The term “refractive surgery” applies to surgical procedures intended to correct the refractive error of the eye and reduce or eliminate the need for corrective lenses. Refractive surgery includes all forms of vision correction surgery such as RK, PRK, LASIK, and LASEK — which involve reshaping the cornea — as well as some types of intraocular lens implants.
Radial keratotomy was first performed in the United States in 1978. RK involves making incisions to the cornea in a radial pattern using a hand-held blade. In response to the surgery, the cornea relaxes and flattens, which changes the focus of the eye. A ten-year study of RK patients (PERK) demonstrated refractive instability with a shift toward hyperopia (farsightedness) which continued throughout the study period. RK is associated with loss of contrast sensitivity and visual aberrations (starbursts, halos, and double vision) resulting from clear zones as small as 3 millimeters. RK fell out of favor with refractive surgeons when laser eye surgery was introduced.
The excimer laser was first used in 1988 to perform PRK in the United States on human subjects. PRK involves removal of the surface cells of the cornea by scraping, followed by laser reshaping of the underlying corneal tissue. The surface cells regenerate within a few days of surgery. PRK is associated with postoperative pain and delayed visual recovery, which dampened patients’ enthusiasm. PRK carries risk of dry eyes and haze formation, particularly with deeper ablations which limits its application. Currently, some doctors use a toxic substance called mitomycin-C (MMC) to prevent haze after PRK (and LASEK), and when performing surface ablations to enhance previous LASIK. MMC carries long-term risks and has not been proven safe for this use. Night vision disturbances are frequently reported by patients after PRK, and were more problematic with earlier laser systems utilizing smaller optical zones.
LASIK was first approved in the U.S. in 1998. Like PRK, an excimer laser is used to reshape the cornea. The major difference between the two surgeries is the way that the stroma, the middle layer of the cornea, is exposed before it is vaporized with the laser. LASIK is faster and easier to perform than PRK, also referred to by some as the “flap and zap”. Pain during and immediately after surgery is less common, and visual recovery is usually rapid in comparison to PRK. Some patients may sit up after surgery and immediately be able to see the clock on the wall in the surgical suite, giving LASIK the “wow!” factor that makes the surgery attractive.
The first step in LASIK is creation of a corneal flap with a blade called a microkeratome or with a special laser. The flap is reflected back to expose the underlying corneal stroma. Next, a laser is used to remove tissue, which reshapes the cornea. Finally, the flap is returned to its original position and is held in place by hydrostatic forces and by pumping activity of specialized cells at the back surface of the cornea. Epithelial cells then grow over the flap, and finally a scar forms at the flap margin. The fact that the corneal integrity is never fully restored after LASIK, and the flap never actually ‘heals’ is discussed in other sections of this website.
The downsides of LASIK include the potential for flap complications, late flap dislocation, severing of the corneal nerves leading to dry eyes, the potential for surface cells to grow under the flap, inflammation, infection, and corneal ectasia (progressive forward bulging of the cornea). The speculum used to stretch the eye wide open during LASIK is associated with damage to nerves of the eyelid, sometimes resulting in sagging eyelids and eyelids that will not fully close. The LASIK suction ring may cause damage to structures inside the eye, such as the vitreous, retina, and optic nerve. Like RK and PRK, LASIK is associated with night vision complaints. These typically occur when a patient’s pupils dilate at night, allowing light to enter through a large surface area of the irregular post-operative cornea.