Received: 21 October 2002Revised: 12 May 2003Accepted: 2 July 2003Published online: 6 August 2003© Springer-Verlag 2003

Abstract Accessory ossicles andsesamoid bones are frequent find-ings in routine radiographs of theankle and foot. They are commonlyconsidered fortuitous and unrelatedto the patient’s complaint; however,they may eventually cause painfulsyndromes or degenerative changesin response to overuse and trauma.They may also suffer or simulatefractures. Our aim was to review, illustrate and discuss the imagingfindings of some of the more fre-quent accessory ossicles and sesa-moid bones of the ankle and foot region, with particular emphasis on those that may be of clinical significance or simulate fractures.

Keywords Ankle · Foot · Accessoryossicles · Sesamoid bones · Normalvariant · Computed tomography ·Magnetic resonance

Eur Radiol (2003) 13:L164–L177DOI 10.1007/s00330-003-2011-8 M U S C U L O S K E L E TA L

J. M. MelladoA. RamosE. SalvadóA. CaminsM. DanúsA. Saurí

Accessory ossicles and sesamoid bones of the ankle and foot: imaging findings, clinical significance and differential diagnosis

Introduction

Many skeletal variations of the ankle and foot may befound, including different accessory ossicles and sesa-moid bones, bipartitions and coalitions [1]. They may bebilateral, or coexist in a single extremity, and they areprone to considerable variation; most are developmentalabnormalities and are generally considered incidental ra-diographic findings [2].

Accessory ossicles commonly derive from unfused ac-cessory ossification centers. They may appear to be normalsubdivisions of ordinary bones or nearby additional free el-ements [1]. The most common accessory ossicles of theankle and foot are the os trigonum, the accessory navicularand the os intermetatarseum. There are also other less fre-quent accessory bones, e.g. the os sustentaculi, the os su-pranaviculare, the os vesalianum, the os calcaneus secun-darius, the os subtibiale and the os subfibulare [1, 2, 3].

Sesamoid bones are partially or totally embedded inthe substance of a corresponding tendon. Anatomicallythey are part of a gliding mechanism that reduces frictionand protects the tendon [1]. Various sesamoid bones maybe found in the ankle and foot, e.g. the os peroneus,within the peroneus longus tendon, and the hallux se-samoids, within the slips of the flexor hallucis brevistendon at the level of the first metatarsal head [1, 2, 3].

Most accessory ossicles and sesamoid bones of theankle and foot remain asymptomatic; however, theyhave increasingly been examined in the radiology litera-ture, because they can cause painful syndromes or de-generative changes in response to overuse and trauma.They may also suffer or simulate fractures and restrictthe range of motion [4, 5, 6]. They have traditionallybeen evaluated by means of conventional radiography orscintigraphy [2, 3, 4]. More recently, CT and MR imag-ing have added to the understanding of their clinical rel-

J. M. Mellado (✉) · A. RamosE. Salvadó · A. Camins · A. SauríInstitut de Diagnòstic per la Imatge,Hospital Universitari de Tarragona Joan XXIII,Carrer Doctor Mallafrè Guasch 4, 43007 Tarragona, Spaine-mail: rmidi@hjxxiii.scs.esTel.: +34-97-7250690Fax: +34-97-7250691

M. DanúsDepartment of Nuclear Medicine,Hospital Universitari de Tarragona Joan XXIII,Carrer Doctor Mallafrè Guasch 4, 43007 Tarragona, Spain

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evance and helped to distinguish them from fractures [5, 6].

Our purpose is to review, illustrate and discuss theimaging findings of some of the most frequent accessoryossicles and sesamoid bones of the ankle and foot region,with particular emphasis on those that may be of clinicalsignificance or simulate fractures (Table 1).

Os trigonum

The os trigonum is one of the largest and most commonaccessory ossicles in the ankle and foot region, with anestimated prevalence of 1–25% [1, 3, 4]. It is connectedto the lateral tubercle of the posterior process of the talusby a fibrocartilaginous synchondrosis. Although initiallyinterpreted as an old non-united fracture, it is currentlyviewed as a developmental skeletal variation, most likelyderiving from the failure of a secondary ossification cen-ter to fuse [1, 2, 3, 4, 5, 6, 7, 8, 9].

The os trigonum commonly presents as an incidentalradiographic finding (Fig. 1a) [2, 3]. The CT scans andMR imaging may detect associated bone and soft tissue

abnormalities and thus complement each other in evalu-ating its potential clinical significance [5, 6]. Single orrepetitive forced plantar flexion of the foot may precipi-tate degeneration (Fig. 1b) or tear (Fig. 1c) of the syn-chondrosis. A large os trigonum may associate with flex-or hallucis longus tenosynovitis (Fig. 1d, e) or entrap-ment. Repetitive impingement of the os trigonum againstthe adjacent soft tissues may generate pain and swellingin the posterior aspect of the ankle, leading to os trigo-num syndrome, one of the subtypes of posterior ankleimpingement syndrome. On such clinical setting, thecombination of bone marrow oedema, flexor hallucislongus tenosynovitis, joint effusion and increased tech-netium-99m uptake supports the diagnosis of os trigo-num syndrome (Fig. 1f–h) [4, 5, 6, 7, 8, 9, 10].

