Customized ablation, based either on the corneal surface or on total aberrometry, interests refractive surgeons because of its potential to individualize corrections based on the patient’s wavefront data. Custom ablations optimize the optical system, unmasking and correcting the actual spherocylindrical error through recognition and ablation of corneal higher-order aberrations (HOAs).
Custom Ablation Transition Zone software, available with the Final Fit program of the Nidek EC-5000 excimer laser (Nidek Co., Gamagori, Japan), optimizes the detection of ocular aberrations—particularly spherical aberration— in the central 6 mm of the cornea. The curvature of the remaining cornea is then gradually modified to achieve a constant curvature gradient up to the limit of the ablation (10 mm diameter) and also to maintain the best possible reduction of ocular aberrations. The ablation profile includes an aspheric component for treatment of the spherical defect (radially symmetric aberrations), a toric component for treatment of astigmatism (linearly symmetric aberrations), and a flying spot component for treatment of HOAs (irregular components).
Unlike other excimer laser ablation software, which shows only the portion of the tissue to be removed, the Final Fit program also simulates the real shape of the cornea after the planned tissue removal. This simulation gives the surgeon the opportunity to check the corneal regularity and shape with different algorithms and maps, including instantaneous, axial, refractive, wavefront (in microns or diopters), and the curvature gradient, during each step of the ablation plan. This program also features continuous feedback on the amount of tissue removed to obtain the final shape of the cornea.
ABLATION STEPS
Because all astigmatism has a HOA component, HOAs must be corrected before an eye’s true astigmatism can be identified. Therefore, the segmental ablation for the higher-order component is applied first. Higher-order corneal surface aberrations, which are naturally irregular and asymmetric, influence the measurements of cylinder power and axis taken before surgery. The correction of HOAs may reveal the true cylinder power and axis, eliminating the coma- and trefoil-related portions of astigmatism (Figure 1). However, sometimes what looks like astigmatism is actually a combination of HOAs (Figure 2).
Once the HOAs are treated, it may turn out that there is actually no astigmatism (Figures 3 and 4). At this point, new topographic axis and astigmatism powers are estimated and properly corrected. Finally, the resulting spherical component to be treated is calculated.
If we do not follow the process explained above, we could end up with unexpected complications that are often not well tolerated by patients. These include pseudodecentration (Figure 5); residual astigmatism, if HOAs are treated without addressing the treatment of the masked astigmatism (Figure 6); induction of higher-order errors because the errors in determining axis are usually coupled to power errors (Figure 7); residual refractive errors (Figure 6); and poor vision.
ADVANTAGES, SIDE EFFECTS
The Final Fit method has several advantages. First, it minimizes the amount of tissue removal and distributes that amount across the total corneal surface, dampering the biomechanical effects of corneal weakening. Second, it provides a constant final corneal curvature gradient with a reduction of corneal curvature gradient from the center to the periphery. This is positioned in the extreme periphery (beyond 9 mm), where the cornea is thicker and flatter. The gradient induces further reduction of spherical aberration, providing excellent quality of vision even when pupils are dilated.
Cross-cylinder astigmatism correction limits the amount of tissue ablation; preserves the mean corneal radius of curvature, leaving a prolate and symmetrical cornea with a final shape closer to the normal prolate cornea; and does not induce aberrations. For this type of correction, cylinder power is divided into two symmetric parts. The multizone cross-cylinder method creates a progressive transition. The low dioptric gradient, between the treated and untreated cornea, has the following advantages: a physiologically prolate symmetric corneal shape, less regression, better visual acuity, and tissue sparing by splitting part of the cylinder ablation in the periphery.
A delicate point in the creation of a new surface with excimer laser ablation is the transition zone. In response to a marked variation of curvature in the peripheral cornea, we may aim to reduce this curvature variation; otherwise, this may in turn induce regression and restriction of the effective optical zone.
PROTOCOL
After surgery, we always apply a protective contact lens and one dose of topical cyclopentolate 1%, a preservative free preservativefree steroid, and a fluoroquinolone. The patient is instructed to use the fluoroquinolone eye drops until reepithelialization is complete. We remove the protective contact lens only when the epithelium completely covers the corneal surface and appears stable. We prescribe topical dexamethasone twice daily for 1 week and preservative-free artificial tears four times daily. Postoperative follow-up is routinely performed at 1, 3, 6, and 12 months. We do not administer a longer course of steroid because the surface regularity provided by smoothing greatly reduces postoperative haze and regression.
Detection of residual refractive errors within the early postoperative months requires careful topographic examination. Surface irregularity due to epithelial healing will be transitory.
Rarely, in cases of central topographic irregularities, we apply a soft disposable contact lens for up to 15 days. The pressure applied by the contact lens speeds regularization of the epithelium. Detection of frank undercorrection should be followed by a course of topical dexamethasone for 1 to 2 weeks, and refraction and intraocular pressure should be monitored.
Elena Albé, MD, is a Consultant in the Department of Ophthalmology, Cornea Service, Istituto Clinico Humanitas Ophthalmology Clinic, Milan, Italy. Dr. Albé is a member of the CRST Europe Editorial Board. She states that she no financial interest in the products or companies mentioned. She may be reached at e-mail: elena.albe@gmail.com.
Paolo Vinciguerra, MD, is the Chairman of the Department of Ophthalmology, Istituto Clinico Humanitas, Milan, Italy. Dr. Vinciguerra is a member of the CRST Europe Editorial Board. He states that he has no financial interest in the products or companies mentioned. He may be reached at e-mail: paolo.vinciguerra@ humanitas.it.
TAKE-HOME MESSAGE
• The ablation profile for the EC-5000 includes aspheric, toric, and flying spot components to treat radially symmetric aberrations, linearly symmetric aberrations, and irregular components, respectively.
• Final Fit software simulates the real shape of the cornea after tissue removal and provides continuous intraoperative feedback for the amount of tissue removal.
• Detecting residual refractive errors within the early postoperative months requires careful topographic examination.
