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The Future of Artificial Limbs

Chapter 27

THE FUTURE OF ARTIFICIAL LIMBS

JONATHON SENSINGER, PhD*; PAUL F. PASQUINA, MD†; and TODD KUIKEN, MD, PhD‡

INTRODUCTION

LIMB INTERFACE SYSTEMS

Sockets and Liners

Artificial Condyles

Osseointegration

CONTROL SYSTEMS

Neural–Machine Interfaces

Artificial Intelligence

MECHATRONICS

Power

Actuators and Transmissions

Sensors and Controls

SUMMARY

 

*Assistant Research Professor, Neural Engineering Center for Artificial Limbs, Rehabilitation Institute of Chicago, Room 1309, 345 East Superior Street, 

Chicago, Illinois 60611

†

Colonel, Medical Corps, US Army; Chair, Integrated Department of Orthopaedics and Rehabilitation, Walter Reed Army Medical Center and National 

Naval Medical Center, Section 3J, 6900 Georgia Avenue, NW, Washington, DC 20307

‡

Director of Amputee Services, Department of Physical Medicine and Rehabilitation, Rehabilitation Institute of Chicago, Room 1309, 345 East Superior 

Street, Chicago, Illinois 60611
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Care of the Combat Amputee

INTRODUCTION

In recent years a remarkable convergence of new 

science, technology, public awareness, and funding 

have instigated the development of advanced artificial 

limbs. For the sake of discussion, research into artificial 

limbs can be broadly grouped into three different areas: 

(1) limb interface systems, (2) control systems, and 

(3) mechatronics. This chapter will document recent 

developments in these fields, discussing future directions of various innovations as well as areas in need 

of further research.

LIMB INTERFACE SYSTEMS

Arguably the most important part of any prosthesis 

is the limb interface system, which must provide both 

function and comfort. In terms of function, the limb 

interface system must transfer loads between the prosthesis and the user, and it must allow the user to control 

the position of the prosthesis. In terms of comfort, the 

system must allow expedient donning and doffing 

and be reasonably comfortable while worn, or else the 

device will not be used.

Sockets and Liners 

Advancements have been made to both upper and 

lower extremity prosthesis sockets and liners, primarily through enhanced materials. Carbon graphite sockets now offer greater durability at a lighter weight. The 

incorporation of flexible materials within the socket 

may offer a more adaptable and comfortable socket. 

Custom fitting can be enhanced by the use of 

computer-aided design and computer-aided manufacturing (CADCAM). Computer-aided design has 

the potential to transform socket fitting from an art to 

a science. As the understanding of tissue compliance, 

loading, and force transfer improves, socket-fitting 

performance and reliability should also improve. In 

the future, fitting may be completely automated using 

three-dimensional imagining techniques such as magnetic resonance imagery or computed tomography. 

Computer-aided manufacturing (CAM) technology 

also holds promise for improving prostheses because 

it could reduce fabrication errors and inconsistencies, 

thereby increasing quality. CAM could also greatly 

reduce socket fabrication time, allowing amputees 

the ability to be trained with a custom prosthesis 

sooner. Finally, CAM could reduce fabrication costs. 

“Squirt Shape,” a technique under development at 

Northwestern University, is an example of promising 

CAM research.1 Unlike many other CAM products 

that create a socket by removing material from a blank, 

Squirt Shape places material only where it will be used 

in the socket. As a result, no material is wasted in the 

process of extruding the socket from a block, resulting in increased efficiency and decreased waste, and 

material cost of only $1 per socket. Squirt Shape also 

uses an automated process to acquire the model of 

the residual limb, resulting in an accurate and precise 

model in a short time.

Advances in custom fitting techniques and materials have allowed better suction suspension systems. 

The Vacuum-Assisted Socket System has been introduced by TEC Interface Systems (St Cloud, Minn) 

and Otto Bock HealthCare (Minneapolis, Minn). The 

principle behind this design is to create negative pressure within the socket for suspension, particularly 

during the swing phase of gait, which may improve 

residual limb perfusion, reduce limb volume changes, 

and improve fit and comfort.2,3 Although objective 

clinical trials of this technology are lacking, it clearly 

shows promise for lower limb amputees. Reductions 

in size and mass will be beneficial. The application for 

dynamic vacuum suspension systems in upper limb 

amputees is also being investigated. A simpler means 

to achieve suction suspension has been introduced by 

Ossur (Reykjavik, Iceland) in the Iceross Seal-In liner.4

This system incorporates a membrane lip placed circumferentially around the distal aspect of the liner to 

cause a “plunger” effect and create negative pressure 

when moving from stance to the swing phase of gait. 

