Re-treatment (Enhancements)

Although excimer laser ablation reduces refractive error and improves UCVA in almost all cases, some patients have residual refractive errors and would benefit from re-treatment. The degree of refractive error that warrants re-treatment varies depending on the patient’s lifestyle and expectations. Re-treatment rates also vary, depending on the degree of refractive error being treated, the laser and nomograms used, and the expectations of the patient. One advantage of LASIK over surface ablation is that refractive stability generally occurs earlier, allowing earlier enhancements, typically within the first 3 months after LASIK. With surface ablation, the ongoing activation of keratocytes and the risk of haze after enhancement usually require a wait of 3–6 months before an enhancement surface ablation is undertaken. Typically, re-treatment rates are higher for hyperopia and for high astigmatism than for other indications.

Studies showed that rates of re-treatment are higher for higher initial correction, residual astigmatism, and patients older than 40 years. One should be careful when enhancing a myopic shift in a patient older than 50 years, as this may be due to a lens-induced myopic shift rather than post–refractive surgery regression. Re-treatment rates vary from 1% to 11%, based on surgeon experience, patient demands, and the other factors just described. Surface ablation re-treatment is nearly identical to primary surface ablation treatment, whereas LASIK re-treatment can be performed either by lifting the preexisting lamellar flap and applying additional ablation to the stromal bed or by performing surface ablation on the LASIK flap. In most cases, the flap can be lifted many years after the original procedure. However, because of the safety of surface ablation after LASIK and the increased risk of epithelial ingrowth with flap lifts, many surgeons now prefer to perform surface ablation re-treatment if the primary LASIK was performed more than 2–3 years earlier. Creating a new flap with a mechanical microkeratome should be avoided because free slivers of tissue, irregular stromal beds, and irregular astigmatism may be produced. Using the femtosecond laser to create a new side cut within the boundaries of the previous flap may facilitate flap-lift enhancements; however, it is important to have an adequate exposed diameter for ablation, and tissues slivers can result if the old and new side cuts overlap. When attempting to lift or manipulate a femtosecond laser–created flap, the surgeon must take care to avoid tearing it, because the femtosecond laser usually creates a thinner flap than traditional microkeratomes.

When a preexisting flap is lifted, it is important to minimize epithelial disruption. A jeweler’s forceps, Sinskey hook, or 27-gauge needle can be used to localize the edge of the previous flap. Because the edge of the flap can be seen more easily with the slit lamp than with the diffuse illumination of the operating microscope of the laser, some surgeons find it easier to begin a flap lift at the slit lamp and complete it at the excimer laser. Alternatively, the surgeon can often visualize the edge of the flap under the diffuse illumination of the operating microscope by applying pressure with a small Sinskey hook or similar device; the edge of the flap will dimple and disrupt the light reflex (Fig 5-11). A careful circumferential epithelial dissection is performed so that the flap can then be lifted without tearing the epithelial edges. Smooth forceps, iris spatulas, and several instruments specifically designed for dissecting the flap edge can be used to lift the original flap.

Once the ablation has been performed, the flap is repositioned and the interface is irrigated, as in the initial LASIK procedure. Special care must be taken to ensure that no loose epithelium is trapped beneath the edge of the flap that could lead to epithelial ingrowth; the risk of epithelial ingrowth is greater after re-treatment than after primary treatment.

Surface ablation may be considered to enhance a previous primary LASIK treatment. Surface ablation performed on a LASIK flap carries an increased risk of haze formation and irregular astigmatism, but it is an appealing alternative when the RSB is insufficient for further ablation; when the LASIK was performed by another surgeon and the flap thickness, or RSB, is not known; or with conditions such as a buttonhole or incomplete flap. Care must be taken when removing the epithelium over a flap to avoid inadvertently lifting or dislocating the flap. Applying 20% ethanol for 20–30 seconds inside a corneal well will loosen the epithelium, after which scraping motions are applied that extend from the hinge toward the periphery. A rotating brush should not be used to remove the epithelium from a LASIK flap. The risk of postoperative haze due to surface ablation over a previous LASIK flap may be avoided or reduced by administering intraoperative topical mitomycin C, 0.02%, and postoperative topical corticosteroids.

Figure 5-11 Indenting the cornea with forceps to visualize the edge of the flap (arrows) through an operating microscope prior to an enhancement procedure.

(Courtesy of Roger F. Steinert, MD.)

The appropriate choice between conventional and wavefront-guided treatment for enhancing the vision of patients who have previously undergone conventional LASIK is not yet established. Some studies report better results in both safety and efficacy with conventional LASIK re-treatment. With wavefront-guided re-treatments, particularly in patients with high spherical aberrations, the risk of overcorrection may be greater. Caution should be exercised in evaluating the degree of higher-order aberrations and the planned depth of the ablation when deciding between conventional and wavefront-guided treatments.

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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.