The os trigonum may be radiographically confusedwith fractures of the lateral or medial tubercles of the pos-terior process of the talus, the so-called Shepherd’s or Ce-dell’s fractures [11, 12]. These fractures are the result ofacute impingement between the posterior lip of the tibiaand the calcaneus on extreme flexion of the ankle. The pa-tient usually complains of pain in the posterior aspect ofthe ankle and tenderness posterior to the lateral mall-

Table 1 Accessory ossicles and sesamoid bones of the ankle and foot region

Accessory/sesamoid bone Prevalence (%) Clinical significance Differential diagnosis

Os trigonum 1–25 [1, 3, 4] Synchondrotic degeneration or tear Shepherd’s fracture [7, 9, 11]. [4, 5, 6, 7, 8, 9]. Posterior ankle Cedell’s fracture [12]. impingement syndrome [4, 5, 6, 7, 8, 9]. Pseudoarthrosis [7, 9]Flexor hallucis longus tendon entrapment [10]

Type-II accessory navicular 2–12 [1, 6] Synchondrotic degeneration or tear Type-I accessory navicular [1]. [4, 5, 6, 13, 14, 15]. Posterior tibial tendon Navicular tuberosity avulsion dysfunction or tear [4, 5, 6, 13, 14, 15, 16] fracture [11]

Os sustentaculi 0.3–0.4 [1, 3] Synchondrotic degeneration [19]. Isolated fracture Painful syndrome [18, 19] of the sustentaculum tali [11]

Os intermetatarseum 1.2–10 [1, 3] Painful syndrome [4, 6, 17, 21]. Lisfranc fracture dislocation [11]Hallux valgus [4, 21]

Os supranaviculare 1 [3] Painful syndrome [22] Cortical avulsion fracture of the navicular or talar head [11]

Os vesalianum 0.1 [3] Painful syndrome [17, 23] Avulsion fracture at the base of the fifth metatarsal [1, 11]

Os calcaneus secundarius 0.6–7 [1, 3] None [2] Avulsion fracture of the anterosuperior calcanealprocess [11, 23, 24]

Os subtibiale 0.9 [3] None [26] Medial malleollus avulsion fracture [11, 26]

Os subfibulare 2.1 [3] Painful syndrome [4, 11, 26, 27]a Lateral malleolus avulsion fracture [11, 26, 27]

Os peroneum 9 [3] Painful os peroneum syndrome [6, 28]. Painful os vesalianum. Fracture, diastasis [6, 28] Bipartite os peroneum [6, 28]

Hallux sesamoid bones Close to 100 [1] Fracture, stress fracture, diastasis [6, 30]. Bipartite tibial sesamoid Chondromalacia, osteonecrosis, infection [6, 30] [1, 2, 6, 30]. Soft tissue

derangements of the forefoot [6, 30]

a True os subfibulare has no clinical significance; however, old non-united avulsion fractures of the lateral malleolus may present with apainful syndrome, causing the so-called painful os subfibulare

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fragments (Fig. 2) [7, 11]. Incomplete healing may lead topainful pseudoarthrosis [9]. If pain persists and there areno signs of healing, the fragment should be removed.

The fusion of the above-mentioned secondary ossifi-cation center gives rise to the lateral tubercle of the pos-

Fig. 1a–h Os trigonum in five different patients. a A 50-year-oldwoman examined for Morton’s neuroma. Lateral ankle radiographshows incidental os trigonum (arrow). b A 45-year-old man suf-fering from chronic tenderness and pain on his left ankle joint.The CT section shows a large os trigonum (arrow). Subchondralcysts adjacent to the synchondrosis (arrowheads) are seen, reflect-ing chondro-osseous degenerative changes. c A 21-year-old manpresenting with right flat-foot deformity and avascular necrosis onthe tarsal navicular. Sagittal short tau inversion recovery (STIR)image shows a fluid-filled interface (large arrowhead) betweenthe os trigonum (arrow) and the talus, reflecting disruption of thefibrocartilaginous synchondrosis. Collapse of the necrotic tarsalnavicular (small arrowheads) is also noted. d, e A 66-year-oldman suffering chronic inflammatory arthritis in his left ankle joint.d Sagittal T2-weighted MR image shows a large os trigonum (ar-row). Significant distention of the flexor hallucis longus tendon

sheath is also noted (arrowheads). e Axial T2-weighted MR imagein the same patient shows a fluid-filled flexor hallucis longus ten-don sheath (arrowheads) and an extremely thin tendon (arrow),consistent with chronic FHL tenosynovitis. The real involvementof the os trigonum in the clinical syndrome and its relationshipwith the MR findings remained unproved, but a loose associationcould not finally be excluded. f–h A 20-year-old man presentingwith tenderness in the posterior aspect of his right ankle. f SagittalT1-weighted image and g sagittal STIR image show an os trigo-num (arrow). Mild bone marrow oedema is found on both sides ofthe synchondrosis. A small adjacent effusion (arrowheads) is alsonoted. h Lateral view of a 99mTc-MDP bone scan in the same pa-tient reveals increased activity in the posterior aspect of the ankle(arrow). Increased uptake is also seen in the anterior aspect of theankle joint (arrowhead). The clinical and radiological backgrounddescribed above is consistent with os trigonum syndrome

eollus. Radiographically, the fragment is rarely displaced,and it may be difficult to distinguish it from true os trigo-num. The CT scans and MR imaging may help to identifytrue fractures by detecting oblique irregular interfaces thatseparate incompletely corticated, eventually comminuted

