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Comparison of interface pressures with pin and suction suspension systems 821
JRRD
JRRD
Volume 41, Number 6A, Pages 821828
November/December 2004
Journal of Rehabilitation Research & Development
Comparison of interface pressures with pin and suction suspension systems
Tracy L. Beil, MS; Glenn M. Street, PhD
Human Performance Laboratory, St. Cloud State University, St. Cloud, MN
AbstractA common mode of limb suspension for transtibial
amputees is the pin liner/shuttle lock system. Despite its popu-
larity, some clinicians question its use because of observed
daily and chronic changes to the residual limb. For this study,
we measured limb interface pressures during ambulation with
pin and suction suspension systems. No pressure differences
were seen between the modes of suspension during stance
phase. However, during swing phase, pin suspension main-
tained an average occlusive compressive pressure of 6.7 kPa on
the proximal tissues, as compared to the subocclusive pressure
of 1.1 kPa with suction suspension. Simultaneously, pin sus-
pension elevated the peak magnitude of suction to 39.5 kPa at
the distal residual limb, compared to 26.1 kPa with suction
suspension. During swing phase, the pin liner squeezes proxi-
mally while creating a large suction distally on the residual
limb and is the likely cause of daily and chronic skin changes
observed in pin users.
Key words: interface pressure, pin suspension, prosthesis,
residual limb, suction suspension, transtibial amputee, urethane
liners, verrucous hyperplasia.
INTRODUCTION
Prosthetists attempt to fit transtibial amputees with
sockets that provide a firm connection to the artificial
limb without causing skin disorders or limb pain. The
challenges lie in the large loads placed on the previously
non-weight-bearing tissues of the residual limb. As a
result, all socket designs place undue stress on the resid-
ual limb. The traditional patellar-tendon-bearing (PTB)
socket is designed to impose high pressures on the patel-
lar tendon and medial tibial flair, which can reach peak
pressures of 300 to 400 kPa [13]. In contrast, total
contact PTB sockets without gel liners and total sur-
face-bearing sockets with gel liners attempt to distribute
more evenly the pressures across the residual limb with
peak pressures less than 200 kPa [46].
Evidence suggests that skin adapts to these stresses
[79] but also that skin health is clearly compromised.
Ulcers [1011], epidermoid cysts [8,10], Kaposi-like sar-
coma [1213], and verrucous hyperplasia [10,1415] are
skin conditions that are attributed to external pressures
applied to the residual limb. Sustaining proper circulation
and fluid exchange in the soft tissues is imperative for
maintaining a healthy residual limb. Pressures applied to
the limb by socket systems, chiefly during ambulation,
complicate this task. Therefore, clinicians need to be
aware of the pressures applied to the residual limb when
prescribing socket systems to patients, particularly physi-
cally active amputees or those amputees with compro-
mised circulation.
Pin and suction suspension prostheses are commonly
used by amputees. Both systems use gel liners within
undersized total surface-bearing sockets, although their
Abbreviations: A/D = analog-to-digital, ANOVA = analysis
of variance, PTB = patellar-tendon-bearing, SACH = solid
ankle cushion heel.
This material was based on work supported by TEC Inter-
face Systems, Waite Park, MN.
Address all correspondence to Tracy L. Beil, MS; Kaiser Per-
manente Center for Health Research, 3800 N. Interstate Ave-
nue, Portland, OR 97227; 503-335-2400; fax: 503-335-2424;
email: Tracy.Beil@kpchr.org.
DOI: 10.1682/JRRD.2003.09.0146 822
JRRD, Volume 41, Number 6A, 2004
modes of suspension are quite different. Pin suspension
uses a metal pin extending distally from the liner that
locks into a receptacle at the bottom of the socket. Suction
suspension does not use a pin. Instead, suction develops
in the slight air space between the gel liner and socket
when the liner attempts to slide proximally relative to the
socket during swing phase of ambulation. The air space is
sealed by a gel sleeve covering the proximal socket and
liner along with a one-way valve at the distal expulsion
port of the socket.
The pressures associated with pin suspension have yet
to be reported in the literature, despite its prevalent use.
This lack of pressure data is of some concern because of
an apparent link between observed limb changes and pin
use. The symptoms most commonly seen in amputees
who use a locking pin system for suspension are daily red-
dening and swelling of the distal residual limb. Long-term
changes with pin use include general thickening and dis-
coloration of the distal tissues [16]. This condition can
sometimes develop into verrucous hyperplasia in
long-time pin users, which is a hyperplastic disease of
warty papules (10,1415,17).
Prosthetists attribute these symptoms to the liner
being stretched during swing phase, thus squeezing the
limb proximally and creating a heavy localized suction
distally. Manufacturers of pin liners have attempted to
reduce this effect by reinforcing the pin liner with various
stiffening materials to supplement the support already
provided by the fabric covering. These stiffeners extend
from the pin toward the proximal end of the liner. Despite
these attempts and regardless of the type of liner material
(silicone, thermoplastic, or urethane), daily distal swell-
ing and discoloration of the residual limb continue to be
observed with pin suspension.
We designed the current study on transtibial ampu-
tees to measure and contrast pressures applied to the
residual limb when using two methods to anchor the gel
liner to the socket:
1. Distal pin.
2. Suction created between the socket and liner during
unweighting with a sealing suspension sleeve.
We conjectured that the positive (compressive) pressures
during the stance phase of walking will be the same with
both modes of suspension. However, during swing phase,
we hypothesized that the pin-anchored liner will maintain
compressive pressures on the proximal tissues, creating a
squeezing effect while increasing the magnitude of the
negative pressure distally. In contrast, we theorized that
suction suspension will relieve compressive pressures on
proximal tissues and decrease the magnitudes of the neg-
ative pressures distally during swing phase, as illustrated
in Figure 1.
METHODS
To compare the effects of two modes of suspension,
we outfitted urethane liners with pressure sensors such
that limb and liner interface pressures could be measured
during ambulation. Instrumentation consisted of five
force-sensing resistors (Model 402, Interlink Electronics,
Camarillo, CA) that measured positive pressures through
contact and one air pressure sensor (Model 8515c,
Endevco, San Juan Capistrano, CA) that measured the
negative pressure at the distal end of the limb with a full
bridge configuration.
We conducted calibration of the 0.6 mm-thick,
18 mm-diameter contact sensors by applying pressure with
an inflatable air bladder while the sensor was placed on a
flat piece of Shore
®
type OO durometer urethane.
Pressures from 0 to150 kPa were randomly applied twice
Figure 1.
Illustration on left shows pressure application of pin suspension
during swing phase as compared to suction suspension of illustration
on right. The pin effect created by pin suspension is a squeezing of
proximal tissues while suctioning distal tissues. 823
BEIL and STREET. Comparison of limb suspension systems
in increments of 10 kPa while the voltage output was
recorded for each sensor. A piecewise regression was fit-
ted to the data in which an exponential equation was
applied from 0 to 30 kPa, and an equation of the fourth
power was fitted from 30 to 150 kPa. The curvilinear out-
put resulted in decreasing precision with increasing pres-
sure. From 0 to 80 kPa, the average residual was ±0.95 kPa
(0.402.63) and ±2.45 kPa (1.04.07) from 80 to 150 kPa.
We used a sealed chamber attached to a syringe to cal-
ibrate the air pressure sensor. The syringe was drawn to
create pressures in 10 kPa increments from 0 to 80 kPa.
A linear regression equation was fit to the output voltages
with an average residual of ±0.12 kPa (0.040.26).
To ensure sensors would be flush with the inner liner