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Updated: Sep 17 2023

Lower Extremity Os

Images
https://upload.orthobullets.com/topic/7049/images/lateral xr os trigonum.jpg
https://upload.orthobullets.com/topic/7049/images/f02314bd-21bd-4eea-bc39-ed0890fc142b_os_vesalianum..jpg
https://upload.orthobullets.com/topic/7049/images/posterior process fracture.jpg
https://upload.orthobullets.com/topic/7049/images/xray - lateral - wheeless_moved.jpg
https://upload.orthobullets.com/topic/7049/images/os subfibulare ap xr.jpg
https://upload.orthobullets.com/topic/7049/images/os peroneum1.jpg
https://upload.orthobullets.com/topic/7049/images/bipartite os peroneum xr.jpg
  • summary
    • Lower Extremity Os are secondary ossification centers that remain separated from the normal bone and may be confused with a fracture.
    • Diagnosis requires plain radiographs of the foot and ankle.
    • Treatment is generally observation as most are completely asymptomatic. In the event of symptoms, treatment depends on location of Os and chronicity of symptoms.  
  • Epidemiology
    • Up to 40 accessory ossicles and multiple sesamoids have been described in the foot and ankle
  • Etiology
    • Definition
      • accessory ossicles
        • secondary ossification centers that remain separated from the normal bon
      • sesamoids
        • are bones that are incorporated into tendons and move with normal and abnormal tendon motion
    • Most common ossicles
      • os trigonum
      • accessory navicular (os tibiale externum)
      • os intermetatarseum
    • Most common sesamoids
      • os peroneum
        • located in the peroneus longus tendon
      • hallux sesamoids
        • located in the flexor hallucis brevis tendon at the base of the 1st metatarsal head
  • Classification
      • Accessory Ossicles and Sesamoids of the Foot and Ankle
      • Accessory bone
      • Prevalence
      • Clinical significance
      • Differential 
      • Os trigonum
      • 10-25%
      • Posterior ankle impingement, FHL entrapment
      • Shepherd's fracture
      • Type II accessory navicular
      • 2-12%
      • Posterior tibial tendon dysfunction
      • Navicular tuberosity avulsion fx, type I accessory navicular
      • Os subfibulare
      • 2%
      • Painful os subfibulare
      • Lateral malleolus avulsion frx
      • Os peroneum
      • 9-20%
      • Painful os peroneum, fracture, diastases
      • Painful os vesalianum, bipartite os peroneum
      • Os vesalianum
      • 2%
      • Painful os vesalianum
      • Avulsion frx of the 5th metatarsal base
      • Hallux sesamoids
      • ~100%
      • Fracture, stress fracture
      • Bipartite tibial sesamoid
  • Os Trigonum
    • Definition
      • accessory ossicle representing the separated posterolateral tubercle of the talus
      • usually asymptomatic, but can become symptomatic and cause os trigonum syndrome
    • Epidemiology
      • incidence
        • 10-25% of the population have os trigonum
        • commonly symptomatic in ballet dancers due to extreme plantar flexion ("en pointe" toe position)
    • Pathophysiology of os trigonum syndrome
      • repetitive microtrauma (ankle plantarflexion)
        • may present as a stress fracture
      • acute forced plantarflexion
        • may present as an acute fracture
    • Associated conditions
      • FHL tenosynovitis or entrapment
    • Anatomy
      • osteology
        • the secondary ossification center forms posterior to the talus between 8-13yrs
        • normally fuses with talus within 1yr
        • if the ossicle fails to fuse, it articulates with the talus through a synchondrosis
        • the os lies lateral to FHL, tibial nerve, PTT, and posterior tibial artery
    • Presentation
      • symptoms
        • pain in "en pointe" position
      • physical exam
        • posterolateral ankle pain with passive ankle plantar flexion
          • differentiate from FHL tendinitis where ankle pain is posteromedial and there may be triggering
        • may have swelling and tenderness over FHL if associated with FHL tendinitis
    • Imaging
      • radiographs
        • recommended views
          • lateral radiograph with foot in plantar flexion
        • findings
          • shows os trigonum impinged between posterior tibial malleolus and calcaneal tuberosity
          • os trigonum can be round, oval or triangular and of variable size
      • MRI
        • findings
          • shows os trigonum and associated inflammation and edema in FHL tendon
    • Differential diagnosis
      • fracture of the posterior process of the talus (Shepherd's fracture)
      • FHL and posterior tibialis tendinitis
        • produce posterior medial ankle pain and tenderness
    • Treatment
      • nonoperative
        • NSAIDS, rest, immobilization, restricted weightbearing
      • operative
        • surgical excision
          • indications
            • if nonoperative management fails
          • techniques
            • through open lateral approach or posterior ankle arthroscopy
  • Os Tibiale Externum (Accessory navicular)
    • See Accessory Navicular
  • Os Subfibulare
    • Definition
      • small piece of bone adjacent to inferior fibula
    • Epidemiology
      • incidence
        • 1-2% of population
    • Pathoanatomy
      • may represent avulsion fx of ATFL that secondarily ossifies
      • or accessory ossification center
    • Presentation
      • symptoms
        • may be asymptomatic
        • may have ankle pain (symptomatic os subfibulare)
        • may be associated with chronic ankle instability and present with recurrent ankle sprains
      • signs
        • focal tenderness and swelling at the site of the ossicle
        • laxity with anterior drawer and inversion/eversion stress testing
    • Imaging
      • radiographs
        • recommended views
          • standard ankle series (weightbearing AP, lateral, mortise)
          • varus stress view
        • findings
          • accessory ossicle
          • talar tilt on varus stress view
            • suggesting ankle instability
            • increased separation of os fragment from fibula tip
    • Differential diagnosis
      • acute lateral malleolus avulsion fracture (by the ATFL)
    • Treatment
      • nonoperative
        • NSAIDS, rest, immobilization, restricted weightbearing
          • indications
            • initial treatment for symptomatic os subfibulare
      • operative
        • surgical excision
          • indications
            • failed nonoperative management
  • Os Peroneum
    • Definition
      • sesamoid bone found within the peroneus longus tendon near the base of the 5th MT
      • may represent avulsion or rupture of peroneus longus
    • Epidemiology
      • incidence
        • 9-20% of adults
        • bilateral in 60%
        • bipartite in 30%
    • Pathophysiology of injury or fracture
      • direct trauma
      • indirect trauma (sudden inversion and supination)
        • associated with peroneus longus tendon rupture
    • Imaging
      • radiographs
        • findings
          • normal os peroneum
          • acute os peroneum fracture
          • peroneus longus rupture
      • MRI
        • findings
          • normal os peroneum
          • acute os peroneum fracture
          • peroneus longus rupture
    • Differential diagnosis
      • painful os vesalianum
      • bipartite os peroneum
    • Treatment
      • nonoperative
        • NSAIDS, rest, immobilization, restricted weightbearing
          • indications
            • initial treatment for painful os peroneum syndrome
            • minimally displaced os peroneum fractures
      • operative
        • surgical excision
          • indications
            • painful os peroneum syndrome (with minimal tendon involvement) refractory to conservative treatment
            • os peroneum fracture with displaced fragments
        • surgical excision and repair of peroneus longus tendon or tenodesis to peroneus brevis
          • indications
            • os peroneum associated with peroneus longus tendon rupture
  • Hallux Sesamoids
    • See Sesamoid Injuries of the Hallux
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