Sesamoids and accessory ossicles of the foot: anatomical variability and related pathology

Hallucal sesamoids

The hallucal sesamoids are always present at the plantar aspect of the first metatarsal head, and are a constant in humans. The AP and axial view radiographs of the foot best depict the hallucal sesamoids (Fig. 2). The medial sesamoid commonly shows bipartite variation [2]. Bipartite sesamoid fragments tend not to fit together perfectly, which aids differentiation from a fracture (Fig. 3) (Table 2). Associated pathology related to the hallucal sesamoids is not uncommon and in addition to fracture includes infection, arthritis and osteonecrosis [3].

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Table 2

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

Anatomically, sesamoids at the second through fifth metatarsals appear to be embedded in the plantar aspect of the joint capsule and may also be multiple or multipartite. The prevalence of sesamoids at the second through fourth metatarsals has been documented at 0.4 % at the second metatarsal, 0.2 % at the third, 0.1 % at the fourth, and up to 4.3 % at the fifth metatarsal [4]. If present, these sesamoids are best evaluated on AP and oblique radiographs of the foot (Fig. 4). Pathology associated with these sesamoids is very rare, although infection from direct spread from adjacent soft tissue is possible.

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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 [5]. The prevalence of this sesamoid has been reported variably at 2–13 % in its ossified form [4]. A review of post-mortem cases reports an anatomically identified IP joint nodule in up to 73 % of cases [6]. 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

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 [12]. 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].

Os peroneum

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 [13], 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 % [14]. 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.

Accessory navicular

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].

Os intermetatarseum

This ossicle is most commonly located between the first and second metatarsal [8]. Radiographic studies report a prevalence of 1–7 %, while anatomical studies demonstrate a higher prevalence of up to 13 % [9, 13]. It is best visualised on an AP foot radiograph and usually appears oval or round, but may also be spindle-shaped and attached to a metatarsal. Although this os rarely has associated pathology, its location may cause it to be confused with a fracture of the second metatarsal such as in Lisfranc injuries (Fig. 6). Lack of a donor site, soft tissue swelling and osseous malalignment my help differentiate this normal variant from a fracture.

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Os vesalianum

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 % [17]. 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 [18]. 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 [4]. 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 % [21]. 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 [21]. However, its location makes it difficult to differentiate from a fracture of the anterosuperior calcaneal process [22]. MRI may be useful to detect bone marrow oedema related to a fracture in this location.

Trauma

Both sesamoids and accessory ossicles may simulate a fracture of a neighbouring bone or may undergo fracture themselves, either from acute or chronic repetitive trauma. As these osseous structures are themselves small, identification of injury may be difficult. Certain radiographic clues can be used to diagnose acute injury. A well-corticated structure with smooth borders is less likely to be secondary to acute injury, while an irregular fragment with poorly corticated margins is suggestive of a fracture (Fig. 8). Other clues one might look for include evidence of displacement, presence of a donor site, and soft tissue swelling. For example, differentiating between an os vesalianum and an avulsion fracture of the apophysis of the fifth metatarsal can be difficult. If the suspected bony fragment is separated from the metatarsal by a radiolucent line of constant width with a well-corticated edge on radiography, it is more likely to be an os vesalianum than a metatarsal avulsion fracture [24]. As another example, it may be challenging to distinguish between the os trigonum and a fracture of the lateral tubercle of the posterior process of the talus, the so-called Shepherd’s fracture [25, 26]. The fragment is rarely displaced, making it difficult to differentiate it from a true os trigonum on radiography alone. Clinical correlation with focal tenderness is also useful in determining acute injury. When radiographic and clinical findings are equivocal, CT and MRI may help to identify true fractures by detecting oblique irregular interfaces which separate incompletely corticated fragments [10, 25, 26].

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Due to their locations in the foot, the hallucal sesamoids are more susceptible to trauma and fracture, with the medial sesamoid fractured more frequently [3, 27]. Acute injury to the hallucal sesamoids can be seen in deceleration injuries transmitting a large amount of force through the sesamoids (Fig. 9), secondary to axial loading as in falling from a height, and also in association with plantar plate injuries [3]. Fractures of the os trigonum may be seen in forced plantar flexion [10, 11]. A direct blow or inversion injury may result in fractures of the os peroneum [13], and proximal migration of an os peroneum suggests a tear of the peroneus longus tendon [23, 28].

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Sesamoiditis

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 [13]. Chronic pain can be caused by stress fracture, stress reaction, osteoarthritis and osteonecrosis of the sesamoids, as well as tendinosis and capsular inflammation [27]. 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 [3], also increasing the risk of stress fracture and osteonecrosis.

