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Section 2: Upper Limb
the muscle, thus providing length sufficient for a coaptation while limiting the
overall reinnervation length and, as a
result, the time required for reinnervation. Under loupe magnification, a nerve
coaptation is done by bringing the small
motor nerve into the center of the much
larger median nerve; 7-0 polypropylene
suture is used. To guard against dehiscence, nearby epineurium is sutured
to adjacent muscle epimysium using
additional small-caliber polypropylene
sutures. Successful neurotization of the
short head after the median nerve transfer will result in an intuitive hand-close
signal, and preservation of the musculocutaneous nerve innervation to the long
head of the biceps is maintained as an
elbow flexion signal. If the brachialis
muscle is present, the ulnar nerve can be
identified posterior to the median and
medial antebrachial cutaneous nerves,
through the same anterior approach, and
it is mobilized to the motor nerve of the
brachialis muscle. Because the brachialis has retained its musculocutaneous
innervation, a portable nerve stimulator
can be used to track the continuation of
the musculocutaneous motor nerve to
its entry into the brachialis. The ulnar
nerve transfer is useful for control of a
prosthetic wrist. Before the anterior incision is closed, the previously elevated
adipofascial flap is positioned between
the short and long heads of the biceps to
encourage spatial differentiation of the
myoelectric signals. The elevation of this
flap and deeper transposition also may
improve myoelectric signal conduction
through the thinned overlying skin flaps
and limit aberrant reinnervation.
The distal radial nerve is then transferred. Although the posterior dissection
can be done with the patient supine,
with the shoulder hyperextended it is
much easier to reposition the patient
and perform the radial nerve transfer
with the patient in the prone position.
A generous longitudinal incision is made
between the long and lateral heads of
the triceps, and a proximally based
adipofascial flap is again elevated. It
is helpful to begin the deep dissection
relatively cephalad because the interspace is best found where the deltoid
insertion overlies the proximal triceps.
Elevation of the long head of the triceps
typically reveals the major trunk of the
radial nerve, with one or two small motor nerves branching to supply the lateral head (Figure 3). The radial motor
branch to the long head of the triceps
arises significantly more proximally
and usually is not seen. This exposure
is analogous to that used for radial
nerve transfers intended for restoration
of shoulder abduction. The radial nerve
typically enlarges after the amputation
and feels relatively firm. Stimulation of
the main trunk should fail to produce
any muscle contraction. As with a median nerve transfer, the radial nerve
is divided and coapted to the motor
nerve(s) to the lateral head of the triceps
close to the motor entry point. A handopen signal will be created in the lateral
head of the triceps, with preservation
of the elbow extension signal mediated
by proximal radial nerve innervation of
the long head of the triceps. As with the
anterior approach, the adipofascial flap
is positioned along the raphe between
the long and lateral heads of the triceps.
Postoperative drains and a mildly compressing dressing are applied. Therapy
to maintain shoulder motion can be initiated 2 weeks after the nerve transfer
procedure.
8
Several technical pearls have emerged
from experience with the transhumeral
TMR procedure. First, it is helpful to
avoid a distal division of the biceps or
brachialis muscle because the muscle
will retract proximally and may become
buried beneath the deltoid muscle; as a
result, an EMG signal can be obscured
by the overlying deltoid muscle. Second, it is important to widely explore
the space between the target muscle
bellies to identify all of the individual
motor nerve branches to each muscle. To
encourage reinnervation and eliminate
Figure 3
exposure in a p atient undergoing transhu meral
targeted muscle reinnervation. Th e radial nerve
(solid arrow) and a motor b ranch from the radial
nerve to the lateral head of the triceps (dashed
arrow) have been identied. The motor branch
will be transected and will serve as the recipient for a coaptation with the distal radial nerve
proper. The patient also was treated with an
angulation osteotomy of the humerus.
Photograph show ing a posterior
cross-talk competition from remnant
native innervation, it is critical to completely denervate the target muscle
before performing the nerve transfer.
The skin flap thinning that occurs as a
byproduct of adipofascial flap elevation
serves to improve signal detection by
limiting the distance between the reinnervated muscle and cutaneous electrode. Liposuction or direct excision of
the subcutaneous tissues can facilitate
future signal detection at sites remote
from the access incision.
