Sesamoids and accessory ossicles of the foot: anatomical variability and related pathology
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- Nwawka, O.K., Hayashi, D., Diaz, L.E. et al. Insights Imaging (2013) 4: 581. doi:10.1007/s13244-013-0277-1
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Sesamoids and accessory ossicles seen in the foot vary widely in their prevalence and appearance. Occasionally, these bones may be associated with painful syndromes, due to various pathologies, including trauma, infection, inflammation, degeneration and others. However, symptomatic accessory and sesamoid bones are rare, and search for additional pathology should be performed. Although the clinical significance of these osseous structures is probably minor, clinicians very commonly ask about these bones, which may originate an unnecessary work-up. Therefore, knowledge of their presence and morphological variations is important to prevent misinterpreting them as fractures—a common error. Finally, it may be very difficult to distinguish between incidental variants and truly symptomatic ones. Radiological studies provide insight regarding the presence and pathology involving these bones. This review describes an overview of the anatomy of sesamoids and accessory ossicles in the foot, and provides a pictorial review of their pathological conditions, including trauma, sesamoiditis, osteomyelitis, osteoarthritis and pain syndromes. Radiological studies including radiography, ultrasound, scintigraphy, computed tomography (CT) and magnetic resonance imaging (MRI) provide useful information which should be used in concert with clinical findings to guide patient management.
• Sesamoids and accessory ossicles seen in the foot vary widely in their prevalence and appearance.
• Pathology of these bones includes trauma, sesamoiditis, infection, osteoarthritis and pain syndromes.
• Radiography, ultrasound, scintigraphy, CT and MRI provide information regarding the pathology of these bones.
KeywordsSesamoids Accessory ossicles Foot MRI CT Radiography
Sesamoids are osseous structures partially or totally embedded in a tendon. Their function is to protect the tendon from injury by reducing friction. In contrast, accessory ossicles are supernumerary bones that commonly derive from unfused primary or secondary ossification centres . They are thought to be normal variants with no definite known function. However, both sesamoids and accessory ossicles may be associated with pathological conditions.
Sesamoids and accessory ossicles share several imaging characteristics: They are usually small, well-corticated, ovoid or nodular, may be bipartite or multipartite, and are found close to a bone or a joint. The presence of these osseous structures is usually incidental. Both sesamoids and accessory ossicles may be unilateral or bilateral, and are subject to significant morphological variations. Understandably, this may make the recognition of pathological conditions challenging. Although it is difficult to determine the symptomatic nature of these bones, imaging provides important diagnostic information to be considered in the clinical work-up. This article aims to review the normal anatomy of these bony structures, and to discuss their most common associated pathological conditions.
Location and reported prevalence of the sesamoid and accessory ossicles of the foot
Reported prevalence [references]
Plantar aspect of the first metatarsal head
Plantar aspect of the second through fifth metatarsals
0.4 % (second);
0.2 % (third);
0.1 % (fourth)
Up to 4.3 % (fifth) 
Plantar aspect of the interphalangeal joint of the first digit of the foot
2–13 % 
Posterior to the talus
At the cuboid tunnel adjacent to the calcaneocuboid joint, within the peroneus longus tendon
Adjacent to the tarsal navicular, within the distal posterior tibial tendon
Between the first and second metatarsal
Proximal to the base of the fifth metatarsal, within the peroneus brevis tendon
0.1–1 % 
Above the talonavicular joint
Above the neck of the talus
Anterior to the tibiotalar joint
0.5 % 
Os calcaneus secundarium
Adjacent to the anterior calcaneal process
0.6–7 % 
Radiological clues to differentiate a fractured hallucal sesamoid and bipartite hallucal sesamoid
Single medial sesamoid with a fracture
Bipartite medial sesamoid
Slightly larger than the lateral sesamoid, whereas the
Much larger than the lateral sesamoid
Show a sharp, radiolucent, uncorticated line between the two fragments
Two corticated components
The two fragments often fit together like pieces of a puzzle
The two components do not fit like pieces of a puzzle
A 99mTc-MDP bone scan will show increased uptake
A 99mTc-MDP bone scan should be normal
MRI may show bone marrow oedema in a recently fractured sesamoid
No MRI signal abnormality
Lesser metatarsal sesamoids
Interphalangeal joint sesamoid
As its name implies, the interphalangeal joint sesamoid is seen at the plantar aspect of the interphalangeal (IP) joint of the first digit of the foot. It is embedded within the joint capsule and the presence of an ossified sesamoid may alter biomechanics and limit motion in the joint . The prevalence of this sesamoid has been reported variably at 2–13 % in its ossified form . A review of post-mortem cases reports an anatomically identified IP joint nodule in up to 73 % of cases . These bones are best imaged on an AP radiograph of the foot or toes. A potential serious pathology associated with this sesamoid is its interposition into a dislocated IP joint, making it irreducible [7, 8].
