Elbow Joint: Distal Humerus, Ligaments and Nerve Relations

Articular surfaces and ligaments

The elbow is formed by two humeral surfaces, the trochlea and the capitulum, meeting the ulna and the radius, with three fossae above them that receive the forearm bones at the ends of movement.

Distal humerus

• Capitulum: the rounded lateral knob, on the front and lower surface only, for the concave upper surface of the radial head.
• Trochlea: the pulley-shaped medial surface, wrapping round to the back of the bone, for the trochlear notch of the ulna.
• Coronoid fossa: above the trochlea in front, for the coronoid process in full flexion.
• Radial fossa: above the capitulum in front, shallower, for the rim of the radial head in full flexion.
• Olecranon fossa: the deep hollow behind, for the olecranon in full extension.
• Epicondyles: the medial epicondyle is larger and carries the common flexor origin; the lateral epicondyle carries the common extensor origin.

Ligaments

• Ulnar collateral ligament: a triangle of anterior, posterior and oblique bands from the medial epicondyle to the coronoid process and olecranon. The anterior band is the main restraint to valgus stress.
• Radial collateral ligament: from the lateral epicondyle to the annular ligament, leaving the radius free to rotate.
• Annular ligament: a band encircling the radial head, attached to the front and back of the radial notch of the ulna and narrower below than above.

The long axis of the extended forearm angles laterally away from the arm, the carrying angle, which is usually larger in women. Fat pads lie between the capsule and synovial membrane over the fossae.

Relations

Each of the three main nerves of the forearm passes the elbow at a fixed point, and each is injured there.

• In front: the cubital fossa, bounded by brachioradialis laterally and pronator teres medially, holds from lateral to medial the biceps tendon, the brachial artery and the median nerve. The brachial artery divides into radial and ulnar arteries at about the level of the radial neck. The median nerve leaves between the two heads of pronator teres; the deep (ulnar) head separates it from the ulnar artery.
• Laterally: the radial nerve lies in the gap between brachialis and brachioradialis with the radial recurrent artery. In front of the lateral epicondyle it divides into the superficial branch and the deep branch, which pierces supinator and becomes the posterior interosseous nerve.
• Behind the medial epicondyle: the ulnar nerve, in contact with the ulnar collateral ligament, entering the forearm between the heads of flexor carpi ulnaris.
• Superficially: the cephalic vein laterally, the basilic vein medially, and the median cubital vein joining them across the bicipital aponeurosis. The medial cutaneous nerve of the forearm runs with the basilic vein, and the lateral cutaneous nerve of the forearm lies deep to the cephalic vein.

Blood supply and innervation

The elbow is supplied by an arterial anastomosis that links the vessels of the arm with those of the forearm.

The anastomosis keeps the forearm alive if the brachial artery is tied off below the profunda brachii, but it does not protect against the sudden kinking of a supracondylar fracture. Following Hilton's law, the joint receives articular branches from the nerves of the muscles that cross it: musculocutaneous, radial, median and ulnar.

Clinical relevance

Elbow injuries are dangerous mainly because of the artery and nerves that lie against the bone.

• Supracondylar fracture: the commonest elbow fracture in children, usually the extension type from a fall on the outstretched hand. The proximal fragment can injure the brachial artery and the median nerve, most often its anterior interosseous fibres; the radial nerve is at risk with posteromedial displacement, and the ulnar nerve in the flexion type. Missed ischaemia leads to forearm compartment syndrome and Volkmann's ischaemic contracture.
• Fat pad sign: on a lateral radiograph a raised anterior fat pad, or any visible posterior fat pad, indicates an effusion and suggests an occult fracture.
• Elbow dislocation: usually posterior. Dislocation with a radial head fracture and a coronoid fracture is the terrible triad.
• Pulled elbow: in young children a sudden pull on the hand draws the radial head partly out of the annular ligament.
• Epicondylitis: tendinopathy of the common extensor origin (tennis elbow, chiefly extensor carpi radialis brevis) or common flexor origin (golfer's elbow).
• Medial epicondyle avulsion: in adolescents the fragment can be trapped in the joint with the ulnar nerve.

On the specimen

The elbow stations combine a dry distal humerus with an anterior dissection of the cubital fossa and the radial nerve branches.

• Bone: the capitulum is lateral and does not reach the back of the bone; the trochlea is medial and does. The coronoid fossa is the deeper hollow above the trochlea; the radial fossa is the shallow one above the capitulum. The olecranon fossa is behind. The medial epicondyle is the bigger, more prominent one.
• Veins: the cephalic vein is lateral, the basilic medial, and the median cubital vein runs obliquely upwards and medially between them, over the bicipital aponeurosis. The thin nerve lying on the basilic vein is the medial cutaneous nerve of the forearm.
• Fossa contents: the thick biceps tendon, then the brachial artery medial to it, then the median nerve most medially, disappearing between the heads of pronator teres. The deep (ulnar) head of pronator teres is seen crossing between the median nerve and the ulnar artery.
• Radial side: part brachioradialis away from brachialis to see the radial nerve, with the radial recurrent artery running up beside it. Follow the nerve down: the superficial branch continues under brachioradialis; the deep branch passes into supinator as the posterior interosseous nerve.
• Behind: the ulnar nerve lies in the groove behind the medial epicondyle.