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EDUCATION EXHIBIT
481
Accessory Muscles:
Anatomy, Symptoms,
and Radiologic
Evaluation1
ONLINE-ONLY
CME
See www.rsna
.org/educaion
/rg_cme.html
Paul A. Sookur, MRCP • Ali M. Naraghi, FRCR • Robert R. Bleakney,
FRCPC • Rosy Jalan, FRCR • Otto Chan, FRCR • Lawrence M.
White, MD
the
gross anatomy of
the commonly
occurring accessory
muscles.
A wide array of supernumerary and accessory musculature has been
described in the anatomic, surgical, and radiology literature. In the
vast majority of cases, accessory muscles are asymptomatic and
represent incidental findings at surgery or imaging. In some cases,
however, accessory muscles may produce clinical symptoms. These
symptoms may be related to a palpable swelling or may be the result
of mass effect on neurovascular structures, typically in fibro-osseous
tunnels. In cases in which an obvious cause for such symptoms is not
evident, recognition and careful evaluation of accessory muscles may
aid in diagnosis and treatment.
■ Identify
©
LEARNING
OBJECTIVES
After reading this
article and taking
the test, the reader
will be able to:
■ Describe
the characteristic imaging
features of various
accessory muscles.
RSNA, 2008
■ Discuss
the potential role of accessory
muscles in symptom
causation.
TEACHING
POINTS
See last page
Abbreviations: ADM = abductor digiti minimi, AIN = anterior interosseous nerve, ECR = extensor carpi radialis, EDBM = extensor digitorum
brevis manus, FCR = flexor carpi radialis, FDAL = flexor digitorum accessorius longus, FDS = flexor digitorum superficialis, FHL = flexor hallucis longus, FPL = flexor pollicis longus, PAES = popliteal artery entrapment syndrome, PCI = peroneocalcaneus internus, TCI = tibiocalcaneus internus
RadioGraphics 2008; 28:481–499 • Published online 10.1148/rg.282075064 • Content Code:
1
From the Department of Medical Imaging, Royal London Hospital, London, England (P.A.S., R.J.); Joint Department of Medical Imaging of
University Health Network and Mount Sinai Hospital, Toronto Western Hospital, 399 Bathurst St, Toronto, ON, Canada M5T 2S8 (A.M.N.,
R.R.B., L.M.W.); and Department of Medical Imaging, London Independent Hospital, London, England (O.C.). Recipient of a Certificate of
Merit award for an education exhibit at the 2006 RSNA Annual Meeting. Received April 4; revision requested June 26 and received August 14;
accepted October 3. All authors have no financial relationships to disclose. Address correspondence to A.M.N. (e-mail: [email protected]).
©
RSNA, 2008
482
RG ■ Volume 28 • Number 2
March-April 2008
Introduction
Muscle anatomic variants are commonly encountered. These variants may consist of absence of a
muscle, supernumerary muscles, deviation from
the normal course, or an anomalous origin or insertion. Accessory muscles are anatomic variants
representing additional distinct muscles that are
encountered along with the normal complement
of muscles.
Historically, the majority of data regarding
accessory musculature has been based on serendipitous findings at surgery. However, with the
advent of modern cross-sectional imaging techniques such as ultrasonography (US), computed
tomography (CT), and magnetic resonance
(MR) imaging, accessory muscles are regularly
encountered and can be accurately identified
noninvasively.
Accessory muscles are commonly overlooked
at imaging evaluation (1). Although they are typically asymptomatic and encountered as incidental
findings, accessory muscles have been implicated
as a potential source of clinical symptoms. Such
symptoms are usually due to mass effect of the
supernumerary muscle, with the patient presenting with either a palpable swelling or secondary compression of adjacent structures such as
nerves, vessels, or tendons.
Cross-sectional imaging can accurately demonstrate accessory muscles and help differentiate them from other soft-tissue masses. In this
article, we describe the gross anatomic and radiologic appearances of the normal musculature
and the more commonly encountered accessory
muscles in the shoulder, elbow, wrist, knee, and
ankle, with emphasis on features that help distinguish between normal and accessory muscles. In
addition, we discuss and illustrate the patterns of
clinical symptoms associated with specific accessory muscles.
Accessory Muscles of the Shoulder
The biceps brachii muscle has two heads: a long
head, which arises from the supraglenoid tubercle
within the capsule of the shoulder joint; and a
short head, which lies medial to the long head
and arises from the coracoid process. The two
muscular bellies fuse, and a flattened distal ten-
Figure 1.
Drawing illustrates the posterior surface of
the elbow, with
the anconeus
epitrochlearis
(arrow) extending from the
olecranon to the
medial epicondyle superficial to
the ulnar nerve
(arrowhead).
don attaches to the posterior margin of the radial
tuberosity.
Additional heads of the biceps brachii resulting in a three- or four-headed variant have been
described. These variants are extremely rare, and
the data are limited to case reports.
In a case report of a cadaveric dissection of a
four-headed biceps (2), the first supernumerary
head arose from the humerus in the area between
the lesser tuberosity and the coracobrachialis
and brachialis muscles and joined the long head
of the biceps at the level where the short head
joined. The second supernumerary head originated from the humerus at the site of insertion of
the coracobrachialis and joined the biceps tendon
at the bicipital aponeurosis in the distal third of
the arm.
Other accessory heads have been described
that originate from the intertubercular sulcus
adjacent to the insertion of the pectoralis major muscle (3). A muscular slip may pass to the
posterior fascia of the pronator teres muscle,
forming a tunnel that encircles the median nerve
and brachial artery. This entity is postulated as
a potential cause of compression with ensuing
symptoms.
Accessory Muscles of the Elbow
Accessory Brachialis
The brachialis arises from the anterior surface
of the lower half of the shaft of the humerus and
inserts into the tuberosity of the ulna and the adjacent surface of the coronoid process.
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Sookur et al
483
needs to be distinguished from the ulnar head
of the flexor carpi ulnaris muscle, which occurs
more distally and merges with the humeral head
of that muscle.
