Carpal boss

Figures (7)

Patient history

A 32-year-old woman presented to our institution with bilateral dorsal hand swelling that was bony hard on palpation. The prominence was located between the second and third rays at the carpometacarpal level. The swelling occasionally became painful, usually in relation to overuse of the hand. MRI was performed.

Imaging findings — key points

• MRI of the wrist shows a bony protuberance at the dorsal aspect of the second and third carpometacarpal joints, formed by the base of the metacarpal and the adjacent trapezoid and capitate.
• There is bone marrow oedema within the protuberance and the adjacent carpal bones, indicating mechanical irritation at the articulation.
• A small adjacent ganglion cyst is present in the dorsal soft tissues, and the overlying extensor tendons show mild tenosynovial thickening.
• The underlying joint shows mild degenerative changes with small osteophytes but no erosions, fracture or destructive lesion.
• No soft-tissue mass, marrow replacement or aggressive periosteal reaction is identified.

Discussion

Carpal boss is a bony prominence at the dorsal aspect of the second or third carpometacarpal joint, produced by a degenerative osteophyte, an accessory ossicle or a congenital variant of the joint. It becomes symptomatic with overuse, causing dorsal wrist pain, a palpable hard swelling and sometimes an overlying ganglion or bursitis. MRI is useful to confirm the bony protuberance, identify marrow oedema and exclude a soft-tissue mass, while radiographs and CT show the osseous anatomy. Treatment is conservative with rest, splinting and steroid injection, and surgical excision is reserved for persistent symptoms.

Differential diagnosis

Alternative diagnosis	Key features that distinguish it from this case
Ganglion cyst	A ganglion cyst is a fluid-filled lesion that follows water signal on all sequences, does not contain bone marrow or a bony protuberance and may communicate with the joint.
Osteophyte	A simple osteophyte is a marginal bony outgrowth of a degenerated joint without a separate ossicle or the characteristic carpometacarpal boss configuration.
Osteoarthrosis	Osteoarthrosis produces joint space narrowing, subchondral sclerosis and osteophytes around the affected joint, but it does not explain a discrete dorsal bony boss with overlying bursitis.
Avulsion fracture	An avulsion fracture shows a donor site and a displaced bony fragment with marrow oedema after trauma, and it does not have the chronic, corticated appearance of a carpal boss.

The confirmed diagnosis itself is not included as an alternative — it is stated separately below.

See all Musculoskeletal differential diagnoses