Os Trigonum

The term os trigonum refers to a failure of the lateral tubercle of the posterior process to unite with the body of the talus during ossification, producing an impingement with extreme plantar flexion.

An os-trigonum has been reported to be present in approximately 10% of the general population and is often unilateral.

See Also: Ankle Anatomy

Os-Trigonum Anatomy

The origin of this ossicle may be congenital or acquired:

• Congenitally, it can be a persistent separation of the secondary center of the lateral tubercle from the remainder of the posterior talus (Stieda’s process) secondary to repeated microtrauma during development.
• The acquired form may be secondary to an actual fracture that has not united.

The posterior aspect of the talus often exhibits a separate ossification center, appearing at 8–10 years in girls and 11–13 years in boys. Fusion usually occurs 1 year after its appearance. When fusion does not occur, an os-trigonum is formed.

Stieda’s process. A posterior projection off the talus

Os Trigonum Syndrome

A traumatic os-trigonum is formed by a nonunion fracture or stress fracture of Stieda’s process.

Athletes, particularly those whose activity requires sustained plantarflexion, such as dancers, are at an increased risk of sustaining an injury to this structure.

Os trigonum syndrome (also referred to as talar compression syndrome) may involve:

1. an inflammation of the posterior joint,
2. inflammation of the ligaments surrounding the os trigonum,
3. a fracture of the os trigonum,
4. pathology involving the Stieda’s process.

Subtalar and midtarsal joint pronation causes the os trigonum or Stieda’s process to become compressed between the tibia and calcaneus.

The arrow indicates the location of the os-trigonum, a fracture of Stieda’s process. A fracture line can be seen at the midpoint on the process

Symptoms

Inflammatory conditions become symptomatic after activity or with repetitive microtrauma.

Symptoms associated with os-trigonum include:

1. pain with maximum plantar flexion as the posterior talus impinges on the posterior tibia, os trigonum fractures are characterized by the sudden onset of pain after forced plantarflexion or dorsiflexion.
2. Swelling lateral or medial to the Achilles tendon may be noted.
3. During palpation, patients report tenderness anterior to the Achilles tendon and posterior to the talus.
4. Ankle plantarflexion may produce pain during ROM testing.

Imaging

Plain film radiographs are used to identify the presence of an os trigonum.

Plain radiographs should include a lateral view of the ankle and a lateral view in plantarflexion.

A bone scan may be used to determine the reactivity of the os trigonum, but absence of uptake does not exclude impingement.

MRI imaging shows os-trigonum and associated inflammation and edema in FHL tendon.

MRI

Differential Diagnosis

Differential diagnosis includes:

1. posterior ankle impingement,
2. Achilles tendonitis,
3. fibularis tendonitis,
4. flexor hallucis longus tendonitis.

Os Trigonum Treatment

The Os Trigonum Treatment involves symptom reduction with restoration of normal strength, ROM, and biomechanics as needed.

The Os Trigonum Treatment may include

1. Rest,
2. Anti inflammatory medications,
3. avoidance of plantarflexion,
4. injection.

If symptoms persist, Os Trigonum surgery removal of the bone may be required but is delayed until skeletal maturity is achieved.

Os Trigonum surgery is done through open lateral approach or posterior ankle arthroscopy

Os Trigonum surgery excision

References

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9. Abramowitz, Y, et al: Outcome of resection of a symptomatic ostrigonum. J Bone Joint Surg, 85A:1052, 2003.
10. Mouhsine, E, et al: Post-traumatic overload or acute syndrome of the ostrigonum: a possible cause of posterior ankle impingement. Knee Surg Sports Traumatol Arthrosc, 12:250, 2004.
11. Yilmaz, C, and Eskandari, MM: Arthroscopic excision of the talar Stieda’s process. Arthroscopy, 22:225, 2009.
12. Paulos, LE, et al: Posterior compartment fracture of the ankles: a commonly missed athletic injury. Am J Sports Med, 11:439, 1983.
13. Karasick, D, and Schweitzer, ME: The os trigonum syndrome: imaging features. Am J Roentgenol, 166:125, 1996.
14. Masciocchi, C, Catalucci, A, and Barile, A: Ankle impingement syndromes. Eur J Radiol, 27(S1):S70, 1998

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