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Cynthia Thai WRIT340 Section 66806 December 6, 2013

Mechanical Sensation: Prosthetic Arms Capable of Feeling

Abstract

Despite modern technology and ongoing research, prosthetic arms have not changed

much since the late 1960s and restore very little function of a normal hand. For above the elbow

amputees, the few available options were cumbersome to use and it was not until recently that a

technique known as targeted muscle reinnervation enabled prostheses to be more intuitive to

control. Furthermore, current research has shown that the restoration of sensation for amputees

may become a reality in the near future with reinnervation applications.

Introduction

Our arms and hands are versatile instruments that we use everyday for various tasks,

from the menial task of opening a water bottle and bringing it to our mouths, to the particular

task of threading a needle. They enable our independence, from the ability to button up a shirt to

the capacity of tying one’s shoes. They are what we use to engage in our communities, to greet

people with a handshake or a hug, as well as to clap after a speech or performance. As a result,

the lack of a limb, whether it be due to trauma, lost to disease, or congenital, causes physical,

emotional, as well as social impediments. As of 2005, there are nearly two million people living

in the United States with limb loss [1]. Thanks to modern technology and ongoing research, lost

limbs can be replaced with increasingly advanced prosthetics to restore some function. Yet, of all

the extremity prosthetics, the hand has been presented as a more difficult tool to restore due to its

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high degrees of freedom and range of sensory feedback capabilities [8]. As of 2012, nearly half

of upper extremity amputees choose to not use a prosthetic altogether [3].

History of Prosthetics

The loss of a limb has been a problem for centuries

and attempts to replace it with a prosthetic began as early as

218 B.C. in Rome. General Marcus Sergius lost his right

hand during the Second Punic War and had an iron hand

made to support his shield during the continued battle [2].

Yet, the most well known upper extremity prosthetic was the

Iron Hand of German Imperial Knight Gottfried "Götz" von

Berlichingen. After losing his right hand in the 1504 Siege

of Landshut, he had an iron hand constructed using the most

advanced machinery of that time. The iron glove had leather

straps to tighten the iron hand onto the stump (Fig. 1). Gearwheels allowed for the movement of

the fingers which, combined with a rigid thumb, enabled a working grasp [3]. Therefore, the

hand mostly served to support his sword or to hold horse reins but not any more sophisticated

actions [4].

It was not until 1812 that there was a breakthrough in the development of prosthetic arms.

Berlin dentist Peter Baliff altered the design by powering the movement of the fingers using the

shoulder or trunk but his invention only applied to forearm amputees who still had use of the

elbow [3]. After World War II, attention was again brought back to the need for progress in

prosthetics as nearly 15,000 U.S. soldiers came home with the loss of one or more limbs [5].

Figure  1:  The  infamous  Iron  Hand  designed  for  Gottfried  von  Berlichingen,  displaying  the  leather  straps  as  well  as  mechanisms  for  several  joint  movements    

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Technology had progressed to using electric motors to flex and extend the fingers [3]. With its

introduction in the late 1960s, the myoelectric arm stands as the present forefront of upper

extremity prostheses.

Current Prosthetic Arms

Today, upper extremity amputees can choose

from a variety of different prostheses that differ from

the way it is powered to the type of terminal device

they choose to take the place of the hand. The terminal

device may be a cosmetically natural hand, a

cosmetically mechanical hand, a split hook, or a

prehensor [6]. While a cosmetic hand may be more aesthetically pleasing, it weighs much more

than the human hand and tends to have a weaker grip [8]. In comparison, the split hook is a

popular choice due to its minimal weight, ease of use, and durability (Fig. 2). A prehensor,

resembling a lobster claw, is also durable but mostly used for heavy lifting and climbing due to

its grip strength (Fig. 3).

Depending on the lifestyle of the amputee, he or she may opt for a

passive prosthesis, mainly used for cosmetic purposes, or a functional

prosthesis, which may or may not provide a cosmetic appearance but

enables its user to complete varying tasks [6]. Functional prosthetics can be

body-powered, myoelectric, or a combination of both. A body-powered

device employs cables to use the power provided from movement of a

specific part of the body, usually the shoulders, in order to open

Figure  2:  The  split  hook  is  approximately  one  third  of  the  weight  of  a  mechanical  hand.    

