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M. Chen, S. MacRae, G. Yoon; Posterior Corneal Aberrations and Their Compensation Effects on Anterior Corneal Aberrations in Keratoconic Eyes. Invest. Ophthalmol. Vis. Sci. 2008;49(13):987. doi: https://doi.org/.
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© ARVO (1962-2015); The Authors (2016-present)
To characterize posterior corneal aberrations in keratoconic (KC) eyes and investigate compensatory effects between anterior and posterior corneal surfaces.
A total of 113 eyes, classified into four groups (37 advanced, 31 moderate, 14 mild KC eyes and 31 normal eyes) were measured with Orbscan IIz (Bausch & Lomb). Both anterior and posterior corneal topographies for a 6 mm diameter were decomposed by Zernike polynomials to compute corneal higher order aberrations (HOA).
Significantly larger amounts of posterior corneal aberrations were observed in KC eyes than in normal eye group. The averaged higher order RMS wavefront errors of posterior cornea were 0.94, 0.51, 0.22 and 0.19 µm in advanced, moderate, mild KC and normal eyes, respectively. Posterior corneal coma was the most dominant posterior HOA in KC eyes and its magnitude was 7 times larger than that found in normal eyes. In advanced KC eye, the posterior corneal coma was orientated toward superior-nasal direction with an orientation angle of 61±17 degree for OD and 66±17 degree for OS. Positive vertical coma of the posterior cornea was more dominant (averaged magnitude ratio of posterior corneal vertical to horizontal coma was 2 in advanced KC eye) in advanced KC partially compensating for negative vertical coma in the anterior cornea. On average, 23%, 24%, 24% of the anterior corneal coma was compensated by posterior corneal coma in advanced, moderate and mild KC eyes respectively. However, no significant compensation effects were found in normal corneas.
The significant residual uncorrected posterior corneal coma in KC eyes can explain the clinical phenomena that KC patients with RGP lens still had lower visual acuity than the normal vision. Since RGP lenses correct only anterior lower and higher order corneal aberrations, significant amounts of residual posterior corneal higher order aberrations, in KC eyes would limit the visual benefit obtainable with conventional RGP lenses, while whole eye wavefront designed customized contact lenses allows compensation for posterior corneal higher order aberrations.
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