Keywords
brachial plexus - motion analysis - birth palsy - nerve transfer - trapezius - motion
browser
Introduction
For brachial plexus surgeons, there are few areas more intriguing than the dynamics
of how children with brachial plexus birth palsy (BPBP) use their affected arm.[1]
[2]
[3] Functionality of the affected arm is not well understood because the injury occurs
in the perinatal period and the disease transforms as the child grows. Nerve and muscle
weakness develop over time, as well as changes to muscle, joint, bone, and compensatory
muscle recruitment.[4]
One interesting question in children with BPBP is whether the upper or lower trapezius
(UT and LT, respectively) muscles are functionally active during upper extremity use.
It is intriguing because there are three ways in which an innervated trapezius can
be utilized in children with BPBP, leaving it in situ, denervation and use of the
trapezius' spinal accessory nerve (SAN) for distal nerve transfers, or using it directly
as a muscle transfer.[5]
[6]
[7] The prevailing surgical technique of treating BPBP addresses a compromised suprascapular
nerve through a posterior approach of transferring the lower portion of the SAN which
innervates the LT and leaving innervation of the UT intact.[8]
[9]
Because there are no studies looking at the natural contribution of the trapezius
in children with BPBP, the morbidity of harvesting the SAN is largely unknown. It
is known that normal children who sustain iatrogenic SAN injury have limited shoulder
abduction of 70 to 90 degrees, and that divisions of trapezius muscles each have diverse
roles in scapulothoracic movements that contribute to glenohumeral stability.[10]
[11]
[12]
To better understand the functional role of the trapezius in children with BPBP, this
report presents pilot data on UT and LT activity in an 18-year-old female with extended
upper brachial plexus (C5–6–7) birth palsy. It is the first of its kind to demonstrate
use of the Motion Browser, a novel visual analytics system developed at New York University.
Motion Browser is a tool that synchronizes video recordings with kinematic and EMG
data for assessment of muscle groups.[13]
Methods
An 18-year-old female with right-sided extended upper trunk (C5–6–7) BPBP, Mallet
score of 22, and a prior anterior shoulder release underwent bilateral upper extremity
analysis with the Motion Browser. She had no prior nerve transfer surgery and was
free of neurological or metabolic impairment beyond BPBP. Examination at time of presentation
confirmed intact trapezius function; the only physical limitation was right shoulder
motion. She had mild internal shoulder rotation, elbow flexion contracture deformity,
and weaker wrist flexion compared with the contralateral side, but was greater than
the British Medical Research Council (BMRC), grade 3 in all tested motions. In this
institutional review board (IRB) approved study at New York University, the patient
was evaluated with simultaneous three-dimensional motion analysis, 16-channel electromyography
(EMG), and video monitoring.
Data were recorded for eight muscles in each upper limb as follows: (1) biceps, (2)
triceps, (3) pronator teres, (4) pronator quadratus, (5) UT, (6) LT, (7) flexor digitorum
superficialis, and (8) extensor digitorum communis. The following six upper extremity
movements were performed: (1) shoulder abduction, (2) shoulder external rotation,
(3) shoulder internal rotation, (4) elbow flexion, (5) forearm pronation, and (6)
forearm supination. Movements were tested individually in each limb. Only one recording
for each limb was assessed per movement. Shoulder external/internal rotation was performed
with the arm in the adducted position. Nonshoulder movements of elbow flexion and
forearm pronation/supination were called distal movements. EMG, kinematic data, and
video recordings were processed through the Motion Browser.[13]
Data were selected in the Motion Browser, starting from the auditory tone that prompted
the patient to initiate motion and ending when the arm returned to the resting position.
Twelve-movement segments were collected, six movements per limb. Within each movement,
maximal active motion and muscle activity patterns were recorded. [Fig. 1] displays the data extraction steps.
Fig. 1 The Motion Browser data extraction method. 1. The Motion Browser displays the synchronized
video, kinematics, and electromyography (EMG) data for the patient's affected and
unaffected limbs for each movement performed. 2. The user selected segments of interest
based on examining the video clips to fit selection criteria. Selected segments included
all EMG data corresponding to the movement recorded in that video segment. 3. The
Motion Browser isolated the motion data for that segment of interest. 4. The Motion
Brower displayed a bar chart depicting the fraction of each muscle's activation relative
to the total measured activity in the movement in the segment of interest. 5. Individual
bar values for muscle activation were recorded for results analysis.
The Motion Browser calculated active range of motion (AROM) as the angular displacement
(in degrees) between the highest point of upper limb placement and the lowest point
of the patient's resting position. Pronation and supination were measured with supination
scored as negative. The similarity ratio between the maximum AROM in the affected
versus the unaffected limb was calculated as a percentage. If the patient produced
more voluntary motion in the affected than in the unaffected limb, a ratio of greater
than 100% was calculated.
The Motion Browser compiled the EMG data throughout the segment into measures of muscle
activity by the root-mean-square (RMS) envelope and displayed the results of the eight
muscle groups in bar charts.[11] Each bar value demonstrated the fraction of muscle activity relative to the total
measured activity throughout the movement. The higher the bar value, the higher the
individual muscle activity. Muscle activity for UT and LT was isolated for assessment.
Results
Voluntary Active Range of Motion Was Similar in Both Affected and Unaffected Limbs
Similarity of the voluntary affected/unaffected maximal AROM is shown in [Table 1]. All movements excluding shoulder abduction had a similarity ratio (affected/unaffected)
greater than or equal to 90%. Only one movement, shoulder abduction, had an affected/unaffected
ratio <90%, at 60.81%.
