n radiologic case study
Enhance your diagnostic skills with
this “test yourself” monthly column,
which features a radiograph and
challenges you to make a diagnosis.
The case:
A
36-year-old man presented with chronic posterior left ankle pain.
A
B
Figure: Lateral radiograph (A) and sagittal inversion-recovery magnetic resonance image (B) of the left ankle.
Your diagnosis?
For answer see page 63
JANUARY 2013 | Volume 36 • Number 15
n radiologic case study
Section Editor: Terrence C. Demos, MD
S P OT L I G H T O N
foot and
ankle
Diagnosis:
Os Trigonum Syndrome
Albert J. Song, MD; Matthew Del Giudice, MD; Martin L. Lazarus, MD; Laurie M. Lomasney, MD;
Terrence C. Demos, MD; Katherine Dux, DPM
Answer to Radiologic Case Study
Case facts appear on page 5
A 36-year-old man presented with chronic posterior
ankle pain. A lateral radiograph of the left ankle showed
tibiotalar and posterior subtalar joint effusions (Figure
1A). In addition, an isolated
ossicle was observed posterior to the talus—an os trigonum. Infiltration at Kager’s
fat pad was nonspecific. A
sagittal
inversion-recovery
magnetic resonance image
of the left ankle confirmed
a posterior subtalar joint effusion and inflammation of
Kaeger’s fat pad (Figure 1B).
Also, an abnormal bone marrow edema pattern was observed with high signal in the
os trigonum, as well as the adjacent posterior talus and superior calcaneus. In a patient
with chronic posterior heel
pain, this is characteristic of
os trigonum syndrome. Incidentally, the Achilles tendon
has normal imaging characteristics.
Impingement syndrome of
a joint refers to the mechanical obstruction or restriction
of motion by intervening soft
tissues or osseous structures,
resulting in a chronic painful
arc. Impingement syndromes
of the ankle can be anterior,
anterolateral, anteromedial,
posteromedial, or posterior to
the joint.1
The authors are from the Department of Radiology (AJS, MDG, LML,
TCD) and the Division of Podiatry (KD), Loyola University Medical Center,
Maywood, and the Department of Radiology (MLL), Northshore University
Health System, Evanston, Illinois.
The authors have no relevant financial relationships to disclose.
Correspondence should be addressed to: Laurie M. Lomasney, MD,
Department of Radiology, Loyola University Medical Center, 2160 S First
Ave, Maywood, IL 60153 (llomasn@lumc.edu).
doi: 10.3928/01477447-20121217-01
1A
1B
Figure 1: Lateral radiograph of the left ankle showing tibiotalar and posterior
subtalar joint effusions posteriorly (white arrow). Note the isolated ossicle
(yellow asterisk) posterior to the talus—an os trigonum (A). Sagittal inversion-recovery magnetic resonance image of the left ankle showing posterior
joint effusion, inflammation of Kager’s fat pad (white arrows), abnormal bone
marrow edema pattern with high signal in the os trigonum (yellow asterisk),
and adjacent posterior talus and superior calcaneus (red arrows) (B).
Posterior ankle impingement syndrome has been
termed os trigonum syndrome, talar compression
syndrome, posterior tibiotalar
impingement syndrome, and
posterior block of the ankle.2-5 Although extensively
described in ballet dancers,
os trigonum syndrome has
also been reported in soccer,
football, and volleyball players, downhill runners, and
even nonathletes.2,3,5-7
Anatomy and
Pathophysiology
The posterior process of the
talus consists of medial and
lateral tubercles and the intervening groove for the flexor
hallucis longus tendon. An os
trigonum results from failure
of a secondary ossification
center to fuse to the lateral tubercle of the posterior process
of the talus (Figure 2). This
secondary ossification center
appears between ages 8 and
JANUARY 2013 | Volume 36 • Number 163
Cover Story
Cover illustration © Scott Holladay
64
ORTHOPEDICS | Healio.com/Orthopedics
n radiologic case study
13 years and typically fuses
within 1 year.2,5,6 An os trigonum is seen in up to 14% of
the general population.1
The os trigonum has 3 surfaces: anterior, inferior, and
posterior. The anterior surface
forms a synchondrosis with
the posterior process of the
talus. The inferior surface can
articulate with either the calcaneus or the posterior subtalar
joint. The posterior talofibular
ligament inserts onto the posterior surface of the os trigonum.2
Impingement and pain
can result from several specific pathologies affecting this
complex regional anatomy.
