Ray tracing and intraocular lens power calculation

Despite having potential benefits for intraocular lens (IOL) power calculation, ray tracing methods still have limited accuracy, according to research. “Ray tracing has the benefit of eliminating approximation errors and gives the opportunity for more elaborate calculations, such as to optimise refraction while considering corneal aberrations, if they are known from preoperative measurements. However, as in every other method of IOL power calculation, the accuracy of ray tracing is limited by the inaccuracy of the measurements that are used in the calculation and by the unpredictability of IOL position,” said Dr Rallatos, Athens Eye Hospital, Athens, Greece. He presented findings from analyses conducted using data from 53 eyes implanted with an acrylic aspheric IOL. Anatomical measurements were obtained with an optical biometer, refractions were measured with an autorefractor, and IOL power calculations were done with ray tracing software (OpticStudio 15.5). Dr Rallatos reported that the difference between the achieved and attempted correction averaged 0.25D, but it was equal or greater than 1D in about 10% of the eyes. The postoperative IOL position represents the greatest source of unpredictability. However even in the hypothetical case of perfect IOL position prediction there will be a residual refractive error. Analyses undertaken to estimate how the parameters involved in the power calculation contributed to this residual error showed that error in anterior cornea power had the greatest influence, accounting for 22.8% of the total error. It was followed by errors attributable to posterior cornea curvature (14.4%) and axial length measurement (13.6%). “Posterior corneal power is known to be a source of IOL power calculation error in eyes with previous refractive surgery, but it also seems to be an important source of error in intact eyes,” Dr Rallatos told EuroTimes. IOL POSITION Investigating correlations between several anterior chamber anatomic parameters (anterior chamber depth, white-to-white, and C-constant) and known IOL position, indicated that the C-constant, a concept introduced by Thomas Olsen MD, would likely be most useful for improving IOL position prediction. However, even when IOL position was known through postoperative measurement and after taking into account posterior cornea power and inaccuracies in other biometric measurements, there was still residual error. “We call this the lost refraction,” said Dr Rallatos, and he proposed that it can be accounted for in enhancement procedures by using normalised weighting coefficients to distribute it across all of the model parameters. Gerasimos Rallatos: g.rallatos@athenseyehospital.gr

Tags: IOL, ray tracing