Ulnar sesamoid bone of thumb

The ulnar sesamoid bone of the thumb is a small, ovoid bone embedded within the soft tissues on the ulnar (medial) side of the metacarpophalangeal (MCP) joint of the thumb. It is one of two constant sesamoid bones at this location the other being the radial sesamoid on the opposite side.

Anatomy and Function

The ulnar sesamoid is incorporated into the volar plate (palmar plate) of the thumb MCP joint and serves as an insertion point for the adductor pollicis muscle. Together with the radial sesamoid (which receives the insertion of the flexor pollicis brevis), these bones function to:

• Protect the flexor pollicis longus tendon as it passes between them

• Increase the mechanical advantage of the intrinsic muscles acting on the thumb

• Distribute compressive forces across the MCP joint during pinch and grip

Morphology

The ulnar sesamoid articulates with the ulnar palmar condyle of the first metacarpal head, which is characteristically flatter and broader than the radial condyle. This broader articulation may explain why the ulnar sesamoid is less prone to degenerative arthritis compared to the radial sesamoid, which rides on a higher, narrower ridge.

Prevalence

The ulnar sesamoid is nearly universally present, with a radiographic prevalence of approximately 99.6% in adults, making it along with the radial sesamoid (99.9%) one of the most constant sesamoid bones in the human body. Ossification typically begins around age 10-11 years (earlier in females) and is complete by age 1314.

Clinical significance

Fractures of the ulnar sesamoid are uncommon and typically result from hyperextension injuries or direct trauma to the thumb MCP joint. These fractures may be missed on standard anteroposterior radiographs and often require oblique views for diagnosis. Bipartite (incompletely ossified) sesamoids can mimic fractures and must be differentiated radiographically. Posttraumatic sesamoid arthritis and sesamoiditis can also occur, though the radial sesamoid is more commonly affected.