Review Article

Proc IMechE Part H:

J Engineering in Medicine
1–12
Biomechanical design considerations Ó IMechE 2016
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DOI: 10.1177/0954411915624452

A review pih.sagepub.com

Yuanjun Sang, Xiang Li and Yun Luo

Abstract
Traditional function and comfort assessment of transradial prostheses pay scant attention to prosthetic interface. With
better understanding of the biomechanics of prosthetic interface comes better efficiency and safety for interface design;
in this way, amputees are more likely to accept prosthetic usage. This review attempts to provide design and selection
criteria of transradial interface for prosthetists and clinicians. Various transradial socket types in the literature were
chronologically reviewed. Biomechanical discussion of transradial prosthetic interface design from an engineering point
of view was also done. Suspension control, range of motion, stability, as well as comfort and safety of socket designs have
been considered in varying degrees in the literature. The human–machine interface design should change from traditional
‘‘socket design’’ to new ‘‘interface design.’’ From anatomy and physiology to biomechanics of the transradial residual limb,
the force and motion transfer, together with comfort and safety, are the two main aspects in prosthetic interface design.
Load distribution and transmission should mainly rely on achieving additional skeletal control through targeted soft tissue
relief. Biomechanics of the residual limb soft tissues should be studied to find the relationship between mechanical prop-
erties and the comfort and safety of soft tissues.

Keywords
Prosthetic interface, transradial prostheses, prosthetic suspension, prosthetic socket, below-elbow prosthesis, upper-
limb amputation, residual limb, rehabilitation

Date received: 28 April 2015; accepted: 1 December 2015

Introduction restore function and/or appearance of a missing limb. It

would be unrealistic to expect that the prosthesis can
Upper-limb amputation is less common than lower- ever replace the functions of a normal limb from pres-
limb amputation, typically 1 upper-limb patient per ent rehabilitation technology. An estimated 20% of
year for every 30 lower-limb patients experiencing defi- participants with absent upper-limb abandoned pros-
ciencies.1 In 2006–2007, only 7% of all referrals to UK thesis use, whereby prosthetic comfort and function are
amputation services were for upper-limb loss, where critical factors.5 Comfort-related design issues, such as
4% presented for upper-limb amputation and 3% for improved heat dissipation/perspiration, no overload on
congenital absence.2 Although the probability of upper- soft tissues, and most definitively, reduced weight, are
limb amputation is small, the overall number is large in paramount on patient’s wish lists for all levels of pros-
one country and the world. In the United States, it was theses. Function-related design should fulfill the
estimated that there were more than 266,000 upper-limb
amputees in 2008.3 In the developing world, prevalence
State Key Laboratory of Mechanical System and Vibration, Institute of
estimates for upper-limb amputation based on conser- Biomedical Manufacturing and Life Quality Engineering, School of
vative UK and US statistical analyses indicate possibly Mechanical Engineering, Shanghai Jiao Tong University, Shanghai, China
2 million major upper-limb amputees worldwide.4
Unlike lower-limb amputees, almost three-quarters of Corresponding author:
all upper-limb amputees are aged less than 55 years, Xiang Li, State Key Laboratory of Mechanical System and Vibration,
Institute of Biomedical Manufacturing and Life Quality Engineering, School
and they are in a state of good health.2 of Mechanical Engineering, Shanghai Jiao Tong University, 800 Dong
Prosthetic replacement is one of the most important Chuan Rd., Shanghai 200240, China.
rehabilitation methods for upper-limb amputees to Email: xiangliwj@sjtu.edu.cn

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2 Proc IMechE Part H: J Engineering in Medicine