Accessory Head of the
Flexor Pollicis Longus Muscle
Figure 2. Axial T1-weighted MR image of the elbow shows the anconeus epitrochlearis (arrow) extending across the
cubital fossa from the medial cortex of the
olecranon (∗) to the medial epicondyle
(arrowhead).
An accessory brachialis originates from the
medial midshaft of the humerus and the medial
intermuscular septum, with a distal insertion into
the common tendon of the antebrachial flexor
compartment muscles (4). The accessory brachialis runs medial to the elbow and crosses the
median nerve and the brachial artery. The distal
tendon has been observed to split and enclose the
median nerve, which may result in symptoms of
median nerve compression (4).
Anconeus Epitrochlearis Muscle
Teaching
Point
The anconeus epitrochlearis muscle takes the
same course as the cubital tunnel retinaculum,
running from the medial cortex of the olecranon
to the inferior surface of the medial epicondyle
(Fig 1). It runs superficial to the ulnar nerve and
serves to keep the nerve in position. The cubital
tunnel retinaculum is postulated to be the remnant of the anconeus epitrochlearis. The reported
prevalence of the anconeus epitrochlearis from
cadaveric studies is 11% (5). The anconeus
epitrochlearis may be unilateral but was found to
be bilateral in one of four patients with cubital
tunnel syndrome in a study by Masear et al (6).
The relationship to the ulnar nerve explains the
association of the anconeus epitrochlearis with
cubital tunnel syndrome (6,7). The anconeus
epitrochlearis can be identified at US (8) or,
more accurately, at MR imaging (Fig 2) (7) and
The flexor pollicis longus (FPL) muscle arises
from the anterior surface of the radius inferior
to the anterior oblique line and superior to the
pronator quadratus muscle, with additional fibers
arising from the interosseous membrane. At the
wrist, the tendon passes deep to the flexor retinaculum to insert into the base of the distal phalanx of the thumb.
An accessory head of the FPL, or Gantzer
muscle, is relatively common, with a prevalence
from cadaveric studies of 45%–66% (9–11).
The origin of an accessory head of the FPL
can be variable. Some studies have found the medial epicondyle to be the origin in 75%–85% of
cases (9,11), whereas others have demonstrated
the coronoid process as the most common origin
(10). Other described origins of an accessory
head of the FPL include the flexor digitorum
superficialis (FDS) muscle (11) and a dual origin
from the medial humeral epicondyle and the coronoid process (Figs 3, 4) (9). An accessory head
of the FPL inserts distally into the ulnar border
of the FPL in 100% of cases (9,11) and is innervated by a branch of the AIN (9).
The relationship of the accessory head of the
FPL to the median nerve and the AIN is important with respect to potential symptom causation. There are conflicting reports in this regard,
with some investigators finding the AIN typically
coursing posterior to the muscular portion of the
accessory head of the FPL (12) and others finding the AIN coursing anterior (9). A study of 120
cadavers by Mahakkanukrauh et al (11) showed
the AIN coursing anterior to the accessory head
of the FPL in 13.4% of cases, lateral in 65.8%,
posterior in 8.1%, and posterolateral in 12.8%.
An accessory head of the FPL has been implicated in compressive neuropathies of either the median nerve or the AIN. Impingement on the AIN
results in anterior interosseous nerve syndrome,
particularly when the nerve runs deep to the
muscle (11), whereas compression of the median
nerve leads to pronator syndrome (9).
484
March-April 2008
Figure 3.
Drawing illustrates an accessory head of the
FPL (arrow) with
humeral and ulnar origins and
inserting into
the ulnar border
of the muscle
(arrowhead).
Bifurcated Distal Biceps Brachii
The distal insertion of the biceps brachii normally consists of a tendinous insertion onto the
radial tuberosity and the lacertus fibrosus, the latter being an aponeurosis arising from the medial
side of the tendon and continuing onto the deep
fascia covering the common flexor muscles of the
forearm.
Anomalies of the insertion of the biceps
brachii have been described, including a bifurcated tendon inserting onto the radial tuberosity
(13). The bifurcated distal tendon attaches via
medial and lateral myotendinous units onto the
radial tuberosity. Isolated complete tear of one
of the myotendinous units clinically simulating a
partial tear has been described (13).
Accessory Muscles
of the Hand and Wrist
Volar Aspect
Accessory Flexor Digitorum Superficialis
Indicis Muscle.—The FDS arises from humeral
(common flexor origin), ulnar, and radial heads.
It extends through the forearm, dividing into superficial and deep planes, before passing through
the carpal tunnel. The distal tendon to the index
finger arises from the deep plane and inserts into
the base of the middle phalanx.
An accessory FDS indicis muscle is a rare but
well-described accessory muscle originating from
the FDS tendon adjacent to the transverse carpal
ligament and inserting into the index finger, typi-
RG ■ Volume 28 • Number 2
Figure 4. Axial T1-weighted MR image
of the proximal forearm demonstrates a
prominent accessory head of the FPL (arrow) deep to the pronator teres (∗) and
medial to the anterior interosseous nerve
(AIN) (arrowhead). Evaluating successive
images makes it possible to distinguish an
accessory muscle from the ulnar head of
the pronator teres and the proximal flexor
digitorum profundus muscle.
cally in the region of the A1 pulley (14). Several
variants of the accessory FDS indicis tendon
have been reported in the literature, including a
muscle belly located wholly within the palm and
replacing the normal tendon. A second variant
consists of a digastric muscular component with
part of the muscle located in the forearm and
part located in the palm. A third variant is characterized by a muscle belly that is located within
the forearm and extends into but not beyond the
carpal tunnel (15,16).
An accessory FDS with a muscular component in
the palm may manifest clinically with a palpable
soft-tissue mass. In cases involving a digastric
component, more proximal extension of the
Teaching
accessory FDS can be associated with comPoint
pression of the median nerve within the carpal
tunnel (14). To our knowledge, there are no data
regarding the prevalence of an accessory FDS
indicis.
