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Page 4 of 10 Amin. Plast Aesthet Res 2022;9:24 https://dx.doi.org/10.20517/2347-9264.2021.119

movement. A clinical study of 27 patients with 238 electrodes placed in this way recorded a failure rate
[40]
below 2% after 16 years . However, muscle fatigue and neighbouring “noise” recorded from adjacent
[41]
electrical activity limit the quality of information processed . Further critique includes many months of
requisite training, meaning 30% of individuals abandon their myoelectric prostheses . This approach is
[42]
limited when few muscle groups remain in the residuum, such as that seen in more proximal amputations.
[43]
In a bid to increase muscle targets, targeted muscle reinnervation (TMR) was introduced by Kuiken et al.
in 2003 to augment upper extremity function (TMR). TMR has been less investigated and applied for the
lower limb. The premise is that remaining nerves in the amputated limb when surgically coapted to target
nerves supplying a regional muscle, provides intuitive contraction of the recipient’s muscle when the
desired function of the native nerve is required (targets are completely denervated to avoid the generation of
unwanted signals). For example, the tibial nerve, when coapted to semitendinosus and the common
peroneal to the biceps femoris, contractions are recorded months later upon the desired movement of the
absent muscle group . Moreover, a reduction in neuroma pain, opioid use, and neuromodulator
[44]
medication has been observed . A surgically related technique employs a regenerative peripheral nerve
[45]
interface, whereby residual motor fascicles are coapted with non-vascularised muscle grafts acting as a
bioamplifier once innervated. Multiple fascicles can provide more signals over that from TMR, but the
[46]
signals are smaller and less easily recorded .
Herr has described the agonist-antagonist myoneural interface (AMI). By having muscles of opposing
functions, surgically brought together as a single unit, the proprioceptive relationship is preserved. High
fidelity control of a prosthetic device is seen in a transtibial amputee. Functional electrical stimulation (FES)
gives feedback to the AMI of the prosthetic position . Healthy tissues are required making it a more
[47]
reliable option at the time of primary amputation. The approaches described are still limited in sensory and
proprioceptive feedback and using muscle targets from unrelated neural inputs make myoelectric systems
feel unnatural and non-intuitive .
[48]
THE EVOLVING ROLE OF BIONIC TECHNOLOGY AND NERVOUS SYSTEM CONTROL
A less investigated approach is to directly communicate with peripheral nerves [Electroneurography
(ENG)] , and may offer the solution to restore sensibility. This is important given grasp, and object
[49]
manipulation is reliant upon afferent feedback. To enable a richer, natural feel and interaction with one’s
environment require an appreciable understanding of the nervous system. Different levels within the
somatosensory system are current research targets ranging from peripheral nerves to the brainstem.
Peripheral nerves have both efferent (motor) and efferent (sensation via cutaneous mechanoreceptors,
proprioceptors, thermoreceptors, and nociceptors) fibres, broadly divided into unmyelinated or myelinated
axons (Schwann cells develop insulating myelin sheath). Large, myelinated fibres have the lowest resistance
due to the increased density of voltage-dependent ion channels. Afferent fibres transmit signals from
mechanical, thermal or noxious stimuli and similarly are unmyelinated or myelinated. The biomimetic
approach faces two major challenges. Firstly, neural interfaces are limited in the number of channels and for
individual channels to reliably stimulate distinct afferents is challenging. Aside from recording, algorithms
for meaningful processing into human translation is complex. Secondly, the nature of the implants is called
into question. As electrodes become more invasive, greater selectivity is achieved in addition to the quality
of recorded signals. This comes at a cost, given invasive electrodes can induce injury to the nerve. The
placement of the electrode can be broadly defined by its relationship with the epineurium [Figure 2].

Extraneural electrodes
The perineurium is not penetrated with the best-known type referred to as the “cuff electrode” that encircles