Clinical History
This 32-year-old female patient presented at our institution with bilateral swelling at hands dorsum, with bony consistence at palpation. The protuberance was located between II and III ray, at carpo-metacarpal level. Swelling became occasionally painful, often in association with hand overuse. A MR examination was performed.
Imaging Findings
MR examination of both wrists, performed with a low-field (0.24 T) extremity-dedicated MRI system, shows signal alteration and a bony protuberance on the dorsal side between the base of the third metacarpal bone and the capitate, with aspect of neoarticulation: on sagittal images a bony bridge can be seen; alteration of the normal joint profile can be observed too (Fig. 1, 2). The tendons and the bone marrow seemed to return normal signal.
Discussion
Carpal boss is a rare disease, without gender predominance, characterised by the presence of a bony protuberance localised on the dorsum of the hand, in the carpal-metacarpal zone; it presents as a tough, painless swelling at physical examination.
The term “carpal boss” was coined by Fiolle in 1931: he firstly described this lesion as an exostosis at the base of II and III metacarpal bones. The origin of carpal boss is unknown and many theories were proposed during the years: the condition may represent a degenerative osteophytic process as well as the presence of an accessory bone.
In 1953, O’Rahilly described the presence of accessory ossicles in the carpal zone: one of this, the os styloideum, normally disappears during fetal development. The persistence of an accessory ossification centre can give rise to accessory bones like an “os styloideum”, located in the quadrangular trapezoid–capitate–metacarpal joint: it can interfere with the normal biomechanics of surrounding joints, leading to degenerative osteoarthritis, especially after repetitive stress [1].
The “os styloideum” may occur either as an isolated unit (articulated with one or more bones), or fused with one or more bones of the carpal or metacarpal zone; it has been proposed that this lesion could interfere with the normal biomechanics of adjacent joints, and lead to degenerative osteoarthritis, which can be symptomatic or not.
This bony prominence may dislocate extensor tendons and induce inflammation of these tendons and of surrounding soft-tissues [2].
Plain radiography, especially in lateral view, and computed tomography are fundamental modalities: they can visualise the bone protuberance, the joint profile and cortical erosions.
MR can provide a panoramic visualisation and highlight bony abnormalities that could lead to the formation of the bony bridge between the metacarpal base and the carpal bones and to subsequent complications: degenerative osteoarthritis can be visualised as an alteration of the normal joint profile on T1-weighted sequences, with an area of bone marrow oedema, hyperintense on T2-weighted and STIR sequences. MR can also show acute soft-tissues and extensor tendons inflammation [3].
Differential Diagnosis List
Carpo-metacarpal bossing
Ganglion cyst
Osteophytes
