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S. Anand, K. Pesudovs, D. Coster; Wavefront Aberrations Arising at the Posterior Corneal Surface in Normal and Keratoconus Eyes as Reported by the Oculus Pentacam. Invest. Ophthalmol. Vis. Sci. 2008;49(13):1031. doi: https://doi.org/.
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© ARVO (1962-2015); The Authors (2016-present)
To describe the wavefront aberrations (WFAs) arising at the anterior and posterior corneal surfaces as reported by the Oculus Pentacam proprietary software in normal and keratoconus eyes.
Two hundred eyes of 200 patients, 100 with keratoconus, and 100 normal were measured. Corneal topography was taken with the Oculus Pentacam rotating Scheimpflug camera system. This system reports Zernike coefficients calculated about the corneal vertex for both the anterior and posterior corneal surfaces. Wavefront aberrations calculated for an 8th order polynomial over a 6.0mm pupil were manually extracted and reported as root mean square (RMS) of the 3rd through 6th orders and total higher order RMS (tHORMS).
The groups were matched for age (normals (mean±SD) 36.2±14.0, keratoconus 40.0±15.8 years, p=0.12). In normal eyes, WFAs (tHORMS, 3rd, 4th, 5th, 6th order) arising at the anterior corneal surface were (0.69±0.16µm, 0.27±0.14µm, 0.60±0.13µm, 0.12±0.07µm, 0.08±0.04µm) and the posterior corneal surface were (1.62±0.37µm, 0.66±0.40µm, 1.39±0.29µm, 0.34±0.15µm, 0.23±0.09µm). In eyes with keratoconus, the magnitude of WFAs were significantly higher (p<0.01); anterior corneal surface (4.40±4.03µm, 3.88±3.54µm, 1.57±1.84µm, 0.73±1.13µm, 0.34±0.54µm), posterior corneal surface (9.03±6.16µm, 7.95±5.63µm, 3.36±2.75µm, 1.45±1.47µm, 0.79±0.95µm).
The small refractive index difference between the posterior cornea and the aqueous humour should give rise to low levels of WFA; approximately one-seventh of that which occurs at the anterior corneal surface. However, Oculus Pentacam proprietary software reports WFAs arising at the posterior surface to be of the order of 2-3 times the magnitude reported for the anterior surface in both normal and keratoconus eyes. This is an interesting and unexpected result. The pentacam is the first instrument to directly measure the posterior corneal surface and report posterior corneal aberrations. The results of this study, however, call into question the reliability of these measurements. An alternative explanation would be that current understanding of corneal optics underestimates the role of the posterior corneal surface.
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