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terior talar process, also named trigonal or Stieda pro-cess. A particularly prominent Stieda process may alsocause posterior ankle impingement syndrome whenforceful or repetitive plantar flexion of the foot occurs[9].

Accessory navicular

The accessory navicular, also known as os tibiale, os tib-iale externum and naviculare secundarium, is adjacent tothe posteromedial tuberosity of the navicular bone in4–21% of individuals [1, 2, 6]. Three types of accessorynavicular bones have been described.

The type-II accessory navicular is a persistent acces-sory ossification center. It is bridged by a fibrocartilagi-nous synchondrosis to the navicular bone, and serves asthe point of attachment for the posterior tibial tendon. Itis one of the most common accessory bones in the ankleand foot, with an estimated prevalence of 2–12% [1, 2, 3,

4, 5, 6]. Type-II accessory navicular bones are a com-mon, fortuitous radiographic finding (Fig. 3a, b) [2, 3];however, their occasional clinical relevance has beenemphasized in the radiology literature [4, 5, 6, 13, 14,15, 16, 17]. When such a bone is present, the distal por-tion of the posterior tibial tendon may straighten, thuscausing adduction forces. These altered biomechanicsmay lead to flat-foot deformity [5, 16, 17]. They mayalso cause tendon impingement on dorsiflexion of theankle, which may be followed by tendon attrition or tear(Fig. 3c, d) [4, 5, 16, 17]. Repetitive shearing stress forc-es acting on a type-II accessory navicular may disrupt itssynchondrosis, which may be followed by flat-foot de-formity [14]. Despite the well-documented association oftype-II accessory navicular bones with posterior tibialtendon dysfunction and flat-foot deformity, most patientsremain asymptomatic; however, the prevalence of type-IIaccessory navicular bones in patients with posterior tibi-al tendon dysfunction is significantly higher than in thegeneral population [16].

Occasionally, a type-II accessory navicular may be-come painful, and cause the so-called symptomatic ac-cessory navicular. This clinicoradiological entity is be-lieved to be caused by osteonecrosis of the accessory navicular. It presents more commonly in middle-agedwomen and causes pain and tenderness in the medial aspect of the foot. The MR imaging may be particularlyuseful in such clinical settings because it can demon-strate a pattern of bone marrow oedema on the accessorynavicular (Fig. 3e) [4, 5, 6, 15, 17].

Type-II accessory navicular bones should be distin-guished from type-I sesamoid bones within the distalsegment of the posterior tibial tendon (Fig. 3f, g) [1]. Al-though it has been suggested that they are involved inthe pathogenesis of flat-foot deformity, this has not beensufficiently proved [17]. In addition, type-II accessorynavicular bones should be differentiated from avulsionfractures of the navicular tuberosity [11]. Fractures ofthe navicular tuberosity are the result of acute eversionof the foot and increased tension of the posterior tibialtendon. They are best seen on anteroposterior andoblique lateral radiographs, and commonly present withlocal tenderness associated with pain on passive eversionor active inversion. Avulsion fractures of the naviculartuberosity may associate with impaction fractures of thecuboid bone, which has been termed “the nutcrackerfracture”. For radiographic distinction, it should be re-membered that the type-II accessory navicular is usuallylarger and well-corticated, commonly presents bilateral-ly, and the line of separation from the adjacent bone isgenerally smoother than in a true avulsion fracture [11].Occasionally, detachment of a type-II accessory navicu-lar may clinically and radiographycally simulate an avul-sion fracture [14].

The accessory navicular may incorporate to the navic-ular tuberosity, thus forming the so-called type-III acces-

Fig. 2a, b A 28-year-old man presenting with recent hyperflexioninjury in his left ankle and foot. He mentioned diffuse ankle pain.a Sagittal T1-weighted and b STIR images show an oblique,slightly irregular and partially intraarticular fracture line (arrow-heads) involving the lateral tubercle of the posterior process of thetalus (arrow). A pattern of conspicuous oedema is noted on the ad-jacent bone marrow, reflecting the subacute nature of the process

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Fig. 4 Cornuate navicular in a 25-year-old man referred for medi-al malleollus fracture. Axial T1-weighted MR image shows aprominent navicular tuberosity (arrow), consistent with type-IIIaccessory navicular, also known as cornuate navicular

