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Os Trigonum / Posterior Talar Impingement


- See:
      - Diff Dx: Posterior Ankle Pain 
      - Fractures of Posterior Talar Process

- Discussion:
    - accessory bone found just posterior to talus (accessory ossification center of the posterior process of the talus);
    - it may be considered a non united portion of the lateral tubercle;
    - os trigonum is round, oval, or triangular and of variable size;
    - present in 2.5% to 14% of normal feet;
    - arises from separate ossification center just posterior to lateral tubercle of posterior talar process;
    - it may fuse w/ lateral tubercle or remain as separate ossicle; 
    - may be injured with forced dorsiflexion injuries - causing disruption of the synchondrosis bewteen the Os and the Talus;
    - signs and symptoms
            - symptoms are worse when she assumes the en pointe position
            - tenderness to palpation at posterolateral aspect of ankle posterior to the peroneal tendons which is made worse with passive plantar flexion;
    - operative management through a medial approach;     

- Radiographs:
    - edges appear smooth, with dense cortical bone;
    - need to distinguish between ossicle & frx of lateral tubercle
    - MRI:
            - look for fluid surrounding the Os and associated marrow edema (absence of talar marrow edema)
            - if there is marrow edema in the talus then consider posterior talar process fracture;  
    - in the report by Sopov et al, the authors sought to determine the clinical significance of an increased uptake of 99mTc methylene-diphosphonate (Tc-MDP) in the os trigonum region seen on bone scintigrams of soldiers on active duty;
            - radionuclide whole-body skeletal imaging and physical examination of the foot were carried out in 100 consecutive soldiers on active duty referred for evaluation of suspected stress-injury of the lower limbs, back pain, and different skeletal trauma;
            - lateral radiographs of the foot were performed in those with increased uptake of Tc-MDP at the site of os trigonum;
            - among 200 feet, 27 (13.5%) showed an increased uptake of Tc-MDP in the os trigonum region;
            - right side was affected in six patients, the left side in 11 and five patients had bilaterally increased uptake;
            - on X-rays, 31 of 44 feet showed the os trigonum. Only 10 of these 27 feet (37%) had a symptomatic os trigonum;
            - increased uptake of Tc-MDP in the os trigonum region is a frequent finding among active soldiers and is of limited value in detecting symptomatic os trigonum;
            - Bone scintigraphic findings of os trigonum: a prospective study of 100 soldiers on active duty


- Posterior Compression Syndrome: (see: diff dx);
    - arises from forced plantar flexion in Ballet dancers;
    - impingement of posterior aspect of talus between tibia & calcaneus may cause block to plantar flexion;
    - results in painful lateral compression between calcaneus and posterior aspect of tibia;
    - forceful passive plantar flexion should reproduce the patients symptoms;
           - in contrast, both FHL tendinitis and posterior tibial tendinitis cause posteromedial tenderness;
    - a differential lidocaine/steroid injection (injected laterally at the posterior process of the calcaneus) should relieve symptoms;
    - note that this condition may occur along with FHL tendonitis;
    - non-operative treatment:
           - local steroid injections can be effective;
           - cast immobilization might reduce soft tissue swelling, but it will not reduce block to motion;
    - operative treatment:
           - excision of bony block will often improve motion and eliminate pain;
           - for isolated posterior impingement, consider lateral approach;
           - performed at the level of the ankle joint, just posterior to the peroneal tendons;
           - identify the sural nerve and the FHL tunnel;
           - after a capsulotomy has been performed, identify the os trigonum or the offending osteophyte;
           - an adequate decompression should allow the foot to be plantar flexed w/ no bony impingement



Anatomy of the os trigonum.

Stenosing tenosynovitis of the flexor hallucis longus tendon and posterior impingement upon the os trigonum in ballet dancers

Outcome of Resection of a Symptomatic Os Trigonum.

Bilateral os trigonum syndrome associated with bilateral tenosynovitis of the flexor hallucis longus muscle.