Table 1
Results for comparing the ratio of angular displacement of active motion between affected
and unaffected limbs in upper extremity movements (cm)
|
Movement
|
Affected
|
Unaffected
|
Similarity ratio (affected/unaffected) %
|
|
Shoulder abduction
|
74.8
|
123
|
60.81
|
|
Shoulder external rotation
|
57.6
|
60.6
|
95.05
|
|
Shoulder internal rotation
|
67.4
|
64
|
105.31
|
|
Elbow flexion
|
162
|
152
|
106.58
|
|
Forearm pronation
|
128
|
142
|
90.14
|
|
Forearm supination
|
113
|
123
|
91.87
|
Lower Trapezius Was More Active in the Affected Limb during Distal Movements
Trapezius muscle activity shown in [Table 2] displays the percentage of EMG activity contributed by UT and LT in all movements
in each limb. UT had no activity in affected or unaffected limbs in shoulder external
rotation, shoulder internal rotation, elbow flexion, forearm pronation, and forearm
supination. UT accounted for 20.0% of muscle activity in the affected limb during
shoulder abduction and 0.5% in the unaffected during shoulder abduction.
Table 2
Percentage of EMG activity contributed by the UT and LT in the six tested movements
in both affected and unaffected limbs
|
UT activity (%)
|
|
Movement
|
Affected
|
Unaffected
|
|
Shoulder abduction
|
20.0
|
0.5
|
|
Shoulder external rotation
|
0.0
|
0.0
|
|
Shoulder internal rotation
|
0.0
|
0.0
|
|
Elbow flexion
|
0.0
|
0.0
|
|
Forearm pronation
|
0.0
|
0.0
|
|
Forearm supination
|
0.0
|
0.0
|
|
LT activity (%)
|
|
Movement
|
Affected
|
Unaffected
|
|
Shoulder abduction
|
35.0
|
85.0
|
|
Shoulder external rotation
|
6.0
|
30.0
|
|
Shoulder internal rotation
|
7.9
|
61.0
|
|
Elbow flexion
|
35.0
|
17.0
|
|
Forearm pronation
|
59.0
|
7.0
|
|
Forearm supination
|
87.0
|
23.0
|
Abbreviations: EMG, electromyography; LT, lower trapezius; UT, upper trapezius.
LT was most active (>50%) in the unaffected limb during shoulder abduction (85.0%)
and shoulder internal rotation (61.0%). It was most active (>50%) in the affected
limb during the distal movements of forearm pronation (59.0%) and forearm supination
(87.0%). Activity in the unaffected limb was more than that in the affected limb during
shoulder abduction (85.0% unaffected and 35.0% affected), shoulder external rotation
(30.0% unaffected and 6.0% affected), and shoulder internal rotation (61.0% unaffected
and 8.0% affected). Activity in the affected limb was more than that in the unaffected
during elbow flexion (35.0% affected and 17.0% unaffected), forearm pronation (59.0%
affected and 7.0% unaffected), and forearm supination (87.0% affected and 23.0% unaffected).
Discussion
The effect of the trapezius muscle, while known to be vital for upper limb function
in healthy children,[11] has never been studied in children/adolescents with BPBP. Over the past decade,
studies have discussed the transfer of the trapezius' SAN for a compromised suprascapular
nerve in treating BPBP.[14]
[15]
[16] However, a necessary prerequisite to perform a nerve transfer is the consensus that
the donor nerve is expendable and will not downgrade the patient's upper extremity
function.[17]
[18] Morbidity of trapezius innervation or transfer should first be determined based
on knowledge about its function in the natural state of injury.
Most authors who study upper extremity function in neurological diseases utilize kinematics
or combined EMG analysis.[18]
[19]
[20]
[21] The Motion Browser is unique because it expands upon that methodology by synchronizing
video recordings with kinematic and EMG data, thus corresponding information at each
time point can be isolated. EMG data are normalized by comparing muscle activity within
the limb, and the browser displays percentages of each muscle activity.[13]
This patient's Motion Browser results suggest that her LT is more active in the affected
limb during the movements of forearm pronation (59.0%) and supination (87.0%) than
in the unaffected (17.0 and 23.0%, respectively). The AROM of the affected limb when
compared with the unaffected during these movements had a similarity ratio of >90.0%.
In these movements that are not typically expected to provoke trapezius activity,
the muscle likely plays a part in isokinetic stabilization of the scapulothoracic
region.[10]
[11]
[12] Conversely, for UT, activity was nearly zero for all movements in the affected limb
except for shoulder abduction (20.0%) and minimally active in the unaffected for all
movements. In the video footage, the patient appeared to lift the affected shoulder
to complete shoulder abduction. UT activation can thus be attributed to compensating
for the abduction deficit, a sequela of the brachial plexus injury.
This report demonstrates the use of the Motion Browser for analysis of the trapezius
muscle in an adolescent with BPBP. This patient's results in the affected limb showed
LT active a decreased amount during functional shoulder and elbow motion, but relatively
active during the most distal movements of forearm supination and pronation. Results
suggested that in this specific patient, transferring the SAN could potentially compromise
distal function at the expense of shoulder restoration.
Limitations
Limitations of the Motion Browser include the constraints of data collection. This
setup is expensive because analyses need to be completed in a laboratory with adequate
equipment. Furthermore, EMG signals are collected through surface electrodes, which
are prone to background noise and difficulty with normalization.
Conclusion
In conclusion, clinicians may wish to exercise caution when considering the current
surgical treatment of the transfer of the SAN for the suprascapular nerve. Studies
in patients who did and did not receive nerve transfer have found no significant differences
in external rotation of the arm[22] or in recovery of ROM or strength[23]; postsurgical follow-ups have found no trapezius atrophy/weakness.[24] Motion systems, like the Motion Browser, can help clinicians better evaluate the
native use of the trapezius muscle in BPBP patients before considering surgery.
