It is common to see partite tibial and fibular sesamoids; typically, they are bipartite. Partitioning may involve one or both sesamoids and be unilateral or bilateral. Furthermore, if only the tibial sesamoid is partite in one foot, only the fibular sesamoid may be partite in the opposite foot. The presentations (partitioning, shape, size, and number) are extremely variable and follow no specific rule. For example, a bipartition may divide the sesamoid into equal or unequal halves; it typically is either transverse or oblique. The bipartition is rarely longitudinal.
It is difficult to differentiate a fractured sesamoid from a partite sesamoid in most instances, especially in the dorsoplantar view. The presence of jagged edges alone is not a useful distinguishing feature because both the bipartite and the fractured sesamoid can appear to have jagged edges. A partite sesamoid that is complicated by degenerative joint disease will have spurs that give the appearance of a jagged edge, similar to that of a fracture. Furthermore, the coarse trabeculations normally found in the first metatarsal distal metaphysis are superimposed on the sesamoids. These shadows can exagerrate the perception of a jagged fracture line. Nor can fracture be determined by the amount of separation between the two segments. It is not uncommon to see apparent "distraction" in the asymptomatic, non-traumatized bipartition. Fracture is best differentiated from variant bipartition with the sesamoid isolated, that is, with no superimposition on the metatarsal head (this can only be accomplished with the sesamoid axial and lateral oblique or modified non-weight bearing lateral –Causton– views). Certainly, correlation with clinical history is important in these instances.
The sesamoids are initially evaluated with the dorsoplantar and sesamoid axial views. The lateral oblique view is invaluable for assessing tibial sesamoid pathology. The tibial and fibular sesamoids are superimposed on the first and second metatarsals, respectively, in the medial oblique view; this view generally does not provide any additional information apart from the dorsoplantar, lateral oblique, and axial views. The standard lateral view is useless for imaging the sesamoids; they are superimposed on each other in addition to other metatarsal and phalangeal bones.
The medial cuneiform will seldom present as two entities in the adult skeleton, known as the bipartite medial cuneiform. The partition classically divides the bone into upper and lower halves. It is best seen in the lateral view.
The classic bipartition, when present, is not readily identified by casual glance. It is fully superimposed on the remaining cuneiform bones and easily mistaken for other bone shadows. However, the tranverse joint space identified in the center of the medial cuneiform is its characteristic radiologic feature. This joint space typically is complete from anterior to posterior.
The bipartite navicular is a rare entity. The division separates the bone into a smaller superolateral segment and a larger inferior and medial segment. It may be found either bilaterally or unilaterally. The smaller bipartite segment appears to be closely associated with the intermediate cuneiform. The bipartition is best seen in the lateral view as a transverse linear lucency (small arrowhead).
Another, oblique, linear lucency may also be seen (long arrow); however, after closer viewing and correlation with a normal gross anatomical specimen, this latter entity appears to represent the articulation between the lateral cuneiform and the larger navicular segment. The navicular will appear deformed in the dorsoplantar view; it is wedge-shaped (apex laterally, base medially).
The os interphalangeus is classically found along the inferior aspect of the hallux interphalangeal joint. It is rare to see this ossicle at the interphalangeal joints of the lesser toes. The position of the os interphalangeus is either central or eccentric; central is more common. These ossicles are round or oval-shaped. The centrally-located os interphalangeus is considered a sesamoid bone because of its location in the plantar capsule and attachment to the flexor tendon. (McCarthy et. al. have identified its location as in the joint capsule and the flexor hallucis capsularis interphalangeus.) The position of the eccentric os interphalangeus probably is intracapsular. Rarely, an ossicle may be encountered along the superior aspect of an interphalangeal joint.
A 
B 
The eccentric os interphalangeus appears to have a different genesis than the centrally-located sesamoid. The basal epiphysis of the distal phalanx occasionally has multiple ossification centers during development. A segment of this ossification center may remain separate into adulthood and persist as the eccentric os interphalangeus. A defect in the adjacent phalangeal base is frequently observed that corresponds to the size and shape of the unfused ossification center in this case. Rarely, the the ossicle is adjacent to a defect along the proximal phalanx head. The eccentric os interphalangeus may also be the sequella of an old, unhealed fracture (i.e., nonunion). However, the majority of these patients do not recall any history of trauma and many times the radiographic finding is bilateral in presentation.
The os interphalangeus can be identified in either the dorsoplantar or isolated lateral view of the hallux. Its transverse plane position is best determined with the dorsoplantar view. It is superimposed on the proximal phalanx head and appears as a fairly well-defined, oval increased density. It can be clearly identified with the lateral view if the hallux is isolated from the lesser digits. The lateral or medial eccentric os interphalangeus can be isolated with the medial oblique or lateral oblique view, respectively.
