Optically, 3.00 D of accommodation should allow comfortable reading at a focal length of 33 cm in a phakic or pseudophakic individual. In a human eye, however, one-third to one-half of the accommodative amplitude must be left in reserve to allow comfortable reading. Thus, reading comfortably at 33 cm requires 4.50 to 6.00 D of accommodative amplitude. In support of this assertion, we can cite the natural accommodative amplitude of the human eye, demonstrated by F.C. Donders, MD, and later by others including A. Duane.1 It was found that in the human eye, the natural accommodative amplitude between 20 to 30 years of age is approximately 6.00 D to 8.00 D.
CONCEPTS BEHIND THE LENS
Concept. The NuLens Accommodating IOL (NuLens, Ltd., Herzliya Pituah, Israel) is unique in its concept of operation, structure, and performance. On a conceptual level, this accommodating lens was designed to change its true power during accommodation through the alteration of lens curvature. This is unlike other accommodating IOLs, which use the axial movement of one or two fixed-power optical surfaces.
Construction. Structurally, the NuLens uses the capsular bag as a component of a moving diaphragm, consisting of the collapsed capsular bag, zonules, and the ciliary processes. The dynamic diaphragm transfers force from the contracting ciliary muscles to the device attached to it. A piston, actuated by the capsular diaphragm, pressurizes a small rigid chamber containing a silicone gel. The chamber is fixated to the eye wall at the ciliary sulcus so that movements along the optical axis are avoided. The silicone gel is pressurized by forward movements of the capsular diaphragm and depressurized by backward movements of the diaphragm. The pressurized gel is displaced through a round hole in the anterior (or posterior) chamber wall to form a lens-shaped bulge continuously changing its curvature in correlation with the ciliary muscle movements.
Such IOL prototypes, even with silicone gel of a relatively low refractive index (1.40), have been shown in a primate eye to deliver more than 40.00 D of accommodation using only 500 µm of vertical diaphragm displacement.
Of interest, a similar accommodating mechanism has been demonstrated in the natural lenses of waterfowl, wherein a high accommodative range is needed during underwater dives. In these birds, a rigid muscular iris acts as a hard surface through which the crystalline lens bulges during the act of accommodation. This extreme increase in the lens' anterior radius of curvature is responsible for the large range of accommodative amplitude in waterfowl.
Primates implanted with the initial prototypes of the NuLens Accommodating IOL have demonstrated more than 40.00 D of change in anterior curvature power. This amplitude could easily be further modified for greater or lesser effect by changing the refractive index of the soft gel and/or the diameter of the opening through which the gel is pressed.
We conducted a clinical pilot investigation including 10 patients with advanced age-related macular degeneration and low vision. We tested the new sulcus fixation method as well as the IOL's performance for short- and long-term ocular reaction.
After 12 months, no safety issues were identified. Four eyes had uneventful Nd:YAG capsulotomy for posterior capsular opacification (PCO). All patients were able to focus for far, obtaining the best visual acuity for each eye limited only by retinal pathology (Figure 1).
Significant improvement was noted in near vision performance (Figure 2) including a low vision reading chart, identifying time on a watch, and cell phone dialing at an average uncorrected distance of 10 cm (= 10.00 D = 2.5X magnification). Repeated ultrasound biomicroscopy examinations during the 12-month follow-up demonstrated backward shift of the lens piston following pilocarpine-induced cyclospasm.
The amount of observed change correlated with the preclinical optical bench performance of the IOLs. On average, the observed change was 8.00 D. PCO resulted in reduction of IOL performance as noted by piston movements, but significant recovery of piston movements (although not full) was noted after Nd:YAG capsulotomy.
New generations of the NuLens Accommodating IOL are emerging. Improved design and manufacturing capabilities have enabled significant improvement in the surgical procedure, decreasing the surgical corneal incision from 9 mm for the first prototype implanted in human eyes to a 5-mm incision in human cadaver eyes at present. With recent modifications to the rigid components of the less, sub–4-mm incisions are expected in the near future.
NuLens Ltd. is developing silicones of a higher refractive index (1.49 and 1.53) and improved elasticity for future-generation IOLs. Another promising improvements is the use of the new IOL design to construct a dynamic component that will be implanted in already pseudophakic eyes. This new concept will restore accommodation, even years after standard cataract surgery.
Jorge L. Alió, MD, PhD, is a Professor and Chairman of Ophthalmology at the Miguel Hernández University, in Alicante, Spain, and Medical Director of VISSUM Corp., in Spain. Professor Alió states that he has no financial interest in the products or companies mentioned. He may be reached at +34 96 515 00 25; firstname.lastname@example.org.
Joshua Ben-Nun, MD, is Chief Medical Officer of NuLens, Ltd. He may be reached at email@example.com.