Ординатура / Офтальмология / Английские материалы / Jaypee Gold Standard Mini Atlas Series CORNEALTOPOGRAPHY_Agarwal, Jacob_2009

MINI ATLAS SERIES: CORNEAL TOPOGRAPHY

FIGURE 8.9: Custom-CAP ablation plan parameters are displayed numerically, with the simulated result of the planned ablation pictured in the ablated map, for the patient in Figure 8.8

Topography-guided treatment of decentration may be advantageous compared to wavefront, which addresses topographical irregularities secondarily. Decentration is a topographical phenomenon, which is directly corrected using topographically-driven treatments. Because the ablation is derived from corneal topography data directly,

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CHAPTER 8: DECENTERED ABLATION

FIGURE 8.10: The topographical maps following Custom-CAP treatment show a significantly improved topographical regularity

restoration of the natural aspheric shape of the cornea is possible. Topographical treatments may incorporate tissue saving algorithms, important in patients with limited stromal bed thickness. Published reports find topographyguided treatment to be effective, improving UCVA, BSCVA and the regularity of the corneal surface. However, residual refractive error, under-correction of topographic abnormalities and regression have been reported.

Case III illustrates the efficacy of the MEL80 with TOSCA software (Asclepion-Meditec, Jena, Germany) and the CRS-Master (Carl Ziess Meditech, Jena, Germany). This

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MINI ATLAS SERIES: CORNEAL TOPOGRAPHY

system includes algorithms to correct decentrations and enlarge small optical zones. The CRS-Master features improved control of sphere and cylinder while correcting topographical irregularities. The system incorporates corneal anterior surface wavefront information derived from topography, and whole-eye optical data to determine the refraction of the front surface and the ablation required to remove the irregularities independent of the refractive change. This patient presented with a history of RK rather than LASIK, reporting stable vision for the first 12 years, with progressive hyperopia and reduced visual quality more recently. Her refraction was +3.75–1.00 × 165, which corrected her vision to 20/20. Topography is shown in Figure 8.11. Surgery was performed with a target of plano, with the intention of improving the corneal surface regularity. PRK was performed with an optical zone of

FIGURE 8.11: Topography maps for Case III, a patient with a decentered optical zone following RK corrected with the Mel80 and CRS-master (Carl Zeiss Meditech, Jena, Germany). (courtesy of Dan Reinstein, MD and Tim Archer)

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CHAPTER 8: DECENTERED ABLATION

FIGURE 8.12: Contrast sensitivity testing before (blue) and after (red) PRK custom treatment to correct the decentrated optical zone. (courtesy of Dan Reinstein, MD and Tim Archer)

7.00 mm. Six months later, the patient was 20/32 uncorrected, and +1.00 – 0.75 × 85 improved her vision to 20/16. Figure 8.12 shows the improvement in contrast sensitivity.

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MINI ATLAS SERIES: CORNEAL TOPOGRAPHY

Two systems incorporate pupil size, topographic elevation data, and refractive status with surgical recreation of the morphological axis. The morphological axis is defined as the axis of corneal symmetry approximating the best match between the axis of ideal shape and that of the current form of the cornea. Following a decentration, the visual axis shifts in an attempt to correct the induced visual effect of the decentration. Any measurement taken using the patient’s fixation, such as autorefraction, wavefront aberrometry, and placido topography, will then measured along the incorrect axis. This is the reason decentration is difficulty to correct using wavefront aberrometry measurements, and why conventional treatments often increase rather than improve the decentration.

Ablation software combines pupil, refractive and corneal elevation information, creating topographicallydriven ablations with tissue saving algorithms. The two systems are the Corneal Interactive Programmed Topographic Ablation (CIPTA) (Ligi, Taranto, Italy) and AstraScan (LaserSight, Orlando, FL). The following two cases illustrate the two systems.

Case IV demonstrates a successful treatment using CIPTA. A patient presented with a decentered ablation following LASIK, which manifested as 2.10 D of astigmatism. Preoperative topography and pachymetry maps are shown in Figures 8.13 and 8.14.

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FIGURE 8.13: Preoperative topography map for a patient with a decentered ablation following LASIK. (Courtesy of Chuck Stewart, OD and Ligi, Taranto, Italy)

FIGURE 8.14: Preoperative pachymetry map for the patient in Figure 8.13. (Courtesy of Chuck Stewart, OD and Ligi, Taranto, Italy)

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MINI ATLAS SERIES: CORNEAL TOPOGRAPHY

Incorporation of the pupillometer data, topography, and refraction is illustrated in Figure 8.15A which displays the imported topographic map and the idea shape with restoration of the morphological axis. The ablation plan is shown in Figure 8.15B, which details the corrective ablation (right) and the predicted elevation topography (left). The actual resulting topography and pachymetry maps are shown in Figures 8.16 and 8.17, demonstrating improved centration and decreased pachymetry.

Case V illustrates a case of decentration corrected using AstraScan. Figure 8.18 shows a superior-temporal decentration on a sagital curvature map. The axial power map is shown in Figure 8.19, where it has been imported into the Astrascan software. The patient’s manifest refraction was –3.75 – 0.25 × 95 with a BSCVA of 20/20, and custom LASIK surgery was planned to correct the decentration and address the refractive error.

Figure 8.20 illustrates the planned ablation profile. Note that the pre-enhancement elevation (lower left), when using the optimized postenhancement corneal vertex as the reference axis, shows the same elevated pattern as the enhancement ablation profile (upper right). In this example, the optimized axis is offset of 0.035 mm temporally, and 0.044 mm superiorly from the pre-enhancement corneal vertex axis. Figure 8.21 shows the postenhancement axial power map showing the decentration has been completely

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MINI ATLAS SERIES: CORNEAL TOPOGRAPHY

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FIGURE 8.16: Actual elevation map following CIPTA treatment. (Courtesy of Chuck Stewart, OD and Ligi, Taranto, Italy)

FIGURE 8.17: Actual postoperative pachymetry map following CIPTA treatment. (Courtesy of Chuck Stewart, OD and Ligi, Taranto, Italy)

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