Advances in upper extremity sockets allow selfsuspension at the long transradial and wrist disarticulation level and minimize restriction of elbow flexion 

as well as pronation and supination. Additionally, 

inventive ways have been developed to incorporate 

myoelectric sensors and metal connections within 

silicone and elastomeric liners to improve the consistency of electromyogram (EMG) signal acquisition and 

improve control of myoelectric prostheses.5,6

Artificial Condyles

There are significant advantages to joint disarticulation amputation, including improved suspension from 

remaining condyles, fixation of the condyles (an important feature in fleshy limbs), rotational stabilization, 

better weight bearing through the end of the residual 

limb, and preservation of distal muscle attachment (eg, 

the adductors in a knee disarticulation). However, joint 

disarticulations are not commonly performed because 
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The Future of Artificial Limbs

when a socket and hinges are placed over the residual 

limb, the socket is bulky, the limb is functionally too 

long, cosmesis is poor, and the hinges cause problems 

with clothing (although some polycentric designs can 

minimize these effects).

Alternative surgical approaches have been used 

to preserve the advantages of a joint disarticulation 

and mitigate the disadvantages. Bone modification 

can sometimes be done surgically. A midhumeral or 

midfemoral osteotomy can be done in conjunction 

with an elbow or knee disarticulation, respectively. 

This preserves the condyles (for suspension), rotational 

control, and distal muscle insertions while providing 

room for a prosthetic joint. Similarly, a humeral angulation osteotomy can sometimes be performed to create a 

bony element for suspension and rotational control of a 

transhumeral prosthesis. Unfortunately, these options 

are rarely available for traumatic amputees due to the 

inability to perform a disarticulation or lack of bone 

available for an angulation osteotomy.

Another approach under development is implantation of artificial condyles. Termed “subfascial implant 

supported attachments,” these implants have been 

inserted in seven patients with transhumeral amputations.7

 This concept presents a viable alternative for 

individuals whose amputation was caused by trauma, 

leaving them without sufficiently long bone to undergo 

other surgical approaches. Initial implants have met 

with success in five out of the seven subjects, and future 

refinements of the condyle geometry should provide 

improved success.

Osseointegration 

All the systems described above rely on some type of 

socket interface. The inherent problem with socket interfaces is that soft tissues are between the load-bearing 

skeletal structures and the rigid socket systems. These 

soft tissues are very compliant and thus not efficient in 

transferring loads. Soft tissues are ill suited for localized 

high pressure and load bearing, which generally leads 

to some level of discomfort and frequently causes skin 

breakdown. Finally, socket systems encase the residual 

limb, retaining heat and moisture and providing an 

environment conducive to bacterial growth. 

An appealing alternative is the concept of directly 

attaching prostheses to the skeletal structure, called 

“osseointegration.” Direct skeletal attachment could 

alleviate the inherent problems of sockets and provide 

a very efficient mechanical interface for a prosthesis. 

The first successful model for osseointegration was the 

dental implant method developed by Swedish bioengineer Per-Ingvar Brånemark. Today, the integration of 

titanium dental implants is used worldwide. Dr Brånemark’s laboratory also performed the first successful 

osseointegration procedure for artificial limbs, with 

direct skeletal attachment of prostheses in amputees 

with short transfemoral amputations.8 Osseointegration has now been performed in many levels of both 

upper and lower limb amputations. Clinical trials are 

ongoing in Sweden, England, Australia, Germany, and 

Spain, and animal studies are being performed in the 

United States. 

