Fundamentals of Excimer Laser Photoablation
All photoablative procedures result in the removal of corneal tissue. The amount of tissue removed centrally for myopic treatments using a broad beam laser is estimated by the Munnerlyn formula:
Clinical experience has confirmed that the effective change is independent of the initial curvature of the cornea. The Munnerlyn formula highlights some of the problems and limitations of laser vision correction. The amount of ablation increases by the square of the optical zone, but the complications of glare, halos, and regression increase when the optical zone decreases. To reduce these adverse effects, the optical zone should be 6 mm or larger.
With surface ablation, the laser treatment is applied to the Bowman layer and the anterior stroma, LASIK, on the other hand, combines an initial lamellar incision with ablation of the cornea, typically in the stromal bed (see Chapter 5 for further details of surgical technique). Theoretical limits for residual posterior cornea apply the same as they do for PRK. Flaps range in thickness from ultrathin (80–100 μm) to standard (120–180 μm). The thickness and diameter of the LASIK flap depend on instrumentation, corneal diameter, corneal curvature, and corneal thickness.
Treatments for myopia flatten the cornea by removing central corneal tissue, whereas those for hyperopia steepen the cornea by removing a doughnut-shaped portion of midperipheral tissue. Some lasers use a multizone treatment algorithm to conserve tissue by employing several concentric optical zones to achieve the total correction required. This method can provide the full correction centrally, while the tapering peripheral zones reduce symptoms and allow higher degrees of myopia to be treated. For an extreme example, 12.00 D of myopia can be treated as follows: 6.00 D are corrected with a 4.5-mm optical zone, 3.00 D with a 5.5-mm optical zone, and 3.00 D with a 6.5-mm optical zone (Fig 1-20). Thus, the total 12.00 D correction is achieved in the center using a shallower ablation depth than would be necessary for a single pass (103 μm instead of 169 μm). For hyperopia, surface ablation and LASIK use a similar formula to determine the maximum ablation depth, but the ablation zone is much larger than the optical zone. The zone of maximal ablation coincides with the outer edge of the optical zone. A transition zone of ablated cornea is necessary to blend the edge of the optical zone with the peripheral cornea.
Care must be taken to ensure that enough stromal tissue remains after creation of the LASIK flap and ablation to maintain adequate corneal structure. The historical standard has been to leave a minimum of 250 μm of tissue in the stromal bed, although the exact amount of remaining tissue required to ensure biomechanical stability is not known and likely varies among individuals. See Chapters 2 and 5 for further discussion of these issues.
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.