Thorac Surg Clin 17 (2007) 511–520
Anatomy of the Thoracic Outlet
Harold C. Urschel, Jr., MD, LLD (Hon), DS (Hon)a,b,*
a
Baylor University Medical Center, 3600 Gaston Avenue, Suite 1201, Barnett Tower, Dallas, TX 75246, USA
b
University of Texas, Southwestern Medical School, Dallas, TX, USA
The term ‘‘thoracic outlet’’ refers to the area
between the neck and shoulder, over the top of the
thorax and under the clavicle. It is a ‘‘wastebasket’’ term, however, in that only the subclavian
artery actually proceeds out through the top of
the thorax toward the arm. The subclavian vein
passes into the thorax from the arm through the
thoracic ‘‘inlet.’’ The nerve roots (C5–T1) proceed
across the top of the thoracic ‘‘outlet,’’ never
coursing in or out of the thorax [1–4]. There are
four major areas of compression that can produce
the thoracic outlet syndrome (TOS). Precise
knowledge of the thoracic outlet is cardinal for
surgical decompression to minimize morbidity,
complications, and malpractice litigation.
Multiple anomalies of the rib may occur, which
include a bifid first rib.
Cervical ribs usually elevate the neurovascular
structures, decreasing the thoracic outlet space,
and are either extensions of the C-7 transverse
process (O2.5 cm) or articulated like any other rib
to the C7 vertebra (Fig. 2). The distal end of the cervical rib may be free, fuse directly with the first rib,
or connect to it through fibrous ligaments (Box 1).
The incidence of cervical ribs in the population is
roughly 0.5% to 1%, whereas those associated
with TOS occur in approximately 5% to 10% of
patients. There is a slightly higher incidence in
women and a greater occurrence on the left side.
Clavicle
Bony outlet
The anatomy of the thoracic outlet is defined by
the bony circle of the sternum in front, connected to
the first rib laterally, which attaches to the vertebra
posteriorly. The clavicle attaches to the first rib and
sternum anteriorly. This is the bony outlet and inlet
of the thorax superiorly (Fig. 1).
The first rib attaches to the vertebra with the
head of the rib lying on the vertebral body, the
neck of the rib articulating with the transverse
process and the main body of the rib forming
a circle to attach to the sternum anteriorly. The
bony circle of vertebra, rib, and sternum form
a plane that is tipped so that the sternal level is
usually inferior to the vertebra. This bony circle
may be relatively flat or it may have a very steep
angle in its passage toward the sternal notch.
The clavicle attaches to the sternum and to the
first rib anteriorly through the costoclavicular and
clavicular-sternal ligaments medially (see Fig. 1).
It proceeds laterally to articulate with the acromial process of the scapula. The medial part of
the clavicle moves very little; however, the lateral
part is more mobile because of the scapula. Removal of the clavicle provides decompression of
the thoracic outlet and was used early in the
therapeutic history of TOS; however, removal of
the first rib achieves the same end without as
much cosmetic morbidity. There are marked variations in the ligaments, tendons, and muscles attaching to the bony structures in the thoracic outlet.
Roos [3] has listed 10 types of anomalous congenital bands and ligaments within the thoracic outlet,
most of which are not frequently seen clinically.
Subclavian artery
* Baylor University Medical Center, 3600 Gaston
Avenue, Suite 1201, Barnett Tower, Dallas, TX 75246.
E-mail address: drurschel@earthlink.net
The subclavian artery, the last major branch of
the aortic arch directed cephalad, makes its exit
1547-4127/07/$ - see front matter Ó 2007 Elsevier Inc. All rights reserved.
doi:10.1016/j.thorsurg.2006.12.004
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512
URSCHEL
Thoracic
outlet
Head and
neck
of 1st rib
Vertebra
1st rib
Clavicle
Manubrium
of sternum
Costal
cartilage
Fig. 1. The bony circle consists of the sternum in front, the first rib laterally, and it’s attachment to the vertebra
posteriorly.
through the top of the chest (Fig. 3) and passes
over the first rib posterior to the anterior scalene
muscle and anterior to the middle scalene muscle. It becomes the axillary artery after it passes
beneath the pectoralis minor muscle.