Fig. 5a–d Painful os sustentaculi in a 49-year-old man (from [19]).a Lateral radiograph shows an os sustentaculi (arrow) located posteri-or to the sustentaculum tali (asterisk). A faint radiolucent interface isseen in-between (arrowhead), reflecting the existence of a fibrocarti-laginous synchondrosis. b Oblique coronal CT scan shows an os sus-tentaculi (arrow) united to the sustentaculum tali (star) and articulat-ing with an elongated medial tubercle of the talus (asterisk). A nar-row and irregular interface in-between the three bones is seen, alongwith subchondral cyst formation and hypertrophic margins. c AxialT2-weighted MR image shows mild bone marrow oedema on the ossustentaculi (arrow). d Sagittal STIR image shows subchondral bonemarrow oedema adjacent to the synchondrosis between the os sus-tentaculi and the sustentaculum tali (arrows). The articulation be-tween the os sustentaculi and the talar body (arrowhead) is also seen

Fig. 3a–g Accessory navicular bones in five different patients.a, b A 43-year-old-woman presenting with acquired flat-foot de-formity. a Dorsoplantar and b lateral radiographs show a type-IIaccessory navicular (arrow) bridged to the posteromedial aspect ofthe navicular by a fibrocartilaginous synchondrosis, representedby a narrow radiolucent interface (arrowheads). c, d A 62-year-old woman presenting with painful acquired flat-foot deformityand type-II accessory navicular. The MR images show a c type-IIaccessory navicular (arrow) and a d interstitial tear of the posteri-or tibial tendon (arrow). A bony spur sharpening the medial aspectof the tendon groove (arrowhead) is also seen. e A 51-year-oldwoman presenting with pain and tenderness in the medial aspectof her left foot. Axial STIR image reveals a type-II accessory na-vicular (arrow). Obvious oedematous changes are noted on bonemarrow and adjacent soft tissues, consistent with symptomatic accessory navicular. f Dorsoplantar radiograph and g axial T1-weighted MR image reveal incidental type-I accessory navicularbones (arrows) in two different patients presenting with a historyof ankle sprain. On the g MR image the sesamoid bone is seenwithin the distal portion of the posterior tibial tendon (small ar-rowheads). The distal portion of the spring ligament (large arrow-head) is also seen

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Fig. 6 Os intermetatarseum in a 59-year-old woman presentingwith hallux valgus deformity and forefoot pain. Dorsoplantar ra-diograph shows a small os intermetatarseum (arrowhead). Associ-ated hallux valgus deformity is also noted (thick arrow). The realinvolvement of the os intermetatarseum in the painful syndromeremained undetermined. Incidental bipartite tibial hallux sesamoidbone is seen (thin arrow)

sory navicular, also known as the cornuate navicular(Fig. 4). The cornuate navicular may occasionally asso-ciate with painful conditions, adventitial bursa formationor flat-foot deformity [5, 6, 16].

Os sustentaculi

The os sustentaculi is a small accessory ossicle connect-ed to the posterior aspect of the sustentaculum tali by afibrocartilaginous synchondrosis. It is a very rare skele-tal variant of the ankle and foot region, with an estimatedprevalence of 0.3–0.4%. The os sustentaculi is generallyconsidered an uncommon fortuitous radiographic finding(Fig. 5a) [1, 2, 3, 18].

The os sustentaculi may also become painful whenchronic shearing stress forces lead to early degenerativechanges across its synchondrosis. In such cases, CTscans accurately show signs of chondro-osseous degen-eration, including cortical hypertrophy and irregularity,subchondral cyst formation and vacuum phenomenon(Fig. 5b). Complementary MR imaging may improve un-derstanding of its clinical relevance because it revealssubchondral bone marrow oedema adjacent to the syn-chondrosis (Fig. 5c, d) [19]. Similar findings have beendescribed in painful os trigonum or symptomatic acces-sory navicular.

The os sustentaculi should not be confused with rareisolated fractures of the sustentaculum tali, which occurwhen the foot is supinated at the moment of impact.Their radiographic detection may be challenging, and ad-ditional oblique projections or tunnel views may beneeded. Fractures of the sustentaculum tali cause painand tenderness along the inner border of the foot. In clin-ical practice, fractures of the sustentaculum tali are morecommonly seen in severely comminuted intraarticularcalcaneal fractures. The absence of complete cortication,the slightly irregular interface of the fracture, and thehistory of trauma should allow ambiguous cases to becorrectly diagnosed [11]. The diagnosis of isolated frac-tures of the sustentaculum tali should not be overlooked,as some of them may require screw fixation.

The os sustentaculi may fuse to the calcaneus, thusoriginating the so-called assimilated os sustentaculi,which may also associate with early degenerative chang-es and painful conditions [20].

Os intermetatarseum

The os intermetatarseum is found between the medial cu-neiform and the base of the first and second metatarsals. Itis one of the most common accessory ossicles of the foot,with an estimated prevalence of 1.2–10%. The os interm-etatarseum may be round or spindle shaped. It may befound as an independent ossicle, articulating by a synovial

joint, or fused with any of the adjacent bones to form anexostosis-like process. Nevertheless, its aetiology and realprevalence remain controversial [1, 2, 3, 4, 6, 21].

