March 2009

 

OPINIONS AND COMMENTARY

 

eyeCONNECTIONS

Crystalens—how does it work?


by J. E. “Jay” McDonald II, M.D.

 

 

CTR/IOL question prompts thoughtful analysis

A ponderance around the effect of a capsular tension ring (CTR) opened the door for one of my favorite discourses on the eyeCONNECT listserv. Our discussion “focused” on the action of the Crystalens (Bausch & Lomb Surgical, San Dimas, Calif.). I think this discussion “encapsulates” the best current thinking and is a must read for anyone working in or considering the presbyopic lens channel. I hope it helps “crystallize” your thoughts on this topic.

Let’s start with a question posted by a colleague on the email list:

Has anyone ever placed a CTR in the bag of a routine case (without any zonular dehiscence) to evaluate its effect on pseudophakic accommodation? Another physician commented a few weeks ago on a Crystalens IOL case that turned out great with a CTR, and it has really puzzled me. However, I think that a CTR in an uncompromised bag could act like a spring that increases the inferior, superior diameter of the bag, whereby pulling the posterior capsule anteriorly as the posterior zonules relax. Also, the plate haptic along with the CTR would prevent anterior and posterior capsular fusion. This also has to be an important feature as the bag can’t really move if the anterior and posterior capsule fuses. Any thoughts on the CTR and plate lens combo for accommodation?

Here were the replies and analysis:

I am not aware of this being evaluated in a controlled fashion. I have done a few Crystalens HD lenses with CTRs, and the results seem satisfactory. As an aside, in our phaco/IOL series in ocular hypertensive patients and glaucoma patients, using a CTR did not increase the IOP drop post-op. It would be worth looking at accommodation with and without CTR in a controlled series.

Richard Lindstrom, M.D. Bloomington, Minn.

My belief is that the Crystalens doesn’t function by moving anterior and posterior but by changing shape based on vitreous displacement combined with pressure applied to the haptics. I believe it is impossible for the lens/capsule diaphragm to actually move forward or backward as a whole in a reversible fashion because it would imply a change in vitreous and aqueous volume during the act of accommodation; therefore, the Crystalens does not move by a change in vertex distance.

If you think about it, vitreous volume and anterior chamber volume (along with the total volume of the eye) must remain constant throughout the act of accommodation. When the ciliary muscle contracts, the plates on the Crystalens, which are arched posteriorly, are pushed back and displace vitreous. The displaced vitreous has to go somewhere so it finds the path of least resistance. If the peripheral capsule is sufficiently tense or fibrosed and the central capsule is not an impediment, the optic will be bowed forward, especially as pressure is applied to the haptics. It is this change in shape of the optic that leads to an increase in central power.

It is my belief that a patient with loose zonules will not generally have a good outcome with Crystalens. By placing a CTR in such an eye that is receiving a Crystalens, I do believe you accomplish some anatomic goals that likely facilitate pseudoaccommodation by preserving the function of the capsule/IOL diaphragm as a functioning entity. My theory assumes that with any change in vitreous shape induced by pressure of the ciliary muscle on the Crystalens plates, there must be a corresponding change in the shape of the capsule/IOL diaphragm and the anterior chamber. For this to occur, some anatomic prerequisites need to exist: 1. The IOL must be arched posteriorly.

2. The IOL must be flexible.

3. The capsule must be flexible without too much scarring and fibrosis.

4. The capsule must be rigid enough in the periphery where the plates are not present to prevent this from herniating forward in reaction to vitreous displacement.

5. The central optic should be the path of least resistance for vitreous volume displacement to move.

6. The ciliary muscle needs to function (this was demonstrated in an article in the July 2008 American Journal of Ophthalmology that showed the ciliary muscle returns to function after cataract surgery).

7. The relationship between the ciliary muscle and capsular bag must be intact via good zonular integrity.

8. Something I’m probably not aware of or forgetting to mention. I believe that in a patient with poor zonules, a CTR will stiffen up the posterior chamber and help preserve the function of the capsule/IOL diaphragm so that most of the anatomic issues above are more likely to be met rather than dealing with a sloppy floppy mess that fibroses irregularly and doesn’t respond well to ciliary muscle contraction.

How all this would play out with a plate lens would depend on the design of the lens, but I believe it would work in some eyes under the right conditions, with the right lens a bit like a Crystalens but not as well because of the limited flexibility of the optic to move (change shape) independently of the plates.

Steven Safran, M.D. Lawrenceville N.J.

