Notes on Phalanges (Attachments & Ossification)

There are 14 phalanges in each hand, 3 for each finger and 2 for the thumb. Each phalanx has a base, a shaft and a head.

Base

In the proximal phalanx, the base is marked by a concave oval facet for articulation with the head of the metacarpal bone. In the middle phalanx, or a distal phalanx, it is marked by two small concave facets separated by a smooth ridge.

Shaft

The shaft tapers towards the head. The dorsal surface is convex from side to side. The palmar surface is flattened from side to side, but is gently concave in its long axis.

Head

In the proximal and middle phalanges, the head has a pulley-shaped articular surface. In the distal phalanges, the head is nonarticular, and is marked anteriorly by a rough horseshoe-shaped tuberosity which supports the sensitive pulp of the fingertip.

Attachments

1. Base of the Distal Phalanx

(a) The flexor digitorum profundus is inserted on the palmar surface.

(b) Two-side slips of digital expansion fuse to be inserted on the dorsal surface. These also extend the insertion of lumbrical and interossei muscles.

2. The Middle Phalanx

(a) The flexor digitorum superficialis is inserted on each side of the shaft.

(b) The fibrous flexor sheath is also attached to the side of the shaft.

(c) A major part of the extensor digitorum is inserted on the dorsal surface of the base.

3. The Proximal Phalanx

(a) The fibrous flexor sheath is attached to the sides of the shaft.

(b) On each side of the base, parts of the lumbricals and interossei are inserted.

4. In the thumb, the base of the proximal phalanx provides attachments to the following structures.

(a) The abductor pollicis brevis and flexor pollicis brevis are inserted on the lateral side.

(b) The adductor pollicis and the first palmar interosseous are inserted on the medial side.

(c) The extensor pollicis brems is inserted on the dorsal surface.

5. In the little finger, the medial side of the base of the proximal phalanx provides insertion to the abductor digiti minimi and the flexor digiti minimi.

Ossification

The shaft of each phalanx ossifies from a primary centre which appears during the 8th week of development in the distal phalanx, 10th week in the proximal phalanx and 12th week in the middle phalanx.

The secondary centre appears for the base during 2-4 years and fuses with the shaft during 15-18 years.