METACARPALS

Introduction

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.

·       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. 

2ND 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 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.

·       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.

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.

·       Metacarpus (yellow). Insertions are shown in red. Left hand, posterior (dorsal) 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 

The metacarpal bones ossify through a process of endochondral ossification, beginning in the embryonic period and continuing into early childhood. Each metacarpal bone has a primary ossification center located in the shaft and a secondary ossification center located at the base (for the first metacarpal) or the head (for the second to fifth metacarpals).

1. Primary Ossification Center: This appears during the 9th week of fetal development in the diaphysis of the metacarpals.
2. 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.
3. 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.

COMPILED & WRITTEN BY DR. PALAK SHAH