VISION QUALITY METRICS

New aberrometry-based vision quality metrics can provide an objective measure of a patient’s through-focus acuity, said Raymond A Applegate OD, PhD, University of Houston College of Optometry, Houston Texas, USA.
“What is particularly powerful about using aberrometry-based vision quality metrics is that they are much more sensitive to subtle changes in vision than an acuity chart,” Dr Applegate told the XXXIII Congress of the ECSRS in Barcelona, Spain.
Dr Applegate noted that interactions between different optical aberrations can improve or decrease visual performance. The classic example is defocus and spherical aberration, which degrade retinal image quality, but in the correct combination provide improved image quality (despite the increase in the total amount of aberrations) over the adverse effect of each aberration independently. (see Figure 1) 
Therefore, wavefront error (WFE) measured by a typical wavefront aberrometer expressed as RMS WFE is not a good metric for predicting visual performance as measured by acuity, he said. On the other hand, by calculating the appropriate visual image quality metrics from the measured WFE, it is possible to predict acuity in through-focus experiments evaluating various presbyopic corrections.
Dr Applegate analysed WFE data from a through-focus experiment conducted by a research team at Aston University in the UK, headed by Prof James Wolffsohn. The goal of Dr Applegate’s analysis was to determine if he could predict the acuity actually measured by the research team, using the measured WFE in patients implanted with one of three presbyopic intraocular lenses (IOLs) – the Lenstec Tetraflex, an accommodating IOL, the Rayner M-flex concentric multizonal bifocal IOL, and the Oculentis Mplus, a standard multifocal.
In the single-blind study, Dr Applegate was only given wavefront data obtained from the patients as they looked through a wavefront sensor at an eye chart placed at four metres distance, while defocus lenses were placed in front of their eye. Only after Dr Applegate made his acuity predictions did the Aston group provide actual acuity measurements made at the time of WFE measurement.

DEFOCUS CURVE
As can be seen in Figure 2, through-focus variation in measured visual acuity is mimicked by the predicted visual acuity from WFE measurement, with the predicted acuity being slightly better than the measured acuity. Dr Applegate noted that this small over-prediction of acuity can be easily corrected given the results of the experiment, in the same way as cataract surgeons use actual surgical results to improve their outcomes. 
For comparison, the red line in Figure 2 is the anticipated through-focus acuity of subjects having a state-of-the-art monofocal IOL implanted and wearing glasses with a 2.5D add in a transition design. 
In eyes with the M-flex and Mplus IOLs, the through-focus curves were very similar (to Figure 2), although the vision quality metrics missed a slight improvement in acuity, intended and achieved by the IOL design, in through-focus acuity, which was detected by subjective measurement of acuity. 
“It is very hard for Zernike polynomials to represent the abrupt changes in WFE in these two designs, but I predict that when we resolve this issue the prediction will be even better,” Dr Applegate said.

Raymond A Applegate: raappleg@central.uh.edu