Introduction

The upper extremity is composed of its associated muscles, nerves, and vessels, which are organized into anatomical compartments.[1] The muscles cross joints to provide tone, maintain dynamic joint stability, and perform dynamic functions of the entire extremity.  In addition, the arteries and veins provide nourishment and remove waste, and the nerves provide motor and sensory innervations.[1]

Structure and Function

The shoulder muscles originate from the axial skeleton and the scapula and insert onto the humerus. The muscles that originate from the anterior compartment of the brachium act to flex the forearm, while the muscles that originate from the posterior compartment of the brachium extend the forearm. The anterior muscles of the forearm act to flex the hand at the wrist joint, and the posterior muscles act to extend the hand at the wrist joint.[1]

The hand is comprised of eleven separate compartments. These are the four dorsal interossei, three volar interossei, the thenar, the hypothenar, the adductor, and the mid-palm compartments, respectively.[2] The dorsal interossei act to abduct the digits, and the palmar interossei act to adduct the digits*. The thenar and adductor compartment muscles act on the thumb, while the muscles of the hypothenar compartment act on the small finger.

*Memory trick PAD-DAB palmar interossei adduct and the dorsal interossei abduct the digits. 

• Interossei - Latin = between the bones
• Thenar - Greek = the mound at the base of the thumb

Embryology

The embryology of the muscles in the upper extremity is a step-wise process. In the first step, the myogenic progenitor cells located in somites give rise to the primary myotomes.[3] The second wave of myogenic progenitors arises from dermomyotomes that give rise to embryonic myoblasts.[3] The myoblasts then proliferate, fuse, and ultimately give rise to the primary muscle fibers.[3]

Blood Supply and Lymphatics

The anatomical components of the upper extremity receive their vascular supply via the subclavian artery and its branches. The subclavian artery arises from the brachiocephalic arterial trunk on the right side. The brachiocephalic arterial trunk is the first branch of the arch of the aorta. The left subclavian artery arises as a direct branch from the arch of the aorta. The subclavian artery becomes the axillary artery as it passes over the lateral border of the first rib. The axillary artery supplies the shoulder muscles via its six branches: the superior thoracic artery, the thoracoacromial arterial trunk, the lateral thoracic artery, the subscapular artery, and the anterior and posterior circumflex humeral arteries.

The axillary artery ends at the lower border of the teres major muscle, where it gives rise to the brachial artery. The brachial artery supplies the arm and then divides to form the radial and ulnar arteries, which supply the forearm and hand. The deoxygenated blood of the upper extremity drains via the cephalic, basilic, and brachial veins, which then flow into the subclavian vein and ultimately to the heart. The extracellular fluid is cleared by the lymphatic system. The right upper extremity feeds the right lymphatic duct, and the left upper extremity empties into the thoracic duct.[1] 

• Brachium - Latin = arm
• Subclavian - Latin = beneath the clavicle; "little key")
• Brachiocephalic - Latin brachium = arm + cephalic (Latin from Greek = head)

Nerves

Innervation to the upper extremity derives from the brachial plexus. It is composed of the ventral rami from the C5 to T1 nerve roots. The anterior muscles of the upper extremity receive their innervation from the musculocutaneous, median, and ulnar nerves. The posterior muscles are innervated by the axillary and the radial nerves.[4] 

The spinal cord gives off dorsal (sensory) and ventral (motor) roots. These join to form the mixed spinal nerve. The mixed spinal nerve then gives off a dorsal primary ramus (which feeds the area of the back adjacent to the vertebrae -perhaps three inches on either side). The much larger ventral rami then supply most of the body at the appropriate level. For the brachial plexus, the roots of the plexus are formed by the ventral primary rami from C5 through T1. The primary rami of C5 and C6 are termed roots of the brachial plexus. They join to form the superior trunk of the plexus. The C7 primary rami form the C7 root of the plexus. It forms the middle trunk of the brachial plexus. The C8 and T1 ventral primary rami form the C8 and T1 roots of the plexus. They join to form the inferior trunk of the brachial plexus. 

The next step involves separating the neural components that supply the anterior part of the limb from the posterior part of the limb. The anterior divisions supply the anterior part, and the posterior part of the upper limb is supplied by the posterior divisions. 