following requirements: improve range of motion

(ROM), provide smooth load and motion transition
between the remnant and prosthesis, ease of don and
doff, provide self-suspension, and so on. Improvement
in comfort and function is considered high priority for
individuals of all ages and all prosthesis types.6
Considering that the prosthetic interface or socket is
the only point of force and motion transfer between the
residual limb and prosthesis, the function and especially
the comfort of the prosthesis have a close relation with
prosthetic interface design. Therefore, prosthetic inter- Figure 1. The Muenster-type socket.8
face or socket design is the most important aspect in the
decision criterion of upper-limb amputees.
The design of the prosthetic socket is not the same
for different upper-limb amputation levels. There are
seven amputation levels related to prosthetic socket
design: (1) forequarter, (2) through shoulder, (3) trans-
humeral, (4) elbow disarticulation, (5) transradial, (6)
wrist disarticulation, and (7) partial hand. Statistics
show that the transradial level is the most common level
of upper-limb amputation seen by the prosthetist.2,7
Transradial amputation is the transverse loss of a part
of the forearm. The remaining length of the forearm
varies for each amputee. Having retained the elbow
joint, the amputee has greater control for supporting
and activating the prosthesis, with this level of amputa-
tion offering the most useful residual limb for successful
prosthetic limb use. This article focuses on the study of Figure 2. The Northwestern-style socket.10
prosthetic socket types for the transradial amputation
level.
To the best of our knowledge, there are few studies compression stability proximal to the epicondyles.
reviewing transradial prosthetic socket types in the lit- Unlike the Muenster design, the Northwestern design
erature. The purpose of this article was to review recent relies primarily on compression in the M/L plane super-
research literature to contribute to the development of ior to the epicondyles with less restrictive A/P trim lines.
a guideline for transradial prosthetic interface system. The Northwestern technique takes great care to the bio-
Furthermore, a biomechanical discussion from an engi- mechanics of socket design as it relates dynamically to
neering point of view was done to provide perspective the ROM of the residual limb. The degree of investiga-
for prosthetists and clinicians. tion of the Northwestern design is the initiation of the
prosthetist to look beyond the brim of the socket and
consider the biomechanics of socket design.9,10,12
Transradial socket designs Although there are significant advantages in both of
The Muenster-type socket (Figure 1), an older socket these self-suspending designs, patients who have worn
designed by Hepp and Kuhn in the mid-20th century, them often have certain limitations. Impingement of the
was specifically designed for short transradial amputa- skeletal substructure by the interface in the epicondylar
tions and focused on anterior/posterior (A/P) compres- region limits ROM. High, inflexible trim lines on the
sion stability at the proximal brim (in the cubital fold). A/P aspect also restrict the ROM and can be proble-
This kind of socket is self-suspending with a supracon- matic in donning and doffing of prosthesis. Conversely,
dylar rim enveloping the epicondyles of the humerus. reduction in A/P compression can lead to loss of sus-
The Muenster-type socket is a full-contact socket, pension and stability, loss of skin-to-electrode contact
enveloping both epicondyles and fixed by muscle ten- for myoelectric prostheses, and excessive concentrated
sion.8–10 load to the cut end of the bone.13 In addition, the fitting
The Otto Bock Muenster-type socket with the procedure is difficult, the socket has a rigid topographi-
MyoBock system developed in 1968 by Otto Fruzinsky cal shape, and perspiration as well as skin irritation
was based on the original Muenster-type socket.9,11 may be serious problems for the envelope of the resi-
The Northwestern University (NWU) Supracondylar dual limb.
Suspension Technique (Figure 2), designed by Billock in Ultimately, these design considerations and clinical
1972, was specifically designed for longer residual limbs experiences allowed a transition into more anatomically
and is considered to obtain more medial/lateral (M/P) considerate designs. The initial concept of the 3/4

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Sang et al. 3

Figure 3. (a) First and second quadrants are the guiding portions of the sockets, the third quadrant provides suspension and
stability, and the fourth quadrant is noncontributory14 and (b) the 3/4 transradial socket.9

