Fundamentals of a retinal prosthesis: a surgical

perspective of suprachoroidal / epiretinal

prostheses

P.J.Allen

Bionic Vision Australia

Medical Bionics Conference 2011

prostheses

Surgical Aims

• Safe

– For the eye

– Overall patient health

• Easily reproducible procedure• Easily reproducible procedure

– Able to teach surgeons worldwide

• Stable for the lifetime of the patient

– Device stable in position

– Minimal risk longterm surgical complications

Our approach

• Cadaver trial surgery

• Animal model

• Human surgery

Human anatomy

• Axial length 24mm (21 – 26mm)

• Coats of the eye

– Cornea and sclera

– Uveal tract

– Retina

• Need to develop a surgical approach which

satisfies the surgical aims

Anatomy of the eye

Surgical approaches

• Suprachoroidal space

– Potential space between the sclera and the

choroid

– Traversed by the vortex veins and posterior ciliary

nerves / blood vesselsnerves / blood vessels

• Subretinal space is beneath the retina

• Epiretinal approach on the surface of the retina

• Within the sclera

Vitreous cavity

Within the sclera

Suprachoroidal space

Subretinal space

Cat model

• Axial length 22mm

• Coats of the eye

– Band of anterior thick sclera approx 7mm

wide

– Thin sclera posterior to this

• Under developed extraocular mucles

• Large anterior chamber

• Greater lens volume

Cat model

• Third eyelid

• Tapetum

– Reflective layer within the choroid

• Area centralis

• Vortex veins and posterior ciliary blood

vessels

Feline surgical model

Human eye Feline eye

External view of cat eye

Cat cadaver eye

Suprachoroidal surgery - cadavers

• Extensive use of cadaver eyes

• Initially anatomical dissections to familiarise

the surgeons with the cat eye

• Surgery performed in cadavers as much as

possible prior to live animals

• Any form / major changes surgery returned to

cadavers prior to proceeding on with live

animals

Surgical approach development

Suprachoroidal surgery

Implant Body Evolution

pat. pend.

Allen, Villalobos & Williams

Suprachoroidal surgery

• The surgical plan initially worked up in the

cadaver

• Then used in the acute studies

• Modified for chronic passive studies

• Modified for chronic active studies

• Preparation for humans

Suprachoroidal surgery

Design retuned to cadaver for further lead modification Allowing final design iteration

Suprachoroidal surgery

• Advantages– Ease of surgery

– Anatomical stability

– Choroid may act as a heat sink

• Disadvantages– Proximity to neural elements

• Unknowns– Spatial resolution

Epiretinal surgery

• Requires vitrectomy to be performed

– Posterior vitreous detached

– Core and peripheral vitreous removed

• Feline model• Feline model

– Large lens so lensectomy also performed

– Able to detach gel compared to rabbit

• Device tacked to retina

Vitrectomy surgery

Cadaver “open sky” approach

Demonstration of tacking

Evolution of device

Assessment in acute surgery guides

changes to form

With these changes being evaluated

in cadavers prior to further live animal

work

Epiretinal surgery

• Advantages

– Proximity to neural elements

– Vitreous cavity fluid can be utilised as a heat sink

– Prosthesis may be visualised through a dilated

pupilpupil

• Disadvantages

– Adhesion, possible difficulties in chronic

attachment

• Unknowns

– Lack of stability may cause variable proximity to

neural elements long-term

– Lack of stability may cause variable thresholds

Conclusions

• The development of a retinal prosthesis requires a

combination of cadaver and animal work

• Surgical team needs to work closely with the

engineering team to determine device specificationsengineering team to determine device specifications

• Feedback with iteration important

• Device evolution occurs in parallel with evolution of

surgical techniques