Thirty-four muscles act on the hand. Intrinsic muscles of the hand contain the origin and insertions within the carpal and metacarpal bones. Muscles originating in the forearm are the extrinsic muscles of the hand. A common rule of thumb is that any muscle tendon that crosses a joint will act on that joint. For example, muscles of the forearm that cross the carpometacarpal joint will produce flexion or extension at the wrist joint. This article presents an overview of the intrinsic muscles of the hand and briefly touches on blood supply.
The primary blood supply of the hand comes from the deep palmar and superficial palmar arches. The superficial branch of the radial artery forms anastomoses with the superficial palmar arch. The deep branch runs through the dorsal interossei muscle and anastomoses at the deep palmar arch. The ulnar artery divides into the deep branch which anastomoses with the deep palmar arch, and the superficial branch which ends at the superficial palmar arch. This anastomosing network allows for blood supply coming from multiple directions, which becomes important when ischemia or other insults occur.
The hand muscles are grouped into extrinsic and intrinsic compartments. The extrinsic muscles of the hand originate in the forearm and are located on the anterior and posterior aspect of the forearm, with flexors positioned anteriorly and extensors posteriorly. These muscles perform the gross movements of the hand and wrist while intrinsic muscles primarily produce fine motor movements.
Intrinsic hand muscles originate and insert from the bones, ligaments, and fascia of the hand. These muscles divide into thenar, hypothenar, and adductor compartments.
The thenar muscles are a group of three muscles that act on the thumb. The positioning of these muscles forms the bulge on the palmar surface of the thumb and proximal, lateral palm, called the thenar eminence. The largest of the three muscles, opponens pollicis, originates at the tubercle of the trapezium and inserts at the lateral margin of the metacarpal of the thumb. It permits the thumb to reach across the palm towards the little finger, by flexing and medially rotating the metacarpal on the axis of the trapezium. The abductor pollicis brevis muscle is positioned anterior to the opponens pollicis and is the primary muscle providing the act of opposition. It originates at the tubercles of the scaphoid and trapezium and inserts at the lateral aspect of the proximal phalanx of the thumb. Abductor pollicis brevis also acts by drawing the thumb away from the midline, which is the action of abduction for all muscles. Flexor pollicis brevis originates at the tubercle of the trapezium via the deep head, and the associated flexor retinaculum via the superficial head, and inserts at the base of the proximal phalanx of the thumb. These three muscles are all innervated by the recurrent branch of the median nerve. The flexor pollicis brevis receives dual innervation with fibers from both the median and ulnar nerves.
The hypothenar muscles act on the little finger and form a bulge on the medial palmar surface, called the hypothenar eminence, which is not as prominent as the thenar eminence. The opponens digiti minimi originates at the hook of hamate and associated transverse carpal ligament and inserts at the ulnar side of the fifth metacarpal. Contraction of the opponens digiti minimi draws the small finger radially, reaching across the palm by flexion and supination. The action of the opponens pollicis and opponens digiti minimi allow for the thumb and the little finger to touch. Abductor digiti minimi originates from the pisiform bone and the tendon of flexor carpi ulnaris and inserts at the ulnar base of the proximal phalanx of the small finger. Contraction of this muscle allows a movement away from the midline, just as the abductor pollicis brevis muscle directs the thumb away from the midline. Flexor digiti minimi brevis originates at the hook of hamate and the transverse carpal ligament and has its insertion located at the base of the proximal phalanx of the small finger. Palmaris brevis originates at the transverse carpal ligament and inserts on the skin of the medial palm. It permits wrinkling of the skin on the palmar surface of the hand and protects the ulnar nerve. The ulnar nerve innervates all muscles of the hypothenar compartment.
The adductor pollicis muscle occupies the adductor compartment. The adductor pollicis muscle originates from two places which we refer to as the oblique and transverse heads. The oblique head originates at the capitate, second and third metacarpals, and inserts at the ulnar base of the proximal phalanx of the thumb. The transverse head arises at the third metacarpal and also inserts at the medial aspect of the proximal phalanx of the thumb. The ulnar nerve provides innervation, permitting adduction and flexion of the metacarpophalangeal joint.
If you make the shape of an L by straightening your second through fifth fingers, via the extension at the proximal interphalangeal joints, and flexing at the metacarpophalangeal joints, you are using muscles of the hand called lumbricals. There are four lumbricals. The first two lumbricals originate from the radial aspect of the first and second tendons of flexor digitorum profundus and insert at the radial lateral bands. The median nerve provides innervation to the two radial lumbricals. The third and fourth lumbricals originate from the ulnar aspect of the medial three flexor digitorum profundus tendons and insert at the radial lateral bands. The ulnar nerve innervates the two ulnar lumbricals. The first and second lumbricals are unipennate, meaning that all muscle fascicles positioned on the same side of a particular tendon. The third and fourth lumbricals are bipennate, meaning run on both sides of the ligament.
The interossei muscles act by adducting and abducting the fingers. The mnemonic PAD and DAB helps to remember the actions of each muscle. The "P" in PAD stands for the palmar interossei. Interossei muscles on the palmar surface adduct the fingers, bringing them towards the midline. The "D" in DAB stands for dorsal interossei. The dorsally placed interossei permit abduction of the phalanges, moving them away from the midline.
To continue the mnemonic, the second and third letter of PAD and DAB stand for the first two letters of the direction where the fingers are moving. The dorsal interossei originate from the adjacent sides of two metacarpals. They insert at the extensor hood and proximal phalanx of each phalange. There are three palmar interossei, which each originate at the medial or lateral surface of the 2, 4, and fifth metacarpals and insert at the base of digits 2-4 and the extensor hood of each finger. These muscles receive innervation from the deep branch of the ulnar nerve. The dorsal interossei are bipennate, and the palmar interossei are unipennate.
The intrinsic muscles of the hand provide the fine motor movements while the extrinsic muscles permit strength. There are several cases where extrinsic muscles and tendon interruption result in hand deformity and malfunction. Situations, where intrinsic hand muscles are solely affected, are less common but are still relevant. Ulnar nerve compression at Guyon's canal can manifest as atrophy and weakness in interosseous muscles, the third and fourth lumbricals, and adductor pollicis. The physical exam will reveal weakness with abduction and adduction of the fingers, flexion at the MCP and extension at the PIP, and adduction and flexion at the MCP of the thumb. Ulnar nerve compression will also affect the hypothenar compartment because most intrinsic hand muscles receive innervation from the ulnar nerve. Median nerve compression at carpal's tunnel will affect the muscles of the thenar compartment. Carpal tunnel syndrome is elicited on physical exam by Tinel's test, where the practitioner taps on the medial aspect of the wrist at the median nerve. Numbness and tingling during this maneuver is a positive sign. A positive Phalen's sign test is also indicative of carpal tunnel syndrome. The patient raises the hands out in front of himself, flexes the wrist and places the dorsal wrists together, while flexed, for about 60 seconds. Numbness and tingling with this maneuver is also a positive sign. Other injuries of note can be of a traumatic nature, lending to lesions at tendon insertion which would alter the mechanics of the intrinsic muscle involved. For example, if blunt trauma struck the insertion of the opponens pollicis muscle, the action of thumb opposition and supination would be significantly altered despite intact innervation.
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