Figure 4. Ergonomic socket design was constructed with

stainless steel tubes that were placed in anatomical regions to
maximize the biomechanical efficiency of the design.9
Figure 5. High-performance variable suspension prosthesis and
3S suspension systems for short- to mid-length transradial
socket design was proposed by Sauter14 in 1986 amputees who demand high-performance prosthetic function.16
(Figure 3), and more radical concepts such as the ergo-
nomic socket design from the Netherlands in 1985
emerged (Figure 4).15 Both designs recognize the sock- After experiences with each of these systems, Radocy
et’s purpose and the need to provide cooler socket tem- designed the high-performance variable suspension
peratures. For Sauter’s 3/4 socket, the problem of prosthesis (HPVSP) in 1995 (Figure 5), which is a trans-
ventilation is overcome through the removal of the radial supracondylar socket (modified Muenster) that is
proximal-posterior portion of the socket, suspension is combined with 3S or Icelandic Roll-On Silicone Socket
also improved, and a definite increase in flexion range (ICEROSS) suspension technologies. The variable sus-
can be observed. The Netherlands team focused on cre- pension prosthesis (VSP) is a viable alternative for
ating an economical socket that was easy to don. The short- to mid-length transradial amputees who demand
resulting frame design considers anatomical structures rigorous sporting activities and need increased prosthe-
and biomechanical principles with a unique connection sis’ versatility. When using the prosthesis in normal
for the structure distal to the socket.9 Open windows of activities, a normal liner (e.g. a fresh sock) is used, and
both designs allow for greater heat dissipation and when using the prosthesis for sporting activities, a sili-
enhanced proprioception. Additional advantages of cone liner with lock is used to provide superior suspen-
both designs are the increased ease of donning and sion.16 The combination of the supracondylar socket
greater ROM. However, the disadvantages are the and silicone sleeve can provide superior suspension for
increased probability of debris and perspiration improved performance. The disadvantages of this com-
between the frame and the flexible inner that may dam- bination technology, such as heat dissipation, moist
age the delicate electronics. In addition, some patients conditions, and so on, may be mainly related to the
may prefer the olecranon and other open areas to be development of the silicone sleeves or inner liner, which
protected by an encapsulated design. will be discussed later in this article.

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4 Proc IMechE Part H: J Engineering in Medicine

Figure 6. The WILMER open socket design.18

Figure 7. ACCI in diagnostic phase.10

The WILMER open socket is a tubular open socket
design introduced by Plettenburg17 in 1998 (Figure 6).
This open socket designed by Delft University of
channels to the distal volume, which offers additional
Technology in the Netherlands was a completely new
rotational stability, as well as an extended brim up to
socket with an accompanying new fitting procedure to
the cubital fold, and the radial channels also ensure suf-
meet the disadvantages of the Muenster socket.15 This
ficient soft tissue contact along the length of the entire
socket design, made of soft-covered stainless steel
limb throughout all ranges of motion reducing distal
tubes, supports the arm only at the minimal fitting
end discomfort.
areas that are required for good fixation of the socket.
The transradial anatomically contoured (TRAC)
Meanwhile, the socket is strong enough to transmit
interface modeled after the ACCI is a socket based on
force and motion between the residual limb and pros-
Muenster and Northwestern interfaces with more
thesis. The socket can be easily put on and off and is
aggressive contouring of the anatomy to maximize load
continuously adjustable during the day. Unlike full-
tolerant areas of the residual limb (Figure 8). TRAC
contact sockets, the steel tubes cause the resulting con-
interface was studied since the late 1980s and was
struction to be open, providing good ventilation of the
described in 2003 by Miguelez et al.13 The socket looks
skin. In conclusion, the main advantages of the like the Muenster socket, but the fitting technique is
WILMER open socket are the open structure, the con- different because the inner shell of the socket is made
tact of the elbow with its surroundings, and the easy of flexible material. The TRAC uses both A/P and M/
donning and doffing. The main problem of the L compression augmented by contouring of the muscu-
WILMER open socket is the unaesthetic look and the loskeletal presentation distal to the cubital fold for
difficult manufacturing technique. The WILMER open enhanced comfort and stability. The elbow’s ROM is
socket has been accepted as a standard prosthetic increased compared to the Muenster and NWU socket
socket in several rehabilitation centers in the because of the special fitting technique and the flexible
Netherlands.18 inner shell. When having a long forearm stump, rota-
Traditional self-suspending interface designs at tion of the radius and ulna is not possible due to the
transradial level typically focus on three distinct areas: skeletal lock built into the socket design. The TRAC
(1) the supracondylar region, (2) the cubital region, and addresses the deficits of previous designs by contouring
(3) the olecranon region. The anatomically contoured five key areas: (1) the antecubital region, (2) the olecra-
and controlled interface (ACCI) adds relief and modifi- non region, (3) the epicondylar region, (4) the distal
cation to the antecubital region in addition to these radial region, and (5) the wrist extensor and flexor mus-
three regions (Figure 7). The ACCI increased A/P and culature.13 Therefore, the advantages of this socket are
M/L compression but focuses them in different areas. decreased displacement of radius and ulna when the
ACCI was used for short transradial amputation by socket is loaded and more stable relationship between
Alley10 in 2002. The ACCI introduced by the author in the radius and ulna angle in relation to the posterior
the early 1990s controls axial rotation by extending the plane of the interface. Based on the 3/4 socket design,
medial and lateral stabilizers more proximally than in modification to the olecranon region is applied on
earlier designs and adding supracubital fossae anterior TRAC, with more aggressive supra-olecranon contour-
and proximal to the humeral epicondyles, avoiding the ing needed to retain stability and suspension.
often reported discomfort in earlier designs that sus- Additional advantages of this modification include ease
pended directly proximal to the epicondyles. The tradi- of donning and greater ROM. The indicators suggest
tional interfaces and the ACCI differ in how well the that aggressive muscle contouring has the potential to
interfaces perform under heavy loading, particularly in improve load transfer performance as well as comfort
the initial stages of flexion. The ACCI adds radial and should be adequately considered in the decision

