ACCOMMODATING IOL

Developing an extruded gel accommodating intraocular lens (IOL) that provides excellent optical quality over a wide refractive range using natural physiological accommodative effort is a complex engineering challenge. A prototype lens with a corresponding mathematical model suggest it can be done, Sean J McCafferty MD of the University of Arizona, Tucson, AZ, US, told the 2014 American Society of Cataract and Refractive Surgeons (ASCRS) in Boston.

The design is based on the NuLens concept, a sulcus-positioned lens in which gel extruded through a central aperture interface creates lenticular deformation under contraction of the ciliary muscles leading to accommodation, Dr McCafferty said. However, the NuLens design works counter to the natural direction of accommodative effort, increasing rather than decreasing power with relaxation.

The new prototype uses a bicameral chamber filled with a high-index fluid in front of the deforming surface, Dr McCafferty said. This creates a counter force vector from the sulcus that decreases lens power under relaxation, which is physiologically similar to the action of the natural crystalline lens.

Developing a mathematical model

The primary development goal of the new design is to minimise the force required to accommodate, Dr McCafferty said.

“There is a very limited amount of force available inside the eye to produce accommodation. But accommodation accompanied by major optical aberrations isn’t worth much, so the secondary goal is optimising image quality on the retina, expressed as a visual Strehl ratio.”

Small changes in lens design and materials, such as the size of the deformable aperture, and the axial thickness and elasticity of the gel material, have major effects on the force required and the optical quality achieved, Dr McCafferty explained. For example, decreasing axial gel thickness from 2.0mm to 0.5mm with a 3.0mm aperture changes the lens interface profile from a smooth lenticular curve with very low aberrations to a wavy surface with extreme aberrations.

The mathematical model developed for the lens correlates closely with the observed characteristics of the prototype, Dr McCafferty said. A wide range of design and material variables can be plugged into this validated model for testing without manufacturing hundreds of varying prototypes, speeding the development process.

So far, the model suggests that maximising the refractive index change at the interface reduces actuation force. Minimising the gel’s Poisson’s ratio, which is a measure of how much a material deforms in one direction in response to force exerted in another direction, also reduces force and improves image quality – though Poisson values for gels run in a narrow range and are all fairly high. Reducing the elasticity, or Young’s modulus, and total disc area compressed also lower force requirements – though this runs counter to improving image quality by maximising the ratio of axial gel thickness to aperture diameter.

Multiple input parameters can be combined using a Monte Carlo simulation to derive a root sum square merit function that provides precise answers for the diameter, Young’s modulus and other design and materials trade-offs that will optimise force and visual quality outcomes, Dr McCafferty said.

“This is the process that any deformable interface IOL should go through before final development and FDA approval.”

Sean McCafferty: sjmccafferty66@hotmail.com