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.
Wavefront Guided (Custom) Treatments
As discussed earlier, conventional treatments are based on eyeglass prescription data from the phoropter, and custom treatments are derived from wavefront maps.
However, to truly understand the benefits of custom treatments, the concepts of lower order and higher order aberrations must be introduced.
The total distortion in any individual's vision consists of a combination of lower order and higher order aberrations. Lower order aberrations are more symmetric and can be measured with the phoropter and corrected with standard eyeglasses. Lower order aberrations include nearsightedness, farsightedness and astigmatism.
Higher order aberrations are irregular, often asymmetric forms of distortion that are left over after the lower order aberrations are subtracted out. Higher order aberrations cannot be measured by the phoropter or corrected with standard eyeglasses, and they carry unfamiliar names like coma, trefoil and spherical aberration. Most individuals have minimal higher order aberrations.
The relationship between lower order aberrations, higher order aberrations and the clarity of vision is simple. Lower order aberrations determine whether an individual needs glasses, while higher order aberrations determine the quality of vision. Individuals with significant lower order aberrations need to wear glasses, while those with small amounts don't. Low levels of higher order aberrations are associated with crisp, sharp vision, good contrast sensitivity and night vision free of glare and halos, while significant higher order aberrations are associated with washed-out vision, poor contrast sensitivity and nighttime glare and halos.
Examples of the relationship between aberrations and visual clarity:
Lower Order Higher Order: This is the typical scenario of a patient who seeks laser vision correction. The elevated lower order aberrations signify a need to wear corrective lenses, while the low level of higher order aberrations mean that the patient's quality of vision through glasses is good.
Lower Order Higher Order: This is the goal of laser vision correction. Because the lower order aberrations have been reduced, the patient no longer needs corrective lenses, and because the higher order aberrations remain low, the patient will enjoy a good quality of vision.
Lower Order Higher Order: This is a situation that laser vision correction patients seek to avoid. Though the lower aberrations are reduced and glasses are no longer needed, the presence of higher order aberrations can lead to a low quality of vision, poor contrast sensitivity and nighttime glare and halos.
Lower Order Higher Order: This is a relatively uncommon scenario sometimes found in patients seeking laser vision correction. The elevated lower order aberrations signify a need to wear glasses, while the elevated higher order aberrations mean that the patient's quality of vision is poor, even with glasses. This finding can be due to dryness, failure to remain out of contact lenses for an adequate period of time or a degenerative eye condition known as keratoconus, so affected patients must be carefully screened to determine if laser vision correction is appropriate.
The goal of modern laser vision correction is to reduce the patient's lower order aberrations so that glasses are no longer needed, while holding down the higher order aberrations, so that the quality of vision remains high.
Conventional laser treatments have successfully treated millions of patients. However, conventional technology causes an increase in higher order aberrations in most patients. Fortunately, the increase is usually small enough that the quality of vision remains good, but patients who notice a decrease in sharpness or nighttime vision disturbances after surgery are often found to have elevated higher order aberrations.
With the technology available in 2006, custom-wavefront treatments still lead to an increase in higher order aberrations in most patients. However, the increase is significantly less than with conventional treatments, and controlled studies have shown that custom-wavefront patients enjoy sharper vision, a greater ability to see contrasts and better night vision.
Wavefront Optimized Treatments
Wavefront-optimized treatments contain both conventional and custom features and are only available on the Wavelight Allegretto laser. Wavefront-optimized treatments are not the same as the wavefront-guided treatments just described. The logic behind wavefront-optimized treatments requires a detailed explanation.
Because several opposing factors are in play, higher order aberrations are still increased with custom laser treatments. On the beneficial side, the patient's existing higher order aberrations are measured with the wavefront analyzer and treated by the laser. However, on the detrimental side, new higher order aberrations are induced by the laser treatment itself and, when LASIK or IntraLASIK is performed, by creation of the corneal flap. Therefore, most patients end up with a small net increase in higher order aberrations.
With conventional treatments, the higher order aberrations are increased even more because the preexisting higher order aberrations are not measured or treated.
Wavefront-optimized treatments can be thought of as a product of reverse engineering. A large number of conventionally treated patients were studied, and it was found that most of the increase in higher order aberrations was due to spherical aberration. Therefore, instead of measuring and treating an individual's unique aberration pattern, wavefront-optimized treatments aim to keep higher order aberrations down by simply adding in additional treatment for spherical aberration.
Proponents of wavefront-optimized treatments point to data that shows patients with normal levels of preoperative higher order aberrations end up with the same amount of postoperative higher order aberrations, regardless of whether a wavefront-optimized or wavefront-guided approach is used. Detractors point out that the additional spherical aberration treatment is based on population averages, and because some eyes deviate significantly from the average, it is better to use a wavefront analyzer to measure and treat each individual's exact aberrations.
