The thenar musculature consists of four muscles located on the radial side of the palm. Together they form the 'ball' of the thumb known as the thenar eminence. They originate at different carpal bones and distally attach to the thumb. The thenar muscles are:
Abductor pollicis brevis
This muscle arises from the tubercles of the scaphoid and trapezium, and from the flexor retinaculum. From there, its short tendon courses to the base of the proximal phalanx and the dorsal aponeurosis of the thumb via the radial sesamoid bone. The abductor pollicis brevis lies quite superficially, underneath the skin.
This muscle has two origin surfaces. The transverse head originates from the palmar base of the third metacarpal bone, while the oblique head arises from the capitate bone and the palmar bases of the second and third metacarpal bones. The common tendon attaches distally to the proximal phalanx and the dorsal aponeurosis of the thumb via the ulnar sesamoid bone. The adductor pollicis is the deepest of all thenar muscles.
Flexor pollicis brevis
It has two heads separated by the tendon of the flexor pollicis longus. The superficial head originates from the flexor retinaculum and the deep head from both the capitate and trapezium bones. The tendon runs to the base of the proximal phalanx of the thumb via the radial sesamoid bone.
This muscle courses from the tubercle of the trapezium bone and the flexor retinaculum to the radial surface of the first metacarpal bone. It is mostly covered by the abductor pollicis brevis.
The recurrent (or thenar) branch of the median nerve (C8-Th1) is responsible for the innervation of the abductor pollicis brevis and opponens pollicis. The adductor pollicis is supplied by the deep branch of the ulnar nerve (C8-Th1).
The flexor pollicis brevis is the only thenar muscle receiving a double innervation by both nerves due to its transformation during the course of embryogenesis. Most frequently, the median nerve supplies the superficial head and the ulnar nerve the deep head. However, the innervation pattern is quite variable due to the fact that most people have an interconnection between the recurrent branch of the median nerve and the deep branch of the ulnar nerve at the 'ball' of the thumb (Riches-Cannieu anastomosis).
The main function of each thenar muscle is associated to their names:
- The abductor pollicis brevis moves the thumb away from the hand (abduction at the carpometacarpal joint [CMC]).
- The powerful contraction of the adductor pollicis moves the thumb towards the hand (adduction at the CMC).
- The flexor pollicis brevis is mainly responsible for bending the thumb (flexion at the CMC).
- The opponens pollicis does a combination of flexion, abduction and a medial rotation in the CMC, which all together result in opposition.
By those means, the thumb is able to touch the other fingers and fulfill grip movements. Since the abductor pollicis brevis, adductor pollicis and flexor pollicis brevis attach more distally at the thumb, they can also perform an extension or flexion at the metacarpal joint of the thumb (MCP I).
A classic test used to examine the ulnar nerve is the Froment's sign. During that exam, the patient is asked to hold a sheet of paper between his thumb and index fingers, whereupon the examiner tries to pull it away against the resistance. Those affected by an ulnar lesion, and thus a weakened adductor pollicis, will be forced to make use of their flexor pollicis instead (supplied by the median nerve!), which can be recognized visually by the flexion of not only the CMC but also the IP of the thumb (positive Froment's sign). Common causes for a positive Froment’s sign are an ulnar nerve entrapment or injuries of the elbow.