Ultimately, the success of reinnervation is contingent on the ability to
achieve normal fascicular architecture
by sufficient proximal trimming of the
donor nerve before transfer. Two factors favor successful reinnervation of
the target muscle: TMR (unlike hand
transplantation, in which maximal
nerve length must be preserved) almost
always requires coaptations at a location
proximal to the site of nerve injury, and
Atlas of Amputations and Limb Deciencies, Fourth Edition © 2016 American Academy of Orthopaedic Surgeons
332
Chapter 27: Targeted Muscle Reinnervation for Prosthetic Control and Treatment of Neuroma Pain
there are substantially more fascicles in
the donor nerves than in the coapted
recipient nerves. The second factor has
been called hyperinnervation. In the experience of this chapter’s authors, more
than 95% of these transfers yield detectable EMG control sites, without the need
for nerve wraps or fibrin glue. Theoretic
failures of TMR would occur from a nonviable muscle segment after isolation, a
poor donor nerve with an unsuspected
higher injury, a donor nerve that does
not comfortably reach the target motor
nerve, and aberrant reinnervation. Over
the past decade, this chapter’s authors
have seen each of these issues only once.
Shoulder
Disarticulation TMR
General Considerations
TMR at the shoulder level is primarily
indicated to improve poor prosthetic
function, despite adequate rehabilita
tion, in a patient with a standard prosthetic device. After amputation at the
shoulder level, poor function is ubiquitous with currently available prostheses.
In some respects, TMR at the shoulder
level has even greater potential than at
the transhumeral level because of the
greater loss of native innervation and
the limitations of conventional prosthetic rehabilitation. The preoperative
workup is similar to the workup for a
patient with a transhumeral amputation,
although the physical examination must
particularly evaluate the function of the
pectoralis major, serratus anterior, and
latissimus dorsi muscles. A history of
limb avulsion should heighten suspicion that the nerve endings may be too
proximally located to allow tension-free
transfer to the chest wall muscle targets.
Although nerve grafts are not desirable,
they can be used to add length to donor
nerves. Alternatively, free muscle transfer has been used to bring the muscle
target to short donor nerves.9 Brachial
plexopathy is the only absolute contraindication to surgery. Relative contraindications include a lack of muscle
Figure 4
incision markings in a patient undergoing targeted muscle reinnervation after shoulder disarticulation. The clavicle has been outlined to
serve as a reference point.
Photograph showing curved
targets, a lack of distally located nerve
-
endings, poor-quality local soft tissue,
and inability of the patient to tolerate a
3- to 5-hour surgical procedure.
Surgical Technique
Access to the brachial plexus and proximal nerve branches is achieved using
an infraclavicular approach through
the interspace between the sternal and
clavicular heads of the pectoralis major.
A transverse incision is designed approximately two fingerbreadths below
the clavicle (Figure 4). As in the transhumeral approach, a medially based
adipofascial flap is elevated (Figure 5).
In addition, the subcutaneous tissue
overlying the pectoralis major muscle
is thinned in an area of approximately
100 cm2 extending from the sternum
to the anterior axillary line and from
the clavicle inferiorly toward the nipple. The motor nerve to the clavicular
head usually is the first to be found in
the space between the two heads of the
pectoralis. This nerve enters the muscle
in a vertical direction and almost always
lies adjacent to the vascular pedicle to
the muscle. Occasionally, a second small
motor nerve innervates the muscle more
Figure 5
fraclavicular approach for targeted muscle
reinnervation in a patient with a shoulder
disarticulation. A medially based adipofascial
ap has been elevated and reected medially.
The space between the clavicular and sternal
heads of the pectoralis major muscle has been
opened an d is being maintained with th e use of
a self-retaining retractor.
Photograph showing an in-
laterally. Dissection proceeds inferiorly,
where medial, middle, and lateral motor
branches can be found innervating the
sternal head. The middle branch emerges medial to the pectoralis minor tendon,
and the lateral branch commonly travels
through the substance of the pectoralis
minor muscle. All nerve branches to
the pectoralis major must be identified
to ensure complete denervation of the
muscle before the nerves are transferred.
