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Imaging Quiz Answer

Posterior Ankle Pain in a Recreational Athlete

Carlos E. Jiménez, MD; Ernesto Torres, MD



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The bone scan demonstrates a small area of intense radiotracer uptake on the posterior aspect of the right ankle (figure 2), suggesting the possibility of a pathologic os trigonum. The scan of the asymptomatic left ankle shows focal uptake in the anterior aspect of the tibiotalar joint, suggesting a bony spur. Retrospective radiographic examination confirmed the presence of an os trigonum ossicle in the right ankle and bony spurs in the left one. The patient was diagnosed as having posterior impingement syndrome of the right ankle, also known as os trigonum syndrome or talar compression syndrome.

Os Trigonum Pathophysiology

Anatomy. During the foot's skeletal development, the normal ossification of the talus progresses posteriorly. Sometimes a separate ossification center on the posterior aspect of the talus develops in children between 8 and 13 years. This center usually fuses with the remainder of the talus within 1 year of its appearance. However, an accessory bone, the os trigonum, forms if this secondary ossification center fails to fuse after skeletal maturation (figure 3). An articulation may develop between the ossicle and talus, or the os trigonum may be connected to the talus by a synchondrosis or fibrous tissue (1).

The foot has up to 21 accessory bones. The os trigonum is one that can sometimes cause foot pain. The prevalence of os trigonum in various groups has been reported to range from 1.7% to 20%, and the overall prevalence has been estimated at 7% to 8% (2). The bone is usually triangular but may also be round or oval, is typically less than 1 cm in diameter, and can occur in one or both feet. Other sesamoids associated with clinical manifestations in the foot include the hallucal interphalangeal sesamoids (3) and the os tibiale externum or accessory navicular bone (4).

Mechanism of injury. Physical activities that result in forceful and/or repetitive plantar flexion, such as ballet and downhill running, can cause impingement of the os trigonum between the calcaneus and the posterior rim of the distal tibia. This can also lead to soft-tissue inflammation of the medially adjacent flexor hallucis longus tendon and to avulsion injuries of the posterior talofibular and talocalcaneal ligaments (figure 3). The os trigonum can also become symptomatic from direct trauma or from disruption of its attachment to the talus (5).

The Clinical Picture

Signs and symptoms. Patients who have os trigonum syndrome usually complain of pain on plantar flexion and sometimes on weight-bearing, as well as swelling and stiffness of the posterior ankle. These symptoms may develop gradually or may begin after an acute injury, such as an ankle sprain or a direct blow.

The physical exam reveals tenderness to direct palpation anterior to the Achilles tendon and posterior to the talus. Pain can often be elicited by forced plantar flexion and sometimes by resisted plantar flexion or dorsiflexion of the great toe (6). In chronic cases, the hallux may have reduced motion as a result of fibrosis of the flexor hallucis longus tendon, which sits between the medial and lateral talar tubercles.

Imaging studies. A lateral ankle radiograph can frequently indicate the presence of an os trigonum ossicle but may not differentiate a clinically relevant pathologic process from normal variants or old, inactive lesions (1,2). A technetium bone scan is very useful in diagnosing symptomatic os trigonum by demonstrating increased radiotracer uptake in the region of the os trigonum. A normal bone scan virtually eliminates this diagnosis (6). Magnetic resonance imaging can also demonstrate a pathologic os trigonum by showing bone marrow edema within the os trigonum accompanied by adjacent soft tissue inflammatory changes (1,2).

Differential diagnosis. The differential diagnosis includes Achilles tendinitis, calcaneal or talar fracture, retrocalcaneal bursitis, posttraumatic arthritis, ankle ligament sprain, osteochondritis dissecans, tarsal coalition, and posterior tibialis, peroneal, and flexor hallucis longus tendinitis. Osseous spurring from repetitive and forceful dorsiflexion can also produce a painful joint impingement at the ankle but most commonly at the anterior rim of the distal tibia, the tibiotalar joint, and the talonavicular joint. This is clinically known as the anterior impingement syndrome (7).


The initial treatment of the os trigonum syndrome is conservative and includes anti-inflammatory agents, activity modification, weight-bearing immobilization, and physical therapy. If conservative measures fail, open or arthroscopic surgical excision of the abnormal accessory bone is recommended because of good results (6,8).

Our patient was treated with anti-inflammatory agents and a weight-bearing short leg cast on his right ankle for 4 weeks. He then began a progressive exercise program that included isokinetic resistance training of the ankle musculature and progressive stationary cycling and running. After 6 weeks of rehabilitation, he was asymptomatic and playing competitive soccer.

The opinions or assertions contained herein are the private views of the authors and are not to be construed as official or as reflecting the views of the US Dept of the Army or the Dept of Defense.


  1. Karasick D, Schweitzer ME: The os trigonum syndrome: imaging features. Am J Roentgenol 1996;16(1):125-129
  2. Wakeley CJ, Johnson DP, Watt I: The value of MR imaging in the diagnosis of the os trigonum syndrome. Skeletal Radiol 1996;25(2):133-136
  3. Roukis TS, Hurless JS: The hallucal interphalangeal sesamoid. J Foot Ankle Surg 1996;35(4):303-308
  4. Maffulli N, Lepore L, Francobandiera C: Traumatic lesions of some accessory bones of the foot in sports activity. J Am Podiatr Med Assoc 1990;80(2):86-90
  5. Brown GP, Feehery RV Jr, Grant SM: Case study: the painful os trigonum syndrome. J Orthop Sports Phys Ther 1995;22(1):22-25
  6. Paulos LE, Johnson CL, Loyes FR: Posterior compartment fractures of the ankle: a commonly missed athletic injury. Am J Sports Med 1983;11(6):439-443
  7. Martire JR, Levinsohn EM: Imaging of Athletic Injuries: A Multimodality Approach. New York City, McGraw-Hill Health Professions Division, 1992, pp 109-111
  8. Marumoto JM, Ferkel RD: Arthroscopic excision of the os trigonum: a new technique with preliminary clinical results. Foot Ankle Int 1997;18(12):777-784

Dr Jiménez is a staff physician in the nuclear medicine service in the department of radiology and Dr Torres is a radiology resident, both at Walter Reed Army Medical Center in Washington, DC. Dr Jiménez is also an assistant professor of radiology/nuclear medicine at the Uniformed Services University of Health Sciences in Bethesda, Maryland, and a member of the Society of Nuclear Medicine and the American College of Physicians. Dr Torres is a member of the American College of Radiology and the American Roentgen Ray society. Address correspondence to Carlos E. Jiménez, MD, 12007 Bernard Dr, Silver Spring, MD 20902.



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