Lasers 101

An Introduction to Vision Correction Lasers

IBM invented the excimer laser technology used in vision correction surgery as a means of etching delicate patterns onto computer chips. Excimer is short for "excited dimer" - in this case one consisting of argon and fluorine gas. When properly stimulated, this dimer releases 193-nanometer ultraviolet light.

Excimer laser light is ideal for laser vision surgery because it removes corneal tissue with great precision (25 100,000ths of a millimeter per pulse) without collateral damage to adjacent tissue.

The basic concept behind laser vision correction is simple. If the corneal contour is too steep, laser pulses flatten the center. Conversely, if it is too flat, pulses are placed to steepen the periphery.

Over the years, a number of significant advancements in laser technology have led to improved patient outcomes:

Mid 1990's: flying spot lasers - The first FDA-approved lasers were broad-beam. With broad-beam technology, the laser beam remains stationary, while the spot size varies in size and shape around a central reference point. Nearsightedness is treated with circular spots of gradually increasing diameter, and astigmatism is treated with rectangular spots of gradually increasing width, with the long end of the rectangle in-line with the direction of the astigmatism.

The downside to broad-beam technology is that spots can only be placed symmetrically around the central reference point, so asymmetric forms of distortion cannot be treated. Flying spot lasers overcome this limitation by moving the spot, so any location on the cornea can be precisely targeted.

Mid to late 1990's: trackers - Most patients are able to hold their eyes virtually motionless during laser treatment. However, small movements occasionally still occur. Trackers use specialized cameras to measure any movement and retarget the laser to the proper location. Because modern trackers take measurements hundreds or even thousands of times per second, even small, quick movements are easily tracked.

2002 to present: wavefront guidance - Before wavefront guidance, all laser treatments were based on a patient's eyeglass prescription as determined by the phoropter test - the familiar "Which is better one or two?" series of questions. Although data from this testing method are generally very good, only symmetric forms of distortion (nearsightedness, farsightedness and astigmatism) can be measured.

In 2002, wavefront analyzers entered the marketplace. These devices use a low-powered laser to directly scan the eye for the necessary correction, conveniently bypassing the subjective trial-and-error phoropter test. Wavefront analyzers measure visual distortion with unsurpassed precision and produce a fingerprint-like map of the subtle irregularities unique to each individual eye.

Wavefront-guided refers to laser treatments that are based on wavefront data. Because each eye receives a unique pattern of laser spots based its unique wavefront map, wavefront-guided treatments are also known as custom laser treatments.

2005 to present: automated registration - There are three basic steps to a wavefront-guided laser treatment. First, the eye is measured with the wavefront analyzer and a wavefront map is produced. Next, software within the wavefront analyzer reads the wavefront map and designs a unique pattern of laser spots to be placed on the eye. Finally, the proposed treatment is loaded into the laser and delivered.

Because wavefront maps are complex three-dimensional shapes, a potential difficulty lies in delivering each laser spot to the exact location on the cornea from which the wavefront data was obtained.

Before automated registration, surgeons obtained wavefront data in reference to the center of the pupil and aligned the laser with the center of the pupil during treatment. This approach was imperfect because the center of the pupil often shifts position under the bright lights of the laser and because misalignment can be induced by clockwise or counter-clockwise rotation of the eye around a stationary pupil center.

With automated registration, a picture of the eye is taken concurrently with the wavefront reading. Stationary reference points such as blood vessels or iris landmarks are identified. Once under the laser, another picture is taken, the reference landmarks are located, and the laser's targeting computer compensates for any shift in the location of the pupil center or rotation of the eye.

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