For the superior trunk of the brachial plexus C5, C6),  the anterior division separates from the posterior division. The anterior divisions of C5 and C6 join to form the superior trunk of the plexus.

For the middle trunk, the anterior division separates from the posterior division. The anterior division joins with the superior division to form the lateral cord of the brachial plexus. The anterior division of the inferior trunk continues as the medial cord. All the posterior divisions join to form the posterior cord. 

The three cords - lateral, medial, and posterior - are named for their relationship to the axillary artery. These cords then give off branches which are specific nerves. 

• A memory trick for this arrangement is "Randy Travis drinks cold beer" - roots trunks division cords branches. 

Branches of the Brachial Plexus

The dorsal scapular nerve arises from the C5 root, often with a contribution from C4. It innervates the rhomboid and (sometimes) the levator scapulae.

The suprascapular nerve arises from the superior trunk. It innervates the supraspinatus and infraspinatus muscles.

The lateral cord is formed by the ventral roots of C5, C6, and C7. It gives rise to the musculocutaneous nerve, the lateral pectoral nerve, and the lateral root of the median nerve. 

The musculocutaneous nerve gives rise to motor branches that innervate the coracobrachialis, biceps brachii, and brachialis muscles. It is referred to as the "BBC nerve." It then passes into the forearm as the lateral cutaneous nerve of the forearm.

The lateral pectoral nerve innervates the clavicular head of the pectoralis major. 

The lateral cord contributes to the lateral root of the median nerve. The medial root of the median nerve is formed from the medial cord of the brachial plexus. The median nerve has no branches in the arm. It passes into the forearm medial to the brachial artery, where the nerve can be damaged by a needle used to sample arterial blood from the brachial artery. The median nerve passes beneath the flexor digitorum superficialis and innervates it. It then passes into the hand beneath the flexor retinaculum (which forms the roof of the carpal tunnel). In the hand, it gives rise to the recurrent branch of the median nerve, which innervates the three thenar muscles (abductor pollicis brevis, flexor pollicis brevis, and opponens pollicis). The median nerve supplies sensory function to the thumb, index finger, middle finger, and half the ring finger. This pattern is useful for testing for lesions of the median nerve. The median nerve can be damaged in the carpal tunnel, giving rise to carpal tunnel syndrome. The three muscles of the thumb supplied by the median nerve will be denervated in this lesion.

The sensory distribution involves the thumb, index finger, middle finger, and the lateral half of the ring finger. One can test for carpal tunnel syndrome by forced flexion of the hands at the wrist. One has the patient place the dorsal surfaces of the hands together and straighten the forearms to a horizontal position. The patient is then instructed to hold this position for 30 to 60 seconds. If carpal tunnel syndrome is present in either hand, this provocative test will cause pain because it puts additional stress on the median nerve in the carpal tunnel. One can also use the reverse Phalens' test in which the palmar surfaces of the hands are placed together rather than the dorsal surfaces. If a carpal tunnel is severe, one can divide the flexor retinaculum to relieve the pressure on the median nerve. One can also microtunnel the flexor retinaculum to relieve pressure on the nerve. 

While in the forearm, the median nerve gives rise to the anterior interosseus nerve, which innervates the flexor pollicis longus and the radial half of the flexor digitorum profundus. It ends by innervating the pronator quadratus. One test for the anterior interosseus nerve is to have the patient touch the tip of the thumb to the tip of the index finger ("OK sign"). This action requires the use of the flexor digitorum profundus (to flex the distal phalanx of the index finger) and the flexor pollicis longus (to flex the distal phalanx of the thumb).   

The medial cord is formed by the ventral roots of C8 and T1. The medial cord gives rise to the ulnar nerve, the medial root of the median nerve, the medial pectoral nerve, the medial cutaneous nerve of the arm, and the medial cutaneous nerve of the forearm.

The ulnar nerve has no branches in the arm. It passes into the forearm by passing posterior to the medial epicondyle, where it can easily be injured by fracture of the medial epicondyle or sitting with the elbows on a desk while studying ("student's palsy"). In the forearm, it innervates the flexor carpi ulnaris and the medial half of the flexor digitorum profundus. It passes through Guyon's canal between the pisiform and the hook of the hamate bone. It can be injured here by a fracture of the hook of the hamate bone. It gives rise to a superficial branch that innervates the skin of both sides of the little finger and the ulnar half of the ring finger. The deep branch innervates the abductor digiti minimi, opponens digiti minimi, and flexor digiti minimi, the four dorsal interossei muscles, the three palmar interossei muscles, lumbricals 3 and 4, and ends by supplying the adductor pollicis.