Branches of the subclavian artery include the
internal mammary artery proceeding inferiorly
under the sternum. Just before passing over the
first rib, the subclavian gives off the vertebral
artery, which is directed cephalad toward the brain.
The thyrocervical trunk branches distally. The
subclavian artery is joined by the nerves of the
brachial plexus before passing beneath the clavicle
over the top of the first rib. It also carries branches
of the sympathetic nerves peripherally. Pressure on
this artery from the scalenus anticus or any other
area of compression in the four major areas may
produce poststenotic dilatation if it is a minor
compression, or a subclavian artery stenosis, aneurysm, or occlusion if it is major compression.
Emboli may occur distally in the axillary-subclavian artery from stenosis, occlusion, or aneurysm.
After removing the compressed area, poststenotic
dilatations generally regress gradually over a period
of time and do not usually require further therapy.
All of the other anomalies of compression usually
need medical or surgical treatment (Box 2).
Subclavian vein
The axillary-subclavian vein courses posterior
to the pectoralis minor muscle, passes over the top
of the first rib, and under the clavicle (Fig. 4). The
infraclavicular tunnel is bounded medially by the
costoclavicular ligament and laterally by the scalenus anticus muscle. The vein descends through the
thoracic inlet, joining the internal jugular vein to
form the innominate vein on the left and the superior vena cava on the right. Medial to the vein is
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Cervical
nerve roots
Cervical
rib
Post stenotic
dilatation
Clavicle
Subclavian
artery
Subclavian
vein
Fig. 2. The cervical rib on the left is fused to the first rib, and elevates the neurovascular structures decreasing the space
and increasing the compression.
BRACHIAL
PLEXUS
CORDS
Lateral
Posterior
Medial
Subclavian
artery
TRUNKS
Upper
Middle
Lower
Common
carotid
artery
Apex of
pleura
Subclavian
vein
Clavicle
Fig. 3. The subclavian vein and artery pass over the first rib and under the clavicle. The brachial plexus traverses the top
of the bony circle to join the artery. The apex of the pleura (cupula) is depicted on the left side.
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514
URSCHEL
Box 1. Key points pertinent to thoracic
outlet anatomic anomalies of bone
or ligaments
1. Bony abnormalities, such as cervical
rib, bifid first rib, and so forth, may
disturb the thoracic outlet anatomy.
2. Tendinous bands or ligaments may
produce compression in the thoracic
outlet, and are not visible on
radiograph.
3. Muscle anomalies, such as the
posterior scalenus muscle (which
normally is not present), can produce
compression in the thoracic outlet.
the costoclavicular ligament; laterally is the insertion of the scalenus anticus muscle into the scalene
tubercle of the first rib. The primary area of
the vein compression occurs in the tunnel between
the first rib and the clavicle (Fig. 5). In most patients with thrombosis of the axillary-subclavian
vein (Paget-Schroetter syndrome) the costoclavicular ligament congenitally inserts further lateral
than usual. When the scalenus anticus muscle (lateral to the vein) becomes hypertrophied through
activity, the vein is significantly narrowed in these
patients. With this underlying anatomic abnormality, and the addition of dehydration or other
Box 2. Key points pertinent to thoracic
outlet anatomy and management
of subclavian artery pathology
1. The development of poststenotic
dilation of the subclavian artery in
TOS reverses itself after simply
removing the first rib, which
decompresses the TOS. No arterial
resection is necessary.
2. Subclavian artery aneurysms, in
contrast, in the thoracic outlet require
resection with an interposition graft.
3. Distal emboli may occur from
aneurysms, poststenotic dilatations,
or occlusions of the subclavian artery
in the thoracic outlet, mandating
resection and interposition graft.
positional abnormalities, the vein may clot producing the Paget-Schroetter syndrome or the ‘‘effort
thrombosis’’ of the axillary-subclavian vein.
When the costoclavicular ligament is in a normal
position, rarely does the vein clot even with significant hypertrophy of the scalenus anticus muscle
(Box 3) [5].
Cervical nerves and brachial plexus
Above the bony circle is a ‘‘cone’’ of nerve
roots, which emerge from the vertebral foramina
beginning with C-5 and proceeding down
through T-1 and track inferiorly from the top
of the cone toward the thoracic outlet (Fig. 6).