A pair of sesamoid bones are consistently found at the first metatarsophalangeal joint. This occurs almost without exception. In addition, sesamoid bones may also be found along the inferior aspects of the lesser metatarsophalangeal joints, though not commonly. These latter ossicles are known as accessory sesamoids. They may be seen at any of the lesser metatarsophalangeal joints in varying combinations. For example, accessory sesamoids may appear at either one, two, three, or all four lesser metatarsophalangeal joints. Furthermore, accessory sesamoids may exist as a single entity or occur in pairs. They appear circular or oval in shape and vary in size. Rarely, they are bipartite.
A 
B 
Accessory sesamoids are best isolated in the sesamoid axial view (B), seen along the inferomedial aspect of the metatarsophalangeal joint when solitary. The single sesamoid is superimposed on the medial aspect of the metatarsal head in the dorsoplantar view (A). The medial positioning of these sesamoids may be related to the normally inverted position of the lesser metatarsals relative to the x-ray film.
The os intermetatarseum is situated superiorly between the first and second metatarsal bases. It is best seen in the dorsoplantar view (A). It can occasionally be seen in the lateral view superiorly (B), though it typically is superimposed on the first metatarsal base. This ossicle may be round, oval, kidney-shaped, linear, or even resemble a rudimentary metatarsal (C). Its size also varies. The os intermetarseum may articulate with the medial cuneiform or be attached to the first metatarsal base. Calcification of the perforating branch between the dorsal and plantar metatarsal arteries may simulate an os intermetatarseum. An os intermetatarseum is rarely encountered at the second lesser metatarsal-cuneiform joint (also known as the os cuneometatarsale II dorsale). The os intermetatarseum is infrequently the cause of foot symptomatology.
A 
B 
C 
A sesamoid may infrequently be found in the tibialis anterior tendon near its insertion onto the first metatarsal base. It is located adjacent to either the first metatarsocuneiform joint or the medial cuneiform and is best seen in the dorsoplantar and, if located adjacent to the insertion site, lateral oblique views. This sesamoid may be either round, oval, or linear in shape. It might also be bipartite.
An accessory ossicle located at the posterior tip of the fifth metatarsal tuberosity is known as the os vesalianum. The presence of this ossicle is quite rare, and its size and shape may vary considerably. It typically presents as a small, rounded calcific density. It is best seen in the medial oblique view. Ossification of the peroneus brevis tendon and old, unhealed (nonunion) fracture of the tuberosity may look similar to the os vesalianum. This ossicle is not the same entity as the persistent fifth metatarsal apophysis (described next).
A 
B 
The fifth metatarsal tuberosity apophyseal ossification center may remain unfused into adulthood. This is known as a persistent apophysis. It is large in size and appears to articulate with the metatarsal base. The persistent fifth metatarsal apophysis is occasionally referred to as the os vesalianum, albeit incorrectly. It is clearly identified with the medial oblique view (C) and is frequently bilateral and symmetrical.
A 
B 
C 
The os peroneum is a sesamoid bone found in the peroneus longus tendon. It varies not only in size, but in number: it commonly is partite. The os peroneum is classically situated beside the cuboid bone just proximal to where the tendon runs along the peroneal sulcus, but its position varies considerably. It is best isolated in the medial oblique view. The os peroneum generally is superimposed on the cuboid in the lateral and dorsoplantar views. However, it can be clearly identified in the lateral view if its position anatomically is more distal in the tendon; at this location it articulates with the anterior aspect of the cuboid's inferior tuberosity. This sesamoid may infrequently be found at a more proximal location, adjacent to the calcaneocuboid joint or anterior calcaneus. This latter entity may easily be misinterpreted as an avulsion fracture. Location of the os peroneum adjacent to the calcaneus as opposed to the cuboid may be variant or could be indicative of a ruptured peroneal tendon with posterior sesamoid displacement.
An ossicle uncommonly found along the superior aspect of the naviculocuneiform joint is the os infranaviculare. It is best identified in the lateral view. This ossicle is seen infrequently along the superomedial aspect of the naviculocuneiform joint in the medial oblique view.
The os supranaviculare is found along the supererior aspect of the talonavicular joint. It is much more common than the os infranaviculare. Like the os infranaviculare, the os supranaviculare is typically found along the superior aspect of its respective joint, and infrequently superomedially. The former is identified in the lateral view, the latter in the medial oblique view. This ossicle has a multitude of configurations. It may appear as an entirely separate ossicle, a continuation of the articular subchondral bone, or may even be attached to the navicular. As with many of the accessory ossicles, it may be impossible to differentiate the os supranaviculare from an old, nonunion fracture.
The accessory ossicle located along the superior surface of the talar head is known as the os supratalare. It typically is located over the ridge along the talar head/neck, but may be seen distally over the head. It easily can simulate an old, ununited avulsion fracture and is only identifed in the lateral view. Occasionally, calcification of the talonavicular ligament may appear in the same location and mimick the appearance of an os supratalare.
An ossicle of varying size, shape, and position may be found adjacent to the navicular tuberosity. It is best identified in the lateral oblique view. The terms accessory navicular and os tibiale externum have been used interchangeably to identify these ossicles. However, there has been some discrepancy in the literature as to the definition and identification of this entity.
