Biological Considerations for an Intraocular Retinal Prosthesis - Artificial Sight: Basic Research, Biomedical Engineering, and Clinical Advances - page 5

The rate at which aqueous is produced and drained determines IOP; the
overproduction or more often outflow obstruction will result in raised IOP,
which may in turn lead to retinal nerve fiber layer damage and loss of peripheral
vision - a common eye disease known as glaucoma. Thus, a retinal prosthesis,
which has components in the anterior chamber, risks disrupting the fluid flow in
the eye.
The lens is surrounded by a very thin capsule and is kept in place by zonules,
tiny fibers which attach the capsule to the ciliary body. The lens is normally clear,
but may become opaque following injury secondary to trauma or intraocular
surgery. The opaque lens is known as cataract and more commonly occurs
with age. During cataract surgery, the lens is removed from the capsule and
replaced by an artificial lens made of silicone, PMMA, acrylic, etc. (Figure 1.5).
If necessary, the capsular bag can also be used to accommodate some parts of
the retinal prosthesis.
Posterior Segment
The sclera, or outermost layer of the eye, merges with the cornea and conjunctiva
at the limbus (Figures 1.1 and 1.4). Unlike the cornea which is clear, the sclera is
white and blocks most light from entering the posterior chamber. The conjunctiva
is a partially transparent membrane that contains vessels and covers anterior part
of the sclera (Figure 1.4). The sclera and cornea give the eye its shape.
The choroid is a highly vascular tissue located underneath the sclera, and
merges anteriorly with the ciliary body. This structure supplies nutrients to the
outer third of the retina, including the photoreceptor outer segments which are
particularly metabolically demanding. The inner retina is perfused by a separate
circulatory pathway, retinal vessels (Figure 1.6). The vitreous, which fills the
space between the lens and the retina, is jelly-like in consistency and helps to
keep the retina attached to the underlying layer.
Retina
The retina is positioned as the innermost layer of the posterior segment of the
eye and functions as both the sensory detector and the early processing for the
Artificial lens
Posterior capsule
Figure 1.5. Schematic cross section of the anterior segment after cataract surgery,
showing an artificial intraocular lens inside the capsular bag.