Figure  3:  The  prehensor  is  mainly  used  for  athletic  activities.  

Figure  4:  The  top  drawing  shows  the  anterior  view  while  the  bottom  drawing  shows  the  posterior  view  of  a  typical  harness  used  with  a  body-­‐powered  prosthesis.  

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or close the terminal device. As a result, straps and harnesses attach the prosthetic arm to the

body from the opposite shoulder (Fig. 4).

In contrast, a myoelectric prosthesis uses suction to attach to the stump and involves

electric motors powered by a battery [6]. As a result, it provides a stronger grip but is usually

heavier than a body-powered prosthesis. When we contract a muscle, a small glint of electricity

is generated that can be recorded using sensors. Thus, by placing electrodes on the skin above

that muscle, the myoelectric arm is able to detect the electric signal in a contracted muscle. This

signal is amplified and translated for the electric motor to direct the terminal device into a certain

position [7].

While these systems improve the conditions of amputees below the elbow greatly, the

same benefits are not seen in bilateral shoulder disarticulation amputees since these methods

enable only one action to be carried out at a time. For an amputee who lacks both arms above the

elbow, it is cumbersome and slow to sequentially attain and lock an elbow bend, change modes

to rotate the wrist, and finally switch modes to position the terminal device [9]. In addition, these

actions involve awkward contractions of the muscles of the remaining arm or shoulders and thus

are counterintuitive [9]. One method to make the myoelectric arm more intuitive and natural is to

translate the thought of moving the original arm to a direct order for the prosthesis to carry out

that action, a process achieved through targeted muscle reinnervation.

How Targeted Muscle Reinnervation Works

When we want to contract a muscle, the motor cortex of the brain sends a signal down the

spinal cord and out to the nerves innervating the muscle of interest. In the case of the hand, the

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nerve travels through the brachial plexus, a network of

nerves located in the shoulder that innervate the arm and

eventually reach the fingertips (Fig. 5).

Even when an arm is amputated, the commands

from the brain to that original arm still travel to the

brachial plexus nerves but the targeted muscle is no longer there for stimulation [10]. The

relocation of the brachial plexus nerves to the muscles of the chest enables these commands to be

biologically amplified and programmed for the control of the myoelectric arm. This involves

removing the existing nerves and most of the fat of that chest muscle segment, thus enabling

sensors to better detect minute signals. When the patient thought about opening and closing the

fist, areas of the reinnervated chest muscle would twitch and the command would be passed onto

motors of the myoelectric arm [9]. This results in an intuitive technique to operate the prosthetic

arm simply through the thought of the action. Not only does this allow for minimal training with

the prosthesis, the technique enables several actions to be executed by different parts of the

prosthesis at the same time.

Astonishingly, after the nerves were established throughout the chest muscle, the patient

reported being able to feel his phantom hand just from touching specific areas of the reinnervated

chest muscle [9]. He was able to sense temperature, texture, and touch of a specific part of the

hand due to the nerves reinnervating not only the chest muscle but also the skin of the chest.

Sensation travels the same nerve routes as the sequence of a command to the muscle, just in the

opposite direction by running up the arm, up the spinal cord, to be processed in the sensory

Figure  5:  The  Brachial  plexus  is  a  conglomerate  of  nerve  fibers  in  the  shoulder  that  innervate  the  arm  down  to  the  fingertips.  

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cortex of the brain. Correspondingly, if the brain could still send commands to the muscle, then

theoretically, sensory information could also be sent using the same route back to the brain.

What Constitutes Feeling

While picking a grape off a table and bringing it to our mouths without squishing or

dropping it may seem like a very simple task, it involves a complicated series of events involving

proprioception and mechanoreception. Pressure sensory feedback of the fingertips from

mechanoreceptors informs our hands how much force to use to hold the grape.

Mechanoreceptors are also responsible for informing us the texture of the surface of the grape as

well as how much the fingers are stretching to hold onto the grape. Proprioceptors are sensors for

our spatial position and are responsible for how we reach out to pick up the grape as well as

know exactly where to bring it to our mouths. Unfortunately, prostheses users are unable to sense

where the arm is and must visually ascertain its spatial position. As for mechanoreception,

ongoing research explores how we can use the remaining nerve pathways of the arm to restore

sensory feedback to amputees, but while much is known about how our fingertips feel, little is

known in correspondence to the areas of the brain that process this information.

