Evaluation of a New Design: Body-Powered, Upper-Extremity Prosthesis Without Shoulder Harness
Donna Meeks, O.T.R.
Maurice LeBlanc, M.S.M.E., C.P.
Introduction
About 90% of upper limb amputees wear
body-powered prostheses with a shoulder harness and cable for operation. The shoulder harness is not comfortable, and arm amputees
would prefer not to have it.7
The challenge of designing a new type of
body-powered, upper-limb prosthesis was presented to a graduate engineering class at Stanford University.8 Three students elected to work on this project.3 They designed and built
a Triceps Power Capture (TPC) below-elbow
prosthesis and won one of the 1986 RESNA
Student Design Competition awards for their
effort.4
The three students also arranged for a fellow
student and below-elbow amputee to test the
new TPC prosthesis.6 He helped design the test
protocol and evaluate the prosthesis in use.
The Rehabilitation Engineering Center at
Children's Hospital at Stanford sponsored the
student project. Because the results were encouraging, the Center decided to fit another
below-elbow amputee with the TPC prosthesis
to further assess its potential for practical use.
This article describes the results of the fittings of the TPC prostheses and draws conclusions from these two clinical trials.
Description of the TPC Prosthesis
The conventional below-elbow prosthesis is
operated by a combination of shoulder "shrug"
and gleno-humeral flexion to pull on the cable
and open the prehensor (Figure 1)
.
As shown in Figure 2
and Figure 3
, the TPC prosthesis operates by elbow extension, causing a pull on a cable which opens the prehensor.
About 30° of elbow extension is required to
fully open the prehensor. It is a reach-and-open
type of motion.
The three students developed the TPC prosthesis after having done good background research of prior state-of-the-art. Since it was developed, there has been one published article9
and knowledge of three other prosthetists/researchers1,2,5 who have fit the elbow-extension
control below-elbow prosthesis. However,
none of them have the latch at the elbow which
allows free swing and "uncoupling" from the
prosthesis (Figure 4)
.
Method of Evaluation
The procedure for testing was to have the
amputee subjects (a) perform activities with the
standard below-elbow prosthesis and then with
the TPC prosthesis to compare results, and (b)
wear the TPC prosthesis for two weeks and so
licit subject responses.
A five page test protocol for assessing use of
the standard and TPC prostheses in performing
delineated activites was used. The activities
that were compared represent seven groups of
movement and hand coordination. These activities require two hands and occur in daily life
situations.
Execution of activities at times was limited
by the inability of the prosthesis to perform a
task; i.e., the TPC could not lift a heavy suitcase, whereas the standard 5X hook with harness could, but with much fatigue upon the
wearer.
The ranking was based on a 1 to 5 scale.
This small scale resulted in little differentiation
between the two prostheses. Some of the tests
were not timed because they could be executed
indefinitely. All body parts or inanimate objects that assisted in the task were recorded.
Some tasks were not done because the amputee
stated he would always do it with his other
hand, i.e., reach for a book on a shelf or open a
door with a key.
Results
As judged by test scores, the TPC was better
for:
- Activities across the body (no restriction of
harness)
- Activities overhead (control independent of
harness)
- General comfort (lack of harness)
The standard below-elbow prosthesis was
better for:
- Ease of activation (no latch to operate)
- Agility in task performance (requires little
body movement to operate)
In general, the test scores for the two different prostheses were not remarkably different overall. For most activities, they were
relatively close. For other activities, one or the
other was favored in trade-off fashion (Figure 5)
.
Comments
Subjective comments by the amputee subjects included:
- Having no harness with the TPC prosthesis
was nice, both for comfort and donning/doffing.
- The motion that controls elbow extension for
the TPC prosthesis was easier to learn, but
the operation of the latch and remembering if
it was in the latched or unlatched position
made it more difficult.
- Overall, both amputees would favor continuing to use their present prostheses rather
than opting for the new TPC prosthesis, due
primarily to (1) the extra motion required by
the latch and (2) the malfunction of the latch
during trial use. Both said that the TPC
would be an attractive alternative if an
easier, more reliable latch were developed.
Conclusions
While the test scores were close, the amputee subjects did not favor the TPC prosthesis
for ultimate use and acceptance now.
For amputees who cannot or do not want to
tolerate the harness, the TPC prosthesis offers a
good alternative.
The use and acceptance of the TPC prosthesis would be enhanced if the elbow latch
were improved for easier and more reliable activation.
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