Instrument combines approaches
Aretrospective analysis of data collected with a proprietary diagnostic instrument (iDesign, Abbott Medical Optics) shows that corneal higherorder aberrations (HOAs) are generally, but not always, compensated for by the internal optics of the eye, said Daniel R. Neal, PhD.
The study included data from 100 eyes of 50 patients and a total of 267 combined fullgradient corneal topographic and high-resolution wavefront aberrometry measurements. The patients represented a diverse population with a broad range of age (25 to 65 years) and refractive error (mean sphere −1.78 D, range −13 to −1.25 D; mean cylinder −0.6 D, range −2.0 to 0 D). The HOAs were expressed as 6th-order Zernike polynomials and the measurements were taken over the natural pupil diameter.
RMS values for corneal topography HOAs were almost always less than the total wavefront values, and the mean ratio of wavefront:corneal topography HOA values was about 0.76. There was a statistically significant correlation between the wavefront and corneal topography total HOA RMS values, and 96% of all measurements showed overall compensation.
However, the strength of the correlation between the wavefront and corneal topography HOA values varied for different HOAs, and in contrast to previous studies, compensation was not observed for astigmatism or trefoil.
"It is generally accepted that 60% to 80% of the eye's optical power and its aberrations comes from the cornea," said Dr Neal, director of research and development, Abbott Medical Optics, Albuquerque, New Mexico, USA. "The finding of a high degree of HOA compensation in our diverse population suggests that functional compensation is maintained with age, but it appears there is variability between patients and depending on the HOA term.
"Therefore, we believe this novel diagnostic instrument that can separate the contributions of corneal and internal optics to the wavefront may have a particular role in analysing ocular phenomena and provide valuable information for guiding advanced refractive corrections," he added. "In addition to laser refractive surgery treatment planning, the results have implications for IOL treatment planning and IOL selection as well as for furthering our understanding of how the eye works."
The instrument used to obtain the measurements will become commercially available in Europe later in 2011 and is not yet approved by the FDA. It provides five functions, including aberrometry, autorefraction, corneal topography, pupillometry and keratometry. Relevant to the present study is the fact that the unit obtains the topography and aberrometry measurements nearly simultaneously on a common fixation and measurement axis and correlates them both spatially and temporally.
"Subtraction of corneal aberrations from the total wavefront error of the eye to separate contributions of the cornea from those due to internal ocular components has been done before using data from other instruments that feature a different kind of topographer," Dr Neal said. "Previous studies have used placido ring topographers. Unlike placido disc topographers, fullgradient topography technology measures both radial and tangential gradients and in that respect is similar to HartmannShack aberrometry. Because it relies on a pattern of spots instead of placido rings, the fullgradient topographer potentially provides more accurate information."