16.1 Upper Limb: Bones, Superficial Veins, Shoulder Muscles, Axilla & Brachial Plexus, Arm, Cubital Fossa, Forearm, Hand & Joints

Why this area matters

The TOS lists:

• bones of the upper limb;
• superficial structures (fascia, cutaneous nerves, superficial veins, lymphatic drainage);
• anterior thoracoappendicular and scapulohumeral muscles;
• the axilla (artery, vein, lymph nodes, brachial plexus);
• the arm and cubital fossa;
• the forearm and hand;
• all joints from sternoclavicular to interphalangeal.

Road-traffic and fall injuries, which are frequent in the Philippines, make fracture-nerve correlations very testable.

Bones and common fractures

• Clavicle: the most frequently fractured bone in the body, usually at the middle third. The sternocleidomastoid lifts the medial fragment and the weight of the arm pulls the lateral fragment down. In newborns it may fracture during shoulder dystocia.
• Scapula: its spine, acromion and coracoid process are muscle attachments.
• Humerus:
  • surgical neck (axillary nerve, posterior circumflex humeral artery);
  • midshaft (radial nerve in the spiral groove, profunda brachii artery);
  • supracondylar (median nerve and brachial artery; the classic childhood fall on an outstretched hand; risk of Volkmann ischemic contracture);
  • medial epicondyle (ulnar nerve).
• Radius and ulna: a Colles fracture of the distal radius (dorsal displacement, "dinner-fork" deformity) is common in older women with osteoporosis. A Smith fracture is volar displacement.
• Hand:
  • Scaphoid fracture causes tenderness in the anatomical snuffbox. Blood enters distally, so the proximal pole can undergo avascular necrosis.
  • A boxer's fracture is a fifth metacarpal neck fracture.

Superficial structures

• Cephalic vein: lateral; runs in the deltopectoral groove to the axillary vein.
• Basilic vein: medial; pierces the deep fascia in the arm to join the brachial veins and form the axillary vein.
• Median cubital vein: connects them in front of the elbow; the usual venipuncture site. The bicipital aponeurosis separates it from the brachial artery and median nerve.
• Lymph from the upper limb drains to the axillary nodes; the medial hand and forearm drain first through the cubital (supratrochlear) nodes.
• Dermatomes: C5 lateral arm, C6 thumb, C7 middle finger, C8 little finger, T1 medial forearm.

Muscles of the pectoral region and shoulder

• Anterior thoracoappendicular muscles:
  • pectoralis major (medial and lateral pectoral nerves);
  • pectoralis minor (the landmark for the three parts of the axillary artery and the levels of axillary nodes);
  • subclavius;
  • serratus anterior (long thoracic nerve; paralysis causes a winged scapula, for example after mastectomy with axillary dissection).
• Scapulohumeral muscles:
  • deltoid (axillary nerve; abduction from about 15° to 90°);
  • teres major;
  • the rotator cuff (SITS): supraspinatus (initiates abduction; most often torn or impinged), infraspinatus and teres minor (lateral rotation), and subscapularis (medial rotation).
• Shoulder (glenohumeral) joint: shallow and very mobile, stabilized mainly by the cuff and labrum. Anterior dislocation is the most common type and can injure the axillary nerve (numb "regimental badge" area, weak abduction).

The axilla

• Walls: pectoral muscles in front; subscapularis, teres major and latissimus dorsi behind; serratus anterior medially; the humerus laterally.
• Axillary artery: continues from the subclavian at the outer border of the first rib and becomes the brachial artery at the lower border of teres major. Pectoralis minor divides it into three parts, giving 1, 2 and 3 branches:
  1. superior thoracic;
  2. thoracoacromial and lateral thoracic;
  3. subscapular, and anterior and posterior circumflex humeral.
• Axillary lymph nodes: five groups (pectoral, subscapular, humeral, central, apical). Surgeons describe levels I to III relative to pectoralis minor. Clearing them risks lymphedema and injury to the long thoracic and thoracodorsal nerves.