Researchers at Cleveland Veterans Affairs Medical Center and Case Western Reserve

University have developed a new type of interface that is able to convey the sense of touch from

twenty areas on a prosthetic hand [11]. This is done by carefully discerning which areas of the

skin of the stump correspond to sensations of the palm, fingers, and the rest of the hand. Twenty

electrodes are used to deliver electrical signals to the nerve fibers from outside a protective

sheath of living cells that are located next to those nerve fibers [11]. Tests are performed where

the subject uses their prosthetic hands to pluck the stem off a cherry with the goal of not

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squishing the cherry. The sensation is mostly accurate and these implants have continued to work

after eighteen months, a milestone showing that the electrical interfaces to the nerve fibers have

not degraded in performance [11].

A less invasive and less expensive product was invented in New Zealand to be simply

slipped over a prosthetic arm. Daniel Kamp created “Sensitive Touch,” which are marketed as

“intelligent gloves for prosthetic arms” [12]. It features an intelligent fabric that enables an

electrical current to continuously run through each individual sensor in the thumb, each of the

four fingers, and the palm [12]. These different pressures on each sensor will cause a change in

electrical resistivity, which are then translated into electrical outputs. Finally, electrical pulses

that correspond to the stimulation of the glove are felt on finger, thumb and palm representations

on a cuff [12]. Giving back sensation to amputees helps to rid the disabling practical and

emotional issues caused by the lack of sensitivity.

Reaching Forward

While rerouting the existing sequence of muscle control and sensation feedback can

greatly increase the efficiency of the myoelectric arm, many researchers are searching for

alternative routes that will facilitate a direct connection from the brain to the prosthesis. One

such method involves using the brain-computer interface, a system that is able to skip the spinal

cord and still convey its messages to the nerves of the muscles [13]. This would be particularly

monumental for patients with severe motor disabilities due to the lack of access to the spinal

cord. The system uses electrodes on the scalp to record and translate activity to an external

device that can then deliver the command to the myoelectric arm [13]. The first human patient to

participate in a study involving the brain-computer interface system was a quadriplegic woman.

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She underwent surgery for the implantation of microelectrodes into the motor cortex of the brain

[14]. Within two weeks, she was able to freely move a robotic arm simply using her mind.

However, the robotic arm did not provide sensory or spatial feedback and therefore required

visual assistance.

Further developments can potentially allow this interface system to be applied to

prosthetics but will indefinitely add to the already steep cost of purchasing and maintaining

prostheses. Continued research will inevitably progress the quality of prostheses but is less likely

to drive costs down. Some have already turned to 3D printing as a cheaper means to assemble

artificial limbs, but the ease of production is not enough to advance prostheses. What truly is

needed is a better grasp of how the brain works in order to find a more direct hub where

researchers can utilize for controlling artificial limbs. By combining this knowledge with 3D

printing, perhaps in the near future, amputees will be able to reach out for normal life, one in

which batteries last longer, sensations are a basic component, and costs are lower.

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References

[1] Ziegler-Graham K, MacKenzie EJ, Ephraim PL, Travison TG, Brookmeyer R. Estimating the

Prevalence of Limb Loss in the United States: 2005 to 2050. Archives of Physical

Medicine and Rehabilitation2008; 89(3):422-9.

[2] Meier, R. H., and D. J. Atkins. "History of Arm Amputation, Prosthetic Restoration, and Arm

Amputation Rehabilitation."Functional Restoration of Adults and Children with Upper

Extremity Amputation. New York, N.Y.: Demos Medical Pub., 2004. 1-7.

[3] "Prosthetic Arms Through the Ages." MEDICA Trade Fair. N.p., n.d. Web. 8 Oct. 2013.

<http://www.medica-

tradefair.com/cipp/md_medica/custom/pub/content,oid,23173/lang,2/ticket,g_u_e_s_t/~/

Prosthetic_Arms_through_the_Ages.html>.

