Accessory ossicles and sesamoid bones: Spectrum of pathology and imaging evaluation

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 insignifi cant 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 disorders.

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 intermetatarseum.

Os acromiale

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 ossifi cation 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 ossifi cation 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.1 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.2-4 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 osstyloideum 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.5