Asphärizität der Hornhaut und Astigmatismus

Background: Regarding astigmatism of the cornea, curvature as well as asphericity depend on the meridional axis. Their functional dependence and relation to the quality of the retinal image are still unclear. Methods: Theastigmatic eye was modelled using a biconoid anterior corneal surface by means of a commercially available optical designer programme (Zemax EE, Zemax). The influence of asphericity and astigmatism on the quality of the retinal image was determined by means of ray tracing. Thirty eyes with astigmatism of up to 5.3 D underwent corneal topography (Keratograph C, Oculus) which allowed a numerical evaluation of the cylindric power as well as the asphericity in the main meridians. Results: The quality of the retinal image of an eye with corneal astigmatism can be improved by a factor of 2.28 if the asphericity is optimised. Correction of the central astigmatism only (without considering asphericity) yields only a rather marginal improvement. The average difference of the asphericity in the main meridians is close to zero, however, the individual difference ranges from - 0.372 to + 0.444 which is definitely clinically relevant. Conclusions: Anisotropic asphericity of the cornea may significantly enhance or compensate central corneal astigmatism. Clinically manifest astigmatism is an individually variable combination of asphericity and curvature difference in the two main meridians and is dependent on the pupil size. Laser correction of corneal astigmatism must take meridional asphericity into account.