Carpus

The carpus is composed of eight separate bones arranged around the capitate, but commonly described as forming two rows each of four bones. Three of the bones in the proximal row articulate above with the radius or articular disc at the radiocarpal joint, whilst below they articulate with the radius or articular disc at the radiocarpal joint, whilst below they articulate with the distal row of bones forming the midcarpal joint. The four carpal bones of the distal row articulate with the bases of the five metacarpal bones via the carpometacarpal joints. There are also articulations between the adjacent carpal bones in each of the rows, the intercarpal joints.

The bones are bound together by ligaments and so form a compact mass, which is curved to give a posterior convexity and a pronounced anterior concavity(the carpal sulcus). This sulcus is converted into a canal(carpal tunnel) by the flexor retinaculum.

The individual carpal bones are clinically important because they are often injured, especially the scaphoid and lunate, and because they provide recognizable bony landmarks in the wrist region.

From lateral to medial the proximal and distal rows are arranged as follows:

Proximal: scaphoid, lunate, triquetral, pisiform

Distal: trapezium, trapezoid, capitate, hamate

Proximal row

The three lateral bones to the proximal row are so arranged as to form a convex articular surface facing proximally to fit into the concavity formed by the radius and the articular disc. Individually, each of the bones has a characteristic shape and its own set of articular surfaces.

Scaphoid – The scaphoid is marked anteriorly by a prominent palpable tubercle and a narrowed waist around its centre. Articular surfaces are present on the scaphoid: proximally for the radius, medially for the lunate and more distally for the head of the capitate, and lateral to the tubercle for the trapezium and trapezoid. The small, non-articular surface of the tubercle is the only region available for the entry of blood vessels. It is a common site of fracture.

Lunate – The lunate has a smooth convex palmar surface which is larger than its dorsal surface. On its medial side is a square articular surface for the triquetral, and on its lateral side a crescent – shaped area for the scaphoid. Distally, there is a deep concavity for the head of the capitate, while proximally the bone is convex where it articulates with the radius and articular disc.

Triquetral – The triquetral lies in the angle between the lunate and hamate, with which it articulates via a sinuous surface. The square lateral articular surface is for the lunate. The triquetral is distinguished by a circular articular surface for the pisiform. The proximal part enters the radiocarpal joint during addiction of the hand.

Pisiform – The pisiform is a small round sesamoid bone found in the tendon of flexor carpi ulnaris. It articulates with the palmar surface of the triquetral. The anterior surface projects distally and laterally forming the medial part of the carpal tunnel.

Distal row

The distal row of carpal bones presents a more complex proximal articular surface, being flat laterally and convex medially. Individually, the bones all have a characteristic shape.

Trapezium – The trapezium is the most irregular of the carpal bones, with a palpable tubercle and groove medially on its anterior surface. It has articular surfaces proximally for the scaphoid and trapezoid, which are set at an angle to each other. Its main feature is the articular surface for the base of the first metacarpal. This articular surface is saddle-shaped and faces distally, laterally and slightly forwards, contributing greatly to the mobility of the carpometacarpal joint of the thumb.

Trapezoid – The trapezoid is a small and irregular bone which articulates with the second metacarpal. It lies in the space bounded by the metacarpal, scaphoid, capitate and trapezium, articulating with each.

Capitate – The capitate is the largest of the carpal bones being centrally placed with a rounded head articulating with the concavities of the lunate and scaphoid. Medially and laterally there are flatter articular surfaces for the hamate and trapezoid respectively. The dorsal surface is flat, but the palmar aspect is roughened by ligamentous attachments. The distal surface articulates mainly with the base of the third metacarpal, but also by narrow surfaces with the bases of the second and fourth metacarpals.

Hamate – The hamate is wedge-shaped with a large curved palpable hook projecting from its palmar surface near the base of the fifth metacarpal. The hook is concave on its lateral side forming part of the carpal tunnel. The distal base of the wedge articulates with the bases of the fourth and fifth metacarpals. The wedge passes up between the capitate and triquetral to reach the lunate. The articular surface for the capitate is flat and that for the triquetral is sinuous.

Overall the carpus presents a deep transverse concavity on the palmar surface. The flexor retinaculum bridges the concavity, attaching to the tubercles of the scaphoid and trapezium laterally, and the pisiform and hook of hamate medially, forming the roof of the carpal tunnel.

Palpation

Starting on the medial side of the palmar aspect of the wrist at the proximal part of the hypothenar eminence, the pisiform can be distinquished easily with the tendon of flexor carpi ulnaris running proximally from it. Immediately distal and slightly lateral to the pisiform, the hook of the hamate can be palpated if sufficient pressure is applied through the hypothenar muscles.

On the lateral side of the carpus just proximal to the distal wrist crease, the prominent tubercle of the scaphoid can be palpated, and immediately beyond this, the tubercle of the trapezium. The scaphoid can be “pinched” between the palpating thumb and index finger if these are placed on the tubercle and in the “anatomical snuff-box” at the base of the thumb on its dorsal surface.

Ossification

Each carpal bone ossifies from a single centre, all of which appear after birth. During the first year of life the centres for the capitate and hamate appear. These are followed by centres for the triquetral between 2 and 4 years, the lunate between 3 and 5 years, the scaphoid, trapezium and trapezoid all between 4 and 6 years, and finally the pisiform between 9 and 14 years. Ossification is not complete until between 20 and 25 years. The hook of hamate may be separate. Small additional nodules may also be present. The shape of the individual carpal bones, and not their size, can be used to age an individual.