Osseointegration surgery is performed in two 

stages. First, a metallic fixture is inserted into the medullary cavity of the bone, and the skin is closed over 

the fixture. Bony ingrowth occurs around the fixture 

over 3 to 6 months. A pin-line “abutment” is placed 

into the fixture during the second surgery. An opening through the skin allows the abutment to protrude 

and serve as the interface for the prosthetic device. A 

progressive weight-bearing schedule is started soon 

after surgery. The benefits reported include a secure 

and rigid attachment that allows excellent mechanical control. Osseointegration systems eliminate the 

problems with prosthesis sockets described above, 

providing greater comfort. Additionally, recipients 

report improved sensory feedback from their directly 

skeletally attached limb. “Osseoperception”and pro

prioception are improved because stiffness between the 

residual limb and the prosthesis is greatly increased, 

giving more accurate sensing of the endpoint of the 

prosthesis. A number of technical challenges remain 

to be solved before widespread acceptance and use 

can be expected. The fairly high incidence of infection 

at the percutaneous interface is of significant concern. 

Bone resorption, osteomyelitis, and abutment failures 

are additional complications. Although this procedure 

is not being performed in the United States because 

of the relatively high complication rate,9 the potential 

benefits are enormous and have inspired ongoing basic 

science research.

CONTROL SYSTEMS

The lack of control in upper limb prostheses is a 

severely limiting factor for function, especially for 

high-level amputees in whom disability is greatest. Current devices use primarily shoulder motion 

transmitted through Bowden cables or myoelectric 

control. Body-powered systems allow control of only 

one degree of freedom at a time with shoulder motion. Myoelectric prostheses also allow operation of 

only one joint at a time. Myoelectric prostheses are 

intuitive for transradial amputees to use because 
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Care of the Combat Amputee

hand flexion and extension muscles are present to 

operate the motorized hand. With higher levels of 

amputation, however, control is less intuitive because 

proximal muscles are now used to control distal arm 

functions. With both body-powered and myoelectric 

systems the hand, wrist, and elbow must be controlled 

sequentially, which is cumbersome and slow. Bodypowered and myoelectric control can be combined in 

a hybrid approach that allows simultaneous operation 

of two joints, although the cognitive burden is high. 

Finally, existing prostheses provide very little sensory 

feedback. Body-powered devices give some sensory 

feedback because the user can feel how hard they are 

pulling the cable, but no current device provides touch 

feedback or sensation during object interaction.

The control of lower limb prostheses is also an 

important issue. Although significant dexterity and 

multiple degrees of freedom are not required, as 

with the upper limb, excellent control is required for 

maximal mobility and safety. Once again, the higher 

the amputation level, the greater the challenge and 

need. Significant advances have been made in recent 

years with computerized knee systems that attempt 

to predict the intent of the user and adapt to different 

gait patterns. These systems acquire information only 

from sensors in the prosthesis, however, and so must 

rely on other control sources, such as push buttons or 

key fobs, to allow the user to switch gait modes.

Neural–Machine Interfaces

To address all the above problems, a neural–machine interface that can provide motor commands to 

operate the prosthesis and serve as a conduit to provide 

sensory feedback is needed. Three types of neural 

interfaces are currently being investigated: (1) brain–

machine interfaces, (2) peripheral nerve interfaces, and 

(3) targeted reinnervation. Most limb control signals 

originate in the brain. As a result, it seems intuitive to 

tap directly into the source of this information (Figure 

27-1), rather than capture it en route (peripheral nerve 

interfaces) or translate by-products of its endpoint 

(myoelectric control and targeted reinnervation). A recent increase of encouraging work in this field, termed 

“brain–machine interfaces,” involves individuals with 

spinal cord injury who are able to control a pointer on 

a computer screen.10–12

Two roadblocks must be overcome before this 

technology can be considered for prosthetic use. The 

first involves the complexity of the brain’s information 

structure, and our ignorance of how it works. Although 

understanding of the brain has grown exponentially 

in recent decades, very little is understood, from a 

neuron-to-neuron basis, about how the brain transFigure 27-1. The University of Utah’s cortical array (inset) 

records activity from individual neurons in the brain and 

transmits the signals to a processor outside of the brain. These 

signals may be used to stimulate muscles of a paralyzed limb, 

or to control a prosthesis if the limb is missing.

Drawing: Courtesy of John Hopkins University Applied 

Physics Laboratory and the University of Utah.

mits control signals to execute complex movements. 

