Investigating impact of formula constants on predicted refraction and limitations
of constant optimization for classical and modern intraocular lens (IOL) power calculation
formulae.Tertiary Care Center.Retrospective single centre consecutive case series.Analysis
based on a dataset of 888 eyes before and after cataract surgery with IOL implantation
(Hoya Vivinex). Spherical equivalent refraction predSEQ was predicted using IOLMaster
700 (IOLM) data, IOL power, and formula constants from IOLCon (https://iolcon.org).
The formula prediction error (PE) was derived as predSEQ minus achieved spherical
equivalent refraction for the SRKT, Hoffer Q, Holladay, Haigis, and Castrop formulae.
The gradient of predSEQ (gradSEQ) as a measure for the impact of the constant on refraction
was calculated and used for constant optimization.Results: using initial formula constants
the mean PE was -0.1782±0.4450/-0.1814±0.4159/-0.1702±0.4207/-0.1211±0.3740/-0.1912±0.3449
dpt for the SRKT/Hoffer Q/Holladay/Haigis/Castrop formula. All gradSEQ (except gradSEQ
for the Castrop R) decay with axial length because of interaction with the effective
lens position (ELP). Constant optimization for a zero mean PE (SD: 0.4410/0.4307/0.4272/0.3742/0.3436
dpt) results in a change in the PE trend over axial length in all formulae where the
constant acts directly on the ELP.With IOL power calculation formulae where the constant(s)
act directly on the ELP, a change in the constant(s) always changes the trend of the
PE according to gradSEQ. Formulae where at least one constant does not act on the
ELP have more flexibility to zero the mean or median PE without coupling to a PE trend
error over axial length.