MR imaging has been successfully used to
demonstrate an accessory FDS indicis. The
anomalous muscle belly may be visualized in the
palm, deep to the subcutaneous tissues and volar
to the second metacarpal bone. The distal tendon of the anomalous muscle belly may be seen
extending into the flexor tendon sheath as the
superficial tendon (17). More proximal axial MR
images through the carpal tunnel may help identify the FDS tendon continuous with the muscle
belly in the palm; alternatively, an additional,
more proximal muscle belly may be evident in
cases with digastric morphologic features (18).
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485
Figure 6. Axial T1-weighted MR image
of the wrist shows an accessory ADM (arrowhead) superficial to the ulnar neurovascular bundle (arrow).
Figure 5. Drawing illustrates the flexor
surface of the hand, with an accessory
ADM (straight arrow) arising from the
flexor retinaculum (∗), coursing superficial
to the ulnar nerve (arrowhead), and inserting into the base of the fifth proximal phalanx with the ADM (curved arrow).
Accessory Hypothenar Muscles.—The hypothenar eminence is composed of the abductor
digiti minimi (ADM), flexor digiti minimi, and
opponens digiti minimi muscles; hence, accessory muscles of the hypothenar eminence will
be described together. The ADM arises from the
pisiform bone and flexor carpi ulnaris and inserts
into the ulnar side of the base of the proximal
phalanx of the little finger. The flexor digiti minimi arises from the flexor retinaculum and the
hook of hamate. It inserts into the proximal phalanx with the abductor and acts to flex the fifth
metacarpophalangeal joint.
Of the accessory hypothenar muscles, the
accessory ADM is the most common, with a
prevalence of 24% (19). It originates from the
antebrachial fascia, coursing anterior to the ulnar neurovascular structures in the Guyon canal
and inserting into the ADM or separately onto
the ulnar aspect of the base of the proximal phalanx (Figs 5, 6) (20). Alternatively, the accessory
muscle may originate from the palmaris longus
tendon in the lower third of the forearm (19). At
axial cross-sectional imaging, an accessory ADM
is evident as a muscular structure anterior to the
ulnar neurovascular bundle, on the radial aspect
of the pisiform bone. In contrast, the ADM arises
from the pisiform bone without extending anterior to the ulnar nerve at the level of the pisiform.
Other muscles may be mistaken for an accessory
ADM and include the palmaris brevis muscle,
which can be distinguished from the former owing to its location distal to the pisiform bone and
its insertion into the skin on the ulnar border of
the palm.
An accessory flexor digiti minimi is an extremely rare variant that arises from the intercompartmental septum on the ulnar aspect of the
forearm just proximal to the wrist joint, with a
distal insertion into either the proximal phalanx
of the fifth digit or the flexor digiti minimi (21).
The relationship of these accessory muscles to
the Guyon canal has been implicated in compression neuropathy of the ulnar nerve at this level
(20,22). An accessory ADM is still fleshy as it
crosses the Guyon canal, a characteristic that may
contribute to compression of the ulnar nerve and
helps identify the presence of an accessory muscle, since under normal circumstances no muscular structures are visualized in this location.
Variations in Palmaris Longus Muscle
Anatomy.—The palmaris longus muscle arises
from the common flexor origin and inserts into
the palmar fascia. It consists of a short muscle
proximally and a long tendon distally. The palmaris longus is a vestigial flexor of the wrist and is
the most variable muscle in the forearm. It is commonly used for the surgical repair of tendon tears.
The palmaris longus may have a variable appearance in the forearm and wrist. It may be
duplicated, digastric, entirely muscular, or tendinous proximally and muscular distally (reverse
palmaris longus) (18). An accessory or epifascial
palmaris longus is an extremely rare variant that
arises from the subcutaneous fascia of the forearm, with a distal insertion into the superficial
palmar aponeurosis. In an MR imaging study
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Figure 7.
Drawing illustrates the flexor
surface of the
distal forearm,
with an FCR brevis vel profundus
(arrow) arising
from the distal
radius, traversing
superficial to the
pronator quadratus (∗), and passing deep to the
flexor retinaculum
(arrowhead).
of 42 asymptomatic wrists, Zeiss and GuilliamHaidet (19) reported a prevalence of 7% for a
muscular palmaris longus, but no cases of an accessory palmaris longus were identified.
Palmaris longus variants may manifest clinically with a forearm soft-tissue mass or symptoms
of compression of the median nerve (23) or ulnar
nerve (24). Symptomatic cases of epifascial palmaris longus have been successfully treated with
surgical excision of the accessory muscle (25).
Epifascial palmaris longus, reverse palmaris
longus, and the digastric and completely muscular varieties of palmaris longus are characterized
at axial cross-sectional imaging of the wrist by
excess muscle tissue in the midline immediately
superficial to the flexor retinaculum. Imaging of
the more proximal forearm may serve to fully delineate the morphologic features of the palmaris
longus variant (18). Palmaris longus anomalies
are commonly overlooked at MR imaging (25).
Flexor Carpi Radialis Brevis Vel Profundus
Muscle.—There is very little literature on the
rare short radiocarpal flexor muscle known as the
flexor carpi radialis (FCR) brevis vel profundus
muscle (26,27). This accessory muscle arises
from the volar aspect of the distal radius, distal
to the origin of the FPL. The muscle passes anterior to the pronator quadratus and crosses deep
to the FCR. Distally, the tendon inserts onto
the capitate bone and the base of the third and
fourth metacarpals (Figs 7, 8). Although this rare
accessory muscle has typically been described in
cadaveric specimens, its position may potentially
cause symptoms of carpal tunnel syndrome. To
Figure 8. FCR brevis vel profundus
in a 22-year-old patient who presented
with volar soft-tissue swelling. Axial T1weighted MR image of the wrist shows
no masses, but a prominent accessory
muscle (straight arrow) is seen deep to the
FCR (curved arrow) and superficial to the
pronator quadratus (∗) on the radial aspect
of the FPL (arrowhead).
Figure 9.
