Are IOLs more accommodating?

Not necessarily. Here is a look at existing, and in-the-pipeline, technology.

By Martin L. Fox, MD, FACS

As an inevitable consequence of the aging process, all humans lose the ability to seamlessly adjust the focus of vision from distance to near, introducing the “heartbreak of presbyopia.” This phenomenon makes even the most trivial visual task at near next to impossible. In youth, the human eye can induce up to 15 D of near focus, allowing functional vision as close as 6.7 cm in 350 milliseconds. This focusing phenomenon is accomplished through the elastic alteration in the shape of the crystalline lens. By ages 45 to 50, the blush of robust youthful accommodation reduces to a paltry 2 D, and the near point of vision extends to well beyond required reading range.^1-3

Understanding the mechanism of human accommodation remains somewhat steeped in mystery and the subject of occasional controversy. We know the accommodative mechanism occurs either through reflex or conscious volition and consists of a tripart mechanism that includes: conversion of the eyes to the near point; constriction of the pupillary aperture; and adjustment of shape of the crystalline lens. The latter has been the subject of considerable debate.

In 1855, Hermann von Helmholtz suggested that a constriction of the ciliary body musculature leads to slackening of its attached lens zonular fibers, producing a consequential increase in lens curvature. The theory: this effect was also partially induced by the outward pressure of the vitreous and aqueous humor.⁴ The Helmholtz theory assumes that stiffening of the aging crystalline lens makes it resistant to shape change, thus producing presbyopia. This theory remained in question as recently as 2000, when Ronald Schachar, MD, proposed an alternative albeit controversial theory. He proposed that ciliary muscle contraction resulted in a tightening of equatorial zonular tension, thereby inducing the lens shape alteration necessary to allow for near focus.⁵ He hypothesized that the aging presbyopic lens grows equatorially, inducing a slackening of the lens zonules and rendering ciliary body contraction ineffective. This is based upon scleral expansion procedures designed to correct presbyopia.

Understanding the mechanism of lens shape change in accommodation has become essential to the general ophthalmologist as the array of intraocular lenses (IOLs) designed to restore ciliary body/lens capsular-generated accommodation, under FDA scrutiny, grows. The designers of these lenses all assume that the aphakic ciliary body will continue to function interchangeably from one eye to the next with predictability over time — an assumption that remains unproven. With the advent, and superior safety afforded with femtosecond laser cataract surgery, precision in the architecture of the anterior capsular opening should help in assisting the process along with modern OCT visualization of the ciliary body apparatus to confirm its viability.

Here, I describe the current accommodation-restoring IOLs along with those on the horizon.

Currently available flexing IOLs

First, two capsular-based flex mechanism IOLs claiming to restore accommodation are FDA approved: Crystalens and its toric cousin Trulign (Bausch & Lomb) and Tetraflex (Lenstec). Both function by the mechanism of ciliary body-zonular generated lens flexion.

The Crystalens has a silicone monofocal optic with two flexing hinged plate haptics. Centration in the capsular bag is maintained through polyamide loops that cross each end of the plate haptics. The Crystalens and Trulign are designed to both stabilize the lens in the capsular bag and to allow the lens optic to vault forward with ciliary body contraction in the presence of an increase in vitreous pressure.⁶

The knock against flex mechanism lenses has been their extreme variability in degree and duration of accommodative effect. Additionally, there are no agreed-to standards of true accommodation. Perhaps this will change now that that the FDA and American Academy of Ophthalmology are working to establish extended depth of focus guidelines.⁷

The flexible nature of Crystalens haptics has lead to the tilting phenomenon of the lens optic known as the Z syndrome and its myopic shift over time. More common a few years ago, Z syndrome still occurs.⁸

The flexing characteristic of the Tetraflex lens can also create improved near acuity; however, those involved in its clinical study have noted variability of effect and have indicated that it provides pseudoaccommodation at best.

New mechanism IOLs

One capsular bag, fixated accommodating IOL worth discussing is PowerVision’s FluidVision Lens.

The FluidVision lens is an acrylic IOL with anterior and posterior optics separated by a central cavity. Its two compressible haptics contain a silicone oil-based fluid. When the eye’s normal physiologic ciliary-body contraction occurs, the zonules release tension on the capsule, thus allowing for compression of the two large haptics. This pushes the silicone fluid between the two optics and creates an accommodative effect of the lens.

Another pipeline technology, sulcus fixated accommodating lenses, includes two IOLs: DynaCurve (NuLens Ltd.) and Lumina (Akkolens). DynaCurve is an out-of-the-bag IOL, designed to work in the ciliary sulcus. The lens is comprised of PMMA haptics, a PMMA anterior reference lens providing distance correction and a small chamber containing solid silicone gel that communicates via aperture with a posteriorly situated piston. Internal scleral fixation secures the haptics to the sulcus without sutures. Ciliary muscle force moves the lens’ piston, which pushes the gel through an aperture to form a bulge that functions as a lens that focuses the image on the retina. Change in the gel membrane shape has been reported to provide more than up to 5D of accommodation.

The TRULIGN Toric IOL provides a broader range of vision for astigmatic correction, and 99.2% of patients reported no significant visual disturbances.*
*Data on file, Bausch & Lomb Incorporated. Study 650.

Making assumptions, relying on gravity

The sulcus-fixated Lumina is a dual-optic lens that relies on ciliary body contraction to slide one of the optics over the other creating a continuous change in total lens power. Again, its mechanism comes through an assumption of consistent ciliary body function over time.

Other players

A new player in the premium IOL space is LensGen, based in Irvine, Calif. Currently conducting a proof-of-principal pilot study, LensGen has designed a two component accommodative product. The first lens, placed in the capsular bag, is fixed and works like a standard monofocal IOL. The second component is a fluid lens that is placed into the fixed lens; it is designed to move in concert with the eye’s accommodative demand. This two-system design solves a surgeon’s small-incision challenge and provides a vision correction platform to treat all refractive errors, the company says.

In a category all its own, the ELENZA lens (ELENZA) runs on a rechargeable battery designed to last 50 years. The smart, electroactive liquid crystal lens detects the pupil constriction on accommodation and modifies the lens shape electronically.

Conclusion

While conceptually appealing it remains to be seen whether the current crop of adjusting lenses holds promise.

The ability of these entries to reliably provide accommodation across a range of recipients is problematic for this writer and they will find strong competition from another family of lenses on the horizon designed to increase depth of focus as a mechanism to return reading vision. OM

REFERENCES

About the Author
	Dr. Fox is medical director at the Cornea and Refractive Surgery Practice of New York and at Clarity Refractive Services of West Orange, N.J. E-mail him at foxmd@laserfox.com or by phone 917-207-3147. His website is www.laserfox.com.