Functional analysis of the UC-BL shank axial rotation device
1
L. W . L A M O U R E U X a n d C. W . R A D C L I F F E
Biomechanics Laboratory, University of California, Berkeley
Origins of concept
"Present indications are that transverse rota­
tions of the various segments of the leg a r e an
i m p o r t a n t factor in the ease a n d r h y t h m of
walking of n o r m a l individuals. I n order t o
improve function, reduce fatigue, a n d prevent
m o r e o r less continual abrasion at critical p o i n t s
of t h e s t u m p of the a m p u t e e , provision in t h e
prosthesis for allowing transverse rotations of
the same order of magnitude as present in
n o r m a l legs has the possibility of being a major
contribution t o the improvement of artificial
legs."
T h e above statement is a direct q u o t e of t h e
opening p a r a g r a p h in the 1947 report o n funda­
mental studies of h u m a n locomotion by t h e
University of California Prosthetic Devices
Research Project established at Berkeley in
1945. T h e goal of these studies was t o provide
a basis for improvements in prosthetics tech­
nology. F r o m their results nothing stood o u t
m o r e clearly t h a n the need for a device t o allow
passive axial r o t a t i o n between socket a n d foot
in lower-limb prostheses.
Design criteria
Design criteria for such a device were
established o n the basis of a m p u t e e tests of an
experimental unit with adjustable stops a n d
variable return spring characteristics. T h e final
r e c o m m e n d a t i o n was that a lightweight unit be
designed which would permit rotations of up t o
20 degrees in either direction a b o u t the long
axis of a prosthetic limb, with a centering spring
t o r q u e of approximately 0.23 N m (2 lb in) per
degree of r o t a t i o n . This r a n g e of m o t i o n was
found
to accommodate most
locomotor
activities without contact against t h e rigid
rotation stops. T h e centering spring torque was
a c o m p r o m i s e : a stiffer spring would tend t o
negate the p u r p o s e of t h e device by allowing
large torques t o be applied to the residual l i m b ;
a softer spring would n o t return the foot con­
sistently t o centre during the swing phase.
I n addition t o providing these basic charac­
teristics, any practical axial r o t a t i o n unit m u s t
have bearings capable of supporting t h e
amputee's weight a n d the severe bending
m o m e n t that occurs during the latter p a r t of
stance phase when weight is supported o n t h e
forefoot. T h e friction t o r q u e in the bearings
u n d e r l o a d must b e small c o m p a r e d to t h e
centering spring t o r q u e if the unit is t o perform
its function of allowing axial r o t a t i o n during
stance p h a s e . T h e return spring also should h a v e
a d e q u a t e d a m p i n g t o prevent excessive vibration
of the foot during swing phase. T h e axial r o t a ­
tion device should be c o m p a c t t o permit
inclusion in the widest possible variety of
prostheses, a n d it should be light in weight t o
minimize the increase in inertia of the distal
p a r t of the prosthesis. Finally, a truly practical
device m u s t be simple, reliable, a n d low in c o s t .
Previous designs
Since the need for an axial r o t a t i o n device
h a s been clearly recognized for 27 years, it is
quite a p p r o p r i a t e t o ask why satisfactory
devices have n o t been developed a n d m a d e
widely available t o amputees. T h e reason seems
t o be that the requirements for high strength,
low friction, soft b u t well-damped r e t u r n spring,
a n d light weight a r e difficult t o achieve simul­
taneously. A n u m b e r of designs h a v e appeared,
b u t all have lacked at least one of these essential
features. T h e original University of California
design was t o o large a n d heavy, a n d lacked
a d e q u a t e d a m p i n g of the return spring (Uni­
versity of California, 1947). A subsequent
design was compact but had excessive bearing
friction which prevented r o t a t i o n a t the very
time it was needed (Mullby a n d Radcliffe, 1960).
1Previouslv published in Orthopadie-Technik, 1/75,
7-9.
O t h e r designs have been unable t o withstand
the severe bending m o m e n t which occurs when
weight is b o r n e o n t h e forefoot, o r they have
h a d excessively stiff r e t u r n springs, or they have
been excessively large a n d heavy (Staros a n d
Peizer, 1972, 1973).
The present design
T h e device described here h a s been con­
servatively designed t o satisfy t h e original 1947
design criteria in the h o p e of providing increased
opportunity for clinical evaluation of the con­
cept of passive axial r o t a t i o n in lower-limb
prostheses. It consists of a pair of thin-section,
full-ball-complement ball bearings which s u p ­
p o r t axial loads a n d bending m o m e n t s , and a n
elastomer torsion spring in a ring configuration
as shown in Figure 1. T h e spring is shaped with
conical end plates to allow uniform shear
strains t h r o u g h o u t the volume of the elastomer.
