The authors describe a spectrum of accessory ossicle and sesamoid pathology and suggest appropriate imaging modalities. While they are considered to be normal anatomic variants, accessory ossicle and sesamoid bones may help to identify the source of patient symptomatology. The recognition of key imaging findings can help radiologists determine whether or not clinical symptoms are associated with these structures.
is a Radiology Resident,
is a Professor and Chief, Musculoskeletal Radiology,
is an Assistant Clinical Professor, and
is an Assistant Clinical Professor, Department of Radiology,
David Geffen School of Medicine at UCLA, Los Angeles, CA.
Presented in part at the 106th Annual Meeting of the American
Roentgen Ray Society, Vancouver, Canada, May 2006.
Accessory bones, or ossicles, are considered to be normal
anatomic variants. Sesamoid bones are ovoid nodular bones, often
small, found embedded within a tendon or joint capsule. Although
accessory ossicles and sesamoid bones are generally considered
clinically insignificant anatomic variations, they can become
symptomatic. Traumatic conditions include acute fracture, stress
fracture, and pseudarthrosis. Neoplastic and arthritic conditions
are also encountered, as well as inflammatory and degenerative
This article reviews a spectrum of accessory ossicle and
sesamoid pathology and provides guidelines for the preferred
imaging modality for the suspected abnormality. We discuss some of
the more commonly symptomatic bones, namely the os acromiale, os
styloideum, metacarpal and hallux sesamoids, patella, os trigonum,
os calcaneus secundarius, accessory navicular, os peroneum, and os
There are normally 3 acromial ossification centers that fuse
between 22 and 25 years of age. An os acromiale results from the
failure of 1 of these centers to fuse. The anterior ossification
center is termed the pre-acromion, the middle is the meso-acromion,
and the posterior is the meta-acromion. The basi-acromion forms the
point of attachment of these 3 ossification centers to the scapula.
A number of subtypes of os acromiale have been described. The most
common variant is nonfusion between the meso- and meta-acromion.
The prevalence of os acromiale in radiographic and anatomic studies
ranges between 1% and 15%.
An os acromiale can contribute to shoulder impingement symptoms.
Contraction of the deltoid muscle may pull the os acromiale
downward, causing it to impinge on the rotator cuff.
Abnormal motion may lead to an osteophytic spur at the
pseudarthrosis, which may also impinge on the cuff.
The diagnosis of os acromiale may be made on axillary projection
radiographs, computed tomography (CT), or magnetic resonance
imaging (MRI). MRI may show marrow edema and degenerative changes
at a pseudarthrosis (Figure 1).
Os styloideum (carpal boss)
The os styloideum is an ununited bony protuberance, located on
the dorsum of the wrist at the base of the second and third
metacarpals. The association between dorsal wrist pain or
fatigability and an os styloideum is known as carpal bossing (carpe
bossu disease). Symptoms of carpal bossing may result from
osteoarthritis, an overlying ganglion or bursitis, or from an
extensor tendon slipping over the ossicle.
The lateral radiographic view best profiles the os styloideum
(Figure 2). CT may show degenerative disease at a pseudarthrosis,
and MRI will show edema related to abnormal motion (Figure 2).
Sesamoid bones of the hand
Most people have 5 sesamoid bones in each hand: 2 at the thumb
metacarpophalangeal (MCP) joint, 1 at the interphalangeal joint of
the thumb, 1 at the MCP joint of the index finger, and 1 at the MCP
joint of the small finger.
Traumatic, inflammatory, and arthritic conditions can affect the
sesamoid bones of the hand. Thumb sesamoid fractures usually result
from hyperextension injury to the MCP joint. These fractures may be
associated with rupture of the palmar plate or accessory collateral
Radiography is usually sufficient for diagnosis; however, oblique
views are often necessary for the diagnosis of a fracture (Figure
Patella fractures are usually transverse, often resulting from
rapid flexion against a fully contracted quadriceps. Osteochondral
fractures may result from a direct blow or patellar dislocation.
Children are vulnerable to this type of fracture because of
Differentiation of an acute patella fracture from a bipartite
patella may be difficult. The bipartite patella is a synchondrosis,
found in 1% to 2% of the population (Figures 4 and 5). Bipartite
patellas are classified into 3 types. Type I is located at the
inferior pole (5% of all cases), type II is at the lateral patellar
margin (20%), and type III is at the superolateral margin (75%).
Most (98%) remain asymptomatic, but direct trauma may disrupt the
synchondroses (Figure 4).
