Myopia (Nearsightedness)
 Myopia affects about one of every four people in North America and is the most common refractive problem. It is generally characterized by an eyeball that is too long in the relation to the rest of its parts. In some cases, it may also be characterized by a cornea with more curvature than normal. As a result, light rays converge to a focal point before they reach the retina and then they begin to diverge. Since the retina can only respond to the quality of the image that reaches it, the brain receives unclear messages of visible objects to interpret.
People who are myopic see near objects more clearly than distant objects. The reason for this is that the light rays from near objects are still diverging when they reach the eye, thus counteracting the eye's tendency to cause them to converge too soon and allowing some degree of focused vision. Light rays from distant objects are more parallel, so they converge before they reach the retina of an elongated eye.
Excimer Laser Correction of Myopia
When directed at nearsighted eyes, an excimer laser can vaporize away microscopic layers of tissue to reduce the curvature of the cornea and improve focus.
On the day of surgery, candidates arrive at the surgical center and prepare for surgery. The selected eye is anesthetized with drops. For the procedure, candidates are positioned lying face up under the laser. The surgeon programs the microprocessor responsible for controlling the beam of ultraviolet light.
Since the eye must be held very still while the laser does its sculpting, stabilizing devices may be used to hold the head still. The surgeon spends time training candidates to fixate their designated eye so it will remain still for the procedure. However, the ALLEGRETTO WAVE Eye-Q laser is equipped with an iris recognition tracker. This highly sophisticated safety device locks in on the iris and ensures the treatment to the eye is applied in the same way as the information was acquired. The system tracks tiny eye movements and guides the laser to follow them. This ensures the laser treats the correct areas of your eye.
After the training, the surgeon gently removes the outer protective layer of the cornea known as the epithelium (as in PRK) or cuts a corneal flap to expose the stromal layers (as in LASIK). He then sets the laser controlling devices, verifies the candidate's eye position, and at the appropriate time, presses the control switch. The unique flying spot laser light sculpts the cornea into the planned new shape. The total laser application time is only seconds.
The excimer laser beam can be controlled so precisely that it can remove tissue 0.25 microns at a time. This ability is amazing considering that one-micron equals one millionth of a meter. The cornea is about 550 microns thick at its thinnest point. Most excimer procedures remove less than 50 microns of tissue, which is less than 10% of the original corneal thickness.
After PRK surgery, candidates wear a therapeutic bandage contact lens for one to three days until the epithelium replaces itself. In LASIK surgery there is no need for the contact lenses, as the epithelium is left intact and the procedure is painless. In the LASIK procedure vision improvement is almost instantaneous. With PRK good vision and stability may take several months.
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