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Refractive Surgery

By Deepinder K. Dhaliwal, MD, L.Ac, Professor, Department of Ophthalmology, University of Pittsburgh School of Medicine

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Corneal refractive surgery alters the curvature of the cornea to focus light more precisely on the retina. The goal of refractive surgery is to decrease dependence on eyeglasses or contact lenses. Most people who undergo refractive surgery achieve this goal; about 95% do not need corrective lenses for distance vision.

Ideal candidates for refractive surgery are healthy people aged 18 and older with healthy eyes who are not satisfied wearing eyeglasses or contact lenses.

Contraindications to refractive surgery include

  • Active ocular diseases, including severe dry eye

  • Autoimmune or connective tissue diseases, which can impair wound healing

  • Use of isotretinoin or amiodarone

Refraction should be stable for at least 1 yr prior to surgery. Latent herpes simplex virus may be reactivated after surgery; patients should be advised accordingly.

Adverse effects of refractive surgery include temporary symptoms of

  • Foreign body sensation

  • Glare

  • Halos

  • Dryness

Occasionally, these symptoms persist.

Potential complications include

  • Overcorrection

  • Undercorrection

  • Infection

  • Irregular astigmatism

In excimer laser procedures done on the superficial corneal stroma, haze formation is possible. If infection, irregular astigmatism, or haze formation causes permanent changes in the central cornea, best-corrected acuity could be lost. The overall complication rate is low; chance of vision loss is < 1% if the patient is considered a good candidate for refractive surgery preoperatively.

Types of refractive surgery

The two most common refractive surgery procedures are

  • Laser in situ keratomileusis (LASIK)

  • Photorefractive keratectomy (PRK)

Other refractive surgeries include

  • Small incision lenticule extraction (SMILE)

  • Phakic intraocular lenses (IOL)

  • Corneal inlays

  • Clear lensectomy

  • Intracorneal ring segments (INTACS)

  • Radial keratotomy

  • Astigmatic keratotomy

Laser In Situ Keratomileusis (LASIK)

In LASIK, a flap of corneal tissue is created with a femtosecond laser or mechanical microkeratome. The flap is turned back and the underlying stromal bed is sculpted (photoablated) with the excimer laser. The flap is then replaced without suturing. Because surface epithelium is not disrupted centrally, vision returns rapidly. Most people notice a significant improvement the next day. LASIK can be used to treat myopia, hyperopia, and astigmatism.

Advantages of LASIK over PRK include the desirable lack of central stromal healing response (the central corneal epithelium is not removed, thereby decreasing the risk of central haze formation that occurs during healing), the shorter visual rehabilitation period, and minimal postoperative pain.

Disadvantages include possible intraoperative and postoperative flap-related complications, such as irregular flap formation, flap dislocation, and long-term corneal ectasia. Ectasia occurs when the cornea has become so thin that intraocular pressure causes instability and bulging of the thinned and weakened corneal stroma. Blurring, increasing myopia, and irregular astigmatism can result.

Photorefractive Keratectomy (PRK)

In PRK, the corneal epithelium is removed and then the excimer laser is used to sculpt the anterior curvature of the corneal stromal bed. PRK is used to treat myopia, hyperopia, and astigmatism. The epithelium typically takes 3 to 4 days to regenerate; during this time a bandage contact lens is worn. Unlike LASIK, no corneal flap is created.

PRK may be more suitable for patients with thin corneas or anterior basement membrane dystrophy.

Advantages of PRK include an overall thicker residual stromal bed, which reduces but does not eliminate the risk of ectasia, and lack of flap-related complications.

Disadvantages include the potential for corneal haze formation (if a large amount of corneal tissue is ablated) and the need for postoperative corticosteroid drops for several months. The intraocular pressure of postoperative patients who are using topical corticosteroids should be monitored carefully because corticosteroid-induced glaucoma has been reported after PRK.

Small Incision Lenticule Extraction (SMILE)

In SMILE, a femtosecond laser is used to create a thin, intrastromal lenticule of tissue, which is then removed through a small (2 - 4 mm) peripheral corneal laser incision. SMILE is FDA-approved to treat myopia.