Brachial Plexus Organization & Clinical Plexopathies

Anatomical Architecture of the Brachial Plexus

The brachial plexus is formed by the ventral primary rami of spinal nerves $C_5, C_6, C_7, C_8,$ and $T_1$. It traverses the posterior triangle of the neck and the axilla in five successive structural stages (mnemonic: Roots, Trunks, Divisions, Cords, Branches — "Read That Damn Clean Book"):

1. Roots ($C_5\text{--}T_1$): Emerge between the anterior and middle scalene muscles in company with the subclavian artery. Branches arising directly from roots: Dorsal scapular nerve ($C_5$, rhomboids and levator scapulae) and Long thoracic nerve ($C_5\text{--}C_7$, serratus anterior; injury causes winged scapula and inability to abduct the arm above $90^\circ$).
2. Trunks:
   • Upper Trunk: Formed by the union of $C_5$ and $C_6$. Gives off the suprascapular nerve (supraspinatus and infraspinatus) and nerve to subclavius.
   • Middle Trunk: Direct continuation of $C_7$.
   • Lower Trunk: Formed by the union of $C_8$ and $T_1$.
3. Divisions: Each trunk splits into an anterior division (destined for anterior flexor compartments of the limb) and a posterior division (destined for posterior extensor compartments). All six divisions pass beneath the clavicle through the cervicoaxillary canal.
4. Cords: Named according to their spatial orientation relative to the second part of the axillary artery:
   • Lateral Cord: Formed by the anterior divisions of the upper and middle trunks ($C_5, C_6, C_7$).
   • Posterior Cord: Formed by the union of all three posterior divisions ($C_5\text{--}T_1$).
   • Medial Cord: Formed by the continuation of the anterior division of the lower trunk ($C_8, T_1$).
5. Terminal Branches:
   • Lateral Cord: Musculocutaneous nerve and the lateral root of the Median nerve.
   • Posterior Cord: Axillary nerve, Radial nerve, upper and lower subscapular nerves, and thoracodorsal nerve.
   • Medial Cord: Ulnar nerve, medial root of the Median nerve, and medial cutaneous nerves of the arm and forearm.

Clinical Brachial Plexopathies

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Upper Extremity Peripheral Nerve Injuries & Clinical Deformities

Axillary Nerve ($C_5\text{--}C_6$)

• Anatomical Course: Originates from the posterior cord; winds around the surgical neck of the humerus within the quadrangular space accompanied by the posterior circumflex humeral artery.
• Mechanism of Injury: Fracture of the surgical neck of the humerus; anterior dislocation of the glenohumeral joint; improper crutch use.
• Motor Deficit: Atrophy and paralysis of the deltoid (loss of arm abduction from $15^\circ$ to $90^\circ$; initial $0\text{--}15^\circ$ abduction initiated by supraspinatus) and teres minor (weakened external rotation).
• Sensory Deficit: Cutaneous anesthesia over the lower lateral shoulder ("regimental badge" area).

Radial Nerve ($C_5\text{--}T_1$)

• Anatomical Course: Continuation of the posterior cord; passes posterior to the humerus in the radial (spiral) groove accompanied by the profunda brachii artery (deep brachial artery); pierces the lateral intermuscular septum to enter the anterior forearm.
• Mechanism of Injury: Midshaft humeral shaft fractures; prolonged compression against the back of a chair ("Saturday night palsy"); compression in the axilla from crutches ("crutch palsy").
• Clinical Presentation:
  • Midshaft fracture / Spiral groove lesion: Paralyzes the brachioradialis, extensor carpi radialis, and all wrist/finger extensors $\rightarrow$ Wrist Drop and finger drop; inability to extend fingers at MCP joints. Triceps function and triceps tendon reflex are PRESERVED because branches to the triceps arise proximal to the spiral groove in the axilla.
  • High axillary compression: Complete extensor paralysis including the triceps; absent triceps reflex, wrist drop, and profound sensory loss.
  • Sensory Deficit: Cutaneous loss over the dorsal aspect of the hand, dorsal thumb, and the first dorsal web space.

Median Nerve ($C_5\text{--}T_1$)

• Proximal Lesion (Supracondylar Fracture of the Humerus):
  • Injured as it traverses the anterior cubital fossa medial to the brachial artery.
  • Motor loss: Pronator teres, pronator quadratus (loss of pronation); flexor carpi radialis (weak wrist flexion with ulnar deviation); flexor digitorum superficialis and lateral half of flexor digitorum profundus (loss of flexion of digits 1, 2, and 3); thenar muscles (loss of thumb opposition).
  • Hand of Benediction (Pope's Blessing): When the patient is asked to make a fist, digits 1, 2, and 3 cannot flex and remain extended, while digits 4 and 5 flex normally (intact ulnar nerve).
• Distal Lesion (Carpal Tunnel Syndrome / Wrist Laceration):
  • Compressed beneath the transverse carpal ligament (flexor retinaculum) along with nine flexor tendons (4 FDS, 4 FDP, 1 FPL).
  • Motor loss: Paralyzes the thenar muscles via the recurrent motor branch (OAF: Opponens pollicis, Abductor pollicis brevis, Flexor pollicis brevis superficial head) and the first and second lumbricals.
  • "Ape Hand" Deformity: Thenar eminence atrophy; the thumb is adducted into the plane of the palm by the intact adductor pollicis (ulnar nerve), abolishing opposition.
  • Sensory Deficit: Numbness and paresthesia over the palmar aspect of the lateral $3\frac{1}{2}$ digits. High-Yield Examination Distinction: Sensation over the thenar eminence itself is completely spared because the palmar cutaneous branch of the median nerve arises proximal to the wrist and passes superficial to the flexor retinaculum.