A dorsal defect of the patella is sometimes associated with
bipartite or multipartite patella, which is seen as a focal lucency
in the superolateral patella covered by articular cartilage (Figure
Other categories of disease that affect the patella include
degenerative or metabolic disease and benign or malignant primary
or metastatic tumors (Figures 6 and 7). Reported tumors include
chondroblastoma, histiocytosis, giant cell tumor, simple bone cyst,
brown tumor of hyperparathyroidism, hemangioma, osteochondroma,
lipoma, and osteoblastoma. Malignant tumors include lymphoma and
hemangioendothelioma. Metastatic tumors and myeloma are rare.
Between 8 and 13 years of age, an ossification center forms
posterior to the talus. Normally, this ossification center fuses
with the talus within 1 year. In approximately 7% of the
population, it remains separate and is referred to as the os
trigonum. The os trigonum is corticated and articulates with the
lateral talar tubercle through a synchondrosis. Os trigonum
syndrome results from repetitive microtrauma or acute forced
plantar flexion of the foot. The chondro-osseous border of the
synchondrosis may be injured either as a chronic stress fracture
or, less frequently, as an acute fracture.
An os trigonum is usually round or oval with well-defined
corticated margins, whereas a fracture of the lateral tubercle
typically has irregular serrated margins between the fragment and
the posterior talus. Lateral radiographs obtained with the foot in
plantar flexion may show the os trigonum impinged between the
posterior tibia malleolus and the calcaneal tuberosity (Figure 8).
MRI is very useful in establishing the diagnosis of os trigonum
syndrome (Figure 8). Because of its location between the medial and
lateral talar tubercles, inflammatory changes may be seen in the
flexor hallucis longus tendon in patients with os trigonum
Os calcaneus secundarius
The os calcaneus secundarius is an accessory ossicle of the
anterior facet of the calcaneus, which may be mistaken for a
fracture of the anterior process. Radiographically, the os
calcaneus secundarius is best seen on the medial oblique view
(Figure 9). Although it is uncommon for an os calcaneus secundarius
to cause symptoms, range of motion of the subtalar joint may be
limited by the ossicle, clinically resembling a calcaneonavicular
Accessory navicular (os tibiale externum, os naviculare
There are 3 types of accessory navicular.
Type I, or os tibiale externum, occurs when an ossification center
forms a sesamoid bone within the tibialis posterior tendon, near
the navicular insertion. Type I ossicles are generally well-defined
round or oval bones, measuring approximately 2 to 3 mm in diameter,
and are located up to 5 mm medial and posterior to the medial
aspect of the navicular. In type II accessory navicular, the
ossification center measures approximately 9 to 12 mm in size, and
resides adjacent to the tubercle of the navicular bone. A residual
cartilaginous synchondrosis joins the triangular type II ossicle
approximately 1 to 2 mm medial and posterior to the navicular. The
majority or entire tibialis posterior tendon inserts on the type II
accessory ossicle. The type III accessory navicular, or cornuate
navicular, is a prominent navicular tuberosity, which is
essentially a type II ossicle connected to the medial aspect of the
parent navicular by an osseous bridge.
Of the 3 patterns, the type II accessory navicular is most
commonly associated with medial foot pain.
A valgus stress injury may fracture the synchondrosis, resulting in
abnormal motion. MRI is the most specific imaging modality for
detecting the symptomatic accessory navicular, showing edema in the
bone and soft tissues (Figures 10 and 11).
The os peroneum is an oval or round ossicle located within the
substance of the distal peroneus longus tendon near the cuboid.
Fractures of the os peroneum result from direct trauma or indirect
stress from dorsiflexion of the foot (Figure 12). Fractures of the
os peroneum are typically associated with peroneus longus tendon
dysfunction, which may be either acute or chronic.
Os intermetatarseum is present in approximately 1% of feet. It
is situated superiorly between the first and second metatarsal
It may articulate with the medial cuneiform or be attached to the
first metatarsal base. The ossicle is best visualized on the
anteroposterior view. It may be round, oval, kidney-shaped, linear,
or even resemble a rudimentary metatarsal (Figure 13). Although
infrequently symptomatic, an os intermetatarseum may cause dorsal
midfoot pain from compression of the medial branch of the deep
The hallux sesamoids can be associated with acute or stress
Although much less common than stress fractures, avascular necrosis
of the hallux sesamoid bones should be considered in the
differential diagnosis of persistent forefoot pain.
The tibial sesamoid is more likely to fracture than the fibular
sesamoid because of weight-bearing mechanics (Figure 14).
Compared with a fractured sesamoid, a bipartite bone is usually
larger and has smooth margins.
In addition to standard radiographic views, an axial sesamoid
projection is often helpful. MRI will show edema if the bone is
fractured (Figure 15).
We have reviewed a spectrum of pathology involving accessory
ossicles and sesamoid bones. These normal anatomic variations may,
in fact, represent the source of patient symptomatology. The
identification of key imaging characteristics can help determine
whether or not to attribute clinical symptoms to these