Photoablative ­lasers can be subdivided into broad-­beam ­lasers, scanning-­slit ­lasers, and flying spot ­lasers. Broad-­beam ­lasers have larger-­diameter beams and slower repetition rates and rely on optics or mirrors to create a smooth and homogeneous multimode ­laser beam of up to approximately 7 mm in dia­meter. ­These ­lasers have very high energy per pulse and require a small number of pulses to ablate the cornea. Scanning-­slit ­lasers gen­ erate a narrow-­slit ­laser beam that is scanned over the surface of the tissue to alter the photoablation profile, thus improving the smoothness of the ablated cornea and allow­ ing for larger-­diameter ablation zones. Flying spot ­lasers use smaller-­diameter beams (ap­ proximately 0.5–2.0 mm) that are scanned at a higher repetition rate; they require use of a tracking mechanism for precise placement of the desired pattern of ablation. Broad-­beam ­lasers and some scanning-­slit ­lasers require a mechanical iris diaphragm or ablatable mask to create the desired shape in the cornea, whereas the rest of the scanning-­slit ­lasers and the flying spot ­lasers use a pattern projected onto the surface to guide the ablation profile without masking. The majority of excimer ­lasers in current clinical practice use some form of variable or flying spot ablation profile.

­Because conventional ­laser treatment profiles have small blend zones and create a more oblate corneal shape following myopic corrections, they are likely to induce some degree of higher-­order aberration, especially ­spherical aberration and coma. ­These aberrations occur ­because the corneal curvature is angled more peripherally in relation to ­laser pulses emanating from the central location; thus, the pulses hitting the peripheral cornea are relatively less effective than are the central pulses.

Wavefront-­optimized ­laser ablation improves the postoperative corneal shape by tak­ ing 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 re­ sults in better-­quality vision and fewer night-­vision concerns ­because it maintains a more prolate corneal shape. 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 substantial preoperative higher-­order aberrations. It also has the advantages of being faster than wavefront-­guided technology and avoiding the additional expense of an aberrometer.

In wavefront-­guided ­laser ablation, information obtained from a wavefront-­sensing ab­ errometer (which quantifies the aberrations) is transferred electronically to the treatment ­laser to program the ablation. This pro­cess differs from conventional excimer ­laser and wavefront-­optimized ­laser treatments in which subjective refraction alone is used to pro­ gram the ­laser ablation. The wavefront-­guided ­laser is intended to treat both lower-­order (ie, myopia or hyperopia and/or astigmatism) and higher-­order aberrations by applying com­ plex 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 before obtaining the wavefront patterns or by employing 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 maintain centration during treatment and allows accurate delivery of the customized ablation profile. Another potential advan­ tage is that the planned ­laser treatment is transferred directly from the aberrometer and does not require manual entry, helping to avoid transcription errors.

The results for both wavefront-­optimized and wavefront-­guided ablations for myo­ pia, hyperopia, and astigmatism are excellent, with well over 90% of eyes achieving 20/40 or better uncorrected distance visual acuity (UDVA). Although most visual acuity par­ ameters are similar between conventional and customized treatments (including both wavefront-­optimized and wavefront-­guided treatments), most recent reports demon­ strate improved vision quality when customized treatment profiles are used. Outcomes of wavefront-­optimized treatments are similar to ­those of wavefront-­guided treatments for most patients, except for ­those with substantial preoperative higher-­order aberrations.

Miraftab M, Seyedian MA, Hashemi H. Wavefront-­guided vs. wavefront-­optimized LASIK: a randomized clinical trial comparing contralateral eyes. J Refract Surg. 2011;27(4):245–250.

Perez-­Straziota CE, Randleman JB, Stulting RD. Visual acuity and higher-­order aberrations with wavefront-­guided and wavefront-­optimized ­laser in situ keratomileusis. J Cataract Refract Surg. 2010;36(3):437–441.

Topography-guided (TG) lasers are similar in concept to wavefront-guided lasers, but TG laser ablations use data from refraction and corneal topography rather than wavefront data from the whole eye. TG ablations have been successful in regularizing the cornea and improving vision in patients with decentered ablation, small optical zone, and irregular corneal surface. A TG laser is now approved for performing ablations in eyes with normal corneas, as well. Analysis of the visual outcomes reported to the FDA for 249 myopic eyes with or without astigmatism that underwent TG laser ablation with one system showed excellent outcomes in terms of safety, efficacy, accuracy, and stability. Postoperatively, 93% of eyes had UDVA of 20/20 or better, and 95% had a manifest refraction spherical equivalent within ±0.5 D of emmetropia. A significant increase in mesopic (41.3%) and photopic (31.9%) contrast sensitivity was found 6 months postoperatively.