Eye surgery is one of the oldest and most familiar applications of laser technology, and as laser technology continues to improve, the results of laser eye surgery become increasingly predictable. The most fundamental improvements are found in the areas of laser control and a better understanding of the physics involved.
One of the most common medical applications of the laser occurs in the area of vision correction such as Laser-Assisted in situ Keratomileusis ( LASIK ). With respect to vision correction, the problem laser surgery seeks to address is called refractive error. To appreciate how ophthalmologists seek to correct refractive error through the use of lasers, it is important to appreciate precisely what refractive error is and how it arises. A mechanical, very unromantic, way of describing the eye is as a device for converting electromagnetic energy from the visible part of the electromagnetic spectrum into electrical signals.
The electrical signals are conveyed to the brain via the optic nerve, and the brain interprets the signals received. Essentially, eyes are for looking, and brains are for seeing. At the front of the eye is the cornea. It covers and protects the front of the eye and also helps focus the light that enters the eye. Behind the cornea is the opening that permits light to enter the eye. That opening is called the pupil. The pupil is small and circular. It looks black, but it is not. The pupil seems dark for the same reason that windows on a house appear dark on a sunny day. The windows are transparent, but the rooms behind them are darker than the outside environment. Similarly, the pupil is the window of the eye. It appears dark because the interior of the eye, which is transparent, is darker than the outside. Surrounding the pupil is the iris, the colored part of the eye. The iris regulates the amount of light entering the eye by opening and closing. Behind the pupil is the lens, which further focuses the incoming light so that a sharp image appears on the retina. The shape of the lens can be changed somewhat as the ligaments around it are tightened or relaxed. This enables one to bring a slightly blurred object into focus. There are, however, limitations to what the lens can do, and despite its name, much of the work in focusing the incoming light is done by the cornea rather than the lens. (The interior of the eye is filled with a transparent material, made mostly of water, called the vitreous humor.) When the image that is formed on the retina is not clear, the brain is unable to form a clear picture from the electrical images that it receives.
The most common reason for a blurry image on the retina involves difficulty focusing the image properly. Ophthalmologists have long known that (in theory) one way to correct blurred vision is to operate directly on the cornea with the aim of changing its shape. Changing the shape of the cornea changes where and how the light entering the eye comes to a focus. Early efforts at sculpting the cornea were done mechanically, and they were very crude. The first attempts, developed in the late 1940s and early 1950s, involved slicing a discshaped piece of cornea off the front of the eye, freezing it, grinding the frozen tissue into the appropriate shape on a special-purpose lathe, and then replacing it. The technique was crude, and it was used only in the most extreme cases.
Today, the same concepts involved in cornea sculpting are carried out with lasers. The procedure begins by using a blade called a microkeratome to open a thin flap on the front of the cornea. The flap is opened much like the hood of a car, exposing the tissue underneath. Next, a high-energy, pulsed laser, operating in the ultraviolet portion of the spectrum, begins to cut the cornea. The duration of the pulses are in the nanosecond—one billionth of a second—range. These brief, high-energy pulses enable the user to destroy selected tissue segments without damaging neighboring tissues by thermal conduction. Layers of tissue on the order of a micrometer (one millionth of a meter) are removed with each pulse of the laser. With such small tolerances, it would not be correct to say that the doctor is performing the operation.
Such levels of precision are far beyond what any individual could achieve. The laser is computer controlled. The machine follows its program, directing and firing the laser, which continues to ablate, or remove, portions of tissue until it has pared the cornea down to some predetermined shape. The flap is then closed—no stitches are necessary—and provided all goes well, the incision soon heals, and the patient’s vision is improved.