Thumb sesamoids !

The thumb sesamoids are two tiny bones in the palm side of the thumb knuckle; like small kneecaps, they guide tendons and protect the joint.

Official name: Sesamoid bones of the thumb

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What it is

Two small round bones in the tendons on the palm side of the thumb knuckle.

What it does

Act like tiny kneecaps that guide the tendons and protect the joint.

Key facts

Joints with: Over the palmar head of the 1st metacarpal (MCP joint)
Landmarks: Two sesamoids in the FPB and adductor pollicis tendons

Relations

Deep: Palmar surface of the head of the 1st metacarpal, set into the MCP volar plate (knuckles)
In front: FPL tendon in its sheath, passing between the two sesamoids
Lateral: Flexor pollicis brevis and abductor pollicis brevis to the radial sesamoid
Medial: Adductor pollicis to the ulnar sesamoid

How to find it

Felt on the palm side of the thumb MCP joint, just in front of the flattened head of the 1st metacarpal, as two small firm nodules that move with the volar plate when the thumb is flexed and extended.

On imaging

Seen on lateral and oblique thumb views; after a hyperextension injury, sesamoids displaced away from the metacarpal head suggest a volar plate tear.

Clinical notes

A fracture of a sesamoid follows forced hyperextension of the thumb and can be confused with a split (bipartite) sesamoid
After a hyperextension injury, a sesamoid that has moved away from the metacarpal head on X-ray points to a torn volar plate of the thumb MCP joint

Anatomical variants

Extra sesamoids are common at the index and little finger MCP joints and are sometimes found at the thumb IP joint

Development

They form inside tendons and the volar plate where these cross the joint under pressure, and ossify late; the ulnar thumb sesamoid usually becomes visible around the start of puberty, which is used when reading bone age.

Test yourself

Think of the answer, then tap the question to see it.

Where are the two constant sesamoids of the thumb, and which muscles attach to them?

In the volar plate of the thumb MCP joint: the radial one with the flexor pollicis brevis, the ulnar one with the adductor pollicis.

Which tendon runs between the two thumb sesamoids?

The flexor pollicis longus tendon, in the groove between them.

How are thumb sesamoids told apart from fracture fragments on X-ray?

By their smooth, rounded edges with an intact cortex.

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Educational information, not medical advice. For any symptom, see a health professional.

Its names

Official Latin (TA2): ossa sesamoidea manus TA2 1281
Official Spanish (DeCS): Huesos sesamoideos de la mano
English (TA2): Sesamoid bones of the thumb

Sources: Terminologia Anatomica (TA2); Anatomy, Shoulder and Upper Limb, Hand Metacarpal Phalangeal Joint; Anatomy of the Human Body (20th ed.): Surface Anatomy of the Upper Extremity (Lea & Febiger (Gray's Anatomy, 1918), 1918) · adapted, Public domain; 6.2 Bone Classification (OpenStax (Rice University), 2022) · adapted, CC BY 4.0.

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Representative model, for educational purposes only.