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 increase the difficulty of creating a successful corneal flap. 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; however, 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 microkeratome 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 RSB remains controversial. The following formula is used to calculate the RSB:
RSB = Central Corneal Thickness – Thickness of Flap – Depth of Ablation
Although most practitioners use a minimum RSB of 250 μm as a guideline, this figure is clinically derived rather than based on any definitive laboratory investigations or controlled prospective studies. A thicker stromal bed after ablation does not guarantee that postoperative corneal ectasia will not develop. Moreover, the actual LASIK flap may be thicker than that noted on the label of the microkeratome head, making the stromal bed thinner than the calculated minimum of 250 μm. Consequently, many surgeons use intraoperative pachymetry—especially for high myopic corrections, enhancements, or thin corneas—to determine actual flap thickness.
Determining flap thickness and RSB via intraoperative pachymetry, rather than by estimating thickness based on the markings on the plate, provides the most accurate data. This is accomplished by measuring the central corneal thickness at the beginning of the procedure, creating the LASIK flap with the surgeon’s instrument of choice, lifting the flap, measuring the untreated stromal bed, and subtracting the intended thickness of corneal ablation from the stromal bed to ascertain whether the RSB will be 250 μm or whatever safe threshold is desired following ablation. Flap thickness is then calculated by subtracting the untreated stromal bed measurement from the initial central corneal thickness. It is important to measure the corneal bed thickness quickly after making the flap in order to avoid corneal thinning from exposure to the air.
The surgeon should preoperatively inform patients with thinner corneas or higher corrections that future LASIK enhancement may not be possible because of an inadequate RSB. These patients may be better candidates for surface ablation enhancements if needed.
Many ophthalmologists believe that excessive corneal flattening or steepening after LASIK may reduce vision quality and increase aberrations. Thus, many of them avoid creating overly flat or overly steep corneas, although no established guidelines are available on the specific values to avoid. The surgeon can estimate the postoperative keratometry by calculating a flattening of 0.80 D for every diopter of myopia treated and a steepening of 1.00 D for every diopter of hyperopia treated (see Chapter 2).
If wavefront-guided laser ablation is planned, wavefront error is measured preoperatively, as discussed in Chapter 1. Although wavefront data are used to program the laser, the surgeon must still compare these data to the manifest refraction before surgery to prevent data-input errors. In general, substantial differences between the manifest refraction and the wavefront refraction should alert the surgeon to a potentially poor candidate for the procedure.
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Excerpted from BCSC 2020-2021 series: Section 13 - Refractive Surgery. For more information and to purchase the entire series, please visit https://www.aao.org/bcsc.