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38 | EYEWORLD DAILY NEWS | MAY 17, 2020 DAILY NEWS ASCRS VIRTUAL ANNUAL MEETING Course explores topography and corneal aberrations tangential curvature map is sensitive to local irregularities of the corneal surface and is generally better for showing transitions in curvature than an axial curvature map (typically has less data smoothing). It is used frequently to determine an ideal contact lens design and optic zone size. The elevation map displays corneal height and is often used for contact lens fitting. It is referenced against the theo- retical best fit sphere placed on the corneal surface. Elevation data is derived from corneal curvature data using an algo- rithm. This map shows areas corresponding to depressions and elevations that are above/ below the best fit sphere. The irregularity map dis- plays areas of anterior corneal surface irregularity and uses a best fit toric surface as the limitation, however, is it as- sumes that all refracted light rays pass through a central optical axis, and it typically excludes extreme values. This map displays an average of scaled values with smoothing, so some detail may be lost. It is preferred clinically because it relates corneal shape to power. It's often used to characterize the astigmatism type, align- ment, and location. The axial curvature map is most accurate near the region of the central cornea. Meanwhile, the tangen- tial curvature map calculates measured data points at a 90-degree tangent to the corneal surface. It correctly assumes that all refracted light does not fall on a cen- tral reference axis. This is a detailed representation of the corneal surface and is useful for understanding the size and shape of unusual areas. The thought rather than "button pushing." The presentation provided more information about Placi- do topography. Each topographic map is a different representation of the same corneal shape. View options include: • Axial • Tangential • Elevation • Irregularity • Rings image • Keratometry • Refractive power • Mean • Corneal wavefront • Image simulation • Point spread function • Modulation transfer function • PathFinder II Dr. Hill shared examples of what each of these views would look like. The axial curvature map, he present- ed, is the most commonly used topographic map. One by Ellen Stodola Editorial Co-Director I n a course on understanding topography and corneal ab- errations, Warren Hill, MD, Mesa, Arizona, presented on using this technology to its fullest. Dr. Hill's presentation be- gan by describing how Placido topography can be used. Placido topography can be used for: • Surface elevation, irregulari- ty, and curvature • Anterior corneal aberration profile • Identifying the location of the corneal apex • Multifocal IOL screening • Sophisticated image simula- tion Placido topography can't be used for: • Refractive data • Regional pachymetry • Posterior corneal power mapping His presentation noted that while Placido topography may be used for keratoconus screening, it may not provide an accurate diagnosis. Dr. Hill described the many Placido topography devices available and shared a histor- ical perspective, highlighting Portuguese ophthalmologist Antonio Placido painting alter- nating black and white rings on a flat disk in 1880. Modern topographers use a VKG and fast computers to capture and rapidly process corneal images. Dr. Hill mentioned that both optical biometry and reflective keratometry use a technology base that dates back to the 19th century. Dr. Hill pointed out that an instrument is only as good as the person using it, and mean- ingful use of any ophthalmic instrument involves careful continued on page 40 Some of the many Placido topography devices available Source: Warren Hill, MD, screenshot from presentation MedMont E300 Magellan Mapper EyeSys Vision Oculus Keratograph 5M Handheld Placido disk Zeiss Atlas 9000 Nidek OPD Scan III ReSeevit-Modi

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