Muscles of the hand

The muscles are arranged in groups related to movements of the thumb (the thenar eminence), movements of the digits and movements of the little finger (hypothenar eminence).

The thenar muscles

The muscles forming the bulge of the thenar eminence are the abductor pollicis brevis, the flexor pollicis brevis and the opponens pollicis.  All muscles are innervated by the recurrent branch of the median nerve. 

The abductor pollicis brevis arises from the scaphoid and from the flexor retinaculum and runs up the lateral side of the eminence to insert into the base of the proximal phalanx of the thumb.  Loss of the recurrent branch of the median nerve to this muscle will not result in loss of abduction since the abductor pollicis longus would still be competent.

The flexor pollicis brevis lies in the same plane but medial to the abductor.  The muscle arises from the flexor retinaculum and inserts into the base of the proximal phalanx of the thumb.  The muscle is able to flex the metacarpophalangeal joint.

The opponens pollicis lies deep to the two other muscles, arising from the flexor retinaculum and inserting all along the lateral side of the shaft of the first metacarpal.  The muscle causes the first metacarpal to rotate at the saddle joint with the trapezium, allowing the thumb to be opposed to the fingers.

The adductor of the thumb lies in the palm, together with the adductors of the other digits, and is innervated by the ulnar nerve.

 

The palmar muscles

In the palm lie the lumbricals, the interossei and the adductor pollicis muscles.

A small lumbrical muscle arises from each of the four flexor digitorum tendons as they pass through the palm.  Each lumbrical muscle passes dorsally around the lateral sides of the metacarpophalangeal joints to insert into the dorsal digital expansion.  The muscles aid in flexion of the metacarpophalangeal joint and extend the interphalangeal joints.  The two lateral lumbricals are innervated by the median nerve while the two medial lumbricals are innervated by the ulnar nerve.

The interossei are arranged into dorsal and palmar groups.  The dorsal interossei abduct the index, middle and ring fingers (the middle finger can be abducted both medially and laterally and therefore has two dorsal interossei).  The three palmar interossei adduct the index, ring and little finger towards the middle finger.  The muscles arise from the sides of the 2-4th metacarals and pass around the metacarpophalangeal joint to insert into the dorsal digital expansion.  All interossei in addition to being eith adductors or abductors, flex the metacarpophalangeal joints and extend the interphalangeal joints.  Innervation is by the deep branch of the ulnar nerve.

The  adductor pollicis arises by oblique and transverse heads from the second and third metacarpal bones and inserts into the base of the proximal phalanx of the thumb.  Innervation is by the deep branch of the ulnar nerve, in common with the other adductors of the digits.  

 

The hypothenar eminence

The muscles forming this ridge on the medial side of the palm produce movement of the fifth metacarpal and little finger.  All muscles are innervated by the deep branch of the ulnar nerve.

Abductor digiti minimi lies on the outside of the eminence, arising from the pisiform and inserting into the base of the proximal phalanx of the little finger.  The muscle abducts the little finger.

Flexor digiti minimi lies superficially on the palmar aspect of the hypothenar eminence.  The muscle arises from the flexor retinaculum and inserts into the base of the proximal phalanx of the little finger.  The muscle is able to flex the metacarpophalangeal joint.    

The opponens digiti minimi lies deep to the flexor.  Arising from the flexor retinaculum it inserts into the medial border of the fifth metacarpal bone.  The muscle helps in cupping the hand.  

Applied MSK anatomy of the hand

The hand is at risk in many ways due to its wide range of use and its role in protection against falls etc. The components to be considered here are the tendons and synovial sheaths. Flexor tendons are thick cords of dense regular connective tissue. As the tendons run to their insertions they are restrained by annular and cruciate ligaments at the wrist and in the digits. These ligaments improve the mechanical efficiency of tendon excursion by preventing bowstringing. The tendons run in synovial sheaths lying in the fibro-osseous tunnels. The blood supply to the flexor tendons arises from proximal palmar vessels at the muscle-tendon junction, vincula longa and breva and from vessels at the distal bony insertion. Each tendon has a longitudinally running chief artery. The tendon sheaths of the flexor tendons provide easy movements for the long flexor tendons. They exist as two synovial sheaths at the wrist. One sheath, on the radial side, encloses only the tendon of flexor pollicis longus and continues from the wrist to close to the insertion on the distal phalanx of the thumb. The much larger sheath on the ulnar side is the common flexor tendon sheath beginning proximal to the flexor retinaculum. The common flexor tendon sheath runs deep to the flexor retinaculum and continues to about the middle of the palm. An ulnar portion continues around the long flexor tendons to the little finger. In contrast the flexor tendons to the index, long and ring fingers usually begin blindly near the bases of the fingers, without any connection to the main sheath. The significance of this is that infections within the tendon sheaths on the index, long and ring fingers can extend proximally towards the wrist only by rupture of the sheaths. Infections within the tendon sheaths of the little finger or thumb routinely reach the wrist. Extensor tendons are paratenon tendons and lie in loose areolar tissue on the dorsum of the hand. They are thin and flat. Two of the most serious complications of tendon surgery, adhesion and rupture, can be even more serious in extensor tendons than flexor tendons.