The exact origin of these motor nerves is
not relevant because there is no reason
to preserve native pectoralis function
in the absence of an upper arm. Only
the size and distribution of the motor
entry points is pertinent to the pattern
of nerve transfers. For this reason, an
extensive supraclavicular approach to
the brachial plexus is not justified.
After the pectoralis motor branches are identified, the donor nerves are
located as they course deep to the pectoralis minor tendon (Figure 6). The
donor nerves are differentiated by their
proximal branching pattern and relative
size. The radial nerve is the largest of the
donor nerves. However, accurate identification of each donor nerve is not critical to the success of the procedure. The
pattern of nerve transfers primarily is
© 2016 American Academy of Orthopaedic Surgeons Atlas of Amputations and Limb Deciencies, Fourth Edition
333
Section 2: Upper Limb
Figure 7
nerve trimming in a shoulder disarticulation
targeted muscle reinnervation. The end neuromas are excised f rom the brachial donor ne rves,
and the nerves are trimmed proximally until
healthy individual fascicles are observed.
Figure 6
(U) brachial donor nerves, which are identied as they run deep to the pectoralis minor tendon
(retracted with a Penrose drain).
Figure 8
has been coapte d to the clavicular head motor br anch (solid arrow), and the median ner ve has been coapted to the me dial and middle motor bran ches
to the sternal head (dashed arrow). B, The ulnar nerve will be coapted to the lateral motor branch to the sternal head (arrow). C, The radial nerve will
be coapted to the thoracodorsal nerve (arrow).
Photograph show ing the median (M), musculocutan eous (MCN), radial (R), and ulnar
Photographs showing nerve coaptation in a shoulder disarticulation targeted muscle reinnervation. A, The musculocutaneous nerve
Photograph showing donor
dictated by the inherent spatial arrange
ment of the donor and recipient nerves,
rather than by a preplanned pattern of
transfers based on intended function.
The proximal thoracodorsal nerve can
be found deep to the mobilized donor
nerves if the latissimus dorsi muscle is
to serve as an additional transfer recipient. After adequate dissection, the donor nerves are trimmed with the goal
of achieving normal fascicular architecture (Figure 7). However, the proximal
Atlas of Amputations and Limb Deciencies, Fourth Edition © 2016 American Academy of Orthopaedic Surgeons
334
-
nature of the nerve injury necessitates
tempering the extent of trimming to
maintain length sufficient for avoiding
tension at the nerve coaptation sites. Often it is possible to remove 2 to 3 cm of
damaged nerve without compromising
the nerve transfer. The nerve coaptation
is done in a manner similar to that used
for transhumeral TMR.
The most commonly used pattern
of transfers primarily is based on the
proximity of donor nerves to recipient
sites. The musculocutaneous nerve is
transferred to the motor nerve branch
of the clavicular head, the median and
ulnar nerves are transferred to the motor nerves innervating the sternal head,
and the radial nerve is coapted to the
thoracodorsal nerve (Figure 8). Because
the radial-thoracodorsal coaptation site
is relatively proximal, a relatively long
time period elapses before reinnervation
of the latissimus muscle control site. The
serratus anterior and pectoralis minor
Chapter 27: Targeted Muscle Reinnervation for Prosthetic Control and Treatment of Neuroma Pain
can serve as alternative targets for reinnervation but are less desirable because
of the deep position of the muscles on
the chest wall. The previously elevated
adipofascial flaps and/or free fat grafts
are used to create separation between
the pectoralis segments.
When the humeral head is removed
at the time of the amputation, the pectoralis and all the associated motor nerves
retract medially toward the sternum,
which makes the donor nerve dissection more complicated. If a portion of
the humerus is present, it should not
be excised even if it is too short to serve
as a lever arm for prosthetic control.
Preserving the pectoralis major insertion keeps the pectoralis muscle out
to length, and the broad surface area
facilitates later signal acquisition. In addition, the presence of the humeral head
creates a cosmetically pleasing contour
under clothing. Occasionally, a small
remnant of triceps remains innervated
by the radial nerve and is usable for an
elbow extension signal.
