Trigonous bone syndrome results from excessive injury to the posterior ankle by repetitive plantar flexion. The bulky trigone bone may conflict superiorly with the 3rd malleolus and inferiorly with the superior aspect of the calcaneus by chronic compression. It is often associated with pathology of the flexor hallucis longus tendon . Based on a case of a patient with a bulky trigone bone previously unreported in the literature, which was complicated by tenosynovitis, we will review the pathophysiology, clinico-radiological features, and emphasize the role of imaging in the diagnosis and treatment of this rare entity.
Patient and Observation
This is a 68-year-old female patient with a history of chronic ankle arthritis. She consulted the rheumatology department for exacerbations of inflammatory pain of the right ankle. Magnetic resonance imaging of the right ankle was requested to distinguish between osteoarthritis and synovitis. We performed the anatomical and morphological T1 sequences as well as the DP FATSAT weighted sequence. We found, in T1 weighting, a triangular-shaped ossicle of the posterior space of the talus in heterogeneous T1 intermediate signal with irregularity of its inferoexternal cortex, surrounded by a peripheral hypodensity. It measured 15mm in long axis in relation to a larger trigone bone (See Figure 1). There was a fluid hypersignal on the pre-achillary side with retro talar non-fluid hypersignal in DP FAT SAT in relation to an effusion with soft tissue edema. Then, there was a hypersignal thickening of the tendon sheath of the flexor hallucis proprius in DP FATSAT hypersignal in relation to a tenosynovitis of the flexor halluces (See Figures 2 and 3).
In view of the age, history, clinic, and this imaging, we retained the diagnosis of a larger-than-usual trigone bone syndrome complicated by tenosynovitis of the tendon of the long flexor of the hallux. Treatment initially consisted of rest with corticosteroids to treat the pain and inflammation. Surgical resection was the second option given the volume of this trigone bone,
The trigone bone, a supernumerary bone, is a small ossicle present in 8 to 13% of the population and is bilateral in less than 2% of cases . It is located in the retro talar space in the posterior cartilage of the ankle. It has a triangular shape as seen in Figure 1. It can be reshaped, fragmented, displaced, fractured, bi- or multipartite.
The trigone bone exists from birth as a cartilaginous extension of the posterior part of the talus and appears as a nucleus of ossification between 8 and 11 years of age; its delayed fusion results in a long tail of the talus and the non-fusion with the talus results in the genesis of the trigone bone and a cartilaginous articulation with the posterior part of the talus or synchondrosis . It is small in nature and does not exceed 10 mm in length . In extremely rare cases, as in our case, measured at 15mm long axis, there is a hypertrophy of the trigone bone.
The bony elements and soft tissues of the anatomical region anterior to the Achilles tendon and posterior to the talus are pincered between the lower part of the posterior aspect of the tibia and the upper part of the calcaneus during hyperextension or sudden forced flexion: this is the famous "hammer and anvil" crush. The bulky trigone bone exerts compression on the capsulo-synovial and osteotendinous structures of the ankle at any moment of ankle hypersollicitation, thus increasing the posterior impingement typically referred to as trigone bone syndrome as a result of these chronic repetitive microtraumas . This syndrome is often associated with a pathology of the long flexor tendon of the hallux or the toes, tenosynovitis. It undergoes mechanical constraints in its passage in an osteo-fibrous tunnel between the lateral and medial tubercles of the talus. Other causes of tenosynovitis are: traumatic, inflammatory, infectious, microcrystalline and tumor. Necrosis has also been reported .
The conflict is generated by a repeated plantar flexion observed mainly in soccer players or dancers. The possible attacks are osteoarticular, ligamentary, synovial, tendinous or vasculonervous. The clinic is dominated by pain in the posterior border of the ankle and confirmed by the posterior impaction test, which consists of a brutal percussion of the upper pole of the talus on the posterior margin of the tibia, thus awakening the pain .
MRI, the examination of choice, provides a global view of the lesions, highlights bone damage, analyzes soft tissues, and diagnoses tenosynovitis of the flexor hallucis longus or the often associated pre-achilles bursitis . The normal synovium is not visible. The triad of bone edema, posterior capsulosynovial thickening and sheath effusion of the flexor hallucis proprius are very characteristic [2 ,5]. The thickening of the synovial sheath of the tendon is hyposignal T1, hypersignal T2 intensely enhanced after injection of gadolinium chelates .
Standard radiography and CT reveal the existence of the trigone bone, specifying the number, size, laterality, and focal point. On ultrasound, a thick, hypoechoic tendon with a more or less thick effusion in its synovial sheath is noted. The Doppler shows peripheral vascularization of the synovial sheath .
The clinician and radiologist should be aware of the differential diagnoses of trigone bone syndrome. Many of them are known under the acronym of posterior ankle impingement namely long tail talus syndrome, posterior talar process fracture, short tail talus syndrome, osteochondritis dissecans of the talus , Pauzat calcaneus disease, Achilles tendon bursitis, synovial hypertrophy, posterior capsular hematoma, and tenosynovitis of the hallux longus, which can be a major complication as in our case . In case of chronic or infectious tenosynovitis, reactionary condensations or erosions may be observed and simulate a progressive bone lesion .
Treatment is based on rest, immobilization with a posterior splint, non-steroidal anti-inflammatory drugs or corticosteroid infiltration. In intractable situations, surgical or arthroscopic resection of the accessory ossicle is indicated, followed by physiotherapy for 4 to 8 weeks [1,2,8]. The literature reports satisfactory results.
The syndrome of the bulky trigone bone is little spoken of and often diagnosed late. At this stage, it is frequently associated with tenosynovitis of the flexor hallucis longus, which is its ultimate complication. To complete the diagnosis, MRI is a key examination in the presence of chronic pain of the posterior border of the ankle.
Conflicts of interest.
We declare that we have no conflicts of interest.
All authors contributed to this work.