A metacarpal bone is a group of five bones of the hand metacarpal bones or metacarpus forms the intermediate part of the skeletal hand located between the phalanges of the fingers and the carpal bones of the wrist, which forms the connection to the forearm. Above all 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.
Structure
The metacarpals form a transverse arch to which the rigid row of distal carpal bones is fixed. The peripheral metacarpals (those of the thumb and tiny finger) form the edges of the cup of the palmar gutter and as they’re brought together they deepen this concavity. The index metacarpal is the most firmly fixed, while the thumb metacarpal articulates with the Trapezium and acts independently from the others. the center metacarpals are tightly united to the carpus by intrinsic interlocking bone elements at their bases. The ring metacarpal is somewhat more mobile while the fifth metacarpal is semi-independent.
Each metacarpal consists of a body or shaft, and two extremities: the top at the distal or digital end (near the fingers), and therefore the base at the proximal or carpal end (close to the wrist)
Body
The body (shaft) is prismoid in form, and curved, so on be convex within the longitudinal direction behind, concave ahead. It presents three surfaces: medial, lateral, and dorsal.
The medial and lateral surfaces are concave, for the attachment of the interosseus muscles, and separated from each other by a prominent anterior ridge. The dorsal surface presents in its distal two-thirds a smooth, triangular, flattened area that is roofed by the tendons of the extensor muscles. This surface is bounded by two lines, which commence in small tubercles situated on either side of the digital extremity, and, passing upward, converge and meet a long way above the middle of the bone and form a ridge that runs along with the remainder of the dorsal surface to the carpal extremity. This ridge separates two sloping surfaces for the attachment of the interossei dorsales.
To the tubercles on the digital extremities are attached the collateral ligaments of the metacarpophalangeal joints.
Base
The base (basis) or carpal extremity is of a cuboidal form, and broader behind than in front: it articulates with the carpal bones and with the adjoining metacarpal bones; its dorsal and volar surfaces are rough, for the attachment of ligaments.
Head
The head (caput) or digital extremity presents an oblong surface markedly convex from before backward, less so transversely, and flattened from side to side; it articulates with the proximal phalanx. it’s broader, extends farther upward, on the volar than on the dorsal aspect, and is longer within the anteroposterior than within the transverse diameter. On either side of the top may be a tubercle for the attachment of the collateral ligament of the knuckle. The dorsal surface, broad and flat, supports the tendons of the extensor muscles. The volar surface is grooved within the middle line for the passage of the flexor tendons and marked on either side by an articular eminence continuous with the terminal articular surface.
Neck
The neck, or subcapital segment, is the transition zone between the body and therefore the head.
Articulations
Besides the metacarpophalangeal joints, the metacarpal bones articulate by carpometacarpal joints as follows:
- the first with the trapezium;
- the second with Trapezium, trapezoid, capitate, and third metacarpal;
- the third with the capitate and second and fourth metacarpals;
- the fourth with the capitate, hamate, and third and fifth metacarpals;
- and the fifth with the hamate and fourth metacarpal;
First metacarpal
The metacarpal 1 is the most lateral, thickest, and shortest metacarpal. it’s directed laterally, with its long axis being medially rotated for 90° as compared to the opposite metacarpals. thanks to this axial motion, the edges of the bone are rotated in order 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 primary metacarpal is recognizable by a longitudinal ridge on its medial (palmar) surface, which divides this surface into an outsized lateral and alittle medial part. Also, the top of this bone is flatter than within the other metacarpals. the primary metacarpal features a saddle-shaped articular surface on its base, via which it articulates with Trapezium.
The first metacarpal provides the attaching points to the several hand muscles;
The opponent pollicis muscle attaches to the radial border and therefore the adjacent part of the shaft. The radial head of the primary dorsal interosseous muscle attaches to the ulnar border and therefore the adjacent part of the shaft.
The abductor pollicis longus inserts to the palmar surface of the shaft.
Second metacarpal
Synonyms: Os metacarpals 2
The metacarpal 2 is the one with the most important base and therefore the longest shaft. Its base shows several areas for the articulations with the carpal bones;
A groove within the coronal plane via which it articulates with the trapezoid. Medially to the present groove may be a ridge for the articulation with the capitate, while laterally may be a quadrangular surface for the joint with Trapezium bone. An elongated facet on its medial surface for the articulation with the third metacarpal. Dorsally to the facet for Trapezium may be a shallow impression of the insertion of the extensor carpi radialis longus muscle. The palmar surface of the bottom 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 primary dorsal interosseous attaches from the lateral side, while the second palmar and second dorsal interossei attach from the medial side.
Third metacarpal
Synonyms: Os metacarpals 3
The metacarpal 3 is found at the bottom of the center finger. It differs from the others by a process that projects proximally from the laterodorsal fringe of its base. This process participates within the joint with the capitate. The lateral surface of the bottom articulates with the second metacarpal, while the medial surface articulates with the fourth metacarpal via two oval articular surfaces. The palmar surface of the bottom is where the flexor carpi radialis insert, while the extensor carpi radialis brevis inserts to its dorsal surface.
The shaft is analogous thereto of the second metacarpal, being dorsally convex. Its lateral side is 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
The metacarpal 4 shows a couple of specificities of its base.
It has two oval facets on the lateral surface via which it articulates with the bottom of the third metacarpal. It features one elongated facet on the medial surface for the articulation with the bottom of the fifth metacarpal. The proximal surface is quadrangular and serves for the articulation with the hamate. The shaft provides the insertion point for the third palmar interosseous and therefore the ulnar head of the third dorsal interosseous on its lateral side, while the medial side is where the fourth dorsal interosseous attaches.
Fifth metacarpal
The metacarpal 5 is the smallest of all five metacarpals. Its base slightly differs from the opposite 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 bottom, however, articulates with the hamate.
The proximal part of the lateral surface of the shaft articulates with the fourth metacarpal, while the medial surface serves to connect the opponent’s digit 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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