The os intermetatarseum is a common incidental ra-diographic finding. Occasionally, it may suffer fracturesor be involved in painful conditions [4, 6, 17, 21]. Thesymptomatic os intermetatarseum causes pain and ten-derness when the dorsum of the midfoot is palpated atthe level of the first intermetatarsal space, because super-ficial and deep peroneal nerves are compressed [6]. Inthese cases, radiotracer uptake may increase [6]. It hasbeen suggested that the os intermetatarseum may asso-ciate with hallux valgus deformity (Fig. 6) [4, 21].

The os intermetatarseum should not be confused withsmall fractures involving the base of the second metatar-sal bone, which may occur in Lisfranc fracture disloca-tions. Traumatic injuries of the Lisfranc joint complexare found after violent forced plantar flexion and rota-tion, most commonly during motor vehicle accidents.Lisfranc fracture dislocations are often associated withsoft tissue injuries, and may be generally identified onconventional radiographs when malalignment, fractureor soft tissue swelling are seen [11].

Os supranaviculare

The os supranaviculare, also known as the ostalonaviculare dorsale, talonavicular ossicle or Pirie’s

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bone, is found on the dorsal aspect of the talonavicularjoint, close to the midpoint. It has an estimated preva-lence of 1% and is a rare incidental skeletal variant [1, 2,3]. The os supranaviculare is easily detected on lateralankle radiographs (Fig. 7a) or sagittal MR images(Fig. 7b). Eventually, the os supranaviculare may fusewith the rest of the tarsal navicular to form a bony spurof no clinical significance (Fig. 7c). The os supranavi-culare may rarely become symptomatic [22], thus requir-ing radiographic survey and specific clinical assessment.In such cases, surgical resection may be required.

The os supranaviculare should not be confused withcortical avulsion fractures of the tarsal navicular [11].These fractures occur as the result of a twisting force ap-

Fig. 7a–c Os supranaviculare in two different patients. a A 27-year-old man with chronic posterior tibial tendon tenosynovitis(not shown). The lateral radiograph shows an incidental suprana-vicular bone (arrow) located in the dorsal aspect of the talonavicu-lar joint. b Sagittal T1-weighted MR image in the same patientdemonstrates the round and well-corticated appearance of the ac-cessory ossicle (arrow), unrelated to the patient’s disease. c Later-al ankle radiograph in a 50-year-old man suffering subtalar arthri-tis (not shown) reveals a small pointed os supranaviculare (arrow)fused to the navicular. A small adjacent radiolucency in the dorsalaspect of the navicular bone (arrowhead) simulates an avulsionsite

plied to the foot leading to injuries of the talonavicularcapsule. This type of avulsion fractures most commonlyoccur in middle-aged women. The injury commonly as-sociates with the wearing of high-heeled shoes. Avulsionfractures of the dorsal margin of the talar head may asso-ciate. In order to distinguish avulsion fractures of thedorsum of the navicular and true os supranaviculare, theclinical background should be borne in mind. From theradiological standpoint, the avulsion fracture of the dor-sum of the navicular is commonly represented by a thinflake of bone. Associated soft tissue swelling and lack ofcomplete cortication support the diagnosis of avulsioninjury. Nevertheless, radiographic distinction alone maybe extremely challenging.

Os vesalianum

The os vesalianum is a small accessory ossicle adjacentto the tip of a well-developed tuberosity of the fifthmetatarsal. It is a very rare accessory bone, with an esti-mated prevalence of 0.1% [3]. The original descriptionof the os vesalianum referred to a small bone near the lit-tle toe and probably articulating with the cuboid. Al-though commonly incidental, it has very occasionallybeen reported its association with painful syndromes ofthe forefoot [17, 23].

The os vesalianum should be differentiated from thenormal ossification centre of the tuberosity of the fifthmetatarsal, which is parallel to the metatarsal shaft [1]. Itshould also be distinguished from avulsion fractures ofthe apophysis of the fifth metatarsal, which usually lie ina transverse plane [11]. Avulsion fractures of the base ofthe fifth metatarsal bone involve the insertion site of theperoneus brevis tendon. They are usually associated withinversion injuries and sprains and may be easily over-looked [11].

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row oedema in MR images suggests the existence of ahealed fracture (Fig. 9d, e).

Os subtibiale

Accessory centres of ossification of the medial mall-eollus may appear between 7 and 10 years of age, al-though occasionally they may persist into adulthood,forming the so-called os subtibiale. The true os subtibi-ale is present in only 0.9% of adults. It may be multipleand usually remains asymptomatic [1, 2, 3, 26]. The ossubtibiale should be distinguished from a persistent sec-ondary ossification centre, which usually appears as thenormal subdivision of a completely developed and nor-mally shaped medial malleollus.