Debate remains over the mechanisms involved (eg, 

force control, position control, synergies). These concepts must be much better understood to bridge the 

gap between simple control of a cursor and intuitive 

control of complex trajectories. The second roadblock 

involves physical connection of the sensor to the brain. 

Infection around implanted sensors is still a concern, 

as is heat dissipation for wireless systems. Fixation of 

sensors to a finite neuron for a long period of time is 

also difficult. These roadblocks are substantial, but a 

great deal of basic science research is currently devoted 

to surmounting them. Brain–machine interfaces might 

not be used in prostheses for some time, but they are 

likely to be adopted much earlier for patients with 

spinal cord injuries.

In peripheral nerve interfaces, electrodes are directly 

connected to the residual nerves of the amputee, and 

the electric signal from the nerve is used to control the 

artificial limb13–16 (Figure 27-2). Although this concept 

offers the potential for improved control, it has several 

inherent problems such as the fragility of nervous 

tissue and permanence of electrode array fixation.17

In addition, the neuroelectric signal is very small, difficult to record, and difficult to separate from EMGs of 

the surrounding muscle.18 Additional challenges arise 

in transmitting signals from the nerve to an external 

device, which requires either persistent percutaneous 
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The Future of Artificial Limbs

Figure 27-2. The Utah slant array (inset) records activity 

from individual nerve fascicles of a peripheral nerve. It is 

implanted into the muscle adjacent to the nerves, as shown 

in the image in the lower right quadrant of the figure. The 

slanted tips ensure that nerve fascicles at different depths of 

the nerves are probed. These signals may then be transmitted 

to a controller, which in turn controls a prosthesis. 

Drawing: Courtesy of John Hopkins University Applied 

Physics Laboratory and the University of Utah.

wires (which tend to become infected) or complex 

transmitter–receiver systems. Finally, the durability 

of the implanted hardware is a critical issue. Prosthetic control systems must function for decades, and 

implanted neuroelectric control systems may require 

surgery to repair. At this time several laboratories are 

making progress toward solving these problems. As 

a result, peripheral nerve interfaces may one day be 

widely used, allowing intuitive, finely tuned control.

 Targeted muscle reinnervation (TMR) is a new 

technique that improves the function of myoelectric 

upper limb prostheses by creating new myosites19,20 

(Figure 27-3). TMR transfers residual nerves from an 

amputated limb onto alternative muscle groups that 

are not biomechanically functional due to the amputation. The target muscles are denervated prior to the 

nerve transfer. The reinnervated muscle then serves as 

a biological amplifier of the amputated nerve motor 

commands.14 TMR thus provides physiologically appropriate surface EMG control signals that are related 

to functions in the lost arm. TMR with multiple nerve 

transfers provides simultaneous, intuitive control of 

multiple degrees of freedom via the motoneuron activity originally associated with the amputated muscles. 

Great success has been achieved in clinical practice for 

myoelectric prosthesis control. 

Using simple myoelectric control paradigms based 

on amplitude measurement of the EMG signal deFigure 27-3. Targeted muscle reinnervation reroutes nerves 

to remaining, nonfunctional muscles. Instead of using artificial implantable sensors, this technique allows the muscles 

themselves to act as biological amplifiers. Conventional surface electrodes may then be used to sense these new control 

signals. Signals from these electrodes are transmitted to a 

microcontroller, which controls the prosthesis.

veloped from each discrete target muscle region, the 

first four successful TMR patients have demonstrated 

the ability, for the first time, to control two degrees 

of freedom simultaneously using only EMG signals. 

Functional task performance has been evaluated using the box and block test and clothespin test. The 

subjects have shown a 2.5- to 7-fold increase in speed 

for task completion. Subjectively, they have reported 

significantly easier and more natural control of their 

prostheses.20–23

More recent research combines TMR with advanced 

signal processing techniques to further improve the 

control of artificial arms. Using pattern recognition 

classification algorithms, control of advanced prosthetic arms has been demonstrated, including the 

operation of powered elbows, two-function wrists, 

and multifunction hands. Targeted reinnervation can 
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Care of the Combat Amputee

also provide physiologically appropriate cutaneous 

sensation feedback. The afferent fibers in the residual 

nerves can be directed to reinnervate different residual 

limb skin, for example, the chest skin of a shoulder 

disarticulation patient. When this reinnervated skin 

is touched, it feels like the missing hand is being 

touched. Studies23–25 show that the patient can feel 

very light touch, graded pressure, heat, cold, sharp 

and dull sensations, and pain—all as if it were in the 

missing hand. 