Drawing illustrates the dorsal
aspect of the
wrist, showing
the course of the
EDBM (arrow) of
the index finger.
our knowledge, there are no descriptions of the
imaging characteristics of this muscle.
Dorsal Aspect
Extensor Digitorum Brevis Manus
Muscle.—An extensor digitorum brevis manus
(EDBM) muscle is a well-described accessory
muscle that occurs on the dorsum of the hand.
An EDBM is rare: In a recent study of 128 cadavers by Rodriguez-Niedenfuhr et al (28), this
accessory muscle was found in four limbs (1.6%).
The muscle can be unilateral or bilateral.
The origin of an EDBM can vary, although
the most common origin is the dorsal wrist
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487
Figure 10. EDBM in a 25-year-old man who presented with a palpable swelling of the
dorsal aspect of the wrist. (a, b) Sagittal T1-weighted (a) and axial T2-weighted (b) MR
images of the wrist show a mass (arrowhead) that is isointense relative to muscle and centered at the level of the carpometacarpal joint. (c) Longitudinal US image shows an EDBM
with the characteristic muscle echotexture (arrowhead).
capsule deep to the extensor retinaculum (28).
The muscle may also arise from the distal radius
or the deep carpal fascia (29). It typically inserts
onto the extensor hood of the index finger or
middle finger (Fig 9) (28) but may also insert
into the extensors of the fourth and fifth fingers
by way of either a tendon or a slip (29).
An EDBM can manifest clinically as a softtissue swelling on the dorsum of the hand that
becomes firm on extension of the fingers. It is
often clinically diagnosed as a ganglion, synovitis,
or a carpal boss. An EDBM is usually painless
but may occasionally be associated with exerciseinduced pain or tenosynovitis of the extensor tendons. The EDBM tendon has been used in surgical restoration of function of ruptured tendons.
At MR imaging, an EDBM typically appears
as a masslike lesion with low T1 and T2 signal
intensity, usually located between the second and
third metacarpals, with the bulk of the muscle at
the level of the carpometacarpal joints. Because
of its low T1 and T2 signal intensity, the muscle
may mimic a giant cell tumor of the tendon
sheath (Fig 10) (30). US can be used to demonstrate the normal echotexture of the muscle
as well as morphologic changes of the muscle
upon flexion and extension, resulting in dynamic
contraction-induced protrusion of the muscle between the extensor tendons (31).
Treatment of a symptomatic EDBM may include extensor retinaculum release or excision of
the muscle (32).
Accessory Extensor Carpi Radialis Muscle
and Variations.—The extensor carpi radialis
(ECR) longus muscle originates from the distal
lateral supracondylar ridge of the humerus and
the lateral epicondyle and inserts into the dorsal
surface of the base of the second metacarpal. The
ECR brevis muscle originates from the lateral
epicondyle of the humerus and the radial collateral ligament of the elbow and inserts into the
dorsal surface of the base of the third metacarpal.
The two tendons pass through the second extensor compartment at the wrist, located radial to
the Lister tubercle.
A variety of accessory musculature related to
ECR muscles has been reported in the literature,
including accessory ECR brevis, ECR intermedius, and ECR accessorius muscles.
An accessory ECR brevis arises from the
medial aspect of the normal ECR brevis (33).
The accessory tendon passes deep to the main
tendon and enters the second extensor tunnel
of the wrist. Variable insertions of the accessory
ECR brevis have been described, including the
base of the second metacarpal, the base of the
third metacarpal, and the dorsal digital expansion of the index finger, and may occur bilaterally
(33,34). An accessory ECR brevis may manifest
clinically as a soft-tissue mass in the forearm
that can be accurately distinguished from other
masses at US and MR imaging (35). It may
mimic a split tear of the ECR tendons in the second extensor tunnel, but more proximal images
will demonstrate the presence of the anomalous
muscle belly.
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Figure 11. Drawing
illustrates an ECR intermedius (arrow) originating between the ECR
brevis and ECR longus
and inserting into the
base of the second
metacarpal.
An ECR intermedius muscle originates between the origins of the ECR longus and ECR
brevis and inserts onto the base of the second or
third metacarpal or the abductor pollicis longus
muscle (Figs 11, 12). The reported prevalence
of an ECR intermedius in cadaveric studies has
ranged from 12% to 24%, with bilaterality in
59% of cases (36,37). When present, the tendon
has been used successfully in tendon transfer procedures (36,37).
The ECR accessorius muscle is an extremely
rare accessory muscle that originates from the
ECR longus and inserts into the abductor pollicis
longus or abductor pollicis brevis muscle (38).
When present, the ECR accessorius traverses the
wrist through a separate fascial tunnel beneath
the extensor retinaculum.
Accessory Muscles of the Knee
Accessory Slips of the Medial
and Lateral Gastrocnemius Muscle
The gastrocnemius muscle has two bellies, which
arise from the posterior surface of the femur just
above the femoral condyles and from the adjacent
capsule of the knee joint. The two bellies unite to
form the Achilles tendon.
Reported variations of the origin of the medial
and lateral heads of the gastrocnemius consist
of anomalous origins and accessory slips. The
medial head of the gastrocnemius may have an
aberrant origin, arising from the region of the intercondylar notch rather than the medial femoral
Figure 12. Axial T1-weighted MR image
of the wrist shows an incidental accessory
ECR muscle (arrowhead) located between
the ECR longus and ECR brevis tendons
(arrows) in the second extensor compartment. The patient was completely asymptomatic in this area.
Figure 13.
Drawing illustrates the
popliteal fossa,
with an accessory
slip of the medial
head of the gastrocnemius (arrow) compressing
the popliteal artery (arrowhead).
condyle. Similarly, the lateral head of the gastrocnemius may have an aberrant origin, arising more
medially from the posterior femur but maintaining its position lateral to the popliteal artery. An
accessory slip of the medial head of the gastrocnemius may arise from the intercondylar notch,
passing between the popliteal artery and vein and
inserting into the medial head of the muscle
(Fig 13). An accessory slip of the lateral head of
the gastrocnemius has also been described originating from the posterior cortex of the distal femur, medial to the lateral head. The slip courses
anterolateral to the popliteal vessels, inserting into
the lateral head of the gastrocnemius (Fig 14)
(39,40).
