"Despite improvements in their design, the latest generation multifocal IOLs still have significant trade-offs," says David F. Chang, MD, who is a clinical professor at the University of California, San Francisco, and in private practice in Los Altos, Calif. "Multifocality produces significant aberrations and an inherent reduction in contrast. There will always be halos or ghost images produced by the secondary focal zone. Finally, a multifocal lens cannot provide excellent vision across the entire range of intermediate to near zones. Therefore, accommodating IOLs offer the promising prospect of better contrast, fewer aberrations and ghost images, and a better range of intermediate to near focus."
I. Howard Fine, MD, agrees. "The trend will be toward accommodative lenses, especially as the technology improves," adds Dr. Fine, who is a clinical professor, Oregon Health and Sciences University, and in private practice at Drs. Fine, Hoffman, and Packer, in Eugene, Ore.
Following is a look at some of the newest accommodative IOL designs.
According to Dr. Chang, who is the medical monitor for the Synchrony IOL (Visiogen Inc., Irvine, Calif.), this lens is a single piece, dual-optic accommodating IOL made of the latest-generation silicone. It features a 5.5-mm high-power anterior optic and a 6-mm negative-power optic. The optics are connected by haptics that act like springs.
|Figure 1: The Synchrony IOL and injector.|
The refractive shift produced by any optic movement is proportional to the dioptric power of the lens. "Therefore, the anterior moving optic of the Synchrony is a 32 D (+) lens in order to maximize the near shift produced by its forward movement. The rear optic is a minus lens that has varied power in order to achieve the net IOL power required for emmetropia," he says.
The design of the Synchrony relies on the Helmholtz theory of accommodation—ciliary muscle contraction reduces zonular tension, allowing the capsular bag to become lax. This allows the spring-like connecting struts to push the anterior optic forward. "Ultrasound biomicroscopy imaging has confirmed that the front optic does move enough to produce the approximately 2.5 D of accommodation measured with defocus curves," adds Dr. Chang.
Clinical trials are ongoing in Europe and South America, and the FDA Phase II trial began in November 2005, with five study sites in the United States. Ten lenses were implanted in 2005 in the United States, and more than 200 lenses have been implanted outside the United States.
Ivan Ossma, MD, MPH, a clinical professor of ophthalmology, Universidad Industrial de Santander, Bucaramanga, Colombia, has compiled two-year follow-up data on 24 eyes implanted with the Synchrony lens. At three months, 63 percent had uncorrected distance vision of 20/40 or better, and 100 percent had uncorrected near visual acuity of 20/40 or better. At six months, 79 percent had 20/40 or better uncorrected distance vision. At both 12 and 24 months, 83 percent had uncorrected distance vision of 20/40 or better, and 100 percent had uncorrected near visual acuity of 20/40 or better. Study results were reported at the American Academy of Ophthalmology meeting in Chicago.
Additionally, Burkhard Dick, MD, conducted a prospective clinical trial on 15 eyes of 12 patients.1 He performed surgery on all 12 patients, and there were no intraoperative complications. All eyes have at least six months of follow-up, and no case of interlenticular opacification has occurred. He has not seen any serious complications, and no lenses have been explanted. "All patients were very satisfied with the visual functioning and achieved accommodation ranges between 0.5 D and 2.5 D," says Dr. Dick, who is a professor of ophthalmology at Johannes Gutenberg University, Mainz, Germany.
The FlexOptic lens (Advanced Medical Optics) is an accommodative IOL that conforms to the geometry of the capsular bag and changes curvature. "As the ciliary muscle constricts, the bag will constrict, and the optic will actually change its radius of curvature," says Mark Packer, MD, assistant clinical professor at Oregon Health and Sciences University, and in private practice at Drs. Fine, Hoffman, and Packer in Eugene, Ore. "Unlike the Crystalens and the dual-optic lenses, this IOL is not designed to move axially in the eye. At least in theory, more accommodation is possible from a change in surface curvature than from axial movement, even with two optics."
|Figure 2. The TetraFlex lens by Lenstec.|
Currently, only benchmark testing has been conducted on the latest design of this lens.
The Smart IOL (Medennium Inc., Irvine, Calif.) is a concept for a bag-filling IOL that will allow the ciliary muscle to resume control of lens shape alteration. Filling the bag will eliminate problems with decentration and edge effects, according to Dr. Fine.
"One popular concept has been to develop a gel that could fill the emptied bag and remain flexible," says Dr. Chang. "However, this approach creates many new challenges. How much gel does one inject? How does one control the net resulting lens power? How does one seal the capsulotomy, and can a dense cataract be removed through a micro-capsulorhexis that would be most compatible with such injectable gel technology? Finally, how does one produce a precise optical shape with this method?"
The advantage of the Smart IOL is that the hydrophobic acrylic lens can be manufactured to precise optical specifications, including power and anterior and posterior curvature. However, because of its unique thermoplastic properties, it can then be reconfigured into a thin rod that can be implanted through a phaco incision. After implantation, warming to body temperature causes the shape to transform back to its original designed configuration, which will completely fill the capsular bag. One theoretical advantage of this design is that it is compatible with current phaco methods and a standard-sized capsulorhexis, according to Dr. Chang.
The Smart IOL has been implanted into cadaver eyes, but must undergo further testing before clinical trials can begin.
FluidVision (PowerVision Inc., Belmont, Calif.) is representative of a new generation of IOLs whose goal is to produce enormous accommodative range—possibly exceeding 10 D, according to Dr. Chang. This is accomplished through a hydraulically induced shape change that is controlled by the ciliary muscle.