More importantly, the os subtibiale should be distin-guished from avulsion fractures of the medial malleollus(Fig. 10), which are not uncommon in the clinical settingof ankle trauma and derive from pronation and externalrotation. When these forces are strong enough, avulsionfracture of the medial malleollus may be followed by an-terior tibiofibular ligament tear and fibular fracture. Be-cause the os subtibiale may be found during the radio-graphic evaluation of ankle trauma, it may be erroneous-ly interpreted as a fracture. It has been suggested that thesmooth corticated margins on both sides of the radiolu-cent cleft make it possible to diagnose os subtibiale cor-rectly; however, an acutely symptomatic patient with anaccessory ossification centre in the medial malleollusshould be considered and treated as having a fracture. Anos subtibiale may also cause chronic symptoms. Themost likely source of symptoms is chondro-osseous dis-ruption, which results in a fracture that may heal with a

Fig. 8a, b Os calcaneus secun-darius in a 40-year-old womanwith a painful right forefoot.a Oblique radiograph incom-pletely reveals an os calcaneussecundarius (arrows). A slight-ly irregular radiolucent cleft(arrowhead) is seen betweenthe accessory bone and the ad-jacent anterosuperior calcanealprocess. b Axial T2-weightedMR image demonstrates theround and well-corticated ap-pearance of the os calcaneussecundarius (arrows). The inci-dental accessory bone was con-sidered unrelated to the pa-tient’s complaint

Os calcaneus secundarius

The os calcaneus secundarius is located in the intervalbetween the anteromedial aspect of the calcaneus, thecuboid, the talar head and the tarsal navicular. It may beround, although it is more often triangular. It is a rare ac-cessory ossicle of the foot, with an estimated prevalenceof 0.6–7% [1, 2, 3]. The os calcaneus secundarius has noclinical significance and may be incidentally found onroutine radiographs (Fig. 8a) or MR studies (Fig. 8b).

It has been suggested that most os calcaneus secunda-rius derive from recent or remote fractures of the antero-superior calcaneal process [24]. These fractures are avul-sion injuries of the bifurcate ligament secondary to in-version and forced plantar flexion. More rarely, they maybe caused by eversion injury on a dorsiflexed foot. Frac-tures of the anterosuperior calcaneal process are consid-ered to be rare but can be easily overlooked on conven-tional radiographs. Special projections, including medialand lateral oblique views, have been found useful in thisregard. Early diagnosis and conservative treatment aresaid to be crucial if unnecessary surgery is to be avoided.Anterosuperior process fractures may clinically mimicinjuries of the lateral collateral ligament, fractures of thelateral process of the talus, or fractures of the fifth meta-tarsal. The point of maximum tenderness in anterosuperi-or calcaneal process fractures is 1 cm inferior and 3 cmanterior to the talofibular ligament [24, 25].

When clinical and radiographic manifestations are un-clear, MR imaging may be used (Fig. 9a–c). On MR im-aging, the presence of bone marrow oedema adjacent tothe fracture line (Fig. 9b, c) has been found most usefulfor confirming clinical and radiographic suspicion oftraumatic lesion [24]. As a general rule, an ovoid, small,and well-corticated appearance favours the diagnosis ofaccessory ossicle (Fig. 8b). Conversely, larger shapeswith a wider proximal base and an altered signal on theadjacent bone marrow as seen on MR images suggestfracture (Fig. 9b, c) [24, 25]. The absence of bone mar-

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fibrous union or may form a pseudoarthrosis. In bothacute and chronic conditions, mechanical irritation is be-lieved to be the source of local pain and tenderness. Inthese cases, a technetium bone scan may support the di-

agnosis of traumatic injury when uptake is increased[26].

Os subfibulare

The os subfibulare is an accessory bone located underthe tip of the lateral malleollus (Fig. 11). The ossiclemay be round or comma shaped. Radiographic studieshave found an os subfibulare in 2.1% of individuals. Theos subfibulare represents the persistence of an accessoryossification centre, which should not be confused withthe more common persistent secondary centre [1, 2, 3,27].

Given the relative rarity of the os subfibulare, it hasbeen suggested that most ossicles adjacent to the lateralmalleollus are old non-united avulsion fractures(Fig. 12). Avulsion fractures of the lateral malleollus arecommonly caused by inversion forces in the clinical set-ting of ankle sprains. Avulsion fractures of the lateralmalleollus course with swelling, tenderness, painfulweight bearing, painful range of motion and ankle effu-

Fig. 9a–e Healing fracture and chronic healed fracture of the an-terosuperior calcaneal process in two different patients. a–c A 54-year-old man with history of inversion and plantar flexion injury.a Sagittal T2-weighted and b STIR image reveal a slightly irregu-lar interface (arrowhead) traversing the base of the anterosuperiorcalcaneal process (short arrow). A pattern of bone marrow oede-ma (long arrow) is seen on the STIR image, supporting the diag-nosis of healing fracture. c Axial T1-weighted MR image in thesame patient better reveals the broad-based triangular configura-tion of the fractured fragment (short arrow) outlined by a hypoin-tense line consistent with progressing union (arrowheads), withmild hypointensity on adjacent bone marrow (long arrow) whichreflects bone marrow oedema. d–e A 28-year-old professionalsoccer player presenting with non-specific ankle pain. d The later-al radiograph reveals a faint radiolucency (arrowhead) at the baseof an otherwise normal-shaped anterosuperior calcaneal process(arrow). e The corresponding sagittal T1-weighted MR image re-veals a broad band of hypointensity (arrowheads) traversing thebase of the anterosuperior calcaneal process (arrow). The findingswere thought to represent completed healed fracture, unrelated topatient’s complaint

