Transverse head of adductor pollicis

At a glance

English

Transverse head of adductor pollicis

Latin (TA2)

Caput transversum adductoris pollicis

System

Muscle

Region

Upper limb

Side

Paired, left and right

About the transverse head of adductor pollicis

Adductor pollicis is the muscle that pulls the thumb back in against the palm and the index finger, and it has two heads: oblique and transverse. The transverse head is the lower, more horizontal of the pair. It lies deep in the palm, beneath the lumbricals and long flexor tendons, and over the metacarpals and interossei, forming much of the fleshy web between thumb and index finger.

Its fibres arise along the front of the third metacarpal shaft, over roughly its distal two-thirds, so the head has a broad base on the middle of the palm. From there they converge sideways like a fan towards the thumb. They join the oblique head and the deep part of flexor pollicis brevis in a shared tendon that attaches to the ulnar edge of the thumb's proximal phalanx at its base, with a sesamoid bone in the tendon just before it reaches bone.

The gap separating the two heads is an important doorway. Through it the radial artery reaches the palm from the dorsum of the hand and goes on to make the deep palmar arch. The ulnar nerve's deep branch travels the other way across the palm near the same place, slipping under a thin fibrous arch between the two heads with the arch beside it.

Acting with the oblique head, the transverse head adducts the thumb at the carpometacarpal joint, bringing it from a spread position back alongside the palm, and gives power to a key grip, such as holding a card between thumb and the side of the index finger. Weakness shows as Froment's sign: when the patient grips paper this way, the thumb tip bends because flexor pollicis longus is recruited instead, a sign of ulnar nerve damage. Anaesthetists stimulate the ulnar nerve and watch this muscle twitch to monitor drugs that relax muscles during surgery.

Attachments, action and supply

Origin

Palmar surface of the shaft of the third metacarpal, mainly its distal two-thirds.

Insertion

Ulnar edge of the thumb's proximal phalanx at its base, through a tendon shared with the oblique head, containing the ulnar sesamoid.

Action

Adducts the thumb at the carpometacarpal joint and helps power the key grip between thumb and index finger.

Nerve supply

Ulnar nerve, deep branch (C8, T1).

Blood supply

Deep palmar arch, with branches of the princeps pollicis and palmar metacarpal arteries.