Crystalens, 9 months post-op

Source: Jack Singer, M.D.

patient with implanted Crystalens exhibiting peripheral capsule fibrosis with centrally clear capsule leading to slight decreased vision Source: Steve Safran, M.D.

a patient with Crystalens exhibiting central clear capsule and peripheral capsule fibrosis—seen as striae in the posterior capsule which affected the refraction, corrected and uncorrected vision; she was immediately improved with YAG Source: Steve Safran, M.D.

I agree with much of what you say, but there is pretty irrefutable evidence that the natural lens, or an IOL in the capsule or on the capsule (a la the NuLens [Herzeliya, Israel]) can move forward or occasionally backward with accommodation.

Richard Lindstrom, M.D.

This is only true in a closed system, and although it may be easy to view the system this way (and for the most part true), the eye is technically not a closed system. Based on your theory, if the anterior chamber were to be reduced in volume, the finite and unchanging volume of the aqueous would need to displace the vitreous. This ignores the outflow capacity of the anterior chamber and its fluid (trabecular meshwork, uveoscleral).

Jeffrey Horn, M.D. Nashville, Tenn.

Another participant asked me to explain my ideas a bit further. Suffice it to say that the capsular bag as an entity alone or the capsular bag with an IOL in it can move with accommodation, usually anterior, and can exert a force, and that force can actually be and has been measured in the primate eye. An object can move in a fluid-filled space. You can prove this very simply by putting any object you want into a closed container fully filled with a fluid. It just requires that fluid be displaced from one side of the object to the other. This can occur in many ways in the eye, including fluid moving through the zonules from the anterior chamber into the posterior chamber or vitreous. The eye walls are also somewhat elastic, and fluid can move out through the trabecular meshwork or into the suprachoroidal space. A difference in IOP on the two sides of an elastic or moveable membrane can also move that membrane until the pressure equalizes. So, I find the data powerful in support of the fact that the natural lens, and most IOLs move some with accommodation, usually forward. I am not saying your other ideas are incorrect or that the two are not compatible.

Richard Lindstrom, M.D.

I believe that what is happening with Crystalens is as follows: The haptics are angulated posterior at rest. When the ciliary muscle contracts, the ciliary processes extend anterior and forward (confirmed by ultrasound biomicroscopy). When this happens, the plates get pushed posterior, and vitreous is displaced just as if you are sitting on a waterbed and some volume is displaced. The vitreous volume pushes the peripheral capsule (where the plates are not) forward, and if the peripheral capsule is fibrosed, then the central optic preferentially bows forward. This is why Crystalens patients improve with time, as the peripheral capsule stiffens and then improve further with YAG (which removes the impediment to vitreous deformation of the optic). It is also why the AT50 outperforms the AT45—because the plates are bigger and displace more vitreous. It also explains why the lens has to be posteriorly vaulted to work. It makes no sense to believe that a Crystalens, with its optic behind the haptic insertion, would move forward with CB contraction. If you believe that in the reversible act of accommodation there is aqueous outflow and then when you relax, the aqueous somehow refluxes back into the eye, then you are visualizing things in a very different manner than I do. Certainly, aqueous outflow occurs with pilocarpine drops over the course of 15 minutes or an hour, but in the reversible act of accommodation, I don’t believe this is taking place. I also don’t believe that fluid is refluxing through the zonules from the anterior chamber to posterior chamber and back during accommodation and relaxation. If that were the case, then Vision Blue (Doric, Zuidland, The Netherlands) would easily stain the posterior capsule and run into the vitreous when you use it. My theory assumes that compartmentalization and the inability of the lens to have any net movement forward or back is what actually makes Crystalens work. It assumes the possibility of deformation of the optic because the lens cannot have any net movement, and thus if the plates are pushed back, something else has to move forward.

Steven Safran, M.D.

Astute observations. Here are a few thoughts to complement your reasoning. Implantation of many early generation bulls eye: bifocal IOLs and intracorneal lenses have proven to me that a power change in the central 1.5 mm of the lens is enough to generate a J1+ image at near. I believe that the major power change that occurs during accommodation in the natural lens is secondary to a steepening in the anterior central 2–3 mm of the lens. If you measure the central anterior chamber depth during accommodation, it is reduced, so fluid had to be displaced, at least a few molecules. I agree the total volume of the natural lens is unchanged, but as the shape changes, aqueous molecules are displaced, and the anterior chamber depth is reduced. As for the Crystalens, during accommodation its total volume remains unchanged as well. As the ciliary muscle contracts, there is some residual elasticity of the capsule that compresses the haptics like pistons. These apply pressure at the optic–haptic junction, and the haptics underride and/or compress the optic causing it to arch forward. There can also be some tilt inducing coma or astigmatism. During this “accommodative arching,“ the anterior chamber depth as measured dead center in the optic is reduced in most cases, just as it is in natural accommodation. Some fluid has to be displaced for the optic to arch and move, but since the volume of the IOL and capsule is not changed, the fluid is simply displaced to another position in the eye. Still, anterior chamber depth centrally is reduced, and the principal plane of the central portion of the IOL optic moves forward. The total increase in depth of focus of the Crystalens is, in my opinion, multifactorial. This concept is easy to simulate in a simple fashion on your desk. Just take a piece of paper, and push in on the edges. The paper will “arch” forward in the center and move anterior. It does not always do so symmetrically, but centrally it definitely moves forward.

