Laser in situ keratomileusis (LASIK) was initially reported by Ioannis Pallikaris in 1990 and has become an efficient and most popular procedure in corneal refractive surgery. However, a reduction of visual performance and the occurrence of higher-order aberrations (such as glare and halo under dim conditions, decrease of contrast sensitivity and poor night vision) have been reported after the surgery.
The introduction of wavefront-guided laser technology in 1999 was a significant step forward for the field of refractive surgery, as it allowed not only the correction of spherocylindrical errors, but also higher-order aberrations. Hence this surgical procedure was a great candidate for reducing post-LASIK night vision problems and improved resulting vision acuity.
Fundamentals of the technology
Wavefront-guided LASIK represents a refinement of classical refraction methods where the laser correction is tailored to the specific pattern of corneal refractive aberration in each patient. As the cylindrical aberration in corneas is not always uniform, applying a homogeneous correction over the entire area may not give the optimal result.
The surgeon begins the procedure by using the wavefront device to transmit a safe ray of light into the eye. The light gets reflected back off the posterior portion of the eye and goes out through the pupil before it finally reaches the abberometry device, where the spherocylinder refraction is calculated based on a 4-mm entrance pupil. Such reflected wave of light is then arranged into a unique pattern that measures sphere and cylinder of the eye, as well as the higher-order aberrations.
Acquired visual measurements are then displayed as a 3D map, also known as a wavefront map. Nomograms to independently adjust the amount of sphere and cylinder treatment should also be developed. Formatted and processed information can then be electronically transferred to the laser and computer-matched to the position of the eye, allowing the specialist to personalize the LASIK procedure for specific visual requirements of each patient.
The VISX WaveScan system was the first laser based on this technology to be approved by the American Food and Drug Administration (FDA) for the treatment of myopia (nearsightedness) and astigmatism. It measures the refractive error and wavefront aberrations of the human eye using a Shack-Hartmann wavefront principle. It consists of the S4 excimer laser and the WaveScan wavefront device.
Wavefront-guided laser ablations are superior to conventional LASIK surgery, and some authors indicate that the end-result can even be so-called “super vision”. Ocular or total wavefront-guided ablations have demonstrated effectiveness in minimizing aberrations without previous unsuccessful or non-optimized refractive eye surgeries. Still, this technique is not appropriate for every eye.
There are some unique challenges and obstacles for application of wavefront technology. Human eye is a dynamic system with constant biological fluctuations, such as changes in the pupil size and focus, thus it significantly differs from a telescope which is static in nature. Although wavefront represents an excellent mapping system, it does not necessarily mean that the result will be the exact replica. In other words, ablating corneal tissue is not nearly so precise.
Wavefront-guided ablations may result in a removal of more tissue than conventional ablations, depending on the individual patient. In cases of thin corneas, that could be an issue. In addition, benefits of wavefront guided treatment may be masked by epithelial hyperplasia in such surface ablations.
In conclusion, when considering any type of corneal-based refractive surgery (LASIK, Epi-Lasik, photorefractive keratectomy), a wavefront diagnostic should be considered. This approach will help to determine if critical higher-order aberrations are below normal, normal or elevated. If they are elevated, either wavefront-guided LASIK is required, or no surgery is suitable.