When the adductor pollicis is suspected to be denervated by an ulnar nerve lesion, the examiner has the patient attempt to hold a piece of paper between the thumb and index fingers. The patient then attempts to hold the paper as the examiner pulls the paper from the patient's hand. When the patient attempts to hold the paper without the adductor pollicis, he will automatically flex the distal phalanx of the thumb using the flexor pollicis longus. This is termed "Froment's sign" and signifies the presence of a lesion of the ulnar nerve that has denervated the adductor pollicis.[5] If the patient can still flex the distal phalanx of the little finger when making a fist, the ulnar nerve in the forearm is intact. The lesion of the ulnar nerve is, therefore, in the hand. If the patient cannot flex the distal phalanx, then the lesion is in the forearm or higher (perhaps in Guyon's canal or at the medial epicondyle). If the patient can flex the distal phalanx, then the lesion is in the hand.

The medial pectoral nerve supplies the pectoralis minor and the sternocostal head of the pectoralis major.

The medial cutaneous nerve of the arm supplies sensory function to the skin of the medial surface of the arm. The medial cutaneous nerve of the forearm supplies sensory function to the skin of the medial surface of the forearm. Note that this nerve does not cross the wrist. If the patient has anesthesia or pain in the medial surface of the hand, this must be due to a lesion of the ulnar nerve and not the medial cutaneous nerve of the forearm. If the area of anesthesia or pain is present on both the medial side of the distal forearm and hand, the lesion is due to damage to the C8 root and dermatome. (A dermatome is an area of skin supplied by a single dorsal root for sensory function).

The posterior cord of the brachial plexus is formed from the C5-T1 roots. It gives rise to the axillary and radial nerves. For motor function, the axillary nerve innervates the deltoid and teres minor muscles and terminates by forming the upper lateral cutaneous nerve of the arm, a sensory branch. The radial nerve has a much longer course and function. It passes around the shaft of the radius where it can be injured. It then passes over the lateral epicondyle, which is also subject to injury. It innervates the triceps brachii and anconeus in the arm. In the forearm, it innervates the brachioradialis and extensor carpi radialis longus. These two muscles receive radial nerve branches that form while in the arm. A fracture of the lateral epicondyle may spare these two muscles while damaging the rest of the radial nerve.

The nerve then divides into a superficial and a deep branch. The superficial branch passes beneath the brachioradialis to provide a sensory function to the dorsal thumb and part of the proximal dorsal surfaces of the fingers. The deep branch innervates the supinator by passing through it. It emerges from the supinator in a fascial area termed the arcade of Frohse, where it may be compressed by the fibers of the arcade, damaging the nerve. It then becomes the posterior interosseus nerve, which innervates the extensor carpi radialis brevis, the extensor digitorum, the extensor digiti minimi, and the extensor carpi ulnaris. The radial nerve has no motor function in the hand. Damage to the radial nerve produces a wrist drop in which the patient is unable to extend the hand at the wrist. One can test the degree of damage by holding the patient's hand and having him extend it against resistance. Alternatively, the examiner may try to push the hand into flexion. Resisting the flexion requires extension by muscles innervated by the radial nerve. If the hand radially deviates during this testing, then the innervation to the brachioradialis and extensor carpi radialis longus is intact, and the lesion must be at the level of the lateral epicondyle. Damage to the radial nerve at mid-humeral levels or higher will produce a complete wrist drop. Other sensory branches of the radial nerve are the lower lateral cutaneous nerve of the arm, the posterior cutaneous nerve of the arm, the posterior cutaneous nerve of the forearm, and the superficial branch of the radial nerve in the hand.  

The posterior cord of the brachial plexus gives rise to three nerves before the formation of the radial and axillary nerves. These are the upper subscapular, thoracodorsal, and lower subscapular nerves. The upper and lower subscapular nerves innervate the subscapularis muscle. The lower subscapular nerve also innervates the teres major muscle. The thoracodorsal nerve innervates the latissimus dorsi muscle.

