Biological Considerations for an Intraocular Retinal Prosthesis - Artificial Sight: Basic Research, Biomedical Engineering, and Clinical Advances - page 20

Cable
Array
Tack
Figure 1.18. Schematic diagram of an epiretinal implant showing the array, the cable,
and the tack, and their relation to the surrounding ocular structures.
after insertion of the array. The cable should be made as narrow as possible,
since the final opening depends on the width of cable.
Attachment of the Array to the Retina
With the epiretinal approach, the electrode array is fixed on the retina by means
of a tack. Because the tack penetrates the choroid (which is a highly vascular
tissue), there is a risk of intraocular bleeding at the time of tacking. It is
possible, however, to reduce this risk by briefly increasing the IOP by raising
the infusion line. While massive intravitreal bleeding may be associated with
subsequent complications, minimal bleeding is usually absorbed within days or
weeks without any consequence.
Another concern with the tack is the risk of gliosis (fibrosis), i.e. proliferative
tissue that may grow from the penetration site and may either cover the retinal
surface or encapsulate the array and interfere with retinal stimulation. The use of
only one tack for fixation of the array reduces this risk. In one study which an
epiretinal arraywas fixedonto the retina by two tacks in four dog eyes, retinal hyper-
pigmentation was observed around the site of the tack. In that study, histological
examination revealed minimal fibrosis around the site of implantation [8].
Pressure Effect on the Retina
When the array is tacked to the retina, some of its parts may exert unwanted
pressure, which may subsequently result in retinal damage. Normal IOP is
10-21mmHg, and it is well established that raised IOP can damage the nerve
fiber layer of the retina. Glaucoma is a common cause of blindness in the
elderly and is caused by nerve fiber layer damage secondary to raised IOP. It