Trapezoid
Os Trapezoideum
location_on Distal row of carpal bones, between the trapezium and capitate
The trapezoid is the smallest bone in the distal carpal row, wedge-shaped with a broad dorsal surface and narrow palmar surface. It is firmly locked into the distal carpal row by its articulations with the second metacarpal, trapezium, capitate, and scaphoid, making it the least commonly fractured and dislocated carpal bone. Its rigidity contributes to the stability of the index finger ray.
Key Anatomical Features
- Wedge-shaped: broader dorsally than palmarly
- Distal surface articulates with the base of the second metacarpal
- Tightly bound within the distal carpal row with minimal mobility
- Proximal surface is narrow and concave for the scaphoid
- The least commonly injured carpal bone due to its protected position
Muscle Attachments
| Muscle | Attachment | Action |
|---|---|---|
| No direct muscular attachments | The trapezoid is bound by intercarpal ligaments | Provides structural rigidity to the distal carpal row |
| Flexor retinaculum (indirect) | Spans from trapezium/scaphoid to pisiform/hamate | Bridges over the trapezoid as part of the carpal tunnel roof |
| Extrinsic ligaments | Dorsal and palmar intercarpal ligaments | Lock the trapezoid in position |
| Adductor pollicis (oblique head, partial) | Adjacent carpals including trapezoid area | Adducts the thumb |
Joints and Articulations
| Joint | Type | Connects to |
|---|---|---|
| Second carpometacarpal joint | Synovial plane (nearly immobile) | Second metacarpal |
| Trapeziotrapezoid joint | Synovial plane | Trapezium |
| Trapeziocapitate joint | Synovial plane | Capitate |
Common Pathologies
Trapezoid fracture
The rarest carpal fracture, accounting for less than 1% of carpal fractures. Usually associated with high-energy trauma and often accompanied by other carpal injuries.
Trapezoid dislocation
Extremely rare in isolation. When it occurs, it is usually dorsal and associated with significant perilunate ligament disruption.
Second CMC joint arthritis
Arthritis at the trapezoid-second metacarpal joint can cause dorsal hand pain and swelling, often from repetitive occupational stress.
Clinical Relevance
The trapezoid's protected position within the distal carpal row makes isolated injuries extremely rare. Its tight articulation with the second metacarpal base means that second metacarpal base fractures should prompt evaluation for trapezoid injury. The rigid second CMC joint (compared to the mobile first CMC joint) provides a stable post for the index finger, essential for precision grip.
Development and Ossification
The trapezoid ossifies from a single center appearing between ages 4 and 6, typically slightly after the trapezium. It is entirely cartilaginous at birth.
Did You Know?
- The trapezoid is the least commonly fractured carpal bone due to its protected and tightly bound position
- Trapezoid means table-shaped in Greek, like a small version of the trapezium
- The trapezoid is so firmly wedged into the distal carpal row that isolated dislocation almost never occurs
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