Intraocular lens formula constant optimization and partial coherence interferometry biometry: Refractive outcomes in 8108 eyes after cataract surgery

Abstract

No author has a financial or proprietary interest in any material or method mentioned. Additional disclosure is found in the footnotes.

Optimising biometry for best outcomes in cataract surgery

“…It is best considered as a 'fudge factor' that adjusts IOL predictions for systematic errors arising from the entire clinical environment, including those arising from the biometry measurement devices (and combinations thereof), patient population, and surgical technique. 29,31 The estimated IOL constant published by manufacturers is typically intended for use with contact ultrasound biometry, although increasingly constants specific to optical biometry are also provided. It is well established that, as a result of corneal indentation, contact ultrasound produces shorter AL measurements than the immersion technique 32 and optical biometry.…”

“…33 The use of a contact ultrasound-specific IOL constant with either immersion or optical biometry will result in outcomes more hyperopic than intended. 31 If an IOL constant specific for immersion or optical biometry is not available, then a suitable value may be listed at the User group for Laser Interference Biometry (ULIB); 34 at the time of writing, IOL constants for over 250 IOL models are available, derived from the outcomes of over 50 000 patients, although caution should be exercised when using constants optimised on other patient populations (eg, inspection of the ULIB data shows differences in constants for the same IOL model between Japanese and non-Japanese eyes), and by other surgeons. Furthermore, in some cases, the ULIB data reflect a mixture of optical and immersion axial length measurements, and automated and manual keratometry.…”

“…Furthermore, in some cases, the ULIB data reflect a mixture of optical and immersion axial length measurements, and automated and manual keratometry. 31 Despite these caveats a constant obtained from the ULIB database is likely to be a better starting point for optical biometry than the manufacturer's published contact ultrasound constant when introducing a new IOL into practice, provided that the IOL constant is then optimised according to local clinical outcomes.…”

“…IOL constant optimisation has been shown to improve substantially prediction accuracy for contact ultrasound (from 79.7 to 82.5% within ± 1 D), 10 immersion ultrasound (from 60% to 65% within ± 0.5 D), 16 and with optical biometry (from 76-89% to 92-94% within ±1 D, dependent upon IOL model and formula). 31 The latter study used constants intended for contact ultrasound as the baseline, 31 and in practice, the magnitude of the improvement to be gained from IOL constant optimisation depends on how close the optimised and baseline constants are found to be.…”

“…36 Aristodemou et al evaluated the clinical significance of different degrees of error in the IOL constant and estimated that a minimum of 86 eyes is required to optimise the pACD for the Hoffer Q formula and around 250 for the SRK/T A constant and Holladay 1 Surgeon Factor. 31 All eyes included for optimisation should have a stable refractive error and best-corrected visual acuity of 6/12 or better, 22 and as wide a range of axial length as possible, 35 and preferably all eyes should have been measured using the same devices for keratometry and axial length. Although the optical biometry devices were calibrated against immersion ultrasound measurements and correlate well with them 17 and with each other, 37 and therefore the IOL constants for the different methods should be very similar, inspection of ULIB data suggests that this may not be the case.…”