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Sang et al. 5

Figure 8. (a) The transradial anatomically contoured (TRAC) interface and (b) the TRAC interface with three-quarter
modification.13

compression areas along nearly the entire shaft of the

bone, while the release areas allow for soft tissue to
escape out of the fields of compression; compression
should be a little larger in the center of the bone before
a load is applied at the end of the socket to result in a
uniform load along the residual limb bone.21 The
improved sockets are suitable for patients with transra-
dial, transhumeral, transtibial, and transfemoral ampu-
tations. However, the compression bar length, width, or
magnitude should be adjusted to account for regional
sensitivities of individuals, the blood perfusion should
be studied to find an acceptable static pre-load pressure
allowing adequate blood flow, and both certification
and licensure for clinical training should be considered
in future research.
Figure 9. High-fidelity radial interfaces.19 After a subsequent suspension modification of the
high-fidelity interface developed by Randall Alley, a
new socket style with a dynamic load strap made up of
and rationale of all transradial fittings. The TRAC a fiber braid with an interior pneumatic bladder was
interface may not suitable for patients with congenital used in DEKA Arm and presented by Phillips22 in
deficiency, lacking sufficient radial head and epicondy- 2011 (Figure 10). The strap would shorten after the
lar definition to provide suspension as well as keep bladder inflates providing adjustable mechanism which
ROM and stability simultaneously. This interface may does not require buckles or snaps. One advantage of a
not be suitable for individuals with long residual limbs flexible posterior suspension is the preservation of resi-
for preventing rotation of the radius and ulna. In addi- dual pronation and supination. The extent to which a
tion, some patients prefer an interface that does not dynamic load strap improves ROM at the transradial
encapsulate the olecranon, while some patients prefer level should be evaluated in future work.
the olecranon to be protected by an encapsulated The cost of prosthesis and its maintenance is a pri-
design. mary factor in underdeveloped countries and
The high-fidelity interface, or compression-stabilized regions.23–26 The international transradial adjustable
interface (Figure 9), proposed by Randall Alley in 2011, limb (ITAL) proposed by Alwyn Johnson in 2009
is based on compression–release stabilization (CRS) (Figure 11)4,27,28 provides a new option for economi-
theory, which focuses primarily on control of the under- cally disadvantaged amputees in the United States and
lying bone along their entire length through alternating developing countries. ITAL comprises an innovative
soft tissue compression and release.19,20 The improved variable compression, variable below-elbow interface
sockets created longitudinal depressions added to the geometry, a new body-powered prehensor with adjusta-
socket walls and open release areas between the depres- ble pinch force, control harness, and cable. The below-
sions to receive the displaced tissue. To reduce the lost elbow or transradial interface in the ITAL is also
motion between the bone and socket wall, this tech- dubbed the Johnson-Veatch interface (JVI). The JVI is
nique correctly places three or more longitudinal a simple mechanism incorporating variable

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6 Proc IMechE Part H: J Engineering in Medicine

