Os tibiale externum

An os tibiale externum, also known as accessory navicular or os naviculare accessorium, is a large accessory ossicle that can be present 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 (os naviculare syndrome) is most common presenting feature of accessory navicular bone, the pain is aggravated by walking, running and weight-bearing activities. When large, it can protrude medially and cause friction against footwear.

Gross anatomy

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

    Classification

    This classification was proposed by Geist7 in 1914 and at the time of writing (July 2016) remains the most widely used classification system.

    The Geist classification divides these into three types:

    1. type 1 accessory navicular bone
      • also known as os tibiale externum
      • 2-3 mm 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 5 mm
      • 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
      • typically measures around 12 mm
      • connected to the navicular tuberosity by a 1-2 mm thick layer of either fibrocartilage or 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 

    Radiographs show a medial navicular eminence that is best visualised on the 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.

    History and etymology

    It is thought to have been first described by Bauhin in 1605 6.


    Lower limb anatomy
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    Article Information

    rID: 1785
    System: Musculoskeletal
    Section: Anatomy
    Synonyms or Alternate Spellings:
    • Os naviculare accessorium
    • Accessory navicular
    • Geist classification
    • Os naviculare
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    Figure 1: the three types
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    Case 1
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    Os Naviculare
    Case 2
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    Case 3: bilateral
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    Case 5
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    Case 4: type II
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    Case 6
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    Case 7: fractured type II
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    Case 8
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    Case 9: also with a os calcaneous secundarius
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    Case 10: type II and III
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    Case 11: type II
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    Case 12: with os naviculare syndrome
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    Case 13: type II
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