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Anatomy in Practice
The posterior triangle of the neck: Where is scalenus anterior?
Ewan Kennedy BPhty
PhD candidate, Department of Anatomy and Structural Biology
University of Otago
Dunedin, New Zealand
Susan Mercer BPhty (Hons) MSc PhD
Associate Professor, School of Biomedical Sciences
University of Queensland
Brisbane, Australia
ABSTRACT
In the physiotherapeutic literature scalenus anterior has been depicted as palpable
and measurable by surface electromyography (EMG) within the posterior triangle of
the neck, but without explaining in detail how this is achieved. The purpose of this
study is to present the topographical anatomy of scalenus anterior in the posterior
triangle of the neck, and consider the techniques of palpation and surface EMG from
an anatomical perspective. The location of scalenus anterior within the posterior
triangle was reviewed in anatomical textbooks and research literature, a dissection of
a single embalmed cadaver (male, 64 years) and gross observation of transverse E12
plastinated slices. These resources revealed that the muscle belly of scalenus anterior
is located below the level of C6 deep in the root of the neck, covered by various
structures including the clavicular head of the sternocleidomastoid (SCM) and a
substantial fat pad. This location suggests that neither palpation nor surface EMG
recording of scalenus anterior is straightforward. Clinicians and researchers utilising
these techniques should be aware of the topographic anatomy, and demonstrate
this in a detailed methodology. This will help ensure that the techniques presented
are both realistic and accurate, which are key aspects of evidence based practice.
Kennedy E, Mercer S (2006): The posterior triangle of the neck: Where is scalenus
anterior? New Zealand Journal of Physiotherapy 34(3): 142-146.
Key words: Neck muscles, scalenus anterior, palpation, clinical anatomy
INTRODUCTION
The posterior triangle of the neck (lateral cervical
region) is a complex region, spiralling from the back
of the skull to the clavicle anteriorly between the
trapezius and sternocleidomastoid (SCM) muscles.
Due to its spiral shape illustrations often distort the
appearance of the triangle (Last 1978), making an
accurate perception of depth difcult. Structures in
this region commonly assessed by clinicians include
the cervical lymph nodes, external jugular vein,
brachial plexus, SCM and scalenus muscles. The
surface anatomy of these structures is not simple,
particularly for the scalenus muscles which form
the deepest aspect of the oor of the triangle.
Physiotherapists tend to primarily be concerned
with the muscles in the posterior triangle, which is
reected in the physiotherapeutic literature. Many
authors describe techniques for locating or palpating
the scalene muscles (Brantigan and Roos 2004,
Falla et al 2002a, Gross et al 2002, Kostopoulos
and Rizopoulos 2001, Portereld and DeRosa 1995,
Senjyu et al 2002). However these authors tend
to present palpation of the scalenus muscles as
straightforward, with little reference to the anatomy
of the region. Falla et al (2002a) published a method
for recording electromyographic (EMG) activity in
scalenus anterior using surface electrodes. The
authors positioned the electrodes based on palpation
of the muscle belly of scalenus anterior, without
142
describing in detail where palpation occurred with
reference to anatomical landmarks. A large number
of research articles have since been published
utilising the same methodology (Falla et al 2002b,
Falla et al 2003a, Falla et al 2003b, Falla et al 2004a,
Falla et al 2004b, Falla et al 2004c, Falla et al 2004d,
Falla et al 2004e, Falla et al 2004f), constituting a
substantial proportion of recent physiotherapeutic
literature pertaining to the cervical spine.
A detailed knowledge of topographical anatomy
is essential to accurate palpation, particularly for
scalenus anterior as it has a conceptually difcult
location in the root of the neck. Considering the
complexity of the posterior triangle and the amount
of physiotherapeutic literature utilising palpation to
locate scalenus anterior, this technique deserves to
be examined further. The purpose of this study is
to present the topographical anatomy of scalenus
anterior in the posterior triangle of the neck, and
consider the techniques of palpation and surface
EMG from an anatomical perspective.
ANATOMY OF THE POSTERIOR
TRIANGLE
To appreciate the depth, location, relations
and variations of structures in this area various
anatomical resources were consulted including
anatomy texts, the research literature, a dissection
of a single embalmed cadaver (male, 64 years), and
NZ Journal of Physiotherapy – November 2006, Vol. 34 (3)
transverse E12 plastinated slices. These resources
provide a number of different perspectives, and
complement each other to present a comprehensive
picture of the anatomy of the region.
