Pronation and Supination
Positioning the hand in space is essential for grip, hand movement, and everyday functioning of the upper limb. Our upper limb has developed and evolved immensely, and in addition to our elongated and opposable thumb, our ability to supinate and pronate gives us enormous mechanical advantage. Turning a screwdriver and turning a key are two examples of the functional movements that utilise pronation and supination. There are a number of muscles involved in each of these movements, which work synergistically. In this article we will discuss the anatomy and clinical relevance of these movements.
Bones and joints
Pronation and supination are movements that occur at the proximal radioulnar joint. The head of the radius is discoid and fits with the radial neck within the circular annular ligament, that attaches the proximal radius to the ulna. The wheel like rotation of the head of the radius enables supination (palm facing upwards), and pronation (palm facing downwards).
Proximal radioulnar joint
This is a synovial joint located between the head of the radius and the radial notch of the ulna. It is enclosed within the same articular capsule as the elbow (humeroulnar joint). The head and neck of the radius sit in the tough fibers of the circular annular ligament, which surrounds them like a collar around a neck. The ligament arises from and inserts back onto the radial notch. The superior fibers of the ligament blend with the ligaments of the elbow, and the lower fibers contribute to the quadrate ligament. The inner ligament is lined with a synovial membrane that ensures smooth action during pronation and supination. Superficially, the radial collateral ligament of the elbow supports the annular ligament. The quadrate ligament sits distally to the annular ligament to prevent excessive supination and strengthen the link between the head of the radius and the ulna. Movements at this joint are restricted to supination and pronation. Flexion and extension of the elbow occurs at the humeroulnar joint.
Distal radioulnar joint
This pivot joint is located distally near the wrist joint, and is formed between the head of the ulna, and the ulnar notch of the radius. The anterior and posterior radioulnar ligaments, as well as a triangular fibrocartilaginous plate support this joint. This triangular fibrocartilage connects the bones and ensures they remain together during pronation and supination. It is thicker at its periphery than at its centre. The thick apex of the triangle attaches to the ulnar styloid process, and its thin base attaches to the prominent edge of the radius, just proximal to the radiocarpal articulation. The triangular fibrocartilage also separates the wrist joint (radiocarpal and ulnocarpal joints) from the distal radioulnar joint. The ulnar notch of the radius slides over the head of the ulna during pronation and supination.
This is a tough membrane of connective tissue that connects the interosseus border of the ulna with the interosseus border of the radius. Its fibers run laterally in an oblique fashion along the entire shaft of both bones and are perforated distally to allow vessels to pass between the anterior and posterior compartments of the forearm (aperture for the anterior interosseous artery). The membrane keeps the radius and ulna bonded during supination and pronation. It also transfers forces between the two bones, and numerous muscles such as flexor digitorum profundus arise from it.
This is a flat, thin ligamentous cord that arises from the ulnar tuberosity distal to the annular ligament, and inserts onto the posteromedial aspect of the radial shaft (just inferior to the radial tuberosity) in an oblique fashion. Hence, its fibers run from medial to lateral to stabilize the proximal radioulnar joint, but are absent in some individuals.
The median nerve innervates this muscle of the anterior compartment of the forearm. It has two heads, an ulnar and a humeral. The larger and more superficial humeral head arises from the medial supraepicondylar ridge. The ulnar head is thin and arises from the medial surface of the coronoid process. The median nerve passes through the two heads in order to reach the forearm, and is separated from the ulnar artery by the ulnar head. The muscle inserts onto the lateral surface of the radius distal to supinator, and hence causes pronation when it contracts.
The anterior interosseus nerve, a branch of the median nerve, innervates this square-shaped muscle in the anterior compartment of the forearm. It arises from the distal shaft of the ulna and inserts onto the lateral shaft of the radius to cause pronation when it contracts.
This muscle from the posterior compartment of the forearm is innervated by the deep branch of the radial nerve (arising from the posterior cord, nerve roots C5-T1), which goes on to become the posterior interosseus nerve. It surrounds the proximal part of the radius, and consists of superficial and deep sets of fibers. The deep set arises from the lateral epicondyle of the humerus, the supinator crest of the ulna, and the radial collateral ligament. It inserts onto the lateral proximal radial shaft, and the oblique line of the radius, just below the insertion of pronator teres.
The radial nerve passes down the spiral groove of the humerus, and emerges anterior to the lateral epicondyle, between brachialis and brachioradialis. The nerve then enters the supinator muscle, and divides into the posterior interosseous nerve and the superficial cutaneous branch. This muscle supinates the forearm, at all angles of elbow flexion and extension. Biceps brachii is only able to supinate the flexed elbow.
The musculocutaneous nerve innervates this muscle. The long head arises from the supraglenoid tubercle of the scapula, and the short head arises from the coracoid process. The tendon of the muscle inserts onto the radial tuberosity. The muscle also expands out as the bicipital aponeurosis, which attaches to the shaft of the ulna. Biceps brachii acts primarily as an elbow flexor, and secondarily as a supinator. It is able to supinate when the elbow is flexed. When the elbow is fully extended, supinator performs the action.
This muscle is innervated by the radial nerve. It arises from the lateral epicondylar ridge and lateral epicondyle. The muscle inserts onto the radial tuberosity. As the muscle crosses the elbow joint, anterior to the joint line, it acts as an elbow flexor, and a semi-pronator of the forearm. When the musculocutaneous nerve is damaged, the brachioradialis muscle still enables elbow flexion due to its innervation from the radial nerve. It flexes best when the forearm is in mid-position between supination and pronation. When the elbow is flexed, the brachioradialis semi-pronates the forearm.