A m p u t e e trials have been conducted primarily
with above-knee ( A K ) amputees. Their response
h a s been uniformly favourable with d r a m a t i c
relief of skin abrasions a n d epidermoid cysts in
some cases. T h e design criteria appear t o be well
suited t o t h e needs of t h e A K amputee. S o m e
questions remain whether the design is o p t i m u m
for below-knee (BK) amputees, w h o , unlike the
A K , can safely flex the knee during stance
phase. F u r t h e r clinical trials are needed t o
evaluate the specific requirements of the B K
a m p u t e e . In any event, the need is less critical
for the B K a m p u t e e because he generally h a s a
n o r m a l hip joint which allows him m u c h
greater freedom of axial r o t a t i o n .
Measurement procedure
T h e axial r o t a t i o n device was installed in a n
above-knee prosthesis with a U C - B L polycentric
knee a n d S A C H foot. This prosthesis was then
instrumented t o allow measurement of axial
rotation occurring within the device, axial
t o r q u e in the shank, pelvis r o t a t i o n a b o u t a
vertical axis, a n d relative internal-external
r o t a t i o n between the socket a n d the pelvis.
Measurements were obtained as the subject
walked at a comfortable speed (100 steps/min)
o n a power-driven treadmill, b o t h with the
device operating a n d with its m o t i o n locked out.
Measurement results
Fig. 1. The UC-BL shank axial rotation device in
cross section. Although the unit is shown mounted
on a foot, an alternative and preferred installation
would invert the device and mount it as high as
possible in the prosthetic shank. This preferred
installation moves the mass of the unit proximally
and minimizes bulk at the ankle.
W i t h one exception, the measurements were
m u c h as expected. W h e n t h e axial r o t a t i o n
device was operating, pelvic r o t a t i o n a b o u t a
vertical axis increased slightly t o a total of 6
degrees, t h e s h a n k r o t a t e d externally 8 degrees
over the foot during stance phase, and the step
length increased slightly. W h a t was n o t expected
was that the relative internal-external rotation
between pelvis a n d socket during stance phase
increased from 3 t o 8.5 degrees when the device
was operating.
Because of the unexpected n a t u r e of this
measurement, the measurements were repeated
with another A K case wearing a single-axis
hydraulic knee. T h e results of this second set of
measurements were essentially t h e same as the
first. It is the measurements of this second
a m p u t e e which are shown in Figures 2 a n d 3.
Fig. 2. A comparison of relative axial rotation
between the pelvis and the socket of an above-knee
prosthesis, with and without the axial rotation unit
(torque absorber) operating. Note the unexpected
increase in relative rotation which accompanies use
of the device.
Fig. 3. A comparison of axial torques occurring in
an above-knee prosthesis with and without the axial
rotation unit operating. Note the decrease in torque
which results from use of the device.
Discussion
Previously it h a d been tacitly assumed that
the condition of m i n i m u m strain t o the tissues
a r o u n d the b r i m of the socket would correspond
to m i n i m u m relative r o t a t i o n between the pelvis
a n d the socket brim. T h e measurements d o n o t
support such an assumption.
W h e n the axial rotation device is operating,
axial torques are reduced, as shown in Figure 3.
This reduction in torque, however, is accom­
panied by a n increase in relative axial r o t a t i o n
between the pelvis a n d the A K socket during
stance phase. This increased range of internal
a n d external rotation can be attributed t o t h e
effect of muscle action within the confines of the
quadrilateral A K socket.
At the instant of heel contact o n his prosthesis,
the a m p u t e e is actively extending the hip o n the
a m p u t a t e d side t o assure stability of the
prosthetic knee. This hip extension m o m e n t
must be transmitted t o the socket by m e a n s of
increased contact pressures over theproximal-ant
socket a n d stump. Since the gluteal muscles are
largely responsible for the hip-extension m o m e n t
there is a simultaneous generation of contact
force in the gluteal region due t o bulging of the
contracting gluteal musculature.
T h e combination of these two effects gives
rise t o a contact pressure distribution similar t o
that shown in Figure 4. A s illustrated, this pres­
sure distribution generates an external r o t a t i o n
torque a b o u t the long axis of the socket.
W h e n the prosthesis incorporates a n axial
r o t a t i o n device, this net t o r q u e acting a b o u t the
long axis of the socket is able t o r o t a t e the
socket externally over the fixed foot, the only
resistance t o such rotation being the relatively
weak return spring in the axial r o t a t i o n device.
This axial r o t a t i o n tends t o relieve the contact
pressures which caused the t o r q u e a n d thereby
reduces pressures in the critical antero-medial
region of the brim.