After quilting sutures are placed, the
skin is closed over closed suction drains
to reduce the risk of seroma formation
after the extensive subcutaneous thinning. The patient typically resumes use
of the original prosthesis 4 to 6 weeks after surgery, when postoperative swelling
subsides and the wound has adequately
healed. Prosthetic fitting for new control
sites is done when EMG signals from the
newly neurotized muscles have become
robust; a minimum of 3 to 6 months
usually is required. The latissimus muscle, which is most distant from the coaptation site, can take 9 to 12 months to
show detectable reinnervation.
TMR for Lower Limb
Motor Control
Amputation is more common in the
lower limb than in the upper limb, and
often it is done at a more proximal level
than in the upper limb.
infrequent use of lower limb myoelectric
devices precluded early adaptation of
10,11
However, the
the TMR technique to the lower limb.
Recent improvements in the durability
and functional capabilities of motorized
knees and ankles have justified a thorough evaluation of the possible benefits
of TMR for patients with a lower limb
amputation. The goal of the transfemoral
TMR procedure is analogous to that of
TMR in the upper limb: to use nerve
transfers between distally transected
nerves and functionally redundant
muscles in the residual limb to create
an EMG representation of the amputated lower leg muscles within the residual limb. Although early results have
been encouraging, it remains to be seen
whether salvage of this additional neural
information will lead to improved prosthetic function.
The surgical technique and prosthetic control strategy for lower limb
TMR were outlined in two recent clinical reports.
12,13
The essential principle
is to transfer the tibial division of the
sciatic nerve to a medial hamstring
(semitendinosus or semimembranosus) and the peroneal division of the
sciatic nerve to a lateral hamstring (the
short or long head of the biceps femoris)
while maintaining at least one natively
innervated hamstring muscle to control
prosthetic knee flexion. In this manner,
prosthetic knee flexion-extension and
ankle plantar flexion-dorsiflexion can
be intuitively controlled.
Neuroma Management
Beyond its established role in enhancing myoelectric control, TMR may be an
effective technique for the treatment of
painful postamputation neuromas. TMR
is believed to inhibit neuroma formation by providing both a vascularized
scaffold that can guide regenerating
axons and a denervated muscle target
toward which growth can be directed.
In essence, TMR gives the regenerating
nerve somewhere to go and something
to do; these elements are lacking in
other, largely unproved neuroma treatments. This use of TMR is supported
by histomorphologic data from preclinical studies as well as by retrospective
outcome data from two separate clinical
studies.
14-18
A multicenter randomized
controlled study is under way to rigorously evaluate the role of TMR in the
management of postamputation neuroma pain.
A wide variety of nerve transfers can
be used in TMR done primarily for neuroma management. To treat neuroma
pain at the transhumeral or shoulder
disarticulation level, TMR should be
done as previously described, regardless
of whether the patient complied with
earlier rehabilitation or wear requirements for his or her prosthesis. Residual limb pain is a common obstacle to
prosthetic rehabilitation, and resolution
of the pain may allow the patient to increase use of his or her prosthesis, which
would have greater functional potential
after TMR.
At the transradial level, ulnar and
median neuromas and their motor
nerve targets can be accessed through
a volar longitudinal incision between
the flexor carpi ulnaris and the flexor digitorum superficialis. The radial
nerve is dissected through a separate
incision and passed into this working
space. Motor nerves innervating the pronator quadratus, flexor carpi ulnaris,
and flexor digitorum superficialis are
the most common targets after neuroma
excisions (Figure 9).
For transtibial amputations,
symptomatic neuromas of the deep
and superficial peroneal nerve, the
tibial nerve, and the sural nerve can
be excised and the newly freshened
nerve endings transferred locally to
the motor nerves of the medial and/
or lateral gastrocnemius muscles, the
soleus muscle, the anterior tibial muscle, or even trimmed more proximally
and coapted to a motor branch of the
semitendinosus muscle (Figure 10).
