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 bone 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