The Importance of Quality of Vision

The findings from an ongoing phase 4 premium lens study are explored.

By Jason Stahl, MD

Our practice and Jay Pepose's are currently conducting a prospective, randomized, partially masked study of three IOLs that were bilaterally implanted (53 patients to date). We are studying the Crystalens AO (Bausch + Lomb), the Tecnis multifocal (Abbott Medical Optics) and the Restor +3 (Alcon).

Factors in Choosing a Lens

There is no perfect lens—all have their strengths and weaknesses. Despite their benefits, all require compromises in range of vision or quality of vision. It's well known that multifocals work by splitting light, which decreases contrast sensitivity, so you must ensure that your patient has no corneal or retinal pathology (ie, irregular astigmatism, macular degeneration) that may result in poor visual quality if implanted with these lenses. I perform a preoperative retinal OCT on all patients prior to discussing IOL options to rule out maculopathy, such as early epiretinal membranes that may not be visible by ophthalmoscopy. The aspheric optic of the Crystalens AO does not split light so 100% of the light is delivered, resulting in high visual quality.

In addition, the size of the pupil and its dynamic range can affect the range of vision with multifocal lenses. For example, the Restor has a 3.6-mm diffractive multifocal optic and outside that 3.6 mm, it's purely monofocal. Good light is necessary to constrict the pupil in order to utilize the multifocal optic for near vision. Since 40% of the light focuses for near vision and 40% of the light focuses for distance, you lose 20% of the light, which affects quality of vision. Contrast is lost because 100% of the light is not hitting the retina as it does with the Crystalens AO at all pupil sizes. With the Crystalens AO, there is no loss of light (Figure 1).

The Tecnis multifocal optic exists throughout the entire lens, so it's less dependent on pupil size when it comes to distributing light. At near and distance, patients are still losing 18% of the light due to the scatter from the multifocal optic. In addition, a large pupil size can negatively impact the Tecnis multifocal at intermediate distances.¹ It's necessary to measure a patient's pupil preoperatively, because the size can affect the visual performance of the presbyopia-correcting IOLs.

Figure 1. With the Crystalens AO, there is no loss of light, so quality of vision is not compromised. Figures courtesy of Jay Pepose, MD.

Study Findings

Our study findings have shown that the Crystalens AO has better uncorrected and distance-corrected intermediate vision. Conversely, the Tecnis multifocal and Restor have better distance-corrected near vision than the Crystalens AO. Multifocals have a very near point of focus which is greater for the Tecnis multifocal than the Restor.

With all of the lenses, distance-corrected vision is good and it's not surprising that we have better intermediate vision with the Crystalens AO and better near vision with the multifocal lenses.

Quality of Vision Metrics

Quality of vision metrics were evaluated in the study. The Optical Quality Analysis System (OQAS, Visiometrics), which is based on the double-pass technique, provides an objective measurement of optical quality. The objective scattering index (OSI) is used to evaluate how lens changes affect visual quality. It's easy to understand that a cataract patient will have a higher OSI score due to internal lens scatter but even a 50-year-old with early (trace) nuclear sclerosis and 20/20 best-corrected vision will have an increased OSI due to loss of optical quality of the aging lens. In the study, we're comparing the OSI measurement for the three lenses. At 1-month postop, the smooth optic of the Crystalens AO had better scores (less scatter) compared to the diffractive rings of the two multifocal lenses (Figure 2). Another quality-of-vision metric provided from the OQAS is point-spread functions (PSF). The Crystalens AO had narrower 50% and 10% PSF than the Restor or Tecnis multifocal. The real strength of the OQAS device is that it provides objective quality-of-vision metrics.

We're also using a glarometer to evaluate haloing. There were fewer halos with the Crystalens AO when compared to the Tecnis multifocal (Figure 3). Our study is confirming what we've suspected—that you'll achieve better optical quality with the Crystalens AO than with multifocal IOLs.

Figure 2. Smaller OSI score means less scatter and better visual quality.

Figure 3. There were fewer halos with the Crystalens AO when compared to the Tecnis multifocal.

What Lens to Offer?

I will use a multifocal lens in patients who demand very good near vision and aren't bothered by the idea of some loss of contrast or haloing at night. Each patient must have very pure optics when I examine the cornea and the retina must be healthy as well. In my practice, I use the Crystalens AO in the vast majority of eyes. I cater to a large population of post-refractive eyes—about 40-50% of the eyes in my practice. In those eyes, I almost exclusively use the Crystalens AO because these patients already have complex corneal aberrations, which will likely be worsened with a multi-focal lens. However, even in virgin eyes, I tend to use the Crystalens AO most of the time since my goal is to have the best range of vision but also the best quality of vision.

There's a larger defocus curve with the Crystalens AO than with a traditional lens. With the Crystalens, there is some variability as to how much accommodation an eye can produce. A blended-vision target with the Crystalens (−0.50 D to −0.75 D in nondominant eye and plano in dominant eye) will achieve a nice range of distance, intermediate and near vision in most patients.

Ideally, the IOL will be centered on the visual axis but this is challenging to accomplish. Since the pupil center and capsular bag aren't aligned with the visual axis, and the IOL may move postoperatively from capsule contraction and fibrosis, IOLs are decentered about 0.5 mm on average from the visual axis.² The Crystalens AO has an aspheric optic with zero spherical aberration, and the multifocal lenses have negative spherical aberration. Lens centration is very important. IOL decentration or tilt induces second and third order aberrations. There will be very little effect from decentration or tilt with the Crystalens AO since it has zero spherical aberration, unlike the Restor and Tecnis multifocal.

What I have experienced in my practice over the years and what's being demonstrated in the study is an improved optical quality with the Crystalens versus a multifocal IOL, and that's important to patients. I want my patients to enjoy a good spectacle-free range of vision but I want them to have a good quality of vision too and, in my practice, that requirement is most often fulfilled by the Crystalens AO.

All presbyopia-correcting IOLs are good lenses, but no one lens is ideal for all patients. You must understand your patients' visual needs and expectations, then determine the lens that will serve them best.

REFERENCES

1. Packer M, chu RY, Waltz KL, et al. Evaluation of the aspheric Tecnis mutifocal intraocular lens: One-year results from the first cohort of the Food and Drug Administration clinical trial. Am J Ophthalmol. 2010;149:577-584.
2. Rynders M, Lidkea B, Chisholm W, Thibos LN, Statistical distribution of foveal transverse chromatic aberration, pupil centration, and angle psi in a population of young adult eyes. J Opt Soc Am A Image Sci Vis. 1995;12:2348-2357.

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Dr. Stahl is a cataract and refractive surgeon at Durrie Vision in Overland Park, Kansas and he is an assistant clinical professor in Ophthalmology at Kansas University Medical Center.