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

CHAPTER 2: PENTACAM

Nemeth et al compared the ultrasound and Pentacam Anterior Chamber Depth (ACD) measurements in phakic and pseudophakic eyes. They reported similar ACD measurements with the Pentacam and ultrasound in phakic eyes, with significantly lower measurements using the Pentacam in pseudophakes, however.

Elbaz et al compared ACD measurements taken from Pentacam, ultrasound A-scan and IOLMaster (Carl Zeiss Meditach, Jena, Germany). Measurements of ACD by the Pentacam differed significantly from those of ultrasound and IOLMaster. Author concluded measurements of ACD and corneal curvature using these machines may not be interchangeable. In contrast, when comparing Pentacam and Orbscan ACD measurements, Lackner et al studied the differences of ACD values as well as interand intraobserver variability measured with Orbscan and Pentacam were such that the two modalities can be regarded as interchangeable.

Angle anatomy relative to angle-closure glaucoma may be assessed using the Pentacam as well. Rabsiber et al found the good reliability with anterior chamber assessment, and suggested the Pentacam may be used to classify the potential risk for angle-closure glaucoma.

Pentacam describes the wavefront error based on height data for the anterior and posterior surface. Note this is different from a wavefront aberrometer, which measures

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MINI ATLAS SERIES: CORNEAL TOPOGRAPHY

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CHAPTER 2: PENTACAM

the whole eye wavefront error. Zernicke analysis is performed without a reference body, but rather compared to normative data (from healthy eyes). Abnormal values are highlighted for ease of detection. An example is shown in Figure 2.21. The Zernicke coefficients are used to calculate the aberration coefficient. A value of 0 denotes no aberration. Values greater than 1.0 may be visually significant.

Anterior Chamber Phakic Lens Simulation

The software simulation was created to assist surgeons with implantation of phakic lenses into the anterior chamber. Posterior chamber lenses cannot be imaged, as the Pentacam is not able to image structures behind the iris. Lens type and power can be chosen from the database, and software calculates the refractive lens power based on the manifest refraction entered by the clinician. Distances between the phakic lens, crystalline lens, and endothelium are displayed, and aging simulations are also available. The software assumes the crystalline lens grows at a rate of 18 microns per year, which over time would cause the iris to move toward the cornea with age, crowding the anterior chamber and putting the endothelium at risk.

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MINI ATLAS SERIES: CORNEAL TOPOGRAPHY

The accessibility of the cornea and the non-intraocular designation of its anatomical status makes it the focus of refractive surgery. The cornea has therewith enjoyed this privilege for decades of refractive surgery. These very advantages also have us refractive surgeons constantly vying to make this delicately transparent yet inherently elastic tissue more predictable.

Thus began our search for that perfect tool which could study and analyze the cornea and also determine our consistency for cornea-based refractive surgery. Corneal topographers have been the gold standard for understanding corneal shape which is the basis of laser refractive surgery. Recently, with the advent of aberrometers, the bar has been raised. We have a new technology as well as a new language to address the cornea with and thereby translate the same into effective and accurate surgical outcomes.

This bridge from topography to wavefront technology is actually an adjunct and not about the past or future. These are complementary technologies as of now and this chapter therewith addresses the application of the two.

With the advent of complex topographic systems and wavefront aberrometers, opthalmologists now benefit from a deeper understanding of the optics of the cornea and their effect on vision.

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CHAPTER 3: CTWA: COMPLEMENTARY TOOLS

The axial map is a traditional but poor descriptor of corneal refraction because it does not take into account spherical aberration. Despite its limits, the axial map became known as “the corneal topography map”. With the development of arc-step algorithms, placido systems could not only approximate axial power, but also measure corneal shape. Elevation maps became available. An example of an axial and elevation map of an eye with with- the-rule astigmatism is shown in Figure 3.1.

In the early to the mid 1990s, the explosion of excimer laser refractive surgery necessitated more accurate optical instruments to create more detailed representation of the corneal surface. At this point, the path divided as two parallel paths emerged. Some believed the answers to problems encountered in refractive surgery could be answered using elevation-based topography, while others supported wavefront aberrometry-based platforms.

Placido disc imaging systems are limited to evaluation of the anterior cornea only, and calculate rather than directly measure elevation data. Systems emerged which directly measured corneal elevation, and evaluated anatomy posterior to the anterior cornea. The PAR Corneal topography system (PAR CTS) was the first system to directly measure anterior elevation topography, using principles of triangulation. The distortion of a grid projected on to the cornea was mathematically compared to the true

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MINI ATLAS SERIES: CORNEAL TOPOGRAPHY

FIGURE 3.1: An axial (bottom) and elevation (top) map of an eye showing with-the-rule astigmatism

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CHAPTER 3: CTWA: COMPLEMENTARY TOOLS

grid in a reference plane. Because the geometry of the reference surface and the grid projection is known, rays can be intersected in three dimensional space to compute the X, Y and Z coordinates of the surface. This system was generally accepted to be more accurate when measuring complex bicurve and multicurve test surfaces, but poor reproducibility was reported.

Slit-scanning technology addressed both the need for direct measurement of elevation as well as evaluation of the posterior structures within the eye. The Orbscan II (Bausch and Lomb, Roschester, New York) combined a placido disc to measure curvature and slit scanning to measure both surfaces of the cornea. The most commonly used display for this system is the Quad map, shown in Figure 3.2. A placido image is captured to evaluate curvature data. Then, over 1.5 seconds, two scanning slit lamps project a total of 40 images each at 45 degrees of the video axis. A proprietary tracking system reduced eye movements. It produced pachymetry and anterior chamber depth information for the first time. This system remains the only type capable of evaluation of the posterior surface. Thus, there is no way to validate the information found in the posterior map.

Another technology was developed to address two problems associated with currently used topographic technology: assumptions inherent to power calculation and paracentral measurement of the cornea. The Pentacam

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