Figure 10. Comparison of the rigid diagnostic socket and the

final socket with the dynamic load strap. The anterior and
posterior openings were maintained, the posterior strut was
replaced with the dynamic load strap, and the medial and lateral
openings were removed to improve capture of native pronation
and supination.22

compression and variable geometry with an open

frame, in contrast to a full-contact socket. JVI has a
modular two-component interface comprising a hum-
eral cuff and forearm adaptor that can be manually
adjusted with simple hand tools. The humeral cuff pro-
vides the major portion of suspension and should be
iteratively designed to be comfortable under consider-
able loads. The stability of the interface can be
obtained by the cuff’s three-point contact with the M/P
condyles and olecranon process. The supracondylar
cuff length can be adjusted medially and laterally to
effectively minimize rotation and migration below the Figure 11. (a) ITAL’s epicondyle contact configuration27 and
(b) complete ITAL unit.4
condyles and prevent point loading on the humerus.
Readily replaceable sleeves minimize the inconvenience
of replacing worn parts. The JVI could be adjusted to
fit limb segments of multiple amputees and nonampu-
tees to comfortably suspend static axial load of up to
15.88 kg. The advantages of JVI are its low cost and
easy fit and maintenance.
Another low-cost socket made from an ordinary
soda bottle was presented by Wu in 2009 (Figure 12).
The socket is made by putting a polyethylene ter-
ephthalate (PET) bottle over a plaster model of the
stump and applying heat with a heat gun. The bottle Figure 12. Low-cost socket made of a soda bottle.29
conforms to the shape of the plaster model.29 The low-
cost socket is so crude that it is simply for decoration
without function. eliminate the need for straps in suspension, especially
in myoelectric devices. The Muenster technique, the
NWU socket, the 3/4 transradial socket, the WILMER
Suspension
open socket, the ACCI socket, the TRAC socket, the
For upper-limb amputees, continuous suspension of the high-fidelity interface, the ITAL JVI socket, and so on
prosthesis is needed to overcome the effect of gravity.30 utilize the elbow’s bony anatomy for self-suspension in
There are different kinds of prosthesis suspension: (1) varying degrees. And the high-fidelity interface uses
with straps, (2) enveloping prominent bones, (3) silicone not only elbow anatomy but also compression areas
suspension, and (4) osseointegration. above the release tissue in the relief zone. Silicone sus-
Nowadays, many transradial self-suspending socket pension technology and vacuum suspension technology
designs have been developed and used in an effort to can be used as an auxiliary to enhance suspension

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Sang et al. 7

stability of the interface between the residual limb and disarticulation, and partial hand) amputees (Figure 14).
socket wall. Unlike the outer rigid socket, the inner sili- There are two basic construction methods: the first uses
cone liner is made of soft materials and is generally an all-silicone construction with components molded
called a silicone socket. into the silicone, and the second uses a hybrid construc-
The silicone socket is rolled onto the residual limb tion with silicone for the interface attached to a plastic
excluding air, adhering to the stump by friction and laminated frame. The comfort and cosmetic appear-
vacuum.31 In 1986, Ossur Kristinsson introduced the ance can be improved over previous construction tech-
first ‘‘silicone liner socket’’ in the form of the niques by providing soft, flexible sockets with suction
ICEROSS. Since its introduction in the mid-1980s, suspension.
roll-on silicone sockets with locking mechanisms or However, because all normal mechanisms to cool
lanyards have evolved into a widely used method of the body are hindered by the silicone roll-on sockets,
prosthetic suspension. Roll-on suction suspension liner including convection, radiation, evaporation, and con-
is made of silicone material and is designed as a flexible duction, amputees have less area to allow heat loss,
tube to be rolled up on the residual limb to replace the resulting in warm and moist conditions inside the pros-
rigid inner socket.32 This design provides not only thesis which may be a contributing factor in causing
improved suspension but also better comfort and infections.34 Besides the problem of poor thermal con-
greater ROM for the prosthesis. The locking liner uses ductivity, there is also skin irritation due to the inner
a pin-locking mechanism at the end to secure the outer silicone.31 Individuals who have worn the silicone liner
socket to the liner (Figure 13).33 Roll-on suction sus- for longer periods report higher occurrences of contact
pension liners are incorporated in anatomically con- dermatitis, folliculitis, and residual limb soreness.35
toured sockets. Breathable liners for transradial prostheses was pro-
Custom silicone socket technology introduced by posed by Bertels36 in 2011 (Figure 15). This liner is
Uellendahl11 in 2006 has expanded and improved the made of a special three-dimensional textile spacer fabric
option for long below-elbow (long transradial, wrist combined with partial silicon coating for suspension.
The side facing the skin is provided with bacteriostatic
fibers that include silver ions (Ag + ) to prevent bac-
teria from multiplying. The middle layer is provided
with monofil threads forming a distance with damping
function between the bottom and cover layers. The
monofil threads are provided with Coolmax multifil
fibers lying in between. The humidity and undesired
odors in this new arm liner can be transported and
reduced to ensure comfort. The liner has many advan-
tages: offered in different sizes, minimized shear stress,
avoided distal pain, distributed force transmission to
the whole liner, simply cut to the needed length, easily
Figure 13. Suction locking liner showing roll-on application washable, and so on. The liner becomes usable in com-
(right).33 bination with an open external frame prosthetic socket

