Fig. 6 shows the PSFs for the Crystalens HD IOL in its nonaccommodative and accommodative states and the Akreos Adapt AO IOL for both 3- and 5-mm pupils. The PSF was similar between the Crystalens HD IOL in its nonaccommodative and accommodative states and the Akreos Adapt AO IOL for the 3-mm pupil. However, these images worsened when the pupil diameter increased up to 5 mm for both IOLs. Strehl ratio values for the Crystalens HD IOL in its nonaccommodative and accommodative states and the Akreos Adapt AO IOL for a 3-mm pupil were 0.70 ± 0.08, 0.63 ± 0.06, and 0.62 ± 0.06, respectively. The Strehl ratio decreased slightly with the accommodation state, but no statistically significant differences were found between both states and both IOLs for a 3-mm pupil (p > 0.05). These values for a 5-mm pupil changed to 0.07 ± 0.01, 0.06 ± 0.01, and 0.07 ± 0.01, respectively. Again, no statistically significant differences were found between both states of the Crystalens HD IOL and both IOLs (p > 0.05). Statistically significant differences were found in the Strehl ratio of the same lens comparing both pupil diameters (p < 0.05).
Table 2 shows DVA (nonaccommodative state) and NVA (accommodative state) for the Crystalens HD IOL. In addition, DVA values for the Akreos Adapt AO IOL and p values comparing both IOLs for distance vision are shown. There were no statistically significant differences in DVA between both IOLs for all contrasts and pupil diameters (p > 0.05). The DVA and NVA were slightly better with the 3-mm pupil than those with the 5-mm pupil, but no statistically significant differences were found between both pupil diameters.
The mean values of log10 CS are plotted in Fig. 7. Fig. 7A shows distance CS for the Crystalens HD and Akreos Adapt AO IOLs for 3- and 5-mm pupils. No statistically significant differences were found between both IOLs (p > 0.05). Contrast sensitivity was slightly better with the 3-mm pupil than that with the 5-mm pupil size, but no statistical significant differences were found at any spatial frequency. Fig. 7B shows the results for near conditions (only for Crystalens HD IOL) where no statistically significant differences were found at any spatial frequency (p > 0.05).
Depth of Focus
Fig. 8 shows the depth of focus of the Crystalens HD IOL in its accommodative (Fig. 8A, C) and nonaccommodative (Fig. 8B, D) states for 3- and 5-mm pupils. Fig. 8 shows the real change of focus produced in the case that the IOL would work after moving in the eye. We have considered the depth of focus as the length of the horizontal dashed line arrow above 0 logMAR VA. Depth of focus for the Crystalens HD IOL in its accommodative and nonaccommodative states for a 3-mm pupil was about 1.25 and 1.75 D, respectively. For a 5-mm pupil, these values for the Crystalens HD IOL in its accommodative and nonaccommodative states were about 0.75 and 0.50 D, respectively.
Our outcomes are in agreement with those obtained in the study by Hunter et al.9 They evaluated the MTFs for an accommodating IOL in the posterior lens position (nonaccommodating state) and the anterior lens position (accommodative state) for 3- and 5-mm pupil diameters. They found a little change in image quality between the posterior and anterior lens position. As with our study, they reported a significant decrease in image quality with increased pupil size; for a 3-mm pupil size, the predicted MTFs were much closer to the diffraction limit than the MTFs for a 5-mm pupil size (Figs. 4, 5). Although we have not obtained results of a Crystalens IOL without the central zone (original Crystalens), the MTF results are similar to those obtained for the Akreos Adapt AO IOL (Figs. 3, 4). This indicates that the extra 1 D in the central zone has little impact on the optical properties of the IOL. This result should not surprise us because the addition corresponds to a small center pupil (1.5 mm in diameter). Image quality is dominated by the higher order aberrations present across the full pupil.
Figs. 4 and 5 also include the achromatic retinal contrast threshold values found by Sekiguchi et al.24 at a retinal illuminance of 500 td. Fig. 4 suggests that, for an eye with a 3-mm pupil, spatial frequencies up to about 45 cpd should be recognizable, corresponding to about 20/13 (visual resolution in white light). Note that the cutoff frequency for each IOL comes from the intersection between the MTF of the IOL and the neural curve. Cutoff frequencies for the Crystalens HD IOL in a nonaccommodative state are slightly better (about 45 cpd) than that for the Crystalens HD IOL in its accommodative state IOL (about 40 cpd). For a 5-mm pupil, cutoff frequencies for the Crystalens HD IOL in its two situations are similar and better than that of the monofocal IOL (Fig. 5). Note that, because the retinal threshold curve rises steeply at higher frequencies, differences between the Crystalens HD IOL in its two situations do not greatly increase the cutoff frequency, although it does increase the suprathreshold contrast at lower spatial frequencies.
Distance and near vision of the Crystalens HD IOL was above 20/20 in both pupil diameters evaluated at 100% contrast. The effect of increased aberrations with large pupils25,26 (increases approximately a factor of 4.4; Table 1) did not seriously affect VA (see Table 2 for a full description). To compare the outcomes of the accommodative lens, we analyzed the monofocal Akreos Adapt AO IOL at distance vision. We obtained a DVA better than 20/20. However, other studies27,28 that have evaluated the Akreos Adapt AO IOL showed an uncorrected DVA (UDVA) about 20/25. This difference in DVA between our study and these studies was likely caused by the measurement method because they implanted the IOL and then evaluated visual performance. We have simulated the vision with this IOL, avoiding effects of the surgical procedure such as IOL decentration or tilt and residual refractive errors. The age of patients was also different. In our study, patients ranged from 21 to 30 years old and with experience in psychophysical experiments. In contrast, in the other studies, patients were older, with ages ranging from 50 to 80 years. The outcomes of the Crystalens HD IOL were comparable to those of the monofocal Akreos Adapt AO IOL.