Current research is being performed to integrate 

sensors in the artificial hand and tactors (devices that 

appropriately stimulate the reinnervated skin) into 

advanced limb systems to provide patients with useful 

sensory feedback. Broader clinical trials are in progress 

with current arm systems. Laboratory trials with advanced arm systems are also in progress, and clinical 

trials with advanced arm systems are planned for the 

near future. Targeted reinnervation with the potential 

to improve the function of lower limb prostheses is in 

the very early stages of development.

Artificial Intelligence

Humans adjust their grasp or gait based on many 

low-level, unconscious control decisions. If a glass is 

about to slip, people instinctively tighten their grasp; 

likewise, as water is poured into a cup, the hand reflexively tightens the grasp. When people start to walk 

faster, their muscles fire more strongly to prevent the 

leg from jerking to a stop. This same type of low-level 

control, termed “artificial intelligence,” is even more 

beneficial in the field of prosthetics, where there is 

a paucity of control channels. With so few ways to 

control a prosthesis, it would be ideal for the user to 

direct only the highest level of control, allowing the 

prosthesis to provide lower level decisions. Several 

implementations of this concept have been introduced 

into prostheses. Examples include the following:

•	 the Otto Bock SensorHand Speed hand (Otto 

Bock Healthcare), which senses when an object 

is about to slip and tightens its grasp;

•	 the Otto Bock C-Leg (Otto Bock Healthcare), 

which changes knee resistance depending on 

the activity required as determined by the 

prosthesis; and 

•	 the Ossur RHEO KNEE (Ossur, Aliso Viejo, 

California), which continuously adapts to provide 

better low-level control of the prosthetic knee 

throughout the entire life span of the prosthesis.

Future artificial intelligence technology may 

play a crucial role in orchestrating the control of 

prosthetic hands with multiple actuated fingers, 

the interaction of powered lower limb prostheses 

with the environment to provide proper dynamics, 

the low-level coordination of prostheses for subjects 

with bilateral amputation, and many other areas of 

control.

MECHATRONICS

Powered prostheses consist of three general components: (1) a power source, (2) an actuator and 

transmission system, and (3) some form of feedback. 

These components largely parallel the human body. 

The human arm has two sources of power: (1) the 

complex network of energy in the form of sugars 

and fats, and (2) tendons (often forgotten), which 

absorb and transmit energy during various phases of 

activities. The actuator in the human arm is muscle, 

which generates an incredible amount of force but 

contracts only a minute amount. To move the arm 

great distances, a transmission is needed, which is 

found in the bones and muscle insertions. Arm bones 

and muscle act as lever arms to convert large forces 

and small movements into smaller forces and larger 

movements. Finally, the feedback system consists of 

various layers of sensors and neurons with complex 

decisions made at various levels, including the spinal 

cord and brain. In a robotic prosthesis (Figure 27-4) 

the energy source is typically a battery, the actuator is 

typically an electric motor, the transmission is usually 

a gear drive, and types of feedback vary, from simple 

wires to microprocessors.

Power

The most important characteristic of a power source 

is its energy-to-weight density. Other important characteristics include maximum discharge and recharge 

rate, the ease of recharging, and the safety of the power 

source in the event of a failure.

Nickel cadmium batteries are slowly being replaced 

by lithium ion batteries, which offer triple the amount 

of energy for the same weight. Lithium ion batteries 

are standard for applications such as power drills. 

Lithium polymer batteries offer even better densities 

than lithium ion batteries because they require less 

packaging. They also conform more easily to a given 

space (they aren’t required to be rectangular boxes and 

can follow limb contours), and so represent an ideal 
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The Future of Artificial Limbs

Figure 27-4. The left arm of the mannequin is an initial prototype of a robotic prosthetic arm designed to provide greater 

function to people who have had amputations at the shoulder, whereas the right arm is a traditional body-powered 

prosthetic arm. The robotic prosthetic arm was made by 

DEKA Research & Development Corporation (Manchester, 

NH) for the Defense Advanced Research Projects Agency 

(DARPA). It uses lithium ion batteries for power, brushless 

DC motors, and a variety of sensors including encoders, Hall 

effect sensors, and load cells. 

