The radius has a small round head that articulates with the capitellum. The capitellum is the lateral part of the condyle of the humerus.
Directly below the head is the narrow neck, then the shaft of the radius. The posterior interosseous nerve crosses the upper part of the shaft, and can be damaged in fractures or dislocations of the radial head, or in surgery.
The radial tuberosity at the upper end of the shaft is roughened and this is where the biceps inserts.
The distal end of the radius is widened with the prominent radial styloid process on the lateral aspect. It articulates with the scaphoid and lunate bones at the wrist.
The ulnar is large at its proximal end. It has a curve, concave anteriorly, that articulates with the trochlear portion of the humeral condyle.
This trochlear notch is formed by the coronoid process anteriorly and the olecranon posteriorly. The olecranon can be felt as the ‘tip of the elbow’.
Just below, and lateral to the trochlear notch, is the radial notch which articulates with the radius.
The distal end of the ulna has a rounded head and a projection, the ulnar styloid process.
The elbow joint has three separate articulations and moves in two very distinct ways.
The largest part of the joint is between the ulna and the humerus. More precisely, it is between the trochlear notch of the ulna and the trochlear part of the humeral condyle. It acts as a hinge joint and allows flexion and extension of the elbow.
The radial head articulates with the capitellum of the humerus, and it is this part of the joint that can pivot, allowing pronation and supination.
The third articulation is the proximal radioulnar joint. The edge of the radial head articulates with the radial notch of the ulna during pronation and supination
The anterior compartment of the forearm contains muscles that flex the wrist, fingers and thumb and the pronators.
The main flexors of forearm are:
Coracobrachialis flexes the arm at the shoulder. Brachialis flexes the elbow, and the biceps muscle causes flexion at both joints.
- Brachialis The brachialis muscle starts at the lower end of the shaft of the humerus and attaches to the ulna. Some of the lateral part of the muscle is supplied by the radial nerve. Its movement, supination of a flexed forearm, is a good test of the motor component of the C6 nerve root.
Biceps brachialis Both heads of biceps originate from the scapula, the muscle belly lies alongside the humerus, and the final single tendon inserts onto the radial tuberosity of the ulna.
The biceps tendon reflex tests the C6 spinal segment. Biceps movement tests the motor component of C5 (and, to a certain extent, C6).
The nerve supply to these is the musculocutaneous nerve.
Pronator quadratus, together with flexor digitorum profundus and flexor pollicis longus, is in the deepest layer.
The next layer of muscle is made up of flexor digitorum superficialis, while the most superficial muscles are those whose tendons can be seen at the wrist and provide some of the landmarks for nerve blocks. From medial to lateral they are flexor carpi ulnaris, palmaris longus (only 85% of people) and flexor carpi radialis. Pronator teres crosses just below the cubital fossa.
Also, in the anterior compartment are the radial and ulnar arteries and veins, and the median, ulnar and superficial branch of the radial nerves. Compartment syndrome in the anterior compartment can be devastating and lead to Volkmann’s contracture.
The extensors, adductors and abductors of the wrist joint are found in the posterior compartment of the arm, together with the finger and thumb extensors, aconeus and brachioradialis. The deepest layer is made up of:
- Abductor pollicis longus
- Extensor pollicis brevis
- Extensor pollicis longus
- Extensor indicis
They are all innervated by a branch of the radial nerve, the posterior interosseous nerve.
More superficial lie brachioradialis, extensor carpi radialis longus and brevis, extensor digitorum, extensor digiti minimi, extensor carpi ulnaris and aconeus. They all receive their nerve supply via the radial nerve or its branches.
Branches of the radial artery and vein are found in the posterior compartment. Movement of the wrist extensors is the motor test for C8. The patient holds the wrist
Pronator Teres and Pronator Quadratus
Pronator teres and pronator quadratus both ‘do what it says on the tin’. The former runs between the medial epicondyle of the humerus and the lateral radius, the latter between the distal radius and ulna. During pronation the distal radius moves over the ulnar and turns the hand.
The aconeus muscle is mentioned here for want of a better section to put it in. It adducts and abducts the ulna, so that the palm of the hand does not sway from side-to-side during pronation and supination. It is positioned between the lateral epicondyle and the ulna in the posterior compartment of the forearm.