An anomalous relationship between the
popliteal artery and the proximal gastrocnemius
may manifest clinically with PAES. PAES results
Teaching
Point
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489
Figure 14. Popliteal artery entrapment syndrome (PAES) in a 33-year-old woman. Axial intermediate-weighted MR images of the knee demonstrate an accessory
slip of the lateral head of the gastrocnemius (arrow) arising from the posterior
aspect of the distal femur, crossing anterolateral to the popliteal vessels (arrowhead
in a), and inserting into the lateral head of the gastrocnemius (∗ in b).
Figure 15. PAES caused by an anomalous origin of the medial head of the gastrocnemius in a
35-year-old man. Maximum-intensity-projection
MR angiographic image shows obliteration of
the popliteal artery in the popliteal fossa (arrow).
from the occlusion of the popliteal artery upon
active plantarflexion or passive dorsiflexion of the
ankle. This condition is typically encountered in
young patients, who may present with symptoms
of intermittent claudication following exercise.
PAES has been classified according to the relationship of the popliteal vessels to the gastrocnemius and popliteus muscles (41). In type I anomaly, the popliteal artery takes a medial course
around a normally situated medial head of the
gastrocnemius. Type II anomaly is characterized
by an anomalous medial head of the gastrocnemius compressing the popliteal artery deep to
it. In type III anomaly, the accessory slip of the
medial head of the gastrocnemius forms a sling
around the popliteal artery. In type IV anomaly,
the popliteal artery courses deep to the popliteus, whereas type V anomaly is characterized by
concomitant involvement of the popliteal vein.
Type VI involves a normally located artery that
is entrapped by gastrocnemius hypertrophy. An
anomalous lateral head of the gastrocnemius and
an accessory slip of the lateral head have also
been implicated in PAES (39).
An accessory slip of the medial head of the
gastrocnemius has been found to occur in 21% of
patients with PAES, and an accessory slip of the
lateral head of the gastrocnemius was found in
30% of patients with PAES (39).
CT and MR imaging have been used to identify such accessory slips, as well as an anomalous
relationship of the popliteal vessels to the gastrocnemius. MR angiography has proved effective in
demonstrating popliteal artery occlusion and is
useful in surgical planning (Fig 15) (39,40).
Tensor Fasciae Suralis Muscle
A tensor fasciae suralis muscle is a very rare accessory muscle. Although it may arise from the
distal aspect of any of the hamstring muscles, in
the majority of reported cases it originates from
the distal semitendinosus muscle. It may insert
into the posterior fascia of the leg, into the medial
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March-April 2008
Figure 16. Tensor fascia suralis in a 30-year-old man. Axial proton-density–
weighted MR images show a popliteal swelling caused by an accessory muscle
(straight arrow) posterior to the semimembranosus (∗ in a) and lateral to the semitendinosus (arrowhead in a). More inferiorly, a tensor fascia suralis is seen superficial to the medial head of the gastrocnemius (curved arrow in b).
head of the gastrocnemius, or via a long thin
tendon onto the superficial aspect of the Achilles
tendon (42). A tensor fasciae suralis is located
superficially in the popliteal fossa, between the
semitendinosus and semimembranosus muscles
medially and the biceps femoris muscle laterally.
It is an unusual cause of a popliteal soft-tissue
swelling or mass (43) and is innervated by the
tibial nerve (42).
A tensor fasciae suralis can be accurately identified with US or MR imaging, which delineate
an accessory muscle in the popliteal fossa, lateral to the semitendinosus tendon and posterior
to the semimembranosus (Fig 16) (43,44). A
muscle similar in location but arising from the
semimembranosus has also been described (accessory semimembranosus) (45). In practice,
differentiating between these entities may be difficult, especially if the full extent of the muscle is
not imaged.
Accessory Popliteus
The popliteus arises via a short tendon from
the groove along the lateral cortex of the lateral
femoral condyle and inserts into the posteromedial surface of the proximal tibia, above the
popliteal line.
An accessory muscle termed the accessory
popliteus has also been described in the radiology
literature (46). This variant has a common origin
with the lateral gastrocnemius and extends inferomedially deep within the popliteal fossa, passing
anterior to the popliteal vessels and inserting into
the posteromedial capsule. MR imaging demonstrates an accessory muscle located between the
popliteal vessels and the posterior knee capsule
and coursing obliquely through the popliteal
fossa (Fig 17). Although in this reported case the
accessory muscle was asymptomatic, the authors
postulated that the proximity of the muscle to the
neurovascular bundle might result in compressive
symptoms (46).
Accessory Muscles of the Ankle
Lateral Aspect: Accessory Peroneal Muscles
The peroneus longus muscle (arising from the
proximal fibula) and the peroneus brevis muscle
(arising from the lower two-thirds of the fibula)
contribute tendons, which pass behind the lateral
malleolus. The peroneus brevis tendon lies more
anterior and separates the peroneus longus from
the peroneal groove of the distal fibula. More
distally, the peroneus longus and peroneus brevis
tendons are typically separated by the peroneal
tubercle of the calcaneus.
A third peroneal tendon, the peroneus tertius
tendon, is encountered in 83%–95% of cases in
cadaveric studies (47). The tendon and muscle
are located in the anterior compartment of the
leg and arise from the anterior surface of the distal fibula and the extensor digitorum longus muscle. The tendon passes deep to the inferior extensor retinaculum, either in the same compartment
as the extensor digitorum longus or in a different
compartment. The peroneus tertius inserts onto
the base and the dorsal surface of the shaft of the
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491
Figure 17. Accessory popliteus in a 43-year-old man who was being evaluated for an anterior cruciate ligament
tear. Axial (a), sagittal (b), and coronal (c) proton-density–weighted MR images of the knee demonstrate an accessory popliteus (straight arrow) coursing obliquely across the popliteal fossa deep to the popliteal vessels (arrowhead).
