Overall Bony Structure
– the concept of rays, arches, and motion – what is and what isn’t mobile in the hand, and how this influences function.
– 27 bones: 5 metacarpals, 14 phalanges, 8 carpal bones
– 5 rays: made of the metacarpal and phalanges
The first ray is unique in terms of structure and mobility.
– the first ray consists of the thumb phalanges and 1st metacarpal – important for its movement, and relative autonomy owing to scapholunate mobility and the outward angulation of the trapezium in the carpal plane, so that the first metacarpal makes an angle of about 45o with the second. This and the biconcavity of the metacarpotrapezial joint explains the gap between the first ray and the others, and the potential of the first ray to oppose the others.
The rays have different degrees of mobility and independence
– the second ray bends in the sagittal plane, while the others must rotate to converge towards the scaphoid tubercle.
– at the carpo-metacarpal articulation, the 1st and 5th rays have the greatest mobility, the 4th has some, while the 2nd and 3rd are quite fixed. The 4th CMC joint can flex about 10o, the 5th about 20o.
– The rays differ in mobility and independence – the thumb is very mobile and independent; the 5th is less so, and the remaining have very little independent motion. The index ray is independent at the phalangeal level owing to the arrangement of the tendons.
The rays function differently in grip/pinch.
– The two ulnar rays act in palmar grip for support and static control
– The three radial rays act for precision grip and dynamic control (although the third can be integrated with the ulnar digits in power grip.)
There are three arches -carpal arch, metacarpal arch, and longitudinal arches
Carpal Arch: formed by the proximal and distal rows of carpal bones. The proximal row is more mobile than the distal. The scaphoid bridges the two rows and serves to stabilize the midcarpus.
Metacarpal Arch: formed by the metacarpal heads which are bound together by the deep transverse metacarpal ligament connecting the volar plates of each MCP articulation. This arch is mobile, and the concavity depends on the mobility of the external metacarpals (1, 4, 5) – in full extension, the arch is flattened or reversed. The heads of the second and third are fixed.
Longitudinal Arches: for each ray the longitudinal arch spans from the fixed CMC articulation to the mobile digits, with the MCP articulation acting as the keystone; the MCP is therefore essential for support of the longitudinal arch