Laser Eye Surgery

Quick History:

Corrective eye surgery has been in practice since the 1940’s. Radial keratotomy (RK) and photorefractive keratectomy (PRK) have had the greatest impact on current methods of refractive eye surgery. The most common refractive surgery used today is Laser in Situ Keratomileusis (LASIK). RK uses a diamond tipped knife to flatten the central cornea by making a series of incisions radiating from the cornea. RK can only treat low degrees of myopia and astigmatism, unlike LASIK. PRK uses a laser to correct the refractive area by removing corneal tissue. PRK can be discomforting or even painful because it removes nerve fibers that run through the corneal epithelium. After this surgery, it can take up to six months for vision to stabilize. Immediately following the surgery, vision is blurry until the corneal epithelium healed, and the bandage lens removed. PRK is similar to LASIK because it uses a laser to correct the corneal defects. LASIK has the same benefits, but with fewer, less-sever side effects than PRK. Also, LASIK is less painful, and recovery time is shortened because the corneal epithelium is not removed.

The first lasers used were broad beam lasers (about 6 mm) that acted similar to a showerhead, splitting the main laser into smaller beams. Current laser technology implements the “flying spot laser”, named to describe the technique of creating a pattern with the laser, so that the full width of the flying spot beam hits the cornea with each pulse. Advantages of this type of laser include a smoother corneal shape, and increased accuracy and efficiency of the surgery.

The Role of Physics in Laser Eye Surgery:

The deficiency of the eye can be examined using the physics of refraction. Refraction occurs mostly at the cornea due to the substantial difference between the index of refraction of the cornea, and that of air. The cornea and the lens combine to focus the light at the retina. The eye adjusts the curvature of the lens in order to alter the focal length, enabling it to focus objects at varying distances. A normal eye is able to converge light properly by changing the lens shape so that objects both near and far appear clearly.

In nearsightedness, the cornea is overly curved resulting in a greater index of refraction. The light converges, and the image forms in front of the retina. In order to correct this problem with laser eye surgery the procedure begins with a surgeon temporarily peeling back the epithelium. A laser is then trained on the eye to remove small amounts of the cornea. The cornea is flattened in order to correct nearsightedness. The flattened cornea has a lower index of refraction than previously, focusing the image on the retina instead of in front of the retina.

Another dysfunction of the eye is farsightedness. In farsightedness, the cornea is not curved enough, and thus, the refractive index is lower. This causes the image’s focal point to be located behind the retina. The surgeon corrects this condition by using a laser to remove the outer edge of the cornea, thus steepening it. As a result of this surgery, the eye is able to focus the image directly on the retina, due to the greater index of refraction resulting from the curvature of the eye being increased.