Conversion of disclosed lens power formula constants.

Abstract

Purpose: To present a concept for inter-converting constants of disclosed lens power formulae and to simplify these conversions to linear prediction models for clinical use.

Setting: Augen- und Laserklinik Castrop-Rauxel, Germany.

Design: Retrospective single centre case series.

Methods: Analysis based on a dataset with biometric measurements of 19,472 eyes in a cataractous population performed with the IOLMaster 700. The formula constant conversion from formula A to B includes 2 steps: calculation of the lens power using formula A with variation of formula A constant, and back calculation of the constant for formula B for all constant values of formula A using nonlinear iterative optimization techniques to minimize the sum of squared formula B prediction error.

Results: Formula conversion was calculated for the SRK/T A constant (A = 117 to 121), Hoffer Q (pACD = 4.2 to 6.8), Holladay 1 (SF 0.5 to 3.0), Haigis (a0 0.0 to 2.5 with a1/a2 = 0.4/0.1) and Castrop (C 0.15 to 0.50 with H/R = 0.0) formulae. As the conversions follow linear dependencies, linear regression models were fitted to simplify formula constant conversions in clinical routine.

Conclusion: Conversions of formula constants could be used in cases where, for example, new lens models are launched with only the nominal SRK/T A constant available, or in situations where constants for some of the formulae are not available. Conversions to constant triplets for the Haigis or Castrop formula require additional nonlinear boundary conditions to take full advantage of formulae with multiple constants.