A large variability exists in the ocular wave-front aberrations in different normal subjects.
30 In particular, spherical aberration is small in normal young subjects and becomes larger and positive with age,
31 with individual variation. Some people should therefore experience more improvement in visual performance than others when spherical aberration is corrected. In all four subjects assessed in this study, the simulated implantation of a customized spherical aberration–correcting IOL improved visual performance when compared with the simulated implantation of a spherically surfaced IOL. In a realistic comparison, when investigating the benefit provided to a group of patients by lenses that provide customized correction of spherical aberration, compared with a population conventional spherical lens implants, we should consider that in the spherical case (case 2), there would be a spread in the postoperative ocular spherical aberration values. In this study, we consider only the average difference, and, as such, the conclusions are valid only for this average situation. Performing measurements in a simulator allows you to determine the theoretical maximum benefits. As such, in a clinical situation, we expect the benefit of customized correction of spherical aberration to be less than what has been measured in this experiment.
As only four subjects were assessed, each of which was relatively young, with low degrees of refractive error, generalizations cannot be made to all patients with cataract. Young subjects were used in this experiment due to the desire to collect reliable psychophysical data, but the improvements presented here may not be representative of the improvements attainable in older subjects because of possible differences in modulation threshold functions (retinal contrast transfer abilities). The larger amount of aberrations in the eyes of older subjects,
31 32 compared with younger subjects may also influence the visual improvements attainable in older and younger subjects. Despite these limitations, the results obtained in this study can be compared with already published clinical results obtained with the Tecnis lens.
In the clinical study performed by Mester et al.,
8 the average difference in spherical aberration was the same as occurred in the current study in individual subjects. The average improvement in photopic contrast sensitivity measured using a Functional Acuity Contrast Chart (FACT chart; Vision Sciences Research Corp., San Ramon, CA) at 6 cyc/deg was 30% as opposed to the 47% measured in this study (for the white light condition). This relative increase in visual performance can be explained by a number of factors. First, in this study, all subjects had customized correction of their individual spherical aberration, whereas in the Mester et al. study all patients received an average correction. The luminance conditions and pupil sizes were also different in the two studies. In the clinical study by Mester et al. the photopic contrast sensitivity was measured at 85 while 35 cd/m
2 was used in this study. In addition, the pupil size of the patients with cataract was controlled by the ambient lighting conditions and was therefore a natural pupil size, whereas in the simulator the pupils of all subjects were pharmacologically dilated and an artificial pupil size was imposed (4.8 mm). This most likely played a role in the fact that the contrast sensitivity values measured in this study were lower than values that are typically measured in a clinical setting for contrast sensitivity. These low values can also be explained by the fact that, although the luminance conditions were photopic, they were on the low end of the photopic scale, and the pupil sizes were relatively large for photopic conditions. In addition, although patients are given no time restrictions on identifying a target in clinical measurements of contrast sensitivity (using a contrast sensitivity chart), the identification task performed on the simulator is made more difficult by the fact that subjects are presented with the target for only 0.3 seconds.
The different levels of improvement measured for different psychophysical tests administered during this study draws attention to the fact that the potential benefit derived from improvements in the visual system varies, depending on the target chosen for the assessment. In white light the average improvement in visual acuity associated with customized correction of spherical aberration was 10%, whereas for the same conditions, the average improvement in the CSF was 32% (as an average of the three spatial frequencies). Similarly, in green light, the average improvement in visual acuity associated with customized correction of spherical aberration was 38%, whereas, for the same conditions, the average improvement in the CSF was 57% (again as an average of the three spatial frequencies). This reflects the fact that contrast sensitivity testing is more sensitive to changes in the retinal contrast than is visual acuity testing.
33 Also, retinal images of contrast sensitivity targets contain one pure spatial frequency (even for a defocused eye), whereas high-contrast letters are complex targets that contain more than one spatial frequency. As a result, retinal images of defocused letters may contain phase-contrast reversals. In fact, one subject commented that phase-contrast reversals under defocused conditions made it easier to identify letter orientation. In other studies of the potential of customized wave-front aberration correction,
4 7 higher spatial frequencies were chosen for subjective performance tests, because theoretical optical calculations of the MTF reveal that higher spatial frequencies have more potential for improvement. However, we chose to use lower spatial frequencies or the areas around the peak of the CSF curve, because these peak frequencies are more closely correlated with functional vision.