The accessory muscle courses parallel to the normal popliteus in the coronal plane (curved arrow in c).
Figure 18. Drawing illustrates the peroneocalcaneus
externum type of peroneus quartus (arrowhead) arising from the peroneus brevis (arrow) and inserting into
the retrotrochlear eminence (∗).
fifth metatarsal and may cause snapping over the
lateral dome of the talus (48).
A large number of other accessory peroneal
muscles have been described, including peroneus
quartus, peroneus accessorius, peroneocalcaneus
externum, and peroneus digiti minimi muscles
(49). Occasionally, the term peroneus quartus
is used to refer to several or even all of the accessory peroneal muscles (1,50). The reported
prevalence of peroneus quartus muscles varies
widely, partly due to the variety of terminology
used. Cadaveric studies demonstrate a prevalence
ranging from 13% to 26% (50,51). Radiologic
studies demonstrate a similar range of prevalence,
with 10% in an ankle MR imaging study (52)
and 22% in a US study (53). Peroneus quartus
muscles are frequently bilateral and are more
commonly seen in men (52,54).
In the majority of cases, a peroneus quartus
originates from the peroneus brevis. Other origins that have been described include the posterior surface of the fibula and the peroneus longus
(54). The peroneus quartus descends medial and
posterior to the other peroneal tendons (Fig 18)
(52). The distal insertion of peroneus quartus
tendons varies and has been classified into several
types (51). Insertion into the calcaneus is known
as peroneocalcaneus externum and represents
the most common type (54). The calcaneal attachment may be onto the peroneal tubercle (50)
or the retrotrochlear eminence posterior to the
peroneal tubercle (52). Insertion of the tendon
onto the retrotrochlear eminence may lead to significant hypertrophy of the latter structure (52).
However, other studies have found no difference
in the size of the retrotrochlear eminence in persons with a peroneus quartus and those without
(55). A peroneus quartus may also rarely insert
onto the cuboid (peroneocuboideus) (50,54), the
peroneus longus (peroneoperoneolongus), or the
inferior peroneal retinaculum adjacent to the retrotrochlear eminence (54).
The peroneus digiti minimi arises from the
peroneus brevis to insert via two tendons into the
base and head of the fifth metatarsal and the base
of the proximal phalanx (50,52). The peroneus
492
Figure 19. Axial T1-weighted MR image
of the ankle above the level of the tibiotalar
joint shows a peroneus quartus (arrowhead) posteromedial to the peroneus longus (straight arrow) and peroneus brevis
(curved arrow) and separated from them
by a subtle fat plane.
Teaching
Point
RG ■ Volume 28 • Number 2
March-April 2008
accessorius has been reported to arise from the
muscular portion of the peroneus brevis, inserting onto the peroneus longus (56).
Similar to the other peroneal muscles, a peroneus quartus acts predominantly as a pronator
of the foot. It is commonly encountered as an
asymptomatic variant but may cause lateral ankle
pain or ankle instability, particularly in athletes
(54). Hypertrophy of the bone attachments has
been implicated in peroneal tenosynovitis (50).
The presence of an extra tendon may result in
crowding of tendons deep to the peroneal retinaculum, leading to anterior subluxation, mechanical attrition, or longitudinal tears of the
peroneal tendons (50,57). In some cases, surgical
excision has provided symptomatic relief (54).
Peroneus quartus tendons have been used successfully for surgical repair of superior and inferior peroneal retinacular injuries (58).
US can help identify a peroneus quartus and its
calcaneal insertion (53), but the muscle is optimally delineated at MR imaging (59). On axial
MR images, a peroneus quartus is visualized posteromedial or medial to the peroneus brevis and is
separated from it by a fat plane (Fig 19) (52,53).
The accessory tendon may be mistaken for a
longitudinal split tear of the peroneal tendons.
However, the accessory tendon can be differentiated from a split tear by evaluating the
more proximal images, which will demonstrate an
anomalous muscle belly. The muscle belly
a peroneus quartus may vary in size, and the
an anomalous muscle
Figure 20. Drawing illustrates an FDAL (arrows)
inserting into the quadratus plantae (arrowhead).
myotendinous junction may extend distally to a
variable extent. In the retromalleolar region, the
myotendinous unit may range from completely
tendinous to completely muscular (53).
Medial Aspect
Flexor Digitorum Accessorius Longus.—The
flexor digitorum longus arises from the shaft of
the tibia below the soleal line. Its tendon passes
behind the medial malleolus and, in the sole,
splits into four slips to insert into the distal phalanges of the second through fifth toes.
A flexor digitorum accessorius longus (FDAL)
originates either from the medial margin of the
tibia (60) and the fascia of the deep posterior
compartment, or, with equal frequency, from the
lateral margin of the fibula distal to the origin of
the flexor hallucis longus (FHL) (61). However,
an FDAL can vary widely in origin, arising from
any structure in the posterior compartment, including adjacent muscles such as the FHL (62).
The FDAL tendon descends posterior and superficial to the tibial nerve (63), courses beneath
the flexor retinaculum through the tarsal tunnel,
and is intimately related to the posterior tibial
artery and tibial nerve (61). Within the tarsal
tunnel, it may contain fleshy fibers (62), a factor
that helps identify an FDAL at MR imaging. The
distal tendon inserts into the quadratus plantae
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493
Figure 21. Axial (a) and coronal
(b) intermediate-weighted MR images of the ankle show a fleshy FDAL
(straight arrow) medial to the FHL
(curved arrow in a) and deep to the
flexor retinaculum (arrowhead in a).
The insertion of the FDAL onto the
quadratus plantae (∗ in b) is well visualized in the coronal plane.
Figure 22. Axial intermediateweighted MR images obtained in
a 40-year-old patient with tarsal
tunnel syndrome show an FDAL
(arrowhead in a), along with fatty
infiltration of the FHL (straight arrow in b) and soleus muscle (curved
arrow in b).
muscle or the flexor digitorum longus tendon
(Fig 20) (61,63).
