Decentration is often the cause of patient dissatisfaction—due to reduced visual acuity, glare, halos, and, in the most severe cases, monocular diplopia. Functional deficits are dependant on the amount of decentration (ie, function of the extent of treated refractive defect, degree of astigmatism,1 and optical zone used2). Decentration up to 1 mm (0.8-15% of myopic PRKs)1-3 does not appear to cause significant loss of either UCVA or BCVA. The introduction of eye-tracking technologies in refractive surgery has considerably reduced the percentage of decentration, although it has not altogether eliminated it.4-8
DIAGNOSIS OF DECENTRATION
We still need to understand how to assess decentered ablations. Not all ablation profiles are equally effective in ensuring a satisfactory correction. In theory, the optical zone (OZ) that is useful to vision should coincide with the laser setting, and the transition zone should join the OZ to the nontreated cornea. Actually, not all ablation profiles provide the same refractive effectiveness. If the ablation and transition profiles are different and the diopter power correction and the diameter of OZ are equal, then the tangential map will show a different-sized OZ. The final result will, therefore, be determined by a higher diopter gradient, which causes primary and secondary spherical aberration.
High spherical aberration reduces the functionally useful optical zone and increases the refractive effect of decentration. Therefore, the refractive effect not only depends on the amount of decentration, but it should also be seen in connection with the degree of spherical aberration. Ideally, the ablation profile should generate a new aspherical prolate surface that is larger than the pupil diameter. The technical assessment of decentration is often based on the ablation profile analysis on an axial map. This algorithm determines the refractive impact of a change in shape, but not the zone curvature. A more appropriate way to determine OZ size and quality is with a wavefront map, however, it is still not infallible when measuring decentration, which appears as coma. On the other hand, it is also generated by ectasia, focal scarring, or internal coma abberations due to the natural lens, the retina, or higher-order aberrations.
When decentration is suspected, elevation maps may help identify ectasia or central islands. The actual determination of centration as a geometric effect of ablation is seen in a tangential map. The most reliable and repeatable method to evaluate the real ablation profile is to determine the position of the ablation edge through topography. The ablation edge is the least affected by the repair processes, because a smaller amount of tissue is removed. Therefore, there is a lower repair response, and it becomes the site with the highest curvature variation. In tangential map, the ablation edge is shown in warm colors (ie, red-yellow) for myopic treatments (Figure 1) and in cold colors (ie, blue-green-purple) in hyperopic treatments (Figure 2). The position of a red-blue ring may be related to the pupil center, or even better to the line of sight (LOS).
The tangential map correctly highlights the edge and ablation area, the curvature change between the treated and untreated area, and its centration on the pupil or LOS. The curvature change is labeled with a red-blue ring that outlines the edge of the ablation area. Measure the (1) width, (2) centration and diopter gradient relative to the pupil center or LOS, and (3) distance from the pupil edge of the ablation edge. A treatment is well centered when both the topography-generated red-blue ring and the ablated area are centered over the pupil or the LOS (Figures 1 and 2). A diagnosis of decentration is unequivocal when both the red-blue ring and the ablated area are decentered relative to the pupil center or the LOS (Figures 3 and 4).
This paradigm is essential, as the cases most frequently reported as decentrations are, in fact, wrongly diagnosed as such. When the tangential map shows a decentered optical zone but the red-blue ring is centered relative to the pupil center or LOS, the diagnosis should be pseudodecentration. Therefore, pseudodecentration occurs in the presence of uneven distribution of the corneal dioptric gradient (ie, nasal pupil, astigmatism correction in one meridian, central island), focal scarring, or high corneal dioptric gradient.
In the axial map, a paracentral island may simulate a decentered treatment (Figure 5). Small repair process alterations with OZ haze may change the central corneal power, simulating a decentration (Figure 6). This also occurs in high corneal diopter gradient in high myopic treatment (Figure 7). If analyzed with a tangential map, and the ablation area and red-blue ring are correlated to the pupil center and LOS, the final diagnosis should be pseudodecentration (Figures 5 through 7).
For a postrefractive surgery diagnosis of decentration or pseudodecentration to be correct, therefore, it is necessary to examine the correlation between the ablation area and the red-blue ring relative to the pupil center or LOS on the tangential map (Table 1).
TREATMENT OF DECENTERED ABLATIONS
There are various reports on treating a decentered ablation after refractive surgery.
• Alessio et al9 evaluated the efficacy, predictability, stability, and safety of the Corneal Interactive Programmed Topographic (CIPTA; Ligi, Taranto, Italy). The program provides customized laser ablation for correction of postmyopic PRK decentrations.
• Mrochen10 used wavefront-guided LASIK to treat three patients with previous ablation zone decentrations between 1.5 mm to 2.0 mm.
• Lafond et al11 described a technique combining myopic and hyperopic treatment to recenter the ablation zone. The refractive status obtained by the first does not need adjustment.
• Kymionis et al12 evaluated the efficacy, predictability, and safety of topographically supported customized ablations (TOSCAs) for decentered ablations following LASIK.
• We treated decentered ablations using custom phototherapeutic keratectomy. Only an accurate topographic analysis can prevent incorrect decentration diagnoses, which often mislead surgeons into performing a second procedure. This increases the risk of making the initial situation worse.
Alessandro Randazzo, MD, practices at the Istituto Clinico Humanitas Rozzano, Milano University, in Italy. Dr. Randazzo states that he has no financial interest in the products or companies mentioned. He may be reached at alessandro.randazzo@humanitas.it.
Paolo Vinciguerra, MD, practices at the Istituto Clinico Humanitas Rozzano, Milano University, in Italy. Dr. Vinciguerra is a member of the CRST Europe Editorial Board. He states that he is a paid consultant for Nidek Co., Ltd. (Gamagori, Japan). Dr. Vinciguerra may be reached at paolo.vinciguerra@humanitas.it.
Up Front | Jun 2007
Diagnosis of a Decentered Treatment
An accurate topographic analysis can prevent incorrect diagnoses.
Paolo Vinciguerra, MD, and Alessandro Randazzo, MD
