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Cataract Surgery | Mar 2009

Using Next-Generation Topography to Screen LASIK Candidates

Full-gradient topography will provide more information than Placido-based topography.

Current state-of-the-art LASIK surgery includes the use of high-resolution treatment planning, flap creation with a femtosecond laser, and custom wavefront-guided ablation with cyclotorsional registration. Equally important for a predictable, successful, and safe outcome with modern laser vision correction is the use of the best available screening technology to determine patients' suitability to undergo surgery.

With the advent of wavefront-guided laser treatment, an increasing part of the emphasis in refractive surgery has moved from the operating room to preoperative planning. Good results are achieved only if we have accurate diagnostic data on which to base our treatments. The first, and most vital, stage includes screening and assessment.

Currently available modalities for up-to-date screening of LASIK candidates include corneal topography, pachymetry, autorefraction, pupillometry, and aberrometry.

Topography is important not only to exclude those with forme fruste keratoconus but also to match corneal wavefront data with whole-eye wavefront. This will help determine whether a patient's optical aberrations lie in the cornea or the lens and will become increasingly important as wavefront-guided treatment advances in the coming years.

A new type of corneal topography, termed full-gradient topography,1 has been developed by WaveFront Sciences, the wavefront technology company acquired by Advanced Medical Optics, Inc. (Santa Ana, California), in 2007. This new modality has the potential to refine our screening, diagnosis, and treatment.

NEW TYPE OF TOPOGRAPHY
Corneal topography devices based on Placido-disc keratoscopy are well understood by ophthalmologists and are based on refined algorithms. However, they have well-known limitations. Skew rays are not directly captured in Placido-disc systems, and these systems are more sensitive to the radial component of the curvature gradient. Additionally, the data over the apex of the cornea are commonly extrapolated from the surrounding curvature. As a result, Placido-based topography reconstruction may not be entirely accurate in some patients.

Full-gradient topographers are new to ophthalmology, but they are also based on established concepts. In these systems, Shack-Hartmann grids, like those used in wavefront aberrometry, are used to capture x and y slopes for each spot projected on the cornea. The central 3-mm area is directly measured, rather than extrapolated. The technology is not inherently sensitive to misalignment of the eye, as Placido-based systems are, and it has the ability to capture a wide area of the cornea. Full-gradient topography has the potential to provide increased accuracy of corneal mapping over Placido-based systems.

HOW IT WORKS
In full-gradient topography, a cone-and-shell design is used to produce uniformly illuminated spots on the cornea. The cone, which faces the cornea, is perforated with holes that allow spots of light to be projected onto the eye. A shell behind the cone has a surface with Lambertian reflectance properties, meaning that light falling on it is scattered evenly, producing uniform brightness at all angles (Figure 1).

The uniformly bright spots projected onto the cornea are captured by the camera and are analyzed using pattern-recognition software. Unique fiducials with known cone locations are used as the basis for spot association. Localized Taylor series least-squares refinement, which can be compared with the use of Zernike polynomials in whole-eye wavefront analysis, are used to associate spots measured by the camera with the cone spots.

Model eyes were used in the development of the system, with abnormal surfaces producing spot patterns that deviate from a normal grid. Figure 2 shows a model eye with a tricurve simulating keratoconus.

On test surfaces, full-gradient spot-based topography has shown good correlation with other topography instruments. Accuracy on the order of 0.01 mm can be achieved with simple machine calibration, which is similar to the accuracy of Placido-based topographers. With more sophisticated calibration, taking into account cone position, rotation, and tilt, significant improvement in accuracy may be expected. This will allow the development of better topography-related analyses. The information generated can also be registered directly to whole-eye Shack-Hartmann analyses.

Full-gradient topography should offer clinical advantages over other topography modalities. Unlike Scheimpflug-based systems, all data capture is simultaneous, so there is no time lag. Unlike Placido-based units, there are no information dropouts due to loss of rings. Fourier or zonal reconstruction can be used to fill in data over missing data points, such as a scar due to corneal injury.

SCREENING AND TREATMENT
Full-gradient topography may provide additional topographic information and may be more predictable than Placido–disc-based systems. It is a tool that we need to fine-tune topography and further improve our screening of LASIK patients. Additionally, this next-generation topography may be particularly useful for performing topography-guided ablations in certain challenging cases, including primary and secondary treatments in highly aberrated eyes, eyes in which we are unable to capture wavefront measurements, eyes with lenticular aberrations, injured eyes, and eyes with corneal scarring.

Currently, this technology is in early testing as part of the iDesign Advanced WaveScan Studio (Advanced Medical Optics, Inc.). In addition to full-gradient topography, the iDesign device will incorporate whole-eye Shack-Hartmann wavefront aberrometry, keratometry, and pupillometry. This next-generation diagnostic technology will provide topography along with aberrometry data to clinicians, enhancing both screening and planning for LASIK.

Julian Stevens MRCP, FCRS, FRCOphth, is a Consultant Ophthalmic Surgeon at Moorfields Eye Hospital, London, and is a member of the CRST Europe Editorial Board. Mr. Stevens states that he is a consultant to Advanced Medical Optics, Inc., and to Revision Optics, Inc., and that there is a research agreement between Moorfields Eye Hospital and Advanced Medical Optics, Inc.. He may be reached at e-mail: JulianStevens@compuserve.com.

  1. Stevens J. New technology updates: topographic-based ablations. Paper presented at: Cornea Day, American Society of Cataract and Refractive Surgery; April 4, 2008: Chicago.

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