Title: Carpal Boss

Imaging Findings:
Figure Lateral view demonstrates abnormal bony protuberance/bossing (arrows) along the dorsum of the hand between the second and third carpal-metacarpal joints. Profiling the abnormality helps differentiate degenerative osteophytic changes from the presence of an embryologic ossicle (os styloideum), as well as from more proximal triquetral bone fractures by location.

Clinical:
Unmovable bony protuberance on the dorsum of the hand between the second and third carpal-metacarpal joints which may or may not be symptomatic due to overlying ganglion, bursa, osteoarthritic changes, or tendon slippage across protuberance. More commonly seen in the right hand.

Etiology:
The carpal boss may represents an embryologic accessory ossification center referred to as the os styloideum with varying amounts of fusion (1). The majority (94%) of these ossicles fuse with the second or third metacarpal base (2). Others may fuse with the trapezoid or capitate, and the remaining (2%) do not fuse.

References:
1. Conway WF, Destouet JM, Gilula LA. The carpal boss: an overview of radiographic evaluation. Radiology 1985; 156: 29-31.
2. Bizzaro AH. On sesamoid and supernumerary bones of the limb. J Anat 1921; 55:256-268.