The carpus is made up of 8 carpal bones, which are arranged in two rows.
1. The proximal row contains (from lateral to medial side) (i)the scaphoid, (ii)lunate, (iii) triquetral, and (iv) pisiform bones.
2. The distal row contains in the same order (i) the trapezium, (ii) trapezoid, (iii) capitate, and (iv) hamate bones.

Identification

1. The scaphoid, is boat-shaped and has a tubercle on its lateral side.
2. The lunate, is half-moon-shaped or crescentic.
3. The triquetral, is pyramidal in shape, and has an isolated oval facet on the distal part of the palmar surface.
4. The pisiform, is pea-shaped, and has only one oval facet on the proximal part of its dorsal surface.
5. The trapezium, is quadrangular in shape, and has a crest and a groove anteriorly. It has a concavoconvex articular surface distally.
6. The trapezoid, resembles the shoe of a baby.
7. The capitate, is the largest carpal bone, with a rounded head.
8. The hamate, is wedge-shaped, with a hook near its base.

General Features

1. The proximal row is convex proximally, and concave distally.
2. The distal row is convex proximally and flat distally.
3. Each bone has 6 surfaces:
(i) The palmar and dorsal surfaces are non-articular except for the triquetral and pisiform.
(ii) The lateral surfaces of the two lateral bones (scaphoid and trapezium) are nonarticular.
(iii) The medial surfaces of the three medial bones (triquetral, pisiform and hamate) are nonarticular.

4. The dorsal nonarticular surface is always larger than the palmar nonarticular surface, except for the lunate, in which the palmar surface is larger than the dorsal.

The general points help in identifying the proximal, distal, palmar and dorsal surfaces in most of the bones. The side can be finally determined with the help of the specific points.

Specific Features

1. The scaphoid.

The tubercle is directed laterally, forwards and downwards.

2. The lunate.

(i) A small semilunar articular surface for the scaphoid is on the lateral side, and

(ii) a quadrilateral articular surface for the triquetral is on the medial side.

3. The triquetral.

(i) The oval facet for the pisiform lies on the distal part of the palmar surface.

(ii) The medial and dorsal surfaces are continuous and nonarticular.

4. The pisiform

(i) The oval facet for the triquetral lies on the proximal part of the dorsal surface.

(ii) The lateral surface is grooved by the ulnar nerve.

5.The trapezium

(i) The palmar surface has a vertical groove for the tendon of the flexor carpi radialis

(ii) The groove is limited laterally by the crest of the trapezium

(ii) The distal surface bears a concavoconvex articular surface for the base of the first metacarpal bone.

6. The trapezoid.

(i) The distal articular surface is bigger than the proximal.

(ii) The palmar nonarticular surface is prolonged laterally.

7. The capitate.

The dorsomedial angle is the distal-most projection from the body of the capitate. It bears a small facet for the 4th metacarpal bone.

8. The hamate.

The hook projects from the distal part of the palmar surface, and is directed laterally.

Attachments

There are four bony pillars at the four corners of the carpus. All attachment are to these four pillars.
1. The tubercle of the scaphoid gives attachment to (i) the flexor retinaculum; and (ii) a few fibres of the abductor pollicis brevis.
2. The pisiform gives attachment to the following: (i) Flexor carpi ulnaris, (ii) Flexor retinaculum, (iii) abductor digit minimi, and (iv) extensor retinaculum.
3. The trapezium has the following attachments: (i) The crest gives origin to the abductor pollicis brevis, flexor pollicis brevis, and opponens pollicis (ii) The edges of the groove give attachment to the two layers of the flexor retinaculum, (iii) The lateral surface gives attachment to the lateral ligament of the wrist joint. (iv) The groove lodges the tendon of the flexor carpi radialis.
4. Hamate. (i) The tip of the hook gives attachment to the flexor retinaculum. (ii) The medial side of the hook gives attachment to the flexor digiti minimi and the opponens digit minimi.

Articulations

1. The scaphoid articulates with the following bones: radius, lunate, capitate, trapezium and trapezoid.
2. The lunate articulates with the following bones: radius, scaphoid, capitate, hamate and triquetral

3. The triquetral articulates with the following: pisiform, lunate, hamate and articular disc of the inferior radioulnar joint.
4. The pisiform articulates with the triquetral.
5. The trapezium articulates with the following: scaphoid, first and second metacarpals, and trapezoid.
6. The trapezoid articulates with the following: scaphoid, trapezium, second metacarpal and capitate.
7. The capitate articulates with the following: scaphoid, lunate, hamate, 2nd, 3rd and 4th metacarpals and trapezoid.
8. The hamate articulates with the following: lunate, triquetral, capitate, and 4th and 5th metacarpls.

Text adapted from: Human Anatomy by BD Chaurasia

Image Credit: Wikipedia

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Tags: capitate, carpal bones, hamate, lunate, pisiform, scaphoid, trapezium, trapezoid, triquetral

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