Classically described in ballet dancers who perform enpointe and demi-pointe, os
trigonum syndrome is due to
repetitive dorsi- and plantarflexion of the ankle causing
impingement of posterior
ankle soft tissue and osseous
structures.6 Direct compression of the os trigonum or disruption of the synchondrosis
may be observed. An enlarged
os trigonum increases the
probability of chronic impaction injury.1,6
An elongated lateral tubercle, known as a Steida process,
extends from the posterior process of the talus and is thought
to predispose individuals to
posterior ankle impingement.7
Compression of the Steida
process has been implicated as
a cause of posterior ankle impingement. Distinguishing a
fracture of the Steida process,
called a Shepherd’s fracture,
from os trigonum syndrome
can be a diagnostic dilemma
because both can present
acutely after an injury involving extreme ankle plantar flexion.2
Posterior ankle impingement is also associated with a
thickened posterior intermalleolar ligament and stenosing tenosynovitis of the flexor hallucis longus. The posterior intermalleolar ligament (IML) has
been implicated as a cause of
posterior ankle impingement,
but details of the role of this
ligament in impingement are
not fully understood. Found in
up to 56% of the population,
the IML is located between the
posterior tibiofibular and the
posterior talofibular ligaments
(Figure 3).6 Some consider the
IML to be an extension of the
posterior talofibular ligament.
A similar ligament is found
in primate ankles, so the IML
is also referred to as the tibial
slip of the posterior talofibular
ligament and the marsupial
meniscus. Intermalleolar ligament tears or entrapment can
also cause posterior ankle impingement symptoms.8,9
The relationship of posterior impingement and tenosynovitis of the flexor hallucis
longus is unclear. At the posterior ankle, the flexor hallucis
longus tendon travels within
the sulcus between the medial
and lateral tubercles of the talar posterior process, a region
where direct compression by
the os trigonum may exist.10
However, flexor hallucis longus tenosynovitis is common
in ballet dancers with or without posterior ankle impingement.10 Also, although tenosynovitis of the flexor hallucis
longus can be seen in up to
68% of patients with posterior
2
Figure 2: Superior view volumerendered 3-dimensional computed
tomography scan of the talus with an
os trigonum (black asterisk) showing
the anatomy of the posterior process.
Synchondrosis between the posterior process and os trigonum (white
arrow), the groove for the flexor hallucis longus tendon (white asterisk),
and the medial tubercle of the posterior process (arrowhead) are shown.
ankle impingement, a causal
relationship has yet to be established.2,7,10
Although not included in
os trigonum syndrome due to
the acute presentation, acute
chondro-osseous disruption
can occur as a result of impaction during plantarflexion or
the ossicle can be avulsed due
to tension at the insertion of
the posterior talofibular ligament while in dorsiflexion.2
Interestingly, those who present with acute posterior ankle
impingement symptoms tend
to be nonathletes, suggesting
that the mechanism is acute
disruption of the synchondrosis or osseous contusion rather
than chronic repetitive motion.
Clinical Presentation
Os trigonum syndrome
classically presents as chronic
posterior ankle pain, typically
as a result of an overuse syndrome.1 Ankle stiffness, lock-
3
Figure 3: Coronal proton-density
magnetic resonance image of the left
ankle of a 12-year-old boy showing
the posterior intermalleolar ligament
(arrowhead) as a thin fibrous band
arising from the malleolar fossa
of the fibula coursing between the
posterior tibiofibular ligament (white
arrow) and posterior talofibular ligament (black arrow), inserting on the
posterior tibia.
ing, and posterior soft tissue
swelling are symptoms of os
trigonum syndrome. Physical
examination reveals posterior
ankle tenderness and soft tissue fullness anterior to the
Achilles tendon. Ankle pain
can be exacerbated by forced
plantar flexion or push-off
maneuvers, such as those that
occur during dancing, running downhill, or kicking.1,2
The ankle may be inverted
to prevent the impingement
symptoms, frequently resulting in recurrent ankle sprains.7
Although symptoms are often relieved with rest, pain
frequently recurs with return
to activity.6 Posterior ankle
pain can be complex, with
a differential diagnosis that
includes Achilles pathology,
retrocalcaneal bursitis, flexor
hallucis longus and peroneus
JANUARY 2013 | Volume 36 • Number 165
n radiologic case study
4A
4B
4C
Figure 4: Lateral radiograph of the left ankle of a 12-year-old girl with a great toe fracture (not shown) with incidental
note of multipartite or immature Stieda process (white arrow) posterior to the talus (A). Lateral radiograph of the left
ankle of a 24-year-old man with a similar fragmented appearance of the Stieda process (white arrow) (B). Sagittal computed tomography scan of the same ankle in 4B confirming an acute fracture (white arrow) of the Stieda process (C).