Similarly, sciatic end neuromas in a patient with a transtibial amputation can
be transferred to the semitendinosus or
© 2016 American Academy of Orthopaedic Surgeons Atlas of Amputations and Limb Deciencies, Fourth Edition
335
Section 2: Upper Limb
Figure 9
radial targeted muscle reinnervation (TMR) for
management of neuroma pain. Large median
nerve and smaller ulnar nerve end neuromas
have been dissected through a volar forearm
approach. TMR was performed with transfer of
the median nerve to a motor branch to the exor digitorum supercialis, and the ulnar nerve
was transferred to the motor ner ve of the exor
carpi ulnaris (ner ve coaptations are not shown).
Photograph showing trans-
semimembranosus muscle using a posterior midline approach, as described
in a cadaver feasibility study.19 In these
patients, it seems important to perform
the nerve transfers and for the patients
to actively exercise the target muscles
after neurotization has occurred.
Ultimately, any motor nerve branch
can serve as a nerve transfer recipient
after neuroma excision, provided the
level of morbidity associated with sacrifice of the recipient motor branch is acceptable. This technique is particularly
applicable to patients with a major limb
amputation because of the availability of
multiple muscle targets and absence of
functional donor site morbidity. However, consideration must be given to the
effect of potential atrophy from target
muscle denervation on the overall quality of the residual limb coverage.
Although most TMR procedures have
been performed for either prosthetic
control or in cases of chronic neuroma
pain, a recent case report highlighted
the effective use of TMR in the acute
setting for neuroma prevention after a
traumatic shoulder disarticulation.20
The patient reported no local neuroma pain or phantom discomfort at
8 months postoperatively. The potential
Figure 10
management in a patient with a transtibial amputation. Chronic pain in the common peroneal
nerve end n euroma was treated with ne uroma excision and transf er to a motor branch to the biceps
femoris muscle. A, The sciatic nerve is separ ated into its tibial and pe roneal components, w ith loops
around each. B, The common peroneal nerve end neuroma and motor ner ve to the biceps femoris
muscle were dissected out (each is lying on top of the yellow backgrounds). C, Nerve coaptation
was performed after excision of the common peroneal nerve end neuroma and neurotomy of the
motor nerve to the biceps femoris muscle.
use of TMR as a prophylaxis against the
development of pain after amputation
will only be determined by broad-based
surveys of pain and phantom discomfort
in amputees. Such surveys are currently
being conducted.
Intraoperative photographs showing targeted muscle reinnervation for neuroma
prosthesis (for example, patients are able
to avoid dropping or crushing objects
because they can sense the extent of the
force being applied), and it may allow
patients to better integrate the prosthesis
into their self-image. Mechanisms to enable the prosthesis to capitalize on such
Directions of Future Inquiry
Targeted sensory reinnervation (TSR)
and regenerative peripheral nerve interfaces are experimental adjuncts to
TMR that have the potential to dramatically improve a patient’s functional
capacity. In several patients, a sensory
nerve in the vicinity of the motor nerve
transfers has been coapted end-to-side
to the donor median or ulnar nerve to
achieve TSR.21 Stimulation of the reinnervated skin results in the sensation
of the patient’s hand being touched,
thus providing cortical feedback to the
hand representation on the cortical homunculus. There is restoration of all
modalities of cutaneous sensation, including pressure, vibration, and thermal
sense. These patients gained an ability
to discriminate gradations of force that
information through haptic feedback are
still in development, but integration of
TSR into prosthetic function represents
a major step toward complete restoration
of the amputated limb.
Regenerative peripheral nerve interfaces seek to overcome the loss of signal
fidelity that results from surface recording of a deeper nerve signal. Attempts
at direct nerve signaling have been
plagued by iatrogenic nerve injuries,
poor durability, and neuroma formation.
The newest generation of implantable
electrodes, like TMR, uses muscle as a
biologic amplifier to record neural activity. However, the implantable nature of
the electrodes allows recording immediately adjacent to the nerve and thereby
substantially improves signal quality
without requiring direct nerve contact.
22
matched that of their uninjured skin.
