AJR 2002; 178:1247-1254
© American Roentgen Ray Society
Artifacts, Anatomic Variants, and Pitfalls in Sonography of the Foot and Ankle
Smita Patel1,
David P. Fessell1,
Jon A. Jacobson1,
Curtis W. Hayes1 and
Marnix T. van Holsbeeck2
1 Department of Radiology, University of Michigan Health System, 1500 E. Medical
Center Dr., TC2910, Ann Arbor, MI 48109-0326.
2 Department of Radiology, Henry Ford Hospital, 2799 W. Grand Blvd., Detroit, MI
48202.
Received September 7, 2001;
accepted after revision November 6, 2001.
Presented at the annual meeting of the American Roentgen Ray Society,
Washington, DC, May 2000.
Address correspondence to D. P. Fessell.
Introduction
The sonographic appearance of ankle tendons, ligaments, joints, and other
soft-tissue masses has recently been described
[1,2,3,4].
Familiarity with the anatomic variants and knowledge of sonographic artifacts
and pitfalls improves diagnostic yield and aids in treatment. In this article,
we discuss a variety of artifacts, anatomic variants, and pitfalls that can
simulate abnormality or injury and become a source of confusion and
misdiagnosis.
Pitfalls Associated with Technical Factors
Artifacts
Anisotropy is an artifact commonly seen when ligaments and tendons are
examined with the sonographic beam oblique to the structure being imaged. The
obliquity causes the tendons to appear artificially hypoechoic, mimicking the
appearance of tendinosis or a tear (Fig.
1A,1B).
Anisotropy can be avoided by keeping the beam perpendicular to the tendon or
ligament [1] (Fig.
1A,1B).
All ligaments and tendons should be assessed in both the longitudinal and
transverse planes.
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Fig. 1A. —57-year-old woman with normal peroneal tendons. Transverse
sonogram obtained with beam perpendicular to peroneus brevis (small
arrow) and peroneus longus (large arrow) tendons, which are
adjacent to lateral malleolus (arrowheads), shows normal fibrillar
echotexture of tendons.
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Fig. 1B. —57-year-old woman with normal peroneal tendons. Transverse
sonogram at same location as A but with slight angulation of transducer
shows hypoechoic peroneus brevis (small arrow) and peroneus longus
(large arrow) tendons, caused by anisotropy. Note cortex of fibula
(arrowheads).
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A refraction artifact produces posterior acoustic shadowing at the edge of
a tendon. Refraction may be seen at the site of a tendon tear while the
longitudinal course of a tendon is being scanned, and therefore, its presence
can help to establish diagnosis of a tear
[2]
(Fig. 2). During transverse
scanning of a tendon, refraction can produce shadowing from the curved outer
surface of a normal tendon. Such shadowing could simulate a tear in a deeper
adjacent tendon (Fig. 3).
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Fig. 2. —43-year-old man with Achilles tendon tear. Longitudinal
sonogram reveals refraction artifact (long arrows) at site of
surgically confirmed complete Achilles tendon tear. Note ends of torn tendon
(X markers).
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Fig. 3. —75-year-old woman with normal peroneal tendons. Oblique
longitudinal sonogram shows refraction artifact (straight arrow) from
edge of peroneus longus tendon (curved arrow). Artifact simulates
tear in peroneus brevis tendon (arrowheads). MR imaging (not shown)
confirmed peroneal tendons to be intact.
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Failure to Use Dynamic Imaging
One of the advantages of using sonography rather than other imaging
modalities is its dynamic capability, allowing imaging during joint motion and
direct correlation with the patient's symptoms. It is especially valuable in
examining patients believed to have tendon dislocation, subluxation, or tears
[1,
2] (Fig.
4A,4B).
Failure to use the full dynamic capabilities of sonography decreases
diagnostic accuracy and is an entirely avoidable pitfall.
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Fig. 4B. —33-year-old man with peroneal tendon subluxation. Transverse
sonogram obtained during dynamic assessment with ankle in dorsiflexion and
eversion shows subluxation of peroneal tendons (arrows) superficial
to fibular cortex (arrowheads).
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Pitfalls Associated with Normal Anatomic Variants
Normal Fluid Around Tendons and Joints
Normally, less than 3 mm of fluid can be seen at the dependent portions of
the peroneal tendons, anterior tibiotalar joint, and retrocalcaneal bursa in a
healthy person [3]
(Fig. 5). On sonography, no
fluid is normally seen around the flexor digitorum longus, anterior tibial
tendon, posterior ankle joint, or Achilles tendon.
Distal Posterior Tibial Tendon
On both MR imaging and sonography, the distal posterior tibial tendon can
appear heterogeneous at its navicular attachment
[4]. This appearance is due
both to the fat interspersed between multiple small slips of tendon as they
fan out to insert onto the navicular bone and to the spring ligament insertion
[5] (Fig.
6A,6B).