Photograph: Courtesy of DEKA Research & Development 

Corporation.

power source for cosmetically appealing prostheses. 

Several recent nanoscale advances are now commercially available, and will likely be accepted into prosthetic use in the next 5 to 10 years. These batteries offer 

increased efficiency and hold the promise of increased 

power density. Methanol fuel cells, which have over 

ten times the density of lithium ion batteries, may soon 

be used to power laptops and could be inserted into 

prostheses in the near short term. However, they require improvement in several areas before commercial 

implementation, including temperature management, 

water management, and flow control.

The elbow flexion assist in the Otto Bock Ergo Arm 

(Otto Bock HealthCare) is an excellent example of a 

power supply that counteracts the power lost to gravity. Likewise, clutches resist gravity without consuming power, and so may be thought of as a power source. 

Human tendons are extremely efficient at storing and 

releasing energy, much more so than electrical regenerative systems such as those found in hybrid cars. A 

biomimetic advancement in prostheses is the introduction of artificial tendons, typically through a spring.26

Denser power sources, however, have implications for 

other components. For example, the power density of 

lithium ion batteries makes the added complexity and 

weight of a clutch a detriment; the same result may 

be achieved with less added weight merely by using 

a slightly larger battery. If fuel cells become an option 

in prosthetics, other items such as elbow flexion assists 

may likewise become disadvantageous. 

Actuators and Transmissions

As with the power source, the most important 

characteristic of an actuator is its power density. Several technologies such as hydraulic, pneumatic, and 

electromagnetic motors actually have greater power 

density, and therefore better performance, than human 

muscle. Other cutting-edge technologies, such as nitinol, artificial muscles, and piezoelectric motors have 

significantly lower power density than human muscle. 

As those technologies improve, they may provide new 

avenues of discovery for prosthetics (Figure 27-5).

Hydraulic motors have long been advocated as a 

better actuator than the more conventional electromechanical motor because they have triple the power 

density. Commonly used in large construction equipment such as cranes, hydraulic motors lose much of 

their advantage, however, when they are miniaturized 

to fit into a hand, and recent miniature hydraulics have 

been unable to perform as well as equivalent electromechanical motors.

A possible alternative for prosthetics is the “cobot,” 

a concept developed for the automotive industry27,28

in which a central motor spins continuously, and additional, tiny motors tap into the power created by the 

central motor to actuate numerous joints simultaneously. This setup is appealing because it allows for fast, 

independent movement of multiple joints, as well as 

power grasps by all the joints acting in unity, similarly 

to the human hand. Electromechanical motors waste a 

substantial amount of energy speeding up and slowing down for each individual movement, and cobots 

solve this problem. They are likely to be useful only for 

actuation of the fingers for patients with transhumeral 

amputation.

The most important characteristic of a transmission 

component is its efficiency, which in prosthetic devices 

ranges from 30% to 95%. Planetary gears offer the 

highest efficiency rates, often over 90%, but are unable 

to withstand large torques in a small package. Helical 
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Care of the Combat Amputee

Figure 27-5. Intrinsic hand design, which allows such functions as the pinch-grip, created by the John Hopkins University Applied Physics Laboratory for the Defense Advanced 

Research Projects Agency (DARPA). All of the actuators are 

intrinsic within the space of the hand, allowing the device to 

be fit to patients with a long transradial amputation (actuator 

technologies such as hydraulics and cobots are excluded due 

to their large space requirements). 15 brushless DC motors 

are used to actuate 18 degrees of freedom. 

Drawing: Courtesy of John Hopkins University Applied 

Physics Laboratory.

teeth and ceramic materials improve efficiency and reduce noise, although increasing cost. Harmonic drives, 

used in the LTI Boston Digital Arm System (Liberating 

Technologies Inc, Holliston, Mass) provide significant 

speed adjustment in a single stage. However, they are 

not as efficient (70% maximum) as planetary gears, and 

they are noisier, although recent developments have 

made them significantly quieter in the Touch Bionic 

elbow (Touch Bionics, Livingston, United Kingdom). 