Os trigonum is located posterior to the talus. The reported prevalence of the os trigonum is quite variable, from 7 to 25 % [9, 10]. They are best seen on lateral radiographs of the foot and ankle and may be round, oval or triangular and may have a synchondrosis with the posterolateral talus [10, 11]. The os trigonum may be symptomatic in different forms related to its size, stability and other local issues. Fracture of an os trigonum itself is extremely rare . A common differential for the os trigonum is a Shepherd fracture, i.e. a fracture of the lateral process of the talus. Pathology at the os trigonum may be a cause of posterior ankle impingement syndrome [10, 11, 13].
The os peroneum is located in the region of the cuboid tunnel near the calcaneocuboid joint. It is embedded within the peroneus longus tendon, technically making it a sesamoid. Although present in everyone in at least a cartilaginous form , this ossicle in its fully ossified form is found in up to 26 % of the population [13, 14]. On radiographs, it is best identified in an oblique view of the foot. It may appear round or oval. It is bipartite in approximately 30 % of cases, and bilateral in approximately 60 % . The os peroneum may become painful, may fracture, and also may become displaced. Displacement is an indirect sign of a peroneus longus tendon tear, described later in the article.
There are three reported types based on morphology. Type I (30 %) is an oval or round sesamoid located within the distal posterior tibial tendon. It may be separated up to 5 mm from the navicular tuberosity. Type II is the most common variant (50 %) and is known as the os naviculare. It is a triangular or heart-shaped unfused accessory ossification centre, separated from the tuberosity by a 1–2 mm wide synchondrosis. Type III (20 %) is a prominent tuberosity, thought to be a fused type II accessory navicular bone. The accessory navicular bone is the second most common accessory bone of the foot, with reported prevalence from 2 to 21 % [9, 15]. It is bilateral in 50 % of cases. It is best seen on an AP radiograph of the foot. Of the three variations, type II is most commonly symptomatic [13, 16].
The os vesalianum is located proximal to the base of the fifth metatarsal, found within the peroneus brevis tendon and may articulate with the cuboid. The reported prevalence of an os vesalianum is between 0.1 and 1 % . It is best seen on an oblique radiograph of the foot. The os vesalianum is rarely a source of pathology, but painful conditions similar to os peroneum syndrome have been described . A differential consideration for an os vesalianum is a fracture of the base of the fifth metatarsal.
Os supranaviculare, os supratalare and os talotibiale
These rare ossicles are all located adjacent to the dorsal talus [4, 19, 20]. The os supranaviculare may fuse with the navicular to form a spur of no clinical significance . These ossicles are rarely associated with painful conditions and, although their appearance may occasionally be challenging radiographically, the lack of a donor site, soft tissue swelling and focal tenderness should help to differentiate them from avulsion fractures.
Os calcaneus secundarium
This ossicle is located between the anteromedial aspect of the calcaneus, the cuboid, the talar head and the tarsal navicular. The incidence of the os calcaneus secundarium has been reported between 0.6 and 7 % . This rare ossicle is difficult to detect radiographically due to overlying structures, but can be visualised on CT. The os calcaneus secundarium has not been shown to have any clinical significance . However, its location makes it difficult to differentiate from a fracture of the anterosuperior calcaneal process . MRI may be useful to detect bone marrow oedema related to a fracture in this location.
Pathological conditions in sesamoids and accessory ossicles of the foot
Both sesamoid bones and accessory ossicles can be affected by trauma, infection, degenerative disease and osteonecrosis. Due to their locations in the foot, the hallucal sesamoids and the os trigonum are more susceptible to trauma and fracture. Location also predisposes the hallucal sesamoids to infection, particularly via direct extension. Accessory ossicles with synchondroses such as the os naviculare and os trigonum are predisposed to degenerative change. Sesamoids embedded within high-use tendons such as the os peroneum are also susceptible to degenerative disease.
