Neuromusculoskeletal interface improves precision, comfort, and quality of life

Use of an osseointegrated, self-contained robotic arm with implanted electrodes not only allowed for improved prosthetic control and intuitive touch feedback, it also improved patients’ ability to conduct everyday tasks over prostheses powered by electrodes attached to the surface of the skin, researchers reported.

In a brief report published in the New England Journal of Medicine, Max Ortiz-Catalan, PhD, of Chalmers University of Technology’s Department of Electrical Engineering in Gothenburg, Sweden, and colleagues reported outcomes for four patients who received a neuromusculoskeletal prosthesis over 3-7 years following transhumeral amputation. And, in all cases, precision of prosthetic control improved over time.

“All patients reported having greater trust in their prosthesis since the intervention, referred to it as being part of themselves, and reported positive effects on their self-esteem, self-image, and social relations,” Ortiz-Catalan and colleagues wrote, though they noted that these statements were not assessed using any established measure.

To prepare for the neuromusculoskeletal interface, three of the patients underwent nerve transfers, which consisted of “rerouting the ulnar nerve to the motor branch of the short head of the biceps muscle and re-routing the deep branch of the radial nerve to the motor branch of the lateral head of the triceps,” they explained. “Neuromas at the ulnar nerve and distal branch of the radial nerve were excised. The distal ends of these nerves were coapted to the ends of motor branches of the musculocutaneous and radial nerves.”

Meanwhile, for the fourth patient, prosthetic motor control depended on signals from “natively innervated biceps and triceps muscles.”

The major difference between the neuromusculoskeletal prosthesis and conventional prostheses is that, rather than using surface electrodes, this interface uses electrodes “sutured onto the epimysium of the two heads of the biceps muscles and the long and lateral heads of the triceps,” Ortiz-Catalan and colleagues wrote. “…To obtain sensory feedback, we placed a spiral cuff electrode around the ulnar nerve in all four patients and placed an additional electrode around the median nerve in three patients. The cuff electrodes delivered signals for tactile sensory feedback originating from three sensors on the prosthetic thumb through electrical stimulation of the afferent nerve fibers that had been severed in the amputation.” The patients’ previous osseointegrated implants were altered to fit the new neuromusculoskeletal set-up.

As for the four patients, each had previously used prostheses, with limited success:

  • Patient 1 (male, age 47 years) initially received an electrically driven prosthesis in 2003, which was attached to the body with a socket that used the constant application of pressure on the stump to remain in place and which he only wore sporadically due to the discomfort caused by the socket. This was replaced by an osseointegrated implant in 2009 that used surface electrodes. However, he had trouble operating the prosthetic as the skin-surface electrodes became unresponsive when he was in outdoors in a colder climate, which impacted the electrodes’ ability to record myoelectric signals. In addition, he reported “’distressing’ phantom limb pain with an intensity of 6 on a 10-point visual analogue pain scale on which 0 indicates no pain and 10 the worst possible pain.” He received the neuromusculoskeletal interface in Jan. 2013 at age 41.
  • Patient 2 (male, age 46 years) initially used an electrically driven prosthesis with a socket and surface electrodes in 2011, but back pain and discomfort made it difficult to control the prosthetic limb. He received an osseointegrated implant in 2014, which resolved the discomfort, but the patient reported poor control of the prosthetic hand and preferred to use a prosthetic “gripper.” He received the neuromusculoskeletal interface in Jan. 2017 at age 44.
  • Patient 3 (male, age 44 years) initially wore an electric prosthesis attached via socket, which he used sporadically for five years before ditching it due to “discomfort and poor functionality.” In 2013, he reported back pain resulting from a postural imbalance caused by the missing arm. He received an osseointegrated implant in 2014 and started using an electric hand controlled with surface electrodes, but he reported poor control of the limb and phantom limb pain with an intensity of 3 out of 10 on a visual analogue pain scale. He received the neuromusculoskeletal interface in Jan. 2017 at age 42.
  • Patient 4 (male, age 44 years) wore his prosthetic hand sporadically “owing to discomfort caused by the socket used for attachment and to poor function related to the surface electrodes used to control the hand prosthesis.” He received an osseointegrated implant in 2007, which resolved the discomfort caused by the previous socket. He received the neuromusculoskeletal interface in May 2017 at age 42.

Patients were fitted with their self-contained arm prostheses — the ErgoArm and the SensorHand, manufactured by Ottobock (Duderstadt, Germany) — four to six weeks after they received the neuromusculoskeletal interface, and in Jan. 2017 (one patient) and Sept. 2018 (two patients), “electrical stimulation intended to elicit tactile perception was coupled to force sensors in the thumb of the prosthetic hand, providing graded sensory feedback during grasping of common objects,” Ortiz-Catalan and colleagues reported. “The fourth patient did not participate in follow-up after the initial fitting of the prosthesis and was therefore not provided with sensory feedback.”