Other concepts, such as Ikona gears (Ikona Gear International Inc, Coquitlam, British Columbia, Canada) 

and cycloid gears, face similar problems. 

A promising development is continuously variable 

transmission (CVT), which can optimize the gear ratio 

to achieve maximum performance or efficiency in a 

variety of situations. The Otto Bock DynamicArm 

(Otto Bock Healthcare) uses a CVT, as does the cobot. 

CVTs typically involve increased cost and complexity, and often necessitate an additional motor. Several 

suggestions for CVTs in prostheses without the need 

for additional motors and electronics have recently 

been proposed.

Sensors and Controls

The ability to sense various torques allows such 

features as CVT, auto-grasp during a slip, advanced 

control schemes such as impedance control or minimum jerk trajectories, maximum power efficiency, 

and adaptive stance control of the knee. Sensor-based 

control is what transforms robotics into mechatronics, 

and it is in this area that the field of prosthetics will 

likely grow in the next 10 years. Microcontrollers are 

becoming increasingly small—many powerful microcontrollers are now smaller than a nickel—allowing 

for precise control schemes to optimize any feature. 

As microprocessors become more integrated, different 

components may be able to communicate and provide 

feedback; for example, feedback in the socket–residual 

limb interface could allow adaptation of the socket 

shape based on the type of gait or phase of the gait 

cycle.

Mechatronic technology in which power transmission is isolated from signal acquisition is readily 

available. As researchers and industry alike continue 

the paradigm shift from conventional robotics to 

mechatronics, prostheses will exhibit increased power 

and dexterity without increases in weight. Although 

prostheses will admittedly become more complex, and 

in some ways more fragile, mechatronics, coupled 

with new power sources such as fuel cells, will provide a platform for a new surge of innovation in the 

field of prosthetics.
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The Future of Artificial Limbs

SUMMARY

Many advances in the last 20 years have had a 

remarkable impact on the function of prostheses, 

more so for lower limb than upper limb prostheses. 

Significant recent advances, including the shrinking 

size of microprocessors and new types of batteries, 

have created a new level of technology, now standard 

in the commercial sector, that should have a substantial impact on the future of prosthetics. The one area 

where success has been limited is socket suspension. 

Although commercial companies have devoted attention to this area, prosthetic interfacing requires 

substantial investigation and innovation to be part of 

an entire system superior to today’s prostheses. The 

future of artificial limbs will certainly be both challenging and exciting, and should lead to improved 

quality of life for end users. 

REFERENCES

1. Rolock JS, Tucker K. Squirt shape—development of an automated fabrication technique to enhance prosthetics CAD/

CAM. Paper presented at: Second National VA Rehabilitation Research & Development Conference; February 20–22, 

2000; Arlington, Va.

2. Otto Bock HealthCare Web site. Available at: http://www.ottobock.com/cps/rde/xchg/ob_com_en/hs.xsl/601.html. 

Accessed December 30, 2008.

3. Beil TL, Street GM, Covey SJ. Interface pressures during ambulation using suction and vacuum-assisted prosthetic 

sockets. J Rehabil Res Dev. 2002;39:693–700.

4. Ossur Web site. Available at: http://www.ossur.com/pages/11836. Accessed December 30, 2008.

5. Dietl H. New developments in upper limb prosthetics. In: Smith DG, Michael JW, Bowker JH, eds. Atlas of Amputations 

and Limb Deficiencies. Rosemont, Ill: American Academy of Orthopaedic Surgeons; 2004: Chap 28.

6. Daly W. Clinical application of roll-on sleeves for myoelectrically controlled transradial and transhumeral prostheses. 

J Prosthet Orthot. 2000;12:88–91.

7. Witsø E, Kristensen T, Sivertsen S, et al. Improved comfort and function of arm prosthesis after implantation of a 

humerus-T-prosthesis in transhumeral amputees: the subfascial implant supported attachment (SISA) project. Paper 

presented at: 12th World Congress of the International Society for Prosthetics and Orthotics; July 30–August 3, 2007; 

Vancouver, British Columbia, Canada. 

8. Brånemark R, Brånemark PI, Rydevik BL, Myers RR. Osseointegration in skeletal reconstruction and rehabilitation: a 

review. J Rehabil Res Dev. 2001;38:175–191.

