Proximal phalanx, thumb !

The proximal phalanx of the thumb is the thumb's first finger bone, beyond the knuckle; its base anchors the ligament that steadies pinch.

Official name: Proximal phalanx of the thumb

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What it is

The proximal bone of the thumb.

What it does

One of the finger bones; the joints between them let the finger curl.

Key facts

Joints with: Metacarpal head (MCP joint), next phalanx (IP joint)
Landmarks: Base, shaft, head (condyles)

Relations

Above: Head of the 1st metacarpal at the MCP joint, with the thumb sesamoids, the thumb UCL and the radial collateral ligament
Behind: EPB inserts on the back of the base; the EPL tendon passes over it to the distal phalanx
In front: FPL tendon in its sheath under the oblique pulley

How to find it

On the back of the thumb between the MCP knuckle and the IP joint crease. Tenderness or a lump on the ulnar side of its base, just beyond the MCP joint, is where a skier's thumb or a Stener lesion is felt.

Examination

Ulnar collateral ligament stress test: the thumb MCP joint is pushed radially in extension and in about 30° of flexion; marked opening or no firm end point compared with the other thumb suggests a complete UCL tear

On imaging

PA and lateral thumb views show avulsion fragments; ultrasound or MRI shows a Stener lesion, in which the torn UCL ends up above the adductor aponeurosis and cannot heal in place.

Clinical notes

A displaced bony gamekeeper's fragment or a Stener lesion usually needs surgical repair to restore pinch
The radial collateral ligament on the other side of its base tears less often than the UCL but also leaves the thumb unstable in pinch and grasp

Development

Like the other phalanges, it ossifies from a primary centre in the shaft before birth and a single secondary centre at its base, whose plate closes in adolescence.

Test yourself

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

What does an avulsion fragment from the ulnar base of the proximal phalanx of the thumb indicate?

A bony skier's (gamekeeper's) thumb: the thumb UCL tore off with a piece of bone.

What is a Stener lesion?

The torn thumb UCL ends up above the adductor aponeurosis and cannot heal in place; it usually needs surgical repair.

Which tendon inserts on the back of the base of the proximal phalanx of the thumb?

The extensor pollicis brevis; the extensor pollicis longus passes over it to the distal phalanx.

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

Its names

Official Latin (TA2): phalanx proximalis manus TA2 1277
Official Spanish (DeCS): Falange proximal del pulgar
English (TA2): Proximal phalanx of the thumb

Sources: Terminologia Anatomica (TA2); Anatomy, Shoulder and Upper Limb, Hand Bones; 8.5 Development of the Appendicular Skeleton (OpenStax (Rice University), 2022) · adapted, CC BY 4.0.

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Representative model, for educational purposes only.