Arm and Cubital Fossa

The arm is enclosed by brachial (deep) fascia, continuous with the pectoral, axillary, and deltoid fascia proximally and the antebrachial fascia distally.

• Two intermuscular septa (medial and lateral) extend from the brachial fascia to the humerus, dividing the arm into anterior and posterior compartments.
• Anterior compartment: flexors, supplied by the musculocutaneous nerve.
• Posterior compartment: extensors, supplied by the radial nerve.

Biceps Brachii

• Origin: Long head, supraglenoid tubercle of scapula (tendon runs through the shoulder joint capsule, within the intertubercular sulcus). Short head, tip of coracoid process (with coracobrachialis).
• Insertion: Radial tuberosity (main tendon); bicipital aponeurosis blends with the deep fascia of the forearm medially.
• Innervation: Musculocutaneous nerve (C5, C6).
• Action: Powerful supinator of the forearm (most effective with the elbow flexed); flexor of the elbow; weak flexor of the shoulder.
• Important relations: Long head tendon lies in the intertubercular sulcus, stabilising the humeral head; bicipital aponeurosis protects the underlying brachial artery and median nerve in the cubital fossa (relevant to venepuncture safety).

Brachialis

• Origin: Distal anterior surface of the humeral shaft (lower half).
• Insertion: Ulnar tuberosity and coronoid process of ulna.
• Innervation: Musculocutaneous nerve (C5, C6); also receives a small contribution from the radial nerve (proprioceptive/partial motor, clinically minor).
• Action: The primary flexor of the elbow: strongest and works regardless of forearm position (unlike biceps, unaffected by pronation/supination).
• Important relations: Lies deep to biceps; forms the floor of the lower arm/cubital fossa.

Coracobrachialis

• Origin: Tip of coracoid process (with short head of biceps).
• Insertion: Medial surface of humeral shaft, mid-third.
• Innervation: Musculocutaneous nerve (C5, C6, C7), pierces the muscle, a classic identifying feature.
• Action: Flexion and weak adduction of the shoulder.
• Important relations: Landmark for locating the musculocutaneous nerve (pierces it) and for the axillary artery's relations in the upper arm.

Triceps Brachii

• Origin: Long head, infraglenoid tubercle of scapula. Lateral head, posterior humeral shaft, superior to the radial groove. Medial head, posterior humeral shaft, inferior to the radial groove (deep head, mostly covered by the other two).
• Insertion: Olecranon of ulna.
• Innervation: Radial nerve (C6, C7, C8).
• Action: Primary extensor of the elbow; long head also assists shoulder extension/adduction.
• Important relations: Radial nerve and profunda brachii vessels pass between the lateral and medial heads, within the radial (spiral) groove of the humerus; long head passes between teres minor and teres major, forming the medial border of the quadrangular and triangular spaces.

Anconeus

• Origin: Posterior surface of lateral epicondyle of humerus.
• Insertion: Olecranon and posterior proximal ulna.
• Innervation: Radial nerve (C7, C8).
• Action: Assists elbow extension; stabilises the elbow joint during pronation/supination; abducts the ulna slightly during pronation.

• Origin: Lateral cord of brachial plexus (C5–C7).
• Course: Pierces coracobrachialis, then descends between biceps and brachialis, supplying all three anterior compartment muscles.
• Termination: Becomes the lateral cutaneous nerve of the forearm after piercing the deep fascia lateral to the biceps tendon, near the elbow.
• Motor supply: Coracobrachialis, biceps brachii, brachialis.
• Sensory supply: Lateral forearm (as lateral cutaneous nerve of forearm).

• Origin: Posterior cord of brachial plexus (C5–T1).
• Course: Enters the arm posterior to the brachial artery, passes through the triangular interval, then runs in the radial (spiral) groove of the humerus with the profunda brachii artery, between the medial and lateral heads of triceps. Pierces the lateral intermuscular septum to enter the anterior compartment distally, running between brachialis and brachioradialis toward the elbow.
• Motor supply (in the arm): Triceps brachii, anconeus (and part of brachialis).
• Sensory supply (in the arm): Posterior cutaneous nerve of arm and forearm (branches given off early in the axilla/arm).
• Clinical relevance: Vulnerable to injury in mid-shaft humeral fractures (radial groove), causes wrist drop; also compressed in "Saturday night palsy" from prolonged pressure against a hard edge.