5A
5B
5C
Figure 5: Lateral radiograph of the left ankle of a 49-year-old man with hindfoot pain showing sclerosis and subcortical
cystic change (arrow) at the synchondrosis of the os trigonum (A). Sagittal computed tomography scan of the ankle/
hindfoot showing degenerative arthropathy and subluxation (white arrow) of the posterior subtalar joint. A secondary
abnormal articulation is shown between the os trigonum and the posterior subtalar facet of the calcaneus. Computed tomography confirms degenerated synchondrosis (black arrow) of os trigonum (B). Oblique axial computed tomography
scan of the ankle showing flexor hallucis tenosynovitis (white arrow) with fluid and inflammation around the tendon (C).
pathology, degenerative and
inflammatory arthropathies of
the ankle and hindfoot, ankle
instability, tarsal tunnel syndrome, talar osteochondral
lesions, Haglund syndrome,
coalition, and, in the pediatric
population, Sever’s disease.3
Radiology
Radiography
Radiography is the imaging
of choice for the initial radiologic evaluation of ankle pain.
Unfortunately, findings of posterior ankle impingement can
be nonspecific or inconclusive
on radiographs. The presence
66
of an os trigonum is not diagnostic of posterior ankle impingement because up to 14%
of people without posterior
ankle symptoms have an os trigonum.1 Although an os trigonum larger than 1 cm has been
associated with os trigonum
syndrome, size does not correlate with symptoms.2 Fracture
and fragmentation of the os trigonum are also difficult to distinguish from a normal multipartite ossicle on radiographs
(Figure 4).3 Degeneration of
the synchondrosis is characterized by an irregular, sclerotic, or cystic articulation
(Figure 5).1,2 Dynamic fluoroscopic evaluation or static lateral radiographs of the ankle
in plantarflexion may show osseous impingement, whereas
contrast intravasation at the
synchondrosis during ankle
arthrography confirms disruption.1,5,7
Although potentially contributory, none of the associated
anatomic variations provide a
definitive diagnosis. In a review
by Peace et al,6 a prominent
superior calcaneal tuberosity
was present in 64% of cases of
clinical os trigonum syndrome.
A prominent superior calcaneal
tuberosity is defined as any osseous prominence of the posterior superior calcaneus extending above the superior parallel
calcaneal pitch line, which is a
line drawn from the posterior
superior edge of the posterior
subtalar calcaneal facet parallel to the calcaneal plantar cortex (Figure 6). Furthermore, a
prominent downward sloping
posterior tibia was seen in 25%
of cases in the same study.6 A
downward slope is defined as
a posterior malleolus extending beyond 5 mm distal to a
line drawn tangentially along
the anterior tibial plafond on
lateral views and parallel to the
physeal scar.6
Scintigraphy
Bone scintigraphy is nonspecific and not commonly
performed for the diagnosis of
an os trigonum. Furthermore,
poor spatial resolution makes
localization of uptake in relatively small structures such
as the os trigonum difficult.
However, a negative study
confidently rules out the syndrome.5,7 On positive studies,
a 3-phase bone scan shows
increased blood flow and increased uptake of radiotracer
in the symptomatic os trigonum. Single-photon emission
computed tomography can
also be used to localize uptake and differentiate posterior
ankle impingement from other
nearby abnormalities of the
ankle, such as subtalar osteoarthritis.2,3,5,6
Ultrasound
Ultrasound can show nodular capsular thickening at the
posterior ankle or lateral to the
ORTHOPEDICS | Healio.com/Orthopedics
n radiologic case study
6A
os trigonum. Doppler interrogation is typically negative.
However, ultrasound is most
useful for image guidance during diagnostic or therapeutic
injection, allowing precise
anatomical administration of
anesthetics or steroids (Figure
7).1,5,11
Computed Tomography
Computed
tomography
imaging mirrors radiographic
findings but provides better
anatomic and soft tissue detail.
