Metacarpal Bones – Characteristics of Individual Metacarpal Bones

Metacarpal Bones – Characteristics of Individual Metacarpal Bones

1. The metacarpal bones are 5 miniature long bones, which are numbered from lateral to the medial side.

2. Each bone has a head placed distally, a shaft and a base at the proximal end.

(i) The head is round. It has an articular surface which extends more antero- posteriorly than laterally. It extends more on the palmar surface than on the dorsal surface. The heads of the metacarpal bones form the knuckles; (ii) The shaft is concave on the palmar surface. Its dorsal surface bears a flat triangular area in its distal part; (iii) The base is irregularly expanded.

Characteristics of Individual Metacarpal Bones :

1st (a) It is the shortest and stoutest of all metacarpal bones.

(b) The base is occupied by a concavo-convex articular surface for the trapezium.

(c) The dorsal surface of the shaft is uniformly convex.

(d) The head is less convex and broader from side to side than the heads of other metacarpals. The ulnar and radial corners of the palmar surface show impressions for sesamoid bones.

(e) The first metacarpal bone is rotated medially through 90° relative to the other metacarpals. As a result of this rotation, the movements of the thumb take place at right angles to those of other digits.

Metatarsal:

1. The head and shaft are flattened from side to side

2. The shaft tapers distally

3. The dorsal surface of the shaft is uniformly convex

4. The base appears to be cut sharply and obliquely

(f) It does not articulate with any other metacarpal bone.

2nd The base is grooved from before backwards. The medial edge of the groove is larger.

3rd The base has a styloid process projecting up from the dorsolateral corner.

4th The base has two small oval facets on its lateral side for the third metacarpal, and on its medial side it has a single elongated facet for the 5th metacarpal.

5th The base has an elongated articular strip on its lateral side for the 4th metacarpal. The medial side of the base is nonarticular and bears a tubercle.

Side Determination :

The proximal, distal, palmar and dorsal aspects of each metacarpal bone can be made out from what has been stated above. The lateral and medial sides can be confirmed by the following criteria.

Metacarpal :

1st The anterolateral surface is larger than the anteromedial.

2nd (a) The medial edge of the groove on the base is deeper than the lateral edge, (b) The medial side of the base bears an articular strip which is constricted in the middle.

3rd (a) The styloid process is dorsolateral.

(b) The lateral side of the base bears an articular strip which is constricted in the middle.

(c) The medial side of the base has two small oval facets for the 4th metacarpal.

4th (a) The lateral side of the base has two small oval facets for the 3rd metacarpal, (b) The medial side of the base has an elongated articular strip for the 5th metacarpal.

5th (a) The lateral side of the base has an elongated articular strip for the 4th metacarpal, (b) The medial side of the base is non­articular and has a tubercle.

Main Attachments:

The main attachment from shaft of metacarpals is of palmar and dorsal interossei muscles. Palmar interossei arise from one bone each except the 3rd metacarpal. Dorsal interossei arise from adjacent sides of two metacarpals. The other attachments are listed below.

Metacarpal

1st (a) The opponens pollicis is inserted on the radial border and the anterolateral surface of the shaft.

(b) The abductor pollicis longus is inserted on the lateral side of the base.

(c) The first palmar interosseous muscle arises from the ulnar side of the base.

2nd (a) The flexor carpi radialis is inserted on a tubercle on the palmar surface of the base.

The extensor carpi radialis longus is inserted on the dorsal surface of the base.

The oblique head of the adductor pollicis arises from the palmar surface of the base.

3rd (a) A slip from the flexor carpi radialis is inserted on the palmar surface of the base.

(b) The extensor carpi radialis brevis is inserted on the dorsal surface of the base, immediately beyond the styloid process.

(c) The oblique head of the adductor pollicis arises from the palmar surface of the base.

(d) The transverse head of the adductor pollicis arises from the distal two-thirds of the palmar surface of the shaft.

4th Only the interossei arise from it.

5th (a) The extensor carpi ulnaris is inserted on the tubercle at the base.

(b) The opponens digiti minimi is inserted on the medial surface of the shaft.

Articulations at the Bases

1. With the trapezium.

2. With the trapezium, the trapezoid, the capitate and the 3rd metacarpal.

3. With the capitate and the 2nd and 4th metacarpals.

4. With the capitate, the hamate and the 3rd and 5th metacarpals.

5. With the hamate and the 4th metacarpal.

Ossification

The shafts ossify from one primary centre each, which appears during the 9th week of development. A secondary centre for the head appears in the 2nd-5th metacarpals, and for the base in the 1st metacarpal. It appears during the 2nd-3rd year and fuses with the shaft at about 16-18 years.