W h e n the prosthesis does not contain a n axial
r o t a t i o n device, the hip extension m o m e n t still
gives rise t o the pressure distribution shown in
Figure 4. N o w , however, the prosthesis is very
stiff in torsion a n d pressures c a n n o t be relieved
by external rotation of the socket. Consequently,
the pressure pattern tends t o r o t a t e the s t u m p
internally within the fixed socket with little or n o
relief of pressure. F o r m a n y amputees this com­
bination of slight internal r o t a t i o n a n d high
contact pressure will give rise t o skin t r a u m a a n d
the development of sebaceous cysts in the
antero-medial region of the socket brim o r
chafing in the region of ischial contact.
A s t h e stance p h a s e continues a n d the centre
of pressure moves forward t o the ball of the
foot, t h e hip action changes from active
extension t o active flexion as required to initiate
knee flexion near the end of stance phase. This
flexion m o m e n t tends t o reduce the magnitude
of the anterior force a n d increase t h e magnitude
of t h e posterior force producing t h e socketb r i m pressure pattern. At the same time the
gluteal muscles relax their tension in the
posterior region a n d the rectus femoris actively
bulges against the anterior lateral b r i m area,
allowing a posterior shift of t h e s t u m p within
t h e socket. T h e medially located hamstring
tendons tend to resist this displacement a n d a
pressure distribution similar t o t h a t shown in
Figure 5 results. A s illustrated, this pressure dis­
tribution which results from active hip flexion
generates a n internal r o t a t i o n t o r q u e a b o u t the
long axis of the socket.
Again, without an axial r o t a t i o n device in t h e
prosthesis, t h e socket c a n n o t m o v e t o relieve
these changing forces a n d the torques m u s t be
passed t h r o u g h the skin a t the stump-socket
interface, giving rise t o the aforementioned skin
p r o b l e m s which a r e familiar t o limb fitters.
W i t h an axial rotation device the socket is free
t o respond t o the d e m a n d s of the s t u m p a n d
relieve the pressures a n d torques caused by
cyclic action of the musculature.
Conclusions
T h e installation in an above-knee prosthesis
of a device which allows r o t a t i o n of the socket
over the fixed foot h a s been shown t o offer t h e
a m p u t e e the following a d v a n t a g e s :
1. I m p r o v e d gait symmetry.
2. R e d u c t i o n of axial torques between the
s t u m p a n d the socket.
3. R e d u c t i o n of the frequency of occurrence,
o r elimination of, sebaceous cysts due to
skin t r a u m a a t t h e socket brim.
4. I m p r o v e d freedom of movement when
changing direction of m o t i o n , w o r k i n g at a
bench or counter, a n d in sports activities.
Fig. 4. Estimated pressure distribution around the
brim of an AK socket at heel contact. Note the
resultant external rotation moment.
T h e usefulness of a n axial r o t a t i o n device for
above-knee amputees has been demonstrated
clinically with tests o n research patients over a
period of 30 years. T h e present design h a s been
used successfully b y several amputees for periods
of u p t o 18 m o n t h s without malfunction a n d it
appears t o have overcome t h e shortcomings of
previous units. M o r e extensive a m p u t e e trials
will begin in t h e near future under t h e auspices
of t h e Veterans Administration Prosthetics
Center in N e w Y o r k City.
It is possible that experience m a y d e m o n s t r a t e
t h a t assymetrical spring rates, non-linear
elasticity, o r a change in r o t a t i o n axis m a y
improve future devices, particularly for the
below-knee a m p u t e e .
Acknowledgement
Fig. 5. Estimated pressure distribution around the
brim of an A K socket just prior to toe-off. Note the
resultant internal rotation moment.
This research was m a d e possible by a grant
from t h e Veterans Administration, Prosthetic
a n d Sensory Aids Service, a n d their continued
s u p p o r t is appreciated.
REFERENCES
MULBY, W . C. and RADCLIFFE, C. W . ( 1 9 6 0 ) . An
ankle-rotation device for prostheses. Biomechanics
Laboratory, University of California, San Fran­
cisco and Berkeley. Tech. Rep. 3 7 .
STAROS, A. and PeiZER, E. (1972). Veterans Admini­
stration Prosthetics Center Research Report,
Bull. Pros. Res., BPR 1 0 - 1 8 , 226.
STAROS, A. and PeiJZER, E. (1973). Veterans Admini­
stration Prosthetics Center Research Report,
Bull. Pros. Res., BPR 1 0 - 1 9 , 148.
UNIVERSITY OF CALIFORNIA (1947). Fundamental
studies of human locomotion and other informa­
tion relating to design of artificial limbs. Report to
National Research Council, Committee on
Artificial Limbs. Vol. I, Part I, Sect. 1-1, and
Vol. II, Part IV, Sect. 16.