The Castrop formula for calculation of toric intraocular lenses

Abstract

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.

Biometry and Intraocular Lens Power Calculation in Eyes with Prior Laser Vision Correction (LVC) – A Review

“…The good results with the EVO TK method suggest the potential of formulae like Kane that recently provided formidable results in various studies 23 , 24 , 25 . Among the new generation IOL formulae, the Castrop formula allows for a unique feature in patients after LVC 26 , 27 . It can be accessed via the LPCM (Lens Power Calculation Module) button on the IOLCON website ( ).…”

Comparison of 2 modern swept-source optical biometers—IOLMaster 700 and Anterion

“…Mean keratometric power was derived from the corneal front surface curvature K12 = 0.5 (337.5/R1a + 337.5/R2a). Corneal surface power was expressed in 3 vector components [ 14 , 15 ] including equivalent power (VEQ) and astigmatic vector considered in the 0/90° meridian (V0) and the oblique 45°/135° meridian (V45) for the front surface: …”

“…The corneal front and back surface curvature and the CCT were used as measured with the biometers. The effective lens position (axial position of the IOL in the pseudophakic eye with respect to the corneal front apex) was predicted with a linear regression method using AL, ACD and LT and the formula constants as shown for the Castrop formula [ 13 , 14 , 17 ]. Without loss of generality, we performed our IOLP prediction for the formula constant triplets as optimised for the ZCB00 lens (Johnson & Johnson Vision, Santa Ana, USA) extracted from the IOLCon WEB site ( https://iolcon.org , accessed on 21.07.2022).…”

Update Biometrie und Linsenberechnung – ein Review zu Grundlagen und neuen Entwicklungen

“…Im Gegensatz zu anderen Formeln der neuen Generation wurde die Castrop-Formel vollständig veröffentlicht. Der Aufbau der Formel ist modular, sodass man bei der Berechnung selbst auswählen kann, welches Hornhautmodell verwendet werden soll, welcher Algorithmus zur ELP-Bestimmung herangezogen wird und ob man 2 oder 3 Konstanten verwenden will 34 , 35 , 36 . Die posterioren Radien und die ZHD können direkt in die Formel implementiert werden, falls verfügbar, ansonsten werden die Werte des Liou-Brennan-Augenmodells verwendet.…”

“…Unlike other new generation formulae, the Castrop formula was published in its entirety. The structure of the formula is modular, so that you can choose which corneal model to use, which algorithm to use for ELP determination, and whether to use two or three constants during the calculation 34 , 35 , 36 . The posterior radii and CCT can be implemented directly in the formula if available, otherwise the values of the Liou-Brennan eye model are used.…”