Myoelectric Partial-Hand Prosthesis
Robert Putzi, CPO
Introduction
Due to an unfortunate airplane accident, a
young man endured two traumatic amputations and burns covering 80 percent of his
body. When he first became my patient, he
had a left below-elbow amputation and a
right distal transmetacarpal amputation.
Due to his severe burns, fitting the prosthesis
was a problem because normal procedures
did not apply in his case. After many hours
of thought, patient input, and trial and error,
a solution was reached that provided the patient with practical use and satisfaction.
At the outset, after numerous skin grafts,
the patient, "Mark," was fitted with a left
below-elbow myoelectric prosthesis. A conventional prosthesis could not be used because of problems fitting the harness due to
severe burns in the axilla area. Mark, who is
predominantly right-handed, used this prosthesis to make up for lost dexterity in his still-mending right hand. After a lengthy recuperation and further skin grafts, the right
hand was healthy enough for fitting and the
real challenge began (see Figure 1
).
Design
The first attempt was to use a mechanical
prosthesis, but again, difficulties with the
harnessing weree experienced. It was then decided to use an Otto Bock myoelectric hand,
which was dismantled except for the main
frame. First, the motor and transmission
were reattached at a 45-degree angle relative
to the frame (see Figure 2
).
To minimize the mechanism's length, the
tension spring of the inner hand shell was
eliminated. The amputation in the distal
metacarpal region was 11/4-inch larger than
the wrist section; therefore, to facilitate donning and removing the prosthesis, a socket
with an angled opening and a flap covering
secured with Velcro was made (see Figure
3
).
The stripped frame was then attached to
this socket with carbon and fiberglass in a 45degree angle, which placed the fingers of the
myoelectric hand in a functional position
(see Figure 4
). This combination resulted in
only a 3/4-inch discrepancy from a normal
metacarpal phalangeal joint. The next task
was placing the electrodes.
The electric output of the flexors and extensors was very good. However, there was a
problem with the pronator. When stimulating the extensor in the closed position, the
pronator inadvertently opened the hand.
The patient, himself, found the correct
placement for the extensor electrode, thereby eliminating the problem.
Using experience gained in orthotics, the
socket with the electrodes was connected in
accordance with the principles of the wrist-driven flexor hinge or tenodesis splint. The
goal was to maintain the exact location of the
electrodes in each of the hand's positions.
Three objectives for the upper socket design
were to
- cover a minimal amount of surface area
- provide a method of donning and removing the prosthesis with ease and
- conceal and protect all cable connections to the hand (see Figures 5, 6 and 7
.
These objectives were accomplished by
using a double lamination (see Figure 8
).
After successfully completing lamination,
the electrical hand controls had to be installed. Because Mark's wrist area was still
intact, the existing controls had been removed from their original position in the
hand. The Otto Bock team in Minneapolis,
comprised of Jack Hendrickson, CP, Rick
Schmierer and Dennis Miller, was very helpful in supplying a new electronic control
module that could be installed separately in
any location. They also supplied an on/off
rocker switch, which was installed below the
battery container. The skeleton hand operated perfectly with these controls; however,
some difficulties were encountered when the
inner hand shell was placed in position.
The removal of the tension spring from the
hand shell prevented downshifting of the
transmission, causing loss of grip force and
loss of one-third of the fingers' opening capabilities. To overcome these deficiencies, the
inner hand shell was drastically modified.
Eventually only the finger covers were used.
The cosmetic glove, subsequently added, in
no way impaired the hand's movement (see
Figure 9
).
Results
Upon completion of the new prosthesis,
Mark learned to operate his new hand immediately and mastered its various uses far
sooner than expected (i.e., eating on his
own). Interestingly, Mark commented that
the proprioception of the right hand was
much greater than that of the left myoelectric hand. One could explain such an occurrence by the use of less hardware, thus less
weight distally to the amputation (see Figure
10
).
Conclusion
It is felt that this reconstruction of the myoelectric prosthesis was a satisfactory solution
in providing the patient with as much hand
and arm mobility as possible in light of his
condition. By using basic principles of orthotics and prosthetics, and exercising ingenuity in using existing proven components, it
is possible to provide improvement in function and cosmetics to an individual with a
partial hand amputation.
Acknowledgements
The author would like to thank the Otto Bock
team for its excellent cooperation and inspiration.
Robert Putzi is a certified prosthetist/orthotist
with Hittenbergers, 1117 Market St., San Francisco, CA 94103.
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