Muscles

Anterior Axioappendicular Muscles

Pectoralis Major

• Function: flexion, adduction, medial rotation of the humerus
• Origin: clavicular head: medial clavicle, anterior sternum, costal cartilages of ribs 1 to 6 and external oblique aponeurosis
• Insertion: the lateral edge of the intertubercular sulcus (groove) of the humerus
• Nerve: medial pectoral nerve (C8, T1) for the pectoralis minor muscle and the sternocostal head of the pectoralis major muscle.  Lateral pectoral nerve (C5, C6, C7) for the clavicular head of the pectoralis major

Pectoralis Minor

• Function: depression of the shoulder and protraction of the scapula
• Origin: third, fourth, and fifth ribs
• Insertion: coracoid process of the scapula
• Nerve: medial pectoral nerve (C8, T1)

Subclavius

• Function: depression and stabilization of the clavicle
• Origin: medial side of the first rib
• Insertion: the inferior-middle part of the clavicle
• Nerve: nerve to subclavius (C5, C6)

Serratus Anterior  

• Function: protraction of scapula and rotation of the scapula
• Origin: first through eighth ribs
• Insertion: medial part of the anterior scapula
• Nerve: long thoracic nerve (of Bell) (C5, C6, C7)

Posterior Axioappendicular Muscles

Superficial Layer

Latissimus Dorsi 

• Function: adduction, medial rotation, and extension of the humerus
• Origin: the spinous process of the seventh to twelfth thoracic vertebrae, the iliac crest, the thoracolumbar fascia, and the inferior third and fourth rib
• Insertion: intertubercular groove of the humerus
• Nerve: thoracodorsal nerve (C5, C6, C7)

Trapezius 

• Function: elevation, depression, and retraction of the scapula and rotation of the glenoid cavity
• Origin: superior nuchal line, nuchal ligament, occipital protuberance, and the spinous process of C7- T12
• Insertion: spine of the scapula, the acromion, and the lateral clavicle
• Nerve: cranial nerve 11 (spinal accessory nerve

Deep Layer

Levator Scapulae

• Function: adduction, medial rotation, and extension of the humerus
• Origin: the transverse process of C1 through the C4 vertebrae
• Insertion: superior portion of the medial border of the scapula
• Nerve: dorsal scapular nerve (C4, C5)

Rhomboid Major

• Function: retraction of scapula and depression of the glenoid cavity
• Origin: spinous processes of T2 through the T5 vertebrae
• Insertion: inferior aspect of the medial scapula
• Nerve: dorsal scapular nerve (C4, C5)

Rhomboid Minor

• Function: retraction of scapula and depression of the glenoid cavity
• Origin: nuchal ligament and the spine of C7 and the T1 vertebrae
• Insertion: the superior aspect of the medial scapula
• Nerve: dorsal scapular nerve (C4, C5)

Intrinsic Muscles of the Shoulder

Deltoid 

• Function: anterior portion; flexion and medial rotation of the arm, middle portion; arm abduction, posterior portion; extension and lateral rotation of the arm
• Origin: lateral clavicle, acromion, and the scapular spine
• Insertion: deltoid tuberosity
• Nerve: axillary nerve (C5, C6)

Teres Major

• Function: adduction and medial rotation of the brachium
• Origin: posterior aspect of the scapula at the inferior angle
• Insertion: intertubercular groove on the medial aspect
• Nerve: lower scapular nerve (C5, C6)

Teres Minor 

• Function: lateral rotation of the arm (brachium)
• Origin: posterior aspect of the scapula at the inferior angle
• Insertion: the inferior aspect of the greater tubercle
• Nerve: axillary nerve (C5, C6)

Supraspinatus 

• Function: initiation of arm abduction
• Origin: posterior scapula, superior to the scapular spine
• Insertion: the superior aspect of the greater tubercle
• Nerve: suprascapular nerve (C5, C6)

Infraspinatus

• Function: lateral rotation of the brachium
• Origin: posterior scapula, inferior to the scapular spine
• Insertion: greater tubercle of the humerus, between the supraspinatus and teres minor insertion
• Nerve: suprascapular nerve (C5, C6)

