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Os tibiale externum

An os tibiale externum (or accessory navicular or os naviculare accessorium) is a large ossicle adjacent to the medial side of the navicular bone. The tibialis posterior tendon often inserts with a broad attachment into the ossicle. Most cases are asymptomatic but in a small proportion it may cause a painful tendinosis due to traction between the ossicle and the navicular. Such changes are best seen on MRI.

Epidemiology

  • an accessory navicular bone is present in ~10% (range 4-21%) of the population
  • first appears in adolescence
  • more common in female patients
  • reported bilateral prevalence is ~70% (range 50-90%)

Clinical presentation

Most of the time it is asymptomatic and found incidentally on radiographs, although medial side foot pain is most common presenting feature of accessory navicular bone, the pain is aggravated by walking, running and weight bearing activities.

Gross anatomy

An accessory navicular bone is located posterior to the posteromedial tuberosity of the tarsal navicular bone.

    Classification

    The Geist classification divides these into three types:

    1. type 1 accessory navicular bone
      • also known as os tibiale externum
      • 2-3mm sesamoid bone embedded within the distal portion of the posterior tibial tendon
      • no cartilaginous connection to the naviculam tuberosity and may be separated from it by up to 5mm
      • accounts for 30% of accessory navicular bones
      • usually asymptomatic
    2. type 2 accessory navicular bone
      • accounts for ~55% (range 50-60%) of all accessory navicular bones
      • triangular or heart-shaped
      • measures up to 12 mm
      • connected to the navicular tuberosity by a 1-2-mm thick layer of either fibrocartilage on hyaline cartilage
      • eventual osseous fusion to the navicular tuberosity may take place
    3. type 3 accessory navicular bone
      • an especially prominent navicular tuberosity called a cornuate navicular
      • thought to represent a fused type 2 and is occasionally symptomatic as a result of painful bunion formation over the bony protuberance

    Radiographic features 

    Radiograph shows a medial navicular eminence which is best visualised on lateral-oblique view. A symptomatic accessory navicular may appear as a 'hot spot' on bone scan and on MRI bone marrow oedema can be seen.

    Treatment and prognosis

    Acute pain can be managed can be achieved by corticosteroid injection and immobilisation of the foot for 2-3 weeks. For refractory cases surgical management can be considered.


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