Figure 1-19 Schematic diagrams of thermokeratoplasty and conductive keratoplasty. Heat shrinks the peripheral cornea, causing central steepening (arrows).
Laser Biophysics
Laser–Tissue Interactions
Three different types of laser–tissue interactions are used in keratorefractive surgery: photoablation, photodisruption, and photothermal. Photoablation, the most important laser–tissue interaction in refractive surgery, breaks chemical bonds using excimer (from “excited dimer”) lasers or other lasers of the appropriate wavelength. Laser energy of 4 eV per photon or greater is sufficient to break carbon–nitrogen or carbon–carbon tissue bonds. Argon-fluoride (ArF) lasers are excimer lasers that use electrical energy to stimulate argon to form dimers with the caustic fluorine gas. They generate a wavelength of 193 nm with 6.4 eV per photon. The 193-nm light is in the ultraviolet C (high ultraviolet) range, approaching the wavelength of x-rays. In addition to having high energy per photon, light at this end of the electromagnetic spectrum has very low tissue penetrance and thus is suitable for operating on the surface of tissue. This laser energy is capable of great precision, with little thermal spread in tissue; moreover, its lack of penetrance or lethality to cells makes the 193-nm laser nonmutagenic, enhancing its safety. (DNA mutagenicity occurs in the range of 250 nm.) Solidstate lasers have been designed to generate wavelengths of light near 193 nm without the need to use toxic gas, but the technical difficulties in manufacturing these lasers have limited their clinical use.
The femtosecond laser is approved by the US Food and Drug Administration (FDA) for creating corneal flaps for LASIK and may also be used to create channels for intrastromal ring segments and for lamellar keratoplasty and PKP. It uses a 1053-nm infrared beam that causes photodisruption, a process by which tissue is transformed into plasma, and the subsequent high pressure and temperature generated lead to rapid tissue expansion and formation of microscopic cavities within the corneal stroma. Contiguous photodisruption allows for creation of the corneal flap, channel, or keratoplasty incision.
Photothermal effects are achieved by focusing a holmium:YAG laser with a wavelength of 2.13 µm into the anterior stroma. The beam’s energy is absorbed by water in the cornea, and the resulting heat causes local collagen shrinkage and subsequent surface flattening. This technique is approved by the FDA for treating low hyperopia but is not commonly used at present.
Fundamentals of Excimer Laser Photoablation
All photoablation procedures result in the removal of corneal tissue. The amount of tissue removed centrally for myopic treatments 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, whereas LASIK 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 (130–180 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 mid-peripheral 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.
Figure 1-20 Diagrammatic comparison of single and multizone keratectomies. A, Depth of ablation required to correct 12.00 D of myopia in a single pass. B, Depiction of how the use of multiple zones reduces the ablation depth required. (Illustrations by
Cyndie C. H. Wooley.)
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.
Types of Photoablating Lasers
Photoablating 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 generate 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 allowing for larger-diameter ablation zones. Flying spot lasers use smaller-diameter beams (approximately 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 use utilize some form of variable or flying spot ablation profile.
Wavefront-optimized and wavefront-guided laser ablations
Because conventional laser treatment profiles have small blend zones and create a more oblate corneal shape postoperatively, they are likely to induce some degree of higher-order aberration, especially spherical aberration and coma. These aberrations occur because the corneal curvature is relatively more angled 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 taking 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 results in better-quality vision and fewer night-vision complaints. 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 advantage of being quicker than wavefront-guided technology and avoids the additional expense of the aberrometer.
In wavefront-guided laser ablation, information obtained from a wavefront-sensing aberrometer (which quantifies the aberrations) is transferred electronically to the treatment laser to program the ablation. This process is distinct from those in conventional excimer laser and wavefront-optimized laser treatments, in which the subjective refraction alone is used to program the laser ablation. The wavefront-guided laser attempts to treat both lower-order (ie, myopia or hyperopia and/or astigmatism) and higher-order aberrations by applying complex 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 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.
The results for both wavefront-optimized and wavefront-guided ablations for myopia, hyperopia, and astigmatism are excellent, with well over 90% of eyes achieving 20/40 or better uncorrected distance visual acuity (UDVA; also called uncorrected visual acuity, UCVA). Although most visual acuity parameters are similar between conventional and customized treatments (including both wavefront-optimized and wavefront-guided treatments), the majority of recent reports demonstrate improved vision quality when customized treatment profiles are used. Outcomes with wavefrontoptimized treatments are similar to those of wavefront-guided treatments for most patients, with the exception of patients with substantial preoperative higher-order aberrations.
Topography-guided laser ablations
Topography-guided lasers are currently investigational in the United States. Although similar in concept to wavefront-guided lasers, topography-guided devices link the treatment to the corneal topography rather than to the wavefront data. Although experience is still early, these instruments may offer significant benefit in the treatment of highly aberrated eyes, such as eyes with previous RK or PKP.
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