34 In a normal population, this peak often occurs at ∼6 cyc/deg,
35 whereas recent studies of the contrast sensitivities of pseudophakic individuals reveal that their peak is ∼3 cyc/deg.
8 19 20
It is not surprising that correcting spherical aberration provides a visual benefit.
36 From the results obtained in this study, it seems clear that there are potential benefits to customized correction of spherical aberration in cataract patients who use IOLs. Naturally, patients who have large amounts of spherical aberration, either due to normal aging or to LASIK for myopia correction
6 (in this case, LASIK that does not provide wave-front correction) would benefit more from customized correction. Correction with an IOL is sensitive to tilt and decentration of the IOL. Bench measurements of the Tecnis lens indicate that if it is decentered 0.5 mm and tilted <7°. Its optical performance will exceed that of a conventional spherical IOL.
21 A higher level of spherical aberration correction would be even more sensitive to correct placement in the eye. The potential benefits associated with the customized correction of spherical aberration are reduced when the pupil is small. As a result, the improvement in visual performance would be largest in younger patients where the pupil is large or in situations where the pupil is dilated due to low light conditions such as driving at night. Yoon and Williams
7 revealed that there are large visual benefits associated with the correction of all monochromatic higher-order wave-front aberrations. A study of whether the correction of the remaining higher-order aberrations is practical and realizable using customized IOLs should be performed. The effect of the correction of all these aberrations on depth of focus should also be investigated.
In this study, through-focus measurements of visual performance were conducted to achieve a better understanding of the impact of spherical aberration on pseudoaccommodation due to depth of focus. The classic definition of depth of focus is the range of defocus values for which there is a blur of small enough size that it will not adversely affect the performance of the system. Thus, when the MTF is used for assessment the 80% criterion is often used to determine depth of focus, which is the defocus range over which the MTF is greater than 80% of the peak MTF.
29 Figure 11 illustrates the application of this criterion to model eyes with spherical aberrations that are the same as in cases 1 and 2 in the experiments conducted in this study. Using the MTF of idealistic on-axis eye models (containing no higher-order aberrations other than spherical aberration) and the 80% criterion, depth of focus is increased by adding spherical aberration to the eye model. Similar behavior is observed in the measurements. If the 80% criterion is applied to the contrast sensitivity data collected in the AOVS
(Fig. 9) , depth of focus with spherical aberration corrected (case 1) again is less than depth of focus with spherical aberration introduced (case 2). As a result, we see that this definition of depth of focus may not be the most relevant definition when functional vision is considered. Indeed, with spherical aberration correction our results suggest that the range of functional vision is not decreased and that the average visual performance is as good or better when spherical aberration is corrected for as much as 1 D of defocus. This may be because even in the corrected state other higher order aberrations remain (average RMS error was still 0.25 μm after correcting the spherical aberration). This serves to flatten the MTF versus defocus curves shown in
Figure 11 so that they appear more like those in
Figure 10b and allow the subjects to have more tolerance to defocus. The 80% criterion is an arbitrary criterion that may not reflect the nature of tolerance to defocus in the visual system. It may be more relevant to use an absolute threshold metric that is individually determined. More investigation is needed to determine a functional definition of the subjective depth of focus.
Using monochromatic eye models and the recorded wave-front aberration, we calculated the MTF and used it to predict the improvements possible
(Fig. 10) . These predictions were then compared with the measured monochromatic contrast sensitivity outcomes
(Figs. 6b 9) . In general, there is a reasonable qualitative agreement between the optical (MTF) and visual parameters (CSF) studied in agreement with previous studies of this issue.
37 Quantitatively, the improvement measured when spherical aberration is corrected is underestimated by the predictions provided by the theoretical optical calculations (especially when the eye is defocused). This may be explained by the Stiles-Crawford effect, which was not included in the eye model and which may affect the optical predictions and the psychophysical measurements differently. However, for the pupil size studied (4.8 mm), it is assumed that the Stiles-Crawford effect plays a relatively minor role
38 and does not explain the differences between the two assessment techniques. Because eye models do not always provide us with a complete description of the potential improvements in visual performance associated with new optical innovations, adaptive optics simulators have become useful tools.
Through the use of adaptive optics it is possible to correct precisely the higher-order wave-front aberrations of the eye. Although this technique can only be used in laboratory simulations with complicated and expensive equipment, it provides us with a noninvasive method for exploring the potential of new ophthalmic devices. Adaptive optics also provides us with a powerful tool for the interactive design of new ophthalmic devices and a way to test the relationship between ocular optical quality and visual performance under different conditions.