[4] Phillips, Gordon. "Artificial Arms and Hands." Best Foot Forward. Cambridge: Granta

Editions, 1990. 40-41.

[5] "U.S. Military Builds on Rich History of Amputee Care." Amputee Coalition of America.

N.p., n.d. Web. 10 Oct. 2013. <http://www.amputee-coalition.org/military-instep/rich-

history.html>.  

[6] “Prosthetic Devices for Upper-Extremity Amputees." Amputee Coalition of America. N.p.,

n.d. Web. 10 Oct. 2013. <http://www.amputee-coalition.org/military-instep/rich-

history.html>.

[7] "How Myoelectric Prosthetics Work." Myoelectric Prosthetics. N.p., n.d. Web. 9 Oct. 2013.

<http://www.myoelectricprosthetics.com/>

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[8] Billock, John. "Upper Limb Prosthetic Terminal Devices: Hands Versus Hooks. “Clinical

Prosthetics and Orthotics 10 (1986): 7. The American Academy of Prosthetics and

Orthotics. Web. 10 Oct. 2013.

[9] Kuiken, Todd, Laura Miller, Robert Lipschutz, Blair Lock, Kathy Stubblefield, Paul

Marasco, Ping Zhou, and Gregory Dumanian. "Targeted Reinnervation for Enhanced

Prosthetic Arm Function in a Woman with a Proximal Amputation: A Case Study." The

Lancet 369.9559 (2007): 371-380. Science Direct. Web. 10 Oct. 2013.

[10] Kuiken, T.A., G.A. Dumanian, R.D. Lipschutz, L.A. Miller, and K.A. Stubblefield. "The

Use of Targeted Muscle Reinnervation for Improved Myoelectric Prosthesis Control in a

Bilateral Shoulder Disarticulation Amputee." Prosthetics and Orthotics

International 28.3 (2004): 245-253.  

[11] Talbot, David . "An Artificial Hand with Real Feelings." MIT Technology Review. N.p., 5

Dec. 2013. Web. 5 Dec. 2013. <http://www.technologyreview.com/news/522086/an-

artificial-hand-with-real-feelings/>.

[12] Kamp, Daniel. "Sensitive Touch." Behance. N.p., 4 Jan. 2012. Web. 5 Dec. 2013.

<http://www.behance.net/gallery/Sensitive-Touch-intelligent-gloves-for-prosthetic-

arms/2448037>.

[13] Yahud, Shuhaida, and N.A. Abu Osman. "Prosthetic Hand for the Brain-computer Interface

System." IFMBE Proceedings 15 (2007): 643-646. Link Springer. Web. 10 Oct. 2013.

[14] Choi, Charles. "Quadriplegic Woman Moves Robot Arm With Her Mind." Live Science.

N.p., 17 Dec. 2012. Web. 18 Oct. 2013. <http://www.livescience.com/25600-

quadriplegic-mind-controlled-prosthetic.html>.

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(Fig. 1)  "Prosthetic Arms Through the Ages." MEDICA Trade Fair. N.p., n.d. Web. 8 Oct. 2013.

<http://www.medica-

tradefair.com/cipp/md_medica/custom/pub/content,oid,23173/lang,2/ticket,g_u_e_s_t/~/

Prosthetic_Arms_through_the_Ages.html>.

(Fig. 2) "Hosmer Model 5." Amputee Supplies. N.p., n.d. Web. 8 Oct. 2013.

<http://amputeesupplies.com/products/model5-prosthetic-hook.html>.

(Fig. 3) "Replacement Grip." TRS Inc. N.p., n.d. Web. 12 Oct. 2013.

<www.oandp.com/products/trs/adults/grip_3_fingers.asp>.

(Fig. 4) “Body-Powered Prostheses." Upper Limb Prosthetics Information. N.p., n.d. Web. 12

Oct. 2013. <http://www.upperlimbprosthetics.info/index.php?p=1_9_Body-Powered>.

(Fig. 5) "Complications Brachial Plexus Nerve Block." Health Central. N.p., n.d. Web. 14 Oct.

2013. <http://www.healthcentral.com/chronic-pain/h/complications-brachial-plexus-

nerve-block.html>.