Care must be taken when examining this region. Applying pressure with the
transducer to elicit symptoms at the site of a suspected abnormality can aid
in distinguishing normal asymptomatic heterogeneity from true symptomatic
disease or injury, such as a tendon tear. In addition, one must exercise care
to ensure that an accessory navicular bone (Fig.
6A,6B)
is not misinterpreted as an abnormality.
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Fig. 6A. —55-year-old woman with normal os tibiale externum variant.
Line drawing illustrates distal posterior tibial tendon (arrowheads)
and its relationship to os tibiale externum (arrow). (Courtesy of BMC
Media, Ann Arbor, MI)
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Fig. 6B. —55-year-old woman with normal os tibiale externum variant.
Longitudinal sonogram shows normal heterogeneous appearance of distal
posterior tibial tendon (short arrows) as it fans out to insert on
navicular. Os tibiale externum (long arrow) is also visible.
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Calcaneofibular Ligament
The appearance of the calcaneofibular ligament on transverse scanning can
mimic an intraarticular body deep in relation to the peroneal tendons (Fig.
7A,7B).
Longitudinal scanning shows the normal fibrillar echotexture of the ligament.
Familiarity with the appearance of the normal calcaneofibular ligament
prevents misdiagnosis.
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Fig. 7A. —45-year-old woman with normal calcaneofibular ligament.
Appearance of calcaneofibular ligament (white arrows) on transverse
sonogram mimics that of intraatricular body, deep in relation to peroneus
longus (double black arrows) and brevis (single black
arrow). More distal peroneus longus tendon is incompletely visualized
because of oblique angle of transducer.
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Fig. 7B. —45-year-old woman with normal calcaneofibular ligament.
Longitudinal sonogram shows fibrillar echotexture of calcaneofibular ligament
(long arrows) located deep in relation to peroneal tendons
(arrowheads). Note cortex of calcaneus (short arrows).
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Accessory Muscles and Tendons
The peroneus quartus, a common accessory muscle, is medial and posterior to
the peroneus brevis tendon [6]
(Fig.
8A,8B,8C,8D).
The presence of a peroneus quartus can stretch the retinaculum and compress
the peroneus brevis tendon against the fibula, leading to a split or tear of
the peroneus brevis tendon. A peroneus quartus can also be confused with a
longitudinal tear of the peroneus brevis tendon. Knowledge of the normal
course and common insertion of the peroneus quartus onto the lateral calcaneus
can help one to avoid this pitfall.
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Fig. 8A. —44-year-old man with normal peroneus quartus muscle. Line
drawing illustrates peroneus quartus tendon (arrow) inserting onto
lateral calcaneus. Arrowheads denote peroneus longus tendon. (Courtesy of BMC
Media, Ann Arbor, MI)
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Fig. 8B. —44-year-old man with normal peroneus quartus muscle.
Longitudinal sonogram shows the peroneus quartus muscle belly (short
arrows) and tendon (long arrow) deep in relation to peroneus
brevis and longus tendons (arrowheads).
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Fig. 8D. —44-year-old man with normal peroneus quartus muscle. Axial
T1-weighted MR image shows peroneus quartus muscle belly (black
arrows) medial to peroneus brevis muscle and tendon (long white
arrow). Note peroneus longus tendon (short white arrow).
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Low-Lying Peroneus Brevis Muscle
The low-lying peroneus brevis muscle extends to or beyond the distal tip of
the fibula. If present, a low-lying peroneus brevis muscle belly can also
produce mass effect, pain, and stretching of the peroneal retinaculum,
contributing to peroneal tendon subluxation or longitudinal tear (Fig.
9A,9B,9C).
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Fig. 9A. —50-year-old woman with low-lying peroneus brevis muscle. Line
drawing illustrates low-lying peroneus brevis muscle (arrows) distal
to tip of fibula (arrowhead). (Courtesy of BMC Media, Ann Arbor,
MI)
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Fig. 9B. —50-year-old woman with low-lying peroneus brevis muscle.
Longitudinal sonogram shows low-lying belly (arrows) of peroneal
brevis muscle, superficial in relation to adjacent calcaneus
(arrowheads).
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Fig. 9C. —50-year-old woman with low-lying peroneus brevis muscle.
Transverse sonogram shows low-lying peroneus brevis muscle (short straight
arrows) and tendon (long straight arrow) superficial to
calcaneus (arrowheads). Peroneus longus tendon (curved
arrow) is seen superficial to peroneal brevis tendon (long straight
arrow).
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Accessory Ossicles
Common ossicles found around the foot and ankle include the os tibiale
externum at the distal posterior tibial tendon (Fig.
6A,6B)
and the os peroneum in the peroneus longus tendon (Fig.
10A,10B).
Knowledge of their location and appearance prevents confusing these structures
with true abnormalities, such as tendon calcification or an unusual avulsion
fracture. Conducting direct sonographic correlation with the site of symptoms
and obtaining radiographs aid in establishing the diagnosis.
