Abstract
PurposeTo explain the concept behind the Castrop toric lens (tIOL) power calculation formula and demonstrate its application in clinical examples.MethodsThe 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.ResultsThree 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.ConclusionsThe 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.
Highlights
The first intraocular lens (IOL) power calculation formulae were published in 1967 by Fyodorov [1] and independentlyGraefes Arch Clin Exp Ophthalmol (2021) 259:3321–3331 in 1970 by Gernet, Ostholt, and Werner [2]
The Castrop vergence formula described in this paper offers a single-step solution for the calculation of toric lenses and prediction of residual spherocylindrical refraction, with the potential to overcome the limitations of traditional two-step calculation methods
For long and short eyes, the measured axial length (AL) is transformed to ALcor using a linear regression as described by Cooke et al [33, 34] with ALcor = 1.23854 + 0.95855· AL-0.05467·lens thickness (LT), where LT refers to the central thickness of the crystalline lens
Summary
The first intraocular lens (IOL) power calculation formulae were published in 1967 by Fyodorov [1] and independentlyGraefes Arch Clin Exp Ophthalmol (2021) 259:3321–3331 in 1970 by Gernet, Ostholt, and Werner [2]. A wide range of formulae have been proposed by different scientists [1,2,3,4,5,6,7,8,9,10,11,12,13]. Some of these calculation concepts are purely empirical, others are so-called theoretical-optical formulae, and others are based on full aperture ray tracing. As with theoretical-optical formulae, the axial position of the IOL has to be estimated empirically prior to cataract surgery [15,16,17,18]
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