The metacarpus is a group of five bones of the hand between the phalanges and the carpus. Even though the metacarpal bones are small, they are classified as long bones since they have structural characteristics of long bones; each metacarpal bone consists of a shaft, distal head and a wide proximal base.
The metacarpal bones articulate with the carpal bones with their proximal ends (bases), and with the proximal phalanges via their distal ends (heads). They are labeled with numbers 1-5 going in the radio-ulnar direction.
This article will discuss the anatomy and function of the metacarpal bones.
|Base, shaft, head
- First metacarpal bone
- Second metacarpal bone
- Third metacarpal bone
- Fourth metacarpal bone
- Fifth metacarpal bone
Each metacarpal bone has a base, shaft and head. The bases of the metacarpal bones are wide and they articulate with the bones of the distal carpal row via the carpometacarpal joints;
- The first metacarpal articulates with the trapezium
- The second metacarpal articulates with the trapezium, trapezoid and capitate
- The third articulates with the capitate
- The fourth and fifth articulate with the hamate
The bases of the metacarpals 2-5 also articulate with each other. The shafts of the metacarpal bones are elongated and each features a flat triangular area on the distal part of its dorsal surface, just proximal to the knuckles. The palmar surfaces show the longitudinal concavities intended for accommodating the muscles of the palm, such as the dorsal and palmar interossei.
Each of the metacarpal bones features a rounded head on its distal end. The shape of the head produces the noticeable prominence on the dorsal surface of the hand known as the knuckle. The head of each metacarpal has a convex articular surface that faces distally and articulates with the base of the corresponding proximal phalanx at the metacarpophalangeal joint.
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First metacarpal bone
The metacarpal bone 1 is the most lateral, thickest and shortest metacarpal bone. It is directed laterally, with its long axis being medially rotated for the 90° in comparison to the other metacarpals. Due to this axial rotation, the sides of the bone are rotated so that its anterior surface faces medially towards the palm, the ulnar border faces posteriorly, while the radial faces anteriorly.
Aside from the common features that it shares with other metacarpals, the first metacarpal is recognizable by a longitudinal ridge on its medial (palmar) surface, which divides this surface into a large lateral and a small medial part. Also, the head of this bone is flatter than in the other metacarpals. The first metacarpal features a saddle-shaped articular surface on its base, via which it articulates with the trapezium.
The first metacarpal provides the attaching points to the several hand muscles;
Second metacarpal bone
The metacarpal bone 2 is the one with the largest base and the longest shaft. Its base shows several areas for the articulations with the carpal bones;
- A groove in the coronal plane via which it articulates with the trapezoid bone.
- Medially to this groove is a ridge for the articulation with the capitate bone, while laterally is a quadrangular surface for the joint with the trapezium bone.
- An elongated facet on its medial surface for the articulation with the third metacarpal bone
Dorsally to the facet for the trapezium is a shallow impression of the insertion of the extensor carpi radialis longus muscle. The palmar surface of the base provides the attaching site for the flexor carpi radialis muscle.
The shaft of the second metacarpal is convex towards the dorsum of the hand. The interossei muscles attach to the proximal part of the shaft; the ulnar head of the first dorsal interosseous attaches from the lateral side, while the second palmar and second dorsal interossei attach from the medial side.
Third metacarpal bone
The metacarpal bone 3 is located at the base of the middle finger. It differs from the others by a styloid process that projects proximally from the laterodorsal edge of its base. This process participates in the joint with the capitate bone. The lateral surface of the base articulates with the second metacarpal, while the medial surface articulates with the fourth metacarpal via two oval articular surfaces.
The shaft is similar to that of the second metacarpal, being dorsally convex. Its lateral side serves as an insertion point for the ulnar head of the second dorsal interosseous, while the medial side is where the radial head of the third dorsal interosseous attaches. The distal two-thirds of the medial surface of the shaft provide an insertion point for the transverse head of adductor pollicis muscle.
Fourth metacarpal bone
The metacarpal bone 4 shows a few specificities of its base.
- It has two oval facets on the lateral surface via which it articulates with the base of the third metacarpal
- It features a single elongated facet on the medial surface for the articulation with the base of the fifth metacarpal
- The proximal surface is quadrangular and serves for the articulation with the hamate bone
The shaft provides the insertion point for the third palmar interosseous and the ulnar head of third dorsal interosseous on its lateral side, while the medial side is where the fourth dorsal interosseous attaches.
Fifth metacarpal bone
The metacarpal bone 5 is the smallest of all five metacarpals. Its base slightly differs from the other metacarpals, as its lateral part is non-articular and instead features a tubercle for the attachment of the extensor carpi ulnaris muscle. The lateral side of the base, however, articulates with the hamate bone.
The proximal part of the lateral surface of the shaft articulates with the fourth metacarpal, while the medial surface serves to attach the opponens digiti minimi muscle. The distal part of the lateral surface receives the fibers from the fourth palmar and fourth dorsal interossei muscles.
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