Fundamentals of a retinal prosthesis: a surgical perspective of ...
Transcript of Fundamentals of a retinal prosthesis: a surgical perspective of ...
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