Metacarpal Bones
Ossa Metacarpalia
location_on Palm of the hand, between the carpal bones and the phalanges
The five metacarpal bones form the skeleton of the palm, numbered 1 to 5 from the thumb to the little finger. Each has a base (proximal), shaft, and head (distal). The metacarpal heads form the knuckles visible when making a fist. The first metacarpal (thumb) is the shortest and most mobile, while the third is typically the longest. The fifth metacarpal neck is the most commonly fractured metacarpal site (boxer's fracture).
Key Anatomical Features
- Five bones numbered 1 (thumb) to 5 (little finger), forming the palm skeleton
- Bases articulate with the distal carpal row at the carpometacarpal joints
- Heads form the knuckles and articulate with the proximal phalanges at the MCP joints
- First metacarpal is the shortest, thickest, and most mobile due to the saddle CMC joint
- Third metacarpal has a dorsal styloid process at its base
- Shafts are concave palmarly, forming the hollow of the palm
Muscle Attachments
| Muscle | Attachment | Action |
|---|---|---|
| Dorsal interossei (4) | Adjacent sides of metacarpal shafts | Abduct fingers away from the middle finger axis |
| Palmar interossei (3) | Palmar surfaces of metacarpals 2, 4, and 5 | Adduct fingers toward the middle finger axis |
| Adductor pollicis (transverse head) | Palmar surface of third metacarpal shaft | Adducts the thumb |
| Opponens pollicis | Entire length of first metacarpal shaft | Opposes the thumb |
| Opponens digiti minimi | Ulnar border of fifth metacarpal shaft | Opposes the little finger |
| Extensor carpi radialis longus | Base of second metacarpal | Extends and radially deviates the wrist |
Joints and Articulations
| Joint | Type | Connects to |
|---|---|---|
| Carpometacarpal joints | Synovial saddle (1st) and plane (2nd-5th) | Distal carpal row |
| Metacarpophalangeal (MCP) joints | Synovial condyloid | Proximal phalanges |
| Intermetacarpal joints | Synovial plane | Adjacent metacarpal bases (2-5) |
Common Pathologies
Boxer's fracture
Fracture of the fifth metacarpal neck, the most common metacarpal fracture. Caused by punching with a closed fist. Up to 70 degrees of angulation is tolerable due to the mobility of the fifth CMC joint.
Bennett fracture
Intra-articular fracture-dislocation at the base of the first metacarpal. The small volar fragment remains attached to the trapezium while the shaft is pulled proximally by the abductor pollicis longus.
Rolando fracture
Comminuted intra-articular fracture of the first metacarpal base, a more severe variant of the Bennett fracture with a T or Y shaped fracture pattern.
Metacarpal shaft fracture
Rotational deformity is the most important factor to assess; even small degrees of malrotation cause overlapping of fingers when making a fist.
Clinical Relevance
When assessing metacarpal fractures, always check for rotational deformity by having the patient flex all fingers; all fingertips should point toward the scaphoid tubercle. Acceptable angulation differs by metacarpal: the first and fifth tolerate more angulation (30-70 degrees) because of their mobile CMC joints, while the second and third tolerate very little (less than 10-15 degrees). Bennett fractures are unstable and typically require surgical fixation.
Development and Ossification
Each metacarpal ossifies from two centers: one for the shaft (8th-9th fetal week) and one epiphysis. Metacarpals 2-5 have their epiphysis at the head (distal end), while the first metacarpal uniquely has its epiphysis at the base (proximal end), like a phalanx. Epiphyses appear at ages 1-3 and fuse around ages 14-19.
Did You Know?
- The first metacarpal has its growth plate at the base (like a phalanx) rather than at the head, leading some anatomists to consider it a phalanx rather than a true metacarpal
- The knuckles are formed by the metacarpal heads, not the finger bones
- Boxer's fracture got its name from amateur fighters who strike with the small knuckle rather than the proper second and third knuckles
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