Because conventional laser treatment profiles have small blend zones and create a more oblate corneal shape postoperatively, they are likely to induce some degree of higher-order aberration, especially spherical aberration and coma. These aberrations occur because the corneal curvature is relatively more angled peripherally in relation to laser pulses emanating from the central location; thus, the pulses hitting the peripheral cornea are relatively less effective than central pulses.
Wavefront-optimized laser ablation improves the postoperative corneal shape by taking the curvature of the cornea into account and increasing the number of peripheral pulses; this approach minimizes the induction of higher-order aberrations and often results in better-quality vision and fewer night vision complaints. As in conventional procedures, the patient’s refraction alone is used to program the wavefront-optimized laser ablation. This technology does not directly address preexisting higher-order aberrations; however, recent studies have found that the vast majority of patients do not have significant preoperative higher-order aberrations. It also has the advantage of being quicker than wavefront-guided technology and avoids the additional expense of the aberrometer.
In wavefront-guided laser ablation, information obtained from a wavefront-sensing aberrometer (which quantifies the aberrations) is transferred electronically to the treatment laser to program the ablation. This is distinct from conventional excimer laser and wavefront-optimized laser treatments, in which the subjective refraction is used to program the laser ablation. The wavefront-guided laser attempts to treat both lower-order (myopia or hyperopia and/or astigmatism) and higher-order aberrations by applying complex ablation patterns to the cornea to correct the wavefront deviations. The correction of higher-order aberrations requires non–radially symmetric patterns of ablation (which are often much smaller in magnitude than ablations needed to correct defocus and astigmatism). The difference between the desired and the actual wavefront is used to generate a 3-dimensional map of the planned ablation. Accurate registration is required to ensure that the ablation treatment actually delivered to the cornea matches the intended pattern. Such registration is achieved by using marks at the limbus prior to obtaining the wavefront patterns or by iris registration, which matches reference points in the natural iris pattern to compensate for cyclotorsion and pupil centroid shift. The wavefront-guided laser then uses a pupil-tracking system, which helps to maintain centration during treatment and allows the accurate delivery of the customized ablation profile.
The results for both wavefront-optimized and wavefront-guided ablations for myopia, hyperopia, and astigmatism are excellent, with well over 90% of eyes achieving 20/40 or better uncorrected visual acuity. Although most Snellen acuity parameters are similar between conventional and customized treatments (including both wavefront-optimized and wavefront-guided treatments), the majority of recent reports demonstrate improved visual quality when customized treatment profiles are used. Outcomes with wavefront-optimized treatments are comparable to those of wavefront-guided treatments for most patients, except those who have significant preoperative higher-order aberrations.
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