Trapezoid !
The trapezoid is a small wrist bone in the second row, at the base of the index finger; it is wedged tightly and rarely breaks.
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What it is
A small wedge-shaped wrist bone under the index metacarpal.
What it does
One of the eight carpal bones that let the wrist bend and carry load to the forearm.
Key facts
- Joints with
- Scaphoid, trapezium, capitate, 2nd metacarpal
- Landmarks
- Small wedge-shaped bone
Relations
- Above
- Scaphoid
- Below
- Base of the 2nd metacarpal
- Lateral
- Trapezium
- Medial
- Capitate
- Behind
- Dorsal CMC ligaments; the ECRL tendon inserts on the 2nd metacarpal base just distal to it
How to find it
Hard to feel on its own: on the back of the hand it lies just proximal to the base of the 2nd metacarpal, which is followed back from the index knuckle.
On imaging
Overlaps its neighbours on standard views; the rare fracture or dislocation (usually dorsal) is best shown by CT.
Clinical notes
- Isolated fractures are rare and come from an axial blow through the index metacarpal, often with a carpometacarpal dislocation; CT is usually needed to see them
- Arthritis between the scaphoid, trapezium and trapezoid (STT joint) is a frequent pattern of wrist arthritis and often coexists with thumb-base arthritis
Development
Ossifies around age 5–7, after the trapezium.
Test yourself
Think of the answer, then tap the question to see it.
Which carpal bone is the least commonly fractured?
The trapezoid, because it is wedged tightly between the trapezium, capitate and base of the 2nd metacarpal.
Why is the index carpometacarpal joint almost immobile?
The trapezoid locks the base of the 2nd metacarpal, forming with the 3rd the rigid central pillar of the hand.
What is a carpal boss over the trapezoid region?
A hard bump on the back of the hand at the 2nd–3rd carpometacarpal joints, often mistaken for a ganglion.
Educational information, not medical advice. For any symptom, see a health professional.
Its names
- Official Latin (TA2)
- os trapezoideum TA2 1257
- Official Spanish (DeCS)
- Hueso trapezoide
- English (TA2)
- Trapezoid
- Also called
- lesser multangular
Related
Sources: Terminologia Anatomica (TA2); Anatomy, Shoulder and Upper Limb, Hand Carpal Bones.
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Representative model, for educational purposes only.
