What is the os trigonum?

What is Os Trigonum?

Os trigonum is an accessory bone located at the posterolateral aspect of the talus in the foot, present in approximately 9% of individuals, that forms when the lateral tubercle of the talus fails to fuse during secondary ossification. 1

Anatomical Characteristics

• Location: The os trigonum sits posterolateral to the lateral tubercle of the talus, positioned between the posterior tibial malleolus and the calcaneus 2, 3

• Prevalence: Present in 9.0% of feet overall, with bilateral occurrence in approximately 32.7% of cases 1

• Geographic variation: Highest prevalence occurs in East Asian populations (11.0%) 1

• Detection methods: Prevalence varies significantly by imaging modality:

  • X-ray: 8.2%
  • MRI: 24.2%
  • CT: 21.0%
  • Cadaveric dissection: 5.0% 1

Classification Types

Three distinct types have been identified based on CT three-dimensional imaging 4:

• Type I (1.9% incidence): Smallest volume with macro-axis of 0.65±0.24 cm and minor axis of 0.41±0.23 cm 4

• Type II (10.5% incidence): Largest volume with macro-axis of 0.89±0.31 cm and minor axis of 0.58±0.32 cm 4

• Type III (14.7% incidence): Most common type with macro-axis of 0.74±0.23 cm and minor axis of 0.55±0.16 cm 4

Clinical Significance

Os trigonum is typically asymptomatic but becomes clinically relevant when it causes posterior ankle impingement syndrome (PAIS), particularly in athletes performing repetitive plantar flexion movements. 5

Os Trigonum Syndrome

• Mechanism of injury: Repetitive plantar flexion causes compression of the os trigonum between the posterior tibial malleolus and calcaneus, leading to disruption of the synchondrosis with the lateral tubercle of talus 2, 3

• High-risk activities: Predominantly affects ballet dancers (especially during pointe or demi-pointe positions) and soccer players performing push-off maneuvers 5

• Clinical presentation: Pain localized anterior to the Achilles tendon, exacerbated by plantar flexion activities 5

• Association with PAIS: Patients with PAIS are approximately 16 times more likely to have os trigonum compared to those without PAIS 1

Diagnostic Approach

Imaging Recommendations

MRI is the preferred imaging modality according to the American College of Radiology for evaluating os trigonum and related pathology, using thin-section protocols. 6

• Radiography: Lateral ankle view in maximal plantar flexion typically reveals the os trigonum between the posterior tibial lip and calcaneus; however, it only demonstrates the bone itself and morphostructural changes, not soft tissue involvement 6, 5

• CT imaging: The choice modality when fractures of posterior talus structures are suspected, offering multiplanar capability for detailed characterization of the ossicle and synchondrosis 6, 3

• MRI advantages: Depicts associated soft tissue damage, bone marrow edema patterns, cartilage changes, and synchondrosis disruption that indicate abnormal mobility or pseudoarthrosis 6, 2

MRI Findings in Symptomatic Cases

• Disruption of cartilaginous synchondrosis between the accessory bone and posterior tibial aspect (seen in 7/9 symptomatic patients) 2

• Fluid effusion surrounding the os trigonum and adjacent soft tissues 2

• Pseudoarthrosis with irregular bone margins and high-signal spots within cartilage 2

• Associated flexor hallucis longus tenosynovitis (present in 6/9 symptomatic patients) 2, 5

• Bone marrow edema or necrosis in severe cases 2

Imaging Protocol Specifics

• Forced plantar flexion acquisitions: Particularly useful to demonstrate the mechanism of injury and compression between posterior tibia and calcaneus 2

• Sagittal T2 sequences: Best demonstrate cartilage changes indicating synchondrosis disruption 2

• Contrast administration: Not routinely performed for symptomatic accessory ossicles, though MR arthrography may demonstrate contrast within a disrupted synchondrosis 6

Important Clinical Pitfalls

• Radiographic confusion: Os trigonum may be mistaken for fractures of the posterior process of the talus on plain radiographs 3

• Actual fracture of os trigonum: Extremely rare but can occur with severe hyperplantarflexion injury; requires high index of suspicion and CT confirmation 3

• Missed soft tissue pathology: Plain radiographs and CT have limited assessment of associated soft-tissue pathology or bone marrow edema compared to MRI 6

• Asymptomatic presence: The mere presence of os trigonum on imaging does not confirm it as the pain source; clinical correlation is essential 5