They join each other as the brachial plexus,
pass over the top of the pleura and the first
rib, under the clavicle, and posterior to the pectoralis minor muscle to the arm. The nerves
pass from the ‘‘cone’’ out through the triangle
formed by the anterior and medial scalene muscles in the neck. These muscles are attached to
the transverse process of the vertebra above,
and to the first rib below, and form a triangle between them. The cervical nerves exit from various sides of these muscles and join the
subclavian artery in the scalene triangle to pass
under the clavicle toward the arm. The C-5
through C-8 nerve roots pass from superiorly
to inferiorly over the top of the pleura and first
rib to join the T-1 nerve root, which exits from
under the first rib and passes through the scalene
muscle triangle. Three trunks of the brachial
plexus are formed by fusion of nerve roots: (1)
the superior trunk from C-5 and C-6, (2) the
middle trunk from C-7 alone, and (3) the inferior
trunk from C-8 and T-1. The three trunks separate into the anterior and posterior divisions that
in turn fuse to form the lateral, medial, and posterior cords of the brachial plexus (see Fig. 3).
The phrenic nerve arises from the C-3, C-4,
and C-5 nerve roots and travels from the lateral
edge of the scalenus anticus muscle medially and
inferiorly over the anterior surface into the
mediastinum. The phrenic nerve proceeds down
anterior to the pericardium to the diaphragm,
which it innervates. Injury to this nerve produces
a paralyzed, elevated diaphragm. The long thoracic nerve, formed by branches of C-5, C-6, and
C-7, passes through the body of the scalenus
medius muscle over the lateral edge of the first
rib and innervates the serratus anterior muscle.
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515
ANATOMY OF THE THORACIC OUTLET
Middle
scalene
muscle
Anterior
scalene
muscle
Clavicle
Brachial
plexus
Subclavian
vein
Subclavian
artery
Costoclavicular
ligament
1st rib
Sternum
Fig. 4. The cross-sectional view of the neurovascular structures traversing the thoracic outlet with the clavicle above and
the first rib below.
Injury to this nerve produces ‘‘winging’’ of the
scapula [6].
Sympathetic nerves
The sympathetic nerves and ganglia are located
in the posterior chest on the vertebra over the
head and neck of the ribs. The sympathetic chain
ascends vertically to supply each nerve root with
an afferent and an efferent branch (the gray and
white rami communicans). The ganglia are at the
level of each thoracic nerve. The T-1 sympathetic
ganglia fuses with C-7 and C-8 to form the stellate
ganglion, slightly above the first rib. The stellate
ganglion lies transversely and medially. The sympathetic nerves proceed to the arm in association
with the artery, peripheral nerves, and bone (Box
4) [7].
Major compression areas
There are four separate major areas of
compression in the thoracic outlet (additionally
there are many minor ones and many other
anomalies) [8–11]. The first is the sternal-costovertebral ‘‘bony circle,’’ through which passes
the subclavian artery as an ‘‘outlet’’ and a subclavian vein as an ‘‘inlet.’’ This also influences
the nerve roots and brachial plexus because
they pass over the top of this area with such
structures as cervical ribs, abnormal first ribs,
and other bony variations.
The second level of major compression proceeding distally is the ‘‘scalene muscle triangle’’
(Fig. 7). The scalenus anticus and scalenus medius insert into the first rib widely apart and the
triangle created is where the nerve roots coming
from the ‘‘cone’’ superiorly join the subclavian
artery as they pass through the triangle. The subclavian vein passes medially to the scalenus
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516
URSCHEL
Brachial
plexus
Subclavian
artery
Abnormal
costoclavicular
ligament
Subclavian
vein (cut)
with blood clot
inside vein
Fig. 5. The costoclavicular ligament inserts much farther laterally on the first rib causing vein occlusion.
Box 3. Key points pertinent to thoracic
outlet anatomy and the development of
the Paget-Schroetter syndrome (effort
thrombosis of the axillary-subclavian
vein)
1. Paget-Schroetter syndrome occurs
only when the costoclavicular
ligament inserts into the first rib far
more laterally than normal.