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Fig. 10B. —39-year-old man with normal os peroneum. Longitudinal
sonogram shows normal os peroneum (large arrow) in peroneus longus
tendon (arrowheads). Posterior acoustic shadowing from os peroneum
obscures underlying cortex of cuboid, deep in relation to os peroneum. Small
arrows indicate cuboid cortex.
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Hypoechoic Fat
The echogenicity of predominantly fatty regions varies depending on the
precise composition of the regions. Pure fat appears hypoechoic, whereas a
mixture of fat and other soft-tissue elements has a hyperechoic appearance
[7]. Kager's fat, which is
anterior to the Achilles tendon, is often hypoechoic and should not be
mistaken for a fluid collection in the retrocalcaneal bursa (Fig.
11A,11B)
[8]. Correlation with the
patient's symptoms and examination of the opposite, nonsymptomatic side can
help in determining the diagnosis.
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Fig. 11A. —35-year-old man with normal fat pad deep in relation to
Achilles tendon. Longitudinal sonogram of posterior left ankle shows
hypoechoic Kager's fat (arrowheads) deep in relation to Achilles
tendon (arrows).
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Fig. 11B. —35-year-old man with normal fat pad deep in relation to
Achilles tendon. CT scan with sagittal reconstruction shows normal attenuation
of Kager's fat (long arrow) deep in relation to Achilles tendon
(short arrow).
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Pitfalls Associated with Injury and Disease
Intact Plantaris Tendon with Achilles Rupture
The plantaris tendon is a thin tendon that is present in approximately 90%
of the population. Its normal location is anteromedial to the Achilles tendon.
When the Achilles tendon is completely torn, the plantaris frequently remains
intact and moves posteriorly into the region of the Achilles tendon tear
(Fig. 12). In the context of a
complete Achilles tendon tear, an intact plantaris tendon may lead to a false
diagnosis of a partial Achilles tendon tear, thereby delaying appropriate
treatment [2]. Knowledge of the
normal anatomy and appearance of the intact plantaris in the setting of an
Achilles tendon tear helps one avoid making this error.
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Fig. 12. —28-year-old man with Achilles tendon tear. Longitudinal
sonogram shows intact plantaris tendon (long arrows) at site of
complete tear of Achilles tendon. Short arrows denote tendon ends.
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Flexor Digitorum Tendon Mimicking an Intact Posterior Tibial
Tendon
The flexor digitorum longus tendon is normally slightly medial and
posterior to the posterior tibial tendon. In the setting of a complete rupture
of the posterior tibial tendon, an intact flexor digitorum longus tendon may
be confused with an intact posterior tibial tendon because of their close
proximity [4] (Fig.
13A,13B).
The course of the flexor digitorum longus tendon, however, is immediately
superior to the talocalcaneal joint. Noting this distinction helps prevent
confusing the two structures.
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Fig. 13A. —80-year-old woman with posterior tibial tendon tear.
Longitudinal sonogram shows intact flexor digitorum longus (arrows),
which should not be confused with intact posterior tibial tendon tear in
setting of complete flexor digitorum longus tear. Note talar cortex
(arrowhead).
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Fig. 13B. —80-year-old woman with posterior tibial tendon tear.
Longitudinal sonogram shows refraction (arrowheads) at site of
complete posterior tibial tendon tear (large arrows). Flexor
digitorum longus (small arrows) is noted immediately deep in relation
to posterior tibial tendon and could be confused with intact posterior tibial
tendon if scanned at site of tear.
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Joint Fluid Versus Synovitis
A complex effusion can mimic synovitis because both may appear hypoechoic.
Power Doppler sonography can be useful in revealing the internal flow that can
be present in synovitis but not in an effusion. Dynamic imaging allows
observation of the fluid as it moves during joint motion; no movement is
observed in synovitis. Applying pressure with the transducer can also induce
swirling of debris that helps to distinguish a complex effusion from a simple
effusion [3] (Figs.
14 and
15).
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Fig. 14. —29-year-old man with gout. Longitudinal sonogram of anterior
tibiotalar joint shows anechoic effusion (solid arrows). Note distal
tibial cortex (small arrowheads) and talar cortex long (large
arrowhead). Fluid obtained at aspiration revealed uric acid crystals
compatible with gout. Note anterior tibial tendon (open arrows).
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Fig. 15. —48-year-old woman with rheumatoid arthritis. Longitudinal
sonogram of anterior tibiotalar joint shows hypoechoic joint effusion with
low-level echoes (arrows), compatible with complex joint effusion or
synovitis. Sonographically guided needle aspiration yielded no fluid, a
finding consistent with synovitis. Note distal tibial cortex (small
arrowheads) and talar cortex (large arrowhead).
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Conclusion
Familiarity with normal anatomic variants as well as with sonographic
artifacts and common pitfalls can increase the diagnostic accuracy of ankle
and foot sonography.
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Introduction
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