TECHNICAL FORUM-- Axilla Bypass Ring for
Shoulder harnesses for
Upper-Limb Prostheses
Milo Collier, CP
Maurice LeBlanc, MSME, CP
ABSTRACT
Some arm amputees who wear upper-limb, body-powered
prostheses experience discomfort in the sound-side axilla
due to pressure exerted by the so-called "Figure 8" shoulder
harness. This article reports the development and testing of a
ring to relieve pressure in the axilla. Subjective responses to
initial testing by amputees suggest the ring helps relieve the
discomfort.
Introduction
The standard "Figure 8" shoulder harness (see Figure 1
) for
upper-limb, body-powered prostheses has an axilla loop on
the sound side that commonly is uncomfortable and can
cause numbness and
nerve damage (1). This
problem may account
for the estimated 50 percent of arm amputees
who choose not to wear
prostheses (2).
Attempts to relieve
the pressure of the harness strap in the axilla
have been made by using padding with Dacron felt and pre-shaped
plastic sleeves to cover
the harness strap.
These solutions reduce
but do not alleviate the
problem. When an arm
amputee with a Figure
8 harness drives a car
with arms stretched
forward, his/her arms often fall asleep because of pressure
in the axilla from the tight harness strap. The pressure
problem also exists with the double axilla loop or "butterfly" harness.
The Hessing axilla pad (see Figure 2
) also has been used
in attempting to solve the pressure problem. The Hessing
pad is a cowhorn-shaped pad adapted from the pad on the
Hessing crutch (3). However, it has not found much use in
clinical practice.
Prototype Axilla Bypass Ring
A prototype axilla bypass ring was conceived several years
ago (4) to relieve harness strap pressure in the axilla for
arm amputees using Figure 8 shoulder harnesses (see Figure 3
and Figure 4
). The concept recently was revived, and several test rings were created.
The current ring is made of 1/8-inch-thick acrylonitrile butadiene styrene (ABS) plastic sheet vacuum-formed over a
model of the shoulder (see Figure 5
). It also can be made of
leather or a more flexible material than ABS.
The benefit of the axilla bypass ring is it places the forces
of the harness in the deltopectoral groove and on the scapula where they can be borne more comfortably. The ring effectively bypasses the axilla and therefore places no uncomfortable or potentially damaging pressure there. Relatively high forces can be taken in the deltopectoral groove
and on the scapula.
Trial Fittings
Twelve adult and six child arm amputees were fitted with the
axilla bypass ring. Patient acceptance was high. All 18 amputees subjectively judged the ring to be more comfortable
than and preferable to their standard methods of axilla relief.
Wear of the bypass ring indicated that the line of the control cable pull can be determined and held in place more
easily with the axilla bypass ring because it is more stationary than a sewn Figure 8 or Northwestern ring harness. The
harness of the bypass ring tends to stay in place better
when amputees raise their arms overhead whereas the conventional Figure 8 harness rides up on the shoulders, rendering it less effective.
Discussion
The prototype axilla bypass ring appeared to work satisfactorily for the initial 18 amputees who tested the prototype.
It is envisioned the ring can be made in three standard sizes
for right and left sides and can be fitted as a component off-the-shelf rather than having to be custom-molded for each
individual.
Acknowledgements
This work is an outgrowth of a project supported in part by grant
133MH70021 from the National Institute on Disability and Rehabilitation Research (NIDRR), U.S. Department of Education,
Joseph E. Traub, program manager.
MILO COLLIER is a certified prosthetist in private practice as the principal of MICA Corp., 1152 Douglas St., Longview, WA 98632; (800)444-6049.
MAURICE LeBLANC is a mechanical engineer/certified prosthetist and director of research at Packard Children's Hospital at Stanford, 725 Welch Road, Palto Alto, CA 94304;(415)497-8192.
References:
- American Academy of Orthopaedic Surgeons. Atlas of limb
prosthetics. C.V. Mosby, 1981 ;108.
- LeBlanc MA. Patient population and other estimates of prosthetics and orthotics in the USA. Orth and Pros September 1973.
- Santschi, ed. Manual of upper-extremity prosthetics, 2nd ed.
UCLA, 1958; 167-8.
- LeBlanc MA. Feasibility for innovation and improvement of
body-powered upper-limb prostheses. Research Report for period 9/30/84-9/29/85, NIDRR Fellowship 1 33FH40021.
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