A three-part system for refining intraocular lens power calculations

Abstract

A three-part system that determines the correct power for an intraocular lens (IOL) to achieve a desired postoperative refraction is presented. The three components are (1) data screening criteria to identify improbable axial length and keratometry measurements, (2) a new IOL calculation formula that exceeds the current accuracy of other formulas for short, medium, and long eyes, and (3) a personalized "surgeon factor" that adjusts for any consistent bias in the surgeon's results, from any source, based on a reverse solution of the new formula; the reverse solution uses the postoperative stabilized refraction, the dioptric power of the implanted IOL, and the preoperative corneal and axial length measurements to calculate the personalized surgeon factor. The improved accuracy of the new formula was proven by performing IOL power calculations on 2,000 eyes from 12 surgeons and comparing the results to seven other currently used formulas.

The Cataract National Dataset electronic multicentre audit of 55 567 operations: when should IOLMaster biometric measurements be rechecked?

“…1 Nearly all formulae used for calculation of intraocular lens (IOL) power depend on axial length (AL) measurements and require knowledge of corneal power (K). [2][3][4][5][6] As a result, accurate and precise measurements are a critical part of preoperative assessment and biometric errors are an important source of ophthalmic error. 7 To minimise such risk the United Kingdom (UK) Royal College of Ophthalmologists (RCOphth) Cataract Surgery Guidelines contain advice as to which outlying biometric measurements merit confirmation.…”

“…Similarly, the same guidelines suggest that an interocular difference in K of 41.00 D or an AL disparity of 40.3 mm should only be accepted after verification, a recommendation similar to that of Holladay. 2 Although these guidelines are potentially of very great use, their recommendations are derived from analysis of studies performed in other countries and have not been specifically validated as appropriate for the UK population. The principle aim of this analysis was to address this uncertainty by determining the normal range of biometric variation in the UK cataract surgery population, so enabling the validity of the RCOphth biometry guidelines to be assessed.…”

Optimising biometry for best outcomes in cataract surgery

“…The formulas in widest use utilise two biometric measurements (axial length and keratometry) and a single IOL constant 20 (Hoffer Q, 21 Holladay 1,22 and SRK/T 23 ). The Haigis formula uses three measurements (axial length, keratometry, and pre-operative anterior chamber depth) and three IOL constants, 18 Olsen's formula five uses measurements (those used for the Haigis, plus pre-operative refraction and lens thickness) and one IOL constant, 24 and the Holladay 2 formula uses seven measurements (those used by Olsen, plus patient age, and the horizontal white-to-white measurement) and one IOL constant.…”

“…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.…”

“…22 Modern optical biometry with its low inter-observer variability 9 means that two surgeons measuring the same eye with the same device are likely to acquire almost identical measurements. Small-incision phakoemulsification techniques, with continuous curvilinear capsulorhexis and in-the-bag IOL placement, may reduce inter-surgeon variability and its effects on refractive outcome.…”