The eye is a complex optical system - very similar to a camera. Vision begins when light enters the eye through the cornea, a powerful focusing surface. From there, it travels through clear aqueous fluid, and passes through a small aperture called the pupil. As muscles in the iris relax or constrict, the pupil changes size to adjust the amount of light entering the eye. Light rays are focused through the lens, and proceed through a clear jelly-like substance in the center of the eye called vitreous, which gives it form and shape. When light rays finally land on the retina, the part of the eye similar to film in a camera, they form an upside-down image. The retina converts the image into an electrical impulse that travels along the optic nerve to the brain, where it is interpreted as an upright image.
A myopic (nearsighted) eye is too long. Instead of focusing on the retina, images fall in front of it, and vision is blurry.
Many forms of refractive surgery improve vision by permanently changing the shape of the cornea, the clear front window of the eye. When treating myopia, or nearsightedness, certain refractive surgery techniques reduce the curvature of the cornea to lessen the eye's focusing power. Images that are focused in front of the retina, due to an elongated eye or steep corneal curve, are pushed closer to or directly onto the retina following surgery.
A hyperopic (farsighted) eye is too short. Instead of focusing on the retina, images focus beyond the retina, and vision is blurry.
When treating hyperopia, or farsightedness, certain refractive surgery techniques make the cornea steeper to increase the eye's focusing power. Images that are focused beyond the retina, due to a short eye or flat cornea, are pulled closer to or directly onto the retina following surgery.
Astigmatism occurs when the cornea is more curved in one direction than the other, like the shape of a football. If astigmatism is significant, light passing through the cornea is scattered. Images reaching the retina are distorted and vision is blurred.
When treating astigmatism, refractive surgery techniques selectively reshape portions of the cornea to make it symmetrical and smooth, like the shape of a basketball, so that images focus clearly on the retina.
Presbyopia is part of the normal ageing process and results in blurring of near vision. As we age the muscles supporting the lens of our eye become less able to focus the lens for near vision. This is why most people find that in their 40s they require reading glasses. New surgical techniques such as PRELEX have been highly successful in treating people with presbyopia.
Common refractive surgery procedures include:
- Laser in situ keratomileusis (LASIK)
- Epithelial-LASIK (Epi-LASIK)
- Advanced surface ablation (ASA), including photorefractive keratectomy (PRK) and laser epithelial keratomileusis (LASEK)
- Phakic intraocular lenses (IOLs)
- PRELEX (Presbyopic Lens Exchange)