Before you learn how LASIK works, it's important to know about the structure of the eye. Here you'll find information about how the eye works as well as:
LASIK stands for laser-assisted in-situ keratomileusis. In order to understand how LASIK works, it is important to understand how the eye sees. In its simplest sense, your eye is like a camera. Your eye has:
- A variable opening called the pupil
- A lens system, which includes the transparent covering called the cornea and a spherical lens
- A reusable "film" called the retina
Various sets of muscles (The muscles control the size of the pupil, the shape of the lens and the movements of the eye.)
(The parts of the eye) On the back of your eye is a complex layer of cells known as the retina. The retina reacts to light and conveys that information to the brain. The brain, in turn, translates all that activity into an image. Because the eye is a sphere, the surface of the retina is curved.
In the retina, sensory cells called rods and cones change the photons of light into electrical signals, which are then transmitted to and interpreted by the brain.
Light enters the eye and an image is focused on the retina.
In order to see clearly, images must be focused on the retina. The ability to focus the light on the retina is dependant on the shapes of the cornea and the lens, which are controlled by their inherent shapes, their stretchiness or elasticity, the length of the eyeball and muscles attached to the lens.
As shown below, light passes through the cornea and pupil, then it is bent (refracted) by the cornea and lens, and comes to a point (focus) on the retina, where the image is formed.
As an example, when you look at a near object, muscles attached to the lens must contract to change the shape of the lens and thus focus the near object on the retina. When looking at a distant object, these same muscles relax again changing the lens shape to focus the distant object on the retina.
Most vision problems occur when the eye cannot focus the image onto the retina. Here are a few of the most common problems:
- Myopia (nearsightedness)
- Hyperopia (farsightedness)
In nearsightedness (myopia), the light from distant objects focuses in front of the retina rather than on it. Myopia occurs usually when the eyeball is too long or when the corneal curvature is too steep. The result is that the person can see close-up objects clearly, but distant objects are blurry.
If you are nearsighted, the image comes into focus before it hits your retina.
In farsightedness (Hyperopia), the light focuses behind the retina rather than on it. Hyperopia usually occurs when the eyeball is too short or when the corneal curvature is to flat. The result is that a person can see distant objects clearly, but close-up objects are blurry.
If you are farsighted, the image doesn't come into focus before it hits your retina.
In astigmatism, the shape of the cornea or the lens is distorted so that the light focuses in more than one spot. This is commonly caused by the cornea not being perfectly round like a basketball but being shaped similar to a football.
In Presbyopia, the lens of the eye becomes less elastic, and therefore cannot change shape as readily to bring light to a focus on the retina; this happens naturally as we grow older and is usually observed when people reach their 40s. If you have Presbyopia, you have trouble focusing light from near objects on the retina. To correct this problem, you might get a pair of bifocal lenses to replace your existing glasses. If you don't already wear corrective lenses, you may be able to simply use reading glasses.
LASIK is very effective in treating myopia, astigmatism and Hyperopia. However, Presbyopia is not easily corrected through the use of laser eye surgery. To make up for Presbyopia one eye can be corrected for seeing at distance and the other eye for near. This is called monovision.
Let's take a look at exactly what LASIK is and you'll understand why it works so well.
Refractive Surgery is any surgery of the eye used to correct the focusing abnormalities (Myopia, Hyperopia, Astigmatism, and Presbyopia) in an eye.
The basis for all refractive laser eye surgery is to reshape the cornea so that it changes the focal point of the eye. Ideally, the focal point is changed so that it focuses perfectly on the retina, just like a normal functioning eye.
As stated in the previous section, myopia (nearsightedness) usually results from the eye being too long. The cornea has a more pronounced curve than a normal eye. Laser eye surgery is great for myopia because it is relatively easy to remove a little of the cornea to flatten it.
Hyperopia (farsightedness) normally means the eye is too short, or the cornea is too flat. Hyperopia can often be corrected with the excimer laser.
