The tissue response of nerves to electrodes
include trauma to axons as well as the nature and extent
of the encapsulation surrounding the implant. Traumatized
axons may not conduct propagating actions potentials and
thick encapsulation layers degrade electric field gradients
by increasing the separation between the electrode contacts
and target axons.
Rigid Cuff Electrodes.
rigid split-cylinder type cuff electrode is shown in the
lower right of the Figure.
On the left, connective tissue developing around the nerve
and the interspace can be seen.
AAMI Standards call for
the nerve cuff inner diameter to be 150% of the outside
of the nerve.
Tissue Response to Open Cuff Electrode
of the connective tissue formed around a “half-cuff”.
The horseshoe shaped region in the upper part of the photograph,
which is absent tissue, is the space previously occupied
by the “half-cuff”. The oval shaped region in
the lower part of the figure is the nerve, which has been
pushed out of the cuff. Close inspection of the upper portion
of the connective tissue on the inner aspect of the cuff
reveals layers, like rings on a tree trunk (arrow, right).
These layers of connective tissue are presumed to have developed
in response to mechanical irritation between the cuff and
the local tissues. It appears that the very small blood vessels
are repeatedly injured, probably mechanically, causing fibrous
tissue to proliferate and push the nerve out of the cuff.