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

MINI ATLAS SERIES: CORNEAL TOPOGRAPHY

FIGURE 3.2: An example of a quad map in a patient S/P myopic Lasik

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(Oculus) addressed these deficiencies. It is a rotating Scheimpflug camera which provides a complete picture from the anterior surface of the cornea to the posterior surface of the lens, as shown in Figure 3.3.

As topographic systems advanced, understanding of the optics resulting from changes in shape induced by disease or refractive surgery, and the need to correct optical problems grew. Unfortunately, significantly irregular corneas, dry eyes, and scarring may cause topographic systems to fail.

Programs such as the Custom Corneal Ablation Planner (Custom CAP) utilized elevation data and computer analysis to create custom ablation profiles in an attempt to correct decentered ablations and relieve patients of visual distortion. An example is shown in Figure 3.4. Topographydriven programs fail to address the refractive error, however, and refractive changes induced by such treatments were somewhat unpredictable.

Just as topographic analysis was facilitated by computers, so was measurement of optical aberrations. This advance in technology occurred slightly later and parallel to the growth of topography. The most common types of aberrometers are the Hartmann-Shack and the ray tracing models. Hartmann-Shack models utilize several hundred lenslets to measure the wavefront. Ray-tracing models utililize individual rays of light to measure the wavefront.

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MINI ATLAS SERIES: CORNEAL TOPOGRAPHY

FIGURE 3.3: Scheimpflug image

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CHAPTER 3: CTWA: COMPLEMENTARY TOOLS

FIGURE 3.4: Custom CAP case

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MINI ATLAS SERIES: CORNEAL TOPOGRAPHY

Both models measure aberrations as a deviation from the plane wave in microns, and measurements are limited by pupil size. This is problematic in eyes with smaller pupils.

The shape of the wavefront is then mathematically described, most commonly using Zernicke polynomials. Figure 3.5 shows this polynomial pyramid. Zernicke polynomials are a combination of radial trigometric functions which describe the wavefront mathematically. While second order terms of defocus and astigmatism are

FIGURE 3.5: Zernicke polynomial pyramid

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addressed by the manifest refraction, the irregular astigmatism primarily attributed to the cornea can be described by higher order terms such as coma, spherical aberration, and trefoil.

Just as topographers exhibit difficulty capturing irregular corneal surfaces, aberrometers falter on irregular wavefronts (typically due to irregular astigmatism). This is especially true for diseased eyes and those S/P keratorefractive procedures. Smolek and Klyce studied Zernicke fitting methods for corneal elevation and reported 4th order Zernicke polynomials may not be adequate in their description of corneal aberrations in significantly aberrant eyes.

Because topography failed to address lower order aberrations, and aberrometry failed to address focal topography irregularities, advanced methods were needed. Several case examples later in this chapter demonstrate how using several systems to gain information about the topography and aberrometry combined are beneficial in determination of the etiology of the visual complaint. It is also beneficial in creating a management plan for surgery.

As the limitations of wavefront became apparent, interest turned to corneal wavefront measurements—the ability to measure the amount of aberration attributable to the cornea alone. Systems which subtracted the corneal wavefront from the total ocular wavefront emerged. The

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MINI ATLAS SERIES: CORNEAL TOPOGRAPHY

EyeSys system is capable of using Wavefront aberrometry and corneal topography to develop a corneal Wavefront map Figure 3.6 illustrates the aberrations found in a keratoconic patient with a 4 mm pupil (a) as well as the topography (b) (Figure 3.7) illustrates the corneal and internal aberrations of the same eye.

A

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MINI ATLAS SERIES: CORNEAL TOPOGRAPHY

FIGURE 3.7: Corneal and internal aberrations of the same eye as in Figure 3.6

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It has been suggested that surgeons consider the rule of three when considering corneal surgery: for every 3 microns of distortion from the ideal shape of the corneal, about a +1 micron difference in the OPD map and –1 micron difference in the wavefront error map. It should be noted, however, that the precision of both excimer lasers and wavefront aberrometers far surpasses that of human healing.

CASE 1: DOUBLE VISION COMPLAINTS S/P LASIK

Patient WR presented complaining of “terrible night vision OS after LASIK. Preoperatively, he was significantly nearsighted, and correctable to 20/20. Postoperatively, he refracted to –2.75+1.75 × 160, with a BVA of 20/30. An RGP improved his vision to 20/20 with a significant reduction in monocular diplopia. His preoperative WaveScan map is shown in Figure 3.8A, with the preoperative topography in the upper right corner of the difference map shown in Figure 3.8B. Elevation mapping revealed a decentered ablation. Significant coma is evident in the aberrometry map, as expected with decentered ablations. We performed a Custom-CAP treatment, which treats the decentered ablation directly without taking the refractive error into account, and the visual distortion improved significantly, indicated by the drop in the RMS value shown in Figure 3.9.

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