An FDAL has a prevalence of 6%–8%, is more
common in males, and is infrequently bilateral
(61,62).
Because of the close relationship of the tendon
in the tarsal tunnel to the neurovascular bundle,
it is not surprising that the presence of an FDAL
is associated with tarsal tunnel syndrome (63,64).
An FDAL has a prevalence of up to 12.2% in
patients with tarsal tunnel syndrome (65) and
has also been associated with tenosynovitis of the
FHL tendon (60).
An FDAL can be optimally identified on axial
MR images (62), which demonstrate the muscle
within the tarsal tunnel, typically superficial to
the neurovascular bundle (Fig 21). Attachment
onto the flexor digitorum longus or quadratus
plantae allows differentiation from other acces-
sory muscles within the tarsal tunnel, such as
peroneocalcaneus internus (PCI) and tibiocalcaneus internus (TCI) muscles, which insert onto
the calcaneus. However, demonstration of the
insertion of an FDAL onto the flexor digitorum
longus may be difficult in some cases. MR imaging may also exclude other causes of tarsal tunnel
syndrome, including ganglia, tumors, varicosities,
and FHL tenosynovitis (Fig 22).
Peroneocalcaneus Internus.—The largest series describing the MR imaging characteristics
of a PCI showed the muscle to originate at the
inner aspect of the lower fibula, below the origin
of the FHL, with interdigitation between these
two muscles (66). A PCI descends posterior and
lateral to the FHL and displaces it anteriorly
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RG ■ Volume 28 • Number 2
March-April 2008
Figure 23. Drawing illustrates a PCI (arrow) located
posterior to the FHL (arrowhead) and inserting onto
the medial surface of the calcaneus, anterior and inferior to the sustentaculum tali.
and medially, which may cause encroachment
on the neurovascular bundle as the tendons pass
through the tarsal tunnel (67). Both tendons pass
inferior to the sustentaculum tali, with the PCI
tendon inserting onto a small tubercle on the
medial aspect of the calcaneus below the sustentaculum (Fig 23).
In a study of asymptomatic volunteers, a PCI
was seen in 1% of cases, with bilaterality in 75%
(66).
A PCI is usually asymptomatic, since it is
not directly related to the neurovascular bundle
and typically becomes tendinous 2–3 cm above
the tibiotalar joint. However, cases of ankle pain
and limitation of movement have been described
(66,67), although the exact causative mechanism
is unclear. A PCI has been demonstrated in the
clinical setting of posterior ankle impingement
and FHL tenosynovitis (67).
MR imaging can accurately delineate the
origin and insertion of a PCI. At imaging, a
PCI may be difficult to differentiate from an
FDAL, with both tendons taking a similar course
through the ankle and hindfoot. Distinctive
features of a PCI include insertion onto the calcaneus, with a fat plane between the PCI and
quadratus plantae. In contrast, an FDAL inserts
into the flexor digitorum longus tendon or the
quadratus plantae itself. The calcaneal insertion
of a PCI may be optimally evaluated on coronal oblique images (66). The relationship of the
Figure 24. PCI in a 38-year-old man
with prior lateral ligamentous reconstruction. Axial intermediate-weighted MR
image of the ankle shows a PCI (arrow)
coursing deep to the flexor retinaculum and posterolateral to the FHL
(arrowhead).
Figure 25. Drawing illustrates an accessory soleus (arrow) arising from the
tibia anterior to the soleus (black ∗) and
gastrocnemius (white ∗) and inserting
onto the superior surface of the calcaneus anteromedial to the Achilles tendon
(arrowhead).
accessory muscle relative to the neurovascular
bundle may also help differentiate between an
FDAL and a PCI. The latter is typically located
posterior and lateral to the neurovascular bundle
and separated from it by the FHL (Fig 24); in
contrast, an FDAL lies immediately superficial to
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495
Figures 26, 27. (26) Accessory soleus
in a 29-year-old man. Lateral radiograph
of the ankle shows partial obliteration of
the Kager fat pad. An accessory soleus
(∗) is seen anterior to the Achilles tendon
(arrowhead) and posterior to the FHL
(arrow). (27) Coronal reformatted image
of the ankle from CT data obtained with
a soft-tissue algorithm shows an accessory soleus (∗) inferomedial to the normal
soleus (arrow) with a muscular insertion
onto the medial surface of the calcaneus.
Teaching
Point
the neurovascular bundle. Occasionally, the FHL
may demonstrate two tendinous slips, which may
be mistaken for a PCI tendon (66).
Accessory Soleus.—The soleus lies deep to the
gastrocnemius and arises from (a) the posterior
aspect of the head and upper shaft of the fibula,
(b) the soleal line of the tibia, (c) the middle
third of the medial border of the tibia, and
(d) a fibrous band bridging its fibular and tibial
origins. The fibers converge on a short tendon,
which normally joins the deep surface of the
Achilles tendon.
An accessory soleus arises from the anterior
(deep) surface of the soleus or from the fibula
and soleal line of the tibia (68–70). From its origin, the muscle descends anterior or anteromedial to the Achilles tendon (Fig 25) (68,70).
Five types of accessory soleus have been described on the basis of insertion characteristics.
Insertion points include the Achilles tendon,
the upper surface of the calcaneus with a fleshy
muscular insertion, the superior surface of the
calcaneus with a tendinous insertion, the medial
aspect of the calcaneus with a fleshy muscular
insertion, and finally, the medial aspect of the
calcaneus with a tendinous insertion (68,70,71).
When the accessory muscle inserts separately
into the superior surface of the calcaneus, the
insertion is anterior and medial to the calcaneal
insertion of the Achilles tendon (70,72). The accessory soleus is innervated by the posterior tibial
nerve, and the blood supply is via the posterior
tibial artery (69,71).
According to cadaveric studies, an accessory
soleus has a prevalence of 0.7%–5.5%, with the
muscle most commonly seen as a unilateral finding (69).
