Learn the Keys to Success With Crystalens HD

The Crystalens gives patients increased depth of field without compromising visual quality.

By Jay S. Pepose, MD, PhD

If you want to minimize patient complaints associated with presbyopia-correcting IOLs and maximize patient satisfaction, you must produce high retinal image quality. While patients with residual refractive error after cataract surgery have difficulty reading the lower lines of the eye chart, those who have reduced retinal image quality and decreased contrast sensitivity report that even the larger optotypes on the eye chart appear waxy or ghosty. The most common cause of decreased contrast sensitivity is cataract, partly due to forward scatter of light, but decreased contrast sensitivity also has been reported in some patients following multifocal IOL implantation.

Just as every passenger who steps onto an airplane assumes the pilot knows how to safely fly and land, patients with lenticular dysfunction expect high-quality, uncorrected distance vision when choosing a premium channel IOL. In contrast to refractive or diffractive multifocal IOLs, the Crystalens doesn't simultaneously divide light into multiple focal points or sacrifice a portion of light energy to unfocused higher diffractive orders (Figure 1).

Figure 1. Simulated images generated by a customized paraxial beam tracing program demonstrate a single point of focus with Crystalens IOLs, in contrast to diffractive and refractive multifocal technologies.

By definition, multifocal IOLs project more than one image onto the retina simultaneously, which may result in decreased contrast sensitivity that isn't obviated by the addition of an aspheric optical design (Figure 2). Decreased contrast sensitivity associated with multifocal IOLs, compared with monofocal or accommodating lenses (particularly at lower spatial frequencies), can cause decreased reaction time during night driving, impaired perception of facial expressions, difficulty in judging the edge of a curb and carrying out other daily tasks. That's why FDA labeling for multifocal IOLs states that "a reduction in contrast sensitivity as compared to a monofocal lOL may be experienced by some patients and may be more prevalent in lowlighting conditions. Therefore, multifocal IOL patients should exercise caution when driving at night or in poor visibility conditions." No comparable warning exists for the Crystalens accommodating IOL.

Figure 2. Multifocal IOLs project more than one image onto the retina simultaneously, which may result in decreased contrast sensitivity, and can't be resolved with the addition of an aspheric optical design.

In this article, I'll discuss the unique features of the Crystalens HD and how it compares with multifocal IOLs. I'll also explain how the Crystalens can restore full range of vision for patients and how to choose the best candidates for this accommodating IOL.

Crystalens Difference

Crystalens IOLs are a modified plate haptic design made from silicone of high refractive index with an ultraviolet filter. The lens is hinged adjacent to the optic and has T-shaped polyamide loops on the plate haptics. Since the introduction of the first-generation Crystalens in 2003, a number of modifications have been made to improve its stability and predictability. The first revision created Crystalens AT-45SE, which, with its squareedged optic, showed a reduced incidence of asymmetric capsular fibrosis and therefore capsular contractions and IOL tilt. For the Crystalens Five-0, the third-generation Crystalens, the lens diameter was enlarged from 4.5 mm to 5.0 mm. The shape of the haptic plates became square to optimize capsular support, and the haptic arc was increased to create greater stability. The Crystalens HD, the fourth-generation Crystalens, has been enhanced further to ensure excellent vision at all distances without any loss of contrast sensitivity. It has a reduced radius of curvature in the central 1.5-mm zone of the optic.

This bispheric modification adds approximately another 1D of add to the lens and improves its near (Figure 3) and intermediate vision compared with its predecessors, without reducing contrast sensitivity or distance vision. The increased depth of focus compared with its 4.5-mm predecessor is clearly seen on optical bench testing (Figure 4). Crystalens is designed to undergo a shape confirmation during ciliary muscle contraction, known as accommodative arching, demonstrated on finite element analysis (Figure 5).

Figure 3. Based on a cross-study comparison, eyes implanted with the Crystalens HD had the best monocular uncorrected visual acuity compared with eyes implanted with the Crystalens AT-45 and Crystalens Five-O.

Figure 4. The Crystalens HD provides increased depth of focus compared with the AT-45, as shown on optical bench testing. The peak-to-valley value and the RMS wavefront error values are lower with the Crystalens HD.

Figure 5. The Crystalens HD is designed to undergo a shape confirmation during ciliary muscle contraction, known as accommodative arching, demonstrated on finite element analysis.

Proper targeting is essential to maximize the performance of the Crystalens HD. The recommendation for the Crystalens Five-0 was to target between plano and −0.25D for the dominant eye and −0.50D for the nondominant eye. But this recommendation has changed for the Crystalens HD, because of the effect of negative spherical aberration and its interaction with defocus. The image quality is higher when the dominant eye is targeted slightly plus, and the effect of wound healing averages approximately a −.21D myopic shift over 3 months. This is another reason why it may be prudent to wait 90 days for a laser vision enhancement. The nondominant eye can be targeted a bit minus, according to the patient's response in the dominant eye at distance and near.