TSR improves the usefulness of the
Atlas of Amputations and Limb Deciencies, Fourth Edition © 2016 American Academy of Orthopaedic Surgeons
336
Chapter 27: Targeted Muscle Reinnervation for Prosthetic Control and Treatment of Neuroma Pain
Summary
TMR has been shown to restore intuitive
limb control in patients with upper limb
amputation and is appropriate for use
with transhumeral or more proximal
amputation levels. TMR can improve
the patient’s function and has the potential to alleviate pain from symptomatic neuromas.
References
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Atlas of Amputations and Limb Deciencies, Fourth Edition © 2016 American Academy of Orthopaedic Surgeons
338
Chapter 28
Targeted Muscle Reinnervation:
Prosthetic Management
Robert D. Lipschutz, CP, BSME
Abstract
Targeted muscle reinnervation is a surgical technique that has been developed to improve
an individual’s ability to control his or her myoelectric prosthesis. Prosthetists who attempt
to t individuals with powered prostheses aer target muscle reinnervation must rst
be knowledgeable in the basic principles of tting myoelectric devices. In addition, the
prosthetist should understand the surgical procedure and the intended outcomes. Applications of traditional and advanced myoelectric control strategies are essential to achieve
successful outcomes. e incorporation of emerging technologies will benet both the user
and the prosthetist.
Keywords: antagonistic muscle action; coaptation;
electromyographic (EMG) signal; mode selection; myoelectric control;
shoulder disarticulation; transhumeral amputation
Introduction
For individuals with proximal levels of
upper limb amputation (transhumeral
amputation or shoulder disarticulation),
successful operation of an upper limb
prosthesis requires the performance of
control motions that are rarely analogous to actions performed before the
amputation. For body-powered prostheses, users must incorporate body
movements such as glenohumeral flexion, scapular protraction, or biscapular
protraction to flex the elbow and/or operate the terminal device. In myoelectrically controlled, externally powered
systems, the actions for controlling a
powered elbow can be more physiologic for individuals with transhumeral
amputations if contraction of the biceps
is used to control elbow flexion and
triceps are used to control elbow extension. Beyond this exception, for the
Neither Mr. Lipschutz nor any immediate family member has received anything of value from or
has stock or stock options held in a commercial company or institution related directly or indirectly
to the subject of this chapter.
other powered components of a transhumeral prosthesis (electronic wrists
and terminal devices), and all the motors
in an externally powered prosthesis for
shoulder disarticulation, both the electromyographic (EMG) input signals and
other physical body movements that are
used as control inputs are nonphysiologic strategies. For example, strategies for
using a myoelectrically controlled transhumeral prosthesis may include using
the residual biceps and triceps muscles
to control elbow flexion and extension,
wrist supination and pronation, and terminal device prehension. Various mode
selection strategies are required to allow
the user to switch between these antagonistic pairs of movements. An example
of mode selection may be an intentional
co-contraction of the biceps and triceps
muscles to switch active control inputs
from one component to another (for
example, elbow to hand). Other methods of mode selection could include activating a bump switch mounted on the
exterior surface of the prosthetic socket
or a momentary pull switch incorporated into the harness. Regardless of
the method of mode selection, another
action is required to direct input signals
from one component to another. More
importantly, the muscle signals used to
control these various motors are rarely
consistent with the action being performed. For example, contracting the
biceps and triceps muscles to control
hand closing and opening is not necessarily intuitive.
Targeted muscle reinnervation (TMR)
is a means by which users can operate
their myoelectrically controlled prostheses in a manner that is more intuitive
and physiologically consistent with the
actions and thought processes that users
had before their amputations.
two to three physiologic electrode sites
for the individual with a transhumeral
prosthesis, and four to six physiologic sites for the shoulder disarticulation
prosthesis, creates the potential for a
more natural means of controlling the
prosthesis. TMR also allows the user to
control multiple motors simultaneously, thus reducing the delays associated
with mode selection for most prosthetic
actions and movements.
This chapter discusses the general
principles of fitting individuals with upper limb prostheses who have undergone
TMR, the challenges encountered during
clinical fittings, technological advancements in TMR, and the potential applications of TMR to lower limb prostheses.