An accessory soleus may manifest clinically
as a soft-tissue mass in the posteromedial aspect
of the ankle. Clinically evident accessory soleus
muscles have a male predilection and commonly
manifest in the 2nd and 3rd decades of life (69),
a fact that may be attributable to the increase
in muscle mass and activity during this period.
There may be associated pain, which is typically
exertional, with a higher prevalence in athletes
(69,71). There are various explanations for pain
associated with an accessory soleus, including
development of a localized compartment syndrome due to an increase in intrafascial pressure
(71,73) or an inadequate blood supply from the
posterior tibial artery (69). Alternatively, accessory soleus hypertrophy may cause compression
of the adjacent posterior tibial nerve. Although an
accessory soleus lies outside the tarsal tunnel, it
has been implicated in tarsal tunnel syndrome in
cases in which the muscle attaches to the medial
aspect of the calcaneus (65). Symptomatic cases
have been successfully treated with fasciotomy,
tendon release, excision, or debulking (69,74). In
a case report of an accessory soleus with congenital clubfoot, Chotigavanichaya et al (72) found
the talipes equinus and talipes varus deformities
resistant to percutaneous Achilles tenotomy, necessitating concomitant release of the accessory
soleus.
An accessory soleus may be visualized at
conventional radiography (Fig 26) (68), CT
(Fig 27) (68,73), US (73), and MR imaging
(69,71,74–76). At conventional radiography, the
normal triangular appearance of the Kager fat
pad anterior to the Achilles tendon is obscured
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March-April 2008
RG ■ Volume 28 • Number 2
Figures 28, 29. (28) Axial intermediate-weighted MR image obtained slightly cranial to the tibiotalar joint shows
an accessory soleus (∗) anteromedial to the Achilles tendon (arrow) and superficial to the tarsal tunnel and flexor
retinaculum (arrowhead). (29) Accessory soleus in a 45-year-old woman. (a) Sagittal short inversion time inversionrecovery image of the ankle shows a tear of the Achilles tendon (arrow). (b) Axial intermediate-weighted MR image
of the ankle shows an accessory soleus (arrowhead) with tendinopathy and low-grade partial tearing.
or obliterated by a well-defined area of increased
soft-tissue density. Cross-sectional imaging is
more sensitive and specific in the identification
of an accessory soleus. The accessory muscle is
demonstrated anterior to the Achilles tendon and
superficial to the flexor retinaculum, typically extending medially to the area between the medial
edge of the Achilles tendon and the medial malleolus (Fig 28). In some cases, MR imaging may
demonstrate abnormal signal intensity, a finding
that reflects the presence of trauma, ischemia, or
atrophy (Fig 29).
Tibiocalcaneus Internus.—There is scant
literature concerning the TCI (77,78), a rare
accessory muscle that arises from the medial
crest of the tibia and descends deep to the flexor
retinaculum and posterior to the neurovascular
structures. A TCI inserts distally onto the medial
surface of the calcaneus approximately 1–2 cm
anterior to the Achilles tendon insertion. The origin and insertion of a TCI is similar to the type of
accessory soleus that inserts onto the medial cortex of the calcaneus. However, the two accessory
muscles can be distinguished on the basis of their
location relative to the flexor retinaculum. A TCI
passes deep to the flexor retinaculum, whereas
an accessory soleus is located superficial to the
flexor retinaculum (Fig 30). The location of a
TCI (within the tarsal tunnel and superficial to
the neurovascular bundle) may resemble that of
an FDAL. The distal insertion of an FDAL onto
Figure 30. TCI in a 30-year-old patient
with tarsal tunnel syndrome. Axial intermediate-weighted MR image of the ankle
shows a TCI (arrow), which resembles an
accessory soleus in that it is located anteromedial to the Achilles tendon and inserts
onto the medial surface of the calcaneus
(not shown). However, it differs from an
accessory soleus in that it is located deep
to the flexor retinaculum (arrowheads). Its
insertion onto the medial surface of the calcaneus helps distinguish it from an FDAL.
the flexor digitorum longus or quadratus plantae
may be used as a distinguishing feature.
To our knowledge, there are no radiology reports on the TCI. Given its location within the
tarsal tunnel and its similarities to the accessory
soleus and FDAL, the TCI may be associated
with tarsal tunnel syndrome.
RG ■ Volume 28 • Number 2
Conclusions
A large number of accessory muscles have been
described in the surgical and radiology literature,
primarily in limited case reports. The vast majority of these accessory muscles are asymptomatic
and tend to represent incidental findings at surgery or imaging. However, accessory muscles
may result in symptoms in some cases. Symptoms
may be related to a palpable swelling, and the differential diagnosis of an accessory muscle should
be borne in mind when an obvious mass is not
detected at imaging. Furthermore, accessory
muscles may result in compression neuropathies,
especially in relation to the cubital tunnel, carpal
tunnel, Guyon canal, and tarsal tunnel. Careful
evaluation of fibro-osseous tunnels for an accessory muscle may help identify such a muscle as a
causative factor, which can easily be overlooked
unless accessory muscles are specifically sought
out during the review process.
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Volume 28 • Volume 2 • March-April 2008
Sookur et al
Accessory Muscles: Anatomy, Symptoms, and Radiologic
Evaluation
Paul A. Sookur, MRCP, et al
RadioGraphics 2008; 28:481–499 • Published online 10.1148/rg.282075064 • Content Code:
Page 483
The relationship to the ulnar nerve explains the association of the anconeus epitrochlearis with cubital
tunnel syndrome (6,7).
Page 484
In cases involving a digastric component, more proximal extension of the accessory FDS can be
associated with compression of the median nerve within the carpal tunnel (14).
Page 488
An anomalous relationship between the popliteal artery and the proximal gastrocnemius may manifest
clinically with PAES.
Page 492
The accessory tendon may be mistaken for a longitudinal split tear of the peroneal tendons. However,
the accessory tendon can be differentiated from a split tear by evaluating the more proximal images,
which will demonstrate an anomalous muscle belly.
Page 495
Occasionally, the FHL may demonstrate two tendinous slips, which may be mistaken for a PCI
tendon (66).