Your Guide to LASIK Eye Surgery
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.