Myoelectric Wiring Technique for Children and Young Adults
Mark Moseley, C.P.
Eric Baron, C.P.O.
Current advancements in electronic components have made it not only possible but increasingly desirable to fit myoelectrically controlled prostheses on young children with a
congenital or acquired amputation of the upper
extremity. A variety of electrically operated
prosthetic hands may be considered for these
children when the child has demonstrated readiness to accept an activated prosthesis; the child
may be as young as two years of age. The cosmetic appearance of the prosthesis can be enhanced by the following technique incorporating a terminal device that will also provide
excellent function.
Current myoelectric systems utilize a natural
neuromuscular pattern for controlling grasp and
release of objects by the terminal device. The
flexor and extensor muscles of the remaining
forearm act to deliver the required myoelectric
signals for terminal device operation. This "biological" control is essential for the young
child as no unnecessary learning of the control
system is generally required.
An additional feature often overlooked in
myoelectric prosthetic applications is the functional aspect of the cosmetic glove. Young
children continue to utilize many gross motor
activities in their daily lives that are often not
facilitated by the hard smooth surface of a conventional type of prosthesis. Catching a ball,
holding a large toy or climbing on playground
equipment are just a few activities that children
participate in which require gross motor skills
and a friction surface, such as provided by a
glove (Figure 1)
.
The design and placement of the battery pack
or power source for the myoelectric prosthesis
is critical. The space available, the size or configuration, the location of the battery, and the
weight distribution are all important considerations with myoelectric fittings in general, but
especially important for the young child. The
battery position should not in any way restrict
regular activities that will be performed using
the prosthesis. Careful thought and planning
must be used, as this may be the single greatest
challenge in providing a reasonable and useful
result for the child and the family.
With the above mentioned points in mind,
two versions of a technique dubbed "the wireless battery" are presented. When this technique is properly used, there are no exposed
wires and the electrical connections from the
battery to the hand are allowed to pivot freely
without any twisting and eventual fatiguing of
the wires (Figure 2)
. The technique allows for
reasonable weight distribution, and the battery
location prevents any interference with functional activities (Figure 3)
.
Wireless Battery #1
A clip receptacle battery holder (Otto Bock
13E52) is riveted to the humeral cuff (constructed of lmm thickness ortholen or equivalent) on the posterior portion at the mid-humeral level. The metal clip from the receptacle
is removed from the plastic casing and the wire
is allowed to pass through a small hole at the
corresponding location on the cuff to the inside
of the humeral cuff. The wire is then separated
into positive and negative leads and soldered to
stainless steel snaps set on the elbow axis
where the humeral cuff will attach to the outer
forearm of the prosthesis (Figure 4)
. The positive and negative leads should lay flat along the
inside of the humeral cuff. The entire inside of
the cuff is then covered with moleskin, or
something similar, to protect the wiring and
provide a smooth, comfortable lining which
will lay against the child's arm.
On the distal portion coming from the electrode/hand assembly, the positive and negative
leads are also separated, leaving enough slack
so the inner socket can be easily removed for
servicing and/or maintenance. These two leads
are each soldered to the corresponding snap
which has been fastened to the proximal portion of the forearm (Figure 5)
. The inside surface of each snap at this location should be covered with a piece of friction or plastic tape to
protect against corrosion from moisture or perspiration. With the cuff snapped to the prosthesis and a charged battery properly placed in
the clip receptacle, the proper electrical connection is now complete. The cuff should pivot
freely about the snaps, and the connections
should be secure enough to ensure that the
function of the prosthesis is maintained
throughout the child's wide variety of activities.
Wireless Battery #2
In this version of the "wireless battery" the
ni-cad cells are removed from the battery
casing (Otto Bock 757B8). They are then
placed in a horizontal position on the posterior
portion of the humeral cuff. Positive and negative leads need to be soldered to the battery
cells and passed through the cuff to the snaps as
in the previous version. The cells are then covered with leather or plastic which is fastened
to the posterior portion of the humeral cuff.
This ensures that the battery cells are protected
against exposure and damage (Figure 6)
. The
wiring of the distal portion of the electrode/hand assembly is identical to the first version.
Since the battery portion in the wireless battery #2 is now no longer removable for proper
recharging, an adaptation must be made to the
battery charging unit (Otto Bock 757L11). The
leads on the inside of the charger are disconnected from their original connections and soldered to additional male portions of snaps secured to each side of the charging unit. This
allows for the battery pack/cuff to be removed
from the prosthesis and snapped to the charging
unit for proper recharging of the ni-cad cells
(Figure 7)
. Two battery/cuff assemblies should
be provided so that proper function can be
maintained at all times.
Conclusion
Experience has shown that the "wireless Mark
battery" techniques offer a reliable electrical
connection for myoelectric prosthetic fittings in cases where it is desired to locate the battery
outside the forearm of the prosthesis. By decreasing the distal or terminal weight and maintaining excellent cosmesis, this technique improves the feasibility of myoelectric prosthetic
applications for the young child (Figure 8)
.
Mark Moseley, C.P., is an Associate Specialists at the UCLA Child Amputee Prosthetic Project, 1000 Veteran Avenue, Los Angeles, California 90024; (213) 825-5201.
Eric Baron, C.P.O., is an Associate Specialists at the UCLA Child Amputee Prosthetic Project, 1000 Veteran Avenue, Los Angeles, California 90024; (213) 825-5201.
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