Figure 14. (a) An all-silicone construction socket for long below-elbow (wrist disarticulation) amputation and (b) a finished silicone
socket with laminated struts. The patient has a wrist disarticulation amputation.11

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8 Proc IMechE Part H: J Engineering in Medicine

Discussion
Definitions of terms
There are many descriptive sentences or terms referring
to the interface between the residual limb and prosthe-
sis, such as human–machine interface (HMI), residual
limb–socket (RL-S) interface, prosthetic socket, pros-
thetic interface, and others. The most mentioned terms
are socket and interface. Although the prosthetic socket
or prosthetic interface refers to HMI between the resi-
dual limb and prosthesis, there is a subtle difference
between the two terms. Herein, this article gives defini-
Figure 15. Breathable liner with three-dimensional textile
spacer fabric in patient trial.36 tions based on the aforementioned literature.
Socket is defined as the part of a prosthetic in direct
contact with the soft tissue of upper or lower limb. The
word ‘‘socket’’ usually implies a traditional socket that
is essentially circular in cross section. A traditional
prosthesis consists of an inner socket to interface with
the patient’s skin and an outer socket over it to incor-
porate the mechanisms that comprise the distal struc-
ture which may be a joint to function as a prosthetic
foot or hand. The inner and outer sockets may be sepa-
rate structures or may be unitary consisting of a single
unit.
Interface is defined as a synonym for socket but is
more often reserved for socket-like structures that have
openings in the outer socket and occasionally in both
the outer and inner sockets.41,42 The main design philo-
sophy behind HMI of the upper-limb prostheses should
change from the term traditional ‘‘socket’’ to the more
exact expression ‘‘interface.’’
Figure 16. Body-powered transradial application.
Source: images reproduced from Jönsson et al.37
The interface design criteria
Anatomical and physiological characteristics of the
for easier donning and doffing. The effect of this new upper and lower limbs are not the same, so the design
socket system still needs further investigations including criteria of prosthetic replacements are also different for
patient tests. both limb deficiencies. The prosthetic interface, one of
Instead of using a socket, the prosthesis can be the important prosthetic components, is not the same.
attached directly to the residual limb with direct bone Here, we focus on the prosthetic interface design cri-
anchorage. This technique is called osseointegration teria. The appearance, cost, ease of donning and doff-
(Figure 16), which was named by Swedish professor ing, as well as maintenance are all part of the design
Per-Ingvar Brånemark. Osseointegration is direct criteria for upper- and lower-limb patients.
attachment for the prosthesis by surgically implanting Individualized design should be considered not only
a threaded titanium implant into the bone, with an according to the state of amputation and health of
additional titanium implant connected to the fixture amputees but also according to the prosthetic opera-
and penetrating the skin.37,38 The first transradial-level tion requirements and habits of amputees.
amputee treatment with this technology was in 1992. For upper-limb deficiencies, the major functions of
Osseointegration has the following advantages: stable the upper-limb prosthesis may serve better to describe
fixation of the prosthesis, better proprioception, elimi- mobility control in terms of positional, operational,
nation of skin problems and pressure problems, and and functional control, the reason being that the upper
better control of the prosthesis. Nevertheless, the main limb is more dexterous and flexible to bear heavy body
reason osseointegration is not widely used is that it is weight relative to the lower limb.43 Therefore, suspen-
contraindicated for people with diabetes, which admit- sion control is very important in upper-limb prosthetic
tedly are mostly lower limb, but also active patients, interface design. For transradial amputation, the elbow
due to forces imparted on the abutment and, finally, joint has a certain degree of functionality retained after
simple rejection of additional surgeries and a continu- amputation. The olecranon and supracondyle can be
ous open wound. Osseointegration may still be another adopted as prosthetic suspension areas, particularly for
choice for transradial interface design.39,40 short transradial amputation. For mid- to long

