Download Anterior and Middle Deltoid Are Functionally Critical Targets for

ANTERIOR AND MIDDLE DELTOID ARE FUNCTIONALLY CRITICAL TARGETS FOR NERVE
TRANSFER FOLLOWING C5-C6 ROOT AVULSION INJURY
1,2
1
Dustin L. Crouch, 3Johannes Plate, 3Zhongyu Li and 1,2Katherine R. Saul
Virginia Tech-Wake Forest School of Biomedical Engineering and Sciences, Winston-Salem, NC, USA
2
Department of Biomedical Engineering, Wake Forest School of Medicine, Winston-Salem, NC, USA
3
Department of Orthopaedic Surgery, Wake Forest School of Medicine, Winston-Salem, NC, USA
email: [email protected], web: http://www.sbes.vt.edu/kholzbau/MoBL/
INTRODUCTION
The deltoid and teres minor muscles, which are
innervated by the axillary nerve, are paralyzed
following brachial plexus avulsion injury of the C5
and C6 nerve roots. A nerve transfer to the axillary
nerve is commonly performed to restore abduction
strength. During this procedure, the radial nerve
branch innervating the long head of the triceps is
transferred to the anterior branch of the axillary
nerve. Depending on the axillary nerve anatomy,
either the entire deltoid or the anterior and middle
compartments of the deltoid are restored, while
teres minor remains paralyzed [1]. While this
technique increases the likelihood that useful
abduction strength is recovered, it is unclear
whether overall function can be improved if teres
minor and posterior deltoid are restored in addition
to the anterior and middle deltoid compartments.
Therefore, we used a musculoskeletal model to
assess the biomechanical roles of teres minor and
the three deltoid compartments in the context of
isometric strength and shoulder movement.
METHODS
We used a computational musculoskeletal model of
the upper limb [2] implemented for dynamic
simulation [3] in OpenSim [4]. The model was
simplified to represent 5 degrees of freedom at the
shoulder, elbow, and forearm, and included 32
muscle compartments crossing the shoulder and
elbow joints. Four scenarios were developed in
which muscles were selectively paralyzed to
investigate the biomechanical role of teres minor
and the anterior, middle, and posterior deltoid
muscle compartments (Table 1). Muscles were
assumed to be unimpaired unless otherwise
indicated. Paralyzed muscles were allowed to
generate passive muscle forces only.
Table 1: Scenario descriptions
Paralyzed muscles and muscle
Scenario
compartments*
unimpaired
none
1
teres minor
2
teres minor, posterior deltoid
teres minor, deltoid (all
compartments)
*Muscles not listed were assumed to be
unimpaired.
3
Maximum isometric abduction and external rotation
strength were calculated for each scenario by
summing the maximum isometric joint moment that
each muscle could generate in a given posture.
Abduction strength was calculated at 45° shoulder
elevation in the frontal plane with the elbow fully
extended. External shoulder rotation strength was
calculated with the arm and forearm in neutral
posture and the elbow flexed to 90°.
To understand how differences in shoulder strength
among
scenarios
may
affect
movement
performance, we conducted dynamic simulations of
abduction from 0° to 90° in the frontal plane with
the elbow extended and the forearm in a neutral
posture. We used a computed muscle control
(CMC) algorithm to compute the muscle activations
required to track the movement as accurately as
possible while minimizing a cost function related to
muscle effort [5]. Shoulder elevation and flexion
angles were evaluated to determine whether each
scenario could abduct the shoulder to 90° and
maintain neutral shoulder flexion.
RESULTS AND DISCUSSION
The largest differences in shoulder strength were
observed for abduction (Fig.1). Compared to the
unimpaired scenario, isometric abduction moment
when both teres minor and posterior deltoid were
paralyzed (scenario 2) was only 4.2% lower, while
isometric abduction moment when teres minor and
all deltoid compartments were paralyzed (scenario
3) was 62.0% lower. External rotation moment
decreased only as much as 16.1% when teres minor
and deltoid were paralyzed. These results suggest
that teres minor and posterior deltoid contribute less
to shoulder strength than do the anterior and middle
deltoid compartments.
Figure 2: Scenario 3 exhibited the least accuracy
when attempting to simulate the abduction
movement. Joint angles of scenario 1 and the
unimpaired scenario are coincident.
CONCLUSIONS
Figure 1: Maximum isometric shoulder joint
moments were similar when the anterior and middle
deltoid compartments were not paralyzed
(unimpaired, scenario 1, and scenario 2).
Scenarios in which the anterior and middle deltoid
compartments were not paralyzed (unimpaired,
scenario 1, and scenario 2) could simulate the
abduction movement to within 4° of the desired
shoulder elevation angle with less than 4° of
anterior flexion (Fig.2). However, when the entire
deltoid muscle was paralyzed (scenario 3), the
model was least capable among all scenarios of
accurately simulating the abduction movement
trajectory. Therefore, paralysis of the posterior
deltoid and teres minor had relatively little effect on
the ability of scenarios 1 and 2 to track the
abduction movement.
Based on the findings of this study, the anterior and
middle deltoid compartments contribute most to
shoulder strength and movement ability, compared
to teres minor and posterior deltoid. Therefore,
reinnervating teres minor and posterior deltoid may
yield
little
additional
functional
benefit.
Understanding the biomechanical consequences of
axillary nerve transfers is important for surgical
planning and predicting functional outcomes in
clinical practice, particularly when multiple
orthopaedic procedures are indicated.
REFERENCES
1. Leechavengvongs S, et al. J Hand Surg, 28A(4), 633-
638, 2003.
2. Holzbaur KRS, et al. Ann Biomed Eng 33, 829-840.
3. Daly M, et al. ASB Conf Proceedings, State College,
PA, 2009.
4. Delp SL, et al. IEEE Trans Biomed Eng 54, 19401950.
5. Thelen DG, Anderson FC. J Biomech 39, 1107-1115.
ACKNOWLEDGEMENTS
This work was supported by the National Institutes
of Health (NIH 5R24HD050821-02) and the Wake
Forest School of Medicine.