Subscapularis

• Function: adduction and medial rotation of the arm (brachium)
• Origin: anterior aspect of the scapula
• Insertion: lesser tubercle of the humerus
• Nerve: upper and lower subscapular nerves (C5, C6, C7)

Muscles of the Anterior Compartment of the Brachium

Biceps Brachii

• Function: flexion and supination of the forearm.
• Origin: short head originates from the coracoid process and the long head from the supraglenoid tubercle of the scapula
• Insertion: radial tuberosity and forearm fascia (as bicipital aponeurosis)
• Nerve: musculocutaneous nerve (C5, C6, small contribution C7)

Brachialis

• Function: flexion of the forearm
• Origin: distal anterior humerus
• Insertion: coronoid process and the ulnar tuberosity
• Nerve:  musculocutaneous nerve (C5, C6, C7)

Coracobrachialis

• Function: flexion and adduction of the brachium
• Origin: coracoid process
• Insertion: medial aspect of the middle of the humerus
• Nerve: musculocutaneous nerve (C5, C6, C7)

Muscles of Posterior Compartment of the Brachium

Triceps Brachii

• Function: extensor of the forearm
• Origin: lateral head, above the radial groove, medial head, below the radial groove, and the long head from the infraglenoid tubercle of the scapula
• Insertion: olecranon process of the ulna and the forearm fascia
• Nerve: radial nerve (C6, C7, C8)

Anconeus

• Function: extension of the forearm
• Origin: lateral epicondyle of the humerus
• Insertion: olecranon process and the posterior aspect of the ulna
• Nerve: radial nerve (C7, C8, T1)

Muscles of Anterior Compartment of Antebrachium (Forearm)

Superficial Layer

Pronator Teres 

• Function: pronation of the radio-ulnar joint
• Origin: coronoid process and the medial epicondyle of the humerus
• Insertion: lateral aspect of the radius
• Nerve: median nerve (C6, C7)

Flexor Carpi Radialis

• Function: flexion and adduction of the hand at the wrist
• Origin: medial epicondyle of the humerus
• Insertion: base of the second metacarpal
• Nerve: median nerve (C6, C7)

Palmaris Longus (inconsistent; absent in approximately 15% of the population)

• Function: flexion of the hand at the wrist
• Origin: medial epicondyle of the humerus
• Insertion: flexor retinaculum
• Nerve: median nerve (C7, C8)

Flexor Carpi Ulnaris

• Function: flexion and adduction of the wrist
• Origin: medial epicondyle of the humerus and the olecranon process of the ulna
• Insertion: pisiform, the hook of hamate, and the fifth metacarpal
• Nerve: median nerve (C7, C8)

Intermediate Layer

Flexor Digitorum Superficialis 

• Function: flexion of the middle phalanx at the proximal interphalangeal joint of the second, third, fourth, and fifth fingers
• Origin: medial epicondyle, coronoid process, and the anterior radius
• Insertion: second, third, fourth, and the fifth middle phalanges
• Nerve: median nerve (C7, C8, T1)

Deep Layer

Flexor Digitorum Profundus

• Function: flexion of the distal interphalangeal joint of the second, third, fourth, and the fifth finger
• Origin: medial and anterior aspect of the proximal ulna and interosseous membrane
• Insertion: second, third, fourth, and the fifth distal phalanges
• Nerve: ulnar nerve (C8, T1) for the medial part, anterior interosseous nerve (C8, T1) for the lateral part

Flexor Pollicis Longus

• Function: flexion of the distal phalanx at the interphalangeal joint of the thumb
• Origin: anterior aspect of the radius and the interosseous membrane
• Insertion: base of the distal phalanx of the thumb
• Nerve: anterior interosseous nerve (C7, C8)

Pronator Quadratus

• Function: pronation of the forearm
• Origin: anterior aspect of the distal ulna
• Insertion: anterior aspect of the distal radius
• Nerve: anterior interosseous nerve (C7, C8)

Brachioradialis

• Function: flexor of the forearm
• Origin: the proximal supracondylar ridge on the humerus
• Insertion: lateral aspect of the distal end of the radius
• Nerve: radial nerve (C5, C6, C7)

