Thenar Muscles

Introduction

Introduction to the Thenar Muscles

The thenar muscles are a group of three short muscles located on the radial (lateral) aspect of the palm, forming the fleshy thenar eminence. They are primarily responsible for the finely controlled movements of the thumb, including opposition, abduction, and flexion. These muscles are innervated predominantly by the median nerve, in particular its recurrent motor branch, which makes them vulnerable in carpal tunnel syndrome and other median nerve lesions. A fourth muscle, the adductor pollicis, lies deep in the palm and is not part of the thenar eminence, although it works with the thenar muscles to produce thumb adduction.

This module details the attachments, actions, nerve supply, vascular supply, and clinical significance of the thenar muscles.

Study

1. The Thenar Eminence and Its Muscles

The thenar eminence is the muscular bulge at the base of the thumb, consisting of three muscles arranged in two layers. Superficially lie the abductor pollicis brevis and flexor pollicis brevis; deep to them is the opponens pollicis. A fourth muscle, the adductor pollicis, lies deep in the palm and is not part of the thenar eminence.

2. Abductor Pollicis Brevis (APB)

Origin: Tubercle of the scaphoid, trapezium, and the flexor retinaculum. Insertion: Radial side of the base of the proximal phalanx of the thumb and the dorsal extensor expansion. Action: Abduction of the thumb at the carpometacarpal (CMC) and metacarpophalangeal (MCP) joints (movement perpendicular to the palm). It also assists in opposition. Innervation: Recurrent (motor) branch of the median nerve (C8, T1).

3. Flexor Pollicis Brevis (FPB)

Origin: Superficial head from the flexor retinaculum and trapezium; deep head from the trapezoid, capitate, and palmar ligaments. Insertion: Base of the proximal phalanx of the thumb (radial side). Action: Flexion of the thumb at the MCP and CMC joints. Innervation: The superficial head is supplied by the recurrent branch of the median nerve (C8, T1); the deep head is usually innervated by the deep branch of the ulnar nerve (C8, T1).

4. Opponens Pollicis

Origin: Tubercle of the trapezium and the flexor retinaculum. Insertion: Entire length of the radial border of the first metacarpal. Action: Opposition of the thumb (medial rotation and abduction of the first metacarpal, bringing the thumb pad toward the little finger). Innervation: Recurrent branch of the median nerve (C8, T1).

5. Adductor Pollicis (Related Muscle)

Though not a thenar muscle, adductor pollicis is a key thumb muscle. It has two heads: oblique head from the capitate and bases of the second and third metacarpals; transverse head from the palmar surface of the third metacarpal. Both insert onto the ulnar side of the base of the proximal phalanx of the thumb. It adducts the thumb. Innervation: Deep branch of the ulnar nerve (C8, T1).

6. Blood Supply

The thenar muscles receive their blood supply from the superficial palmar branch of the radial artery, the princeps pollicis artery (from the radial artery), and the first palmar metacarpal artery. The venous drainage follows the corresponding veins into the deep palmar venous arch.

7. Clinical Considerations

Carpal tunnel syndrome: Compression of the median nerve within the carpal tunnel leads to atrophy and weakness of the thenar muscles (particularly APB and opponens pollicis), resulting in loss of thumb opposition and flattening of the thenar eminence.

Recurrent branch injury: The recurrent motor branch of the median nerve may be lacerated in superficial palmar wounds near the thenar crease, causing isolated thenar paralysis with intact sensation.

Ape hand deformity: Seen in median nerve palsy (e.g., proximal lesion), where the thumb lies in the plane of the palm due to unopposed adductor pollicis (ulnar nerve) action, and thenar wasting occurs.

Froment’s sign: Tests adductor pollicis function (ulnar nerve). The patient flexes the thumb IP joint during key pinch to compensate for adductor weakness. This sign relates to the adductor pollicis, not the thenar muscles directly, but is important in differential diagnosis.

Summary

Summary of Thenar Muscles

The thenar eminence consists of abductor pollicis brevis, flexor pollicis brevis, and opponens pollicis. They act on the thumb to produce abduction, flexion, and opposition, innervated principally by the recurrent branch of the median nerve. The adductor pollicis, supplied by the ulnar nerve, lies deep and is not part of the thenar group. Lesions of the median nerve produce thenar wasting and loss of opposition, while ulnar nerve lesions affect the adductor and most intrinsic hand muscles.