All Hands on Deck: Advances in Upper Extremity Prosthetics

Written By: Justin Perry
Edited By: Elana Balch

Modular Prosthetic Limb. Applied Physics Laboratory [image on the Internet]. [created 2010, cited 2020 May 25]. Institute of Electrical and Electronics Engineers.

The human hand, an intricate appendage consisting of 27 joints, 34 muscles, and over 100 ligaments and tendons, is something most able-bodied people take for granted [1]. Unfortunately, common debilitating health conditions such as cancer and cardiovascular disease have contributed to approximately 425,000 people in the U.S. currently living with arm or hand amputations, with an additional 25,000 per year [2]. Many amputees who use conventional prostheses are unable to regain adequate hand function. However, recent advances in prosthetic technology have overcome many mechanical and cost-associated limitations and show increased promise in imitating the function of a hand. ​

The most sophisticated robotic prostheses utilize myoelectricity, a process by which surface electrodes on the prosthesis detect electrical signals from muscle contractions in the residual limb through contact with the skin. Often, these electrical signals are difficult to detect, so targeted muscle reinnervation (TMR), a surgical technique developed in 2002 at Northwestern University, is needed to amplify the signals [3]. By connecting nerves that would control movement in the missing limb to muscle within or surrounding the residual limb, a larger electrical signal is generated. These enhanced electrical signals enable amputees to more effectively control advanced myoelectric prostheses [3].

The first myoelectric prosthesis, developed by Russian scientist Alexander Kobrinsky in 1960, enabled users to perform movements as precise as unscrewing a lightbulb [4]. Even so, the device had weak grip strength and it was slow, heavy, and easily susceptible to damage [5]. Persistent engineering limitations in later models such as the 1980 Utah arm relegated the idea of a robotic arm to science fiction [4]. Fortunately, a new hope arose when the DEKA arm, or Luke arm, was introduced in 2006 and approved by the FDA in 2014. The DEKA arm allows amputees to use methods in addition to myoelectricity, such as wireless foot controls, to operate the prosthesis [6]. These methods enable amputees to perform programmed grip patterns, which allow them to perform movements like holding an egg or buttoning a shirt [6]. However, the mechanical sophistication of the DEKA arm is rivaled by the modular prosthetic limb (MPL), which was completed in 2010 by the Johns Hopkins Applied Physics Laboratory and has undergone clinical testing since then. The MPL features 100 sensors, 26 joints, 17 motors, and a tiny computer that enables a variety of fine motor movements, such as picking grapes from a stem and holding hands [8].

The DEKA arm and MPL are also remarkable in their compatibility with the human body. The two prostheses can undergo osseointegration, a process in which a prosthesis is attached to the remaining bone in the residual limb. This can prevent joint, skin, and muscular problems experienced by amputees who use socket prostheses. The DEKA arm and MPL also enable amputees to sense touch, position, temperature, and pain, which allows them to more easily control the prostheses [9,10]. These sensations are possible because targeted sensory reinnervation surgery (TSR) can relocate cutaneous nerves—which would normally be connected to the skin of the missing limb—to the surrounding skin [11]. Through contact with the skin, the prostheses can deliver electrical stimulation to these nerves, thus providing somatosensory feedback to the brain. Somatosensory feedback is beneficial to amputees with phantom limb pain, which is the perception of pain in the missing limb. Various studies have suggested that neural stimulation may prevent the reorganization of the somatosensory cortex, which occurs due to lost signals from the missing limb and may cause phantom limb pain [12, 13, 14].

While the DEKA arm and MPL offer unparalleled integration with the human body, both devices are incredibly expensive, with estimated prices of $100,000 and $400,000, respectively [15, 16]. As a result, recent innovations have sought to reduce the cost of myoelectric prostheses without sacrificing their functionality. For instance, the Dexus arm, a device developed by BrainCo and Harvard University that was unveiled earlier this year, costs only around $10,000 and could be approved by the FDA as soon as 2021 [17]. The Dexus arm is lightweight and, unlike the DEKA arm and MPL, does not require much training to recognize myoelectric signals and perform the appropriate movement [17]. In addition, the Dexus arm can be used to perform tasks similar to the other prostheses, like writing and playing the piano [18]. Regardless of the successes of the Dexus arm during testing, it is a socket-based prosthesis, so there is no ability for osseointegration. Most importantly, the device alone cannot provide somatosensory feedback. Even so, the e-dermis—an “electronic skin” made of fabric, rubber, and sensors that transmit electrical information to the peripheral nervous system—could be used to cover the Dexus arm and allow amputees to sense temperature and pain [19].
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In spite of the limitations and design changes needed to maximize the functionality and cost effectiveness of recently developed prosthetic technology, devices like the DEKA arm, Dexus arm, and MPL are much more effective than the most common upper extremity prostheses, which often use sockets, hooks, and cables that can be uncomfortable and cumbersome to operate. As a result, many amputees forgo using a prosthesis and choose to carry on with their daily lives, in which simple tasks like brushing one’s teeth can take an exorbitant amount of time. For amputees who have lost the ability to perform such basic tasks, robotic prostheses that enable them to regain limb function can restore their sense of independence. Fortunately, the development of the bionic arm may be yet another instance in which science fiction becomes science fact.

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Works Cited:

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