In human anatomy, the metacarpal bones or metacarpus form 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. The metacarpal bones are analogous to the metatarsal bones in the foot.
Structure
The metacarpals form a transverse arch to
which the rigid row of distal carpal bones are fixed. The peripheral
metacarpals (those of the thumb and little finger) form the sides of the cup of
the palmar gutter and as they are 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
middle 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 bone consists of a body or shaft,
and two extremities: the head at the distal or
digital end (near the fingers), and the base at the proximal
or carpal end (close to the wrist).
Body
The body (shaft) is prismoid in
form, and curved, so as to be convex in the longitudinal direction behind,
concave in front. It presents three surfaces: medial, lateral, and dorsal.
·
The medial and lateral
surfaces are concave, for the attachment of the interossous muscles,
and separated from one another by a prominent anterior ridge.
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The dorsal surface presents in
its distal two-thirds a smooth, triangular, flattened area which is covered in
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 some distance above the center of the
bone and form a ridge which runs along the rest 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 is broader, and extends farther upward, on the
volar than on the dorsal aspect, and is longer in the antero-posterior than in
the transverse diameter. On either side of the head is a tubercle for the
attachment of the collateral ligament of the metacarpophalangeal joint.
The dorsal surface, broad and flat, supports the tendons of
the extensor muscles.
The volar surface is grooved in 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 the head.
1st 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.
·
The opponens pollicis muscle attaches to
the radial border and the adjacent part of the shaft.
·
The radial head of the first dorsal interosseous
muscle attaches to the ulnar border and the adjacent part of the shaft.
· The abductor pollicis longus inserts to the palmar surface of the shaft.
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;
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A groove in the coronal plane via which it
articulates with the trapezoid bone.
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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.
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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 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.
3RD 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 palmar surface of the base is where the flexor carpi
radialis insert, while the extensor carpi radialis brevis inserts to
its dorsal surface.
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.
4TH METACARPAL BONE
The metacarpal bone 4 shows a few specificities of its base.
It has 2 oval facets on the lateral surface via which it
articulates with the base of the third metacarpal.
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It features a single elongated facet on the
medial surface for the articulation with the base of the fifth metacarpal
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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.
Articulations
Besides the metacarpophalangeal joints, the metacarpal
bones articulate by carpometacarpal joints as follows:
1. the first with
the trapezium;
2. the second with the
trapezium, trapezoid, capitate and third metacarpal;
3. the third with the capitate
and second and fourth metacarpals;
4. the fourth with the
capitate, hamate, and third and fifth metacarpals;
5. and the fifth with the hamate
and fourth metacarpal
· 
Carpometacarpal joints of the left hand. Thumb on left.
Insertions
Extensor Carpi Radialis Longus/Brevis: Both insert on
the base of metacarpal II; Assist with wrist extension and radial flexion of
the wrist
Extensor Carpi Ulnaris: Inserts on the base of
metacarpal V; Extends and fixes wrist when digits are being flexed; assists
with ulnar flexion of wrist
Abductor Pollicis Longus: Inserts on the trapezium
and base of metacarpal I; Abducts thumb in frontal plane; extends thumb at
carpometacarpal joint
Opponens Pollicis: Inserts on metacarpal I; flexes
metacarpal I to oppose the thumb to the fingertips
Opponens digiti minimi: Inserts on the medial surface
of metacarpal V; Flexes metacarpal V at carpometacarpal joint when little
finger is moved into opposition with tip of thumb; deepens palm of hand.
·
Metacarpus (yellow). Insertions are shown in
red. Left hand, anterior (palmar) view.
Blood and Lymphatic Supply of Metacarpal Bones
The blood supply of the metacarpal bones is primarily derived from the dorsal and palmar metacarpal arteries, which are branches of the radial and ulnar arteries. These arteries form a rich anastomotic network on both the dorsal and palmar surfaces of the hand. The nutrient arteries, typically one for each metacarpal, penetrate the diaphysis (shaft) of the bone through the nutrient foramen. These vessels supply the medullary cavity and the inner two-thirds of the cortex, while the periosteal arteries supply the outer one-third of the cortical bone. The venous drainage mirrors the arterial supply, with blood flowing into the dorsal and palmar venous networks.
Lymphatic drainage of the metacarpals is part of the larger lymphatic system of the upper limb. The lymph from the metacarpal region is collected by superficial and deep lymphatic vessels. The superficial vessels primarily follow the cephalic and basilic veins to drain into the axillary lymph nodes, while the deep lymphatic vessels accompany the deep veins and drain into the lateral group of axillary lymph nodes.
Ossification
- Primary Ossification Center: This appears during the 9th week of fetal development in the diaphysis of the metacarpals.
- Secondary Ossification Center: For the first metacarpal, the secondary center appears at the base, whereas for the second to fifth metacarpals, it appears at the head. The timing of their appearance varies, typically between 2 and 4 years of age.
- Fusion: The secondary ossification centers fuse with the primary center around 17 to 20 years of age. This fusion corresponds to the end of skeletal growth in the hand.