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Fig. 10a, b Avulsion fracture of the medial malleollus in two dif-ferent patients, to be differentiated from os subtibiale. a A 29-year-old man presenting with vague heel pain after eversion inju-ry. Anteroposterior radiograph reveals a small ossicle (arrow) ad-jacent to the medial malleollus. The irregular contour of the radio-lucent interface in-between (arrowhead) and the clinical settingsupports the diagnosis of avulsion fracture of the medial mall-eollus. b A 59-year-old man with history of repetitive ankle

sprains. Coronal proton-density-weighted MR image reveals mildamputation of the medial malleollus (black arrowhead). A smallirregular ossicle is found in its vicinity (thick arrow), consistentwith chronic displaced avulsion fracture. The deep fascicle of thedeltoid ligament (white arrowhead) appears mildly heterogeneous.Chronic avulsion fracture of the lateral malleollus (thin arrow) isalso present

Fig. 11 Os subfibulare in a 34-year-old man who was referred sothat osteochondritis dissecans of the talar dome could be ruled out.Anteroposterior radiograph reveals a small, round and well-mar-ginated ossicle (arrow), adjacent to the lateral malleollus, consis-tent with os subfibulare

Fig. 12 Avulsion fracture of the lateral malleollus in a 20-year-oldman with a history of ankle sprain and instability. Sagittal T1-weighted MR image reveals an anteriorly displaced avulsed bonyfragment (white arrow) at the tip of the lateral malleollus. Corticalirregularity is noted at the avulsion site (black arrow). An adjacentsoft tissue lesion is also noted (arrowheads), consistent with repar-ative granulation tissue

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sion. Ankle instability may also be present. In fact, avul-sion fractures of the lateral malleollus may be associatedwith laxity of the anterior talofibular ligament, givingrise to the so-called symptomatic os subfibulare. Individ-uals with avulsion fracture of the lateral malleollus aregenerally older than patients with interstitial tear of theanterior talofibular ligament. The avulsed bony fragmentmay have well-defined cortical margins, thus makingdistinction from os subfibulare difficult [4, 27, 28]. Fromthe clinical and radiographical standpoint, the use of ta-lar-tilt stress projections may reveal ankle instability,which does not generally associate with true os subfibul-are, and therefore contribute to correct management [28].

Os peroneum

The os peroneum is a round or oval-shaped sesamoidbone imbedded within the peroneus longus tendon. It islocated near the calcaneocuboid joint, and may be bipar-tite or multipartite. The os peroneum is probably alwayspresent in a cartilaginous, fibrocartilaginous or ossifiedstage, although its reported radiographic prevalence isonly 9%. The os peroneum is best evaluated in theoblique-lateral view of the foot [1, 2, 3, 6, 28].

The os peroneum is a common and generally fortu-itous radiographic finding (Fig. 13); however, its poten-tial ability for precipitating painful conditions has beenwell documented. The so-called painful os peroneumsyndrome causes lateral pain, tenderness and swellingalong the course of the peroneus longus tendon, and lat-eral pain with resisted plantar flexion of the foot [6, 28].Pain may radiate proximally along the peroneus longusmuscle. The clinical syndrome may be acute or chronic,and is caused by os peroneum fracture, hypertrophichealing of a fractured os peroneum, diastasis of a bipar-tite os peroneum, attrition or tear of the peroneus longustendon proximal or distal to the sesamoid bone, or en-larged peroneal tubercle [6, 28]. Acute onset is lesscommon and usually requires emergency care, due to ei-ther frank rupture of the peroneus longus tendon or frac-ture of an os peroneum, including diastasis of multipart-ite os peroneum. Chronic presentation may mimic asprained ankle, and complaints may be intermittent andnon-specific. The chronic syndrome is associated withattrition of the peroneus longus tendon, diastasis of amultipartite os peroneum or healing of an os peroneumfracture [28].

Tears in the peroneus longus tendon at the midfoot aresaid to represent a diagnostic dilemma for the clinician[29]. It has been suggested that the os peroneum may bea useful radiographic marker of peroneus longus tendontears. When such tears occur, the radiographic study mayshow the displacement or migration of the os peroneum[17, 28]; however, care should be taken not to overem-phasize the radiographic importance and clinical signifi-

cance of the os peroneum, which in most cases is inci-dental. In fact, none of the patients in a 9-patient seriesof peroneus longus tendon tears diagnosed with MR im-aging were found to have an os peroneum [29].

Finally, the os peroneum should be distinguishedfrom os vesalianum and avulsion fractures of the fifthmetatarsal, which usually lie slightly more distal.