Since all patients were familiar with use of a prosthetic hand with surface electrodes, they didn’t need any training to use the new neuromusculoskeletal interface.

Ortiz-Catalan and colleagues assessed both the minimum increment of force that could be applied to an object while closing the prosthetic hand and the minimum incremental activation of the hand during opening and closing movements. Evaluations were performed both before surgery — when the prosthetic was controlled through surface electrodes — and one month after the prosthetic fitting — when the prosthetic was controlled by the epimysial electrodes. Sensory acuity was measured using psychometric tests.

The study authors found that precision in prosthetic control improved in all patients — “Patient 4 did not participate in follow-up but had documented use of his neuromusculoskeletal prosthesis in daily life for 2 years 6 months,” they explained.

“The smallest increment in grip force the patients could exert using eEMG was in average 12.3% of what was possible with sEMG (eEMG = 0.52 N; sEMG = 4.26 N),” they wrote. “The associated standard deviation, a proxy of precision, was reduced to 17.4% of sEMG (eEMG = ±0.48 N; sEMG = ±2.73 N). Similarly, the minimum actuation using eEMG for hand open (HO) and hand close (HC) was 47.4% and 36.3% of sEMG, respectively (eEMG = 1.41 mm HO and 1.64 mm HC; sEMG = 2.98 mm HO and 4.51 mm HC). Their associated error was reduced to 56.5% and 53.3% of sEMG, respectively (eEMG = ±1.07 mm HO and ±1.22 mm HC; sEMG = ±1.89 mm HO and ±2.28 mm HC) [P <0.01 for all].”

In all cases, patients with follow-up data reported sensations from the direct nerve stimulation in the phantom hand, which were described as similar to a “touch by the tip of a pen” before acquiring a more “electric” character at higher intensity. “Initially, patients could perceive a difference in the intensity of sensations when the frequency of stimulation was increased or reduced by 50%,” the study authors reported. “After a month of daily use of sensory feedback, a change of approximately 30% in the frequency of stimulation could be perceived as an increase or decrease in intensity of tactile sensation.”

Patients 1 and 3 reported complete relief of phantom limb pain with the new interface. And, the new implanted electrodes allowed more active participation in the activities of daily life.

“Patient 1 has become employed full-time as a result of the improved functionality of the prosthesis, which has also allowed him to ski, go ice fishing, and ride a snowmobile,” they wrote. “The preferred terminal device of Patient 2 became a myoelectric hand rather than a gripper owing to the superior control provided by the implanted electrodes. He has been able to engage in rally-car racing and to repair cars with his neuromusculoskeletal prosthesis. Patient 3 has been able to orienteer, canoe, and ski while using his neuromusculoskeletal prosthesis.”

While these results constitute a success in terms of increased functionality and sensation, Ortiz-Catalan and colleagues pointed out that prostheses controlled via neural interface are still a work in progress: “Ideally, the number of sensors in the prosthetic hand would match the resolution of the interface, so the patient would have feeling in all the locations on the artificial hand where the sensors are capable of detection. The relevance of the work presented here is not in the number of perceived and measured sensations but in the achievement of an integrated and fully self-contained prosthesis with implanted electrodes that can be used reliably in daily life, enabling intuitive control and somatosensory feedback of the hand.”

They added that, in the future, the new musculoskeletal interface will incorporate other types of electrodes, “potentially allowing for the use of more sophisticated neural interfaces.”

  1. Prosthetic arms controlled via implanted electrodes improved precision prosthetic control, physical comfort, and ability to participate in everyday life versus prostheses controlled via surface electrodes. The neuromusculoskeletal interface allowed somatosensory feedback to the prosthetic hand, allowing for limited physical sensation in the hand.

  2. Be aware that these prostheses are still works in progress and the technology is evolving.

John McKenna, Associate Editor, BreakingMED™

Supported by Integrum, the Promobilia Foundation; the IngaBritt och Arne Lundbergs Foundation; Vinnova, an agency in the Swedish Ministry of Enterprise and Innovation; the Swedish Research Council; the ALF (Avtal om Läkarutbildning och Forskning) through a grant from the Västra Götaland region; and grants from the European Research Council under the European Union’s Horizon 2020 research and innovation program.

Ortiz-Catalan disclosed grant and research support from Integrum, which holds a patent for percutaneous gateway for signal transmission between implanted electrodes and limb prostheses.

Cat ID: 438

Topic ID: 437,438,438,737,730,192,925,159