Outstanding refractive outcomes with high quality vision
By Jay S. Pepose, MD, PhD
The Crystalens AO is the first FDA-approved, aspheric, aberration free, accommodating IOL. The advanced zero aberration design is relatively immune to the optical effects of IOL tilt or decentration with respect to the visual axis and neither adds to nor subtracts from the inherent corneal spherical aberration. Together, the optical impact of this new design results in enhanced image quality over previous iterations of Crystalens, without compromising depth of field. In the next section, the 3-month refractive outcomes of the Crystalens AO are reviewed.
Restoration of High Quality Vision at 3 Vergences
The Crystalens AO IOL is an intraocular image enhancing system, optimized for bilateral implantation, that provides excellent uncorrected distance, unmatched intermediate, and good quality near vision when the surgeon completes the task of hitting the refractive target (i.e. Plano to −0.25D in the dominant eye and between −0.25D to −0.5D in the nondominant eye). Figure 1 shows the excellent 1- and 3-month uncorrected monocular distance vision for Crystalens AO eyes with 3-month data in the SurgiVision database. Figure 2 shows eyes within 0.5D of Plano and less than 1.5D of postoperative astigmatism. Figures 3 and 4 show the same data for binocular uncorrected distance acuity for all patients with 3-month data and the filtered data close to emmetropia, respectively, demonstrating that all patients were 20/40 or better and could obtain an unrestricted driver's license. Uncorrected monocular intermediate vision, both unfiltered (Figure 5) and filtered around Plano (Figure 6), is unsurpassed among presbyopiacorrecting IOLs and the binocular uncorrected vision, unfiltered (Figure 7) and filtered (Figure 8), is even better, with 100% at 20/30 or better. With the growing popularity of computers, cellular phones, and numerous other handheld electronic devices, intermediate vision is becoming increasingly important for daily functions. Monocular uncorrected near vision is also very good and shows further improvement in the percentage of patients at J1 or J2 or better between 1 and 3 months in the unfiltered (Figure 9) and filtered (Figure 10) cohorts. Binocular uncorrected near vision, unfiltered (Figure 11) and filtered (Figure 12) reveals that 100% of patients are J3 or better at 3 months. Finally, Figure 13 shows the combined uncorrected binocular results at near (16 inches), intermediate (32 inches) and far, with all patients at 20/40 and J3 or better at all distances tested.
Importance of High Retinal Image Quality
The introduction of the Crystalens AO — the fifth generation of Crystalens —incorporates an aberration-free design with aspheric front and back surfaces to further enhance retinal image quality without compromising depth of field. In contrast to multifocal IOLs that split light energy between multiple foci and simultaneously cast near and far images onto the retina, the zero spherical aberration Crystalens AO focuses 100% of light at all object vergences (Figure 14). In comparison to multifocal IOLs, this results in uncompromised contrast sensitivity even under mesopic conditions with glare, while minimizing complaints of halos or photic phenomenon. This translates into better performance during night driving and other important activities.
Impact of Spherical Aberration on Visual Performance
The Crystalens AO is a zero-aberration lens, which means it neither adds to nor subtracts from the inherent corneal spherical aberration of the eye. The IOL therefore, by definition, has uniform power across the entire optic, making it more immune to the detrimental effects of IOL tilt or decentration. Decentration of an IOL with either positive or negative spherical aberration with respect to the visual axis (Figure 15) can lead to the induction of asymmetrical third order aberrations such as coma, along with defocus and astigmatism, all conspiring to degrade retinal image quality (Figure 16).
In addition, the aspheric design on both anterior and posterior surfaces of the Crystalens AO optic results in a thinner optic than the same power Crystalens 5-0 predecessor. The thinner lens optic may be more malleable during accommodative effort, further expanding one's range of vision via accommodative arching. By not adding to or subtracting from the cornea's natural spherical aberration, the Crystalens AO provides an optimal balance between extended depth of field and excellent retinal image quality (Figure 17).
Figure 14. Simulated images generated using a custom paraxial beam tracing program shows a single point of focus with Crystalens, as compared with the diffractive and refractive multifocal technologies.
Figure 15. The visual axis does not pass through the center of the cornea, the iris or the lens. The decentration between the visual axis and the pupil has been measured, it's about a third of a millimeter. The decentration between the center of the IOL and the center of the pupil is also about a third of a millimeter. Taken together, this indicates that the mean optical decentration between the visual axis and the center of the IOL is about a half millimeter.
