Applied Radiology

Diagnosis

Posterior-ankle impingement syndrome due to os trigonum syndrome

Findings

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.

Discussion

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 subtalar joints.³

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

Patients 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 talar tubercles.

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

Radiographs can 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.

Additional 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

Conservative 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

Conclusion

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.

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