Textbook Anatomy
The posterior triangle of the neck is formed
between the posterior edge of the SCM and the
anterior edge of the trapezius muscle (Figure 1).
The base of the triangle is formed by the medial
third of the clavicle, and the apex by the superior
nuchal line of the occiput. The oor of the triangle
comprises several parallel muscles covered by the
prevertebral fascia (Bruce et al 1967, Drake et al
2005, Grant and Basmajian 1965, Last 1978, Moore
and Dalley 2006, Wood Jones 1953, Woodburne
and Burkel 1988, Zuckerman 1961).
Figure 1. Lateral view of the posterior triangle of the
neck (shaded area) formed between trapezius and
the sternocleidomastoid. Notice the orientation of the
muscles forming the oor the triangle.
Within the triangle, between the overlying skin
surface and the muscular oor, are numerous
important structures. These include cutaneous
branches of the cervical plexus supplying the skin
overlying the posterior triangle; the platysma and
omohyoid muscles; the spinal accessory nerve,
which innervates the SCM and trapezius muscles;
the deep cervical lymph nodes; the brachial plexus;
and numerous arteries and veins – prominent
is the external jugular vein (see Figure 2). For
physiotherapists these structures are not usually
of primary interest yet are important to note,
particularly in post-surgical cases where these
structures may have been affected.
NZ Journal of Physiotherapy – November 2006, Vol. 34 (3)
The muscular oor of the triangle comprises
(from superior to inferior): splenius capitis, levator
scapulae, and scalenus medius and posterior
(Bruce et al 1967, Drake et al 2005, Grant and
Basmajian 1965, Last 1978, Moore and Dalley
2006, Wood Jones 1953, Woodburne and Burkel
1988, Zuckerman 1961) (see Figure 1). Some texts
also include scalenus anterior in the oor of the
triangle (Drake et al 2005, Grant and Basmajian
1965, Wood Jones 1953), while others note that a
small portion of semispinalis capitis may be visible
at the apex of the triangle (Grant and Basmajian
1965, Last 1978, Wood Jones 1953, Zuckerman
1961).
The location of scalenus anterior with respect
to the posterior triangle is consistently reported
in anatomical texts. While some texts include
scalenus anterior in the muscular oor of the
triangle all that specically state the location of
scalenus anterior describe it as found deep to
the posterior edge of the SCM (Bruce et al 1967,
Grant and Basmajian 1965, Last 1978, Moore and
Dalley 2006). Further detail is provided in Gray’s
Anatomy (Standring 2005), which states that the
clavicle, subclavius, SCM and omohyoid, lateral
part of the carotid sheath, phrenic nerve, transverse
cervical, suprascapular and ascending cervical
arteries, subclavian vein, prevertebral fascia, and
phrenic nerve are all anterior to scalenus anterior.
Last (1978) observes that scalenus anterior is in
fact a very slender muscle, a useful point when
considering locating the muscle via palpation.
Small details such as this are crucial to clinicians,
yet are not often included in anatomical texts.
Research Literature
Consultation of the surgical literature conrms
that scalenus anterior lies deep to the clavicular
head of SCM, and gives further insight: Mattson
(2004) in an article on the surgical approach to
anterior scalenectomy clearly describes how the
surgeon must progress through the skin, platysma,
the clavicular head of SCM and a fat pad before
scalenus anterior becomes visible. This type of
description gives a precise insight into the layers of
tissue lying between the skin and scalenus anterior,
and is fundamental to good surgical practice.
Clearly this approach to anatomy is also essential to
realistic and accurate palpation in physiotherapy.
Variations
The scalene muscles are especially variable,
and physiotherapists can clearly benefit from
understanding some of the common variations
that exist and how they may impact on clinical
practice. Scalenus anterior is typically described as
arising from the scalene tubercle of the rst rib and
inserting into the anterior tubercles of C3-6 (Bruce
et al 1967, Drake et al 2005, Grant and Basmajian
1965, Last 1978, Moore and Dalley 2006, Standring
2005). However the superior attachment sites of
scalenus anterior are well known to vary in cervical
level (Le Double 1897), while Wayman et al (1993)
143
also describe some variation in the position of the
inferior attachment to the rst rib. In a study of
variance in the relations of the brachial plexus and
scalene muscles in cadavers Harry et al (1997)
found such wide variation that only 14% of the
specimens studied could be considered ‘normal’.