Pathological conditions associated with sesamoids and accessory ossicles of the foot and radiological clues to diagnose them
Irregular bony fragment with poorly corticated margins; evidence of displacement; presence of a donor site; soft tissue swelling
Bone marrow oedema isolated to the sesamoid without changes in the metatarsal head on MRI; no increase in sclerosis on CT; increased uptake on 99mTc-MDP bone scan
Increased sclerosis on CT; bone marrow oedema pattern (early) or diffusely decreased signal (late) on MRI
Cortical destruction and adjacent soft tissue inflammation on radiograph (late stage disease); T2 hyperintensity of the marrow with corresponding T1 hypointensity, in addition to signs of inflammation in the surrounding soft tissues on non-enhanced MRI; intraosseous enhancement on contrast-enhanced MRI; possible fluid-equivalent signal changes with peripheral enhancement representing intraosseous and soft tissue abscess formation
Degenerative disease (osteoarthritis)
Joint space loss, subchondral sclerosis and cysts, and osteophytes formation on radiograph; cartilage loss, signal changes suggestive of bone marrow and soft tissue changes on MRI; sclerosis and fragmentation on CT
Posterior ankle impingement syndrome
Presence or absence of os trigonum, soft tissue swelling and infiltration of fat on radiograph; fractures or fragmentation, and degeneration at the synchondrosis on CT; intrinsic osseous pathology and associated synovitis/tenosynovitis on MRI
Painful os peroneum syndrome
Presence of os peroneum, displacement from its normal location, fracture or distraction of a bipartite sesamoid on radiography and CT; tendinosis and abnormal bone marrow signal within the ossicle and the adjacent osseous structures on MRI; tendon tears
Painful accessory navicular
Always type II; degenerative changes at the synchondrosis and abnormal osseous density on CT; increased uptake on technetium bone scan; abnormal signal within the ossicle in the synchondrosis and navicular tubercle, within the adjacent soft tissues and in the posterior tibial tendon on MRI
Painful os intermetatarseum
Increased radiotracer uptake on scintigraphy
Chronic pain at the hallucal sesamoids may clinically be described as “sesamoiditis”, a term that encompasses the symptoms from pathology at the hallux sesamoids complex . Chronic pain can be caused by stress fracture, stress reaction, osteoarthritis and osteonecrosis of the sesamoids, as well as tendinosis and capsular inflammation . Repetitive and excessive axial loading in plantar flexion, such as in ballet dancing, running and even wearing high-heeled shoes, have also been postulated as risk factors for sesamoiditis [3, 27]. Excision of one sesamoid may cause abnormal biomechanical stress on the residual sesamoid , also increasing the risk of stress fracture and osteonecrosis.
Specific ossicle-related painful syndromes
Posterior ankle impingement syndrome
Painful os peroneum syndrome
Painful accessory navicular
Painful os intermetatarseum
Compared with the aforementioned ossicles, the os intermetatarseum is less commonly reported as a cause of pain. However, compression of the superficial or deep peroneal nerve by this ossicle has been described as a source of dorsal foot pain [13, 30]. Patients present with pain and/or numbness at the dorsum of the foot that is exacerbated by standing or jumping, and tenderness to palpation at the first intermetatarsal interspace [13, 30]. Scintigraphy may reveal increased radiotracer uptake in a symptomatic os intermetatarseum .
Sesamoids and accessory ossicles seen in the foot vary widely in their prevalence and appearance. Although the clinical significance of these osseous structures is probably minor, clinicians very commonly ask about these bones, and this may incite an unnecessary work-up. Therefore, becoming familiar with these bones is important to prevent misinterpreting them as fractures, a common error. Finally, it may be very difficult to distinguish between incidental or clinically irrelevant bones and truly symptomatic ones. Radiological studies including radiography, ultrasound, scintigraphy, CT and MRI provide useful information regarding the presence and pathology involving these bones, and should be used in concert with clinical findings to guide patient management.
A.G. is the President of Boston Imaging Core Lab (BICL), LLC, and a consultant to TissueGene, Sanofi-Aventis, and Merck Serono. F.W.R. is a shareholder of BICL, and is a consultant to Merck Serono and the National Institute of Health. Other authors have nothing to disclose.
No funding was received for this work.
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