Astigmatism can also be corrected using the excimer laser by treating an elliptical pattern on the cornea.
Laser eye surgery works by pulsing a tightly-focused beam of light (laser) onto the surface of the eye. Upon contact with the surface of the cornea, the laser vaporizes a microscopic portion of the cornea (more on this later). By controlling the size, position and number of laser pulses, Dr. Davis can precisely control how much of the cornea is removed.
When LASIK is performed, a flap of the outer corneal tissue is created either with a metal blade (a microkeratome) or with a laser (Intralase). This flap is then folded out of the way during the laser treatment. Once the flap is folded out of the way, the excimer laser is used to reshape the underlying corneal tissue. Then the flap is replaced over the reshaped area and conforms to the new shape.
The great thing about the cornea is how quickly it heals. As soon as that flap is replaced, it begins to naturally seal itself to the rest of the cornea without the need for sutures.
Custom or Wavefront LASIK is a newer LASIK treatment than standard LASIK. Standard LASIK or PRK has been available in the U.S. since 1995. It has worked beautifully well, however like most things; we are always trying to improve. The improvement is custom or Wavefront LASIK.
Every eye has optical irregularities. The large irregularities most people are familiar with namely nearsightedness (Myopia), farsightedness (Hyperopia), and astigmatism. Each eye has smaller irregularities as well and like a fingerprint, no two eyes are exactly the same. These smaller irregularities, when grouped together, are referred to as “higher order aberrations”. Until Wavefront came along, we were unable to detect or treat these higher order aberrations. Wavefront is a technology that has been used in Astronomy to improve the irregularities in images coming from distant objects. With respect to the eye we use it to improve the higher order aberrations. Using Wavefront technology, we are able to detect the higher order aberrations and relay this information to the excimer laser. The laser than treats these aberrations and thus we are able to customize the laser treatment.
Many studies have indicated that using custom or Wavefront LASIK increases your likelihood of obtaining sharper vision postoperatively.
The development of the excimer laser is the key element that has made refractive laser eye surgery possible. Created by IBM, excimer lasers (the name is derived from the terms excited and dimers) use reactive gases, such as chlorine and fluorine, mixed with inert gases for example argon, krypton or xenon. When electrically stimulated, the gas mixture emits light in the ultraviolet range.
The excimer laser is a cool laser, which means that it does not heat up the surrounding air or surfaces. Instead, a very tightly-focused beam of ultraviolet light is emitted. The ultraviolet light is absorbed by the upper layer of the surface that it contacts. The sheer amount of ultraviolet light is too much for most organic materials (such as the cornea of the eye) to absorb, resulting in the breakdown of the molecular bonds of the material. This process is known as photoablation.
The excimer laser is incredibly precise. It has the ability to focus a beam as small as 0.25 microns. Considering that a typical human hair is 50 microns in diameter, which means that the excimer laser is capable of removing 0.5 percent of a human hair's width at a time!
There are many excimer lasers currently on the market. The VISX is the most used laser in the U.S. There are many versions of the VISX laser as it has been available for many years. Many laser centers use older VISX models. At Davis Vision Center we only offer the most advanced version the VISX Star S4 with Iris registration. This version is able to precisely place each laser pulse even if your moves during the procedure by track small spots on your iris. This version also uses fourier transform algorithms to treat your higher order aberrations.
When the LASIK procedure is performed, a flap of corneal tissue is created. This flap is subsequently lifted and an excimer laser is used to treat your prescription. The flap is then replaced. This flap has traditionally been created using a metal device called a microkeratome. The INTRALASE uses a laser to create the flap instead of the microkeratome allowing the LASIK procedure to be totally blade-free.
Some Lasik centers still offer only the bladed microkeratome for your flap creation. At Davis Vision Center we offer the INTRALASE laser. Even though Dr. Davis has performed thousands of procedures using the microkeratome, he still feels that the INTRALASE reduces the risk of flap complications and is thus safer.