Bones of the wrist are called carpal bones. They are small bones, 8 in number that make synovial joints with each other and thus add to the mobility of human hand. The carpal bones are arranged in two rows:
First (Proximal) Row:
The first or proximal row is made of: (from radial to ulnar side)
- Triquetral (Cuneiform)
Second (Distal) row:
The second or distal row is made of: (from radial to ulnar side)
Carpal bones of Proximal row:
The scaphoid is the largest and the most lateral bone of the first row.
Named because of its deeply excavated form, the lunate lies between the scaphoid on the lateral side and the triquetral on the medial side.
This bone may be recognized by the small oval or circular facet for the pisiform on its anterior surface. The bone is placed obliquely, so that its surfaces cannot be accurately described as distal, proximal, etc. It is attached to Lunate on the lateral side and pisiform of the antero-medial side.
About the size and shape of a large pea, the pisiform bone rests on the anterior surface of the triquetral, with which it articulates by an oval or circular facet on its dorsal aspect.
Carpal Bones of Distal row:
The trapezium is the most lateral bone of the distal row of the carpus. It may be readily recognized by the oval saddle-shaped facet on its distal surface for articulation with the metacarpal bone of the thumb.
With the exception of the pisiform, the Trapezoid is the smallest of the carpal bones. Its rough anterior surface is small and pentagonal in outline.
This is the largest of the carpal bones. Its anterior surface is rough and rounded. The proximal portion of the bone forms the head, and is furnished with convex articular facets which fit into the hollows on the medial surface of the scaphoid and distal surface of the lunate.
The hamate can be readily distinguished by the hook-like process (hamulus) which projects from the distal and medial aspect of its anterior surface.
The Carpus as a whole:
When the carpal bones are articulated together they form a bony mass, the dorsal surface of which is convex from side to side. Anteriorly they present a grooved appearance, concave from side to side. This arrangement is further emphasized by the forward projection, on the medial side, of the pisiform and hamate, while laterally the tuberosity of the scaphoid and the ridge of the trapezius help to deepen the furrow by their elevation. To these four points the transverse carpal ligament is attached, which stretches across from side to side, and thus converts the furrow into a canal.
Ossification of Carpal bones:
At birth the carpus is entirely cartilaginous. Different bones of the carpus take different length of time to ossify. The following data shows the time taken by each bone to ossify.
- Capitate = 11 to 12 months
- Hamate = 12 to 14 months
- Triquetral = 3 years
- Lunate = 5 to 6 years
- Traprzium = 6 years
- Scaphoid = 6 years
- Trapezoid = 6 to 7 years
- Pisiform = 10 to 12 years