Muscles of Posterior Compartment of the Antebrachium

Superficial Layer

Extensor Carpi Radialis Longus

• Function: extension and abduction of the hand at the wrist
• Origin: the proximal supracondylar ridge on the humerus
• Insertion: dorsal base of the second metacarpal
• Nerve: radial nerve (C6, C7)

Extensor Carpi Radialis Brevis 

• Function: extension and abduction of the wrist
• Origin: lateral epicondyle of the humerus
• Insertion: dorsal base of the third metacarpal
• Nerve: the deep branch of the radial nerve (C7, C8)

Extensor Digitorum

• Function: extension of the middle phalanx at the proximal interphalangeal joint of the second, third, fourth, and fifth fingers
• Origin: lateral epicondyle of the humerus
• Insertion: extensor expansions of the dorsal aspect of the second, third, fourth, and fifth phalanges
• Nerve: posterior interosseous nerve (C7, C8)

Extensor Digiti Minimi

• Function: extension of the little finger at the metacarpophalangeal joint and interphalangeal joints
• Origin: lateral epicondyle on the humerus.
• Insertion: extensor expansion on the dorsal aspect of the fifth phalanx
• Nerve: posterior interosseous nerve (C7, C8)

Extensor Carpi Ulnaris

• Function: extension and adduction of the wrist
• Origin: lateral epicondyle of the humerus and the posterior ulna
• Insertion: fifth metacarpal base
• Nerve: posterior interosseous nerve (C7, C8)

Deep Layer

Extensor Indicis

• Function: extension of the index finger
• Origin: the dorsal aspect of the distal ulna and interosseous membrane
• Insertion: extensor expansion of the second finger
• Nerve: posterior interosseous nerve (C7, C8)

Supinator 

• Function: supination of the forearm
• Origin: lateral epicondyle and supinator crest of the ulna
• Insertion: lateral aspect of the radius
• Nerve: deep branch of the radial nerve (C7, C8)

Abductor Pollicis Longus 

• Function: abduction of the thumb
• Origin: dorsal aspects of the proximal radius, ulna, and interosseous membrane
• Insertion: base of the first metacarpal
• Nerve: posterior interosseous nerve (C7, C8)

Extensor Pollicis Longus 

• Function: extension of the thumb
• Origin: dorsal aspects of the middle ulna and interosseous membrane
• Insertion: distal phalanx of the thumb
• Nerve: posterior interosseous nerve (C7, C8)

Extensor Pollicis Brevis

• Function: extension of the thumb
• Origin: dorsal aspects of the middle radius and interosseous membrane
• Insertion: distal phalanx of the thumb
• Nerve: posterior interosseous nerve (C7, C8)

Intrinsic Muscles of Hand

Thenar Muscles

Opponens Pollicis 

• Function: opposition of the thumb
• Origin: flexor retinaculum and the tubercle of the trapezium
• Insertion: lateral aspect of the thumb
• Nerve: recurrent branch of the median nerve (C8, T1)

Abductor Pollicis Brevis

• Function: abduction of the thumb.
• Origin: flexor retinaculum and the tubercle of the scaphoid
• Insertion: lateral aspect of the proximal phalanx of the thumb
• Nerve: recurrent branch of the median nerve (C8, T1)

Flexor Pollicis Brevis 

• Function: flexion of the thumb.
• Origin: flexor retinaculum and the tubercle of the trapezium.
• Insertion: lateral aspect of the proximal phalanx of the thumb.
• Nerve: recurrent branch of the median nerve (C8, T1)

Adductor Compartment

Adductor Pollicis 

• Function: adduction of the thumb
• Origin: second and third metacarpal and the capitate bone
• Insertion: proximal phalanx and extensor expansion of the thumb
• Nerve: deep branch of the ulnar nerve (C8, T1)

Hypothenar Muscles

Abductor Digiti Minimi

• Function: abduction of the little finger
• Origin: pisiform
• Insertion: medial aspect of proximal phalanx of the fifth finger
• Nerve: deep branch of the ulnar nerve (C8, T1)

Flexor Digiti Minimi Brevis

• Function: flexion of the little finger
• Origin: flexor retinaculum and the hook of the hamate
• Insertion: medial aspect of the proximal phalanx of the fifth finger
• Nerve: deep branch of the ulnar nerve (C8, T1)