The efficacy, predictability, and safety of SMILE are similar to those of laser in situ keratomileusis (LASIK), with the additional benefit that it eliminates flap creation and the attendant risks. Another benefit of SMILE is the reduced degree of postoperative corneal denervation and higher-order ophthalmologic aberrations and an accelerated rate of corneal nerve regeneration relative to LASIK.

Disadvantages include increased incidence of suction loss (an operative complication during refractive surgery) and difficulty with enhancements (additional ophthalmic surgery).

Phakic Intraocular Lenses (IOLs)

Phakic intraocular lenses (IOLs) are lens implants that are used to treat severe myopia in patients who are not suitable candidates for laser vision correction. Unlike in cataract surgery, the patient’s natural lens is not removed. The phakic IOL is inserted directly anterior or posterior to the iris through an incision in the eye. This procedure is intraocular surgery and must be done in an operating room.

Risks include cataract formation, glaucoma, infection, inflammation, and loss of corneal endothelial cells with subsequent chronic corneal edema that eventually becomes symptomatic.

Because phakic IOLs do not correct astigmatism, patients can undergo subsequent laser vision correction to refine refractive results in a technique known as bioptics. Because the bulk of the myopia is corrected with the phakic IOL, less corneal tissue is removed with the excimer laser, and the risk of ectasia is thus low.

Corneal Inlays

Corneal inlays are implants placed into the corneal stroma via a lamellar pocket or flap to treat presbyopia. One type of corneal inlay is made of hydrogel material and creates a hyperprolate anterior corneal surface. A prolate shape is one in which the steepest radius of curvature is central. A hyperprolate surface exaggerates this difference and results in sharper central near vision. Another type of corneal inlay is made of polyvinylidene fluoride and carbon and is a small aperture inlay which improves near vision by increasing depth of focus. These inlays are placed only in the non-dominant eye of presbyopic patients.

Advantages of corneal inlays are improved near vision with a 1-2 line decrease in distance vision in the corrected eye. Also, corneal inlays are removable.

Disadvantages include risk of corneal haze or inflammation, which requires long-term topical steroid use and can result in glare, halo, and difficulty reading in dim light. Complications can include inlay decentration, dry eye, and epithelial ingrowth.

Clear Lensectomy

Clear lensectomy can be considered in patients with high hyperopia who are already presbyopic. This procedure is identical to cataract surgery except the patient’s lens is clear and not cataractous. An extended depth-of-focus, multifocal or accommodating IOL, which allows the patient to focus over a wide range of distances without external lens correction, can be inserted.

The main risks of clear lensectomy are infection and rupture of the posterior capsule of the lens, which would necessitate further surgery. Clear lensectomy should be done with great caution in young patients with myopia because they have an increased risk of postoperative retinal detachment.

Intracorneal Ring Segments (INTACS)

Intracorneal ring segments (INTACS) are thin arc-shaped segments of biocompatible plastic that are inserted in pairs through a small radial corneal incision into the peripheral corneal stroma at two-thirds depth. After INTACS are inserted, the central corneal curvature is flattened, reducing myopia. INTACS are used to treat mild myopia (< 3 diopters) and minimal astigmatism (< 1 diopter). INTACS maintain a central, clear, optical zone because the 2 segments are placed in the corneal periphery. INTACS can be replaced or removed if desired.

Risks include induced astigmatism, undercorrection and overcorrection, infection, glare, halo, and incorrect depth placement. Currently, INTACS are mostly used for treatment of corneal ectatic disorders such as keratoconus and post-LASIK ectasia when glasses or contact lenses no longer provide adequate vision or are uncomfortable. Best-corrected vision and contact lens tolerance improve in 70 to 80% of patients.

Radial Keratotomy and Astigmatic Keratotomy

Radial and astigmatic keratotomy procedures change the shape of the cornea by making deep corneal incisions using a diamond blade.

Radial keratotomy has been replaced by laser vision correction and is rarely used because it offers no clear advantages over laser vision correction, has a greater need for subsequent retreatment, can lead to visual and refractive results that change through the day, and can cause a hyperopic shift in the long term.

Astigmatic keratotomy is still commonly done at the time of cataract surgery. The incisions are referred to as limbal relaxing incisions because the optical zone is much larger and closer to the limbus.