MRI is particularly useful in the imaging of clinical sesamoiditis, with the ability to evaluate intrinsic osseous and soft tissue abnormalities. On MRI, both sesamoiditis and osteonecrosis may show changes in the pattern of bone-marrow oedema isolated to the sesamoid, without changes in the metatarsal head to suggest osteoarthritis (Fig. 10). CT may be useful in differentiating between these two entities, as subtle increases in sclerosis detected on CT would favour a diagnosis of osteonecrosis (Fig. 11). ^99mTc-MDP bone scan may help in identifying the presence of sesamoiditis as a focally increased uptake of the radioactive tracer [26].

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Infection

Osteomyelitis affecting the sesamoids and accessory ossicles is most frequently secondary to direct extension [3]. The hallucal sesamoids may be infected secondary to extension from a soft tissue infection or from a septic joint [27]. Radiographs classically are only sensitive to late stage osteomyelitis, showing cortical destruction and adjacent soft tissue inflammation (Fig. 12). MRI offers a more sensitive evaluation of early disease involvement. MRI findings include T2 hyperintensity of the marrow with corresponding T1 hypointensity, in addition to signs of inflammation in the surrounding soft tissues (Fig. 13). Intravenous contrast should reveal avid intraosseous enhancement in osteomyelitis [27].

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Degenerative disease

As the hallucal sesamoids have a true articular joint with the metatarsal head, they are susceptible to osteoarthritis [3]. Classic findings of osteoarthritis may be seen on radiographs, including loss of joint space, subchondral sclerosis and cysts, and osteophyte formation [27] (Fig. 14). In addition to these changes, MRI may reveal cartilage loss and hyperintense subchondral signal [27]. Accessory ossicles with synchondroses and those within high-use tendons may also be predisposed to degenerative changes [10, 16, 28]. CT is sensitive in the evaluation of sclerosis and fragmentation (Figs. 15 and 16), and MRI can evaluate associated marrow and soft tissue changes.

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Specific ossicle-related painful syndromes

Posterior ankle impingement syndrome

Pathology attributed to the os trigonum is encompassed by the term os trigonum syndrome, which falls within the spectrum of posterior ankle impingement syndrome. Pain associated with the os trigonum may be related to degenerative changes at the synchondrosis, fracture of the posterior lateral talar process, local synovitis, flexor hallucis longus tenosynovitis or intra-articular loose bodies [10, 11, 29]. Repetitive plantar flexion predisposes to this syndrome, as seen in ballet dancers, basketball players and football players [11, 13, 29]. Clinically, patients may experience chronic or recurrent pain with stiffness, soft tissue swelling and tenderness to deep posterolateral palpation [10, 11, 29]. On imaging, radiographs demonstrate the presence or absence of an os trigonum, soft tissue swelling and infiltration of fat (Fig. 17). CT may demonstrate fractures or fragmentation, and degeneration at the synchondrosis [11]. MRI is the superior imaging modality in evaluation of posterior ankle impingement syndrome, however, as intrinsic osseous pathology and associated soft tissue abnormalities such as synovitis and tenosynovitis are readily apparent [11, 29] (Fig. 16).

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Painful os peroneum syndrome

Fracture or attrition of the os peroneum and degeneration or tearing of the peroneus longus tendon at the site of the ossicle may cause painful os peroneum syndrome [28], which presents as lateral pain and tenderness along the course of the peroneus longus tendon. Radiography and CT will demonstrate the presence of an os peroneum, displacement from its normal location, fracture or distraction of a bipartite sesamoid. As mentioned previously, proximal migration of the os peroneum indicates a tear of the peroneus longus tendon, and wide separation (>2 mm) of os peroneum fragments is less likely to represent a normal bipartite os peroneum [23]. MRI may reveal tendinosis and abnormal bone marrow signal within the ossicle and the adjacent osseous structures (Fig. 18). Ultrasound may also be used to locate tendon tears and tenosynovitis associated with the os peroneum [23].

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Painful accessory navicular

A symptomatic accessory tarsal navicular is most commonly seen with a type II accessory navicular and is thought to be the result of altered biomechanics, presenting as shoe irritation and pain localising to the navicular bone [15, 16]. Clinical symptoms may be attributed to tension and repetitive shearing stress at the synchondrosis from the posterior tibial tendon, causing disruption of the synchondrosis, posterior tibial tenosynovitis and even osteonecrosis [13, 16]. CT may demonstrate degenerative changes at the synchondrosis and abnormal osseous density in detail. Technetium bone scan may reveal increased uptake in a symptomatic accessory navicular, although this finding may not be specific [13]. On MRI, there may be abnormal signal within the ossicle, in the synchondrosis and navicular tubercle, within the adjacent soft tissues and in the posterior tibial tendon [13, 16] (Fig. 19).

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