Biological Considerations for an Intraocular Retinal Prosthesis - Artificial Sight: Basic Research, Biomedical Engineering, and Clinical Advances - page 12

Figure 1.13. The basic waveform of the ERG (adapted with permission from

http://webvision.med.utah.edu/index.html).

often non-recordable in advanced degenerative retinal disease, it has little utility
for assessment of blind patients with retinal implants. The ERG can aid in the
development of retinal prostheses by providing a functional measure during
preclinical safety testing [8, 9]; however, its lack of spatial sensitivity makes it
unable to detect small non-functional areas in the retina. At best, the ERG can
serve as a gross measurement that indicates that the retina is not experiencing a
large scale, negative reaction to implantation.

Retinal Surgery
Retinal surgery enables the implantation of a device in the vitreous cavity. Due
to the nature of the vitreoretinal surgery, intravitreal implants are limited in size
and shape. A significant number of retinal diseases are treated surgically. In
some cases, surgery can be performed without entering the eyeball. For example,
in certain cases of retinal detachment, the retina can be reattached by scleral
buckling, i.e. the suturing of a silicone prosthesis over the sclera, which results
in indentation of the sclera, choroids, and RPE toward the retina. However,
in most conditions, surgery is performed by vitrectomy (i.e. the insertion of
surgical instruments into the vitreous cavity and removal of the vitreous). First,
the conjunctiva is cut at the limbus and the sclera becomes exposed. The vitreous
cavity is then accessed via scleral incisions (sclerotomy) made 3.5-4mm behind
the limbus (Figure 1.14), the area overlying the pars plana (Figure 1.1). Anterior
entry to this point may injure the lens and thus cause cataract or damage the
ciliary body or iris; on the other hand, entry behind this point may cause retinal
detachment or choroidal hemorrhage.
In a standard vitreoretinal surgery case, three scleral incisions are made:
one for the infusion terminal, through which fluid constantly flows into the
eye to maintain IOP and prevent the collapse of the eye; a second one for
the insertion of an intraocular light probe; and a third one is used to cut the
vitreous and for intraocular manipulation. The size of a scleral incision is usually
approximately 1mm (between about 0.6mm and 1.4mm). Since the vitreous
is gelatinous and is attached to some parts of the retina, it is not possible to