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Sang et al. 9

transradial amputation, suspension can also be distribution with its corresponding interface stability
achieved through A/P and M/P compression or should be considered in static and alternating loads.
vacuum adsorption. These considerations can be summarized as smooth
The ROM of retained elbow should be considered force and motion transfer between the residual limb
for transradial interface design. The maximum range of and prosthetic interface, and the concomitant problems
elbow flexion and extension is defined as ROM of the can be summarized as comfort and safety of the resi-
elbow, and the total ROM of the elbow without an dual limb and material durability of the prosthesis.
interface was about 146°.13 The ROM of the elbow Force and motion transfer function contradicts with
may interfere with a prosthetic interface; thus, the total comfort and safety in prosthetic interface design.
range angle of elbow flexion and extension may be
decreased. Another ROM consideration is the residual
pronation and supination of the radius and ulna, espe-
Force and motion transfer principle
cially for long transradial amputation. Both ROMs of Transradial interface design should utilize the anatomic
the residual limbs should be considered in prosthetic features of the residual hard and soft tissues, as well
interface design for either myoelectric hand–computer as the biomechanics of soft tissues, to obtain rational
or brain–computer interface control hand. load distribution and smooth force transmission.
The stability of the prosthetic interface should be Distribution and transmission of the applied load are
considered particularly in the bearing and motion state. extremely important in upper-limb interface design
The soft tissues between the underlying bone and the criteria.
prosthetic interface should be compressed to gain more The basic principles of current load distribution
stiffness for transferring force and motion. However, models revolve around two simple concepts: uniform
because the soft tissues possess viscoelastic properties distribution of load over the entire limb and load con-
with low elastic modulus in a conventional socket, con- centration on load tolerant areas of the limb, with con-
siderable motion is lost before the limb can move the comitant relief for areas deemed ‘‘load sensitive.’’ The
socket and prosthesis. There are mainly three motion two basic load distribution models may have a number
aspects lost related to upper-limb interface stability: of problems: decreasing ROM of the elbow, motion
axial rotation, slip, and translation.19 Therefore, the lost between the bone and the prosthetic structure dur-
stability to prevent motion lost in the interface very ing active lifting, and not loading the bone uniformly
importantly relates to the interface shape. but rather concentrating on the load near the ends. An
The comfort and safety of the transradial residual alternative load distribution model, known as the
limb are also important in the design criteria. Most ‘‘High-Fidelity’’ or ‘‘Compression-Stabilized’’ interface,
patients abandon prosthetic use due to its discomfort. which was discussed by Alley et al.,21 is a significant
Comfort and safety considerations have two aspects: departure from current interface design protocol. It
one caused by normal force with friction force and the involves achieving additional skeletal control through
other caused by the interface physiological environ- targeted soft tissue relief. The interface can be stabi-
ment. The normal force may lead to deep pressure inju- lized with respect to the underlying bone. For instance,
ries near the bone,44,45 and the friction force may lead in the ideal transhumeral or transradial prosthesis, the
to skin injury.46 Moreover, the interface physiological terminal device would move the humerus with no
environmental deterioration may also lead to discom- motion lost.49 The ‘‘Compression-Stabilized’’ interface
fort and soft tissues damage. Sweat, waste products, load distribution model may be a better in principle for
and bacteria may rise in the interface due to lack of ven- further transradial interface in force and motion trans-
tilation.47 Comfort and safety conditions under alter- fer design.
nating loads may deteriorate than under static load.
Additional items may be considered for first fit and
long-time fit. These items should include but are not
Comfort and safety
limited to prosthetic control strategies, volumetric Comfort and safety of the prosthetic interface are
changes, and concomitant skin or underlying soft tis- bound with the biomechanics of residual limb tissues.
sues injuries. The progressive upper extremity practi- Biomechanical understanding of the interface between
tioner should possess a comprehensive understanding the prosthesis and the residual limb is fundamental to
of the spectra of socket designs and material character- the improvement of interface design. However, soft tis-
istics in order to optimize prosthetic suspension, stabi- sue properties are very complex with biomechanics
lity, comfort and ROM, and, ultimately, function.1,48 properties of soft tissue showing viscoelasticity: stress
In conclusion, in transradial prosthetic interface relaxation, strain creep, and hysteresis. Moreover, the
design, the prosthetic interface design criteria include soft tissues of the residual limb are not particularly
suspension control, ROM, stability, comfort and safety, adapted to high applied forces, abrasive relative
and appearance. Suspension control should be designed motions, and the other physical irritations encountered
to keep effectiveness in static and dynamic states. at the prosthetic interface.
Moreover, ROM of the residual limb should not be The deformation of soft tissues is very complex. Soft
decreased with the prosthetic interface, and load tissues with low elastic modulus properties have large-