Comparison With Multifocal IOLs

The modulation transfer function (MTF) is a ratio of relative image contrast to relative object contrast. Figure 6 compares the MTF of a 3-mm pupil plotted over a wide range of spatial frequencies for the Crystalens HD and Crystalens Five-0 compared with the aspheric and standard ReSTOR and ReZoom IOLs. The higher MTF for the accommodating lenses reflects superior image contrast.

Figure 6. This slide compares the modulation transfer function of a 3-mm pupil plotted over a wide range of spatial frequencies for the Crystalens HD and Crystalens Five-O compared with the aspheric and standard ReStor and ReZoom IOLs.

The increasing longevity of our patients creates additional concerns about the use of multifocal IOLs to improve near vision at the expense of contrast sensitivity. Contrast sensitivity decreases with each decade of life, so the way a multifocal IOL functions today in a particular patient may not be the case in the same patient 10 or 15 years from now. Also, it's not possible to determine which patients may go on to develop age-related macular degeneration, glaucoma or other comorbidities that may further reduce contrast sensitivity in the future. Finally, pupil size, shape and dynamics often are affected by age, so some multifocal IOLs that have pupildependent distribution of light energy between various foci may function differently as the patient ages. With all lenses, image quality and tolerance to defocus is pupil-dependent. For example, Figures 7 and 8 show through-focus images of United States Air Force (USAF) 1951 resolution targets, using monochromatic light at varying vergence for ReSTOR spherical (top row), aspheric ReSTOR (middle row) and Crystalens HD (bottom row). As shown, the USAF 1951 targets demonstrate better image quality with the Crystalens HD. Aspheric optics have only small beneficial effects on image quality until the pupil exceeds 4 mm, but maximum pupil dilation lessens with age.

Figure 7 (2-mm pupil [left]) and Figure 8 (4-mm pupil [right]). These slides illustrate through-focus images of United States Airforce 1951 resolution targets, using monochromatic light at varying vergence for ReSTOR spherical (top rows), aspheric ReSTOR (middle rows) and the Crystalens HD (bottom rows). These resolution targets demonstrate better image quality with the Crystalens HD.

Choosing the Best Candidates

Most patients with lenticular dysfunction are candidates for the Crystalens HD if their goal is to reduce their dependence on eyeglasses postoperatively. However, when first starting out, it may be best to begin with patients who are the easiest to satisfy — those who have significant cataracts, 1D or less of astigmatism, easygoing personalities, positive attitudes and reasonable expectations. In general, hyperopes often have lower preconceived expectations compared to low-to-moderate myopes with regard to near vision postoperatively, since the latter are used to reading without eyeglasses. Patients with map-dot-fingerprint dystrophy, severe dry eye, irregular corneal astigmatism, macular pathology or advanced glaucoma aren't ideal candidates. Post-LASIK patients generally are good candidates for the Crystalens, but they need to understand that the accuracy of IOL power calculations isn't fully predictable in their case and that they may require a laser vision enhancement. I often use the Crystalens Five-0 for posthyperopic LASIK patients, because they tend to have higher negative spherical aberration, although I rely on the Crystalens HD for all of my other cases. Similarly, patients with very high levels of corneal astigmatism preoperatively must be made aware of the greater likelihood of requiring laser vision enhancement after surgery.

The key to success is establishing achievable, realistic expectations. You must explain to patients that reading the stock page or a menu in a dimly lit restaurant likely will require reading eyeglasses, and that there's no guarantee with any lens implant that they'll be completely free of eyeglasses all of the time. I emphasize that these new lenses are superior to the monofocal IOLs of the past, but they still won't have the depth of focus of the natural crystalline lens in a 20-year-old. I explain that a reasonable expectation would be like having the near vision of someone around age 40, not age 20. I've found that the best approach is to underpromise and then overdeliver. Most patients readily comprehend that there's no prosthetic device that can compare to that of a 20-year-old. I use terms such as "walk around vision" to establish realistic expectations. I tell patients that it's possible to drive to the supermarket, look down the aisle and read what's on the front of a soup can without eyeglasses, but they'll probably need eyeglasses to read the smallest ingredients on the back of the can.

Finally, you'll need to inform patients before surgery that they may require additional procedures, such as Nd:YAG laser capsulotomy or laser vision correction, to achieve their desired visual acuity, and that this process from start to finish can span up to 3 months. The assessment for laser vision correction (ie, corneal topography, regional pachymetry, dry eye status) is integrated into the initial cataract evaluation for the Crystalens patient. Comorbid conditions that can negatively impact outcomes, such as dry eye syndromes, must be treated preoperatively. It's important to monitor patients for the development of posterior capsular fibrosis, capsular contraction and cystoid macular edema. Keeping patients on a lengthy postoperative tapering regimen of NSAIDs and corticosteroid drops may minimize these complications. OM