Purpose To explain the concept behind the Castrop toric lens (tIOL) power calculation
formula and demonstrate its application in clinical examples. Methods The Castrop
vergence formula is based on a pseudophakic model eye with four refractive surfaces
and three formula constants. All four surfaces (spectacle correction, corneal front
and back surface, and toric lens implant) are expressed as spherocylindrical vergences.
With tomographic data for the corneal front and back surface, these data are considered
to define the thick lens model for the cornea exactly. With front surface data only,
the back surface is defined from the front surface and a fixed ratio of radii and
corneal thickness as preset. Spectacle correction can be predicted with an inverse
calculation. Results Three clinical examples are presented to show the applicability
of this calculation concept. In the 1st example, we derived the tIOL power for a spherocylindrical
target refraction and corneal tomography data of corneal front and back surface. In
the 2nd example, we calculated the tIOL power with keratometric data from corneal
front surface measurements, and considered a surgically induced astigmatism and a
correction for the corneal back surface astigmatism. In the 3rd example, we predicted
the spherocylindrical power of spectacle refraction after implantation of any toric
lens with an inverse calculation. Conclusions The Castrop formula for toric lenses
is a generalization of the Castrop formula based on spherocylindrical vergences. The
application in clinical studies is needed to prove the potential of this new concept.