1-8
Adding
© 2016 American Academy of Orthopaedic Surgeons Atlas of Amputations and Limb Deciencies, Fourth Edition
339
Section 2: Upper Limb
General Principles
Although TMR was initially developed
for humans, it is a relatively foreign concept to most in the medical community.
Therefore, it is likely that the prosthetist
or even the individual with the amputation might propose this form of surgical
intervention to other members of the
healthcare team outside of those few
centers where surgeons routinely perform these procedures. Thus, it is essential that the entire team—including
the patient, the surgeon, the physiatrist,
the prosthetist, the occupational therapist, and nurses—fully comprehends
the surgical procedure, any benefits and
risks associated with the process, the
postoperative protocol, and prosthetic
fitting principles and training.9
Muscle Recovery Period
After a successful TMR procedure, the
patient will be asked to pay particular
attention to his or her neuromuscular
development of reinnervation, most
notably what occurs when attempting
to perform a muscle contraction that
was absent before TMR (for example,
closing and opening a hand).10 Reinnervation will occur gradually, reaching a consistently measurable EMG
level approximately 6 months after the
procedure. However, the bipolar voltage
potential may continue to increase in
magnitude, and the optimal electrode
alignment may reorient as further development occurs in subsequent months
and years after the procedure.11 During
this period of reinnervation, the patient
should be given a protocol of how to
exercise both the reinnervated and natively innervated muscles. In addition
to the typical practices of wound care
and healing maintenance of the residual
limb, overall strength and range of motion also should be pursued.
One principle often overlooked in
this transition period is prosthetic wear
after TMR for individuals who had been
previously fit with prostheses. Generally, it is expected that after a brief surgical
10,11
recovery period, the prosthetic user will
regain the pattern of prosthetic use that
existed before TMR. This use may be
complicated by the fact that the overall
limb volume may change substantially
because of the removal or movement
of adipose tissue during the TMR procedure as well as transient atrophy of
muscles that have been deinnervated
and reinnervated. Although the repositioning of adipose tissue, termed
debulking, is a benefit for signal acquisition, the removal or movement
of subcutaneous fat will also alter the
shape of the residual limb to the degree
that either major socket modifications or
socket replacement becomes necessary.
If possible, planning the design of the
preoperative prosthesis should include
considerations for limb volume changes
and postoperative fittings.
12
Myotesting Principles
Fitting individuals with traditional
myoelectric control requires that the
prosthetist follow the basic strategies
for bipolar EMG control. Signal thresholds, antagonistic muscle pairs, the
appropriate alignment of the electrode
poles, and maintenance of electrode
contact on the skin surface are essential
components of these fittings. The addition of TMR sites does not alter these
strategies. However, because more than
two electrode sites are available, the
equipment used for traditional myoelectric site evaluation and location is
limited in its utility. Myotesters provide
a means of viewing only a single pair
of bipolar signals, whereas TMR provides an expected minimum of at least
two pairs of sites. At first, the site selection of antagonistic pairs of muscles,
including isolation, attaining thresh
olds, and proportional control, can be
identified with traditional equipment.
However, it may be necessary to use
the software germane to the particular
powered prosthetic components to simultaneously appreciate all the signals
(Figure 1).
Coactivation of Signals
TMR has created an avenue for discrete EMG signals to control elbow
flexion and extension independently
from the opening and closing of a terminal device. These individual signals
are comparatively easy to attain when
selecting them in isolation and testing
the residual limb for control of the desired motions. However, when all the
electrodes are in contact with the user’s
body, eliminating unintended coactivation of muscles is quite difficult. Usually,
this is not the case within the antagonistic muscle pairs, but occurs more
frequently within muscles that are part
of the same synergistic pattern (elbow
flexion and hand close, elbow extension
and hand open). In addition to the surgical attempt to separate these muscles
by means of adipofascial flaps,8 two distinct signal strategies for addressing this
coactivation have been used.