Murakami et al (2003) even report a case of total
absence of the right scalenus anterior muscle, a
rare occurrence also observed by Le Double (1897).
For more detailed information about the variations
found in this region refer to Bergman et al (1984)
or Le Double (1897).
Cadaveric material
In order to further examine the position of
scalenus anterior in the posterior triangle of the
neck we dissected this region in a single embalmed
cadaver (male, 64 years). The skin, subcutaneous
tissue, platysma and SCM were reected laterally to
expose the posterior triangle, noting the insertions of
the SCM for reference. The fat pad, lymphatic tissue
and small vessels and nerves were then removed to
expose scalenus anterior and other adjacent major
structures. The extent to which the clavicular
head of SCM covers scalenus anterior was clearly
seen in situ (Figure 2). Immediately underneath
the clavicular head of SCM the fat pad described
by Mattson (2004) amounted to a signicant layer
of tissue that took some time to remove cleanly.
The superior aspect of scalenus anterior tapered,
diverged and became tendinous before inserting
into the anterior tubercles of the cervical vertebrae,
and was covered by the lateral part of the carotid
sheath. The muscle belly of scalenus anterior was
located deep in the root of the neck surrounded
by other signicant structures. The muscle was
wedged between the carotid sheath antero-medially;
and the brachial plexus and scalenus medius
postero-laterally as shown in Figure 2. This deep,
cramped location of scalenus anterior makes it an
important landmark in dissection; Last (1978) in
particular refers to scalenus anterior as the key to
the root of the neck and describes its relations in
great detail. However this will also make it very
difcult to differentiate scalenus anterior from
surrounding structures using palpation.
Transverse E12 plastinated slices
The thickness and depth of the structures
overlying scalenus anterior are clearly demonstrated
by transverse E12 plastinated slices (Figure 3). In
this view the layers of tissue are clearly visible, as
is the relative size and position of each structure.
The clavicular head of SCM is revealed as a fairly
substantial muscle bulk, and again seen to largely
overlie scalenus anterior. Deep to the SCM the
fat pad described by Mattson (2004) is 2-3cm
thick, rich with small nerves and vessels. Also
conrmed was the position of scalenus anterior
between the carotid sheath and brachial plexus/
scalenus medius seen in our dissection. Viewing
the transverse slices at T1, C7 and C6 reveals
144
Figure 2. Anterolateral view of a neck dissection
showing the topographical position of scalenus anterior
(ScA) in situ, wedged between the carotid sheath (CS)
antero-medially; and the brachial plexus (BP) and
scalenus medius (ScM) postero-laterally. The clavicular
head of the SCM has been reected (dotted line marks
the in situ position) and fatty and lymphatic tissue
overlying the muscle removed to reveal the surface of
scalenus anterior. Also note the position of omohyoid
(Om) and the external jugular vein (EJV).
the vertical extent of the muscle belly of scalenus
anterior. The muscle belly of scalenus anterior can
be clearly seen at the level of T1 soon after leaving
the rst rib, dividing at C7, and greatly diminished
at C6 as it tapers and becomes tendinous towards
its insertions into the cervical transverse processes
(Figure 3). Given this arrangement it should not be
expected that the muscle belly of scalenus anterior
could be found above the level of C6.
CLINICAL ANATOMY
Having considered the topographical anatomy
of scalenus anterior in the posterior triangle, we
can now reect on our ability to locate, palpate, or
record EMG activity in this muscle using surface
electrodes.
When the location of scalenus anterior
is considered, palpation no longer appears
straightforward. Firstly the therapist must
negotiate the clavicular head of SCM to reach the
underlying tissues: palpating through this muscle
will confound the accuracy of the technique, while
palpating lateral to the clavicular head of SCM
one is more likely to encounter scalenus medius/
NZ Journal of Physiotherapy – November 2006, Vol. 34 (3)
will also make differentiating between scalenus
anterior and these structures difcult.