Hallux sesamoid bones

The medial and lateral hallux sesamoids are embeddedwithin the medial and lateral slips of the flexor hallucisbrevis tendon at the level of the first metatarsal head.The purpose of the hallux sesamoid bones is to providemechanical advantage during hallux flexion, by reducingfriction and reinforcing adjacent soft tissues [1, 6, 30].The size and shape of the hallux sesamoids vary consid-erably. The tibial sesamoid tends to present a bipartitemorphology (Figs. 6, 14a) and is more common than itslateral counterpart [6]: it can be found in as much as33.5% of the population [30].

The pathological conditions of the hallux sesamoidcomplex include traumatic, degenerative, inflammatory,infectious, and ischaemic processes. Most painful condi-tions involving the hallux sesamoid bones are, however,related to acute trauma or chronic stress.

From the radiological standpoint, distinguishing be-tween bipartite sesamoid and true fracture of a tibial hal-lux sesamoid can be particularly challenging, and severalsuggestions have been made as to how to do so efficient-ly (Fig. 14a). The single medial sesamoid with a fractureis slightly larger than the lateral sesamoid, whereas thebipartite sesamoid is much larger. In addition, a fracturetends to show a sharp, radiolucent, uncorticated line,

Fig. 13 Incidental os peroneum in a 32-year-old patient with ahistory of repetitive ankle sprains. The oblique radiograph revealsa small os peroneum (arrow) adjacent to the calcaneocuboid joint

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whereas the bipartite sesamoid has two corticated frag-ments. The two fragments in a fractured sesamoid oftenfit together well, whereas the two components of a bipar-tite sesamoid do not. Comparison with the opposite footand serial radiographic survey have also been proposedas ways of improving lesion characterization. In somecases, technetium bone scans and MR imaging may beused for improving diagnostic accuracy. A 99mTc-MDPbone scan should be normal in a bipartite hallux but willshow increased uptake in a fractured bipartite. The MRimaging may also show bone marrow oedema in a re-cently fractured sesamoid [6, 30].

Chronic stress may also cause painful conditions inthe hallux sesamoids, and is most commonly associatedwith chondromalacia, osteochondritis, osteonecrosis andstress fracture. All these entities are part of the samepathologic spectrum, share a common aetiological factorand present clinically with a painful syndrome which hasbeen termed sesamoiditis. The MR imaging (Fig. 14b, c)has been advocated as a useful procedure for assessingsuch lesions. The MR imaging has been described as be-ing particularly valuable for detecting abnormalities inbone marrow signal and associated soft tissue derange-ments in this particular region [30]. In some cases, a

Fig. 14a–d Painful hallux sesa-moid bones in four different pa-tients. a Dorsoplantar radio-graph in a 47-year-old womanpresenting with long-standingsesamoiditis in her right foot. A bipartite medial sesamoid ispresent (arrowhead). Mild cys-tic changes are found adjacentto the synchondrosis betweenthe small (long arrow) and large(short arrow) components, con-sistent with synchondrotic de-generation. b A 62-year-old-woman referred for evaluationof calcaneal spur and Morton’sneuroma. Coronal T1-weightedMR image of her left foot showsosteoarthritic changes in the lat-eral aspect of the hallux sesa-moid complex, including jointspace narrowing and altered sig-nal intensity of the subchondralbone marrow, consistent withchondromalacia (arrowheads).Ill-defined thickening of theflexor tendons of the second toeis also noted (arrow), consistentwith tendinopathy. c A 48-year-old woman presenting with aclinical diagnosis of sesamoidi-tis on her left foot. Coronal T1-weighted MR image reveals het-erogeneous replacement of nor-mal bone marrow on the tibialhallux sesamoid (arrowheads),consistent with bone marrowoedema. d A 50-year-old manpresenting with clinically sus-pected sesamoiditis in his rightfoot. The plantar projection of99mTc-HDP bone scan revealsabnormally increased uptake onthe right hallux sesamoids (ar-rowheads), consistent with sesa-moiditis. Abnormal increaseduptake is also present on thebase of the proximal phalanx ofthe left hallux (arrow), mostlikely reflecting hallux valgusdeformity

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99mTc-MDP bone scan (Fig. 14d) may help to identifythe hallux sesamoid bones as the source of painful condi-tions of the forefoot.

Conclusion

We review the imaging findings, potential clinical signif-icance and differential diagnosis of some of the mostcommon accessory ossicles and sesamoid bones of theankle and foot. The MR imaging is particularly usefulfor assessing their associated painful conditions, but ra-

diographs, CT scans and scintigraphy can also make avaluable contribution. We also discuss the potential ofthese accessory ossicles and sesamoid bones for simulat-ing fractures. Once again, we emphasize that most acces-sory ossicles and sesamoid bones of the ankle and footmerely represent fortuitous imaging findings; thus, clini-cal correlation and cautious interpretation of imagingstudies are instrumental in the management of these pa-tients.

Acknowledgements Acknowledgements. We express our grati-tude to J. Bates and the Language Service of the Rovira i VirgiliUniversity for their assistance in preparing the manuscript.

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