Hunter et al.9 showed that the maximum achievable theoretical (not measured) shift within the capsular bag was 2 mm. This produced maximum accommodation ranging between 1.0 and 4.1 D, depending on the corneal and lens powers. They concluded that an IOL movement within the capsular bag could theoretically produce up to 2.4 D of accommodation (not including depth of focus) for average corneal and IOL powers. Nawa et al.10 also found that, in an eye with medium axial length and an IOL of 20.0 D, a theoretical forward movement of 1.0 mm of an IOL would generate an objective change in the refractive state of 1.3 D. Thus, following calculations by Nawa et al.,10 a theoretical movement of 1.4 mm would then produce a refractive change of about 1.8 D, a value which is slightly lower than the one found in our experimental study (2 D). Wavefront refraction showed that the forward movement produced artificially in the lens in vitro generates 2 D of objective accommodation. Experimental measurements when simulating that wavefront change have found a slightly larger subjective refractive change (∼2.5 D). Disagreement can be explained in terms of the relative large step used in the depth-of-focus measurements.
It is important to point out that the values of the movement of the IOL used in this model eye study have not, as far as we know, been measured in the living eye. In contrast, recently, Marcos et al.29 measured the changes in the anterior segment in eyes implanted with the Crystalens IOL using an OCT and found no significant movement of the IOL. Also, the one peer-reviewed publication that evaluates the visual performance with the Crystalens HD IOL11 found a low acuity (0.35 logMAR, ∼20/40) inconsistent with an accommodative change of 2.0 D.
Distance CS was also evaluated with the Crystalens HD IOL and compared with the Akreos Adapt AO monofocal IOL for 3- and 5-mm pupils. No statistically significant differences were found between IOLs (p > 0.05). Lee et al.28 evaluated the CS of the Akreos Adapt AO IOL obtaining similar CS values to those found in our study. Then, CS of the Crystalens HD IOL was good and comparable to that of the monofocal IOL. Near CS was also good and comparable to that of distance.
It has been suggested that the Crystalens HD IOL is characterized by increasing the depth of focus because the ciliary muscle’s contraction produces a transfer of energy exerted through the haptics, along with forces from increased vitreous pressure, which causes the lens’ optic to arch. This arching induces positive spherical aberration, with a resulting increase in depth of focus as well as anterior movement of the best plane of focus.30 In addition, this effect is accentuated by the aspheric modification of the central optic. This 3- to 5-µm central thickening has a very small optical zone and further extends the depth of focus. Our results show a greater degree of depth of focus when the IOL nonaccommodates and for a small pupil in relation to the accommodating sate: 1.75 versus 1.25 D (Fig. 8).
The present study is the first that allows a comparison of the visual outcomes of different IOLs in the same patient. We have to consider three limitations of our study: the effect of the surgery, the subjects’ age, and mainly that we have evaluated near vision when the Crystalens HD IOL achieved 2.00 D of accommodation. Surgery effects, such as IOL decentration or tilts, may vary the outcomes reported here. In addition, changes in corneal optical quality may affect too. The experimental subjects (∼24 years old) were much younger than the typical patients who will be implanted with the Crystalens IOL. However, as mentioned in the Methods section, the optics of the subjects is corrected by the adaptive optics system, so it will not play a role in the results. Nevertheless, deterioration of neural visual functions with age could also influence the performance of this lens. However, studies comparing VA in young and older groups have found either no difference (Adams et al.31 [24.6 vs. 57]) or small reductions (Elliot et al.32 [18 to 24 vs. >75 years]). Mordi and Ciuffreda33 have shown that subjective depth of focus increases with age in a rate of 0.027D per year, but, as far as we know, the real reason of this increase has not been established and probably can be explained in terms of senile myosis. Then, results, depth-of-focus results obtained in our experiment, should represent a lower limit of the value that would be obtained in older subjects. Our experimental in vitro study was able to produce an IOL movement of 1.4 mm, which was sufficient to generate an additional 2 D of power. Our aberration measurements indicate that if such a movement can be produced in an eye, this lens should provide excellent visual performance at near. Currently, no such lens movements have been reported with this IOL.
In summary, the optical and visual quality with the Crystalens HD IOL was comparable to a monofocal IOL for distance vision. Our results indicate that if an IOL of the type of the Crystalens HD would move 1.4 mm axially while in the eye, a real refractive change of 2 D would be found. Visual simulations of the IOL optics under this condition (artificially forward moved IOL) shows similar values of VA and CS to the one obtained for distance vision. Because our study shows image quality of the Crystalens HD, IOL is not compromised by forward movement, the poor near acuity observed with eyes implanted with this lens must, therefore, indicate that this type of IOL fails to move such a distance within the eye.
University of Valencia
C/Dr. Moliner 50
This research was supported in part by Ministerio de Ciencia e Innovación Research Grant to Robert Montés-Micó (no. SAF2009-13342), Fundacion Seneca de la Region de Murcia grant 15312/PI/10 to NLG, and a VALi+D research scholarship to Cari Pérez-Vives (Generalitat Valenciana; ACIF/2012/099). We thank Sergio Bonaque and Paula Bernal for their help in different aspects of the wet-cell construction and software development, respectively.
The authors have no proprietary interest in any of the materials mentioned in this article.
Received September 11, 2012; accepted June 12, 2013.
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Keywords:© 2013 American Academy of Optometry
Crystalens HD IOL; optical quality; visual quality; adaptive optics; visual simulation