Aixa Alarcon

Preclinical metrics to predict through-focus visual acuity for pseudophakic patients

This study compares the clinical through-focus visual acuity (VA) in patients implanted with different intraocular lens (IOL) to optical bench testing of the same IOLs to evaluate the suitability of optical metrics of predicting clinical VA. Modulation transfer function and phase transfer function for different spatial frequencies and US Air Force pictures were measured using an optical bench for two monofocal IOLs, three multifocal IOLs and an extended range of vision IOL. Four preclinical metrics were calculated and compared to the clinical through-focus VA collected in three different clinical studies (243 patients in total). All metrics were well correlated (R2≥0.89) with clinical data and may be suitable for predicting through-focus VA in pseudophakic eyes.

Laboratory-Measured MTF of IOLs and Clinical Performance

We read with interest the article by Esteve-Taboada et al.1 The authors measured the modulation transfer function (MTF) of three intraocular lenses (IOLs) using a corneal eye model with zero spherical aberration using monochromatic light. This means that corneal higher order aberrations and chromatic aberration were not taken into account for the evaluation, even though it has been shown that spherical aberration and chromatic aberration have a significant effect on visual acuity and contrast sensitivity. The exclusion of spherical aberration in the measurement set-up follows the general guidelines of the International Organization of Standardization (ISO) for IOLs (ISO 11979-2:1999). The purpose of ISO standards, in general, is to ensure quality, safety, and efficiency, as well as to facilitate international trade.2 For this particular part of the standard, the primary purpose is to ensure manufacturing quality, and not in vivo performance.3 The current ISO standard provides the reader with a warning by stating that when it comes to MTF measurements “No inference should be made to performance in real eyes” (ISO 119792:2014, Clause C.2). In a previous commentary on a similar study by Artigas et al.,4 Norrby warned that the aberration-free ISO model eye is not valid for assessing aspherical lenses.3 The three lens designs in the study of Esteve-Taboada et al. are refractive/diffractive designs, which are known to affect chromatic aberrations. The TECNIS Symfony IOL is designed to correct chromatic aberration5 for all distances, whereas the trifocal lenses are expected to influence chromatic aberration for near and/or intermediate vision.6 Excluding chromatic aberration and spherical aberration in the measurement set-up has a significant influence on the measured values. To illustrate this effect, Figure 1 shows MTF measurements7 similar to those of Esteve-Taboada et al., but now using a model eye that includes corneal spherical aberration of 0.27 micrometers, and that also includes chromatic aberration by using white light and having suitable dispersive properties of the cornea and fluid medium in which the IOL is immersed. As such, the model eye represents the spherical aberration and chromatic aberration that is also found in an average human eye. The difference in MTF obtained in this eye model and the results obtained by Esteve-Taboada et al. is obvious, and cannot be explained by the slight difference in pupil size in both studies. The difference can only be explained by the fact that the measurement conditions for MTF in the chromatic eye model were more representative for the clinical situation. The authors provide far-reaching statements concerning the implications of their findings toward clinical behavior, even though the understanding of the correlation with clinical outcomes is still limited for metrics based on measurements in a model eye. It must be understood that whatever correlation there may be, it will depend greatly on how the lenses were measured. An optimal correlation can only be achieved if the measurement conditions for MTF are representative for the clinical situation.

Hyperopic Q-optimized algorithms: a theoretical study on factors influencing optical quality

In this work, we analyze the way in which pupil size, optical zone, and initial hyperopic level influence optical quality for hyperopic Q-optimized corneal refractive surgery. Different Q-optimized algorithms and the Munnerlyn formula were tested to analyze the optical quality of the final retinal image for initial hyperopic errors from 1D to 5D. Three optical zones (5.5, 6, and 6.5 mm) and two pupil diameters (5 and 7 mm) were considered. To evaluate optical quality, we computed the modulation transfer function (MTF) and the area under MTF (MTFa). Q-optimized values at around Q = -0.18 were found to provide the best optical quality for most of the conditions tested. This optimum final asphericity for hyperopic ablation was not depending on the degree of hyperopia corrected, the optical zone or the pupil size being this information important for clinical practice.

Clinically Relevant Interpretations of Optical Bench Measurement of Intraocular Lenses.

Clinically Relevant Interpretations of Optical Bench Measurement of Intraocular Lenses We read with interest the article by Gatinel and Loicq in the April 2016 issue.1 This study draws conclusions concerning the behavior of intraocular lenses (IOLs) based on optical bench measurements. One must always be careful when correlating the optical behavior of IOLs with their corresponding clinical behavior. For example, although the optical bench measurements of multifocal IOLs show a modulation transfer function (MTF) of essentially 0 in the intermediate range, clinically measured visual acuity values can be 20/32 or greater. In addition, when monofocal lenses are measured following the multifocal ISO standard, some models show bifocal behavior. Therefore, we must be mindful that optical bench measurements may not be fully representative of clinically relevant vision, and great care must be taken not to draw faulty conclusions based on these data. Alarcon et al.2 described improvements in methods of optically testing IOLs to predict visual acuity. This and several other recent studies2-4 highlight the importance of three components that should be included in eye models if one hopes to provide an accurate explanation of clinical behavior: