Sources of error in intraocular lens power calculation

Abstract

Improvement in refractive outcome requires better methods for predicting the postoperative IOL position. Measuring AL by partial coherence interferometry may be of benefit. Autorefraction increases precision in outcome measurement. Reducing these 3 major error sources with means available today reduces the MAE to 0.4 D. Using IOLs that compensate for the spherical aberration of the cornea would eliminate the influence of pupil size. Further improvement would require measuring the asphericity of the anterior surface and radius of the posterior surface of the cornea.

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

“…Compared with acoustic biometry with a standard 10 MHz ultrasound probe, PCI AL measurements have greater precision (20 mm 10 vs 120 mm 11,12 ), are negligibly affected by velocity assumptions 13 and consistently measured along the visual axis. 14 In addition, applanation ultrasound measurements are further affected by an inevitable but variable amount of corneal indentation.…”

Repeatability and Reproducibility of a New Partial Coherence Interferometer; AL-Scan Optic Biometer

“…Accurate measurement of ocular biometry is critical to achieving optimal refractive outcome in patients undergoing cataract surgery [10,11]. Optical biometry is a rapid, easy to use and non-contact method to assess the eye's biometry and calculate the IOL power, so the method is preferred by many ophthalmologists [12][13][14].…”

Optimising biometry for best outcomes in cataract surgery

“…12 The biggest source of error (35.5%) is inaccuracy in the IOL formulas' predictions of the post-operative IOL position. Inaccuracies in axial length and keratometry measurements, either arising from the measurements themselves or associated underlying assumptions, account for 17.0% and 10.1% of the error, respectively.…”

“…Norrby estimated that the lowest MAE achievable with current methods is between 0.36 and 0.40 D, 12 implying a maximum proportion within ±1 D of target of between 95 and 97%. 13 Norrby's limits are not absolute, however, and many of the strategies outlined below allow improved predictions by implicitly refining systematic errors or biometric assumptions, thereby reducing the magnitude of their contribution to the final PE.…”

“…For an IOL labelled between 0 and 15 D, its true power must be within ± 0.3 D of the labelled power, between 15.5 and 25 D the allowed tolerance is ± 0.4 D, between 25.5 and 30 D it is ± 0.5 D, and above 30 D the true power is permitted to be up to ±1 D from the labelled power. For the lowest power IOLs, the resulting error at the spectacle plane may be up to ±0.2 D, but for IOLs greater than 30 D the error resulting from manufacturing tolerance may be as high as ± 0.7 D. Norrby, however, suggests that 'the error should be even smaller because a conscientious manufacturer should apply an internal tolerance of ± 0.25 D', 12 and estimates that the proportion of the PE because of IOL manufacturing is just 1.2% and therefore clinically insignificant.…”