Development of an artificial lens as a solution to cataract, ametropia and presbyopia

The Turtle lens project

Erik. A. Hermans¹, T.T. Terwee², S.A. Koopmans³, M. Dubbelman¹, G.L. van der Heijde¹, R.M. Heethaar¹

 

¹  VU University Medical Center, Amsterdam, The Netherlands
²  AMO Groningen BV, Groningen, The Netherlands

³  Ophthalmology, University Medical Center Groningen, University of Groningen, Groningen, The Netherlands

 

Concept design of the accommodative Turtle lens

 

Aims of the present study

In order to accommodate, the optical power of the human lens increases. However, with age

the human eye loses its ability to accommodate; this is referred to as presbyopia, the cause

of which mainly consists of an increase in stiffness of the lens.

 

            The optical power of the human lens increases with

accommodation at a young age. 

 

In most cases, presbyopia and ametropia are currently corrected by (multifocal) spectacles or

contact lenses. However, at an elderly age there can be cataract formation, and the lens becomes opaque.

Most intra-ocular lenses (IOLs) that are implanted to restore vision have a fixed focal power, and therefore do

not accommodate, but an accommodative IOL could not only provide a solution for cataract and ametropia,

but also for presbyopia. However, the requirements for an IOL that could be used to restore accommodation

and guarantee spectacle-independence are stringent. Firstly, the accommodative power of the accommodative

IOL must be large, in order to ensure comfortable reading. Secondly, it is necessary to have a predictable outcome

of the patient’s refractive error. An accommodative IOL that restores accommodation, but has no predictable

disaccommodated refractive power after implantation, does not guarantee spectacle-independence.

Moreover, an accommodative IOL should preferably also be capable of compensating corneal astigmatism,

and should have an optical quality that is similar to the quality of a present generation monofocal IOL in all

accommodative states. Finally, the accommodative IOL should preferably be operated by the human ciliary muscle.

 

 

Development of an accommodative IOL

The development of an accommodative IOL that meets the requirements for a spectacle-independent solution

for presbyopia is shown. Therefore, an accommodative IOL (“the Turtle lens”) with a rotating focus mechanism

and a mechanical frame that could operate within the range of ciliary muscle contraction has been designed.

The concept design was optically and mechanically optimized for a typical 60 year-old human eye, and ray-tracing

showed that the modulation transfer function (MTF) of the Turtle lens in different accommodative states did not

deviate to any great extent from the MTF of a monofocal IOL.

 

How does it work ?

 

A pair of rotating focus lenses from the top

Example of ray-tracing a pair of rotating focus lenses in an eye model

 

 

 

Finite Element (FE) method used to compute the deformation and displacements

of a concept design inside a capsular bag of a typical 60 year-old human eye

 

Squeezing of the frame results in a rotation of the lenses

and consequently a change in optical power

The change in optical power of the Turtle lens can be induced

by the ciliary muscle

 

We produced prototypes to test the mechanical performance in an enucleated pig’s eye,

using a laboratory lens-stretching device that mimics the action of the human ciliary muscle.

Changes in focal length during stretching were determined by means of laser-based ray-tracing

and video-recordings.

 

  Assembled prototype of the accommodative Turtle lens

  concept with silicon lenses and silicon frame

 

Stretch ring with the lens-zonula-ciliary body specimen

and the Turtle lens in situ, implanted before stretching

 

Experimental set-up of the stretching device

 

During stretch experiments the focal length of the Turtle lens changed according to

the design for 8 D of accommodation, equivalent to a reading distance of 16 cm.

 

 

The lens power of the prototype as a function of stretch (diameter) of the ciliary body

 

In conclusion, most of the requirements for an accommodative IOL, through which spectacle-independence

can be achieved, can be met. The mechanical frame, in combination with the rotating focus principle, can be

used to develop an IOL that restores accommodation with a large and predictable accommodative amplitude.

By using stop-devices, the base power can be varied in steps of 0.5 D, according to the required refractive outcome

of the patient. Astigmatism and higher order aberrations, such as spherical aberration, could be compensated by

the optical design. However, both the mechanical and the optical design need further optimization to improve the

optical quality and functionality of an accommodative IOL.

 

The Turtle lens has been developed in collaboration with Advanced Medical Optics Inc. www.amo-inc.com