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Page 4 of 9 Burke et al. Plast Aesthet Res 2022;9:48 https://dx.doi.org/10.20517/2347-9264.2022.26

[6]
monkeys using the median, ulnar, and radial nerves . RPNI EMG signals were recorded for finger flexion,
[6]
finger extension, and finger rest with an accuracy of over 96% . Long-term stability was demonstrated with
efferent motor action potentials transduced into large-amplitude CMAPs with high SNRs up to 20 months
[6]
postoperatively in this model . Building on this work, subsequent experiments on rhesus macaques were
performed to control continuous and real-time finger flexion and finger extension of a prosthetic hand
using indwelling bipolar electrodes . Due to the positive results from these animal studies, clinical
[7]
translation and subsequent testing of RPNIs were next carried out in humans.

REGENERATIVE PERIPHERAL NERVE INTERFACE FOR NEUROPROSTHETIC CONTROL
A pilot study in humans with upper extremity amputations demonstrated the successful use of RPNIs for
real-time, volitional prosthetic device control . The study included four participants with varying levels of
[9]
amputation including transradial, wrist disarticulation, and shoulder disarticulation. RPNIs in these patients
were created using transected residual nerves (including the median nerve, ulnar nerve, and radial nerve)
and muscle grafts harvested from the patient’s vastus lateralis [Figure 3]. Participant 1 (P1) elected to have
temporary fine wire electrodes placed in their RPNIs under ultrasound guidance. However, in this
[9]
participant, RPNI EMG signals recorded immediately after electrode insertion had a mean SNR of 4.21 . In
contrast, participants 3 (P3) and 4 (P4) elected to have indwelling bipolar electrodes surgically implanted in
their RPNIs. The SNR of the recorded signals in these participants were significantly larger with a mean of
68.9 and 21.0, respectively, and were effective in providing high-fidelity control of thumb and finger flexion
[9]
and extension, as well as intrinsic finger control (finger abduction and adduction) . This difference is likely
attributed to the bipolar electrode design and the relatively small space between electrode sites in the
[9]
percutaneous fine wires (1 mm) compared to the indwelling electrodes (10 mm) . Of note, the future
direction of RPNIs includes electrode implantation at the time of RPNI surgery, as opposed to using
percutaneous wire electrodes into existing RPNIs.

In these patients, RPNIs demonstrated effective transduction of efferent motor action potentials into large
amplitude CMAPs, with high SNRs, for high-fidelity prosthetic control of both extrinsic and intrinsic hand
and finger movements . The patients who underwent surgical implantation of indwelling bipolar EMG
[9]
electrodes into their RPNIs had substantially higher SNRs for prosthetic control, especially when compared
to other methods of harnessing signals directly from peripheral nerves such as cuff and intraneural
electrodes . In addition, RPNIs did not cause iatrogenic nerve injury, nor were there any problems with
[9]
signal stability or degradation commonly seen with electrodes that directly interface with peripheral
[9]
nerves .
This human study also showed the functional stability of RPNIs through ultrasound imaging. For example,
independent contractions of distinct RPNIs, in the same patient with multiple median nerve RPNIs, could
be seen on ultrasound during thumb flexion, index finger flexion of the proximal interphalangeal (PIP)
joint, and index finger flexion of the distal interphalangeal (DIP) joint. On ultrasound, subsections of the
same RPNI and alternative RPNIs of the median nerve fascicles contracted based on the patient’s varying
motor intent. This finding shows that an RPNI contains individual motor units that contract independently,
thus allowing for the accurate recording of discrete signals. Importantly, electrodes can be spaced
millimeters apart on the large surface area of the RPNI to enable selective and functional prosthetic control,
as theoretically, electrodes with more recording sites within RPNIs can provide enhanced signal
selectivity . The use of indwelling bipolar electrodes in RPNIs overcomes a major barrier inherent to
[9]
peripheral nerve interfaces that use intraneural electrodes which require micrometer spacing of numerous
recording sites [9,15] . Another distinct advantage of RPNIs over other techniques is that RPNIs do not rely on
recording EMG signals from residual innervated muscles because signals can be recorded directly from