Some authors identify scalenus anterior with
palpation during resisted neck exion (Falla et
al 2002a) or with the neck laterally exed to the
opposite side (Gross et al 2002). Contracting
scalenus anterior is unlikely to assist in identifying
this muscle unless the overlying clavicular head
of SCM remains inactive, which is unlikely during
resisted neck exion. Similarly any activity in
platysma will make palpation of deeper structures
extremely difcult. Contralateral exion of the
neck will stretch all the tissues of the palpated side
(i.e. muscles, fascia etc), an effect not specic to
scalenus anterior. When stretched the supercial
tissues (platysma in particular) will become more
rigid and rm to the touch, making palpation of
the deeply placed scalenus anterior more, rather
than less, difcult.
Recording EMG activity in scalenus anterior
using surface electrodes appears even more
complex than palpation. Positioning electrodes
on the skin surface in such a way that scalenus
anterior is the muscle in closest proximity appears
extremely difcult (see Figures). The electrodes
must be lateral to the clavicular head of SCM, below
the level of C6 and able to differentiate the activity
of scalenus anterior from surrounding muscles. In
this small window scalenus anterior remains quite
deep to the skin surface, while the curved skin
surface likely precludes orienting the electrodes to
the bre direction of the muscle.
CONCLUSIONS
Figure 3. Transverse E12 plastinated slices at the levels
of C6, C7 and T1. These views demonstrate the size,
shape and position of structures overlying scalenus
anterior. Note how these features change at different
levels, in particular the depth of scalenus anterior at
the level of T1, and how its size diminishes by the level
of C6. White arrowhead – scalenus anterior; Black
arrowhead – scalenus medius/posterior; White asterisk
– Sternocleidomastoid.
posterior and levator scapulae (Figures 1, 2 and
3). Deep still to the clavicular head of SCM remain
omohyoid and 2-3cm of fat pad. Also given that
scalenus anterior tapers and becomes tendinous
superiorly, the muscle belly of scalenus anterior
will be found only below approximately the level
of C6 (inferior to the cricoid cartilage) in the root
of the neck. Considered together the muscle belly
can be visualised as deep to the clavicular head of
SCM and below the level of C6 in the root of the
neck, a small window of opportunity. The wedged
position of scalenus anterior between the carotid
sheath and the brachial plexus/scalenus medius
NZ Journal of Physiotherapy – November 2006, Vol. 34 (3)
The topographical location of scalenus anterior
was found to be deep and low in the root of the neck,
underneath the clavicular head of SCM. When
the anatomy of the posterior triangle is considered
locating, palpating or recording EMG activity of
scalenus anterior using surface electrodes appear
far from straightforward. Clinicians and researchers
utilising these techniques should be aware of the
topographic anatomy, and demonstrate this in a
detailed methodology. This will help ensure that
the techniques presented are both realistic and
accurate, which are key aspects of evidence based
practice.
Clinical anatomy remains a cornerstone of
the physiotherapy curriculum, yet is perhaps
undervalued in the physiotherapeutic literature.
This article demonstrates the way in which anatomy
relates to clinical and research practice, and
continues to serve the profession as an important
aspect of evidence based practice.
Key points
• In the physiotherapeutic literature scalenus
anterior has been depicted as palpable and
measurable by surface electromyography
(EMG), but with little explanation of how this
achieved
• Scalenus anterior is located deep in the root of
the neck, underneath the clavicular head of
sternocleidomastoid
145
•
When the anatomy of the region is considered
locating, palpating or recording EMG activity
of scalenus anterior using surface electrodes
appear far from straightforward
ACKNOWLEDGEMENTS
Many thanks to the Department of Anatomy and
Structural Biology, University of Otago for access
to the anatomical resources used in this study,
in particular Russell Barnett for preparation of
the E12 plastinated slices. Special thanks also to
Robbie McPhee for his illustrative work.
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ADDRESS FOR CORRESPONDENCE
Mr Ewan Kennedy: [email protected],
Department of Anatomy and Structural Biology, University of
Otago. Dunedin, New Zealand. Tel: +64 3 479 5145
NZ Journal of Physiotherapy – November 2006, Vol. 34 (3)