• Origin: Continuation of the axillary artery at the lower border of teres major.
• Course: Descends along the medial side of the arm (with the median nerve initially medial, then crossing anterior to the artery in the mid-arm), superficial to triceps and brachialis, within the medial bicipital groove.
• Relations: Median nerve crosses from lateral to medial to the artery around mid-arm; ulnar nerve separates from the artery proximally to pass posterior to the medial epicondyle; basilic vein lies medial, superficial to the deep fascia.
• Branches:
• Profunda brachii (deep artery of the arm): largest branch, accompanies the radial nerve through the triangular interval and radial groove; contributes to the elbow anastomosis.
• Superior ulnar collateral artery: accompanies the ulnar nerve, contributes to the elbow anastomosis (posterior to medial epicondyle).
• Inferior ulnar collateral artery: arises near the elbow, contributes to the elbow anastomosis.
• Nutrient artery to the humerus and muscular branches.
• Termination: Divides into radial and ulnar arteries at the neck of the radius, approximately 1 cm distal to the elbow crease (within or just distal to the cubital fossa).
• Surface marking: A line from the midpoint of the clavicle (or the axilla) along the medial border of biceps to the midpoint of the cubital fossa, with the elbow extended and forearm supinated.
• Clinical significance: Standard site for measuring blood pressure (brachial pulse, medial to biceps tendon in the cubital fossa); vulnerable in supracondylar humeral fractures (can cause Volkmann ischaemic contracture if compromised); compressible against the humerus for arm haemorrhage control.

• Boundaries:
• Lateral: medial border of brachioradialis.
• Medial: lateral border of pronator teres.
• Base (proximal): imaginary line between the medial and lateral epicondyles.
• Apex (distal): where brachioradialis and pronator teres converge.

• Roof: Skin, superficial fascia (containing the median cubital vein), deep fascia reinforced by the bicipital aponeurosis.

• Floor: Brachialis (proximally) and supinator (distally).

• Contents (lateral to medial): Radial nerve (and its deep branch, at the lateral edge, between brachioradialis and brachialis), Tendon of biceps brachii, Brachial artery (dividing into radial and ulnar arteries), Median nerve (mnemonic TAN or "Really Tired? Better Move", lateral to medial: Radial nerve, Tendon of biceps, Brachial Artery, Median nerve).

• Arrangement of contents (lateral to medial): Radial nerve → Biceps tendon → Brachial artery → Median nerve.

• Median cubital vein: Connects the cephalic and basilic veins obliquely across the roof of the fossa; lies superficial to the bicipital aponeurosis, which separates it from the underlying brachial artery and median nerve, the anatomical basis for the safety of this site in venepuncture.

• Bicipital aponeurosis: Fibrous expansion from the biceps tendon, sweeping medially over the brachial artery and median nerve before blending with the deep (antebrachial) fascia, protects these deep structures during venepuncture and clinical examination.

• Clinical relevance: Site for brachial pulse palpation, blood pressure measurement (stethoscope placement), venepuncture (median cubital vein), and brachial artery puncture for arterial blood gas/angiography.

• Type: Synovial hinge joint (compound, humero-ulnar, humeroradial, and functionally linked to the proximal radio-ulnar joint sharing the same synovial cavity).

• Articular surfaces:
• Humero-ulnar: trochlea of humerus with trochlear notch of ulna (main hinge, most stable articulation).
• Humeroradial: capitulum of humerus with head of radius.
• Proximal radio-ulnar: radial head with radial notch of ulna (within the same capsule, permits pronation/supination independent of flexion/extension).

• Capsule: Encloses all three articulations in a single synovial cavity; anteriorly and posteriorly thin and lax (permits flexion/extension); reinforced medially and laterally by collateral ligaments.

• Ulnar (medial) collateral ligament: Triangular, three bands (anterior, posterior, oblique/transverse); anterior band is the primary stabiliser against valgus stress; runs from medial epicondyle to the coronoid process and olecranon.

• Radial (lateral) collateral ligament: Fan-shaped, from lateral epicondyle to blend with the annular ligament; primary stabiliser against varus stress.

• Annular ligament: Encircles the head of the radius, attaching to the anterior and posterior margins of the radial notch of the ulna; holds the radial head against the ulna, permitting pronation/supination while allowing hinge movement at the elbow.

• Movements: Flexion and extension (hinge) at the humero-ulnar/humeroradial joints; pronation and supination occur at the radio-ulnar joints, not strictly "elbow" movements but functionally linked given the shared capsule.

• Stability: Primarily from the bony congruency of the trochlea/trochlear notch (most stable joint articulation in the body by bony fit), reinforced by the collateral ligaments (medial > lateral in importance, given valgus stresses of throwing/lifting) and dynamic muscular support (anconeus, flexor/extensor origins).

• Blood supply: Elbow anastomosis: a rich collateral network formed by branches from the profunda brachii (radial collateral, middle collateral), superior and inferior ulnar collateral arteries (from brachial artery), and the recurrent branches of the radial, ulnar, and interosseous arteries (from below). This anastomosis maintains distal perfusion even if the brachial artery is occluded around the elbow.

• Innervation: Musculocutaneous, radial, ulnar, and median nerves all contribute articular branches (Hilton's law).