Deep head of flexor pollicis brevis

At a glance

English

Deep head of flexor pollicis brevis

Latin (TA2)

Caput profundum flexoris brevis pollicis

System

Muscle

Region

Upper limb

Side

Paired, left and right

Arises from

The first metacarpal bone

About the deep head of flexor pollicis brevis

Flexor pollicis brevis is one of the three short thenar muscles at the base of the thumb, and it is divided into a superficial head and a deep head. The deep head is the smaller and more medial portion. It is variable in size and sometimes missing, and it lies deep to the tendon of flexor pollicis longus, close to the oblique head of adductor pollicis, which makes it hard to separate in dissection.

It arises from the carpal tunnel's floor, on the trapezoid and capitate bones and the palmar ligaments binding the distal carpal bones. Its fibres run outward and downward, passing under the long flexor tendon of the thumb, to join the superficial head. The combined tendon attaches to the thumb's proximal phalanx, at the radial side of its base, and the radial sesamoid bone of the thumb is embedded in it at the metacarpophalangeal joint. Some descriptions also give it fibres from the base of the first metacarpal.

Acting with the superficial head, it bends the thumb at its knuckle (metacarpophalangeal) joint and helps to flex and turn the first metacarpal inward at the carpometacarpal joint, a part of opposition. It is used in pinching and in gripping objects between the thumb and the side of the index finger, and it helps to steady the thumb during power grip.

The nerve supply of the two heads differs. The superficial head is usually supplied by the recurrent branch of the median nerve, while the deep head is often supplied by the ulnar nerve's deep branch (C8, T1), though the pattern varies and many muscles receive fibres from both. This explains why thumb flexion may be partly preserved after a median or ulnar nerve injury alone. Its blood comes from the superficial palmar branch of the radial artery and branches of the deep palmar arch.

Attachments, action and supply

Origin

Trapezoid and capitate bones and the palmar ligaments joining the distal carpal bones, in the carpal tunnel's floor.

Insertion

Proximal phalanx of the thumb, radial side of its base, via the tendon shared with the superficial head and the radial sesamoid.

Action

Flexes the metacarpophalangeal joint of the thumb; helps flex and medially rotate the first metacarpal in opposition.

Nerve supply

Usually the ulnar nerve through its deep branch (C8, T1); sometimes the median nerve.

Blood supply

Superficial palmar branch of the radial artery and branches of the deep palmar arch.