The thumb metacarpal is shorter and thicker. Its base has a saddle-shaped facet for the trapezium. The shaft is also set at a right angle to the plane of the other four metacarpals such that the axis of thumb flexion and extension is across the palm .
For the remaining four metacarpals, each has the following features :
- body or shaft
- ridge on flexor surface
- long flat triangle on the extensor surface with its base against the head and its apex prolonged proximally as a ridge along the shaft
- the four metacarpals form a gentle concavity for the palm
- the sub-capital region inferior to the head
- it is the region between the shaft and the head
- nutrient foramina of the metacarpal is often located around the neck region
- round articular surface to the proximal phalanges
- extends further on the flexor than extensor surfaces
- adjacent surfaces of the heads are pitted by deep smooth fossae
- behind these fossae lie tubercles for the attachment of the collateral ligaments for the metacarpophalangeal joint
- the four heads form a convexity distally and dorsally make the knuckles of the fist
Proximally, the metacarpals articulate with the distal row of carpal bones :
- first (thumb) metacarpal: articulates with the trapezium via a saddle-shaped synovial joint
- second (index) metacarpal: articulates with the trapezoid as well as the trapezium via a small tubercle
- third (middle) metacarpal: articulates with the capitate
- fourth (ring) metacarpal: articulates with the hamate as well as a small part of the capitate
- fifth (little) metacarpal: articulates with the hamate
The four metacarpals other than the thumb metacarpal also articulate proximally with each other .
Distally, each metacarpal head articulates with the base of its corresponding proximal phalanx .
- opponens pollicis arises from the flexor retinaculum and inserts into the ridge along the radial border of the thumb metacarpal
- the transverse head of adductor pollicis arises from the ridge along the palmar surface of the shaft of the middle metacarpal
- the oblique head of adductor pollicis arises from the bases of the index and middle metacarpals
- opponens digiti minimi inserts into the palmar surface of the fifth metacarpal along the ulnar border of the shaft
- the palmar interossei arise from the flexor surfaces of the metacarpal shafts, from the groove that faces towards the middle metacarpal
- the dorsal interossei arise from the longitudinal grooves on the flexor surfaces of the metacarpal shafts and extend around to the dorsal surface
- flexor carpi radialis attaches to tubercles at the bases of the index and middle metacarpals
- extensor carpi ulnaris attaches to a tubercle at the base of the little metacarpal
- extensor carpi radialis brevis attaches to the styloid process of the middle metacarpal
- extensor carpi radialis longus attaches to the base of the index metacarpal
- abductor pollicis longus attaches to the base of the thumb metacarpal
- carpometacarpal joints are reinforced by strong anterior, posterior and interosseous ligaments
- at the thumb, this is especially reinforced by a ligament that passes from the tubercle of the trapezium to the dorsal prolongation of the base of the metacarpal
- at the 5th metacarpal, pisohamate and pisometacarpal ligaments attach the pisiform to the hook of hamate to the base of the metacarpal
- metacarpal heads are united by the deep transverse ligament of the palm
- collateral ligaments for the metacarpophalangeal joints attach to the pits and dorsal tubercles at the heads of the metacarpals
Three palmar metacarpal arteries arise from the deep palmar arch. They anastomose with the common palmar digital branches of the superficial arch at the metacarpal heads. They also perforate the interosseous spaces to anastomose with the dorsal metacarpal arteries. The dorsal metacarpal arteries are branches off the posterior carpal arch .
Sesamoid bones can sometimes be found at the metacarpal heads, especially the second and fifth, within the palmar capsule of the metacarpophalangeal joints .
The thumb metacarpal ossifies from two centers: one for the body and one for the base. The other four metacarpals also ossify from two centers: one for the body and one for the distal extremity. At the eighth to ninth week of foetal life, ossification commences in the middle of the body, with the centers for the second and third metacarpals being the first, and that for the first metacarpal the last to appear. At the third year the distal extremities of the metacarpals of the fingers and the base of the metacarpal of the thumb begin to ossify; they unite with the bodies at about the twentieth year .