Radial sesamoid bone of thumb

The radial sesamoid bone of the thumb is a small, ovoid bone embedded within the tendon of the flexor pollicis brevis on the radial (lateral) side of the metacarpophalangeal (MCP) joint of the thumb. It is one of two constant sesamoid bones at this joint the other being the ulnar sesamoid on the medial side.

Anatomy and Function

The radial sesamoid articulates with the radial condyle of the first metacarpal head, forming the radial subsesamoid joint. It is incorporated into the palmar plate (volar plate) of the MCP joint and serves several biomechanical roles:

• Protects the flexor pollicis longus tendon as it passes over the MCP joint

• Increases the mechanical advantage of the thenar muscles (abductor pollicis brevis and flexor pollicis brevis) by acting as a fulcrum

• Contributes to MCP joint stability alongside the collateral ligaments, accessory collateral ligaments, and palmar plate

Prevalence

The radial sesamoid at the thumb MCP joint is essentially universal, present in 99.9% of adults radiographically. This makes it one of the most constant sesamoid bones in the human body, along with the ulnar sesamoid at the same joint.

Clinical Significance

• The radial subsesamoid joint is the most frequently involved area of degenerative arthritis in the thumb MCP joint complex, found in over 85% of adult cadavers, owing to the asymmetry of the palmar condyles the radial sesamoid rides on a high, narrow ridge compared to the flatter ulnar condyle.

• Fractures of the thumb sesamoids are uncommon but occur with hyperextension injuries, particularly in sports. They are often missed on standard AP radiographs and require oblique views for detection.

• Sesamoid entrapment can cause a locked thumb MCP joint, where the radial sesamoid becomes displaced and trapped in a cartilage defect on the metacarpal head, preventing flexion.

• Sesamoid arthritis can mimic trigger thumb and should be considered in the differential diagnosis of persistent MCP joint pain.