The first strategy may seem counterintuitive to the prosthetist and/or
the therapist who is adjusting the EMG
signal gains and thresholds within the
prosthetic system. Although the user
can typically generate signals of greater
magnitude and control within native
muscles, the signal gain on these electrodes may increase while the threshold
may decrease. This signal overamplification will make it much easier for the user
to cross the now decreased threshold
for that particular movement. By concentrating on providing only a small
signal from the natively innervated
muscle, the decreased effort may minimize the extent of coactivation with the
reinnervated muscle that is part of the
same synergistic pattern. Having a large
gain/low threshold on the native elbow
-
flexor muscle will allow the user to flex
his or her elbow with minimal EMG
amplitude, thus reducing the chance of
coactivation of the muscle used to close
the hand.
The second strategy for eliminating
the confusion caused by the unintended
coactivation of muscles uses a different
Atlas of Amputations and Limb Deciencies, Fourth Edition © 2016 American Academy of Orthopaedic Surgeons
340
means of signal acquisition and processing, which is referred to as pattern
recognition. Pattern recognition is discussed later in this chapter.
Visualization of Desired Movement
A primary goal of TMR is to enable the
user to provide an intuitive neuromuscular signal that is native to the action
desired from the prosthesis. For the individual with a transhumeral amputation, the native neural pattern for elbow
flexion and extension signals are present
both before and after TMR. However,
hand and wrist movements require a
“rerouted” neural signal from a different muscle. For the individual with a
shoulder disarticulation, this rerouting
occurs for all elbow, wrist, and hand
signals. When motor reinnervation
occurs, it is unclear as to exactly what
neural information reaches its destination in the motor point of the muscle.
For example, after reinnervation of the
median nerve to a motor point on a
targeted muscle, it is expected that user’s attempt to elicit the EMG signal for
“hand close” will send the desired neural signal to contract the targeted muscle. Generally, this attempt is successful.
However, some factors may necessitate
slight variations to the user’s visualized
movement of his or her amputated limb
to generate the proper corresponding
EMG signals. First, the portion of the
median nerve that reinnervated the
target muscle may have been biased toward thumb movements more so than
the second and third digits. In this case,
the patient may have to visualize more
thumb movement for “hand close” versus visualizing the entire hand closing.
In other cases, the patient may think
that his or her phantom limb cannot
completely move into the desired position, again necessitating a variation of
the desired visualization. Both circumstances require the patient, the prosthetist, and the occupational therapist
to be flexible enough to try alternative
motions that are normally innervated
Chapter 28: Targeted Muscle Reinnervation: Prosthetic Management
Figure 1
ing of multiple electromyographic channels. (Courtesy of Rehabilitation Institute of Chicago,
Chicago, IL.)
by the reinnervated peripheral nerve to
determine what is most effective.
in all myoelectric control, consistency
in the activation patterns is the key to
successful prosthetic operation.
Illustration of a graphical user interface screen that allows the simultaneous view-
somewhat standardized surgical princi-
11,12
As
ples, the results from TMR will depend
on what is discovered both before and
during the procedure. For example, a
muscle transfer from another region of
the body may be necessary to provide
Electrode Placement
and Socket Designs
Prosthetists usually have their own preferred electrodes and socket design for
transhumeral and shoulder disarticulation myoelectric fittings that they have
found successful. Such components and
design concepts should remain within each prosthetist’s repertoire, adding
the possibility of minor modifications
caused by the surgical method used, the
location and number of EMG sites, significant movement of superficial tissue
during muscle contraction, and the ease
of relocating electrodes.
12
Similar to prosthetists, surgeons
have their own ideas of how to achieve
successful results. Although TMR has
a viable site for reinnervation of the peripheral nerve. Such transfer may also
be necessary to provide coverage over
bony prominences, such as within an
interscapulothoracic amputation. Alternatively, the originally targeted muscle
may be determined as nonviable intraoperatively. In other surgical variations,
the surgeon may opt to detach the origin
of the muscle tendon to prevent proximal migration of the muscle during
contraction, or a humeral angulation
osteotomy may be included during
the same surgical procedure as TMR.
In all of these cases, it is essential for
the prosthetist to review the surgical
report and/or discuss the case with the
surgeon to ensure that both appreciate
© 2016 American Academy of Orthopaedic Surgeons Atlas of Amputations and Limb Deciencies, Fourth Edition
341
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