2. Subsequent hypertrophy of the
scalenus anticus muscle (eg, in weight
lifters), with the more lateral insertion
of the costoclavicular ligament,
causes severe external compression
and thrombosis of the axillarysubclavian vein.
anticus muscle and laterally to the costoclavicular
ligament, not passing through the scalene muscle
triangle.
The third primary compression space is where
the neurovascular structures pass over the top of
the first rib, below the clavicle and subclavius
muscle. As they transverse this space, they are
situated from medially to laterally as follows: the
costoclavicular ligament, subclavian vein, scalenus anticus muscle, subclavian artery and brachial
plexus, and the scalenus medius muscle (see Figs.
4 and 5).
The fourth and final area is where the neurovascular structures pass under the various fascial
structures and behind the pectoralis minor muscle
(Fig. 8). Neurovascular compression, either primary or recurrent, may occur at any or all of these
major or minor anatomic areas of the thoracic
outlet (Box 5) [12–16].
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517
ANATOMY OF THE THORACIC OUTLET
C5
C6
NERVE
CONE
C7
C8
T1
Brachial
plexus
Fig. 6. The ‘‘cone’’ of nerve roots (C-5, T-1) track inferiorly toward the thoracic outlet, joining each other as the brachial
plexus.
Box 4. Key anatomic points pertinent to the surgical approaches for TOS neurovascular
decompression
1. For primarily nerve (most frequent) and vein decompression, the transaxillary approach is
favored because the neurovascular structures are away from the first rib and do not require
retraction for rib removal. Also, it is easier to remove the first rib completely.
2. For arterial resection or repair the supraclavicular and infraclavicular approach provides
easier arterial control for resection and graft.
3. For recurrent TOS symptoms, the posterior, high thoracoplasty incision allows the best
exposure for removal of first rib remnants and neurolysis and decompression of the brachial
plexus.
4. Dorsal sympathectomy, necessary in traumatic TOS, causalgia, and the sympathetic
maintained pain syndrome, may be performed through the supraclavicular, transaxillary,
posterior, or transthoracic approach.
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518
URSCHEL
Scalene
muscle
triangle
Middle
scalene
muscle
Anterior
scalene
muscle
Brachial
plexus
Clavicle
Axillary
(subclavian)
artery
1st rib
Fig. 7. The scalene muscle triangle is the ‘‘second major level of compression.’’
Thoracic duct
The thoracic duct begins in the upper
abdomen, passes through the diaphragm, and
proceeds up the right side of the thorax to
approximately the level of the T-4 vertebra.
Here it crosses to the left side of the esophagus
proceeding superiorly and passes through the
thoracic outlet medial and slightly dorsal to
the left subclavian artery. It proceeds above the
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519
ANATOMY OF THE THORACIC OUTLET
Brachial
plexus
Subclavian
artery
Subclavian
vein
Clavicle
Pectoralis
minor
muscle
Fig. 8. The neurovascular structures pass behind the pectoralis minor muscle, which is another major area of
compression.
clavicle and enters the
jugular and subclavian
are possible in the
anatomy is important
the duct.
posterior junction of the
vein. Multiple variations
thoracic duct and its
so as to avoid injuring
Summary
Knowledge of the thoracic outlet anatomy and
its many variations is essential for surgical decompression to avoid injuries to vital structures,
minimizing complications and lawsuits.
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520
URSCHEL
Box 5. Key points pertinent to TOS
recurrence
1. Recurrent symptoms are much more
frequent with incomplete removal of
the first rib (particularly the head and
neck posteriorly).
2. This occurs because the osteoblasts
and osteocytes grow out from the end
of the first rib remnant and develop
‘‘fibrocartilage’’ that produces
neurovascular compression.
3. If the head and neck cannot be
removed safely, the end of the rib
remnant should be covered with
muscle to prevent fibrocartilage
formation.
4. Regeneration of the rib from residual
periosteum rarely produces recurrent
thoracic outlet symptoms.
[4]
[5]
[6]
[7]
[8]
[9]
[10]
[11]
Acknowledgment
The author thanks Rachel Montano for her
excellent assistance in organizing, preparing, and
transcribing this manuscript. Without her help it
would not have been possible.
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