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10 Proc IMechE Part H: J Engineering in Medicine

scale deformation. To transfer force and motion effec- research; the prosthetic interface may be divided into
tively, the soft tissues between the underlying bone and an inner flexible structure with outer rigid structure or
prosthetic interface should be compressed tightly to a sole structure with different properties in different
obtain high interface stiffness; thus, the force transfer layers.
error and energy loss will be small enough to realize From clinical prosthetists’ point of view, the ease of
smooth transmission. fit, training, and maintenance of prosthetic interface
The temperature, humidity, and waste products of are decided by a well-designed interface. Therefore, the
the soft tissues accumulate to affect the comfort and prosthetic interface’s clinical application should be
safety of the interface. In traditional encapsulated studied by the engineer together with the clinical pros-
socket systems, physiologic metabolism of the soft tis- thetist to realize a better clinical efficiency.
sues is influenced by lack of ventilation. The interface From the patients’ point of view, cost, individualiza-
temperature may rise along with sweat and humidity tion, and durability of the prosthetic interface are part
may increase accordingly; the bacteria activity in this of the decision for a well-designed interface. There
warm and moist environment may lead to infections, should be two or three interfaces for patient use in dif-
pressure ulcers, pain, odors, blisters, and so on. ferent conditions, such as general living habits, special
Opening windows in the socket wall, especially the and heavy-duty working habits, and so on.
frame-type socket design, can increase ventilation In summary, there is no ‘‘standard’’ prosthetic inter-
between the residual limb skin and air, which can face system for all transradial amputees. Biomechanical
reduce the rate of discomfort and harm caused by phy- considerations from an engineering point of view are
siologic metabolism of soft tissues. given in this article and hope to provide a reference for
engineers, prosthetists, and patients.

Conclusion Declaration of conflicting interests

For transradial amputees, the prosthetic interface ser- The author(s) declared no potential conflicts of interest
ving as HMI is the key component for prosthesis. with respect to the research, authorship, and/or publi-
Performance of prosthetic interface, fitting procedure, cation of this article.
and training procedure are all important for prosthe-
tists and patients. However, the anatomical and physio-
logical characteristics and the biomechanics of soft Funding
tissues of the residual limb are complex. This article The author(s) disclosed receipt of the following finan-
reviewed the recent research literature on transradial cial support for the research, authorship, and/or publi-
prosthetic socket or interface types and analyzed the cation of this article: This work was supported by a 973
advantages and disadvantages of these prosthetic sock- basic research grant from the Ministry of Science and
ets and interface types. Technology of China (no. 2011CB013306), the Natural
From anatomy and physiology to biomechanics of Science Foundation of China (30970704, 51075263,
the transradial residual limb, the force and motion 51121063, 50821003), and the foundation from the
transfer, together with comfort and safety, are the two State Key Laboratory of Mechanical Systems and
main aspects considered in prosthetic interface design. Vibration, China.
The force and motion transfer are affected by the inter-
face suspension and stability in static and alternating
loads. Comfort and safety are affected by the normal References
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