Computed tomography may
better demonstrate degenerative
synchondrosis, fragmentation,
or fractures of involved osseous structures, osseous pressure
erosions, and soft tissue infiltration (Figures 4, 5).1 Computed
tomography can play a role in
presurgical planning.
Magnetic Resonance Imaging
Magnetic resonance imaging (MRI) is the imaging mo-
7
6B
Figure 6: Lateral ankle radiograph of the left ankle a 35-year-old man with
chronic posterior ankle pain exacerbated by plantar flexion showing os trigonum (asterisk) and soft tissue attenuation in Kaeger’s fat pad (black arrow).
An osseous prominence (white arrow) of the posterior superior calcaneus extending superior to the upper parallel pitch line (top white line) represents a
prominent superior calcaneal tubercle (A). T2-weighted fat saturated magnetic
resonance imaging showing bone marrow edema of os trigonum (asterisk)
and opposing calcaneus (block arrow) with increased signal. Note joint effusions in the posterior recess of the ankle joint. Subtle intermediate soft tissue
signal linear filling defects within the joint effusion represent synovial hypertrophy is shown. Infiltrative high T2 signal in Kaeger’s fat pad represents additional inflammation (B).
dality of choice for the diagnosis and evaluation of an os
trigonum. Pain usually occurs
after development of soft tissue
inflammation and synovitis of
the posterior ankle and posterior subtalar joints (Figure 6),
and synovitis has been reported
in up to 100% of posterior impingement cases.1,6 Synovitis
and inflammation are characterized on MRI as increased
T2 signal in soft tissues with
capsular and synovial thickening. The normally high T1 signal in Kager’s fat pad and the
adjacent lipomatous tissue is
decreased when inflamed. In
the presence of a joint effusion,
intermediate T2 signal intraarticular soft tissue filling defects indicate synovial proliferation. Imaging the ankle
while in plantarflexion may
demonstrate the entrapment of
soft tissues.
Opposing posterior ankle
osseous structures may have
Figure 7: Longitudinal ultrasound image of the ankle obtained during guided
injection of os trigonum in a 28-year-old man with posterior ankle impingement. Notice the needle (arrows), os trigonum (asterisk), and calcaneus (C).
patchy areas of increased T2
and decreased T1 signal due
to bone marrow edema. This
is most common in the os trigonum, but the posterior tibia,
talar posterior process, and
calcaneus can also be involved
(Figure 6).1,5,6 Hyperintense
fluid at the synchondrosis
of the os trigonum on T2weighted images indicates
disruption.1,2,5 Areas of bone
sclerosis are hypointense on
all sequences. Chronic disruption of the synchondrosis is
characterized by sclerosis and
cystic change (Figure 8).
With regard to other pathologies seen with posterior
impingement, the flexor hallucis longus tendon and sheath
should be inspected on MRI.
The flexor hallucis longus
tendon sheath normally communicates with the subtalar
joint and the flexor digitorum
longus tendon sheath.1 Also,
the flexor hallucis longus tendon sheath communicates with
the ankle in 20% of normal patients.10 Therefore, fluid within
the flexor hallucis longus tendon sheath can be physiologic.
However, fluid in the tendon
sheath out of proportion to
ankle or subtalar joint effusion is suspicious for tenosynovitis.10,12 Septations within
the distended tendon sheath
also suggest tenosynovitis.
Occasionally, edema is seen at
the distal flexor hallucis longus myotendinous junction.
This may be secondary to impingement of this region with
dorsiflexion (Figure 5).6
Although the posterior
IML was identified in 56%
of dissected cadaveric ankles,
Rosenberg et al8 detected
the IML in only 19% of 97
ankle MRI examinations. It
is thought that an IML 1 to 8
mm wide may be too small
for the spatial resolution of
MRI. Therefore, this ligament
is not routinely inspected.
However, easy identification
of this ligament is potentially
pathologic. In the same study
by Rosenberg et al,8 the IML
was found to be thickened in
up to half of patients with an
os trigonum.8
Treatment
Os trigonum syndrome
is initially treated conservatively with rest, activity modification, nonsteroidal anti-
JANUARY 2013 | Volume 36 • Number 167
n radiologic case study
syndromes of the ankle: role of
imaging in diagnosis and management. Radiographics. 2002;
22(6):1457-1469.
