Page 387 - Handbook of Biomechatronics

Current Advances in the Design of Retinal and Cortical Visual Prostheses 381

the surgical procedure simpler and safer compared with the subretinal
approach, there is a difficulty in attaching the array to the retina, and sharp
metal tacks are required to hold it in place (Fernandes et al., 2012). Finally,
another potential benefit is that the possibility for heat (generated from elec-
trical stimulation) dissipation through the gel mass of the vitreous cavity
(Opie et al., 2012).
On the contrary, in subretinal implantation strategy, it is potentially pos-
sible to gain from the biological signal processing that occurs in the middle
layers of the retina (Chow et al., 2010). The constrained space between the
RPE and the neurons (Fig. 2B) contributes to fixation of the implant and no
tacks are required (Stingl et al., 2013a). It was further suggested that
improved electrode-tissue attachment accounts for lower stimulation
thresholds observed with subretinal compared with epiretinal stimulation
(Jensen and Rizzo, 2006). Avoiding the activation of RGC axon bundle,
often distorting visual perception is another advantage of this approach
(Humayun et al., 2003; Rizzo et al., 2003). A potential disadvantage is
the risk of retinal atrophy in vicinity of the implant due to blocking of
the retina-choroid fluid link, preventing heat dissipation and nutrient
transport (Peachey and Chow, 1999; Sailer et al., 2007).
While in the subretinal and epiretinal approaches, the electrodes are in
direct contact with the neurons in the retina, in the suprachoroidal or intra-
scleral approaches, the array is placed at a distance and there is no direct con-
tact with neurons (Fig. 2B). The main advantage of these approaches is that
the surgical procedure is significantly less invasive and less technically chal-
lenging. There is no need for removal of the vitreous humor (vitrectomy) or
for direct manipulation to the retina (Gekeler et al., 2010; Zrenner et al.,
2011). These surgical manipulations may contribute to complications such
as conjunctival erosion, endophthalmitis, hypotony, and retinal detachment
detected with epiretinal prostheses (Keser€u et al., 2009, 2012; Menzel-
Severing et al., 2012; Humayun et al., 2012), or gliosis (Butterwick et al.,
2009), retinal microaneurysms (Stingl et al., 2013c), and retinal atrophy
(Peachey and Chow, 1999) reported with subretinal prostheses. Further, fix-
ation of the prosthesis requires only sutures to stabilize the implant (no metal
tacks), and a larger array can be implanted (Fujikado et al., 2007, 2011;
Saunders et al., 2014). This means that potentially a large FOV can be
addressed (Villalobos et al., 2012, 2013). The close proximity to blood ves-
sels in the choroid also contributes to reducing the risk for heat induced
damage (Parver et al., 1980). However a major disadvantage is the distance
from the neurons, with approximately 250–400μm vs 180μm reported for a