The Os Trigonum Syndrome
The Os Trigonum Syndrome refers to pain posterior of the ankle and reduced plantarflexion caused by “the nutcracker-phenomenon”. When an os trigonum is present, this accessory ossicle together with surrounding soft tissues can become wedged between the tibia, talus and calcaneus. This can lead to inflammation of the involved structures. 
The os trigonum syndrome can also be named the symptomatic os trigonum, the talar compression syndrome or posterior tibial talar impingement syndrome.
Clinically Relevant Anatomy
Embryologically, the body of the talus and the posterior talar process are separate ossification centers. Between the 7th and the 13th year of life, the posterior talar process appears as a separate ossicle: the os trigonum. Normally, within a year of its appearance, it fuses with the talus, but about 7% of the adult population has still this os trigonum. It can be present unilaterally or bilaterally, with smooth or serrated margins. The os trigonum is usually seen as an individual bone, but can also exist of two or more pieces. It is less than 1cm in size, but this can vary.  
The os trigonum is usually triangular with an anterior, inferior and posterior surface. The anterior surface connects with the lateral tubercle by cartilaginous synchondrosis. The inferior side may articulate with the calcaneus. The posterior surface is nonarticular, but is used as a point of attachment for capsuloligamentous structures. The os trigonum may also be round or oval. 
The flexor hallucis longus tendon is situated medial to the os trigonum, in the sulcus between the medial and lateral tubercle.
There are three mechanisms for the development of an os trigonum:
- fusion failure of an ossification center
- fracture of the posterior margin of the tibia
- fracture of the posterior process of the talus.
The presence of an os trigonum isn’t sufficient to create the syndrome. It must be combined with a traumatic event. 
The os trigonum syndrome can be caused by overuse or trauma. When it’s due to overuse, it’s mostly found by ballet dancers and runners. The forceful plantar flexion that happens during an “en pointe” or “demi-pointe” position, as well as by running downhill, produces compression on the posterior aspect of the ankle joint. In cause of a trauma, the os trigonum can be displaced by forced plantarflexion. 
A load-dependent, persistent pain between the Achilles tendon and the peroneal tendons is the first indicator of the syndrome. Stiffness, weakness and swelling can also be observed in this zone. The second main symptom is a decrease in plantarflexion compared with the unaffected ankle. In some cases the bony prominence may be palpable.
The following diagnosis must be considered:
- Tendinitis flexor hallucis longus;*Tarsal tunnel syndrome;*Subtalar pathology;*Achilles tendinopathy;*Peroneal tendinopathy;*Achilles tendon bursitis;*Osteochondritis dissecans of the talus. 
- Lateral X-ray, possibly weight-bearing, with the foot in full plantarflexion.
- MRI is the preferable technique for establishing the presence and the size of the ossicle, coexisting pathologies and soft tissue and bone damage. Flexion/extension MRI gives information about the mobility of the os trigonum.
- Technetium bone scan shows increased uptake in the region of the os trigonum.
The passive forced plantarflexion test: It should be executed with repetitive quick and passive hyperplantarflexion movements in a neutral position, possibly with exo- or endorotation movement on the point of maximal plantarflexion. Thereby grinds the ossicle between tibia and calcaneus.
If symptoms persist, surgery is applied. (high evidence, all studies mention good results.)
This involves the removal of the os trigonum. Postoperatively we apply a plaster cast for 5days. Hereafter physiotherapy is started for 4-8 weeks. Afterwards full sports activities can be resumed. It will take up to 6 months until full recovery.
Physical Therapy Management
Isometric and eccentric exercises to strengthen and stretch the lower-leg muscles are used in a physiotherapeutic treatment.
Also exercises to improve deep muscle action during plantarflexion are designated. The deep muscles of the lower leg, such as tibialis posterior, flexor digitorum longus, flexor hallucis longus and the peroneals are the opposites of the M.gastrocnemius. By using the deep muscles, the talus is shifted forward during plantarflexion, what will reduce the impact of the os trigonum on the posterior tibia, contrary to using the M.gastrocnemius, which results in lifting the calcaneus and compression of the os trigonum.
Fig. 5: Strengthening of the deep muscles. Performing demipointe (2) and en pointe (3) while holding the calcaneus in place with the hands. Knee flexion at 90° to prevent contraction of the M.gastrocnemius.
Also proprioceptive exercises on a tilt board are applied to correct malalignments of the lower limb.
All of these exercises were found in only one study of 11 dancers with a posterior ankle impingement including 6 cases with an os trigonum. Nine of them had good results with these exercises, the other two ones underwent surgical excision.
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