6. Peace KAL, Hilier JC, Hulme
A, Healy JC. MRI Features of
posterior ankle impingement
syndrome in ballet dancers: a
review of 25 cases. Clin Radiol.
2004; 59(11):1025-1033.
8A
8B
8C
Figure 8: Axial T2-weighted magnetic resonance image of the ankle of a 13-year-old boy with acute posterolateral ankle
pain showing bone marrow edema at the posterior talus and within the os trigonum (black arrow). Note the posterior
talofibular ligament (white arrow) inserting at the lateral facet of the os trigonum (A). Sagittal proton-density magnetic
resonance image showing disruption (black arrow) of synchondrosis (B). Sagittal fat-saturated T2-weighted magnetic
resonance image showing the opposing bone marrow edema at synchondrosis (white arrow).
inflammatory drugs, and physical therapy.7 Immobilization
may be initiated in severe
cases. Ultrasound-guided injections play diagnostic and
therapeutic roles in patients
with posterior ankle impingement.1 Although variable responses to steroid and anesthetic injection have been reported, Robinson and Bollen11
reported a series of 10 patients
who had immediate symptom
relief after ultrasound-guided
injections.
When conservative treatment fails, posterolateral
arthroscopic resection with
or without capsulectomy or
debridement of the os trigonum4,7,13 or resection of a torn
IML1 may result in improved
symptoms. Arthroscopy is less
aggressive than open surgery
and offers the added advan-
68
tage of less patient morbidity
and less postoperative pain to
allow for early functional rehabilitation. This is an uncommonly performed surgery.
Conclusion
Os trigonum syndrome is
the result of impingement of
posterior ankle structures,
most notably the os trigonum.
The evaluation of posterior
ankle pain can be complex and
has a lengthy differential diagnosis. Radiographic findings
can suggest the diagnosis, but
the mainstay of imaging evaluation is MRI. Detection of
bone marrow edema in opposing posterior ankle osseous
and soft tissue structures differentiates an inconsequential
os trigonum from one causing
posterior ankle impingement.
In addition, superior soft tissue
evaluation by MRI allows
identification of associated
findings, including synovitis,
soft tissue inflammation, tenosynovitis of the flexor hallucis
longus, and IML pathology. References
1.Donovan A, Rosenberg ZS.
MRI of ankle and lateral hindfoot impingement syndromes.
AJR Am J Roentgenol. 2010;
195(3):595-604.
2. Karasick D, Schweitzer ME.
The os trigonum syndrome:
imaging features. AJR Am J
Roentgenol. 1996; 166(1):125129.
3. Bureau NJ, Cardinal E, Hobden
R, Aubin B. Posterior ankle impingement syndrome: MR imaging findings in seven patients.
Radiol. 2000; 215(2):497-503.
4. Abramowitz Y, Wollstein R,
Barzilay Y, et al. Outcome of resection of a symptomatic os trigonum. J Bone Joint Surg Am.
2003;85(6):1051-1057.
5. Robinson P, White LM. Softtissue and osseous impingement
7. Wukich DK, Tuason DA.
Diagnosis and treatment of
chronic ankle pain. J Bone Joint
Surg Am. 2010; 92(10):20022016.
8. Rosenberg ZS, Cheung YY,
Beltran J, Sheskier S, Leong M,
Jahss M. Posterior intermalleolar ligament of the ankle: normal anatomy and MR imaging
features. AJR Am J Roentgenol.
1995; 165(2):387-390.
9. Fiorella D, Helms CA, Nunley
JA II. The MR imaging features
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of the ankle. Skeletal Radiol.
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10. Lo LD, Schweitzer MR, Fan
JK, Wapner KL, Hecht PJ. MR
imaging findings of entrapment
of the flexor hallucis longus
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11. Robinson P, Bollen SR. Posterior
ankle impingement in professional soccer players: effectiveness of sonographically guided
therapy.” AJR Am J Roentgenol.
2006; 187(1):W53-W58.
12. Rosenberg ZS, Beltran J,
Bencardino JT. From the
(Wukich DK, 2010) RSNA refresher courses. Radiological
Society of North America.
MR imaging of the ankle and
foot. Radiographics. 2000;
20:S153-S179.
13. Nickisch F, Barg A, Saltzman
CL, et al. Postoperative complications of posterior ankle
and hindfoot arthroscopy. J
Bone Joint Surg Am. 2012;
94(5):439-446.
ORTHOPEDICS | Healio.com/Orthopedics