Arm and cubital fossa

Compartments of the arm

The medial and lateral intermuscular septa divide the arm into an anterior (flexor) and a posterior (extensor) compartment.

• Distal biceps rupture (sudden pop lifting a weight): the muscle bunches in the arm ("Popeye" bulge), and supination is weak with the elbow flexed. Elbow flexion survives because brachialis and brachioradialis are intact.
• Supination is produced by biceps (musculocutaneous nerve) and supinator (radial nerve, deep branch). Pronation is by pronator teres and pronator quadratus (median nerve).
• The musculocutaneous nerve pierces coracobrachialis, runs between biceps and brachialis, and ends as the lateral cutaneous nerve of the forearm. A crush of the whole anterior compartment removes elbow flexion by biceps and brachialis, weak supination, and numb lateral forearm; brachioradialis still flexes weakly.

Radial nerve in the arm

• From the posterior cord, it passes through the triangular interval (below teres major, between the long head of triceps and the humerus) with the profunda brachii artery, then winds in the spiral (radial) groove of the humerus between the lateral and medial heads of triceps.
• It pierces the lateral intermuscular septum and enters the front of the elbow between brachialis and brachioradialis, where it divides into the superficial branch (sensory) and the deep branch (posterior interosseous nerve).

A patient with numb lateral arm, weak triceps and lost triceps jerk therefore has a radial lesion in the axilla, above the spiral groove.

Cubital fossa

• Boundaries: above, a line between the epicondyles; medially, pronator teres; laterally, brachioradialis. Floor: brachialis and supinator. Roof: skin, superficial fascia with the median cubital vein, deep fascia and the bicipital aponeurosis.
• Contents from lateral to medial: radial nerve (lying deep under brachioradialis), biceps tendon, brachial artery, median nerve.
• The bicipital aponeurosis separates the median cubital vein from the brachial artery and median nerve, protecting them during venepuncture. Venepuncture is difficult when the vein is deep under fat or when the arm is not extended.
• The brachial artery divides into radial and ulnar arteries at the level of the neck of the radius, about 2.5 cm below the elbow crease.

Median nerve above the elbow: ligament of Struthers

About 1% of people have a supracondylar process, a bony spur on the anteromedial humerus about 5 cm above the medial epicondyle. A fibrous band, the ligament of Struthers, runs from it to the medial epicondyle, and the median nerve (with the brachial artery) may be compressed beneath it.

• Because the lesion is above the elbow, pronator teres and all long flexors supplied by the median nerve weaken (thumb and index flexion, pronation), and the thenar skin is numb (palmar cutaneous branch leaves above the wrist). This distinguishes it from carpal tunnel syndrome, which spares thenar skin.
• Do not confuse it with the arcade of Struthers, a fascial band over the ulnar nerve in the distal medial arm.

The elbow joint

• Hinge joint (humeroulnar and humeroradial) sharing one capsule with the superior radioulnar joint, a pivot in which the annular ligament holds the radial head.
• Pulled elbow (radial head subluxation) in children under 5: a sudden pull on the arm draws the radial head out of the annular ligament; the child holds the arm pronated and will not use it.
• Carrying angle about 10–15°, larger in women. A reduced or reversed angle is cubitus varus ("gunstock" deformity); an increased angle is cubitus valgus, which can stretch the ulnar nerve over years (tardy ulnar palsy).
• Ossification centres appear in order: capitulum (about 1 year), radial head (3), internal (medial) epicondyle (5), trochlea (7), olecranon (9), external (lateral) epicondyle (11). The order matters when reading a child's elbow film.
• Three-point relationship: the two epicondyles and the olecranon tip form a triangle with the elbow flexed. It is disturbed in posterior dislocation and preserved in a supracondylar fracture.

Supracondylar fracture in children

• Fall on the outstretched hand; the distal fragment is usually displaced posteriorly (extension type).
• At risk: the brachial artery (cold pulseless hand; untreated forearm ischaemia leads to Volkmann ischaemic contracture of the flexors), the anterior interosseous branch of the median nerve (cannot make an "OK" sign: FPL and FDP to index lost), and the radial nerve.
• Gunstock deformity: malunion with the distal fragment tilted medially (varus) and often rotated. Children remodel well in the plane of joint movement, but not in the coronal plane or in rotation, so varus persists.

Ulnar nerve at the elbow: cubital tunnel syndrome

• The ulnar nerve passes behind the medial epicondyle in the cubital tunnel, roofed by Osborne's ligament (the arcuate band between the two heads of flexor carpi ulnaris).
• Elbow flexion stretches the nerve around the epicondyle and tightens the roof, narrowing the tunnel and raising pressure. Sleeping with the elbows bent causes night tingling in the little and ring fingers; tapping behind the epicondyle reproduces it.
• It is the second commonest compression neuropathy after carpal tunnel syndrome. Fracture of the medial epicondyle injures the nerve directly.
• Lateral epicondylitis (tennis elbow) affects the common extensor origin; medial epicondylitis (golfer's elbow) the common flexor origin.

Remember The ulnar nerve lives behind the medial epicondyle; flexing the elbow stretches it and squeezes its tunnel.

Roots tested around the elbow

A C5/C6 disc compresses the C6 root: weak elbow flexion, reduced biceps or supinator jerk, numb thumb.

Exam trap In a midshaft humerus fracture triceps still works but the wrist drops; a weak triceps puts the radial lesion higher, in the axilla.

Exam trap The median nerve lies medial to the brachial artery in the cubital fossa; the biceps tendon is lateral to the artery.

How FCPS asks about this

• Pop at the elbow, Popeye bulge, weak supination: distal biceps rupture; biceps inserts into the radial tuberosity.
• Midshaft (radial groove) fracture of humerus: wrist and finger drop, numb dorsal first web, triceps spared.
• Contents of the cubital fossa: lateral to medial, radial nerve, biceps tendon, brachial artery, median nerve; aponeurosis protects the artery.
• Supracondylar fracture: brachial artery and anterior interosseous nerve at risk; malunion with medial tilt gives cubitus varus.
• Night tingling in ring and little fingers with elbows bent: cubital tunnel syndrome; flexion stretches the nerve and narrows the tunnel.
• Spur above the medial epicondyle with median signs including pronator weakness: ligament of Struthers.
• Triceps jerk root: C7; biceps and supinator C5–6.