The first metacarpal bone or the metacarpal bone of the thumb

Thumb

The thumb is the first digit of the hand. When a person is standing in the medical anatomical position , the thumb is the lateral-most digit...

is the first bone of the thumb. It is connected to the trapezium

Trapezium (bone)

The trapezium bone is a carpal bone in the wrist.The trapezium is distinguished by a deep groove on its palmar surface. It is situated at the radial side of the carpus, between the scaphoid and the first metacarpal bone...

 of the carpus

Carpus

In tetrapods, the carpus is the sole cluster of bones in the wrist between the radius and ulna and the metacarpus. The bones of the carpus do not belong to individual fingers , whereas those of the metacarpus do. The corresponding part of the foot is the tarsus...

 at the first carpometacarpal joint

Carpometacarpal joint

The carpometacarpal joints are five joints in the wrist that articulates the distal row of carpal bones and the proximal bases of the five metacarpal bones....

 and to the proximal thumb phalanx

Phalanges of the hand

The phalanges of the hand are commonly known as the finger bones. They are fourteen in number, three for each finger, and two for the thumb.Each consists of a body and two extremities....

 at the first metacarpophalangeal joint

Metacarpophalangeal joint

The metacarpophalangeal joints are of the condyloid kind, formed by the reception of the rounded heads of the metacarpal bones into shallow cavities on the proximal ends of the first phalanges, with the exception of that of the thumb, which presents more of the characters of a ginglymoid joint...

.

Characteristics

The first metacarpal bone is short and thick with a shaft thicker and broader than those of the other metacarpal bones. Its narrow shaft connects its widened base and rounded head; the former consisting of a thick cortical bone

Cortical bone

Cortical bone, synonymous with compact bone, is one of the two types of osseous tissue that form bones. Cortical bone facilitates bone's main functions: to support the whole body, protect organs, provide levers for movement, and store and release chemical elements, mainly calcium. As its name...

 surrounding the open medullary canal; the latter two consisting of cancellous bone surrounded by a thin cortical shell.

Body/Shaft

The body or shaft is thick and broad — averaging 6 to 11 mm (0.236220472440945 to 0.433070866141732 ). On its dorsal side, the shaft is flat and wide, while the anteroposterior side is less pronounced; usually resulting in a oval-triangular cross-secion.

The dorsal surface of the shaft is weakly convex longitudinally, while its palmar, radial, and ulnar surfaces tend to be concave. The palmar (medial) surface exhibit a blunt ridge which separates a larger lateral part — the origin of the opponens pollicis muscle

Opponens pollicis muscle

The opponens pollicis is a small, triangular muscle in the hand, which functions to oppose the thumb. It is one of the three thenar muscles, lying deep to the abductor pollicis brevis and lateral to the flexor pollicis brevis.-Structure:...

 — and a smaller medial part — the origin of the lateral head of the first dorsal interosseous muscle.

Base

The base is significantly different from the bases of the other metacarpals. It is trumpet-shaped and ends in a saddle-shaped articular surface matching that of the trapezial articular surface. The configuration of the thumb carpometacarpal joint plays an important role in the mechanism of opposition. The articular surface is delimited by a thick, crest-like ridge extending around its circumference.

On the palmar (lateral) side of the base is the insertion of the tendon of the abductor pollicis longus muscle

Abductor pollicis longus muscle

The abductor pollicis longus muscle is one of the extrinsic muscles of the hand. It lies immediately below the supinator muscle and is sometimes united with it.-Origin and insertion:...

, usually featuring a small tubercle. The origin of the first palmar interosseous muscle

Palmar interossei muscles

The palmar interossei are small muscles in the hand that lie on the anterior aspect of the metacarpals. They are smaller than the dorsal interossei of the hand, which lie between the metacarpals.-Structure:...

 is on the ulnar aspect of the base, and can sometimes extend onto the shaft. In contrast to the other metacarpals, the first metacarpal has no articular facets on the sides of its base (for intermetacarpal articulations) because it articulates exclusively with the trapezium.

Head

The head is less rounded and less spherical than those of the other metacarpals, making it better suited for a hinge-like articulation.

The distal articular surface is quadrilateral, wide, and flat; thicker and broader transversely and extends much further palmarly than dorsally. On the palmar aspect of the articular surface there is a pair of eminences or tubercles which articulate with the radial and ulnar sesamoid bone

Sesamoid bone

In anatomy, a sesamoid bone is a bone embedded within a tendon.Sesamoids are found in locations where a tendon passes over a joint, such as the hand, knee, and foot. Functionally, they act to protect the tendon and to increase its mechanical effect. The presence of the sesamoid bone holds the...

s of the thumb metacarpophalangeal joint; the lateral eminence is larger than the medial.

Ossification

The metacarpal bone of the thumb has two centres of ossification: a primary centre in the shaft and a secondary centre in the base. This contrasts to the other four metacarpal bones where the secondary centre is found in the head. The ossification process begins in the shaft during the ninth week of prenatal life, and in the base during the second year of life in girls and the third year in boys. These centres unite before fifteenth year in girls and seventeenth year in boys.
The thumb has several associated accessory bone

Accessory bone

An accessory bone is a bone which does not occur frequently in the body but is still present in a significantly large number of people.- Examples :...

s. When present, these bones are usually found near the base and trapezium

Trapezium (bone)

The trapezium bone is a carpal bone in the wrist.The trapezium is distinguished by a deep groove on its palmar surface. It is situated at the radial side of the carpus, between the scaphoid and the first metacarpal bone...

 and are the products of an additional centre of ossification, usually of the trapezium, that failed to fuse with the associated bone. Named accessory bones include:

• os trapezium secundarium - between the ulnar base of the first metacarpal and the distal margin of the trapezium
• os praetrapezium - between the thumb metacarpal and the distal aspect of the trapezium
• os paratrapezium - between the radial base of the first metacarpal and the distoradial aspect of the trapezium

Fractures

Fractures to the first metacarpal bone account for 30-40% of all hand fractures and 25% of all metacarpal fractures (second to fractures to the fifth metacarpal bone). 80% of fractures to the first metacarpal occur at its base.
Common fractures to the thumb metacarpal include Bennett's fracture

Bennett's fracture

Bennett's fracture is a fracture of the base of the first metacarpal bone which extends into the carpometacarpal joint. This intra-articular fracture is the most common type of fracture of the thumb, and is nearly always accompanied by some degree of subluxation or frank dislocation of the...

 and Rolando's fracture.