"Fluid is pushed through tiny hydraulic actuators. As the ciliary body constricts and the zonules relax, the lens thickens like the human crystalline lens. Upon relaxation, the zonules become taut, and the lens becomes thinner. So, through fluid movement, the lens achieves shape change, which is how we think the human lens works through the Helmholtz theory," says Louis D. Nichamin, MD, medical director of the Laurel Eye Clinic in Brookville, Pa.
"Such a system would be more forgiving of power miscalculation and of any physiologic or anatomic degradation of its accommodative power over time," Dr. Chang adds.
The company has completed a preliminary product design. The next step will be completion of a 1:1 scale prototype followed by the first primate implant.
The TetraFlex IOL (Lenstec, Inc., St. Petersburg, Fla.) is an acrylic lens with square-edge technology that was developed by Robert E. Kellan, MD, assistant professor of ophthalmology at Boston University and associate ophthalmologist at Tufts University School of Medicine. "It was designed not so much to use vitreous movement as to use the ciliary zonular capsular dynamic to ensure maximum forward movement for clear near vision. It is important to note that all of the theories of accommodation are merely theories. No one knows the true mechanism of accommodation," he says.
Sunil Shah, MD, from the Midland Eye Institute, Solihull, UK, has assessed both the objective and subjective performance of this lens, and both studies were presented at the AAO meeting in Chicago.
The subjective study included 100 consecutive eyes that underwent phaco and implantation of the TetraFlex lens. Patients ranged in age from 40 to 86 years, with a mean age of 71 years. Patients' postoperative spherical refraction was 0.23 ±0.69 D, and their postoperative cylindrical refraction was –0.83 ±0.48 D. Mean IOL power was 21.5 D, with a range of 19.5 D to 25.5 D. Best-corrected acuity was 0.06 ±0.13 logMAR at distance and 0.58 ±0.20 logMAR at near. Patients' subjective amplitude of accommodation at one month was 3.1 ±1.5 D, with a range of 1 D to 6.4 D, and their amplitude of accommodation at six months was 1.7 ±2.2 D, with a range of 1 to 6 D.
The objective study included 50 patients, and their objective amplitude of accommodation was 0.4 ±0.6 D, with a range of 0 to 2.3 D.
|Figure 3. The NuLens IOL.|
"Techniques used in this study to assess the objective accommodation did not pick up significant changes that would explain the subjective findings. However, further early work has demonstrated focal rather than global changes, which are more in keeping with the subjective findings," Dr. Shah says.
The TetraFlex has been approved in Europe, Australia, the Middle East and other markets since early 2003. Approximately 6,000 lenses have been implanted to date. The lens is currently in FDA clinical trials in the United States, and 138 eyes have been implanted with the lens in this country. Of the 80 cases that have been followed for six months or longer, 48 (60 percent) had 20/40 or better distance corrected near vision.
Additionally, this IOL effectively caused 94.7 percent of 94 reported cases to be within ±1 D of emmetropia, and 70.2 percent of cases were within ±0.5 D at 6 months or later. Target refractions ranged between emmetropia and 0.25 D.
Of 91 reported cases, 95.6 percent had accommodative amplitudes of more than 1 D, 69.2 percent could accommodate more than 2 D, and 19.8 percent could accommodate more than 3 D, according to Jim Simms, vice-president of Lenstec.
The company hopes that the TetraFlex will enter the U.S. market by 2008.
NuLens (NuLens Ltd., Herzliya Pituach, Israel) is composed of a flexible polymer between two rigid plates, one of which has an opening. When the plates are compressed, the flexible polymer bulges through the opening in the front surface, creating a much more plus-powered lens, explains Dr. Fine, who is a member of NuLens' scientific advisory board.
"The lens is designed to be placed in the ciliary sulcus and promises between 30 D and 50 D of accommodation. This lens is not a slam dunk yet, but it's very promising," he says. The lens was developed by Joshua Ben-Nun, MD, who recently published a feasibility study of this lens.2
The study was coauthored by Jorge Alio, MD, PhD, and was conducted at the Vissum-Instituto Oftalmologico de Alicante and Miguel Hernandez University in Alicante, Spain. A laboratory lens model was used to assess the feasibility of the concept. An implantable measuring device was designed and implanted in monkey eyes to measure the lens action and other parameters. These measurements were used to build an accommodating IOL prototype that was then implanted in monkey eyes.
Drs. Ben-Nun and Alio used pharmacologic agents to achieve ciliary relaxation and spasm. Additionally, they used ultrasound biomicroscopy imaging to document the active changes of the IOL flexible lens curvature as related to the ciliary muscle's status.
They found that the laboratory model produced more than 50 D of accommodation, and the ultrasound biomicroscopy imaging demonstrated changes in lens curvature between 9 D and 53 D.
LiquiLens is an accommodative IOL being designed by Vision Solutions Technologies (Rockville, Md.), according to Dr. Fine. It is a C-loop lens that contains two immiscible fluids in the center of the lens. "The two fluids will have different refractive indices. The lens will be filled three quarters of the way with one of the fluids, and a higher refractive index fluid will float above that," he says.
The refractive index in three quarters of the lens is designed for distance. If patients want to read, they can tilt their heads down, and gravity will force some of the heavier fluid to flow along the increasingly lower front surface of the lens. The result will be a higher refractive index fluid above a lower refractive index fluid and a more plus-powered lens.
1. Dick HB. Accommodative intraocular lenses: Current status. Current Opinion in Ophthalmology 2005;16:8-26.
2. Ben-Nun J, Alio JL. Feasibility and development of a high-power real accommodating intraocular lens. J Cataract Refract Surg 2005;31:1802-1808.