The correlation between anterior and posterior corneal curvature was lower in keratoconus, although the correlation between anterior and posterior astigmatism was maintained.

The visual limitation in keratoconus could be explained by different alterations that occur in these corneas and allowed development of a new grading system for this condition.

Purpose:The aim was to evaluate the anterior and posterior corneal aberrations provided by the Pentacam system in normal and early to moderate keratoconic eyes. Methods: Fifteen normal eyes (normal group, mean age 32.60 Ϯ 7.45 years) and 15 eyes with the diagnosis of keratoconus (keratoconus group, mean age 29.07 Ϯ 8.66 years) were included in this study. All eyes received a comprehensive ophthalmologic examination including corneal topographic analysis with the Pentacam system. Keratometric, best-fit sphere and corneal aberrometric data were recorded and analysed. Root mean square values (RMS) were calculated for primary coma and coma-like aberrations. Results: Statistically significant differences were found in all anterior aberrometric parameters (all p < 0.02), except for horizontal primary and secondary coma Zernike terms (p = 0.61 and 0.72). Regarding posterior corneal surface, statistically significant differences among groups were found in primary spherical aberration, primary vertical coma, coma RMS and coma-like RMS (all p < 0.01). In the normal group significantly higher levels of primary vertical coma, spherical aberration, coma and coma-like RMS were found for the posterior corneal surface (all p < 0.04). In the keratoconus group, only significant differences among anterior and posterior corneal surfaces were found in coma RMS, coma-like RMS and vertical coma (all p < 0.01). These higher levels of aberrations for the posterior corneal surface do not agree with the theoretical optical properties of this surface. Conclusions: With the Pentacam system, higher levels of vertical coma, primary coma and coma-like aberrations are present in keratoconic eyes compared to normal eyes. The values provided by this device for posterior corneal aberrations are erroneous. Keratoconus is an ectatic corneal disorder characterised by a progressive corneal thinning that results in corneal protrusion, irregular astigmatism and decreased vision.1 The hallmark of this ectatic disorder is irregular corneal astigmatism. This significant irregularity is the consequence of the changes induced in anterior corneal geometry which can be assessed by means of corneal topography with increased area of corneal power surrounded by concentric areas of decreasing power, inferior-superior power asymmetry, and skewing of the steepest radial axes above and below the horizontal meridian.2,3 These topographic alterations are mainly responsible for the significant induction of higher order aberrations and then for the visual quality degradation. 4The first ocular refractive interface (aircornea) is the most important contributor to the total refraction due to the existent large difference in refractive index. Therefore, in highly aberrated corneas (high

No author has a financial or proprietary interest in any material or method mentioned.

In unilateral and bilateral amblyopia, lower order aberrations are the main refractive factors leading to amblyopia. Higher order aberrations could have a bilateral amblyogenic effect in those cases where isometropia is present.