Figure 16. When IOLs are decentered 1.0 mm, there is far more image degradation with an IOL with negative spherical aberration (Tecnis) compared to zero spherical aberration (AO).
Figure 17. At peak performance, the AO has 20% better image quality than the 5.0, while the depth of field between the AO and the 5.0 is about the same. So the AO is providing a better image quality over the same depth of field as the 5.O. In comparison, the HD is sacrificing some best image quality in return for getting greater depth of field for near vision and there is also a little less tolerance to residual myopia.
Effect of Pupil Shape and Dynamics on Retinal Image Quality
In a busy clinical practice, preoperative measurement of pupil size, shape and dynamics may not be part of the standard pre-cataract evaluation, but can have a profound effect on retinal image quality for objects at near, intermediate or far. This should be considered in customized premium IOL selection for each patient and discussed with patients during preoperative counseling.
Various mathematical algorithms to determine the quality and sharpness of an image are routinely employed in photography, in autofocus cameras and, recently, in establishing automated methods to evaluate the quality of digital retinal fundus and optic disc photographs. One important component of this assessment is the image sharpness metric. As part of a Bausch + Lomb team, I recently used benchmark testing to compare the imaging qualities of 7 FDAapproved presbyopia-correcting IOLs through a range of object vergences (i.e., through-focus) and various pupil diameters. We used an anatomically and optically accurate eye model and compared Crystalens AO and HD, Tecnis multifocal and ReSTOR 3.0 and 4.0 by imaging a 1951 Air Force Target in this bench model. Sharpness of each digital image was quantified using a 2 dimensional gradient function.
We found substantial differences in image sharpness at various object vergences between the accommodating and multifocal IOLs at different pupil diameters. For each specific IOL, the graphs peak around Plano and the height of each curve is predictive of image sharpness. Crystalens AO had the best distance image quality for all pupil diameters, while maintaining a broad depth of field. The bench test cannot simulate accommodative changes, which is why the near vergence curve appears to drop off with Crystalens. At the average cataract patient's pupil diameter of between 3 and 4 mm, the Crystalens AO has superior image quality compared to the other IOLs reviewed during bench testing. While inferior to the image quality of the Crystalens AO, only when the pupil aperture reached 5 mm did the aspheric ReSTOR 4.0 approach a distance image quality similar to the Crystalens HD, a product of the ReSTOR's apodization. The ReSTOR 3.0 and Tecnis multifocal showed a comparable, poorer distance image quality at all pupil diameters than either the Crystalens AO or HD. The Tecnis showed less negative impact of larger pupil size on the quality of the near image. The modulation transfer function associated with these 7 IOLs paralleled the image sharpness data, with Crystalens AO superior to the multifocal IOLs at all pupil diameters and across all spatial frequencies.
With each presbyopia-correcting IOL design strategy, there are inherent tradeoffs with regard to contrast sensitivity, loss of light energy to useless foci, night glare and photic phenomenon, near and distance image quality at any given pupil diameter. The "take home" message is that if a patient has a pupil of a given size and limited dynamic range, this can dramatically and selectively impact retinal image quality for near or far objects, and this effect is more striking with multifocal IOLs than with accommodating IOLs, which do not split light between multiple foci.
Crystalens AO combines the advantages of an accommodating IOL that does not split light between multiple energy foci with an aspheric, aberration-free design and uniform lens power across the entire optic. The result is that patients enjoy superior retinal image quality at all pupil sizes, immune to the negative optical effects of IOL decentration and tilt, and enhanced depth of field. For an aging population with increased longevity, the impact of age-related ocular co-morbidities, such as glaucoma or macular degeneration can result in decreased contrast sensitivity independent of the impact of the IOL. Surgeons can have confidence that they are not further compromising contrast sensitivity in their patients by implantation of a Crystalens AO. This is particularly reassuring, since we have no way of knowing which cataract patients may go on to develop maculopathies or optic neuropathies with age.
Dr. Pepose is Director of Pepose Vision Institute and Professor of Clinical Ophthalmology, at the Washington University School of Medicine, St. Louis, Mo. He is a consultant to Bausch + Lomb.
Ophthamology Management, Issue: August 2010