9. Marks LJ, Michael JW. Science, medicine and the future: artificial limbs. BMJ. 2001;323:732–735.

10. Musallam S, Corneil BD, Greger B, Scherberger H, Andersen RA. Cognitive control signals for neural prosthetics. 

Science. 2004;305:258–262. 

11. Hochberg LR, Serruya MD, Friehs GM, et al. Neuronal ensemble control of prosthetic devices by a human with tetraplegia. Nature. 2006;442:164–171.

12. Taylor DM, Tillery SI, Schwartz AB. Direct cortical control of 3D neuroprosthetic devices. Science. 2002;296:1829–1832.

13. De Luca CJ. Control of upper-limb prostheses: a case for neuroelectric control. J Med Eng Technol. 1978;2:57–61.

14. Hoffer JA, Loeb GE. Implantable electrical and mechanical interfaces with nerve and muscle. Ann Biomed Eng. 1980;8:351–360.

15. Edell DJ. A peripheral-nerve information transducer for amputees: long-term multichannel recordings from rabbit 

peripheral-nerves. IEEE Trans Biomed Eng. 1986;33:203–214.
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Care of the Combat Amputee

16. Andrews B. Development of an implanted neural control interface for artificial limbs. Paper presented at: 10th World 

Congress of the International Society of Prosthetics and Orthotics; July 2–6, 2001; Glasgow, Scotland.

17. Childress D, Weir RF. Control of limb prostheses. In: Smith DG, Michael JW, Bowker JH, eds. Atlas of Amputations and 

Limb Deficiencies. Rosemont, Il: American Academy of Orthopaedic Surgeons; 2004: 173–195.

18. Upshaw B, Sinkjaer T. Digital signal processing algorithms for the detection of afferent nerve activity recorded from 

cuff electrodes. IEEE Trans Rehabil Eng. 1998;6:172–181.

19. Kuiken TA. Consideration of nerve-muscle grafts to improve the control of artificial arms. J Tech Disabil. 2003;15:105–111.

20. Kuiken TA, Dumanian GA, Lipschutz RD, Miller LA, Stubblefield KA. The use of targeted muscle reinnervation for improved myoelectric prosthesis control in a bilateral shoulder disarticulation amputee. Prosthet Orthot Int. 2004;28:245–253.

21. Lipschutz RD, Miller LA, Stubblefield KA, Dumanian GA, Phillips ME, Kuiken TA. Transhumeral level fitting and 

outcomes following targeted hyper-reinnervation nerve transfer surgery. In: Proceedings of the Myoelectric Control 

Symposium. Fredericton, New Brunswick, Canada: University of New Brunswick; 2005: 2–5.

22. Ohnishi K, Weir RF, Kuiken TA. Neural machine interfaces for controlling multifunctional powered upper-limb prostheses. Expert Rev Med Devices. 2007;4:43–53.

23. Kuiken TA, Marasco PD, Lock BA, Harden RN, Dewald JP. Redirection of cutaneous sensation from the hand to the 

chest skin of human amputees with targeted reinnervation. Proc Natl Acad Sci U S A. 2007;104:20061–20066.

24. Hijjawi JB, Kuiken TA, Lipschutz RD, Miller LA, Stubblefield KA, Dumanian GA. Improved myoelectric prosthesis 

control accomplished using multiple nerve transfers. Plast Reconstr Surg. 2006;118(7): 1573–1578.

25. Kuiken TA, Miller LA, Lipschutz RD, et al. Targeted reinnervation for enhanced prosthetic arm function in a woman 

with a proximal amputation: a case study. Lancet. 2007;369:371–380.

26. Paluska D, Herr H. The effect of series elasticity on actuator power and work output: implications for robotic and 

prosthetic joint design. Robot Auton Syst. 2006;54:667–673.

27. Peshkin MA, Colgate JE, Wannasuphoprasit W, Moore CA, Gillespie RB, Akella P. Cobot architecture. IEEE Trans Robot 

Autom. 2001;17:377–390.

28. Gillespie RB, Colgate JE, Peshkin MA. A general framework for cobot control. IEEE Trans Robot Autom. 2001;17:391–401.