The Crystalens Changes Its Radius of Curvature

The Crystalens accommodative IOL (Eyeonics, Inc., Aliso Viejo, CA) is the only IOL approved by the FDA that restores accommodation after implantation. It has been approved for “approximately one diopter of monocular accommodation.”1 The math appears to be straightforward. A 20.00D IOL in aqueous moves forward a small distance upon contraction of the ciliary muscle and thus changes its effective lens position within the eye. Theoretically, a 30.00D IOL should provide more near focusing power than a 10.00D IOL moving the same distance. Although the Crystalens moves axially in response to the ciliary body's contraction, it has been reported to move posteriorly on occasion.1 This posterior movement is inconsistent with an improved near focus.

The answer to this inconsistency may be in the Crystalens' ability to change its radius of curvature or arch in response to an accommodative stimulus. The concept of accommodative arching is similar to how the natural crystalline lens responds to an accommodative stimulus. Although some axial displacement of the natural crystalline lens occurs during accommodation, the change in its radius of curvature is what gives the power of accommodation. The Crystalens can also change its radius of curvature to create accommodation.

BACKGROUND

Why are some patients thrilled with replacing their lost accommodation with multifocal IOLs, whereas others are not? Wavefront technology is helping ophthalmologists understand accommodation by showing that the pattern of higher-order aberrations changes with accommodation.2,3 These aberrations almost always increase with accommodation. Figure 1 represents a typical change in the wavefront during accommodation. This individual, a medical student, reads for 12 hours every day and has no problem with his distance or near vision. His distance wavefront is what would be expected—almost uniformly in the green with minimal higher-order aberrations. His near wavefront has significantly increased higher-order aberrations, including a big increase in coma and astigmatism. The student reports that his near vision is completely normal. To the extent that most ophthalmologists have thought about it, this is not the image most of them have of a near wavefront.

In almost every patient, higher-order aberrations increase significantly during accommodation. The amount of aberration induced by accommodation tends to grow with age as accommodation is lost. It is very likely that the increase in higher-order aberrations facilitates near vision by creating a range of smooth focus rather than a point of optimal focus.3 This range decreases the eye's need for continuous accommodative fine-tuning. Most of these changes are caused by alterations of the lens' radius of curvature. Ideally, an accommodating IOL would behave similarly.

ACCOMMODATIVE ARCHING OF THE
CRYSTALLINE LENS

The accommodative process does not create a uniform response within the lens, the arching or flexing of which often results in an asymmetrical wavefront (Figure 1). It is unlikely that an eye will have a round lens, equally taut zonules, and a doughnut-shaped ciliary body. Any significant variation in the size or power of these components would result in an asymmetric response, typical with near wavefronts. The observed wavefront changes are generally in proportion to the accommodative stimulus.2-5

As a patient becomes presbyopic, the crystalline lens tends to respond to the loss of overall accommodative ability by creating a central zone of enhanced accommodative response (Figure 2). Although this zone may not be as efficient as the response of a younger eye, it can effectively preserve near vision.

The goal of an accommodating IOL is to replace enough lost accommodation for the patient to function at an acceptable level for normal daily activities.3,5 Some patients will need more accommodative ability than others. Consequently, ophthalmologists will soon define the quality of vision during accommodation rather than only quote the smallest Snellen letter patients identify. An important step in this process will be to understand and replace what each patient's crystalline lens does during accommodation.

ACCOMMODATIVE ARCHING OF THE
CRYSTALENS

The accommodative response of the Crystalens can create localized areas of intense refractive power (Figure 3), something that can only be explained by a change in the IOL's curvature. Compensatory flattening of the lens occurs around those areas. The Crystalens was originally designed to provide accommodation through axial movements, and the IOL may be able to change its radius of curvature in response to the ciliary body's contraction. The smaller size of the Crystalens' optic may contribute to better near vision than that attained with a standard 6-mm optic. Jack Holladay, MD, of Houston has suggested to me that the 4.5-mm optic of the Crystalens increases an eye's depth of field by 25% compared with an optic of 6mm in diameter.

An increase in the IOL's radius of curvature requires very little anterior displacement of the lens. For example, if the Crystalens arches forward 0.3mm over a 2.0-mm diameter, it can provide as much as 10.00D of increased power. This form of accommodative arching may explain how the IOL can move posteriorly and still accommodate. When a patient accommodates, the lens as a whole can move 1mm posteriorly while a small section of it increases its radius of curvature and moves anteriorly, as described earlier. The net result would be a dramatic increase in power over the smaller area and a net posterior movement of the entire Crystalens.

Conclusion
The degree of accommodative arching of the Crystalens varies significantly among patients, with some exhibiting none at all. In my patients, the presence of accommodative arching is associated with good distance and near vision as well as high satisfaction. In April 2005, Michael Breen, OD, of Eyeonics, Inc., told me a group of patients from the original FDA study demonstrated accommodative arching that was present 4 years after the IOL's implantation.

There are other components of the accommodative response, such as pupillary constriction, that also merit evaluation. Based on my experience with the Crystalens, I suspect that the orientation of its haptics is a factor in whether or not a patient experiences accommodative arching. Preoperative near wavefronts may help determine the optimal axis on which to place the haptics of an accommodative IOL within an eye as well as predict the best type of IOL for a given patient.

Kevin L. Waltz, OD, MD, is in private practice with Eye Surgeons of Indiana in Indianapolis. He is a paid consultant to Advanced Medical Optics, Inc., and Eyeonics, Inc. Dr. Waltz may be reached at (317) 845-9488; klwaltz@aol.com.

1. Food and Drug Administration approval letter. Available at: http://www.fda.gov/cdrh/pdf3/P030002a.pdf. Accessed on May 6, 2005.
2. Ninomiya S, Fujikado T, Kuroda T, et al: Changes of ocular aberration with accommodation. Am J Ophthalmol. 2002;134:924-926.
3. Cheng H, Barnett JK, Vilupuru AS, et al. A population study on changes in wave aberrations with accommodation. J Vis. 2004;4:4:272-280.
4. Fiendl O, Baikoff G, Cumming S, et al. Presbyopia: is surgery able to compensate for loss of accommodation? Instructional course presented at: The XXII Congress of the ESCRS; September 11, 2004; Paris, France.
5. Vergés C, Cumming S, Waltz KL, et al. Presbyopia treatment by IOLs. Instructional course presented at: The XXII Congress of the ESCRS; September 11, 2004; Paris, France.