for more detailed discussion of higher-order aberrations.

In an effort to reduce preexisting aberrations and minimize the induction of new aberrations, wavefront-guided ablation creates ablation profiles that are customized for individual patients. In addition to addressing higher-order aberrations, wavefront-guided treatments can correct the lowerorder aberrations of spherical error and astigmatism. Wavefront-optimized lasers have changed the ablation profile of conventional treatments by adding more prolate peripheral ablation, thereby reducing spherical aberration; however, they have no effect on other higher-order aberrations.

Compared with conventional excimer laser ablation, wavefront-guided ablations and wavefrontoptimized ablations appear to offer better contrast sensitivity and induce fewer postoperative higherorder aberrations. Although advances in aberrometry and registration systems have led to improved outcomes, patients who undergo photoablation may still have more higher-order aberrations postoperatively than they did preoperatively.

Wavefront-guided ablation appears to have clear-cut benefit compared with wavefront-optimized ablation only for patients with significant preoperative higher-order aberrations; thus, wavefrontguided ablation is not suitable for all patients and may not be appropriate for use after cataract surgery, particularly with multifocal intraocular lenses. In addition, wavefront data may be impossible to obtain in highly irregular corneas or in eyes with small pupils. In the future, patients with highly irregular corneas that cannot be treated with wavefront technology may be treated with topography-based ablations. Topography-based ablations are not yet approved by the US Food and Drug Administration (FDA) but are widely used in Canada, Europe, and other countries. In general, wavefront-guided ablations remove more tissue than conventional ablations do.

Nuijts RM, Nabar VA, Hament WJ, Eggink FA. Wavefront-guided versus standard laser in situ keratomileusis to correct low to moderate myopia. J Cataract Refract Surg. 2002;28(11): 1907–1913.

Stonecipher KG, Kezirian GM. Wavefront-optimized versus wavefront-guided LASIK for myopic astigmatism with the ALLEGRETTO WAVE: three-month results of a prospective FDA trial. J Refract Surg. 2008;24(4):S424–S430.

Patient Selection for Photoablation

The preoperative evaluation of patients considering refractive surgery is presented in detail in Chapter 2. Table 5-1 lists relative contraindications to photoablation.

Table 5-1

Special Considerations for Surface Ablation

In general, any condition that significantly delays epithelial healing is a relative contraindication to surface ablation. Although keloid scar formation was listed as a contraindication to PRK in FDA

trials, 1 study found that African Americans with a history of keloid formation did well after PRK, and keloid formation is no longer considered a contraindication to surface ablation or LASIK. Historically, patients taking isotretinoin or amiodarone hydrochloride were excluded from undergoing excimer laser procedures, although there is little evidence that these drugs adversely affect laser keratorefractive outcomes.

Patients with epithelial basement membrane dystrophy (EBMD) are better candidates for surface ablation than for LASIK because surface ablation may be therapeutic, reducing epithelial irregularity and improving postoperative quality of vision while enhancing epithelial adhesion. In contrast, LASIK may cause a frank epithelial defect in eyes with EBMD, especially when performed with a mechanical microkeratome.

Any patient undergoing excimer laser photoablation should have a pachymetric and topographic evaluation (see Chapter 2). Younger patients and patients with thin corneas, low predicted residual stromal bed thickness, or irregular topography may be at increased risk for the development of ectasia with LASIK. As such, these patients may be better candidates for surface ablation. Patients with subtle topographic pattern abnormalities need to be evaluated on a case-by-case basis. In some circumstances, patients who are stable may be offered surface ablation but with a clear acknowledgment, as well as a signed informed consent form, that they understand there may still be a risk of progression to corneal ectasia.

Special Considerations for LASIK

The preoperative evaluation of patients for LASIK is similar to that for surface ablation. A narrow palpebral fissure and a prominent brow with deep-set globes both increase the difficulty of creating a successful corneal flap, and the presence of either may lead a surgeon to consider surface ablation over LASIK.

Many reports indicate that postoperative dry eye due to corneal denervation is more common with LASIK than with surface ablation. This difference is important to remember when considering refractive surgery in a patient with known dry eye syndrome. Nevertheless, many patients undergoing PRK will also experience postoperative dry eye, but it is believed that this occurs to a lesser extent than for LASIK patients.

Corneal topography must be performed to assess corneal cylinder and rule out the presence of forme fruste keratoconus, pellucid marginal degeneration, or contact lens-induced corneal warpage. Corneas steeper than 48.00 D are more likely to have thin flaps or frank buttonholes (central perforation of the flap) with procedures using mechanical microkeratomes. Corneas flatter than 40.00 D are more likely to have smaller-diameter flaps and are at increased risk for creation of a free cap due to transection of the hinge with mechanical microkeratomes. These problems may be reduced by using a smaller or larger suction ring, which changes the flap diameter; modifying the hinge length; slowing passage of the microkeratome to create a thicker flap or using a microkeratome head designed to create thicker flaps; applying higher suction levels and creating a higher intraocular pressure (IOP); or selecting a femtosecond laser to create the lamellar flap. If a patient is having both eyes treated in a single session, the surgeon must be aware that using the same blade to create the flap in the second eye typically results in a flap that is 10–20 µm thinner than the flap in the first eye. In addition, there is some concern about transferring epithelium and/or infectious agents between eyes. These specific concerns are greatly minimized with the use of a femtosecond laser for flap creation.

Preoperative pachymetric measurement of corneal thickness is mandatory because an adequate stromal bed must remain to decrease the possibility of postoperative corneal ectasia, although the definition of what constitutes an adequate residual stromal bed (RSB) remains controversial. The