First Metacarpal

Definition of the Primary Metacarpal Bone

The primary metacarpal (1st Metacarpal) is the metacarpal or palm bone connected to the thumb and is the briefest, and most flexible of all the five metacarpal bones [1].


As the metacarpal of the thumb, it is positioned distally on the radial side of the human palm, between the distal carpal row, and the initial proximal phalanx [12].

Primary Metacarpal

Growth and Ossification

The primary metacarpal begins to ossify from two centers, the first for the body or shaft, and another for its base [11]. This indicates that this metacarpal ossifies in a similar manner as a phalanx. This observation led some anatomists to propose that the thumb actually has three phalanges instead of two phalanges and a metacarpal [10].

Primary Metacarpal X-Ray Picture

The ossification centers for the shafts for all metacarpals start appearing around the 8th or 9th week of fetal growth, with that of the primary metacarpal appearing last. Its base begins to ossify when the child is about 3 years old [9].

Anatomy of the Metacarpal of Thumb

Surfaces and Joints

On its head, it connects with the proximal phalanx of the thumb [1] to form the first metacarpophalangeal joint, while the base of the primary metacarpal connects with the carpal bone trapezium to form the carpometacarpal joint of the thumb or the trapeziometacarpal joint (TMJ) [2].

Carpometacarpal Joint (CMC Joint) of the Thumb

The CMC joint of the thumb is of fundamental importance in structuring the hand in humans and other primates, allowing the use of the thumb to hold objects [3]. The articulation between the primary metacarpal and trapezium forms a saddle joint [2]. It is a type of synovial joint where the trapezium forms a concave or saddle-shaped articular facet for the base of the primary metacarpal so the latter can sit into the space like a rider on horseback [4]. As a result, the metacarpal of the thumb makes almost a right angle to the metacarpals of the other digits, allowing it to rotate and move in nearly all directions [5].

Muscle Attachments

There are three muscles attaching to the primary metacarpal, namely, the abductor pollicis longus, opponens pollicis, and the first dorsal interosseous. Additionally, it is also partially the spot of origin for the flexor pollicis brevis, which primarily originates from the trapezium [6].

Common Injuries and Associated Conditions

Thumb Carpometacarpal Arthritis: Since over half of all functions of the hand involve the thumb, the primary CMC joint is subjected to a lot of stress and pressure every day. As a result, it is the most common CMC joint, as well as thumb joint, to get arthritis [7]. Characteristic symptoms include pain and swelling around the base of the thumb, gradually radiating to the wrist, causing difficulty in any twisting movement like rotating a doorknob. Treatment may include exercise, anti-inflammatory medication, and surgery [3].

Fracture of the Primary Metacarpal: A fracture in this area can occur as a result of an accident or some sports injury, with the most serious type of fractures involving the carpometacarpal, and metacarpophalangeal joints of the thumb. The fractures may have specific names depending on their nature and location (e.g. the Bennett fracture at the base of the primary metacarpal), with the treatment methods varying accordingly as well [8].


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