Os Trigonum Syndrome


  • The ostrigonum is an accessory bone that sometimes develops behind the talus. It is connected to the talus by a fibrous band.
  • The presence of an ostrigonum in one or both feet is congenital (present at birth).
  • It becomes evident during adolescence when one area of the talus does not fuse with the rest of the bone.
  • Ostrigonum syndrome is characterized by pain behind the ankle joint.
  • There are three mechanisms for the development of an ostrigonum: 1. fusion failure of an ossification center; 2.fracture of the posterior margin of the tibia; 3.fracture of the posterior process of the talus.
  • The ostrigonum syndrome can be caused by overuse or trauma. Soft tissue structures, including the ankle joint capsule and surrounding ligaments, may react by forming a hypertrophic mass after a trauma.

Diagnostic tips

  1. Pain behind the ankle is the primary symptom (aggravated by pointing the toes or forced plantar flexion).
  2. Stiffness, weakness and swelling can be observed between the Achilles tendon and the peroneal tendons.
  3. Decrease in plantar flexion compared with the unaffected ankle.
  4. Bony prominence may be palpable behind the ankle.
  5. Eversion or inversion movements may cause discomfort

Tests and Imaging

  1. Passive forced plantar flexion test
  2. Plain x-rays of the foot and ankle (lateral) may demonstrate ostrigonum.
  3. Bone scan may be used as a non-specific means of localizing the inflamed and irritated area.
  4. MRI can give more detailed resolution to the bony and soft tissue structures in the region.

Immediate Treatment

  1. Nonsteroidal anti-inflammatory medication or corticosteroid injections are used to reduce soft tissue inflammation.
  2. In case of fracture, a below-knee cast or cam walker is used for 4-6weeks.

Possible Referral

  1. Podiatry for cam walker and rehabilitation with isometric and eccentric exercises to strengthen and stretch the lower-leg muscles.
  2. Foot and ankle surgeon for possible removal of the ostrigonum if symptoms still persist after nonoperative treatment.