Opponens Digiti Minimi

• Function: opposition of the little finger
• Origin: flexor retinaculum and the hook of the hamate
• Insertion: medial aspect of the fifth metacarpal
• Nerve: deep branch of the ulnar nerve (C8, T1)

Short Muscles

Lumbricals 

• Function: flexion of the metacarpophalangeal joints with the extension of the interphalangeal joints.
• Origin: arise from tendons of flexor digitorum profundus.
• Insertion: extensor expansions of the second, third, fourth, and fifth fingers
• Nerve: median nerve (C8, T1) for the lateral two lumbricals, deep branch of the ulnar nerve (C8, T1) for the medial two lumbricals

Dorsal Interossei 

• Function: abduction of the second, third, and fourth fingers
• Origin: adjacent metacarpals
• Insertion: extensor expansions and proximal phalanges of the second, third, and fourth fingers
• Nerve: deep branch of ulnar nerve (C8, T1)

Palmar Interossei

• Function: adduction of the second, third, and fourth fingers
• Origin: palmar aspect of the second, fourth, and fifth metacarpals
• Insertion: extensor expansions and proximal phalanges of the second, fourth, and fifth fingers
• Nerve: deep branch of the ulnar nerve (C8, T1)[1]

Physiologic Variants

Many physiological variants may manifest in the upper extremity. These variations are often a cause for concern because they may lead to a misdiagnosis. The following are a few that may occur.

Under normal anatomical circumstances, the extensor indicis (EI) arises from the dorsal aspect of the distal part of the ulna and interosseous membrane. It inserts into the expansion hood of the index finger. It may present as a double tendon and lead to clinical symptoms within the hand.[6]

The flexor digitorum superficialis typically originates from the medial epicondyle, coronoid process, and anterior aspect of the radius. It inserts at the base of the middle phalanx of the second, third, fourth, and fifth phalanges. There have been reports of the flexor digitorum superficialis with two muscle bellies, which may lead to volar forearm compression and pain.[7]

The extensor digitorum brevis manus is a variant muscle located in the dorsum of the hand. It leads to a variation of the fourth extensor compartment within the hand.[8]

Surgical Considerations

Surgical considerations are important and need to evaluate anatomical variations of the upper extremity.

The double tendon of the EI leads to an increased volume of the fourth dorsal tunnel that may lead to clinical symptoms. Surgical excision to equalize the thickness of the medial slips of the two tendons relieves mechanical stress and may correct the symptomatology.[6]

If the flexor digitorum superficialis has two muscle bellies, then the increased mass may lead to clinical symptoms in the anterior forearm. Many previously reported cases received surgical treatment but depending on the level of symptoms; they may be treated non-operatively.[7]

The extensor digitorum brevis manus anatomical variant can be treated with either decompression or excision surgery. Recent studies have shown that the excisional method of complete removal of the anatomical variant proved beneficial because it is the most effective treatment. Excision leads to improved outcomes compared with decompression.[8]

Clinical Significance

Compartment syndrome is an emergent clinical condition that is well-documented in the literature. It is a clinical diagnosis that is often difficult to determine because it must be assessed in a timely manner. Although the lower leg and the forearm have the most common incidence, compartment syndrome may occur in any muscle compartment in the body.

Even though compartment syndrome of the hand is rare, it is important to have a high index of suspicion. The hallmark of diagnosis is pain with passive stretching, yet one may see pain and swelling as well. Determining the pressure in the compartment is key in the diagnosis. Pressures above 30 mmHg warrant an emergent surgery. Failure to act may lead to debilitating consequences.[2][9]

Other Issues

Neuropathies are a common manifestation in the hand compartments. Carpal tunnel syndrome (CTS) is the most common neuropathy of the hand compartments, affecting 1 to 3 persons in 1000 per year. The specific cause is unknown, but it is a multifactorial syndrome.[10][11][10]

CTS diagnosis is via electromyography and nerve conduction testing. Treatment begins conservatively, usually starting with a reduction in provoking factors. The next step is implementing non-steroidal anti-inflammatory medications and a nightly wrist splint. The subsequent step is to use a local glucocorticoid injection to decrease the inflammation. The final intervention is to release the carpal tunnel surgically to decompress the components.[10]