Richard Lindstrom, M.D.

I agree with you completely that the central optic of both the natural lens and the Crystalens moves forward. The anterior chamber simply changes shape by deepening in the periphery. In the case of the human lens, there is no change in volume, so as the lens steepens centrally it becomes less thick at the equators. In the case of the Crystalens, the peripheral haptics move back as the optic bulges forward. In either case, the amount of movement is very small to create the power changes we are talking about. I think the analogy to indentation gonioscopy is a good one, whereby compressing on the central cornea to shallow the anterior chamber centrally, you can deepen the angle in the periphery to better see the structures. Here are some articles that provide food for thought and at least for me, better visualization: Dorairaj S, Oliveira C, Fose AK et al. Accommodation-induced changes in iris curvature. Exp Eye Res. 2008 Feb;86(2):220-225.

Hermans E, Dubbelman M, van der Heijde R, Heethaar R. The shape of the human lens nucleus with accommodation. J Vis. 2007 Jul 31;7(10):16.1-10. Dubbelman M, Van der Heijde GL, Weeber HA. Change in shape of the aging human crystalline lens with accommodation. Vision Res. 2005 Jan;45(1):117-132.

Steven Safran, M.D.

Voila! We agree. Now to convince the thousands of ophthalmologists (and a few research scientists) that remain unconvinced, skeptical, or in some cases even cynical, that: 1. The ciliary muscle continues to contract with accommodation in the pseudophakic eye, moving inward and anterior. 2. The zonules are relaxed in the accommodated state, a la Helmholtz. 3. If there is any residual capsular elasticity a force can be applied to the lens inside or on the capsule, whether natural or artificial (so capsular fibrosis is the enemy of an accommodating IOL). 4. That the force applied, depending on IOL design, can cause the lens (or optics) to move forward, arch, tilt, separate or change shape, depending on the nature and design of the material in or on the capsule, natural or artificial. 5. Thus, an accommodating IOL is possible and already exists in a functional form in the Crystalens HD. 6. Amazing advances in our ability to simulate natural accommodation with artificial lenses and perhaps pharmacologic or surgical modification of the natural lens to restore its accommodative ability are on the way today. 7. Finally, each of these technologies will likely be multifactorial in the way they increase depth of focus and enhance near vision, like natural accommodation, with some emphasizing one mechanism of action and others another. For example, if one could change the size of an aperture (artificial pupil) inside the eye or cornea from 6 mm to 1.5 mm, increasing depth of focus to allow J1+ near vision, and if this change in aperture size were under the patients control, is that accommodation? To me, the answer is yes. Great observations and discussion!

Richard Lindstrom, M.D.

Editors’ note: Dr. Lindstrom has financial interests with Advanced Medical Optics (Santa Ana, Calif.), Alcon (Fort Worth, Texas), and Bausch & Lomb (Rochester, N.Y.), among other ophthalmic companies. Dr. Safran has no financial interests related to his postings. Dr. Horn has financial interests with Alcon.

If you are not following these threads on the ASCRS Web discussion, you are missing the latest developments in cataract, refractive, glaucoma, and business practices. To join ASCRS eyeCONNECT, where you can receive and exchange the most current thoughts about the hottest topics in ophthalmology, search archives, and more, log onto www.ascrs.org or www.eyespacemd.org.

Contact information

Horn: Jeff.Horn@bestvisionforlife.com
Lindstrom: rllindstrom@mneye.com
Safran: safran12@comcast.net

ABOUT THE AUTHOR

J.E. “Jay” McDonald II, M.D., is the EyeMail editor. He is director of McDonald Eye Associates, Fayetteville, Ark. Contact him at 479-521-2555 or
mcdonaldje@mcdonaldeye.com.

Crystalens—how does it work? Crystalens—how does it work?
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