Thumb UCL !

The thumb's ulnar collateral ligament is on the inner side of the thumb knuckle; it lets the thumb press firmly against the fingers in a pinch.

Official name: Ulnar collateral ligament of the thumb MCP joint

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What it is

The band on the index-finger side of the thumb knuckle.

What it does

Lets the thumb press firmly against the fingers in a pinch.

Key facts

Attaches: Ulnar side of the 1st metacarpal head to the base of the proximal phalanx
Function: Resists valgus (pinch) stress of the thumb MCP
Tight in: Proper collateral in ~30° flexion; accessory collateral in extension

Relations

Superficial: Adductor aponeurosis, the expansion of the adductor pollicis to the extensor hood; branches of the superficial radial nerve run over it
Deep: Capsule of the thumb MCP joint and the ulnar sesamoid in the volar plate, where adductor pollicis inserts
Medial: First web space and the adductor pollicis

How to find it

With the thumb MCP slightly flexed, the ligament lies on the ulnar side of the joint, from the metacarpal head to the base of the proximal phalanx. A Stener lesion may be felt as a small tender lump on the ulnar side of the metacarpal head, just proximal to the edge of the adductor aponeurosis.

Examination

Valgus stress of the thumb MCP at 30° of flexion: more opening than the other thumb, or no firm end point, means a tear of the proper collateral
Valgus stress in full extension: laxity here as well means the accessory collateral and volar plate are also torn

On imaging

PA and lateral X-rays first, looking for an avulsion fragment from the ulnar base of the proximal phalanx and for palmar subluxation. Ultrasound shows a Stener lesion as a rounded, retracted ligament stump lying proximal and superficial to the edge of the adductor aponeurosis; on MRI this gives the 'yo-yo on a string' look.

Common problems

Skier's thumb: a fall onto a ski pole or the hand forces the thumb outward and tears the ligament.
Gamekeeper's thumb: slow stretching from repeated stress.
The torn end can flip above the adductor tendon (Stener lesion), which stops it healing on its own.

How it is diagnosed

Pain and looseness when the thumb knuckle is stressed sideways, compared with the other hand. X-rays check for a bone fragment; ultrasound or MRI looks for a Stener lesion.

Treatment

Thumb splint or cast for partial tears.
Surgical repair (often with a small anchor) for complete tears and Stener lesions.

Clinical notes

A Stener lesion or a displaced bony avulsion is generally an indication for surgical reattachment
Chronic laxity weakens pinch and leads to MCP osteoarthritis; late reconstruction uses a free tendon graft
The radial collateral ligament of the thumb MCP is injured less often, by forced adduction

Development

Before the physis of the proximal phalanx closes, the same valgus force tends to pull off a Salter–Harris III fragment from the ulnar corner of its base instead of tearing the ligament; a displaced fragment behaves like a Stener lesion.

Test yourself

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

What stress does the ulnar collateral ligament of the thumb MCP resist?

Valgus (pinch) stress, from the ulnar side of the 1st metacarpal head to the base of the proximal phalanx.

Why are X-rays taken before stress testing the thumb ulnar collateral ligament?

A non-displaced avulsion fragment could be displaced by the stress test.

In which two positions is the thumb ulnar collateral ligament stress-tested, and what does each assess?

At 30° of flexion for the proper collateral, and in full extension for the accessory collateral and volar plate.

From which end does the thumb ulnar collateral ligament usually tear, and what lesion can result?

From its distal attachment on the proximal phalanx; it can displace into a Stener lesion, generally an indication for surgical reattachment.

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

Its names

Official Latin (TA2): ligamenta metacarpophalangea collateralia TA2 1836
Official Spanish (DeCS): Ligamento colateral ulnar de la articulación metacarpofalángica del pulgar
English (TA2): Ulnar collateral ligament of the thumb MCP joint
Also called: skier's thumb ligament · gamekeeper's thumb ligament

Sources: Terminologia Anatomica (TA2); Anatomy, Shoulder and Upper Limb, Metacarpophalangeal Joints; Anatomy, Shoulder and Upper Limb, Hand Metacarpal Phalangeal Joint.

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Representative model, for educational purposes only.