Ulnar Nerve ($C_8\text{--}T_1$)

• Proximal Lesion (Fracture of the Medial Epicondyle / Cubital Tunnel Syndrome):
  • Injured posterior to the medial epicondyle ("funny bone").
  • Motor loss: Flexor carpi ulnaris (weak wrist flexion with radial deviation); medial half of flexor digitorum profundus (FDP to digits 4 and 5); hypothenar muscles; medial two lumbricals (3 and 4); all interossei (PAD [palmar adduction] and DAB [dorsal abduction]); and adductor pollicis.
  • The Ulnar Paradox: The claw hand deformity is LESS pronounced in proximal lesions than in distal lesions because the medial half of the FDP is paralyzed; without FDP pulling the DIP joints of digits 4 and 5 into flexion, the fingers appear straighter despite severe intrinsic paralysis.
• Distal Lesion (Fracture of the Hook of the Hamate / Guyon Canal Compression in Cyclists):
  • Passes through Guyon's canal between the pisiform and hook of the hamate.
  • Motor loss: All intrinsic muscles innervated by the ulnar nerve are lost, but FDP is fully functional.
  • Ulnar Claw Hand (at rest): Unopposed action of intact long flexors (FDP) and extensors produces severe hyperextension at the MCP joints and profound flexion at the PIP and DIP joints of the 4th and 5th digits.
  • Froment Sign: Tests for adductor pollicis paralysis. When asked to grasp a sheet of paper between the thumb and index finger against resistance, the patient cannot adduct the thumb and instead forcefully flexes the distal interphalangeal joint of the thumb using the flexor pollicis longus (FPL), innervated by the anterior interosseous nerve (median nerve).

Peripheral Nerve Lesions of the Upper Extremity

Arm, cubital fossa and forearm

• Arm:
  • The anterior compartment (biceps, brachialis, coracobrachialis) is supplied by the musculocutaneous nerve and gives elbow flexion and supination.
  • The posterior compartment (triceps) is supplied by the radial nerve.
  • The brachial artery runs medial to biceps and is used for blood pressure measurement.
• Cubital fossa:
  • bounded by brachioradialis laterally, pronator teres medially and a line between the epicondyles above;
  • contains, from lateral to medial, the biceps tendon, brachial artery and median nerve (memory aid "TAN");
  • the radial nerve lies just lateral, under brachioradialis.
• Forearm:
  • The anterior compartment contains the flexors and pronators, supplied by the median nerve, except flexor carpi ulnaris and the medial half of flexor digitorum profundus, which the ulnar nerve supplies.
  • The posterior compartment contains the extensors and supinator, supplied by the radial (posterior interosseous) nerve.

The hand

• Carpal bones (proximal row: scaphoid, lunate, triquetrum, pisiform; distal row: trapezium, trapezoid, capitate, hamate).
• The carpal tunnel is roofed by the flexor retinaculum and transmits the median nerve with nine tendons.
• Guyon's canal (between the pisiform and the hook of hamate) transmits the ulnar nerve and artery.
• Intrinsic muscles:
  • thenar (median recurrent branch, the "million-dollar nerve");
  • hypothenar (ulnar);
  • interossei (ulnar: palmar adduct, dorsal abduct);
  • lumbricals (lateral two median, medial two ulnar), which flex the MCP joints and extend the interphalangeal joints.
• Blood supply: superficial palmar arch (mainly ulnar artery) and deep palmar arch (mainly radial artery). The Allen test checks that the ulnar artery can supply the hand before radial artery puncture.
• Infections: paronychia, felon (pulp space) and flexor tendon sheath infection (Kanavel signs) can spread to the midpalmar and thenar spaces.

Joints