Optimizing Biometry for Best Outcomes in Cataract Surgery
Optimizing Biometry for Best Outcomes in Cataract Surgery
The term 'IOL constant' is a misnomer as it does not relate solely to the IOL and is not necessarily a constant. 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.
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 and optical biometry. The use of a contact ultrasound-specific IOL constant with either immersion or optical biometry will result in outcomes more hyperopic than intended.
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); 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. 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.
Constant optimisation is the process by which the IOL constant is adjusted to minimise the systematic errors listed above, as indicated by a ME of zero. The process of constant optimisation has little effect on the dispersion of outcomes around the mean (ie, the standard deviation, SD), but maximises the proportion of eyes within a particular target range and minimises the MAE (see Appendix for a discussion of the relationship between ME, MAE, and SD). IOL constant optimisation has been shown to improve substantially prediction accuracy for contact ultrasound (from 79.7 to 82.5% within ±1 D), immersion ultrasound (from 60% to 65% within ±0.5 D), and with optical biometry (from 76–89% to 92–94% within ±1 D, dependent upon IOL model and formula). The latter study used constants intended for contact ultrasound as the baseline, 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.
IOL constant optimisation may be performed by entering refractive outcome data into the IOL master, or by using one of the online services provided by Dr Haigis or Dr Hill. Haigis recommends using data from more than 50 eyes, and Hill more than 200. 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.
All eyes included for optimisation should have a stable refractive error and best-corrected visual acuity of 6/12 or better, and as wide a range of axial length as possible, 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 and with each other, 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. It is not clear whether the observed differences are due to random or systematic errors, due to differences between populations or surgeons, or represent true differences between the devices, but optimisation should perhaps distinguish between immersion ultrasound, IOL master, and Lenstar axial lengths.
IOL Constant Selection and Optimisation
The term 'IOL constant' is a misnomer as it does not relate solely to the IOL and is not necessarily a constant. 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.
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 and optical biometry. The use of a contact ultrasound-specific IOL constant with either immersion or optical biometry will result in outcomes more hyperopic than intended.
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); 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. 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.
Constant optimisation is the process by which the IOL constant is adjusted to minimise the systematic errors listed above, as indicated by a ME of zero. The process of constant optimisation has little effect on the dispersion of outcomes around the mean (ie, the standard deviation, SD), but maximises the proportion of eyes within a particular target range and minimises the MAE (see Appendix for a discussion of the relationship between ME, MAE, and SD). IOL constant optimisation has been shown to improve substantially prediction accuracy for contact ultrasound (from 79.7 to 82.5% within ±1 D), immersion ultrasound (from 60% to 65% within ±0.5 D), and with optical biometry (from 76–89% to 92–94% within ±1 D, dependent upon IOL model and formula). The latter study used constants intended for contact ultrasound as the baseline, 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.
IOL constant optimisation may be performed by entering refractive outcome data into the IOL master, or by using one of the online services provided by Dr Haigis or Dr Hill. Haigis recommends using data from more than 50 eyes, and Hill more than 200. 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.
All eyes included for optimisation should have a stable refractive error and best-corrected visual acuity of 6/12 or better, and as wide a range of axial length as possible, 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 and with each other, 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. It is not clear whether the observed differences are due to random or systematic errors, due to differences between populations or surgeons, or represent true differences between the devices, but optimisation should perhaps distinguish between immersion ultrasound, IOL master, and Lenstar axial lengths.
Source...