A healthy crystalline lens accommodates by increasing its optical power. However, as the eye ages and approaches a presbyopic state, its ability to accommodate declines and the crystalline lens stiffens. Although there is no gold standard for the treatment of presbyopia, in most cases patients are fitted with either a multifocal or accommodating IOL. Both options provide some correction, but neither feature excellent visual acuity at all distances. Currently available accommodating IOLs do not restore complete accommodative amplitude. However, we are developing a ciliary muscle-driven accommodating IOL that may provide full accommodation after implantation.
TWO ROTATING LENSES
The Turtle Lens, being developed in collaboration with AMO Groningen (Groningen, Netherlands; Figure 1), uses the range of ciliary muscle contraction to operate a rotating focus mechanism and a mechanical frame, both made of silicone (Figure 2). Its design is composed of two rotating lenses with variable optical power. When the ciliary muscle contracts, it squeezes the mechanical frame and causes the lenses to rotate, changing the optical power. Rotation occurs via the mechanical frame, which uses the capsular bag to transfer the displacement force from the ciliary muscle. Therefore, it is the ciliary muscle that induces the optical change. This IOL is optimized for use in 60-year-old patients. Special stop devices on the Turtle Lens allow the base power to be varied in 0.50 D steps according to the patient's required refractive outcome.
We have produced prototypes for use in an enucleated pig's eye. Using a laboratory lens-stretching device, as designed by Adrian Glasser, University of Houston (Figure 3), which mimics human ciliary muscle contraction, we tested the mechanical performance of the Turtle Lens and calculated changes in focal length during stretching with a ray-tracing device and video recordings. The focal length stretched to the equivalent of a 16-cm reading distance and achieved 8.00 D of accommodation. Additionally, the modulation transfer function (MTF) in different accommodative states was similar to the MTF of a monofocal IOL.
During our study, we concluded that the combination of the mechanical frame and the rotating focus principle is a promising method to restore accommodation and provide predictable accommodative amplitude. The optical design of the Turtle Lens could also compensate for astigmatism and higher-order aberrations, including spherical aberration.
Although this lens is still in development, we believe that the Turtle Lens will provide a large and predictable range of variable power. This is a step in the right direction to offer patients spectacle independence after IOL implantation. However, optimization of the mehanical and optical design is needed to improve its functionality. If these improvements can be made, the Turtle Lens may be a promising technology to provide patients with better optical quality and accommodative amplitude after lens implantation.
Michiel Dubbelman, PhD, practices at the Free VU University Medical Center. Dr. Dubbelman states that he has no financial interest in the products or companies mentioned.
Rob M. Heethaar, PhD, practices at the Free VU University Medical Center. Professor Heethaar states that he has no financial interest in the products or companies mentioned.
Erik A. Hermans, PhD, practices in the Department of Physics and Medical Technology, VU University Medical Center, Amsterdam, Netherlands. Dr. Hermans states that he has no financial interest in the products or companies mentioned. He may be reached at tel: +31 628235760; e-mail: email@example.com.
Steven A. Koopmans, MD, PhD, practices in the Department of Ophthalmology, University Medical Center Groningen, University of Groningen, Netherlands. Dr. Koopmans states that he has no financial interest in the products or companies mentioned.
Thom T. Terwee, MSc, is an employee of AMO Groningen BV, Groningen, Netherlands.
Rob G.L. van der Heijde, PhD, practices at the Free VU University Medical Center. Dr. van der Heijde states that he has no financial interest in the products or companies mentioned.