The ossicle that is found adjacent to the navicular tuberosity may either represent a sesamoid in the posterior tibial tendon or an accessory ossification center for the navicular tuberosity. The sesamoid is characteristically round, small in size, and located at a distance from the navicular tuberosity. The accessory ossification center, in contrast, is larger, oval or semi-circular in shape, and in close apposition to the tuberosity. It may be attached by cartilage or fibrocartilage, or, it may articulate with the tuberosity, containing true synovial tissue. Occasionally, the accessory ossification center is fused to the tuberosity. This latter instance has been referred to as a "wrap-around" navicular, cornuate navicular, gorilliform navicular, and the Kidner foot type. With so many varied presentations, one can see why there has been confusion in the use of terminology for this ossicle!
Three distinct types of ossicles may be identified adjacent to the navicular tuberosity. Lawson has classified them as follows:
(lateral oblique view)
A 
B 
C 
(A) DP view, (B) lateral oblique view, (C) lateral view
A 
B 
(A) DP view, (B) lateral oblique view
The above classification system best distinguishes between the varying forms of this enigma. The term os tibiale externum should be used cautiously and applied only to the sesamoid entity (accessory navicular type I).
The accessory navicular type I may be partite. The partition could be either transverse or longitudinal. The articulation between the accessory navicular type II and navicular may be quite irregular and sclerotic. These findings most likely represent degenerative joint disease and can simulate the appearance of a hypertrophic nonunion fracture. Bone scintigraphy has been advocated for distinguishing between symptomatic and asymptomatic accessory navicular.
The os calcaneus secundarius is best seen in the medial oblique view, adjacent and in close apposition to the calcaneal anterior process along its superomedial surface. The ossicle is occasionally located centrally between the anterior calcaneus, talar head, cuboid, and navicular, especially when larger in size. It is not clearly seen in the lateral view because it is superimposed on the calcaneus and talus. The os calcaneus secundarius can be mistaken for a fracture of the anterior calcaneal process.
An accessory ossification center can be found along the posterior aspect of the talar posterolateral process. It is known as the trigonal (Stieda's) process when it is fused to the talus. If it remains unfused and separate, it is known as the os trigonum. In either case, its inferior surface typically articulates with the calcaneus. The os trigonum may have a fibrous, fibrocartilaginous, or cartilaginous attachment to the talus. A joint space may be identified between it and the posterior talus. Occasionally, it may exhibit degenerative-like findings simulating osteoarthritis. The size of this ossicle ranges from small to large. It is best seen in the lateral view, but is infrequently viewed in the medial oblique view.
Bone scintigraphy may be a valuable diagnostic study for differentiation between symptomatic and asymptomatic os trigonum. Focal intense uptake suggests degenerative disease and/or unhealed fracture.
The os supracalcaneum is an extremely rare ossicle. It is found along the superior surface of the posterior calcaneus. It can be mistaken for the os trigonum; however, the os supracalcaneum is not in direct apposition to the posterior talar process. The os supracalcaneum, when present, is large in size and not easily missed in the lateral view. Occasionally, it is found additionally with an os trigonum just anterior to it.
An accessory ossicle is infrequently seen along the anterior aspect of the ankle joint in the lateral view. This ossicle is known as the os talotibiale. It is found in close apposition to the most anterior and inferior aspect of the tibia. It can easily be mistaken as a loose osseous body. However, the latter entity is usually associated with other degenerative findings. Rarely, an ossicle might present along the posterior aspect of the ankle joint. This unnamed ossicle, like the os talotibiale, is closely apposed to the tibia.
The os subtibiale is an ossicle found just inferior to the tibial malleolus. It varies considerably in size and shape, and can be mistaken for the following conditions: unfused malleolar accessory ossification center, loose osseous body, acute or nonunion avulsion fracture, and deltoid ligament ossification. It is directly apposed to the posterior colliculus of the tibial malleolus. In contrast, malleolar avulsion fractures involve the anterior colliculus. This relationship is best appreciated in the lateral view. The os subtibiale and avulsion fracture may be impossible to differentiate with the anteroposterior and mortise ankle views alone.
An accessory ossicle found beneath the fibular malleolus is known as the os subfibulare. This can easily be misinterpreted as an avulsion fracture. Avulsion fractures of the fibular malleolus are usually ill-defined and irregular. However, old, nonunion avulsion fractures may be identical in appearance to the accessory ossicle. The os subfibulare is seen in both the AP and mortise ankle views. It is not readily visible in the lateral view, but can appear as a fairly well-defined increased density superimposed on the talus.
A large accessory ossification center for the fibular malleolus may be encountered in the same location as the os subfibulare. It has been shown to be directly related to ankle joint instability due to insertion of the calcaneofibular and/or anterior talocalcaneal ligaments and abnormal movement between the accessory ossification center and distal fibula.
© Copyright 1998, Robert A. Christman, D.P.M.
These articles and figures may not be published, reposted, or redistributed without permission from Dr. Christman.
This page was updated May 5, 1998.