The Muscle Cuff Regenerative Peripheral Nerve Interface for the Amplification of Intact Peripheral Nerve Signals

Physiologic nerve signals are on the level of microvolts, which is far too small to record with currently available electrodes reliably. The MC-RPNI can interface with intact peripheral nerves, amplifying the small signals over a hundred times to facilitate reliable and accurate detection of motor intent. The primary advantage of this technique is its biological nature.

It can interface with intact peripheral nerves long term without causing a detrimental effect on the nerve itself or distally innervated muscle targets. Powered exoskeleton devices capable of restoring function to those with extremity weakness are rarely utilized to their full potential due to an inability to detect motor intent reliably and accurately. Due to its biological nerve signal amplification capabilities, the muscle cuff RPNI is the answer to these shortcomings.

Make a longitudinal incision along the anterior aspect of the desired donor hindlimb, extending from just above the ankle to just below the knee with a number 15 scalpel. Dissect the underlying subcutaneous tissue with sharp iris scissors to expose the underlying musculature and distal tendons just proximal to the ankle joint. Isolate the EDL muscle and its distal tendon from the surrounding musculature.

Ensure isolation of the correct tendon by inserting both tines of a forceps or iris scissor underneath the distal tendon just proximal to the ankle joint. Open the forceps or scissors to exert upward pressure on the tendon. Perform a distal tenotomy of the EDL muscle at the ankle level with sharp iris scissors and dissect the muscle free from surrounding tissues working proximally towards its tendinous origin.

Once the proximal tendon is visualized, perform a proximal tenotomy with sharp iris scissors to free the graft, trim both tendinous ends of the muscle graft and cut to the desired length with sharp iris scissors. Make a longitudinal incision along the entire trimmed length on one side of the muscle graft to facilitate placement of the nerve within the muscle graft and provide contact of the nerve with the endomysium. Place the prepared muscle graft in a saline-moistened gauze to prevent tissue desiccation.

Mark the surgical incision, extending from a line approximately 5 millimeters from the sciatic notch to the inferior knee joint. Incise through the skin and subcutaneous tissues along the marked incision line with a number 15 blade. Carefully incise through the underlying biceps femorous fascia, taking care not to extend through the entire depth of the muscle as the sciatic nerve lies just below.

Then dissect carefully through the biceps femorous muscle with blunt-tipped small scissors or a hemostat. Identify the common perineal or CP nerve and carefully isolate it from the surrounding nerves with a pair of micro forceps and micro scissors. Remove any surrounding connective tissue from the middle 2 centimeters of the nerve, taking care not to crush the nerve.

Hold the proximal epineurium with micro forceps over the central most portion of the freed CP nerve and cut into the epineurium immediately with microdissection scissors. Then, travel distally along the nerve to make an epineurial window matching the muscle graft length and remove around 25%of the epineurium. Take care to remove this segment in one piece.

Remove the muscle graft from the saline-moistened gauze and place it under the central portion of the CP nerve where the epineurial window was created. Rotate the nerve 180 degrees so that the epineurial window section touches the intact muscle and does not underlie the eventual suture line. Suture the CP nerve epineurium with an 8-O nylon suture proximally and distally to the muscle graft within the groove with simple interrupted sutures to secure epineurium to endomysium.

Circumferential wrap the edges in the muscle graft surrounding the now secured nerve and suture in place with interrupted 8-O nylon stitches. Once hemostasis is achieved, close the biceps femorous fascia over the construct with 5-O chromic suture in running fashion. Close the overlying skin with a 4-O chromic suture in a running fashion.

H&E staining of an MC-RPNI cross section shows the muscle and the nerve. Cross section of the ipsilateral distally innervated EDL muscle in a rat with an MC-RPNI demonstrated viable nerve and muscle tissue without any significant fibrosis or scarring compared to the cross section in a control rat without an MC-RPNI. Immunohistochemistry of a longitudinal section of an MC-RPNI specimen shows nuclei stained blue with DAPI and nerve tissue stained green.

A closeup of another successful MC-RPNI shows multiple innervated neuromuscular junctions stained with alpha-bungarotoxin in red for acetylcholine receptors. Electrophysiologic testing performed at the MC-RPNI constructs showed that the generated compound muscle action potentials or CMAPs waveforms are similar in appearance to native muscle, further supporting that they have become reinnervated by their contained nerve. CMAPs generated by physiologic EDL muscle following proximal CP nerve stimulation typically range from 20 to 30 millivolts.

EDL CMAPs in rats with implanted MC-RPNIs are not significantly different, averaging 24.27 millivolts plus or minus 1.34 millivolts. It is crucial to practice making an epineurial window before using it in a study. The epineurium is a very thin, fragile covering on the nerve, and damaging the underlying nerve fibers can alter results in highly unpredictable ways.

These constructs are considered mature in rats after 3 months, and their customization capabilities are endless. Following maturation, the MC-RPNI can be utilized in physiologic assessments, electrophysiologic analysis and muscle force testing to name a few. End-to-side neurorrhaphy is a relatively newer concept within surgery where a transected nerve is coapted to an intact peripheral nerve inducing collateral sprouting from the intact nerve into the grafted segment.

We’ve discovered this phenomenon also occurs in muscle grafted directly to the intact nerve and the possibilities for further research are fascinating.