The first metacarpal bone or the metacarpal bone of the

thumb The thumb is the first digit of the hand, next to the index finger. When a person is standing in the medical anatomical position (where the palm is facing to the front), the thumb is the outermost digit. The Medical Latin English noun for thu ...

is the first bone proximal to the thumb. It is connected to the trapezium of the carpus at the first

carpometacarpal joint The carpometacarpal (CMC) joints are five joints in the wrist that articulate the distal row of carpal bones and the proximal bases of the five metacarpal bones. The CMC joint of the thumb or the first CMC joint, also known as the trapeziometa ...

and to the proximal thumb

phalanx The phalanx ( grc, φάλαγξ; plural phalanxes or phalanges, , ) was a rectangular mass military formation, usually composed entirely of heavy infantry armed with spears, pikes, sarissas, or similar pole weapons. The term is particular ...

at the first

metacarpophalangeal joint The metacarpophalangeal joints (MCP) are situated between the metacarpal bones and the proximal phalanges of the fingers. These joints are of the condyloid kind, formed by the reception of the rounded heads of the metacarpal bones into shallow c ...

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 A bone is a rigid organ that constitutes part of the skeleton in most vertebrate animals. Bones protect the various other organs of the body, produce red and white blood cells, store minerals, provide structure and support for the body, a ...

surrounding the open

medullary canal The medullary cavity (''medulla'', innermost part) is the central cavity of bone shafts where red bone marrow and/or yellow bone marrow (adipose tissue) is stored; hence, the medullary cavity is also known as the marrow cavity. Located in the ma ...

; the latter two consisting of

cancellous bone A bone is a rigid organ that constitutes part of the skeleton in most vertebrate animals. Bones protect the various other organs of the body, produce red and white blood cells, store minerals, provide structure and support for the body, and ...

surrounded by a thin cortical shell.

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 In anatomy, a sesamoid bone () is a bone embedded within a tendon or a muscle. Its name is derived from the Arabic word for 'sesame seed', indicating the small size of most sesamoids. Often, these bones form in response to strain, or can be prese ...

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

Body/Shaft

The body or shaft is thick and broad — averaging . On its dorsal side, the shaft is flat and wide, while the anteroposterior side is less pronounced; usually resulting in an oval-triangular cross-section. The dorsal surface of the shaft is weakly convex longitudinally, while its palmar, radial, and ulnar surfaces tend to be concave. The palmar and medial surface exhibits a blunt ridge which separates a larger lateral part – the insertion of the

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. It lies deep to the abductor pollicis brevis and lateral to the flexor pollicis brevis. Structure The ...

– 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 and lateral side of the base is the insertion of the tendon of the abductor pollicis longus muscle, usually featuring a small tubercle. The origin of the first dorsal interosseous muscle 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.

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. When the both sexes were considered together, the ossification of base of first metacarpal was seen to start between 13 and 41 months. These centres unite before fifteenth year in girls and seventeenth year in boys. The thumb has several associated

accessory bone An accessory bone or supernumerary bone is a bone that is not normally present in the body, but can be found as a variant in a significant number of people. It poses a risk of being misdiagnosed as bone fractures on radiography. Wrist and hand ...

s. When present, these bones are usually found near the base and trapezium 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 metacarpal bones account for 30-40% of all hand fractures, of which 25% occur in the first metacarpal (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 and Rolando's fracture

Additional images

File:First metacarpal bone (left hand) - animation01.gif, First metacarpal bone of the left hand (shown in red). Animation. File:First metacarpal bone (left hand) - animation02.gif, First metacarpal bone of the left hand. Close up. File:Gray219 - First metacarpal bone.png, Palmar view of the left hand (first metacarpal shown in yellow). File:Gray220 - First metacarpal bone.png, Dorsal view of the left hand (first metacarpal shown in yellow). File:Rolando fracture.jpg, Fracture of the first metacarpal ( Rolando's fracture). File:Slide4val.JPG, First metacarpal bone. Deep dissection.

Notes

References

* * {{DEFAULTSORT:First Metacarpal Bone Skeletal system Bones of the hand Metacarpus