Posterior-ankle impingement syndrome due to os trigonum syndrome
As a ligament injury was suspected to be the cause of the patient’s
symptoms, a magnetic resonance (MR) imaging scan of the ankle was
performed with a high-resolution surface coil on a 1.5-tesla (T)
scanner. The scan revealed a well-corticated, triangular bone posterior
to the talus. This represented an os trigonum (accessory bone). The
image also revealed patchy, altered marrow signal, which appeared
hypointense on T1-weighted images and hyperintense on fat-suppressed
T2-weighted images, suggesting bone marrow edema (Figure 1). The
talus/os trigonum synchondrosis appeared intact, although subchondral
erosions were present along the articular margins (Figure 1).
Ill-defined hyperintense signal was seen in the soft tissue around the
os trigonum on fat-suppressed, T2-weighted images (Figure 1). The
anterior talofibular ligament appeared swollen and hyperintense,
suggesting a contusion, but otherwise intact (not shown). Ill-defined
hyperintense signal was also seen in the subcutaneous tissue along the
lateral aspect of the ankle on fat-suppressed, T2-weighted images,
suggesting edema (Figure 1). The other bones and ligaments surrounding
the ankle joint appeared normal.
Posterior-ankle impingement (PAI) syndrome describes a group of
pathological entities that result from repetitive plantar flexion of the
foot that causes repeated compression and entrapment of soft tissues,
bony processes or unfused ossicles between the posterior-tibial plafond
and the superior surface of the calcaneum. This repeated compression and
entrapment, like nuts in a nutcracker, results in bone contusions and
local synovitis involving the posterior recess of the tibiotalar and
Variations in normal osseous and
soft-tissue anatomy that predispose one to PAI syndrome include a
prominent down-slope of the posterior tibia, the presence of an os
trigonum, a prominent posterior-talar process (Stieda process),3 prominent tuberosity arising from the superior calcaneum,3 and the presence of the posterior-intermalleolar ligament (PIML).4
When the os trigonum is the cause, the condition is known as os
trigonum syndrome. A separate ossification center forms at the
posterolateral aspect of the talus, within the cartilaginous extension
from the posterior talus, between the ages of 11 and 13 years in boys,
and 8 and 10 years in girls. Normally,this ossification center fuses
with the rest of talus within one year. But if it fails to fuse, an os
trigonum is formed (in 7% to 14% of the cases) that articulates with the
talus via a synchondrosis.3,5,6 Although common in ballet
dancers, os trigonum syndrome is also encountered in those who
participate in other sporting activities that involve forced plantar
flexion of the foot, such as soccer, basketball, and volleyball, as well
as in those who participate in non-sport-related activities.1,3
typically present with recurrent posterolateral ankle pain, especially
on plantar flexion, and sometimes on weight bearing,along with swelling
and stiffness of the posterior ankle. Symptoms are relieved with rest.
During clinical examination, reproduction of pain on forceful plantar
flexion and sometimes on resistive plantar flexion or dorsiflexion of
the big toe are considered hallmarks of PAI syndrome.7
Symptoms resolve with injection of local anesthetic into the
posterolateral aspect of the ankle. In chronic cases, the range of
motion of the hallux may be reduced as a result of fibrosis of the
flexor hallucis longus tendon, which sits between the medial and lateral
The posterior-ankle impingement due to os
trigonum can develop after disruption of the os trigonum through a
significant acute injury (for example, fracture, fragmentation, and/or
pseudoarthrosis). However, it usually develops insidiously as a result
of repeated forced plantar flexion of the foot and chronic injury to
posterior osseous and soft tissues.5,6
reveal an os trigonum or Stieda process. Lateral radiographs obtained
with the foot in plantar flexion may show the os trigonum or lateral
talar tubercle impinged between the posterior-tibial malleolus and the
calcaneal tuberosity.3 MR imaging is the modality of choice
for further evaluation of the bony and soft tissue structures. MR
imaging demonstrates bone marrow edema within the os trigonum and at
synchondrosis with the posterior talar tubercle, a reliable sign of PAI
syndrome.3,8 Other common sites of edema include the
posterior talus (40%) and the posterior calcaneum (24%); diffuse
patchy-marrow edema can appear throughout the hind foot.8 MR imaging can reveal fracture through the os trigonum or fluid in synchondrosis, indicating os trigonum fracture.
inflammatory changes in the adjacent soft tissues can also be seen on
MR imaging. These include edema or enhancement of posterior soft tissue,
indicating posterior tibiotalar joint synovitis (100%) due to
repetitive compression and posterior- capsular thickening; fluid around
the flexor hallucis longus (FHL) tendon or synovial enhancement,
suggesting tenosynovitis of the FHL (68%); and high signal changes
and/or enhancement within the musculotendinous junction of FHL muscle
belly due to impingement.8
An os trigonum should be
differentiated from a fractured lateral-talar tubercle on a radiograph.
An os trigonum is usually round or oval, with well-defined corticated
margins, while a fractured lateral tubercle has irregular serrated
margins between the ossicle and the posterior talus. However, a
fractured fragment may also have smooth borders.3
treatment includes anti-inflammatory agents, activity modification,
weight-bearing immobilization, and physiotherapy. If conservative
measures fail, open or arthroscopic surgical excision of the abnormal
accessory bone is recommended.3
Tendinitis of the flexor hallucis longus and posterior impingement of
the ankle are familiar to the orthopedic surgeon who treats professional
dancers. However, a lack of familiarity with these conditions, a low
index of suspicion with regard to patients who are not dancers, and the
usual resolution of symptoms after modification of activity or rest
probably contribute to the low reported prevalence in non-dancers. MR
imaging has a marked effect on clinical care by enabling the
determination of the exact nature of the osseous- and soft-tissue
lesions and by excluding other causes of posterior ankle pain. In
conclusion, soft-tissue abnormalities and bone contusions of the lateral
talar tubercle and ostrigonum are findings of PAI syndrome, which can
be clearly depicted on MR imaging.
- Wredmark T, Carlstedt CA, Bauer H, Saartok T. Os trigonum syndrome: A clinical entity in ballet dancers. Foot Ankle. 1991;11:404-406.
- Masciocchi C, Catalucci A, Barile A. Ankle impingement syndromes. Eur J Radiol. 1998;27: S70-S73.
- Bureau NJ, Cardinal E, Hobden R, Aubin B. Posterior ankle impingement syndrome: MR imaging findings in seven patients. Radiology. 2000;215:497-503.
ZS, Cheung YY, Beltran J, et al. Posterior intermalleolar ligament of
the ankle: Normal anatomy and MR imaging features. AJR Am J Roentgenol. 1995;165:387-390.
- Robinson P, White LM. Soft-tissue and osseous impingement syndrome of the ankle: Role of imaging in diagnosis and management. Radiographics. 2002;22:1457-1469.
- Karasick D, Schweitzer ME. The os trigonum syndrome: Imaging features. AJR Am J Roentgenol.1996;166:125-129.
Hamilton WG, Geppert MJ, Thompson FM. Pain in the posterior aspect of
the ankle in dancers. Differential diagnosis and operative treatment. J Bone Joint Surg Am. 1996;78:1491-1500.
Peace KA, Hillier JC, Hulme A, Healy JC. MRI features of posterior
ankle impingement syndrome in ballet dancers: A review of 25 cases. Clin Radiology. 2004;59:1025-1033.