incision-to- steepening ratio in the opposite meridian, or coupling ratio, is usually 1:1. Patients experience less irregular astigmatism, flare, and foreign body sensation as compared to their more central counterparts.
Technically, PCRIs are easier to perform and more forgiving as well.
Corneal relaxing incisions are most commonly performed intraoperatively, at the time of cataract surgery, but are also performed as a separate procedure to treat corneal astigmatism. The procedure is fairly quick to perform and the patient experiences little discomfort. Postoperatively, a topical antibiotic is used for 4 to 7 days, and a topical analgesic is used as needed.
Regarding complications of corneal relaxing incisions, patients commonly experience a foreign body sensation during the first few days. Less-common complications include glare, undercorrection or overcorrection, irregular astigmatism from the incisions, wound gape or perforation, decreased corneal sensation, and dry eye syndrome.
Refractive Lens Exchange
Unlike the corneal procedures that have been discussed, refractive lens exchange (RLE) is a surgical option that can correct refractive errors by removing and replacing the crystalline lens. In essence, RLE is cataract surgery without a visually significant cataract, which is the clouding or opacification of the natural lens. RLE is a viable option for high myopes and hyperopes, where laser vision correction is not an option, and for correcting presbyopia.
The natural crystalline lens is removed via emulsification with ultrasound energy. This lens is then replaced by an acrylic or silicone intraocular lens (IOL) implant that can effectively and accurately correct refractive errors. Preoperative biometric measurements, including the corneal curvature and axial length, are used in different formulas to calculate the proper intraocular lens power.
Since the turn of the century, a variety of IOLs has been brought forth to address presbyopia correction. There are different designs, including multifocal designs and accommodating IOLs. Monovision, popularly used with presbyopic contact lens wearers (one eye is corrected for distance vision and the other eye for near), can also be reproduced permanently with RLE and cataract surgery.
Although currently available presbyopia-correcting IOL technology is effective in providing a greater range of vision and freedom from spectacle use, the IOLs are not without their faults. Although improved, the multifocal IOL can cause glare and halos from its inherent concentric ring design, and diminished contrast sensitivity can result from the light’s being “split” for distance and near vision.
The only currently FDA-approved accommodating IOL has neither of these disadvantages of the multifocal IOLs, but it does not provide UV light protection and also provides less predictable near vision (Figure 8).
FIGURE 8 Examples of currently FDA-approved intraocular lens implants. A and B, Multifocal. C, Accommodating. (A, From Alcon Labs, Fort Worth, TX. B, From Abbott, Abbott Park, IL. C, From Bausch and Lomb, Rochester, NY.)
Because RLE is intraocular, it is much more invasive than the previous extraocular corneal procedures. Hence, although both eyes can undergo corneal procedures simultaneously, elective intraocular procedures should always be performed as two separate staged surgeries. Being intraocular, RLE procedures likely have a similar risk of endophthalmitis (severe postoperative intraocular infection) as modern cataract surgeries do, which is between 0.05% to 0.10%. Other postoperative complications of RLE include retinal detachment in high myopes (1.85%-8%) and a likely need for a laser capsulotomy procedure to treat capsular haze that commonly occurs behind the IOL after surgery.
In addition to the postoperative risks of RLE, other common obstacles can be encountered intraoperatively because operating on soft lenses, very long eyes (high myopes), and very short eyes (high hyperopes) has its own set of potential complications. Preoperative surgical planning and strategy are key to a successful RLE.
Phakic Intraocular Lens Implant
Surgically implanted lenses, or phakic intraocular lens implants, may be used only to treat myopia in the United States. Phakic IOLs are an alternative to laser vision correction and can successfully correct myopia in patients for whom keratorefractive surgery is not an option. As compared to laser vision correction for myopia, some studies suggest that the quality of vision with a phakic IOL is superior. Phakic IOLs function very similarly to contact lenses that are permanently implanted inside the eye. Regarding the two FDA-approved phakic IOLs available today, a phakic IOL can be implanted to sit in front of and attached to the iris (see Figure 8) or just behind the iris. As in RLE, the implantation of a phakic IOL is intraocular, and surgery should be performed on only one eye at a time to respect the risks involved in the more-invasive procedure. Unlike RLE, all structures inside the eye, including the crystalline lens, are left untouched when a phakic IOL is implanted.
As do all surgical procedures, phakic IOLs are subject to various complications, of which cataract formation is a more common one (up to 9%). Other surgical complications include infection, retinal detachment, acute glaucoma, and loss of corneal endothelial cells, which are the nonregenerating cells on the back surface of the cornea that are responsible for maintaining corneal clarity.
Corneal Inlays for the Correction of Presbyopia
Presbyopia is the most ubiquitous refractive error, and the presbyopic population is undergoing rapid growth worldwide. In recent years, corneal inlays have gained popularity worldwide, and they can be an effective option for correcting presbyopia in appropriate patients. The 3 types include small aperture (KAMRA) (Figure 9), corneal reshaping (Raindrop) (Figure 10), and refractive (Flexivue Microlens) inlays. The KAMRA inlay received US FDA approval in 2015, followed by the Raindrop inlay in 2016. The pivotal clinical trial data from the
Flexivue inlay are expected to be submitted to the FDA toward the end of 2017. Each provide greater near vision while minimally sacrificing the distance vision, and each technology provides such near vision differently.
FIGURE 9 Kamra inlay. Uses pinhole effect to increase depth of field.
FIGURE 10 Raindrop inlay. Reshapes cornea to a hyperprolate state.
Small Incision Lenticular Extraction (SMILE)
Regarding corneal refractive surgery, there has been a lot of focus on procedures that involve removing an intracorneal lenticule of stromal corneal tissue to change the shape of the cornea in order to create refractive correction. Small incision lenticular extraction (SMILE), recently US FDA-approved in September 2016 for the correction of myopia, removes the lenticule through a small incision without the creation of a corneal flap. SMILE appears to cause less dry eye syndrome in a statistically significant manner. In a study by Sekundo et al., 1.1% of subjects after the SMILE procedure had superficial punctate keratitis at 1 week follow-up with none having subjective complaints of dry eye syndrome. SMILE may also lead to less complications of corneal ectasia, as flap-based procedures (like LASIK) appear to produce a 49% greater reduction in effective corneal stromal collagen stiffness than in SMILE cases.
Refractive surgery technology continues to evolve, particularly in presbyopia correction. Topical drop technologies are in clinical trials, as are other surgical procedures and lasers that are targeting the sclera and ciliary body of the eye. Numerous intraocular lens technologies are on the horizon, including more advanced bifocal and trifocal IOL designs and true accommodating IOLs.
For intraocular refractive surgery options, phakic lens technology, which is currently only approved in the United States for the correction of myopia, may be available for hyperopic and astigmatism correction, as it is in Europe.
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